Introduction:

            In this example you will learn to model slightly more complex situations, specifically involving composite media, using simplification techniques including symmetry. The illustration below is a 2 dimensional representation of a 3 dimensional furnace (example shown below also), with obvious boundary conditions including constant temperatures. 

            Using ANSYS will allow you to output the temperature distribution in an extremely simple and accurate way.  One important question to be answered is whether the layers of the composite insulation material are thick and appropriately conductive such that the outside of the furnace is not too warm.

 

Problem Description:

·         We are modeling heat transfer from a 2-D furnace. Using symmetry, we can limit the scope of modeling to only one section of the furnace, as shown in the illustration. This implementation of symmetry greatly simplifies the effort required for thermal analysis.

·         All units are S.I.

·         Boundary Conditions:

            1) The symmetry lines of the corner piece can be assumed to be insulated.

            2) The outer most boundary has a constant temperature of 300 K.

            3) The inner most boundary has a constant temperature of 1000 K.

·         Material Properties:

            Fire Brick(inner layer): K_FB = 0.3 W/m*K

            Red Brick(middle layer): K_RB = 0.63 W/m*K

            Magnesia(outer layer): K_M = 1.41 W/m*K

·         Dimensions

            Length = 1.5 m

            Width = 1 m

            Thickness of each Layer = .05 m

·         Objective: Find the nodal temperature distribution and the rate of heat loss from the furnace.

·         Figure:

Basic Outline of the Problem:

Preprocessing:

1. Start ANSYS.

2. Create areas using keypoints and lines.

3. Define the material properties.

4. Define element types. (Quad 8node 77 element, which is a 2-D element for heat transfer analysis.)

5. Specify meshing controls / Mesh the areas to create nodes and elements.

Solution:

6. Specify boundary conditions.

7. Solve.

Postprocessing:

8. Plot the temperature distribution.

Exit:

9. Exit the ANSYS program, saving all data.

Starting ANSYS:

·         Click on ANSYS 6.1 in the programs menu.

·         Select Interactive.

·         The following menu comes up. Enter the working directory. All your files will be stored in this directory. Also under Use Default Memory Model make sure the values 64 for Total Workspace, and 32 for Database are entered.  To change these values unclick Use Default Memory Model.

·         Click RUN

Modeling the Structure:

·         Go to the ANSYS Utility Menu (the top bar). Click Workplane>WP Settings…

·         The following widow comes up: (notice the numbers are different)

·         Check the Cartesian and Grid Only buttons

·         Enter the values shown in the figure above. Click OK

·         Go to the ANSYS Utility Menu (the top bar). Click Workplane>Display Working Plane. This will display the working grid on the workspace.

·         Use Utility Menu>PlotCtrls>Pan Zoom Rotate to display the grid as shown in the next step below.

·         Next, go to the ANSYS Main Menu (on the left hand side of the screen) and click Preprocessor>Modeling>Create>Keypoints>On Working Plane.

·         The following window comes up:

·         Click on the working plane below to select the points (they follow the dimensions explained in the beginning, (.5m x .75m). After setting the workplane settings in the beginning, you should be aware that every five lines on the plane equals to .25m. When done, click OK.

·         Now you have created the points to make the block. 

·         Now click Preprocessor>Modeling>Create>Lines>Lines>Straight Line.

·         Use the mouse to connect each of the points you have created and form the corner of the furnace layer by layer. Connect the lines as shown in the figure, then Press OK in the window that appeared on the side to finish. (note that each layer is created separately and the ends of the L section are formed by 3 separate lines each.)

·         If you encounter any problems with connecting the points and need to delete a line click Preprocessor>Modeling>Delete>Lines Only.

·        

·         Now select Preprocessor>Modeling>Create>Areas>Arbitrary>By Lines.  A window will now appear on the left of the screen.

·         Select all the lines that form the innermost layer.  Click Apply such that it is formed separate from the other two areas.  

·         Repeat the step of selecting the lines that make up each area, and clicking Apply until all three layers are defined.

·         The model should look like this now: (note, you have a black background)

Material Properties:

·         Now that we have built the model, material properties need to be defined such that ANSYS understands how heat travels through this composite solid.

·         Go to the ANSYS Main Menu

·         Click Preprocessor>Material Props>Material Models.

·         The pop-up window will now look like this:

·         In the window that comes up choose Thermal>Conductivity>Isotropic.

·         The following window comes up:

·         Fill in 0.3 for Thermal conductivity. Click OK.  This is the Thermal Conductivity of Fire Brick.

·         Now refer to the Define Material Model Behavior main menu and click Material>New Model.  The following window will appear:

·         Click OK if the window reads 2, otherwise, enter this value.  (This means that you are defining a second material)

·         Now repeat the steps of clicking Thermal>Conductivity>Isotropic and then defining the Thermal Conductivity as 0.63 in the ensuing pop-up window.

·         You have now defined the k value of Red Brick.

·         Finally, repeat the steps for creating a new material, and defining its Thermal Conductivity.  This time use K = 1.41. This is the Thermal Conductivity of Magnesia.

·         Now exit the “Define Material Model Behavior” Window.

Element Properties:

·         Now that we’ve defined what material ANSYS will be analyzing, we have to define how ANSYS should analyze our block. 

·         Click Preprocessor>Element Type>Add/Edit/Delete... In the 'Element Types' window that opens click on Add... The following window opens:

·         Type 1 in the Element Type reference number.

·         Click on Thermal Mass>Solid and select Quad 8node 77. Click OK. Close the 'Element Types' window.

·         Now we have selected Element Type 1 to be a Thermal Solid 8node Element.

·         This finishes the section defining how the part is to be analyzed.

Meshing:

·         This section is responsible for telling ANSYS how to divide the block such that it has enough nodes, or points, to produce accurate results.

·         Go to Preprocessor>Meshing>Size Controls>Manual Size>Lines>All Lines. In the menu that comes up type 0.01 in the field for Element edge length and 1 for the Spacing Ratio.

·         Click on OK. Now when you mesh the figure ANSYS will automatically create square meshes that have an edge length of 0.01m along the lines you selected.

·         Now go to Preprocessor>Meshing>Mesh Attributes>Default Attributes. The window is shown below:

·         Make sure that the window matches the one above, click OK, and proceed to Preprocessor>Meshing>Mesh>Areas>Free

·         A popup window will appear on the left hand side of the screen.  This window allows you to select the area to be meshed.

·         Choose the inner area and then click OK in the pop-up window.  This both meshes the area and defines it as Material 1.  Material 1 (as you recall from before) was set to Fire Brick originally by defining the k value of the material as 0.3 W/m*K.

·         Now return to Preprocessor>Meshing>Mesh Attributes>Default Attributes. This time, select Material Number 2 from the dropdown menu and click OK.

·         Once the pop-up window appears, select the middle layer and click OK. 

·         Repeat this process of defining each layer as a different material for Material 3 and mesh it so that all three layers have been meshed.

·         The block should now look like this when you are done meshing:

Boundary Conditions and Constraints:

·         Now that we have modeled the block and defined how ANSYS is to analyze the block we will apply the appropriate Boundary Conditions.  ANSYS refers to all Boundary Conditions under the Loads category, so remember that when looking for commands within the main menu…

·         Go to Preprocessor>Loads>Define Loads>Apply>Thermal (from here one can apply any of the loads, or Boundary Conditions, offered by ANSYS.)

Apply Constant Temperature

·         Now we’ll apply the given temperature boundary condition on the right side of the block.

·         This time, within the Thermal Load category select Temperature>On Lines.

·         A popup window will appear on the left hand side of the screen.  This window allows you to select the line you wish the load to be applied to.

·         Click the innermost boundary of the block and then OK.

·         Enter 1000 in the popup window as the set temperature for the innermost edge of the wall section:

·         Click OK and repeat the process to apply a uniform temperature of 300K to the outermost edge of the furnace.  This temperature is the ambient temperature of the room.

·         Once that is complete, the block should look like this:

Solution:

·         Go to ANSYS Main Menu>Solution>Analysis Type>New Analysis.

·         Select Steady State and click on OK.

·         Go to Solution>Solve>Current LS.

·         An error window may appear. Click OK on that window and ignore it.

·         Wait for ANSYS to solve the problem.

·         Click on OK and close the 'Information' window.

Post-Processing:

·         This section is designed so that one can present the results of their analysis in the most appropriate way. This presentation can be in the form of tabulated nodal values, curves, etc.

·         Go to the ANSYS Main Menu.  Click General Postprocessing>List Results>Nodal Solution. The following window will come up:

·          

·         Select DOF solution and Temperature. Click on OK. The nodal temperatures will be listed as follows:

·         Within this window one can numerically find the maximum and minimum value of the temperature within the block.

Modification / Plotting the Results:

·         The last section displayed the numerical results, but some people prefer a plot presentation of the temperatures on the block over the numerical results.  This is how you go about doing that…

·         First go to General Postprocessing>Plot Results>Contour Plot>Nodal Solution. The following window will come up:

·         Select DOF solution and Temperature to be plotted and click OK.  The output will be like this:

·         This is the Final Solution

·         To find extra information on Saving an ANSYS model see the Appendix on the ANSYS tutorial mainpage.

Saving Projects

Please refer to Tutorial 1 for questions pertaining to alternative boundary conditions!

If at any time you cannot see the complete Workplace then go to Utility Menu>Plot Controls>

Pan Zoom Rotate and zoom out to see the entire Workplace. It is also helpful to note that you can replot certain items such as lines or areas or keypoints at any time through Utility Menu>

Plot>

(Lines or Areas or Volumes or etc)

Please note in this Create Areas, Arbitrary section, there is also an option to create areas only using

Key Points (Through KP’s). This is another option. In general, chose what ever is most comfortable and convenient, depending on the design.

Please note that the order in which you create material models has nothing to do with the order in which you created the areas. You do not define the material properties of the areas until the meshing stage.  Simply define material properties for material numbers, then set which material number corresponds to which area.