Exhaust Port Challenge

Objective: In this challenge, we will be conducting a conjugate heat transfer analysis on an exhaust port with four inlets having an inlet velocity of 5m/s with air at 700K and one outlet with guage pressure 0. A conjugate heat transfer analysis is used when the fluids and solids involved in the simulations undergo a variation in temperature due to the thermal interactions between them at the interfaces. This type of analysis can be used in the simulation of heat exchangers, analysis in cooling of electronic equipment, and general heating and cooling system.

Solving and modeling approach: The provided solid part is imported in space-claim. An enclosed volume is created using the inlet and outlet edges. Then we ensure that the analysis is set to share topology so that the mesh formed are connected between the volume and the solid part. A fine tetra mesh is created both in the fluid volume and solid volume. As we are going to analyze the wall HTC we use an element of smaller size for the solid volume. We also add an inflation layer using the first layer method at the fluid-solid interface. This is done to apply the appropriate wall yplus or wall shear in the simulation. Then we move forward with setup, where we use a pressure-based formulation in a steady-state analysis. In the model, we set energy on and used the k-epsilon viscous method. For the materials, we use aluminum in the solid part and air as the fluid. Here, we use a velocity inlet at 700K and pressure outlet as boundary conditions. After the simulation is run for around 150 iterations we obtain the heat transfer coefficient at the interface.

Pre-processing:

Geometry:

• Open ANSYS> from the analysis system in the toolbox drag Fluid Flow (Fluent) on to the project schematic.
• Right-click on geometry> open Spaceclaim.
• Drag the solid part geometry to the SpaceClaim window. Wait for the geometry to import.
• Now go to prepare > select enclosed volume> then choose the edge method> select the four inlets and the outlet > click the green tick. The volume is created.
• Drag the created volume inside the initial solid model.
• In the main design,> change share topology to share.
• Click on workbench > click share > select the body.
• Ensure that there are no duplicates or interference.
• Close the space claim window.

Mesh:

• In project schematic double click on mesh and wait for the mesh window to open.
• Select the face selection tool > then select the faces > create named selection for inlet, outlet, outer wall convective, and interface inflation layer.
• Click on mesh > change element size to 30mm.
• In inflation, change the named selection to the created interface inflation layer > inflation option: first layer thickness > set the first layer height as 2mm which comes from wall y+ value.
• Right click mesh > insert > sizing > in geometry selection select the body > set the element size to 8mm (this gives the maximum number of elements) > set capture curvature on.
• Click generate. After it has meshed > click update.

Setup:

• We start with mesh check. In domain > go to mesh > select check > Wait for mesh checking results.
• In general, we will be using a pressure-based model with absolute velocity formulation for steady-state analysis.
• Go to models > turn on energy > in the viscous model: set it as K-epsilon.

• In materials the fluid material is air and the solid material is aluminum.
• Now we move on to the boundary conditions. For inlet it will be a velocity inlet, the outlet will be pressure outlet and the outer convective wall will be wall.
• For the inlet, we set the velocity as 5m/s and set the temperature at 700 K.

• The outlet will have a guage pressure of 0Pa with a temperature of 300 K.

• In the outer convective wall, the thermal conditions of the wall will be convection with a heat transfer coefficient of 20W/m2K.

• Now, in the adiabatic surfaces check if the heat transfer coefficient is set to zero.

Solution:

• Now we move to initialization and initialize using a hybrid method.
• We go to results > graphics > contours > new > crate a temperature contour at the inlet, outlet, convective wall, and the adiabatic surfaces.
• In solutions > go to activities > create > select scene animation > then select the temperature contour as the entity.
• Run the calculation for 150 iterations.

Post-processing or Results: After the setup simulation is completed we double click results. The results to be obtained are:

1. Velocity contours at 3 cross-sections.
2. Temperature contours at the same 3 cross-sections.
3. Velocity and temperature contours at the outlet of the exhaust port.
4. Streamline plots of air-flow in the exhaust port.
5. Heat transfer coefficient plot at the inner wall.
6. Temperature contour at the outer wall.

Creation of the planes:

• Right-click > select insert > select plane. Now three planes have to be created, two at the outlet pipe and one near the inlet pipe.

• Follow the same steps to create the plane at the outlet of the exhaust port.

** Plane 3: Next to the intersection of the inlet and outlet pipe, Plane 1: Between the plane 1 and 3, Plane 2: Near the outlet**

Extraction of velocity and temperature contours:

• To extract the velocity plots; click on the plane in the outline tab > go-to color > change the mode to variable > then set the variable as velocity > click on apply.
• Do the same for all the other planes too and the outlet pipe plane.

• To extract the temperature plots; click on the plane in the outline tab > go-to color > change the mode to variable > then set the variable as temperature > click on apply.

Velocity Plot: The velocity increases as we moved in the direction of the flow. When we observe the contour at the exhaust port then we see that at the entrance region the velocity is the least and as the flow from both directions converges in the exhaust port the velocity increases as we move towards the outlet port. Here, an important observation is made .i.e. a red region is observed at the lower region of the bend where the velocity is maximum.

Temperature Plot: The temperature at the mid-plane of the exhaust port increases in the direction of the flow. If we observe the three-sectioned contours we observe that the temperature of the solid part in the cross-section is lesser at plane 3 than at plane one. However, the fluid temperature in plane 3 is higher and the temperature in plane 2 is lower. This is because the volume flowing through plane 1 is half of the volume flow through the plane 3. This is also due to the heat transferred to the solid part from the solid.

Extraction of heat coefficient plot:

• To extract the HTc plots; click on the outlet exhaust port plane in the outline tab > go-to color > change the mode to variable > then set the variable as wall heat transfer coefficient > click on apply.

1st image: Top of the bend.

2nd image: Bottom of the bend.

3rd image: At the entrance region of the outlet pipe in the exhaust port.

Conclusion:

Using the area-weighted average method the surface heat transfer coefficient is computed to be 28.394 W/m^2-K. The evidence that the computed HTC value is correct can be found from the contours of the heat transfer coefficient, and relating these contours with the temperature and velocity contours. If we notice the velocity contour of the exhaust port section the velocity is less in the entrance and the velocity is maximum at the lower bend. Now, notice the temperature contour, at the entrance region the temperature near the wall is low, the same is observed at the region close to the outlet port but the region of low temperature is greater, whereas the temperature near the wall at the bend is the highest. When we see the heat coefficient contours the three regions show different variations. At the top of the bend, the temperature near the wall is low and the velocity is in that region is low too so the HTC value is also low, which is why the first image has a broad green zone. At the bottom bent, the temperature near the wall is high and the velocity is high too, so the heat transfer coefficient is highest, thus it has a thick red contour. As there is heat exchange involved there should be a significant increase in HTC value at the interface, which is obtained from the computation.