Tutorial 3 Natural Convection ansys CFX

8/10/2019 Tutorial 3 Natural Convection ansys CFX

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CFX Tutorial


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Geometry Consider a flat plate of zero thickness

standing vertically. The surrounding airis at 40F while the plate itself is at 53F.

Due to the temperature differencebetween the plate and the surroundingair, and also because gravity is present,plate will loose heat by naturalconvection to the surroundings.

Our goal in this simulation is to findthe amount of heat that is going to belost, and also to find the heat transfercoefficient as a function of distancefrom the plates leading edge.


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Mesh As the geometry is very similar to

flow over a flat plate of zerothickness, and as people have

different preferences regardingtheir meshing program, we willnot discuss the meshing methodhere.

We will need seven regions

shown in the picture to apply theappropriate boundary conditions.

After creating geometry andmesh, start a new simulation inCFX-Pre and import the mesh.


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Setting Material Properties

All these properties arenecessary for naturalconvection problems.Note the extraThermal Expansivitywhich is ThermalExpansion Coefficient.


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Creating the Solution Domain

Reference temperature is the temperature at whichthe fluid has the density provided in the materialproperties section.

Change this option to Buoyant to see the otheroptions. We will start the solution with aacceleration of gravity of ~1/10 of the actual gravityand will increase it as we get converged solutions

along the way.


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Creating the Soln. Domain (contd.)

Select Thermal Energy for heat transfer

We expect the flow to be laminar. If platewas more than 1.5 ft high, instead of 1 ft,then the flow would have been turbulent.

You can either specify the initial conditions

using the last tab in the Domain option, oruse the following icon on the toolbar tospecify them later, since there is only onedomain in this problem.


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Applying Boundary Conditions


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Applying BCs (plate)


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Applying BCs (bottom wall)


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Applying BCs (outlet)


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Applying BCs (opening)


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Setting up Solver Parameters


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Setting up Solver Params. (contd.)CFX as default calculates heat transfer coefficient by assuming thereference temperature being the temperature of wall-adjacent nodes. Wewill change this value to the ambient temperature by means of expertparameters.

As the solution units for temperature isKelvin, we need to convert 40F toKelvin.We could change the solution units toRankin if we wanted to, but again we

had to convert 40F to Rankin.


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Solution


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Solution (contd.)Now that we have a converged solution for diminished gravity, wewill increase gravity step by step to get to the actual gravity.

Edit current results file>Flow>Domain>Domain Models>Buoyancy Model>Gravity Y Component

> OK> File > Save> File > Exit

The same process can be done in CFX12.0 ifwe launch CFX in standalone mode.Otherwise, we just need to edit the setup cellin the project and update the solution cell.


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Solution (contd.)After getting a converged solution at each step, we have to repeat thesame process to increase the gravity a little more. We may need to

repeat this procedure as little as two to three times, or for moreunstable cases more than ten times.

Note:If you think the convergence rate is very fast, you may can increasethe rate at which you increase the gravity.

Warning:If you increase the gravity too much the solution may diverge. If thishappens, reload the last converged solution and continue thesolution procedure using smaller increment steps.


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Solution (contd.)


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Post-ProcessingTemperature contoursStreamlines Velocity contours


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Post-Processing (contd.)Notes: The non-zero velocity

below the plate Maximum velocitylocation shifts to right as yincreasesAbove the plate,maximum velocity is at

x=0.Velocity continues toincrease even after theplate ends.


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Post-Processing (contd.)This graph shows the variation of heat transfer coefficient as a function ofdistance from the upstream edge of the plate.

The average heat transfercoefficient is:0.55 Btu/hr.ft^2.F

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Tutorial 3 Natural Convection ansys CFX