Section

B:

A

diffuser is used in this CFD problem. Its purpose is to reduce the velocity of

the working fluid. Hence the increase in the pressure present in the pipe. An

efficiently designed diffuser would be able to minimize the total pressure loss

while providing nearly uniform flow from the inlet to the outlet of a pipe. CFD

was used in conducting this experimental study with intention to investigate

the effects of sudden contraction of pipe. The objective is to study the

pressure from the inlet to the axial of the sudden contraction. It is also to

determine the suitable mesh size that produces stable grid independent results.

Simulation

Setup:

1) Geometry

a) X-Y

plane is selected. Click Z axis.

a) Draw

the dimension of the geometry.

Lextend / D2 =

0.2mm*(default)

Lextend / 2 =

0.2mm

Lextend =

0.4mm

H3 = 0.4mm

The geometry dimensions:

V1

(inlet)

4

mm

H2

(axis 1)

10

mm

H3

(axis 2)

0.4

mm

V4

(outlet)

1

mm

Table

1: Model Dimension

1) Mesh

a) Define

boundary with proper name as shown below.

b) Select

mesh control and click inflation. Select wall as a boundary.

c) The

following Table is the mesh size that was used in this assignment:

Min Size

0.007mm – 0.1 mm

Max Face Size

0.02mm – 0.5 mm

Max Size

0.02mm – 0.5 mm

Example picture of mesh where its (min size=

0.06mm, max face size & max size = 0.1mm, mesh size = 6520)

d) Since

there are multiple mesh sizes to be studied and investigated upon, therefore a

single size was chosen to be the best average for this experiment. Each mesh

size is recorded after the completion of generating mesh process into data and

then updated.

2) Setup

Some settings at

setup mode were not altered and left in default mode. The only settings that

were changed is stated in Table below:

General

Type

Pressure-Based

Velocity

Formation

Absolute

Time

Steady

2D space

Axisymmetric

Models

Viscous

Standard k-e, Standard Wall Fn

Materials

Fluid

Water-liquid

solid

aluminium

Boundary

Condition

Inlet Velocity

3 m/s

Outlet Pressure

0 Pa

Calculation

Number of iterations

1000

3) Solution

–

Run calculation to gain

result.

PART

1

Table of average pressure at different number

of elements.

Point

Min

size

(mm)

Max face

(mm)

Max

Size

(mm)

No.

of elements

Average

Pressure (Mpa)

A

0.1

0.5

0.5

672

1.50

B

0.09

0.4

0.4

890

1.26

C

0.08

0.3

0.3

1270

1.06

D

0.07

0.2

0.2

2259

2.00

E

0.06

0.1

0.1

6520

1.40

F

0.05

0.09

0.09

7735

1.83

G

0.04

0.08

0.08

9528

1.94

H

0.03

0.07

0.07

11869

2.08

I

0.02

0.06

0.06

15434

1.97

J

0.01

0.05

0.05

21154

1.96

K

0.009

0.04

0.04

31428

1.90

L

0.008

0.03

0.03

53128

1.92

M

0.0075

0.025

0.025

74726

1.90

N

0.007

0.02

0.02

113436

1.84

Table

2

Plotted graph for X-Y-plot of average

pressure against number of elements.

Based

on the tabulation result that was calculated, a graph of pressure against number

of elements is plotted. From the graph drawn, it is to be observed that the

line fluctuated and is not stable until when it reached to a point where the number

of elements (Mesh size) is 31 428. Then, the pressure only finally be stable

with minimum fluctuation where one can consider it be negligible as the

percentage difference among them are so small, proving that the percentage

different among each different point from the number of elements of 31 428

until 113 436 is lower than 3% when calculated.

To

further understand this fluid flow simulation, further research had to done for

result’s greater accuracy to real-life application. The following diagram was

obtained from the research material submitted by (Shabbir, 2015)

Diagram 1 – Sudden contraction of flow

Reference

to the diagram above, it can be seen sudden contraction of flow is affected by

the sudden decrease in pipe diameter. This sudden contraction would cause a

section where flow was narrower than usual. This narrow flow is called a ‘Vena

Contracta’ and it has the maximum velocity and minimum pressure at this part as

it obeys the law of Conservation of Energy. It is after this ‘Vena Contracta’

region where flow velocity would slow down and stabilize. The turbulence zones

were due to backflow occurring at those regions, in which the smaller diameter

pipe had a virtual diffuser acting at the flow that causes ‘Vena Contracta’.

Following formula was given as to calculate the loss of head (Sellens, 2014):

Where,

Ac = cross-section area

of Vena Contracta

Cc = coefficient of

contraction

And Cc can be

expressed in,

But the usual expression

of hL is as follows:

Where K is

represented by,

From

the graph, it can be observed that point K is the ideal choice to be the best

grid resolution for the pressure to be grid independent. This is because, at

this point, the pressure different between this point and the point after this

is already considered relatively small and is labelled as stable. Although the

point after this has smaller pressure difference than the next point, but

obviously need a much longer time to generate the mesh. Hence, time is wasted

as there is no need for it since point ‘K’ was good enough. Following is the

mesh of the chosen point: