Multiphase Flows

Dr. Rajesh Kumar Upadhyay

Lecture - 17
Measurement Techniques: Velocity Measurement

Welcome back to this course. Now what we have discussed in the last class is about the
different modelling technique, we have discussed about the algebraic slip model, we
have discussed about the Aurelian Eulerian model, we have discussed about the Eulerian
Lagrangian model.

And while presenting those models I always say that there is a limitation for each
modelling method. And why there is a limitation because you assume something which
may or may not valid, or we use some drag closure or some closure like a drag closure,
which may or may not be applicable for the system you are investigating, or you are
trying to understand.

And therefore, we I have shown you a slide where I said that there is a different
techniques available, but each technique have their own limitation you are using some
model, which is not scientifically developed which is developed based on certain
assumption and therefore, it is important to validate experimentally your simulation data.

And for experimentally validation of the data what you need you need some
measurement technique which can perform the or we can measure, the condition at the
condition you want to do the measurement with, the accuracy or with the desired
accuracy and the second thing, you need a technique which can be used at all the scales
if possible with the equal accuracy.

And that will give you the luxury to do your numerical simulation at a smaller scale first,
then you go for a higher scale you see that how your numerical simulation predictions
are being done and, whether those predictions are matching with the experimental
measurements or not.

So, what you need you have to do the first numerical simulation for the say laboratory
scale system because that will be easy to simulate that will take less time to simulate,
then you do the validation at numerical simulation numerical method technique at the
laboratory scale at a smaller scale by using some experimental measurements. Now, you
can use the same model which you have validated at laboratory scale, to do the scale up
see the phenomenon at a larger scale, or say pilot plant scale, or even industrial scale do
some measurement at that scale also to see that your scale up or your model which has
been used to validated a laboratory scale, is working fine even at a larger scale. That
need to be done and that exercise needed one thing, and that is called measurements of
the flow parameters.

Now, as I said that already that in multiphase flow measurement, once I am talking about
the measurements actually I talk about the two quantities, I want to understand the two
quantities.

(Refer Slide Time: 03:04)

One is velocity, another one is the phase fraction. We have already discussed that these
two parameters, whether it is a velocity or the phase faction in any multiphase flow
reactors or process vessels, it is a function of position, it is the function of time it means
you need to measure this parameters and you need to measure the parameter with the
position and with the time.

The thing require is that the measurements should be accurate it should give a higher
spatial resolution it means with the position the accuracy is very high. It should have a
very high temporal resolution it means with the time the accuracy should be higher and
you need the technique which can be used at all the scale with equal accuracy. So, that is
the requirement is there and that requirement will fulfil your job, if we want to validate
your numerical simulations or numerical model which we have developed. And the
validation is must because we are using several things or several closures or several
equations which are empirically developed and because, they are empirically developed
we need experimental validation.

Now, if you want experimental validation, then one need to understand that what are the
methods you can use or what are the technique you can use to experimentally measure
the parameter, what is the limitation of these techniques in the introduction classes, I
have already said that why the multiphase flow is still as a major challenge as a field of
research because, there is the region that there is no single numerical method available or
numerical technique available which can do the prediction at all scale with the equal
accuracy for all the system.

Similarly, there is no measurement techniques available, which can major the

So, what I am going to do I am going to give you a glimpse of the different measurement
techniques which can be used in the multiphase flow reactors, we will try to understand
the capability the limitation of each technique and, the capability of each technique and,
based on that on your system of choice the system you want to study the model, you
have used and the parameter which you want to understand, you can use this technique
you can choose one of the technique to validate your numerical simulation or to validate
your numerical model.

So, therefore what I am going to do in this class, we are going to briefly discuss about
the measurement techniques available for the multiphase flow reactors. Now the major
challenge why you need a dedicated measurement techniques because, most of the single
phase flow measurement techniques, suppose if I want to measure the velocity I want to
measure the mass flow rate. So, most of the technique which is being used cannot be
used here because of the multiphase flow nature of the flow.

So, suppose if you want to use rotameter, you want to use the rotameter of two-phase
flow you cannot do that because rotameter what you do you calculate the drag. Now
once they will be a gas flow drag will be different, once they will be liquid flow the drag
will be different because, they are you have a density of the continuous phase and that
depends on the Reynolds number and again that will be going to be depend on the
density and viscosity of the continuous phase.

So, therefore the based on that you cannot use the rotameter similar problem comes with
the venture similar problems with your orifice meter and all. So, you cannot measure
that. Second thing in the numerical simulation whatever we are doing as I said that we
are solving detailed equation. So, if you just validate it with the superficial velocity, it
will be a very crude validation and, you cannot trust that the validation is correct or not
your model has been rigorously validated or not.

So, the rigorous validation of the model is needed because, you are using those model to
predict the flow phenomena inside the reactors. So, the measuring the global parameter
and validating your model with the global parameters are superficial velocity is not a
good idea, you will not able to conclude whether your model is working fine or it is not
working fine.

So, what you need you need a local information you need a average information, but that
should be inside the system and definitely, you need a validation for the mean velocities
inside mean volume fraction inside, but you also need the validation for the local
condition or the fluctuating velocity and fluctuating component of the volume fraction,
that how the volume fraction is changing with the time. And, then only you can say that
the model is able to predict the flow conditions with a high accuracy and therefore, the
techniques should also have a capability to do so.

And to do that what we need we use some sophisticated technique to measure the flow
parameter. And those measurement techniques actually we divide in two paths one is
velocity measurement technique and, second one is the phase fraction measurement
technique. So, these two technique you are going to discuss and we will briefly discuss
this again I am saying I am not going in detail of this I will just give the overview. So,
that based on your model based on the model which you have developed, the parameter
you want to validate, you can use a suitable technique.
(Refer Slide Time: 08:29)

Let us start with the velocity measurement techniques here, velocity measurement
technique or any measurement technique actually is divided in two path, one is called
invasive, another one is called non-invasive. Now, what does the invasive means as the
name suggest it means you are intruding something inside the flow.

So, suppose there is the pipeline where the fluid is flowing gas and liquid both are
flowing so, gas and liquid both are flowing, you want to measure the velocity what you
do you intrude some probe inside. And that is called intrusive once you intrude
something inside some probe inside of the flow, to measure the velocity field or to
measure the flow parameter that thing is called that technique is called intrusive
measurement technique.

Now, this technique generally is very low cost technique it is cheap it give sometimes a
direct measurement, but the major problem is because you are intruding something inside
you may change the flow physics at the point of measurement itself. So, it means the
flow dynamics of the system may change and, that to be at the point of measurement
itself. And that is the major limitation of the intrusive technique, but still this techniques
are widely used in industry in research mainly because, this techniques are very low-cost
technique, you can easily use this technique, the this gives mostly direct measurement
and therefore, it is relatively simpler to use this technique for the measurements.

So, if you are able to assure that your flow change, in the flow is not big you do not need
that much accuracy and you are able to maintain in such a way that condition that it is
the dynamics is not changing much. You can use intrusive technique for the
measurement of the flow quantities like velocity or volume fraction, but if you are not
very sure about it you need a very high accuracy, the non-invasive technique are the
option where, we used to measure from the outside we do not disturb the flow.

Now, the moment you do not disturb the flow what is you are going to do you are going
to do, indirect measurement it means you measure something else and you will convert
that in terms of the velocity. Therefore, there is always a problem. So, back
reconstruction problems so, the reconstruction becomes an issue and kind of applicability
of each technique, or the accuracy of each technique will be depending upon the position
the reconstruction, algorithm will use and the method you used to measure the flow
quantity, somewhere like to use camera, somewhere we use of laser, somewhere we use
kind of radiation. So, depending upon what is you are using your accuracy of the system
is going to be a get affected because of that.

In invasive technique velocity measurement technique, which is again I said that it is not
that much accurate because, the flow can change at the point of measurement, but is still
very low-cost and that is why it is still very attractive. And the accuracy can be
maintained up to the desired limit, if you are able to understand the condition
requirement and, you are able to maintain the flow condition between that.

So, invasive technique is divided in 3 parts of this is not only 3 there are many others,
but they are mainly 3 technique which is widely used, one is pitot tube hot wire
anemometry and optical fiber probe. Now pitot tube nowadays is not being used widely
because, there are other measurement techniques is there, invasive measurement
technique is there, but still I would like to say the pitot tube because most of you have
already done the pitot tube under graduate studies or in your fluid flow courses.

So, I will trover the pitot tube I will try to give the brief difference between the pitot
tube, whatever you have studied and how it is being used for the multiphase flow, then
we look towards the hot wire anemometer and the optical fibre probe and, then we will
discuss the non-invasive technique, under which I will mainly discuss about the PIV or
LDA laser Doppler anemometer particle image velocimetry positron emission particle
tracking and radioactive particle tracking. So, this technique I will be discussing briefly.
(Refer Slide Time: 12:23)

So, start with the pitot tube as we know that how the pitot tube work, there is a tube
inside there is a flow suppose there is a pipeline you use a pitot tube which will be like
this and, you connect this pitot tube with the YouTube nanometre to measure the
pressure drop that, how it will be the pressure port will be there and how it is being
measure that the streamline.

Suppose the fluid which will be coming here, once it will come and heat at this place it
will achieve this stagnation pressure, once the it is achieved the stagnation pressure it
means the velocity will goes to 0, the pressure will increase and because of that you will
see that the selection in the manometer fluid. And this height change in the manometer
fluid can be correlated with the velocity by using the Bernoulli equation. So, that is the
basic principle of the pitot tube which you have already done and, that can be measure
that velocity can the measure as V will be equal to under root 2  P upon rho.

So, that is the way you measure that this P will be the manometer reading and that can
be calculated as rho of the fluid minus rho of manometer into to g into delta h. So, that is
the way delta h can be  P can be calculated, you can use the Bernoulli equation to
calculate the velocity. So, that is the very basic principle of measuring the flow by using
the pitot tube.

But the major problem with the pitot tube with this kind of a pitot tube is there is a
multiphase flow reactors. So, what we measure that the delta P how we measure that we
measure it at a wall tap, which is at the full condition you are somewhere at this location.
And 1 pressure which we are measuring at the stagnation pressure.

But in multiphase flow actually we assume that this pressure whatever you are measuring
is going to be the static at pressure and, that you can be kind of the constant everywhere,
but in multiphase flow what will happen because the phase distributions are there they
will be bubbles say gas liquid system they will be bubble, they will liquid phase flow.
So, what you need this static the pressure should be measured near the stagnation
pressure.

And to do that what we do we use 5 point probes or 6 point probes, we are we have a
sensor probe where the velocity will be kind of a stagnation pressure will be achieved it
will be measured and other probe we put other holes we put on the periphery of this
probe this central hole like this, you can see that there are different holes here on the
periphery of this and, they are actually 45 degree apart. So, this angle we have
maintained at around 45 degree.

So, that they did not see the stagnation pressure directly and, the stagnation pressure
affect will be there only at the central location. So, this pressure will be realized and this
pressure combined will give you the average pressure around that central probe and, we
measure the delta P by the average of this say there is a 4 hole you will measure P 2, P3,
P4, P5. So, say this is P2, this is P3, this is P4 this is P5 and this say is P P 1. You measure
that pressure as we calculate the average pressure from all this probe, you calculate this P
bar will be P2 plus, P3 plus, P4 plus, P5 upon 4 because the 4 probes and V will be equal
to under root of (2 (P1 –P bar)/) and that way you calculate the velocity.

So, this is the method which we used in the Pitot tube, to measure the velocity of the
flow. The first problem it is a point measurement, it is measuring the velocity will be at 1
location. So, our dream to measure the flow phenomena with the position with the time
will not be fulfil, you can measure with the time, but you cannot measure with the
position. So, if you want to measure with the position what you need to do, you have to
insert several pitot tube inside the flow or, you have to use the same pitot tube and we
have to keep it at the different location for the different time.

So, if you put the several pitot tube inside the flow, definitely the intrusive nature will
increase the disturbance will be very high. Therefore, the error in the measurement will
also be very high, if you use the same pitot tube and place it at different location then,
you will not able to get a cross correlation or the correlation between the change in the
flow at the centre of the column and the change in the flow near the wall at the same
time. That luxury will be missing that information will be missing and therefore, this will
not able to get the detailed information.

Second thing is that this probe because you are intruding inside cannot be used for the
solid system, because what will happen the soluble go and choke all holes they will go
and sit inside all these holes. So, there will be problem will be created. Second we know
that if the bubble coalescence take place, or bubble bursting take place, the bubble dead,
or bubble generation both without bubble generation due to the coalescence, or bubble
death due to the kind of bursting of the bubble.

What will happen, you will see a huge difference in the pressure and that huge difference
in the pressure will be recorded by these 4 probes. Therefore, the P presenting itself
will be very difficult to measure and, that is put another challenge in the Pitot tube. So,
that is the limit the application of the pitot tube for the solid system or very dense system
in the gas-liquid. And the dilute system you can still use it, where the bubble coalescence
or bubble bursting phenomena can be minimised.

So, this is about the pitot tube is there. It is used a lot in the industry or for the
measurement, particularly in the late eighties where people have use the pitot tube for the
measurements in the multiphase flow reactors, but slowly the use of the pitot tube has
been reduced in the multiphase flow reactors and it is limited mainly to the single phase
flow, or a very mild conditions of the multiphase flow reactors like, measuring the
velocity of the aeroplane and all you can still use the pitot tube.
(Refer Slide Time: 18:04)

What we do there is a limitation in the Pitot tube and therefore, a new technique has been
invented and that is called hot wire anemometer. Now hot wire anemometer for single
phase flow it is very accurate and most of the turbulence analysis have been done this by
using the hot wire anemometer, in the single phase flow and because it was very accurate
for the single phase flow, the same technique has been tried to implement in the
multiphase flow reactors to.

So, what we do here in the hot wire anemometer, we use a wire definitely as the name
suggests there is wire which is maintained at a certain temperature or certain condition.
Now, how it is being maintained or what condition it has been maintained this has been,
there is a two type of hot wire anemometer one is constant temperature hot wire
anemometer, another one is constant current hot wire anemometer. So, we say that
constant temperature HWA and there is a constant current hot wire anemometer

So, based on how you are maintaining the condition of this wire, we have a constant
temperature; we have a constant current hot wire anemometer. So, what we do there is
wire which means sort inside and let us assume that we are using the constant
temperature hot wire anemometer, this wire is a part of heat stone bridge circuit. So, we
know that what is the Wheatstone bridge circuit so, suppose there is resistance. So, this is
R 1, R 2, R 3, R 4. So, we know that R 1 upon R 2 will be equal to R 3 upon R 4.

So, the resistance will be an equilibrium and therefore, what will happen some current
will be passed some current, if you connect to the voltmeter this will be there, or constant
current will be maintained. Now, if you change there any of the resistance what will
happen there will be change in the current and, that change in the current can be recorded
in terms of the change in the voltage that is the basic principle is being used

Now, how the resistance will be changed. So, what we do we put 1 of the resistance
edges of the resistance I extend it and. I put this resistance, which is nothing, but a small
wire in the flow. So, like this is the resistance 1; 1 resistance of the Wheatstone bridge
and that is being suspended or that has been put in the flow condition.

Now, what will happen suppose if I give a initial voltage or reference current or
reference voltage, then some current will pass through this wire and because, some
current will pass through this wire there, will be certain energy or certain temperature
will be maintained to the wire. Because, you know that the power will be equal to what I
square R that will be the which will be supplied to the wire. Now, we are supplying some
power we are supplying some current. So, the wire will be maintained at a certain
temperature.

Now, the moment you put this wire into the flow, what will happen because of the flow
the temperature will try to go down, there will be heat loss, which will be taking place
from the wire to the fluid. And because of that heat loss what will happen the wire
temperature will change.

Now, we know that the temperature and the resistance is directly correlated. So, if you
temperature will change your resistance will also change and therefore, the current pass
this whole combination will be different. And because to maintain this condition what
we need to do you have supply the extra current and that extra current supply can be
recorded in terms of the change in the voltage which you need to do, to supply the extra
current.

So, what you will get you will see that your voltage requirement will change. And the
voltage the requirement change is correlated with the velocity by using the king’s law
where it is saying that E square, where E is the voltage output will be equal to A + B into
velocity raise to the power 0.5. Therefore, you record the change in the voltage and that
cost because of the change in the temperature, which is taking place and, if you want to
maintain the same temperature of the wire what you need to do you have to supply the
extra current and to supply the extra current you have to give the extra voltage.

So, that change in the voltage you record and that change in the voltage is being
correlated with the kings law kings law, which is says that E square is equal to A plus B
into u raise to the power 0.5, where U is the velocity U is the velocity and E is the
voltage, do this and you measure the voltage change and that voltage change is correlated
with the velocity

Now, what we need to do here you see that we need a constant A and B, which is the
constant this convective constant of the kings law equation and therefore, what you need
you need a calibration. Therefore, what you are doing is a indirect measurement you are
not measuring the velocity, we are measuring the change in the voltage. And that change
in the voltage. So, what you need to do you need to do the calibration? So, what we do
we put hot wire anemometer in the flow and we operate it at a different known velocities.

So, what we have we know the U we give a reference current. So, we know that what is
the temperature and, then based on the velocity the cooling will take place. And
therefore, you increase to maintain the constant temperature this condition of constant
HWA we need to supply extra voltage. And that voltage to U graph we measure, then we
plot it because this plotted between the U square and U raise to the power 0.5.

Now, if you do this plot you will get a straight line which will intersect on the Y axis that
intersect is A and the slope of this curve will be B. So, you get A and B values and based
on that A and B value, you can find the constant. Now, you can insert this probe into the
flow where you do not know the velocity, you will get the voltage output again and by
using this constant which you have already found, by using the calibration you can
calculate the velocity of the flow. So, that is the basic principle we use in hot wire
anemometer, it can be operated whether you maintain the temperature constant, or you
lead the change the temperature and you maintain the constant current.

So, in both the cases you can operate it there are different equations are available I am
not going in detail of those application, if you are interested you can discuss with me or
you can go through the books or literature, which is based on the hot wire anemometer,
but that is the basic principle as I discuss of the hot wire anemometer to measure the flow
rate.
Now, the major thing of the hot wire anemometer is that we are using only convective
loss. Loss of the current or loss of the temperature because of the convection and, that to
be the post-convection and therefore, this hot wire anemometer cannot be used for very
low velocity, where the natural conviction may also dominate the conduction may also
dominate, the radiation losses may also dominate.

So, we are neglected all these losses while using the kings law and, we have said that
only the loss because of the forced convection is being counted. And because only the
loss because of the forced convection has been accounted you cannot use the hot wire
anemometer for a very low velocity system.

So, generally or typically what the velocity of more than 1 meter per second hot wire
anemometer being used; So, if your flow is having a very low velocity HWA cannot be
used, then this different HWA is available different type of HWA is available depending
upon number of wires we use. So, suppose there is a single wire hot wire anemometer
like this, what will happen you will get only 1 dimensional velocity, because there is
only 1 wire you will get the average velocity of 1 dimensional the average velocity will
get?

But if you want both X and Y velocity what you need you need 2 wires. So, 1 wire will
be it is call X wires. So, there will be 1 flow in this direction, another will be say if the
flow is in this direction say, what will happen for each wire it will form an angle say this
is the flow this flow is coming at it in this direction, it will form an angle and this flow
can be divided into the that this will be 1 component of the velocity say, it will be u sin 
and this will be the u cos, it use the velocity.

So, u sin theta is the X component of the velocity. So, you will Y component of the
velocity it will be uy this is the ux. So, you will get both X and Y component of the
velocity it means you will get both the directional velocity X and Y. If you will have
only 1 wire you will get only 1 directional velocity.

So, to measure the 2 velocity we use X Y similarly if you want 3 directional velocity we
use triple split wire, where there is a 3 wires there and you have this wires are aligned at
a particular angle, you do this component wise analysis again and you get all the 3
component of the velocities. So, that is the way hot wire anemometer is used this is use.
The similar way it is being used in the multiphase flow generally, whatever the way I
told you it is being used in the single phase flow also the multiphase flow there is 1 more
challenge and, that challenge is to identify the phase because both the phases are present
inside.

So, what will happen the cooling of the wire will be happen because of both the phases.
So, what you need to do your job has increased, which is in single phase flow you just
measure the voltage change and, that voltage change we recorded in terms of the velocity
or we calibrate in terms of the velocity.

Now, here two level calibration need to be done. And why the two level calibration need
to be done because, we know that the thermal resistance or thermal property of each
phase or each material is different. So, suppose in a gas liquid system if the gas is
coming in contact of the wire, rate of cooling will be different if the liquid is coming in
contact of the wire, rate of cooling will be different, why because the heat capacity the
thermal conductivities are different.

And therefore, you need another level calibration to first need a calibration, you have to
do the same calibration A and B for the air, you have to do the same calibration for the
water, you need to see that for the velocity, how much for the same velocity change how
much voltage change will be required, once the air comes into the contact what is the
ground level voltage for the air, what is the ground level voltage for the water, and based
on that voltage you do the phase discrimination.

So, like this is a typical example which is taken by the Herringe and Davis people of
1974 and they are try to so, that if you put the hot wire anemometer in two-phase flow,
you will get the voltage at two level, one is once the water comes into the contact another
1 is once air comes into the contacts. So, you will get the two different zone test level,
you need to have the calibration for both the phases. You identify with the level that ok,
if the voltage level at this scale at this range you are actually, you will have air if the
voltage level comes to the ground state of this stage you are having water in contact.

So, it define the phase you find it out the phase here at which phase is coming into the
contact, you use the kings law once the phases in contact, it depend on the that what is
the voltage change required and, you do that calculate the velocity this is the basic
principle is being used in hot wire anemometer, but the problem is your intruding the
flow something inside which may change flow at the point of measurement.
Second thing is that if you use only 1 wire the intrusive nature will be less for sure, but
you will get only 1 direction velocity or average velocity will not get the velocity with
the all the 3 component of the velocity. If you intrude the triple split wire, then this
support systems are very big. So, they though the wire thickness is small the support
which actually hold the wire that dimensions will be big. So, overall assembly size will
increase and that which may change the flow at the point of measurement itself. So, that
is the major problem.

Second thing is that the reverse flow condition is also create a problem because, the
cooling will be the same whether the fluid is moving from bottom to the top at the same
velocity, or it is coming from the top to the bottom at the same velocity. So, the reverse
flow conditions you cannot actually measure it you cannot able to identify whether the
flow is moving up or flow is coming down.

So, therefore, a condition where the reverse flow is possibility of the reverse flow is
there, hot wire anemometer will not able to give you the direction of the flow. So, these
are the major limitation further again the wire thickness is very small, if you are having
solid present in the flow, the solid amount is very high then what will happen the wire
will break because the solid will go and hit the wire.

So, it will erode the wire and because, the wire thickness is very small it will
immediately break. So, you cannot use the system for high solid hold-up system, for the
gas liquid system you can use it, but the intrusive nature will increase if you use the
triple split wire, for the solid anyway you cannot use it.

Of a any hostile medium say very acidic solution very corrosive liquid you cannot use it,
why because a very scaling liquid which is very susceptible to scaling you cannot use it
why because, if some layer will deposited or it will be eroded, then what will happen this
calibration will change and therefore, you cannot use the same calibration for the
measurement. So, that need to be careful and that limits the application of hot wire
anemometer.
(Refer Slide Time: 31:46)

Next is optical fibre probe, now with the generation of optical fibre probe or
development of the optical fibre probes, people have tried to use the same technique or
same thing in the multiphase flow for the velocity measurement. Now what is the
problem in hot wire anemometer, what I said that you would not able to get that whether
the flow is reverse or not. You cannot use this system for a very hostile medium.

Second thing is that the major problem of the hot wire anemometer is that if, suppose the
density is very high, then discrete phase fraction solid particle we are using and solid
discrete phase fraction is very high, then it will break the wire ok. So, hot optical fibre
probe actually gives the better accuracy because it depends on the light ok.

So what we do? We use the 2 probes, one is the light emitting wire another wire is there
inside which is light absorbing wire or light receiving wire. So, in 1 wire this is say
optical fibre probe casing there is a 2 wire inside. So, this is say it work it in this way this
is the casing, there is 1 wire and there is a second wire. So, what happen this wire
actually emits the light.

Now, there will be a phase now each phase will have a different refractive index. So, the
light emitted by the each phase will be different. So, that what will happen this suppose
there is a gas liquid system once the bubble comes into the content of this light, the
refractive index of the air say air water system I am talking about this refractive index of
air is 1, what is 1.34.
So, now the reflection intensity will be changed and what will happen you will see the
different reflection intensity this will the reflection will be different. So, the intensity
recorded on this fibre or with the which is connected to the photo detectors will be
different. So, what will happen again you will get the two level of the this voltage and,
this intensity recorded on the photodetector is directly proportional to the current
generated or current this generated by the photodetectors.

So, photodetector how it works once there is a photodetector in which there is a metal,
which once the photon goes and interact with that material it eliminates some
photoelectrons. That photoelectrons actually goes to the photocathode and generate
electron, that electron is being actually passed through the several diodes. So, that the
intensity that electron intensity can be amplifier, and then it least to the anode and
produce a current.

So, the amount of current produce is proportional to or correlated with the amount of
intensity recorded on the photodetectors. So, we know the phases we know the refractive
index of the phases. So, we know that the reflection index that how much light will be
reflected by a phase, we will find that how much current recorded on the photodetector
how will be produced.

So, from their we can identify the phases. So, do the phase identification by this method,
whether it is a air or it is a water and that phase identification once it is done we measure
the velocity, and how we measure the velocity we measure the velocity by using either
the 2 point optical fibre probe, or 4 point optical fibre probe.
(Refer Slide Time: 34:53)

So, what happens in the optical fibre probe, this 2 point optical fibre probe, again we
have the 2 probe. So, if you see there is a 2 needle and they are having distance this is the
second needle, this is the first needle, there are delta l and distance we know that what is
the delta l between that 2 needles.

So, what will happen you insert it in inside the flow? Now suppose there is a gas liquid
system ok. So, there is a gas liquid system the gas has been completely continuously
sparged it is forming some bubble. Now you insert say 2 pin probe I am just introducing
the 2 pin probe. Now this is the pin the distance is delta l we know that.

What will happen now both the probes is emitting light emitting also light and absorbing
also light both the probes are doing the same job. So, once a bubble will come in contact
with this, what will happen you will see that in lout my the intensity recorded on the
detector will change because, now the bubble is coming which have a different refractive
index. So, we will see the similarly the change in the current.

Now, the change in the current or change in the voltage will be termed as which phase is
coming in contact with that needle or that phase needle. Now, this bubble will pears
through this needle because this needle thickness is very very small, it is in the range of 5
to 15 micrometer or even lower, now with the more development this that trying to
reduce the needle diameter. So, this bubble will actually penetrate so, this needle or is to
be spears through this needle and it will go to the second needle.
Now, again what will happen you will on the second needle, which was earlier in the
water you are recording a different current or different voltage that voltage will change.
And it will go or the output signals in terms of the voltage will change. So, earlier say it
was going on this way it will increase and it will go to a new level. So, moment it will go
to a new level we understand that, now the new phase comes into the contact with the
other, needle also the same phase.

So, say if I am using 2 probe needle, I will get 2 such signal, one single will be like this
for the probe 1, another signal will be like this for probe 2. So, what will happen we
know the delta T that how much delta T, it is taken time taken from by the bubble to
travel from this location of first needle to the second needle, we know that delta T we
know that delta L the velocity can be calculated by delta L upon delta T. So, that will be
the velocity calculation immediately, you can calculate the velocity.

Now, once you this way you can calculate, it the velocity can be calculated and the phase
identification can be done based on the refractive index. Now, again the 2 point optical
fibre probe, the problem is you can measure only 1 component of the velocity, you
cannot measure the all the 3 component of the velocity.

And if the bubble is passing through certain angle suppose this is the 2 pen you have use
and bubble is coming in this way in this direction, what will happen you will see the
single you will record the signal, but now the delta L value is this, this is the delta l value
not the this value. And because of this you are actually travelling more distance and we
are considering the delta L instead of this  L you are calculating this  L. So, you will
get the error in the velocity measurement.

And in the gas liquid system because of the left forces involved as we have already
discussed, the bubble changes it is path it does not move vertically up it move
somewhere it in this direction in this way, or maybe it is changing the path continuously
it is going like a flow in this way, what will happen in this case you will have like error
in the velocity measurement because, you are not measuring the direction through which
the bubble is coming, we are assuming that it is using up vertically upward. So, that is
the way the velocity error has been generated.

So, what we do we use 4 point optical fibre probe, where this is place like this. So, 1.1
needle is being at the they are being kind of put all this 4 needle on a equilateral triangle.
The first needle or the longer needle is placed on the centroid of that equilateral triangle,
and there is other order on the 3 different points of the equilateral triangles like this the
way it has been there. So, this is central needle 1 2 3 is the needle which is on the top.

Now, what will happen the bubble will come it will first see the central nozzle and, then
it will move up. Now once it will move up if the bubble diameter is big enough it will
touch all the 4 probes, if not it will touch only the 2 probes, we will get the delta t and we
will get the velocity.

Now, because it is touching, this if suppose some bubble is moving it it in this way it will
touch only these 2 you will get that component of the velocity, we know that if it is
touching these 2 it will be coming in this direction. So, in this way you can calculate the
all the 3 component of the velocity vx, vy, vz all the 3 component of the velocity, you can
also calculate the diameter of the bubble and you can also calculate the volume fraction
measurement.

How you can calculate the volume fraction measurement because, your identifying the
phase and you can find it out the area under the curve for which the phase is coming in
contact with that probe. So, which that area under the curve it means that the total the
time the fraction of the time, if you are doing this if you take say you will always see this
kind of a curve.

So, area under the curve of this where the bubble is coming into the contact and the total
measurement time this will give you a time area under the curve, will give you a time
total measurement time, and the area under the curve once the bubble comes into the
contact these two the ratio of these two will give you the volume fraction.

So, that is the way to measure the volume fraction also it works also on the transmission
network, we will discuss it once we will do the volume fraction measurement technique
once we will discuss that technique. So, in this way it goes and then what happen you
calculate the velocity ok. So, that is the good thing here that you can calculate all the 3
direction velocity because, optical fibre probe thickness is very less.

The equipment size or this probe size is very less compared to the hot wire anemometer
hot wire anemometer is big here, it comes you reduce the size drastically and therefore,
the intrusive nature of the flow the disturbance in the flow can be minimised. But there is
a another problem and the problem is because of the piercing.

(Refer Slide Time: 41:19)

So, what happen let me show you the photograph which is taken from the Xue et al in
2004. If the bubble comes I do not know whether you are able to see, but if you see that
there is a different pins, the fourth pin you are not able to see this is the 4 point optical
fibre probe. So, this is the size which is there and it is zoom sizes.

So, what will happen this bubble will come and it will pierce through the first needle,
you can see that it has pierce actually through the first needle. Now, once it is pierce
through the first needle, now it will move upward and it will pierce through the all the
other needle the way it has been shown here and it will pass through.

So, what will happen you will see the signal, but what this problem it is not on a single
you are going to see so, it is piercing through? So, it is been there for certain time it is
being in touch with the needle for certain time. And, because of that instead of getting a
off and on single like this, way instead of getting this you actually get a signal which is
of this nature. So, you will see that then it will take certain time to rise to the voltage it
will stay there for certain time and then it will come down.

So, the problem comes now if suppose you have this kind of a output, the problem comes
where we should take the  T. So,  L is fixed, but what should be the  T value and that
create a problem why because this  T value is going to be very small in the order of
microseconds or so. Now, small error in that T that T measurement will cost you a lot
in the velocity why, because you are having using X by T.

So, the error in the velocity is very small error in the velocity will cost you heavily in
terms of the velocity measurement and therefore, this creates a problem with the piercing
also create a problem, different people do it in a different type the way to find the delta t
value, but it has been proven by Mude et al. that who is a professor in at Uu delft; in his
people who is which is published in this chemical engineering science, that if you do not
do the measurement properly, or  T measurement properly you can have a huge error,
which can be in order of 100 to 200 percent that error you can generate and that is the
limitation of the optical fibre probe.

Second limitation of the optical fibre probe is suppose if the discrete phase fraction is
very high, it is a bubble density is very high, then what will happen multiple bubble may
approach the probe at the same time, or before the first bubble pierce out the second
bubble come in to the contact, what will happen you will not get the tuning signal and
you will miss lot of things. So, that is the second problem.

Third problem is again the wire thickness the optical fibre probe thickness is very very
small as I have shown you here, if the solid is present the solid fraction is very high they
may is a possibility that you probe may get break. So, that is the another disadvantage of
this thing this optical fibre probe. Otherwise the accuracy is higher it is relatively less
costly compared to whatever we are going to discuss in the non-invasive measurements.
(Refer Slide Time: 44:18)

The non-invasive measurement is being preferred in that what is the major good point
whatever I am keep on saying on all the measurements that your flow may change at the
point of measurement itself, the intrusive nature will increase that can be reduced by
using the non-invasive velocity measurement techniques.

So, what we do in the non-invasive velocity measurement technique, the first non-
invasive velocity measurement technique which has been developed in the 1984, and all
is the laser of Doppler anemometer. Now, again this is being developed initially for the
single phase flow and, later on it has been utilised to investigate the multiphase flow
reactors.

So, what is the principle of this measurement technique this is very simple, what you can
do you use a laser light. And because the laser light is in the optical range and, it can go
inside of the system what you do you use a laser light you use a beam splitter you break
laser light to the two different with laser light. And that laser lights actually is being
focused by using the sending lens or focal length at a measurement points.

So, then what we do we use a laser light we break it the laser light by using the beam
splitter and, the mirror to the 2 laser light. We put we take a lens we focus these 2 lights
on the lens and, then the lens what it does it converged it to the particular location. And
that location is inside the flow of interest of the inside the region of interest where you
want to measure the flow, it go to that point where the 2 laser light cuts, we know that
once the 2 source laser light cuts or 2 optical light cuts or 2 lights cut, we form a fringes
some bright fringes and dark fringes. So, by bright and dark fringes you get and the
distance between the bright fringes and dark fringes can be exactly calculated, suppose if
this angle is  then the distance between the this bright fringes 2 successive bright
fringes or dark fringes say d if this is the distance d can be calculated as λ/sin (/2),

See you can exactly find that what will be the distance between the 2 fringes. Now what
we do we seed the flow with certain particles, which are literally bind the tray with void
particles and have the ability to track the phase of interest. So, what we do? As we
discuss in the multiphase flow if you are tracking the flow with the seed particles we
have to see that the Stokes number is very less than 1. So, that you can say that they can
follow the path of the fluid.

So, we use a very small seed particles where stokes numbers are very low; so, that they
can follow the path of the flow. So, what will happen that seed particles will pass
through it say these are the seeding I have done, and once this particle will pass through
it this will cut this fringes. And there is another lens which is there which is again and
there is a photodetectors which the light emitted from this measurement thing is again
been focus to the photodetectors.

And the photodetector worked on the light intensity that, amount of the current generated
by the photodetector, will be proportional to the intensity of the light which is recorded
on the photodetector. So, that amount of the current can be converted in terms of the volt.
So, you measure the volt generated by this.

Now, what happen once the particle will cut it, what will happen there is a velocity there
will be some Doppler shift will take place. So, there is an intensity and that nothing is
moving you will record certain frequency here. Once a moving particle will become
what it will do, it will change the intensity of this reflection or this is scatter whatever it
is being there. Now because of that the intensity of backward this is scatter will change
and that change in the intensity or the shift in the intensity or shift in the frequency, will
be depending on the velocity of this particle and that is the way the Doppler’s effect
comes into the picture.

So, if there is a light which is emitting, or wave which is emitting or emitting from a
source and if you are having, if you are moving towards that wave what will happen you
see the intensity recorded on your ear of that wave, or that sound frequency will keep on
increasing, if you are moving towards the source and it will be decreasing if you are
moving out of side of the source. And that shift in the frequency is called the Doppler
shift.

So, similarly if the moving particle will go through this measurement area, it will cut this
fringes and then what will happen the frequency recorded or intensity recorded of the
light on this photodetector will has see a shift. And that shift in the frequency is nothing,
but will be correlated with the velocity of the system ok. So, that is the way we measure
the velocity or we measure the voltage change, if you do that there is 2 ways you
measure the Doppler shift and you will can also measure that we know the distance
between the 2 successive bright fringes and dark fringes which will be like this.

And we know that what is the that how the frequency shift will take place, or if suppose
they will cut this bright fringes, there will be some change in the frequency some change
in the intensity say the frequency shift will be there Doppler shift will take place and
therefore, the intensity will change and the voltage output will change something like
this. And the time required between these 2 change T can be recorded that is the time
between the particle travel from 1 bright fringes to another bright fringes, you will get
that T, you know this d you can calculate the velocity by using d upon T you can
measure the velocity.

So, that is the way you measure the velocity in the laser Doppler anemometer, again it is
nonintrusive. So, this is the major advantage that you can major the velocity with how
disturbing the flow. And you can measure measuring the velocity again by using the
Doppler shift and that Doppler shift is actually proportional to the velocity of the fluid or
velocity of the particle. And that particle is the marker of the phase of interest whose
velocity you want map. So, you can get the velocity of that particular phase ok. So, that
is the way LDA measurement has been done, the problem if the LDA measurement is it
is a point measurement you are cutting this thing at a particular point.

So, whatever the velocity you are measuring you are measuring velocity locally at that
point ok, you are not measuring velocity at all the locations, you are not getting the area
velocity or the how the velocity is changing with the position. If you want to do that you
have to use several lasers or you need to focus on different places. So, you are
experimental time will be very high.

Second thing the problem is this all are of lasers are under the optical range. So, because
of the optical range frequency, they are if my eyes cannot see laser can also not see. So,
what means if the system is opaque either the wall of the system is opaque which is true
for most of the industry, then LDA cannot be used. Or if suppose even if I make the glass
made this wall made of the glass, the system may be the wall is transparent, but if the
system is opaque.

Once I say system is opaque means, if suppose the discrete phase in the gas liquid system
the discrete phase bubble fraction is more than 10 percent or, 15 percent or, 20 percent,
what will happen it will be completely dense bubble as I have shown you the pictures
earlier also.

So, what will happen any light which will fall in will actually get reflected or diffracted.
So, what you will able to measure you will able to measure only near the wall you cannot
measure inside. So, that is the major problem with the LDA that if the discrete phase
fraction goes anything above 5 percent, the use of LDA is limited, though it is very
accurate for a single phase flow, but in the multiphase flow, this is the major limitation.

And most of the reactor will be use in industry is having a discrete phase fraction more
than 5 percent. And therefore, it is being limited the application is being limited in the
industry, for the gas solid flow the problem comes again, if the solid fraction is very high
say you are using for the bubbling fluidized bed also, you all the light will be get
refracted or diffracted and you will not able to any measurement.

So, these are the disadvantages of the LDA, they are issues with the post processing also,
if suppose the size of the particles say in multiphase flow generally, if we use the bubble
we use bubble itself as a seed particles ok. So, in that case if the size of the bubble is too
big, which can cut the 2 fringes together your post processing becomes very typical ok.

So, that also generate some error inside so, this is the disadvantages of the LDA, but the
advantage of the LDA is the spatial resolution is very high, it goes in the order of say less
than point 1 mm or even lower that is the spatial resolution very high, if you use a high
speed laser you can even increase the temporal resolution of this.
So, this can gives you the phenomenon change, with the time how the velocity is
changing, but the major problem is it gives the point measurement it cannot be used for
high discrete phase concentration ok.

(Refer Slide Time: 53:24)

So, this is the post processing is keen which we have already discussed that what you do,
you calculate the speed by the spacing between the fringes and, the time required of the
frequency of this curve. So, what we do, we take this frequency we do the Fourier
transform and we make this frequency we do the filtering of that with the putting the
band pass filter and we can word that in terms of a proper waves. And that the frequency
you will calculate that what is this f this is  t.

So, 1 upon  T is f or you can say this is will be 1 upon f frequency can get, you can get
the speed of this will be fringe spacing into the f, this is the slide which have been taken
from the measurement science enterprise Inc, which supplies the LDA which of the also
supplier of the LDA. So, we do it in this way and there we measure the velocity of the
fluid.
(Refer Slide Time: 54:13)

The only problem I said that is you do the point measurement. So, you get the position
velocity change with the time, but you dendrite the velocity change with the position. So,
that you do not did, second thing you cannot use it for the higher discrete phase fraction,
now to remove the first part that you are doing only point measurement another
technique is being introduced or being used and that is called particle image velocity
material.

Now, what we do in particle image velocity material, we generate a laser seed instead of
a cutting the 2 laser at a particular point, what we do we generate a laser seed say this is
the laser seed, there is optics which is being used I am not going in detail of this, you can
if you are interested you can go and see the optics, they are very interesting to understand
all these optics. So, we use a laser source we use some optics where we generate the laser
seeds. So, this is the laser seed is being generated, we use camera whether a high-speed
camera or the CMOS camera, or this kind of a CCD camera which double pulse camera,
we use that and we use the laser which is the double frequency laser.

Now, or this you the laser frequency can be generated in a small domain. So, what we
do, we do a laser we put a laser seed we generate a laser seed inside the flow, we put
some particle which actually eliminate lights in presence of the laser. So, we use double
pulse laser we use a CCD camera in the older version of the PIV. Now what we do we
use either the continuous laser and this is the high speed camera, or we use still the
double pulsed laser and high-speed camera and we required the velocity kind of images
with the time and we process it.

So, we will discuss about it, but what we do say the combinational of PIV technique, we
use the double pulse laser. So, it is generate the 2 pulse in a very short duration, you
eliminate you make a laser seed, you eliminate the whole flow field, we seed it with the
transfer particle the way we are doing it in the LDA. Now again we are using the seed
particles, it means the stokes number should be very low, you have to see that the seed
particles exactly following the path of the fluid.

Now, once the seed particles will go inside in the presence of the laser light they will
eliminate light we record the image, then you chip the laser and again after certain time
you again kind of switch on the levels, you read the 2 pulses of the laser. Now once you
read the pulse the light will be emitted photographs will be captured, another if the light
will be emitted photographs will be captured. So, you will have 2 images, we cross
correlate these 2 images and try to find it out the velocity.

(Refer Slide Time: 56:49)

Now, how we do that cross correlation we have say image 1 at 1 pulse and image 2 at the
second pulse. So, you understood how we are getting the image. So, lights are being
emitted the laser light is there this particles are emitting the lights, under the laser light.
So, what will happen once you take the photograph, those particles will come as a bright
colour. Now or it will come as a this kind of a dark colour depending upon what is the
particle property, whether it is eliminating the light or whether it is absorbing the light,
both kind of things can be used.

Now, you will get the image you will get the particles ok, now what we need to do we
need to find the velocity. So, this is suppose these 2 images and these are the seed
particles we done lot seed particles so, that image density is very high. Now what we do
we divide the whole picture this whole image into the small pixels ok.

So, this is 1 pixel I am talking about and, there are few number of particles is there then
we take the FFT we do the kind of Fourier transform and, then the second m n image we
take we do the FFT in the Fourier transform of this image. And then we cross correlate
the Fourier transforms of these 2 images. If needed we do the filtering also again I am
not going in detail of all those filters, but what you do not do suppose this is the Fourier
if you do the Fourier transform you will get a function f which is u and v.

The second image again you will get a function G which is the function of u and v you
cross correlate these 2 and, with the cross correlation what you do say you take 1 particle
you try to cross correlate this particle in the second image with all these particles, or vice
versa you take 1 particle in the first image and we will try to do the cross correlation with
the all the other particles in the second image.

And with the cross correlation for each and then you do it for the second particle, third
particle, 4 particle, whatever the particle available and you get a cross correlation index,
you do the inverse FFT and you find that the displacement a specific displacement, for
each particle or the combine displacement for the each particles.

So, each particle cross correlation you will get how well it is cross correlated. So, means
suppose this is the particle I can cross correlating with all the particles, I will find that
which is the particle which is having a better cross correlation coefficient with this
particles in the second image, say this is the particle. So, earlier the image this distance
was this, now the distance is this I will get a small displacement.

Similarly, I will do for all the particles, I will get the combined cross correlation index
and that will give some peak, and with that the main cross correlation index what we did
we will get the average displacement of all the particle. So, you get the displacement we
get that the delta x from the cross correlation and delta t, we already know we know that
what is the time of the double pulse. So, you know the delta t you can calculate the
velocity that is the way the velocity is being calculated in the PIV experiments.

The problem in the traditional PIV experiment is if you are not using a high speed
camera, you will not get a time resolved picture, your spatial resolution will be very
high, but your temporal resolution will be low. And therefore, now a time resolved PIV
experiments of PIV measurement techniques have been developed, where we use a high
speed camera, we eliminate the flow with the laser, we use whether the continuous laser
and keep on observing or the keep on recording the images with the time.

Now, it is a high-speed camera which record the image that different ways and, now you
correlated between these 2 images and you get the time resolved the PIV that how with
the time the flow is changing. So, you have a continuous image now ok. So, that is the
time resolved PIV experiments.

Now, advantage of the PIV is; obviously, it is in it has given the luxury to analyse the
company flow field at 1 shot at least the region of interest complete region of interest,
you can analyse at 1 shot you do not need to do multiple experiments like LDA. So, this
gives you the complete information of the flow field that is the major advantages over
the LDA technique.

The kind of the accuracy is higher the temporal resolution can be improved a lot by using
the high-speed camera, but the major limitation remains same that if the system is
opaque, of wall of the system is opaque or the system itself is opaque though the wall is
transparent, but because of the higher discrete phase fraction, or higher solid
concentration the system is opaque, you will not able to get anything because the camera
will not able to see if your eyes are not able to see.

So, if you are not able to see till the centre of the column, you will not able to the camera
will also not able to see and your information will be limited only near the wall. And that
poses the major problem or you can say the major issue with this technique. So, you can
use it for the dilute phase or even the discrete phase fraction is less than 5 percent, but if
the discrete phase fraction is more than that ideally you cannot use this technique and
that poses the major limitation.

And therefore, the optical phase techniques are used in the multiphase flow reactors a lot,
but cannot be used at all this case, cannot be used at industry or cannot be used when the
discrete phase fraction is really high though their accuracy is better. And therefore,
radiation based technique is being developed.

(Refer Slide Time: 62:02)

And in the radiation based technique the first technique is positron emission particle
tracking.

Now, what we do in the positron emission particle tracking, we track the motion we seed
the particle, or we seed the flow with the positron source and we track the motion of that
particle. So, what we do suppose this is the gas solid system, we take a positron source
say we take a positron particle source which can emit the positron particle and, we match
the size shape and density of this tracer particle, exactly as of the other solids present in
the flow. And if suppose you want to track the motion of the liquid, you will match the
density of this particle exactly as the density of the fluid you want to track ok.

Now, what we do there are certain detectors, which are placed around, and, you track the
motion of the particle. Now, how you track the motion of the particle? We know that 1
positron emission source will actually emit a positron and, electrons are available in huge
amount around the system ok. So, bulk of the electrons are available so, the moment a
positron is emitted it annihilates with the electron.

So, what happen there is a positron it means e+ it annihilates, or it combine with electron
once the comes together, they generate 2 gamma rays ok. And this gamma rays are 180
degree apart back to back, it means 1 gamma suppose this is the positron this is positron
this is electron, they analyse each other and they generate 2 back to back gamma rays,
which would be 180 degree apart and back to back they will move.

We put a complex detector system around, the vessel of interest say we put some
detector system complex this detector systems are really complex, which can detect
simultaneously these 2 gamma rays, which is being emitted from the source. And only
those detection which is simultaneous on both the detectors is considered as a event. So,
you get that this line that how the simultaneous detection is there.

Now, depending upon these certain this source intensity gamma ray kind of beta, this
positron emission will be number of positron emitted per unit time will be huge. So, what
will happen you will see that several such lines ok. And this lines way where they will
cut each other that will be the location of the particle.

So, what we do say this is an illusion, this 1 is the electron, 1 is the positron, once the
annihilate they produce to back to back gamma rays. Now because there gamma rays
they can travel in any medium, they will go and detected on the system this detector
system. So, say this is my detector assembly, they will be detected and their detection
will be simultaneous

If suppose some gamma rays got scattered, or attenuated that detection will not be
counted, if suppose the gamma ray is detected only on 1 detector that will not be
counted. Only the simultaneous detection of the gamma rays on both the detector will be
counted. So, what it will give, it will give you a line. So, you will be knowing that my
particle is within this line.

Now, several such interactions will take place depending upon intensity of the source,
now it means each interaction is going to give me a line. So, you will get these kind of a
line I am showing only 3 lines because, only 3 line is needed to calculate the position of
the particle, but in reality you will get several such lines, you will get several such line
where the particles will be detected. So, what you will get out of all these events you
need only 3 events which will be cutting each other to get the particle positions. So, I
will say that this is my particle position.
Now, particle will be free to move inside and therefore, you will able to get the particle
position with the time you know the particle position with the time by using x /  t you
can calculate the velocity of the particle. Now, the particle will move in all the 3
directions. So, you will get the displacement in x y and z.

So, what you will get you can get the v as a function of x y and z with the time and you
will get all the component of the V, it means you will get the V x, you will get the Vy, you
will get the Vz all the component of the v you will get and how it is changing with the
time. So, that is the major advantage of the PEPT and because you are using gamma
rays, what you are going to get you can use it even for the opaque system.

The only problem here is that you are using gamma rays. So, the safety requirement is
there and, the detection system is very complex, they are very complex detector detection
system. Second thing is that the diameter of the particle labelling particle is generally is
in the range of 100 micrometer, it means around 0.1 mm it cannot be lower than that.

So, any structure which will be lower than that you will not able to track it and then kind
of the time of acquisition is generally limited at around 1 millisecond. So, that is the kind
of the limitation in terms of the spatial resolution and temporal resolution.

But the one of the major advantage is that you do not need any calibration, you do not
need to put the particle inside the way I have said for the invasive technique, you do not
need any calibration here because, you know you are solving it with the triangulation
theory. So, that is the major advantage.
(Refer Slide Time: 67:21)

Second advantage is you can because it is gamma ray system you can use it for the
opaque system also, but the major disadvantage is that the detection systems are very
complex, this is the paperwork has been done by Parker et al which they have
investigated the flow profile inside the rotary clean and a bubbling fluidized bed, but you
can see that the detector systems are very complex very bulky system and, they are very
complex as you require simultaneous detection of the gamma rays.

And that is why the cost of this technique is enormously high and therefore, the use is
very limited and only the parker probe which is the University of Birmingham will place
an industry of Birmingham has this technique available because this is very costly ok.

Second disadvantage is the annihilation generate or gamma ray energy of 511 keV that
will be the energy you will generate. And it means you cannot use the system this energy
is limited. So, if the system diameter is very big, you cannot use it. So, suppose if you
want to use it at industrialist scale fluidized bed, or boiler say diameter may be of several
meters, you cannot use this technique because the gamma ray will be the scattered, or
attenuated inside the system itself. And that is the disadvantage of PEPT.
(Refer Slide Time: 68:31)

Therefore, another technique has been developed which is called radioactive particle
tracking technique, in this technique what we do, we again use the same concept as in the
PEPT instead of now positron emission positron particle, we use a gamma ray particle
directly. Now using the gamma ray particle directly gives you a luxury that, you can use
any intensity of your choice you can use any energy of your choice.

So, it gives you the flexibility to use the technique for any diameter system, for any
volume fraction or phase fraction system, for the opaque system also. So, whether you
made wall is made of wall or anything, the diameter of the system is very high discrete
phase fractions are very high, you are using with the solids or bubbling fluidized bed of
40 percent solid you can use it.

So, that is the major advantage of this technique, it gives you lot of flexibility and that
why this technique is very versatile and, can be used at every scale for the different type
of the system. And what we do in this technique we use a marker of we use a single
radioactive particle, which is marker of the phase of interest whose velocity you want
map.

So, suppose in a gas solid fluidized bed, suppose there is a solid, you want to map the
position or motion of the solid you make one of the particle as the radioactive, or you
make use a radioactive particle which size, shape, and density is exactly same as of the
particle present in the flow. And you track the motion of this particle, in case of the
liquid what you do this is like a picture of the bubble column, where gas liquid flow is
there you make the liquid or neutral this particle neutrally buoyant to the liquid.

So, what will happen? It will follow the path of the fluid. Now the size of the particles
should be smaller. So, that the strokes number of the particle is low and it is following
the path of the fluid exactly, and you track the motion of this particle. And how we track
the motion of the particle?

Now, during because this is a gamma ray source during, it is part it will emit the gamma
rays we put some scintillation detectors, which are being made to absorb those gamma
rays or count those photos which emitted by the particles and, we place this scintillation
detectors strategically around the vessel of interest. So, that you can cover the entire zone
of interest, or zone of investigation, we place it properly so, that we get the proper
resolution and resistivity and, the count emitted by this gamma ray is being recorded on
these detectors.

Now, by using the beer lambert law, we know that the intensity recorded on the detector
will be the function of the distance and the attenuation coefficient. So, I =I 0 e(-µL). And
therefore, because it will be going to the attenuation coefficient, you need that we know
that if the particle is closer to the detector, you will get higher counts on the detector, if
the particle is far you will get the lower counts.

So, by using the suitable reconstruction algorithm, we reconstruction the position of the
particle by measuring the counts on the each detector so, each detector acquired the
counts at the same time, we get again the same principle distance from each detector and
that distance from each detector will concise to a particular location of the particle. So,
we get the location of the particle.

So, what we get is that how the particle position is changing with the time, from there
again by using x /t we calculate the velocity. And then again we can calculate the all
the 3 components say Vx Vy Vz of the particle and, we can find it out that how the
velocity is changing with the position and with the time, that all you can do the
calculation.
(Refer Slide Time: 72:05)

Now, how reconstruct and what we do? Suppose this is a experimental setup photograph,
this is the gas solid column, these are the scintillation detectors, which are being used to
edge of the count and we put a say solid particle inside. Now, this particle will move
inside feeling and you will see this kind of a count.

(Refer Slide Time: 72:21)

So, y axis is a count and x axis is a time. So, we will see that how the counts on each
detector is changing, in this experiment we have used 8 detectors. So, you can see that
there is a 3 detector 1, 2 and 3 on this graph 4, 5 and 6 on this graph which is combined
with the red blue and green colour, and red and green on these 2 so, 8 detectors how the
counts on all the eight detectors are changing.

Now, you do the calibration that is the major disadvantage of this. So, what we do
initially we put the particle at several known location for in situ condition, we find that
how the counts on each detector will change, once you will put the particle at a particular
location for the given operating condition.

So, we know that how the count, will change for a particular location of the particle in
the calibration stage in the real experiment stage, we know that how the count on the
detector is changing we match this 2 counts we get the position. So, understood we do
that say this is my column, what we do initially at operating conditions at in situ
condition it may the condition at which you want to do the measurement, we first place
the particle say anyhow at a certain location. And we record that how much is the count
is being recorded by all this detectors.

Then we change the location we do to it the same thing at the several location. Now we
generated the scale count map for all the particles, all the detectors, for each location of
the particle you all this detector will record some count. So, we generate we note it
down, we generate a this scale count map for all this detector, for the known location of
the particle, we know the particle location, we know the detector, we know the distance
and then we record the count for the in situ condition.

In the real experiment particle is free to move inside, wherever it wants to go. Now the
particle what we do we record the counts on the detector with the time. So, in the
calibration, with the position, how the count is being recorded on each detector in real
experiment with the time how the count is recorded on each detector.

Now, we compared these two and, once you compare these two what you will get you
will get that how the position of the particle is changing with the time. So, you will get
this that suppose this particle has start it in this place, how it is changing it is position,
how it is changing it is position with the time you get that.

Now, by using the x /t you get the velocity, that is the Lagrangian velocity time series,
earlier you get the Lagrangian position time series that how the position of the particle is
changing with the time. Now, we got the Lagrangian velocity time series with x /t you
will get the Vx how the Vx component is changing, with the time how is V y component is
changing with the time, how is Vz component is changing with the time.

Now, to get how the velocity changing with the position we do the Eulerian we form
Eulerian grid, we discretise the column in a virtual the small grid virtually, and we see
that in each grid how many times the particle is coming. So, say in each grid in each cell
if the particle say during the whole time of experiment say 8 or 10 hours, if you are
acquiring the data if the particle comes 100 times we take a average and we say that is
the position velocity at this cell.

Similarly, we do it for all the location and we found that how the particle velocity is
changing with the position. And we get the mean velocity. Now, once we have a mean
velocity I already have the local velocity I can get the fluctuations. So, I can get that how
the fluctuation is changing with the time, how the fluctuation of the particle is changing
with the position, all the things we can calculate.

And that is why this gives this technique gives you a rich database to do the detailed
analysis of the flow. The only disadvantage of this technique is the spatial resolution and
the temporal resolution is lower, it is much lower compared to the LDA and PIV it is in
the order of 0.1 mm or 1 mm in between the 0.1 mm to 1 mm depending upon, how
many number of detectors you are using what is your transfer particle size.

Now, the that is the disadvantage another disadvantage is this is the radiation based
techniques. So, required lot of safety gadgets, you need to take a proper approval before
installing this technique in your laboratory, or before using this technique in your
industry. So, that is the disadvantage the major advantages, it is very versatile technique,
it is kind of it can be implemented at all this scales with the equal accuracy.

The another disadvantage of this technique, is our limitation by this technique, is a major
you can say the limitation of this technique is the calibration required. So, what you need
to do? You need to do the calibration at in situ condition and therefore, in the medium is
very versatile say, it is a very high temperature op high pressure reactor do the
calibration at in situ condition may be difficult and, which limits the application of this
technique.

Neither otherwise this technique is very versatile, for any solid fraction for any kind of a
condition say wall is opaque, or it is a transparent, solid, fraction is smaller higher, it is a
smaller equipment laboratory scale equipment, or it is industrial scale equipment, you
can implement it everywhere.

(Refer Slide Time: 77:23)

So, that gives the versatility. So, with this is a typical photograph of this technique.

(Refer Slide Time: 77:23)

With this your velocity measurement technique is over and, in next class we will briefly
discuss about the phase fraction measurement technique.
Thank you.