advanced cfd modelling – coursework ii

Advanced CFD Modelling –

Coursework II

Module: Advanced Computational Fluid Dynamics (ENGM60)

Student: Rodrigo Folgueira

Lecturer: Dr Xiaogang Yang

Advanced CFD Modelling – Coursework II

2 Rodrigo Folgueira (S1000004)

INDEX

1.- Introduction............................................................................................................................... 3

2.- Background Theory .................................................................................................................. 4

2.1.- CFD ................................................................................................................................... 4

2.2.- Turbulence Flow ................................................................................................................ 5

2.3.- Pressure ............................................................................................................................ 5

2.3.1 Static pressure .............................................................................................................. 6

2.4.- Reynolds Number ............................................................................................................. 6

2.5.- Turbulent Kinetic Energy ................................................................................................... 7

3.- Problem description ................................................................................................................. 8

3.1.- Specifying the geometry of the problem ........................................................................... 8

3.2.- Steps ................................................................................................................................. 8

3.3.- Process ............................................................................. ¡Error! Marcador no definido.

4.- Calculation of flow parameters ............................................................................................... 10

4.1.- Data ................................................................................................................................. 10

4.2.- Calculations ..................................................................................................................... 11

5.- Numerical Implementation ..................................................................................................... 12

5.1.- Define model ................................................................................................................... 12

5.2.- Define materials .............................................................................................................. 12

5.3.- Define units ..................................................................................................................... 12

5.4.- Define zell zone conditions ............................................................................................. 13

5.5.- Define boundry conditions ............................................................................................... 13

6.- Result comments and discussions ......................................................................................... 14

6.1.- Static Pressure ................................................................................................................ 14

6.2.- Velocity magnitude - Velocity vectors ............................................................................. 17

7.- Conclusions ............................................................................................................................ 20

8.- References ............................................................................................................................. 21



1.- Introduction

This report consists in the analysis of the turbulence flow through a computer

cooling axial flow fan.

The aim of this project is to design three different models of a cooling axial flow

fan, using numerical simulation methods and analyzing the aerodynamic flow.

All this models are designed in Gambit and solved in Fluent program.

The differences between these models consist in the number of blades and the

blade installation angle.

The first one will have six blades and 20º angle, for the second one the number

of blades is increased to eight and the angle to 25º and for the last one it will be

the same number of blades but different angle, 20º in this case.

The aim of the project is to make a comparison between the data obtained in

the study of this three fans. The objective is to study what happens with the

static pressure and the velocity distributions in the different profiles.

Finally, it will be necessary to consider a proper design of flow pattern by using

the concept of vortex design.



2.- Background Theory

Below, a brief explanation of equations, theory and elements related to this project.

2.1.- CFD

The aproximation of a continuos variable into a finite number of points is called

discretization.

The main elements of CFD is the discretization of the continuous flow and the

discretization of the appropiate eccuations based on the value of the nodes.

Fluent is the most preferred program for working with CFD for a wide range of

flows, incompressible, highly compressible and medium compressible.



2.2.- Turbulence Flow

Turbulent flow is characterized by erratic circular oarts, like whirpools, occurs

when the flow rates are generally very high or in fluids whose viscous forces are

very small.

It can be caused by the presence of

walls with the fluid or the presence

of layers moving at different

speeds. Furthermore, turbulent flow

can be developed either in a flat

conduit or in a smooth rough one.

According to the viscosity of the

fluid, the flow can be classified as

laminar or turbulent.

In turbulent flows, the particles

move out of the established order.

This flow is characterized by the continuous fluid mixing in a chaotic way, as a

result of the breakdown of an orderly flow of vortices.

2.3.- Pressure

Pressure is defined as force per unit area. The unit in the international system is

Pascal, that is Newton per meter square.



2.3.1 Static pressure

In this coursework it will be necessary to discuss and compare the Static

pressure of the three different fans.

The static pressure is the pressure at a point in a fluid where the fluid not

moving (static).

Where:

P0 is the total pressure (Pa)

Ps is the static pressure (Pa)

Pd is the dynamic pressure (Pa)

2.4.- Reynolds Number

Reynolds number can be defined as a dimensionless number which is very

important because it helps to define whether a given fluid is in a laminar regime

or in the transition between both regimes.

where:

V is the mean fluid velocity (m/s)

L is a characteristic linear dimension (m)



μ is the dynamic viscosity of the fluid (N·s/m²)

ν is the kinematic viscosity (ν = μ / ρ) (m²/s)

ρ is the density of the fluid (kg/m³)

2.5.- Turbulent Kinetic Energy

In fluid dynamics, turbulence kinetic energy (TKE) per unit mass is the total

energy of the turbulence intensity.

It depends on the static stability of the air (stable, neutral or unstable). For

stable air, the standard derivations in the atmospheric boundary layer has been

shown empirically that varies with the height.

where:

Dk / Dt is the mean-flow material.

is the turbulence transport.

P is the production.

ε is the dissipation.

http://en.wikipedia.org/wiki/Dynamic_viscosity

http://en.wikipedia.org/wiki/Fluid

http://en.wikipedia.org/wiki/Kinematic_viscosity

http://en.wikipedia.org/wiki/Density

http://en.wikipedia.org/wiki/Fluid_dynamics



3.- Problem description

This coursework describes the behavior of the flow through a computer cooling axial flow fan.

3.1.- Specifying the geometry of the problem

The aim of this project is to calculate and then compare static pressure and the

velocity of thre fans:

First fan: 6 blades, angle = 25º

Second fan: 8 blades, angle = 25º

Third fan: 8 blades, angle = 20º

3.2.- Steps

For this calculation and subsequent comparison is necessary to follow this

steps:

1. design the fan in Gambit.

2. mesh this fan.



3. open this fan that is already Mesh in Fluent.

4. Define the conditions on Fluent.

5. Solve in Fluent.

6. Run the calculations.

7. Display the results in Fluent.



4.- Calculation of flow parameters

This is the calculation that we need to make before start to implement the fan

into Gambit.

4.1.- Data

The axial flow impeller is mounted inside a circular tube of 64 mm in diameter

with a clearance of 2 mm.

The computer cooling axial flow fan is assumed to deliver a designed flow rate

Q0 = 610-3 m3/s and a pressure head Ptotal = 12 Pa under the design

condition.

The following conditions are given:

Hub Diameter Dh = 30 mm.

Impeller Diameter Dt = 60 mm.

Number of blades n = 6, 8 and 8.

Hub width b = 10 mm.

Average blade installation angle m = 25º, 25º and 20º

Blade thickness = 1 mm.



Tube length = 80 mm.

4.2.- Calculations

Re =

Q =

*Not enough data to calculate



5.- Numerical Implementation

This is the calculation that we need to make before start to implement the fan

into Gambit and Fluent and get the values and results that we need to proceed

with the comparison.

5.1.- Define model

The model should be:

Viscous-laminar

K-epsilon (2equations)

5.2.- Define materials

Data for the air:

Density = 1.18 kg/m3

Viscosity = 1.18x10-5 kg/ms

Data for the material of the fan (aluminum):

Density = 2700kg/m3

5.3.- Define units

Velocity should be defined as

Angular-velocity

rpm

-1500 rpm



5.4.- Define zell zone conditions

For rotating air:

Mesh motion

Speed = -1500rpm

For out fluid:

Speed = 0

5.5.- Define boundary conditions

For intake:

Pressure inlet

Gauge total pressure = 12

Specification method = Intensity and hydraulic diameter

For Outlet:

Pressure outlet

Gauge total pressure = 0

Specification method = Intensity and hydraulic diameter

For the wall:

Moving wall

Rotating

Relative t adjacent zone



6.- Result comments and discussions

This are the different results obtained with the different models that have been designed:

6.1.- Static Pressure

In the blades, is possible to see that there is a stagnation of flow, velocity

increases and decreases with pressure.

High pressures are responsible for the flow.

Static Pressure of the fan 8 blades, 25º.



Static Pressure of the fan 6 blades, 25º

It is possible to observe which the static pressure is higher around the nucleus

(centre) of the fan.

Static Pressure of the fan 8 blades, 20º



Static Pressure Comparison among the three different fans.

8 blades, 25º 6 blades, 25º 8 blades, 20º

Max Static Pressure (N/m2) 4.06x106 2.19x105 1.43x105

Min Static Pressure (N/m2) -6.99x106 -4.57x105 -3.48x105

As it is possible to observe here, the values of the first fan are so different than

the values for the second and the third fan. In these last ones we can

appreciate that decreasing or increasing blades is not making big changes in

the behavoiur of the static pressure.



6.2.- Velocity magnitude - Velocity vectors

It is noted that there is less concentration of velocity in the blade with 8 blades

and 25º.

Is also possible to see that there is higher concentration of velocity in the fan

with 8 blades and 20º

Velocity vectors of the fan 8 blades, 25º.




As we observe in this two profiles there is a huge concentration of velocity

where the fan is located, that is because is there where the flow is moved at big

speed (1500rpm).


Velocity vectors comparison among the three different fans.

8 blades, 25º 6 blades, 25º 8 blades, 20º

Max Velocity magnitude vectors (m/s) 4.89x103 2.02x104 4.76x103

Min Velocity magnitude vectors (m/s) 1.86x101 2.35x10-3 8.66x10-3



Contrary to what happened with the static pressures, what we can see

here is that there is similar values for the maximum velocity in the fan 1

and 2 and different values for the minimum velocity of all of them, even

more when we speak about the first one.



7.- Conclusions

In this project, we have studied the specific pressure and the velocity of three

fans with different characteristics. This report presents the results of the study of

a fan and its characteristics.

There are two different studies, first specific pressure and then Velocity. What

we can get from the first one is that the concentration of pressures is located

around the blades, that is it because this ones (the blades) are the responsible

of moving the air flow, this work create a pressure in this point.

It is possible to observe that the first one is working absolutely different than the

other ones, could be a problem in the mesh when working in gambit or a

problem in the definition of the boundary conditions.

The second investigation show us the velocity of the flow when pass through

the fan. The pressures are locatedat the same level than the fan and near the

wall.

There is a region of stagnation pressure in the wall. This is because the blades

of the fan are moving this air flow and are pushing it to the walls; this is why

losing velocity is at the same time that is going to the outlet.

There is not cohesion between all of them in the results of the study of

maximum and minimum velocities, some of them are similar between a pair of

fans, and some are similar between different pair.

The different conditions (angle and number of blades) are creating this

difference between results.

In summary, the aim of this project was to study the velocity and the static

pressure of three different fans and it was successfully completed.



8.- References

CFD

http://www.vagos.es/showthread.php?t=306363 http://bibing.us.es/proyectos/abreproy/3718/fichero/Parte+I%252FCapitulo+3.pdf

Turbulence flow

http://fluidos.eia.edu.co/hidraulica/articuloses/conceptosbasicosmfluidos/flujotturbulento/flujoturbulento.html

Static pressure

http://www.aireyespacio.com/2010/05/la-presion-estatica-y-la-presion.html

Reynolds number

http://www.sc.ehu.es/sbweb/fisica/fluidos/dinamica/reynolds/reynolds.htm

Turbulent kinetic energy

http://www.tutiempo.net/silvia_larocca/Temas/Consultas12.htm

http://www.vagos.es/showthread.php?t=306363

http://bibing.us.es/proyectos/abreproy/3718/fichero/Parte+I%252FCapitulo+3.pdf



http://www.aireyespacio.com/2010/05/la-presion-estatica-y-la-presion.html

http://www.sc.ehu.es/sbweb/fisica/fluidos/dinamica/reynolds/reynolds.htm

http://www.tutiempo.net/silvia_larocca/Temas/Consultas12.htm

advanced cfd modelling – coursework ii

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