Aim
“Study the flow of compressible fluids”
Aim of this study is to understand the flow behaviour
compressible fluid when they are made to flow under different conditions like
different initial velocity or flow inside pipe of different diameters
Objectives
1.
Understanding the flow of fluid
In order to study the flow of compressible fluids it is
important to first understand the basic about the flow of fluid. This may
include information about what fluid is, what are its types, what are different
types of flow in which a fluid can flow.
2.
Numerical analysis to predict
the flow behaviour
Numerical analysis is done in engineering in order to
predict the behaviour of things when certain condition is applied on them. For
this task this may include the computational fluid dynamics to predict the type
of flow in fluid when it moves inside the pipe.
3.
Experiment of air flowing
through a pipe
An experiment will be performed to study the flow of air
inside a pipe. Experiment will consist of three different pipes with difference
in the diameter of pipe and many different initial velocities of the air
entering the pipe. Experimental data will be collected and used for the
calculation of Reynold number, friction factor and head losses inside pipe.
4.
Compare the Numerical analysis
and experimental data
In order to check the accuracy of the predicted flow
behaviour in numerical analysis a comparison will be made between numerical
simulations and experimental results. Another comparison will also be made
between calculated and graphical value of friction factor in order to check any
variation in experimental data as compared to ideal values.
Introduction
Fluid and Its Types
Fluid is any material that can flow from one point to
another point due any reason or in the influence of any force. Fluid can be either
in the form liquid or gas and in some gasses it can be converted from one form
into another during flow. There two main types of fluids
1.
Compressible fluid
Compressible fluids
are those fluids which can undergo compression at molecular level when a certain
amount of pressure is applied on them.
2.
Incompressible fluid
Compressible fluids
are those fluids which cannot undergo compression at molecular level when a
certain amount of pressure is applied on them.
Types of Fluid flow
There three main types of fluid flow
1.
Laminar flow
It is the type of
flow where each proceeding particle follows the same path in the flow on which
the first particle has moved during flow.
2.
Transient flow
It this types of
flow where each proceeding particle follows the same path with little
disturbance in the flow on which the first particle has moved during flow
3.
Turbulent
It is the type of
flow in which each particle has its own path in flow and no one part follows
the other particle in a stream.
Reynold Number
It is the ratio of fluid inertial forces to the fluid
viscous forces. It’s a dimensional less quantity which is used to show the type
of fluid flow. If the Reynold number of the fluid flow in less than 2100 then
the flow is said to be laminar, If the Reynold number of the fluid flow in
greater than 2100 and less than 4000 then the flow is said to be transient and
If the Reynold number of the fluid flow in greater than 4000 then the flow is
said to be Turbulent.
Computational Fluid Dynamics
Computational fluid dynamics is the branch of fluid
mechanics which involve solving complex equations related to the fluid flow in
in different conditions. Computational fluid dynamics is used to predict the
behaviour of fluid flow under the given condition. Computational fluid dynamics
done in simulation program ansys consist of following steps
1.
CAD modelling
CAD modelling is the
process of making a computer added model of the required system using ansys
workbench. In the current work pipe and fluid which is air will be modelled in
this section and then assembled to make complete system
2.
Meshing
Is this section the
designed system is divided into very small sections call cells joined to each
other through nodes or key points. This is done in order to get solution of
governing equation for each section.
3.
Setup
In this section the
inlet, outlet and boundary walls of the system are setup along with the
required values. In this section material properties of the system is defined.
In this section the material properties of pipe like smoothness and fluid
properties like density and fluidity are defined. Value of initial velocity is
also defined in this section. For this work inlet velocity of 47.1 will be used
with air a fluid and smooth pipe
4.
Solution
In solution section type
of solution is selected for the system and required output parameters are
selected. When started solution section will calculated values of selected
parameters and show them in the shape of colour figures. For this work
transient type hybrids solution will be done.
Results
Based on setup made above simulation of the required
system were run and result in terms of fluid velocity has been shown below. According
to the result flow of fluid inside the pipe will be turbulent as different
velocities can be seen near inlet of the pipe along with the waves on entire
length and especially on the top section of fluid domain.
It can be observed from the below mention figures that flow
of fluid inside pipe is uniform and very smooth. This is due to the fact that
wall roughness of the pipe is kept zero to make smooth pipe.
Experimental Results
Pipe
Diameter

Vo m/s

Change in
P mbar

m kg/sec

Vpipe
m/sec

Re

F

34 mm

51.4

9.8

0.1

51.4

116550.3

0.0210

47.8

8.7

0.1

47.8

108387.3

0.0216


41.9

6.8

0.0

41.9

95008.9

0.0219


35.5

5.1

0.0

35.5

80496.8

0.0229


31.2

4.2

0.0

31.2

70746.5

0.0244


21.3

2.1

0.0

21.3

48298.1

0.0262


15

1.3

0.0

15.0

34012.7

0.0327


24 mm

42.1

23

0.0

84.5

191587.8

0.0182

37.3

14.8

0.0

74.9

169744.0

0.0150


32.1

8.7

0.0

64.4

146080.0

0.0119


27.1

2.7

0.0

54.4

123326.1

0.0052


21.6

0.2

0.0

43.4

98296.8

0.0006


14.5

0.4

0.0

29.1

65986.3

0.0027


16 mm

21.7

60.5

0.0

98.0

222191.8

0.0357

18.1

33.4

0.0

81.8

185330.5

0.0283


16.3

28.4

0.0

73.6

166899.8

0.0297


14.2

16.7

0.0

64.1

145397.4

0.0230


12.4

8

0.0

56.0

126966.7

0.0144


10.2

0.3

0.0

46.1

104440.4

0.0008

Comparison
Moody chart mention below is used to find the frictional
factor of the graphically and it is said to the ideal values of frictional
factor as it does not consider the actual working conditions. For the initial
velocity of 47.1 m/sec the experimental value of fractional factor is 0.0216
but for the calculated Reynold Number of 1*10^5 the graphical value is said to
be 0.018 which quite less than the experimental value. The reason for high
value of frictional factor in experiment is that pipe was considered smooth but
it was not. Wall roughness of pipe add more value to frictional factor.
Discussion
·
The prediction made by the
computational fluid dynamics for the behaviour of the fluid flow is verified by
the experimental data. Numerical analysis predicted that fluid flow will be
turbulent in the pipe and according to the experimental results the calculated
Reynold number for all initial velocities is above 4000 range which is for
turbulent flow. This turbulent flow in smooth pipe is due to the high initial
velocity as compared to the density of air.
·
According to the moody chart
the frictional factor and Reynold number are connected to each other. So in
order to find the relationship between these two a graph was generated which
have friction factor on y axis and Reynold number on x axis. According to the
trend shown in graph frictional factor and Reynold number are directly
proportional to each other. Increase in value of Reynold number will increase
the value of frictional factor and decrease in value of Reynold number will
decrease the value of frictional factor
·
In order to find the
relationship between mass flow rate and pressure drop a graph was generated
which have pressure drop on y axis and mass flow rate on x axis. According to
the trend shown in graph mass flow rate and pressure drop are directly
proportional to each other. Increase in value of mass flow rate will increase
the value of pressure drop and decrease in value of mass flow rate will
decrease the value of pressure drop.
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