1.

Introduction
1.1 Background
When the velocity of a fluid is increased with corresponding drop in pressure by constricting
the flow passage area of the fluid is called the venturi effect. The partial restriction causes a
higher pressure at the inlet than that at the narrow end. This pressure difference causes the fluid
to accelerate toward the low pressure narrow section, in which it thus maintains a higher speed
to satisfy the continuity equation as shown in figure 1 . A venturi is a short tube with a tapering
constriction in the middle due to which venturi effect occurs when a fluid flows through the
pipe causing a suction effect due to the pressure difference .

The venturi tube is used inside a carburetor for the carburation process. The process of forming a
combustible fuel-air mixture by mixing the right amount of fuel with air before admission to the
cylinder of the engine is called carburetion and the device doing this job is called
carburetor[ CITATION DEE11 \l 1033 ] figure 2 shows how the carburetion process takes place. The
pressure drop in the venturi constriction is responsible for suction of fuel from the fuel tube. The
rate of discharge of the fuel depends on the pressure difference between the float chamber and
the throat of the venturi of the carburetor and the area of the outlet of the tube. The opening of
the fuel discharge jet is typically located where the suction is greatest. It is, generally, just below
the narrowest section of the venturi tube. The vaporized gasoline sprayed through the fuel jet
mixes with the air entering through the carburetor venturi tube in the mixing chamber located
below the discharge jet[ CITATION Car19 \l 1033 ].

Figure 1: venturi effect[ CITATION Car19 \l 1033 ]

Figure 2: Venturi tube used in carburetion process[ CITATION
DEE11 \l 1033 ]
The purpose of this report is to simulate the compressible fluid flow across the carburetor
venturi to analyze the parameters like the pressure distribution, change of velocity of fluid and
the effects of such parameters on the atomization of fuel and efficiency of generating a
combustible fuel and air mixture. The process of carburetion is affected by the velocity of the
air at point of injection of fuel and the mass flow rate of the air and the presence of elements like
the fuel tube and throttle body causes reduction in mass flow rate that is required to achieve
desirable pressure differences so the main focus of the simulation is to analyze the effects of fuel
tube at different angles inside the venturi tube to the velocity change and pressure distribution
inside the venturi tubes by comparing with the flow parameters when no extending fuel tube is
present.

A model of a simple venturi tube with fuel tube at different angles is developed using a CAD
software and CFD analysis is done on the venture by varying the fuel discharge nozzle angle on
the flow using ANSYS software.

1.2 Objectives
 Evaluation of Flow field at different angles of the fuel injection tube
 Pressure loss in the throat area
 Pressure distribution form the inlet to the outlet of the venturi tube.
 Measurement of Pressure and velocity as a function of location inside the venturi tube.
 Comparison with simulated flow parameters without the presence of extended fuel tube .

2. Literature review
Venturi of a carburetor have many variations like throttle diameter, angle of throttle body, fuel
nozzle angle and other elements present inside the venturi tube. The variation of these elements
change the air fuel mixture concentrations which effects the engine performance as shown in
figure 3. The graph derived from a book about IC engines by Ganesan V [ CITATION Gan09 \l
1033 ] shows that The mixture corresponding to the maximum point on the power output curve
is called best power mixture and the air-fuel ratio at this point is approximately 12:1. The
mixture corresponding to the lowest point on the brake specific fuel consumption curve is called
the economy mixture and the air-fuel ratio at this point is about 16:1.The best power mixture is
generally richer than the stoichiometric mixture whereas the economy mixture is leaner than the
stoichiometric mixture.

A study conducted on effect of venturi diameter of carburetor on performance of six-stroke 125

cc combustion engine informed that diameter 20 (mm) can increase average of about 21(%) on
torsion, 21 (%) on power, 16 (%) on specific fuel consumption (SFC), and 23 (%) on thermal
efficiency to that of the diameter 18 (mm). if compared to four-stroke CE, the six stroke CE has
lower in SFC and thermal efficiency, however, it has higher in average value of torsion, power,
and engine speed, respectively about 15 (%)[ CITATION Sis16 \l 1033 ]. J. Suresh kumar studied the
effects on the flow field due to various shapes of venturi constriction and concluded that
Trapezoidal venturi shape had a higher mass flow rate when compared to other venturi shapes. In
actual vehicle trials, it was found that this venturi shape has a better acceleration time of 5%
(0∼60 km) compared to circular shape. At wide open throttle, CFD predicted airflow rate with
circular venturi shape was found better than other venturi shapes due to lesser restriction. In
actual vehicle trials, this type of venturi cross section yielded 4% more power than the
trapezoidal venturi carburetors[ CITATION JSu13 \l 1033 ]. To understand the flowrate and pressure
distribution inside a venturi due to various obstacles Ami A. Patel conducted a cfd analysis For
Different Positions Of Modified Aerodynamic Shape Throttle Valve which showed ,pressure at
the throat also decreases with increase in opening of the throttle plate so the flow of air from the
float chamber into the throat increases as the increase in pressure drop at the throat section,
amount fuel entering into the throat section increases and makes the mixture progressively rich
and The Velocity streamline is uniform in aerodynamic shape by Comparing to existing flat
plate throttle valve to aerodynamic throttle valve design , mixing of air fuel is also uniform,
reduced unborn fuel ratio and increase the efficiency of carburetor [ CITATION Ami17 \l 1033 ]
When a Computational Fluid Dynamic Analysis of Compressible Flow across a Complex
Geometry Carburetor Venturi showed that the presence of a fuel tube reduces the pressure
drop at throat in the radial direction and when a the edge of the of the fuel tube acts as sharp
leading edge it develops a separation region, which results in a lower pressure at the tip of the
fuel tube. Downstream of fuel tube, it is almost uniform in radial and axial directions. The
presence of fuel tube effectively reduces the velocity and creates the wake region (fluctuating
velocity field) behind the venturi. This wake zone may be responsible for fuel puddling after the
carburetor[ CITATION Arv17 \l 1033 ] Diego A. Arias and Timothy A. Shedd concluded from their
CFD analysis that the effect of the fuel tube on the airflow is to reduce the effective area used by
the flow behind the venturi. The size of the wake region is increased with the length of the fuel
tube. the wake zone is also the region where the isentropic stagnation pressure is reduced most
significantly but , a fuel tube extending into the venturi throat, beyond the throat wall, is
necessary as It brings the fuel flow near the centerline of the venturi, which is intended to help
generate an even distribution of the droplets in the flow field [ CITATION Die06 \l 1033 ]. When
analyzed for fuel discharge nozzle angle of 30 degree , it was observed that the pressure
distribution inside the body of the carburetor is quite uniform which leads to a better atomization
and vaporization of the fuel inside the carburetor body. But in other cases like where the fuel
discharge nozzle angle was 35 , 40 or 45 degree, the pressure distribution is quite non-uniform
inside the body of the carburetor [ CITATION DEE11 \l 1033 ] . Mathias Romańczyk used an open
source three-dimensional computational fluid dynamics (CFD) modelling software OpenFOAM
to investigate and analyses the influence of different gas in let angles on mixer characteristics
and their performance and concluded that the greater the inclination of the gas inlet, the more
methane → CH, is sucked into the Venturi gas mixer. As a result, there would be a richer air-gas
mixture at the outlet of the Venturi mixer. This also significantly affects the increase of the
efficiency of the whole mixing process. Moreover, smaller pressure loss occurs through the
whole Venturi mixer[ CITATION Mat17 \l 1033 ].

3. Numerical Model
3.1 Governing Equations
The venturi effect can be defined by the use of Bernoulli’s Equations for compressible which
defines how the pressure drop and increase in velocity is caused due to the constriction in the
flow passage of the fluid . The differential energy balance equations for a compressible
isentropic flow is given by eqn(1)[ CITATION Yun06 \l 1033 ]
(1)
ⅆP
+ Vⅆ V =0
ρ

Where is the pressure difference, V is the velocity and ρ is the density of the fluid

This relation is also the differential form of Bernoulli’s equation when changes in potential
energy are negligible, which is a form of the conservation of momentum principle for steady-
flow control volumes. A equation for isentropic flow in venturi tubes which describes the
variation of pressure with flow area is given by eqn(2) and eqn (3) [ CITATION
(2)
Yun06 \l 1033 ]

ⅆA ⅆP 1 dρ
A
=
(−
ρ v 2 ⅆP )
Which can be written as

ⅆA ⅆp (3)
= 2 ( 1−M a2 )
A ρv

Where

dρ 1
=
ⅆP c 2

For Subsonic Flow Ma < 1

3.2 Geometry Development

The Geometric model of the venturi tube was developed in Space Claim in ANSYS 2021. The
model Consists of a circular intel area, converging throat section , circular outlet area and a fuel
tube inserted at a certain angle inside the throat section.
 Throat section

Inlet Outlet

Fuel Tube

Figure 4 : Design of Venturi Tube

Figure 5: Design Of Venturi tube with fuel tube at 90 degree

Figure 6: Design Of Venturi tube with fuel tube at 60 degree

 Figure 7: Design Of Venturi tube with fuel tube at 60 degree

3.3 Mesh Generation

Mesh generation is the division of computational domain into smaller sub-domains. The mesh
was generated using ANSYS .

Figure 8: Mesh Generation for venturi tube

Mesh statistics for fuel tube at 90°

Element Size 0.0002m
Center Span Angle Fine
Element shape Tetrahedrons
Fuel Tube Angle Nodes Elements
90 ° 641160 3175728
Mesh statistics for fuel tube at 60 ° and 30 °
Element Size 0.0003m
Center Span angle Fine
Element Shape Tetrahedrons
60 ° 282480 1079872
30 ° 282358 1079066
3.4 Boundary conditions
The Fluid which flows through the venturi is considered as air ideal gas. The gas is compressible
and the carburation process is assumed to be isentropic in nature. The boundary conditions for all
the venturi models are the same to compare the calculated parameters. boundary conditions use
in ANSYS software for the simulations are given in the table below.

Inlet Boundary conditions

The airflow starts from the inlet of the venturi whose conditions are identical to the
atmospheric conditions.
Inlet pressure 1 Atm
Turbulence Intensity Medium
Inlet Temperature 25° C
Outlet Boundary Conditions
The air and fuel mixture exits from the outlet into the combustion chamber at a fixed
pressure with reduced pressure and increased velocity
Outlet Pressure 94KPa
Wall boundary
No slip walls
There is no heat conduction through the material and convection to the surrounding
Fluid Domain
Material Air ideal gas
Reference pressure 1 Atm
Fluid temperature 25°C
Buoyancy Model Non-Buoyant
Turbulence K-epsilon Model
Solver Control
Iterations 100
Convergence RMS 1 × 10-5
 4. Results and Discussion

Figure 9: Velocity (without fuel tube)

Figure 10 : Stagnation Pressure(without fuel tube)

4.1 Fluid flow without tube

Figure 11 : Static Pressure(without fuel tube)

Figure 9, 10 and 11 shows the velocity, Total

pressure and static pressure for a
compressible air flow across the venturi without Presence of a extending fuel tube. In the Fig 9
the velocity increases as flow converges through the nozzle and then separates from the wall at
the divergent section of the throat in and becomes almost constant after rapid acceleration at the
throat section . The velocity is almost constant behind the venturi. In the Fig. 10 the total
pressure (stagnation pressure) is uniform to form the inlet to the end region of the throat except
at the wall of the throat. Mostly the decrease in stagnation pressure is responsible for the
development of the wake region (turbulence region). The Static Pressure is uniform in the radial
direction except at the throat where the pressure drop takes place which is very important
because the pressure drop in the throat area allows the fuel to be suctioned from the
comparatively high pressure float area. After venturi the pressure variation is almost constant (no
pressure fluctuation) in the radial direction. The Proper and uniform distribution of static
pressure ensures the proper atomization and equal distribution of fuel in the air fuel mixture .

4.2 Fluid flow with fuel tube(90 °)

Figure 13: Stagnation Pressure(with fuel tube at 90 °)

Figure12: Velocity (with fuel tube at 90 °)
 Figure 14: Static Pressure(with fuel tube at 90 °)
Figure 12 shows when a fuel tube is
extended at 90 degrees into the throat section the velocity profile is highly disturbed. The
disturbance created by the tube effectively reduces the velocity and creates the wake region
behind the venturi which is a rapidly fluctuating velocity field. The fluctuating velocity field is
the cause of the loss required kinetic energy to carry the fuel out of the throat section into the
mixing zone . This wake zone is responsible for fuel puddling as once the fuel droplet enters
into this region, the momentum has diminished to the level where it cannot carry the fuel droplet
to the manifold. Figure 13 shows a high reduction of the stagnation pressure at the Fuel tube
region and large wake region formed downstream of the Fuel tube due to high turbulence which
is caused by the fuel tube acting as an obstacle. The fuel tube reduces the cross sectional area
and also causes comparatively lower pressure drop at throat in the radial direction. In addition, a
sharp leading edge of the fuel tube creates a separation region, which results in a lower pressure
at the tip of the fuel tube.