CHT Analysis on an Exhaust Manifold

OBJECTIVE:-

• To perform a Conjugate Heat Transfer analysis on an exhaust manifold.
• To compare the simulation results with default mesh and refined mesh models.
• To show the velocity, temperature and heat transfer coefficient plots.
• To study the factors affecting the CFD solutions accuracy.

CHT ANALYSIS:-

• Wherever there is a temperature difference, there will be a heat transfer from higher temperature body to lower temperature body. Three fundamental modes of heat transfer are conduction, convection and radiation.
• The heat transfer within and through the body by its molecules' collision is called conduction. Conduction is more evident in solid objects. The heat transfer between the body and its surrounding is usually referred to as convection. Convection is the more dominant heat transfer in fluids. Convection happens because of the fluid motion due to density difference. Radiation is the mode of heat transfer by the emission of electromagnetic radiation.
• The combination of heat transfer in solids and fluids is called Conjugate Heat Transfer (CHT). Heat exchanger is a best application of conjugate heat transfer. Most of the time, heat exchanger will be used in a thermal system which effectively combines the two fundamental modes of heat transfer, conduction through solid objects and convection with its surrounding which in turn increase the overall efficiency.

GEOMETRY:-

• Exhaust manifold is one of the important component which affects the engine performance.
• In this project, flow through an exhaust manifold is simulated and conjugate heat transfer analysis is carried out.
• The 3D geometry used is given below. It has four inlets which will accept the hot air. The hot air which entered through four inlets will then combine and exit through a single outlet.
• Different views of the geometry used have been given below.
• From the above geometry, fluid volume extracted. After that operation, the geometry will look like the below.
• Since we have both solid volume and fluid volume in our geometry, there will be some faces and/or edges which are common for the two. We need to share those faces/edges between the solid-fluid volumes for accurate and better information transfer between the solid-fluid interface mesh. Hence, share topology option was enabled and applied to the geometry.

MESHING DETAILS:-

Case - 1: Default Mesh:-

• Default element size: 0.15 m
• Total number of nodes: 29309
• Number of elements: 147601
• Different views of the geometry after applying the above mentioned Ansys default meshing settings are give below.
  
• The mesh quality in terms of element metrics is given below. The lowest quality observed was 22.7%.
  

Case - 2: Refined Mesh:-

• The base element size was given as 5e-2 m with maximum size of 6e-2 m.
• Body sizing feature was applied to the solid part with element size as 1.5e-2 m with curvature capture enabled. Body sizing was also applied to the fluid part with element size as 3e-2 m with curvature capture enabled.
• Moreover, five inflation layers were given for the solid-fluid interfaces with total thickness of 5e-3 m with 1.2 growth rate. Inflation layers were given in order to retain the shapes of the geometry and to capture the physics near the interface more accurately.
• Suitable named selections were provided initially.
• Different views of the geometry after applying all the above mentioned meshing techniques are given below.
• The mesh quality in terms of element metrics is given below. The lowest quality observed was 7.21%.

CASE SET-UP DETAILS:-

• Solver Type: Pressure-Based
• Steady state simulation
• Energy equation was enabled as we are interested in temperatue variation
• Viscous Model: k-epsilon Realizable Turbulent Model with Standard Wall Functions
• Fluid Material: Air
• Solid Material: Aluminium
• Initial and Boundary Conditions:
  • inlet: velocity-inlet type, 5 m/s velocity at 700K
  • outlet: pressure-outlet type, zero gauge pressure
  • outer wall: wall type, no slip condition, heat transfer coefficient as 20 W/m2-k with free stream temperature 300K
  • other solid parts: wall type, adiabatic condition
• Initialization: Hybrid Initialization
• Number of Iterations: 1000
• Time step method: Automatic
• Solution Method Scheme: Coupled

RESULTS:-

In the following, first figure will correspond to the case 1 (default mesh) and the second one will correspond to the case 2 (refined mesh).

• Scaled Residuals Plot:-
• In both cases, the residuals plots started to give similar patterns after few iterations. It means the solutions were converged.
• Average Outer Temperature Value:-
• In both cases, the area weighted average temperature values at outlet were obtained as almost same.
• Case 1 provided 679.73K and case 2 provided 679.81K.
• Temperature Contour:-
• The static temperature distribution over the exhast manifold in 3D have been given above.
• As expected, the temperature at inlet is high and as it goes towards the outlet, it reduces. Also, when all the incoming fluids meet at the center, the temperature value is observed as more.
• In both cases, almost similar contours have been obtained. However, the second case result gave more clarity contour.
• The temperature distributions on the plane at the center plane of the gemotry have been provided above.
• The hot fluid nature and the heat transfer effect through solid component are clearly observed.
• Both cases gave same phenomenon even though the second case gave more clarity contour.
• Velocity Contour:-
• The velocity contours for the fluid volume in 3D have been given above.
• As expected, the velocity increases when all the fluids meet at the center and it attain maximum velocity near the bending region of the outlet valve.
• Both cases indicated the similar behaviour.
• The velocity contours on the center plane of the geometry have been given above.
• These contours help us to visualize the variation of the velocity more clearly.
• As evident from the above contours, the velocity increases when all the fluids meet at the center and when it started to flow through the single outlet valve.
• In the outlet part, more velocity is observed near the bending region due to less area.
• Both cases gave similar behaviour but the second case provided more clarity contour.
• Wall Heat Transfer Coefficient Plot:-
• The wall heat transfer coefficient plots at the center plane of the geometry have been shown above.
• They indicate that the heat transfer coefficient is more at the boundary region.
• The second case gave more realistic plot.
• Velocity Vector Plot:-
• The velocity vectors at the center plane of the geomety have been shown above.
• They clearly indicate that the velocity was less at the fluids meeting point and then increases when it started to flow through the single outlet valve.
• The peek velocity region near the bending are is also clearly visible.
• There is no notable difference observed in the two case results except the small difference in the velocity magnitudes.

PREDICTION ACCURACY:-

• It is very important to verify whether the results obtained by our CFD simulation is correct or not.
• Since the CFD solution is obtained by solving many partial differential equations, we would not get 100% accurate results. Hence, in CFD simulations, we would look how closer we have reached.
• One best method to predict the accuacy of the result is by comparing the result with the experimental values. If there is a considerable difference like 60% error, then probably the solution went wrong. But on the other hand, if our solution has only 3% error, we would consider it as a better solution.
• In CFD, the correctness of the solution depends on many factors. They are listed below.
  • Clean Geometry: Often, the designed CAD file cannot be considered as such for CFD simulation. We have to check whether it has any open edges, interferences etc. If they have, we need to remove/adjust them. Also, the topology sharing has a very big role in obtaining the correct result. Hence, we need ensure that the geometry used for the simulation is always clean.
  • Correct Meshing Techniques: CFD results are nothing but the solution of partial differential equations considering the different nodal points. As more nodes and elements are there, more information can be processed to get a better solution though stability factor also needs to be considered. For this, correct discretization/meshing techniques need to be implemented. Care always needs to be taken to preserve the shape of the geometry while applying these techniques. Refining the mesh in the area of interest and providing inflation layers are examples.
  • Solver Type Selection: Depending on the velocity, correct solver; whether pressure based or density based needs to be selected appropriately. If this selection goes wrong, the result will be wrong.
  • Viscous Model: We should know the nature of the flow in which we are interested in whether it is laminar or turbulent. If it is turbulent, we have to choose the best model to capture the physics. For example, if the case involves more kind of boundary phenomenon, k-omega model will be better or if the case involves the physical changes in the bulk of the fuid domain, k-epsilon model will be better.
  • Y Plus Value: In the cases like calculating drag is involved or having flow near the boundary has more importance, Y plus value will have significant role. It helps to determine the first cell height and with the help of correct wall function, better result can be obtained.
  • Other Factors: There are many other factors which decide the accuracy of the CFD solutions such as the convergance, number of iterations, Time step method, solution method scheme, correct boundary conditions etc.
• When all the above factors are decided appropriately, we will get the best accuracy for our CFD simulations.

CONCLUSIONS:-

• The flow through exhaust manifold have been simulated with default mesh and refined mesh and the results were analyzed successfully.
• The combination of heat transfer through solids and fluids is called conjugate heat trasfer and a CHT analysis was carried out in the exhaust manifold with the help of temperature and velocity contours.
• Althouh the flow was simulated by refining the mesh, it gave almost similar behaviour of default mesh except few clarity improvements. Hence the Ansys Default Meshing was observed as a good one in this case.
• The velocity and temperature vary significantly when the fluids coming from various inlets meet and started to flow together. They were clearly observed in the temperature contours, velocity contours and in th velocity vector plots.
• The factors affecting the prediction accuracy have been studied well.