IMPACT COLLEGE OF ENGINEERING AND APPLIED SCIENCES
SAHAKARA NAGAR SOUTH, BENGALURU - 560092
DEPARTMENT OF MECHANICAL ENGINEERING
HEAT TRANSFER LAB
HEAT TRANSFER BY FORCED CONVECTION
OBJECTIVE: To determine the convective heat transfer coefficient and the rate of
heat transfer by forced convection for flow of air inside a horizontal pipe.
THEORY:
Convective heat transfer between a fluid and a solid surface takes place by the
movement of fluid particles relative to the surface. If the movement of fluid particles
is caused by means of external agency such as pump or blower that forces fluid over
the surface, then the process of heat transfer is called forced convection.
In convectional heat transfer, there are two flow regions namely laminar &
turbulent. The non-dimensional number called Reynolds number is used as the
criterion to determine change from laminar to turbulent flow. For smaller value of
Reynolds number viscous forces are dominant and the flow is laminar and for larger
value of Reynolds numbers the inertia forces become dominant and the flow is
turbulent. Dittus–Boelter correlation for fully developed turbulent flow in circular pipes
is,
Nu = 0.023 (Re) 0.8 (Pr) n
Where, n = 0.4 for heating of fluid
n = 0.3 for cooling of fluid
Nu = Nusselt number = hd
K
Re = Reynolds Number = Vd
υ
Pr = Prandtl Number = μ cp
k
DESCRIPTION OF THE APPARATUS:
The apparatus consists of a blower to supply air. The air from the blower passes
through a flow passage, heater and then to the test section. Air flow is measured by an
orifice meter placed near the test section. A heater placed around the tube heats the air,
Saleha Nadeem B.E., M. Tech (Thermal) Asst. prof., Dept. of ME, ICEAS.
IMPACT COLLEGE OF ENGINEERING AND APPLIED SCIENCES
SAHAKARA NAGAR SOUTH, BENGALURU - 560092
DEPARTMENT OF MECHANICAL ENGINEERING
heat input is controlled by a dimmer stat. Temperature of the air at inlet and at outlet
is measured using thermocouples. The surface temperature of the tube wall is measured
at different sections using thermocouples embedded in the walls. Test section is
enclosed in an asbestos rope where the circulation of rope is avoiding the heat loss to
outside.
PROCEDURE:
1. Put on the heater and adjust the voltage to a desired value and maintain it as constant
2. Allow the system to stabilize and reach a steady state. Start the blower and adjust
the desired flow rate.
3. Note down all the temperatures T1 to T7, voltmeter and ammeter readings, and
manometer readings.
4. Repeat the experiment for different heat input and flow rates.
DATA SHEET
SPECIFICATIONS:
Specimen :
Size of the Specimen ‘D’ : I.D. mm x mm long
Heater : Externally heated, Nichrome wire Band Heater
Ammeter : Digital type,0-20amps, AC
Voltmeter : Digital type, 0-300volts, AC
Dimmer stat for heating Coil : 0-230v, 2amps
Thermocouple Used : nos.
Centrifugal Blower : Single Phase 230v, 50 hz, 13000rpm
Manometer : U-tube with mercury as working fluid
Orifice diameter, ‘do’ : mm
G. I pipe diameter, ‘dp’ : mm
Cd :
Observation table:
Room Temperature TR = ………. + 273.15 K
Saleha Nadeem B.E., M. Tech (Thermal) Asst. prof., Dept. of ME, ICEAS.
IMPACT COLLEGE OF ENGINEERING AND APPLIED SCIENCES
SAHAKARA NAGAR SOUTH, BENGALURU - 560092
DEPARTMENT OF MECHANICAL ENGINEERING
Sl. Heater input Tube surface
No Diff. in Temperature °C
Manometer Air temp. °C
Voltmeter Ammeter VI reading hm Inlet Outlet T2 T3 T4 T5 T6
reading V reading I watts mm T1 T7
volts amps
SPECIMEN CALCULATIONS:
1. Mass density of air ρa = P kg/m³
RTR
Where, P = Atmospheric Pressure = 101325 N/m²
R = Gas constant for air = 287 J/kg K
TR = Room temperature in K
2. Pressure drop across orifice meter in ‘m of air
ρm hm
ha = ρ a
where, ρm = Mass density of mercury = 13600 kg /m3
hm = Differential manometer reading of mercury
3. Volume flow rate of air through the orifice, Q = Cd x Ao √2gha m3 /sec
4. Velocity of air flow through the pipe, V = Q / A m/sec
A = (π /4) D2
Saleha Nadeem B.E., M. Tech (Thermal) Asst. prof., Dept. of ME, ICEAS.
IMPACT COLLEGE OF ENGINEERING AND APPLIED SCIENCES
SAHAKARA NAGAR SOUTH, BENGALURU - 560092
DEPARTMENT OF MECHANICAL ENGINEERING
(Note: Change in density of air with temperature is neglected i.e., ρa = constant)
5. Average surface temperature of the tube
Ts = T2 + T3 +T4 +T5 +T6 C
0
6. Mean temperature of air
Tm = T1 + T7 C
0
Properties of air are taken at Tf = Ts+ Ta ………
0
C
2
At temperature Tf, kinematic viscosity ‘ν’, Prandtl number ‘Pr’ and thermal
conductivity ‘k’ and Cp , are taken from properties of air table
7. Reynolds Number Re = V x D
ν
8. Nusselt number Nu = 0.023 Re0.8 Prn
Where, n = 0.4 for heating of fluid
n = 0.3 for cooling of fluid
9. Nu = hxD
kair
10.Forced convective heat transfer h = Nu x kair W/m²-K
D
Saleha Nadeem B.E., M. Tech (Thermal) Asst. prof., Dept. of ME, ICEAS.
IMPACT COLLEGE OF ENGINEERING AND APPLIED SCIENCES
SAHAKARA NAGAR SOUTH, BENGALURU - 560092
DEPARTMENT OF MECHANICAL ENGINEERING
11.Energy input to the test surface, Q = V x I = Watts
12. Average value of heat transfer coefficient
h = Q W/m²-K
As (Ts - Ta)
RESULT:
The convective heat transfer coefficient in forced convection is ____________
W/m0K by practical approach. And theoretically, by using the correlation for these
conditions, convective heat transfer coefficient is ______________ W/m0K
Saleha Nadeem B.E., M. Tech (Thermal) Asst. prof., Dept. of ME, ICEAS.
