MIET-1081 Advanced Thermo-Fluid Mechanicsbe using FLUENT’s text-user-interface (TUI), to set up...

MIET-1081

Advanced Thermo-Fluid Mechanics

Computer Lab ver 2019

Kiao INTHAVONG, Yihuan YAN, Jiawei MA RMIT University, School of Engineering

Information:

COMPUTER LAB LOCATION: 253.02.14

Computer Lab Sessions:

Tuesday: 11:00 - 12:00

Tuesday: 17:00-18:00

Wednesday: 10:00 - 11:00

Friday: 9:00 - 10:00

*Computer lab is limited to max. 60 students in each session. Please attend your enrolled session. You only need to attend ONE of the above sessions.

INSTRUCTORS

Dr. Kiao Inthavong Email: [email protected] Office: 251.3.41

Dr. Yihuan Yan Email: [email protected] Consultation times: Tuesdays 13:00 - 14:00 Office: 251.3.65

Mr. Jiawei Ma Email: [email protected] Consultation times: Thursdays 11:00 - 12:00 Office: 251.2.44

TEACHING ASSISTANTS: Xiang Fang, Jiang Li

INSTRUCTIONS: Bring in this booklet each week. Fill in the answers in the spaces provided. You will also perform hand calculations to assist you in your learning. At the end of the semester this booklet will serve as revision text for your exam.

MIET-1081 Computer Lab Handout 2019 1

mailto:[email protected]



CL-01 Heat Conduction 1.1 Using FLUENT in MIET1081 Introduction to FLUENTs Text User Interface Ansys-FLUENT is a computational fluid dynamics program that provides colourful predictions of the physical behaviour of fluids, by numerically solving equations that govern the flow. In this course we are not interested in the details of the software, but in visualising the physical behaviour of fluid and heat transfer. Heat transfer cases will be available each week and we will be using FLUENT’s text-user-interface (TUI), to set up and solve the heat transfer problem.

When you first start FLUENT, you are in the root menu and the menu prompt is simply an arrow with a blinking cursor. >

To generate a listing of the submenus and commands in the current menu, simply press <Enter>. You will get the following: define/ grid/ solve/ ...

To execute a command, just type its name (or an abbreviation). To move into a submenu, enter its name or an abbreviation. When you move into the submenu, the prompt will change to reflect the current menu name. For example type disp and press <Enter>. To generate a listing of the submenus and commands in the current menu, simply press <Enter> again. To enter a submenu, enter its name, (e.g., type in set) and press <Enter>: > disp /display/set>

To go back up a menu type q or quit at the prompt and press <Enter>.

For example: Let’s define the material of the model as wood.

a) Type define <Enter> followed by b-c <Enter>. Typing b-c is an abbreviation of the same as typing boundary-conditions.

b) Type solid and <Enter>. A list of options follows. c) Type in the responses as shown in bold font below.

zone id/name [body] body material-name [aluminum]: Change current value? [no] yes material-name [aluminum]> wood Specify source terms? [no] no Specify fixed values? [no] no Frame Motion? [no] no Use Profile for Reference Frame X-Origin of Rotation-Axis? [no] no Reference Frame X-Origin of Rotation-Axis (m) [0] 0 Use Profile for Reference Frame Y-Origin of Rotation-Axis? [no] no Reference Frame Y-Origin of Rotation-Axis (m) [0] 0 Mesh Motion? [no] no Deactivated Thread [no] no

We can use journal text files to issue these commands automatically as a list of inputs. It is simply a text file that sends commands. Enter the following command into Notepad, define/boundary-conditions/solid body yes wood n n n n 0 n 0 n n

and save the file as week1.jou. The file should be saved in the same directory as the FLUENT case (.cas) file. If you can’t find your file in a directory use pwd to show where it should be.


1.2 1D Steady Heat Conduction

Conduction is a heat transfer mode involving transfer of energy from more energetic particles of a substance to its neighbouring less energetic ones. Although conduction can occur in solids, liquids, or gases, it is most significant in solids through the lattice connection of electrons. In gases and liquids, conduction occurs through collisions and diffusion of molecules. The rate of heat conduction is dependent on its geometry, shape, its thickness, the material (which is its conductivity), and the temperature difference across the medium.

Figure 1.1: Parameters influencing heat conduction

Consider heat conduction through a large plane wall such as a brick wall of a house. In such a geometry heat conduction can be approximated as one-dimensional since the conduction will be dominant in one direction and negligible in other directions. Fourier’s law of heat conduction is used to describe this mode of heat transfer

Notes:

● The conduction heat transfer area is always normal to the direction of heat transfer. ● The thickness of the wall is independent of the heat transfer area. ● Since heat transfer occurs in the direction of decreasing temperature, the resulting

temperature gradient becomes negative when temperature decreases in the direction of x i.e. increasing x.

● This means that we need a negative sign to ensure that heat transfer in the positive x direction gives a positive value.

● dT/dx is the temperature gradient, which describes how the temperature changes in the x-direction

The thermal conductivity, k is a measure of a material's ability to conduct heat. A high value for thermal conductivity means that the material conducts heat well, and a low value indicates that the material resists heat conduction acting like an insulator. Intuitively we know that metals get hot very quickly from exposure to heat, while water takes longer to heat up and this is confirmed by their conductivities: iron has k = 80 W/moC, and water has k =0.61 W/moC.

Thermal conductivities of some materials

Material k (W/m.oC) Material k (W/m.oC) Material k (W/m.oC)

Diamond 2300 Aluminium 237 Water, liquid 0.61 Silver 429 Steel 16.27 Wood, oak 0.17 Copper 387.6 Brick 1.31 Air, gas 0.026 Gold 317 Glass 0.78 Insulating foam 0.026


Using FLUENT to Predict a Steady Heat Conduction Problem (Practices) Step-1: Open the file “CL01-2.cas” ensuring that the 2D option is selected. See the “ Appendix A ” at the back of the handout p.21 for loading the case.

a. Figure 1.2: 2D geometry of a plane wall problem

Step-2: Use the Excel journal script maker to create your journal file. Define the material body. The name of the material can be one of the following brick, wood, copper, aluminum.

Step-3: Define the boundary conditions. • Boundaries are left, right which may be temperature, or heat-flux. • We also need to enter a value for the boundary condition selected: T, [oC] for

temperature or q, [W/m2] for heat-flux

Investigate the following cases.

CASE 1 - copper bar (k =387.6 W/moC, you don’t need to enter this value since it is contained within the FLUENT material library) with:

• Left end with a constant heat flux 3250 W/m2

• Right end has a constant temperature 30oC

CASE 2 - copper bar (k =387.6 W/moC) with: • Left end has a constant temperature 250oC • Right end has a constant temperature 30oC

CASE 3 - brick bar (k =1.31 W/moC ) with:

• Left end with a constant heat flux 3250 W/m2

• Right end has a constant temperature 30oC

CASE 4 - brick bar(k =1.31 W/moC) with: • Left end has a constant temperature 250oC • Right end has a constant temperature 30oC

Post-processing Use contours, xy-plots to investigate and answer the questions on the next page, (see the Appendix B at the back of the handout p.22 for post-processing procedures).


QUESTIONS

Q1 Using Fourier’s heat conduction equation calculate the following

Case 1 Case 2 Case 3 Case 4

Tleft

250

250

q

3250

3250

Q2 Plot each of the temperature gradient for the four cases.

Q3 Why are the dT/dx the same for Case 2 and Case 4?

Q4 Which dT/dx has the highest gradient? Which has the flattest or smallest gradient? Why?

Q5 If we apply same wall temperature boundary conditions for the four different materials

(brick, wood, steel, and copper) what would their dT/dx look like. Explain why in terms of Fourier’s conduction equation.


http://www.youtube.com/watch?v=4tgKDa0NHr0

1.3 Conduction and Thermal Resistance Equations Heat conduction through a plane wall can be described as

If we are after a heat flux then the equations become

For heat transfer in series, the resistance is

For heat transfer in parallel, the resistance is

Practices Step-1: Open “CL01-3.cas” file in FLUENT in 2D mode. The geometry is shown below:

Figure 2.2: 2D geometry of a two materials in series Step-2: Using the excel script maker file, investigate the following cases:

Case 1: Body1 = copper Body2 = steel

copper steel

Tleft=175oC Tint Tright=25oC

Case 2: Body1 = copper Body2 = brick

copper brick

Tleft=175oC Tint Tright=25oC

Case 3: Body1 = copper Body2 = brick

copper brick

Tleft=45oC Tint ⬅ q=800W/m2


Questions

Q1 Using the thermal resistance equations (Eqns 2.1 to 2.4) calculate the heat transfer, and the middle interface temperature for each case. The conductivities of the materials used are: kcopper =387.6 W/m.K; ksteel=16.27 W/m.K; kbrick =1.31 W/m.K:

Case 1 Case 2 Case 3

Tint

q 800 W/m2

Q2 Plot the temperature gradient profiles for each case

Q3 Explain what the temperature gradient of copper looks like for all the three cases and why this happens.

Q4 Calculate by hand the heat transfer rate (q) for a i) copper (k = 387.6 W/m.K) and steel (k = 16.27 W/m.K), and ii) copper and brick (k = 1.31 W/m.K), in parallel mode,having the same temperature boundary conditions used earlier (175 0C, and 25 0C).


CL-02 External Forced Convection - Flow over Cylinder 2.1 Introduction The flow over a circular cylinder is one of the most widely investigated engineering problems because of the many fluid dynamics properties that it exhibits. As the Reynolds number (Re) increases, the flow exhibits a series of different fluid structures (Figure 4.1). This includes separation, and vortex shedding in its wake, which all contribute to drag, lift and heat transfer.

Figure 4.1: Regimes of fluid flow across a smooth circular cylinder - image from Lienhard (1966) Synopsis of lift,

drag, and vortex frequency data for rigid circular cylinders, Technical Extension Service

The flow field over the cylinder is symmetric at low Reynolds number. As the Reynolds number increases, flow begins to separate behind the cylinder and vortex shedding occurs. Vortex shedding is an unsteady phenomenon. You will run simulations in steady state or unsteady (time dependent) solver to capture these effects.

In this computer lab we investigate the influence of Re on the flow characteristics and how this influences the heat transfer. This forms part of the external forced convection heat transfer topic.

The lab report requirement consists of two parts: i) experimental measurements and its analysis from the wind tunnel experiments, and ii) CFD modelling.


An infinitely long circular cylinder of diameter D = 0.01m sits in a uniform cross flow with velocity U, modelled in 2D (Fig 4.2). There are two boundary conditions to define. Firstly the flow velocity is defined at the inlet. Also when considering heat transfer we have to define temperature values for the inlet velocity, and the cylinder wall. The heat transfer then becomes an external forced convection heat transfer flow over a cylinder. In the labs you need to simulate the following cases:

● Re = 1 (steady, laminar) ● Re = 100 (unsteady, laminar) ● Re = wind tunnel lab experiment, (steady, laminar) ● Re = 1e6 (steady turbulent; heat transfer)

2.2 Reynolds number and Dynamic Similarity The Re number is a dimensionless number which we use to match flow properties. This means that even if the flow conditions exhibit different material properties, velocities and dimensions - as long as the Re numbers are the same - then the fluid flow characteristics should remain the same. The Re number is defined as

where D = 0.01m.

The fluid material we are using is a customised one, called miet1081 where we will change its properties to easily calculate the Re number. For example, for Re = 1, we can define the material property as ⍴ = 1, D = 0.01, μ = 0.01 which produces

This means that U = 1 to obtain Re = 1 and hence gives us a direct and convenient method for defining Re based on changes in the velocity. Based on the Reynolds number the flow may be laminar or turbulent.

2.3 Running the simulation using the graphical interface. a) Select the Run Calculation option on the left panel b) Set the number of iterations c) Click calculate. d) After the solution has converged save the data. Click File

Write -> Data. This will save the iterations that have been performed.

The number of iterations you choose will depend on the convergence of the drag coefficient. Monitor the drag coefficient in Fluent's console until it converges. If it hasn't repeat the above iterations step and continue running the iterations. Note that if you initialize the case by using your setup journal file, then the solution iterations and the results are reset.


Monitoring the drag coefficient in Fluent console.

2.4 Cases to investigate Open “CL02.cas” file in FLUENT in 2D mode. There are seven cases listed below. The flow conditions (i.e. laminar/turbulent, steady/unsteady and isothermal/thermal) are given in the case descriptions. Follow the below instructions and use the excel script maker to set up the cases. Check the Re in Fluent and simulate.

❏ Isothermal Flows (Laminar vs. Turbulent)-Cases 1 & 2:

***For isothermal flow, set the temperature of the cylinder wall to 20 oC (equal to the inlet airflow temperature)***

Case 1: Re=1, steady, isothermal, laminar:

● Input the density (ρ), viscosity (μ) and velocity (U) in the excel script maker to get Re1.

Case 2: Re=1,000,000, steady, isothermal, turbulent:

● Input the density (ρ), viscosity (μ) and velocity (U) in the excel script maker to get Re1e6.

❏ Steady vs. Unsteady Flows-Cases 3 & 4:

We have solved the steady flow for Re=100, and now will continue the simulation but in an unsteady solution. This means we will look at the flow behaviour at incremental times defined by a time step Δt. If we choose a t that is too large we may not see the fluctuating vortices, and if we choose one that is too small, then this will take much longer than needed. We have to determine the fluctuating vortex frequency by using the dimensionless Strouhal (Sr) number. Experiments have found that the Strouhal number for flow past a cylinder is roughly 0.165 for a Re=100. This means that

D = 0.01, and let U = 1m/s (If you use a different velocity, then your Sr will change)

A shedding cycle time is then 1/16.5 , and the suitable time step to capture the frequency is then


Case 3: Re=100, steady, isothermal, laminar:


Case 4: Re=100, unsteady, isothermal, laminar:

● *YOU MUST RUN STEADY CASE BEFORE RUNNING UNSTEADY CASE* ● In order to sufficiently capture the vortex shedding you should have about 25 time steps

in one shedding cycle, and we should look at 5 cycles. Create an unsteady flow journal file with the following lines:

define/models/viscous laminar y define/models/unsteady-2nd-order y display/contour dynamic-press , , solve/animate/define define-mon , 1 n y 5 contour , ,

● Run the calculation with the below settings.

1. Run calculation 2. Time Step Size: 0.0025 3. Number of Time Steps:

125 4. Max Iterations:20 5. Calculate

❏ Wind Tunnel Lab case-Case 5:

Case 5: Re= Wind Tunnel Lab, steady,isothermal, laminar:

● Calculate the Re based on the wind tunnel lab data. ● Input the density (ρ), viscosity (μ) and velocity (U) in the excel script maker to get the

same Re as the Wind Tunnel Lab.

❏ Flow with Heat Transfer (Laminar vs. Turbulent)-Cases 6 & 7:

When we introduce heat transfer, we now must consider another dimensionless number, the Prandtl number (Pr), which we will cover in the topic of Forced Convection (Week 4-Week 9). The Prandtl number is a ratio of the momentum diffusivity (kinematic viscosity) to thermal diffusivity and it in influences the thermal boundary layer. This means that the Prandtl number must remain constant between the two heat transfer cases we are investigating. The Pr is defined as

where Cp is specific heat, (SI units : J/(kg.K) ), μ is dynamic viscosity, and k is thermal conductivity. Since is also used in the Re number we need to ensure that this value is constant


for both cases to ensure that the thermal boundary layer is not in influenced by the material properties.

μ D ρ U Re Re=20 0.00001 0.01 1 0.02 20 Re=1e6 0.00001 0.01 100 10 1,000,000

***For heat transfer cases, the cylinder wall temperature is set to 100 oC. Plot temperature contour plot***

Case 6: Re=20, steady,heat transfer, laminar:


Case 7: Re=1,000,000, steady, heat transfer, turbulent:

● Input the density (ρ), viscosity (μ) and velocity (U) in the excel script maker to get Re1e6.

2.5 Task to do: For Cases 1, 2 & 3, you need to:

a) Change the velocity, density and viscosity in the excel script maker to get the Re

b) Open “CL02.cas” file in FLUENT in 2D mode

c) Load the journal file and then check the displayed Re from Fluent

d) Run the calculation and then record the Cd values

e) Plot velocity vectors in the vicinity of the cylinder wall

f) Plot streamlines/pathlines across the cylinder

g) Create velocity and pressure contours

h) Plot XY-plots of skin friction coefficient over the cylinder

i) Plot XY-plots of pressure coefficient over the cylinder

For unsteady case 4 at Re=100 ONLY:

j) Plot velocity or pressure contours at four different time steps to demonstrate the vortex

shedding phenomena

k) Post-processing unsteady results Follow the steps shown below

For case 5, Re=Wind Tunnel Lab:

l) Run the calculation using the Re from wind tunnel lab and record Cd values.

For heat transfer Cases 6 & 7:

m) Create temperature contours around the cylinder

* Guides for loading and post processings of the cases are available at Appendix A & B. Please follow the instructions in the Appendix to generate the required results. *

** REMEMBER TO SAVE ALL THE RESULTS! You will present those results in your lab report. **


CL-03 Internal Pipe Flow 3.1 Internal Pipe Flow - Developing Flow Internal forced convection heat transfer occurs in various applications that involve pipe and duct flows, such as blood flow, oil and gas pipelines, and air conditioning and heating units. Convection heat transfer is complicated by the presence of fluid motion, and a boundary layer.

In this exercise, a hot fluid flows through a straight circular pipe (with cold walls) of constant radius of r=0.2m and a length of L=0.8m. The inlet velocity starts with a constant profile and as it progresses downstream changes its profile shape. You are to observe this shape and understand its cause. Similarly a constant temperature profile is introduced at the inlet and as the fluid progresses downstream, a thermal boundary develops. The computational model considers half the circular pipe in 2D shown in Figure 5.1a. The effects of Prandtl number, and Reynolds number on the developing flow profile are investigated.

(a) Fluent computational domain (b) developing velocity boundary layer Figure 5.1: Developing internal pipe flow

Preliminary calculations We investigate three different materials, air, water, and oil, and observe how its fluid properties affects its flow behaviour. We make use of Fluent's material library database to select the predefined material properties. The values extracted from Fluent are given in the below:

µ [kg/ms] Cp[J/kgK] ρ [kg/m3] k[W/mK] Air 1.79×10−5 1006 1.225 0.0242

Water 0.001 4182 998 0.6 Oil 0.486 1910 884 0.144

The Prandtl, and Reynolds numbers are defined as

● Calculate the Pr and Re based on the geometry with diameter D = 2rmax = 0.4m. ● Calculate the required inlet velocity to achieve a Reynolds number of Re=100 for the three

fluids

Pr U, inlet velocity [m/s] Air

Water Oil


Model setup

As an example let's calculate air. The Pr becomes:

and the inlet velocity needs to be:

Record the calculated velocities into the above table. Set-up cases for air, water, and oil flowing in the pipe at Re = 100 using the script maker in Excel. Open “CL03-1.cas” file in FLUENT in 2D mode and load the journal files.

Questions Q1 Draw the velocity and temperature profiles for air at these locations: inlet (x=0m, ‘inlet’), midpoint (x =0.4m), and outlet (x =0.8m) locations on the graphs below. Compare with Fluent results. Also for the water and the oil.

An example of velocity profiles at location X1 (in the plots x-axis is Velocity and y-axis is Radius):

Q2 Describe the relative thicknesses of the velocity boundary layers for the three fluids at the outlet.

Q3 Describe the relative thicknesses of the thermal boundary layers for the three fluids.

Q4 Explain why there are differences or similarities between the relative boundary layer thicknesses between the three fluids.

Q5 What would the thermal boundary layer thickness look like for oil, if the Reynolds number is now Re = 1. Explain why.


3.2 Internal Pipe Flow - Fully Developed Flow In this exercise, a hot fluid flows through a straight circular pipe (with cold walls) of constant radius of r = 0.1m and a length of L=10m. The inlet velocity begins at the inlet with a constant profile and becomes fully developed at some section downstream. Two real fluids, air and water, pass through the pipes. The inlet temperature is constant at 25oC while the pipe surface has a constant surface wall temperature of 5oC. The fluid travels at a Reynolds number of 100. The computational model is similar to that from last week’s class.

Preliminary Calculations Since we are using a real fluid, we make use of Fluent’s material library database and select the predefined material. The values extracted from Fluent are given in the table below:

µ [kg/ms] Cp [J/kgK] ρ [kg/m3] k [W/mK] Air 1.79×10−5 1006 1.225 0.0242 Water 0.001 4182 998 0.6

Based on the pipe diameter of D = 2rmax = 0.2m, calculate the Pr number and the required inlet velocity to achieve a Reynolds number of Re=100 for the two fluids (Pr and Re equations given in 3.1). Note that this week’s pipe diameter (D=0.2m) is half of that from last week (D=0.4m).

Pr U, inlet velocity [m/s] Air 0.744 0.0073 Water 6.97 0.0005

Model setup The boundary conditions will be similar to last week. But this time our pipe geometry is much longer and we want to investigate the fully developed length and fully developed region.

Use the velocities given in the above table, set-up cases for air and water flowing in the pipe at Re = 100 using the script maker in Excel. Open “CL03-2.cas” file in FLUENT in 2D mode. You need to export velocity and temperature profiles at different pipe locations downstream. Then normalise the variable against its maximum value.

Questions Q1 What do you notice about the fully developed temperature profiles? Q2 What would happen if the fluid temperature < surface temperature? Q3 What do you notice about the velocity profiles for water and air in the fully developed region?

* Guides for loading and post processings are available at Appendix C. Please follow the instructions in the Appendix to generate the required results. REMEMBER TO SAVE ALL THE RESULTS! *


CL-04 Natural Convection and Radiation 4.1 Natural Convection (only) ***** Before you start the simulations, Sketch your convection currents first. Then we will run the simulations and compare with your sketches.*****

Many heat transfer applications involve natural convection as the primary mechanism of heat transfer. As its name suggests it is a nature at work, occurring naturally without any forced influences. In this lab we use CFD to visualise different natural convection currents. The Grashof number is a dimensionless number. As before, this means it is ratio of two flow properties. It represents the ratio of the buoyancy force to an opposing viscous force acting on the fluid.

Natural convection heat transfer correlations are usually expressed with the Pr involved. When we combine the Gr with the Pr we get the Rayleigh number.

Model setup

We want to investigate the influence of the Rayleigh number on the convection currents. We use air as the base material but alter its viscosity to get the desired Ra number. use an artificial viscosity. Real air has a viscosity of about 1.79e-5. (Note that we could change the temperatures which also affects the thermal expansion coefficient).

Set-up cases using the script maker in Excel. Open “CL04-1-flatPlate.cas” file for cases 1 & 2 and “CL04-1-rectangle.cas” file for cases 3 & 4 in FLUENT in 2D mode.

Cases to investigate CASE 1: The physical model considered here is a two dimensional square cavity of length L with a heated thin plate of length h placed horizontally (see figure) or vertically at its center. The plate is isothermally maintained at a higher temperature Ts = 320K. The vertical walls are cooler at a constant temperature T1 = 300K while the horizontal walls are insulated.

a. Run the case for a horizontal plate at Ra = 105. Plot its temperature contour and compare it with your sketched natural convection currents.

The Rayleigh number can be rewritten with the kinematic viscosity and the Pr number expanded out:

Taking the characteristic length as L = 0.2m, density as = 1.225, Cp = 1006, and conductivity as k = 0.0242 the required viscosity can be determined by setting the Ra number to 1e7:

Set the viscosity of air with an artificial value of 3.6e-3 and set the gravitational acceleration direction to be vertical (y-axis).


b. Run the case again but for Ra = 107. Change the fluid material properties by increasing its viscosity to 3.6e-5 script.

Plot its vector plot on the cavity fluid surface (select cavity fluid surface), and also the temperature contour (don't select any surface). Compare what you see with your sketched natural convection currents.

CASE 2: This time we want to simulate a vertical plate inside the square cavity. Instead of rebuilding the entire geometry again, we can simply change the boundary conditions and take advantage of the symmetrical features of the geometry. Make the top-bottom wall a temperature boundary while the left-right wall becomes adiabatic with a heat-flux condition of 0 W/m2. Also change the gravitational acceleration direction to be horizontal (x-axis). Run for the two Ra-number cases, a. Ra = 105 and b. Ra = 107.

Plot its vector plot on the cavity fluid surface (select cavity fluid surface), and also the temperature contour (don't select any surface). Compare what you see with your sketched natural convection currents.

CASE 3: A rectangular enclosure is investigated. The side walls are adiabatic. A hot bottom wall has Tbottom=320K and a colder upper wall has Ttop=300K. Use the corresponding excel journal maker to generate the journal file and simulate. Set the gravitational acceleration direction to be vertical (y-axis).

Plot vector and temperature contour plots and compare with what currents you sketched.

CASE 4: As in Case-3, rotate the geometry by changing the boundary conditions and the gravitational acceleration direction to be horizontal (x-axis) so that the case now becomes a vertical enclosure.

Tasks to do:

❏ Practice your hand sketches first on the next page of the handout

❏ Run the simulations, then plot velocity vector and temperature contour

❏ Compare your hand sketches with the graphical results

❏ Discuss the questions provide at the next page of the handout


Sketches Sketch your convection currents first! Then run the simulations and see how many you got right.

Questions Q1. The thinner thermal boundary layer occurs on (1)Ra=105 or (2)Ra=107 case? Why? What does it mean in terms of the heat transfer? Q2. Describe the characteristics of natural convective currents in the horizontal enclosure (case 3)and the vertical enclosure(case 4). Q3. Describe the differences between the external natural convection and the internal convection (natural convection inside enclosures)?


4.2 Natural Convection and Radiation ➝ Before running the simulation, sketch the flow patterns in the results section. ➝ Run the simulation, check your flow patterns the sketch the y-velocity profile.

We have learnt that i) thermal radiation is emitted in the form of electromagnetic waves; ii) it does not require a material medium for the wave to propagate from one surface to another; and iii) it is directional in nature. In the lecture and tutorials we focus on surface to surface radiation only, however we saw that the incident radiation (called irradiation, G) can be absorbed, reflected, and transmitted. These three properties also apply when radiation passes through a transparent medium such as fluids - most evident the scattering of sunlight through the atmosphere producing blue skies during the day and red sunsets.

Problem description A square box has a hot right wall at Thot = 2000K and a cooler left wall at Tcold = 500K, while the upper and lower walls are insulated or adiabatic (i.e. no heat transfer). The default working fluid is air. Gravity is vertically down- wards and creates a buoyant flow due to thermally induced density gradients. The fluid has Pr ≈0.72, and the Rayleigh number based on L is 6 х107. Note that the values of the working fluid and gravity have been adjusted to achieve the desired Prandtl, and Rayleigh for natural convection to occur with radiation heat transfer. For Case-3 the medium contained in the box is assumed to be absorbing and emitting, so that the radiant exchange between the walls is attenuated by absorption and augmented by emission in the medium. All walls are black. Cases to investigate

In this lab we investigate the influence of radiation heat transfer in a 2D square box.

Case-1 Simulate natural convection heat transfer only.

Case-2 Include radiation from surface- to-surface only.

Case-3 Include radiation with 5% absorption by the fluid. Set-up cases using the Excel script maker. Open “CL04-2.cas” file.

The definition of the walls are the same as before where we set them as either a zero heat-flux or a fixed temperature condition. Fluent provides five radiation models that allow us include radiation, with or without a participating medium. We simply make use of the Rosseland model which behaves sufficiently well for our case.

For each case you can: ❑practice the hand sketch of the flow patterns, use velocity vectors to verify your sketch; ❑practice the hand sketch of the velocity profile along the horizontal centerline, and use velocity plots to verify your sketch; ❑use the velocity contours to find the maximum velocity; ❑use temperature contours to see the temperature distribution; ❑use the reports function to find the heat transfer rate at the hot wall (right wall) (screenshot available on lab handout);


1. Report 2. Fluxes 3. Set up 4. right 5. Compute 6. Results

Sketch i) Sketch the flow patterns for the three cases. Record the maximum velocities and q_right for each case.

max velocity: __________ : ______________

max velocity: __________ : ______________

max velocity: __________ : ______________

ii) Sketch the velocity profile along the horizontal centreline for Case 1 on the graph below:

Questions Q1 Which case exhibits the highest heat flux? Which case exhibits the highest flow velocity? Give an explanation for this.

Q2 If the temperature difference remains the same, but now the temperatures are Tcold = 70oC and Thot = 1570oC, what happens to the radiation heat transfer? What happens to the natural convection heat transfer? Which exhibits the greater change?


Appendix A Loading files and running simulations ❏ Open a new .cas file.

1. Double click case file

2. 2D 3. Serial 4. Display mesh 5. Ok

❏ Copy and paste the generated script from excel maker to Notepad and save as .jou file

In Notepad:

1. save as: . jou 2. Save as type:

All Files (*.*).

❏ Read the journal file in Fluent

1. Read 2. Journal 3. Select .jou file 4. Ok

❏ Run the calculation and record the drag coefficient (CL-02)

1. Calculation 2. Number of iterations: 2000 3. Calculate

❏ Check Re and record the drag coefficient (CL-02)

1. Record Cd-1


Appendix B Post-processing guide (CL-02)

❏ Plot vectors 1. Graphics 2. Vectors 3. Setup 4. Scale: the size of

the vector arrows (recommended range: 0.002 to 0.005) Skip: to skip every n number of vectors; (recommended range: 1 to 5)

5. Display

❏ Plot flow streamlines across the cylinder. In Fluent these are referred as Pathlines.

1. Graphics 2. Pathlines 3. Setup 4. Velocity 5. Release from

Surfaces: inlet 6. Display

❏ Plot velocity and pressure contours.

1. Graphics 2. Contours 3. Setup 4. select variable

(e.g. pressure, velocity and temperature)

5. Display


❏ Plot an XY-Plot of the skin friction coefficient over the cylinder.

1. Plots 2. XY plot 3. Setup 4. Unclick Position

on X Axis 5. Y Axis Function:

Wall fluxes→ Skin Friction Coefficient

6. Theta-radians 7. cylinder

Post processing-Skin friction coefficient plot

❏ Plot an XY-Plot of the pressure coefficient over the cylinder.

1. Plots 2. XY plot 3. Setup 4. Unclick Position

on X Axis 5. Y Axis Function:

Pressure→ Pr. Coefficient

6. Theta-radians 7. cylinder

❏ For unsteady case:

* If there is no file shown in Sequences, but you have made the files earlier, then you can read in the file by clicking Read, and the .cxa file within the same folder as the Fluent case file. Use play and stop buttons to view the results.

1. Animations 2. Solution

Animation Playback

3. Setup 4. Select the

sequence* 5. Change write

format to: Picture Files

6. Picture options 7. Picture format:

PNG or JPEG. 8. Apply 9. Close 10. Write


Appendix C Loading and Post-processing guides (CL-03-3.2) 1. Run the simulation with the fluid of “air”

- Set-up cases for air and water flowing in the pipe using the script maker in Excel - Open “CL03-2.cas” file in FLUENT in 2D mode. - Load the journal files and run the calculations. - Generate velocity XY-plots

1. Plot 2. XY-plots 3. Set up 4. Select Velocity 5. Select a location 6. Click Node values

and Position on X axis

- For symmetry view:

1. Viewing 2. Views 3. symmetry 4. Apply

- Extract velocity profile data from Fluent

*Copy all the data (6&7) from Notepad into excel and plot the velocity profiles.

1. Click Write to File 2. Select Velocity 3. Multi-select from

x=0.01m to x=5m. 4. Write 5. File of type➝All

files, rename the file with extension of . txt

6. Location data (in radius)

7. Velocity data*

- Repeat the same approach for temperature profiles

2. Run simulation with same approach for “water”


MIET-1081 Advanced Thermo-Fluid Mechanicsbe using FLUENT’s text-user-interface (TUI), to set up...

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Transcript of MIET-1081 Advanced Thermo-Fluid Mechanicsbe using FLUENT’s text-user-interface (TUI), to set up...