Exhaust Port Challenge

Aim : To observe the thermal behaviour of the hot fluid flowing through the exhaust pipe and also to calculate the wall/surface heat transfer coefficient on the internal solid surface of the manifold.

Objectives :

• To calculate the wall/surface heat transfer coefficient on the internal solid surface & show the velocity & temperature contours in appropriate areas.
• To verify the HTC predicitions from simulation are right or no. Also to expalin on what factors the accuracy of the predicition depend on.

Description: Heat transfer is known as the passage of thermal energy from a hot to the cold material. The passage generally occurs in three different modes i.e. conduction, convection and radiation.

Conjugate Heat Transfer (CHT) is used to describe processes which involve the variations of the temperature with solids and fluids.

The exchange of thermal energy between the two physical bodies is called as the study of heat transfer. The rate of transfered heat is directly praportional to the temperature difference between the bodies.

The conjugate heat transfer corresponds with the combination of heat transfer in solids and in fluids. Forced convection is the most common way to achieve high heat transfer rate. Also provides the velocity and pressure distribution of the fluid inside the solid.

Modes of heat transfer are as follows-

• Conduction- A measure of the amount of conduction for a given gradient is the heat conductivity.
• Convection- When the heat is desipated by moving fluid, the flow can either be caused by external influences such as forced convection or buoyancy forces. Convective heat transfer if coupled to the fluid flow solution.
• Radiation- Transfer of energy by electromagnetic waves between the surfaces with different temperatures, that are seperated by a medium that is least partially transparent to the radiation.This useful at higher temperatures like during combustion processes.

Conjugate Heat Transfer is generally used in heat-exchangers, turbocharges, air conditioning systems , engine exhaust systems etc. CHT analysis helps in prediciting the surface temperatures as well as the rate of heat exchanging.

The conjugate heat transfer analysis helps in measuring the heat transfer through two different regions. Helps to locate/ predicit the hot spot regions i.e. region that have maxximum temperatures during the flow.

It plays a vital role in design optimization by improving the heat transdfer rate and provinding sufficient cooling system to it. With help of CHT analysis visualization of flow patterns and also trimming of geometry can be done based on the temperaturerequirement.

It can be performed to improve the cooling performance of the water jacket and increase engine life.

Advancements in cooling for applications such as gas turbines components required improved understanding of the complex heat transfer mechanisms.

This analysis is used for Motor and Battery to get optimized design for high voltage systems.

The results of CHT analysis are considered as inputs to the structural simulations as thermal loads.

CHT analysis is used for different types of engines like bikes, cars, commercial vehicles, agricultural equipment etc.

Procedure :

• Initially open the space claim and set the units to meters. Then import the Exhaust port geometry to spaceClaim.
• Now extract the fluid volume from the geometry i.e. go to Prepare > select Volume Extract > select Select Edges and select the inner openings of inlets and outlet > click OK.
• When ever if we are not able to select the edges, that means we have some dublicate edges in the geometry.hence we nrrd to clear them.
• Then we can see the the fluid volume is extracted. Once it is extracted just rename the geometry as fluid-volume and solid-volume. Now the geometry is ready for meshing.
• Open Meshing Interface. Once the geometry is imported to mesh, generate a basic mesh to visualize it.
• Check if all the areas are meshed perfectly. It is observed that the inner openings of the inlet ports have a non-conformal mesh. This kind of mesh is obtained because in meshing we both solid-volume and fluid as seperate parts. Non-conformal mesh means that the vertex should be linked each other and faces also should be connected to each other. We have a smooth heat transfer only when these are connected properly.
• For a CHT simulations we need a perfect overlap of mesh. Go to geometry select the two volumes > right click Move to a new component. Name the new generated geometry as the Final Geometry. And delete the empty components.
• To see that mesh is overlapped correctly, go to geometry > click on FFF > in propeties > select Share Topology = Share. Now both the interfaces are shared. Go to Workbench in the top toolbar select Share. This helps to create interfaces between two bodies and helps to see interfaces where information is shared.
• We can see pink circles appear on the inner circles of inlets and out. It is observed that information between solid-volume and fluid-volume is shared perfectly.
• Go to Meshing interface and fist create the names surfaces. Now generate the mesh. Check for the meshing interface at the inner circles of inlet and outlet. We can see that perfect meshing is done between them.
• Also check the number of elements we got for the basile mesh. As we know that for acedamic licence we can go upto half a million cell count. To check the Element Quality go to Details of Mesh > select Quality > click on Mesh Metric. Plot is created to check the quality. In this case we consider minimum of 10%. Check for the quality.
• Now open the Setup and check for the complete geometry. It is observed that ansys automatically displays blue colour to the inlets and red to the outlet i.e. this is done based on the named selections.
• In this case Pressure Basaed Solver is used to do a steady state analysis. Select Energy and for Viscous Model = k-epsilion - turbulence model.
• Materials used in this case are fluid=Air and solid=Aluminium.
• Go to Boundary Condition. Select inlets and edit the parameters. Here for baseline mesh consider Velocity Magnitude=5m/s and Inlet temperature=700k. Now edit the Outer wall convection parameters i.e. consider Thermal conditions= Convection and Heat Transfer Coefficient=20 w/m^2-k. This coefficient value is considered from the experimental data.
• Now directly initialize using Hybrid Initialization. Then create a temperature contour by selecting the required surfaces to display on the geometry. Also create solution animations. Now finally run the simulation for 350 iterations.
• We can see that residuals and contours of static temperature are displayed on the graphics window.
• Click on the Results to open the CFD post. Select the required surfaces that are required for visualization on th ecfd post window and select different parammeters like temperature, pressure and velocity. Also create a streamlines from the inlets to see the effects of flow in the port.
• Create a Plane in the outlet and required contours are generated by changing the parameters. Here Wall Heat Transfer Coefficient is also plottedusing the above created plane.

Mesh Refinement :

• As we observe that the baseline mesh is perfectly ok for the simulatiion, further we can refine the mesh by adding the inflation layers and by maintaining the conditions of the y+ to get more accurate results.

Geometry of the Exhaust Port:

Named Selection:

Case-1 (Baseline Mesh)

Inputs :

Element Size- 0.15m

Velocity Magnitude- 5m/s

Inlet Temperature- 700k

Heat Transfer Coefficient- 20 W/m^2-k.

Meshing:

Conformal Mesh:

Residuals:

Temperature Contour:

Velocity - Streamline Contour:

Velocity Contour at Outlet:

Temperature (outer wall convection) Contour:

Wall Heat Transfer Coefficient:

Pressure Contour:

Case-2 (Refined Mesh)

• For refinement of mesh we add inflation layers so as to find the accurate near the walls. The thickness of inflation layer depends on the turbulence model.When we see the reynolds number , if we take k-epsilon turbulence model the standard wall function y+ range is 30-300 where as for the k-omega sst model y+ range is 1-5. In few cases it may go down below 1.
• As we add inflation layers for refinement , in this case we take total thickness as 0.005m. Further by validating we can either increase or decrease the total thickness as this completely depends on the value of y+.
• Also we add body sizing for refinement. On adjusting the value here we should maintain the maximum number of elements that we are getting. This is because we have restrictions to the academic version of the software.
• To check the Element Quality go to Details of Mesh > select Quality > click on Mesh Metric. Plot is created to check the quality. In this case we consider minimum of 10%. Check for the quality.

Inputs :

Element Size- 0.15m

Body Sizing- 0.0105m

Inflation layers- 5

Total thickness- 0.005m

Velocity Magnitude- 5m/s

Inlet Temperature- 700k

Heat Transfer Coefficient- 20 W/m^2-k.

Refined Mesh:

Conformal Mesh:

Residuals:

Temperature Contour:

Velocity - Streamline Contour:

Velocity Contour at Outlet:

Temperature (outer wall convection) Contour:

Wall Heat Transfer Coefficient:

Pressure Contour:

Observations:

Parameter	case-1 Baseline Mesh	case-2 Refined Mesh
Number of Elements	137728	497072
Nodes	33608	137862
Maximum Velocity (m/s)	39.98	40
Minimum Temperature (K)	464	509
Maximum Temperature (K)	700	700
Wall Heat Transfer Coefficient (w/m^2-k)	105	219.1

Results:

• Initially we assumed that hot exhaust gasses are passing through inlet (high temperatures).But when we observe the solid material temperature close to the inlets the temperature is very low, whereas at the place where all the pipes join(Outlet) the temperature is high. This is mainly due to convection.
• Also observed at inlets the velocity is very low when compared to the outlet. This is because of mass conservation i.e. we have 4 similar size of inlets and a single outlet, so all the air from inlets should flow from a single outlet. Hence the velocity is high.
• As the simulation is performed in steady state,we have high velocity. This leads to high Reynolds Number. Hence we need maximum heat transfer coefficient.
• We know that Wall Heat Transfer Coefficient is calculated at the walls. In the above results we can observe that there is heighest Wall Heat Transfer Coefficient because at the same point the velocity is heighest.
• From the above contours its clearly noted that maximum heat transfer is occuring where there is maximum heat transfer coefficient. As the temperature of fluid is also high we can see that the solid close to that region undergoes heat transfer.
• On refinement of mesh it is also observed that the heat transfer coefficient is present only on the first cell that is close to the wall.
• Comparing both cases its clear that on adding inflation layer we can see a smooth plot.
• Hence on refining the mesh we could see a smooth representation of heat transfer coefficient at the walls and also gave more accurate outputs.

Validating the HTC predictions from the simulations : The obtained results can be verified in multiple ways i.e. we can consider a research work related to the above simulations and compare the obtained result with the research work. This help to understand the different parameters considered to obtain the same result. The convective heat transfer can be derived analytically through dimensional analysis.

Theoretically, using Nusselts number we can find the heat transfer coefficient.

The accuracy of the heat tranfer coefficient mainly depends on the surface wall layer thickness and the refinement of mesh.

Factors affecting the accuracy of the HTC predicitions: The accuracy mainly depends on the following factors-

• Number of elements i.e. more finer the mesh gives more the accuracy in results.
• Choosing the correct turbulence model as per the Reynolds number and the y+ value also important.
• Adding inflation layers around the boundary of the geometry provides smooth representation of HTC as it provides body fitted mesh.
• Always the mesh between two bodies should be conformal, as they help for smooth heat transfer between them. (share topology is used)

Conclusion:

Conjugate heat transfer analysis is used to find the heat transfer between the two different surfaces.The above simulations showed a clear picture on how the velocity is affecting the coefficient values. Several contours are generated to have a clear idea inside a exhaust port when a hot fluid is flowing through the inlet of the port. To visualize the flow the streamlines are used. It is also understood that on refining the mesh we get the most accurate results when compared to the baseline mesh.