ME 448/548 Convection Cookbook for I
Contents
1. cookbook are h 3cm W 15cm H 9cm L 30cm The thermal and flow parameters are Uin 1m s Tin 20 C Q 10W The hydraulic diameter of the duct is 4WH D _ h 9 W H 10 29 cm The kinematic viscosity of air at 20 Cis v 1 51 x 1075 m s so that the Reynolds number of the duct based on hydraulic diameter is UinDpn Lm s 0 1029 m 1 Rep y oea Thus the flow in the duct is turbulent To set the pseudo time step in the ESC solver we need to estimate the time it takes for a particle transit to move past the geometric features of interest The ESC User s Manual recommends a time step of At L 2U where Le is a characteristic length scale for the flow and U is the characteristic velocity scale For this problem Ue is the inlet velocity Uin Two choices of length scale are h the size of the cube or L the length of the duct Thus two candidates for the time step are h L 1 At a 3U 0 015s At IU 0 15s If the model converges with a pseudo time step of At z then there is no need to use a smaller time step If the model does not converge with At At z then a smaller time step should be tried A reasonable lower limit for the time step is 0 01s 1 The pseudo time step is only used to enhance stability The flow is steady Creating the Model 1 Launch I DEAS and open a new model file 2 Switch to the Master Modeler task 3 Select mm newton units 4 Sket2. ME 448 548 Convection Cookbook for I DEAS ESC PSU ME Dept Gerald Recktenwald Winter 2001 gerry me pdx edu Overview This document gives step by step instructions for simulating turbulent flow and heat transfer for a single heated block in a duct To set up and solve this model with LDEAS ESC the following new features are introduced 1 Creation of new solid materials to specify properties for heat conduction 2 Creation of a 3D mesh to model heat conduction within a solid that it is in contact with air Use of a flow blockage Use of flow surfaces to define heat loads oo A w amp Model partitioning for mesh control Physical Problem Figure 1 gives the plan and elevation views of the geometry A cubic block of dimension h is situated near the inlet of a rectangular duct of height H and width W The fluid air entering the duct has a uniform velocity Uin and temperature Tin The dashed box shown in the plan view indicates a Plan View ra eae ee mesh partition z Figure 1 Geometry of the cubic block in the rectangular duct subvolume of fluid around the block that it used to control the mesh density near the block This subvolume is created as an IDEAS partition but does not exist as a physical boundary to the flow A heat load Q is applied to the base of the block This heat is conducted upward through the block and is transferred to the air by convection The geometrical parameters used in this
3. ch a 300mm x 120mm rectangle on the work plane See Figure 1 Fix the dimensions of this rectangle and extrude it a distance H 90mm to create the fluid volume of the duct 5 Select Sketch on Face p1 r1 c1 e Select the bottom surface of the duct e Sketch the plan view of the cube Dimension the square cube to be 30 mm on a side Locate the cube 60 mm from the side walls center it in the duct and 30 mm from the inlet e Select Extrude p1 r5 c1 gt Click the right mouse button and select partition from the pop up menu gt On the Ertrude Partition Create form select distance and enter 30 mm gt Check that partition is selected from the right most pop up menu gt Click OK 6 Select Sketch on Face p1 r1 c1 e Select the bottom surface of the duct e Sketch the plan view of the dashed rectangle in Figure 1 Dimension the rectangle to be 60 mm wide across the duct and at least 100 mm long in the flow direction e Select Extrude p1 r5 c1 gt Click the right mouse button and select partition from the pop up menu gt On the Eztrude Partition Create form select distance and enter 60 mm gt Check that partition is selected from the right most pop up menu gt Click OK At the completion of these steps you have a single part that is partitioned into three volumes as depicted in Figure 2 The innermost volume is the cube The cube is surrounded by a brick shaped volume of air that will have a fine mesh The r
4. emaining volume is the bulk of the air in the duct e Name the part Figure 2 Partitions in the completed model Meshing the Model Overview To mesh this part we will use a combination of mapped and free meshing Mapped meshes are used e on the bottom surface of the cube e in the cube volume e on the inner part of the inlet surface e on the outlet surface The mesh on the bottom surface of the cube is needed to apply the heat load boundary condition The inner part of the inlet surface is the rectangular area formed by the upstream face of the fine mesh partition The mapped meshes enable us to easily control the mesh density in the solid and on faces of the air volume Free meshing is used e on the outer part of the inlet surface e in both subvolumes occupied by the air The combination of the free mesh and mapped mesh on the inlet surface is shown in Figure 3 Other meshing strategies could be used For example free meshes could be used on all surfaces and volumes i E R ty Figure 3 Combination of mapped mesh and free mesh on the inlet surface The major steps in meshing the model are Prepare for meshing name the FE model Define a Null Shell physical property Define a TMG material for aluminum for the block Define a non conducting TMG
5. erial properties o Enter 17700 mN mm mm C sec o Click Modify Value gt Select Specific heat below phase change from the list of material properties o Enter 8 75e 8 mN mm kg C o Click Modify Value gt Click OK e Click Quick Create third button on the right side of the form gt Enter material name Null Conductive gt Select TMG Solid from the drop down menu gt Select Thermal Conductivity from the list of material properties o Enter 0 mN mm mm C sec o Click Modify Value 4 Apply thin shell mesh to the bottom of the cube Use the Null Shell physical property and the Null Conductive material property 5 Apply mapped solid mesh to the cube Be sure to select Aluminum 2024 T6 as the material 6 Mesh the inlet and outlet surfaces with the Null Shell physical property and the Null Con ductive material property 7 Mesh volume with ESC AIR as the material Applying Boundary Conditions Boundary conditions for this problem are e Heat load of 10 W on the bottom of the cube e Fan inlet with a uniform velocity of 1 m s at 20 C e Vent to the ambient at the outlet surfaces e Adiabatic wall boundary conditions e ESC flow surfaces on the cube walls The flow surfaces on the cube walls is implemented with the flow blockage feature of ESC Steps for Applying Boundary Conditions 1 Create the solid as a flow blockage e Click the flow blockage button p1 r2 c3 e Select the cube volume clicking on the b
6. material for surface meshes Apply surface meshes to domain boundaries Apply mapped volume mesh to the aluminum cube Define volume meshes for fluid in two steps First mesh the inner fluid volume adjacent to the cube Then mesh the outer fluid volume that fills the remainder of the duct Note that a surface mesh also needs to be defined on the surfaces of the cube in contact with the air This surface mesh is used by the flow surface to define the thermal connection between the solid aluminum and the fluid air We will use the flow blockage feature of the ESC task to define the aluminum cube as a solid that is impermeable to the fluid The flow blockage feature automatically coats the surface with a surface mesh Steps for Meshing the Model 1 2 Switch to Meshing task second drop down menu in upper right hand corner Click Create FE model p1 r6 c2 e Name the FE model e Set the default part material to be ESC air e Click OK e Click OK to dismiss the warning about air not being a suitable material for stress analysis 3 Click Material Property p1 r5 c1 material selection form opens e Click Quick Create third button on the right side of the form gt Enter material name Aluminum 2024 T6 gt Select TMG Solid from the drop down menu gt Select Mass Density from the list of material properties o Enter 2 77e 6 kg mm o Click Modify Value gt Select Thermal Conductivity from the list of mat
7. ons e KE model e Thermal solve e Set physical time step to 0 2 s e Set working directory gt Click Study Setup p1 r4 cl gt Select the run directory gt Click OK 2 Solve the model 3 View the results
8. ottom surface allows selection of just the cube volume gt Name the blockage Aluminum block gt Verify that the blockage type is solid gt Check convection gt Click create surface properties o Name the surface property smooth surface o select use surface roughness of and enter 0 mm o Click convection heat transfer coefficient This opens the Additional Auto Calcu late Options form uncheck Electronics Enclosure Click OK o Click OK gt Click the button to the right of Surface Properties o Select smooth surface o Click OK gt Click OK 2 Apply the heat load to the cube click Thermal BC Create p1 r1 c2 Select the bottom surface of the cube e Name the boundary condition Block heat load Verify that heat load is selected select W from the pull down menu e enter a value of 2 Verify that the heat load is 2 W e click OK 3 Apply the fan boundary condition to the inlet click Fan p1 r3 c1 e Select both parts of the inlet surface e Inthe Fan create dialog box gt Change the name of the fan inlet fan would be a good choice gt Select the inlet radio button gt Set the flow parameter for the fan o Select velocity from drop down menu o Enter 1 m s gt Explore the options but do not change them gt Click OK 4 Apply a vent boundary condition to the outlet Prepare to Solve and Solve the Model 1 Use standard procedures to set the solver opti
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