Conjugate Heat transfer analysis on Exhaust Manifold

EXHAUST PORT SIMULATION

Conjugate Heat Transfer is a process that denotes the exchange of thermal energy at an interface of fluid and solid domain. It basically corresponds to the combination of heat transfer in fluids and solids.

The heat transfer usually takes place by conduction in solids and is dominated by convection in fluids. There are numerous cases where both conduction and convection occur simultaneously, for example, in applications like heat exchangers, boilers, coolers, heat pipe, IC engines, HVAC, etc. In all these applictions CHT analysis can be performed to find out heat transfer and transport parameters like temperature, heat transfer coefficient, pressure, velocity and other derived quatities like Nusselt number.

Conjugate heat transfer (CHT) refers to the coupling of conduction in solids with the convective and radiative heat transfer in the surrounding fluids. Accurately predicting CHT is an essential part of the design process in many industrial applications including electronics cooling, turbomachinery, and HVAC. With ANSYS CFD, the tools to perform high fidelity CHT analyses are at your fingertips.

So, the Conjugate heat transfer occurred mostly in the cooling process like HEAT SINK, BATTERY COOLING, AIRFLOW OVER THE SOLID OBJECT FOR COOLING.

Whenever the solid and fluid interface appears, this type of CHT analysis can be done to find the optimum result by using the Ansys. It allows the simulation of the heat transfer between solid and fluid domains by exchanging the thermal energy at the interface between them.

CHT ANALYSIS ON A MODEL OF AN EXHAUST PORT:

Here we use the inline four-cylinder engine exhaust manifold, where the conjugate heat transfer is analyzed. Fluid volume is extracted from the port and consider as the fluid region where the air enters into the region. Since it has the interface of the solid and fluid domain so this type is called as CHT analysis.

When we generate the baseline mesh, the non-conformal mesh is generated which affect the result. To make a Conformal mesh Share topology option is used which make the perfect mesh.

SETUP- Pressure based solver is used for this analysis. Steady state and enable the energy to measure the temperature. The turbulent k-epsilon method is used to run the analysis.

BOUNDARY CONDITIONS OF THE GIVEN PROBLEM:

Velocity inlet = 5 m/s,Temperature-700k

Outer wall Heat Transfer coefficient(convection) = 20W/m^2K

Free stream Temperature-300k

Outlet- Pressure outlet.

Use the hybrid initialization and run the analysis to get the result.

CASE-1-BASELINE MESH

33691 nodes

138408 eleements

The mesh between the solid part and fluid volume is non conformal at first.This means that the nodes between the solid and flid interface are not interconnected.To construct a conformal mesh between the two, SHARED TOPOLOGY is turned on.This is done in geometry editing section and then mesh is generated.

RESIDUALS PLOT:

The fine mesh gives the result more accurate. The above residual shows that the solution is slowly in a converging manner.

TEMERATURE CONTOUR:

VELOCITY PLOTS:

HEAT TRANSFER COEFFECIENT ON XY PLANE :

HEAT TRANSFER COEFFECIENT ON CONVECTIVE WALLS :

CASE-2-REFINED MESH

Number of elements =84709

Number of nodes =252370

Element size = 18mm

To make the mesh more refine, reduce the size of the element in the solid region and add the inflation layer which the replica of the part.The inflation layer is generated on the outer layer of the fluid domain to get the optimum wall heat transfer coefficient and a higher resolution/accuracy of the simulation calculations.

RESIDUAL PLOTS:

TEMPERATURE CONTOURS:

WALL HEAT TRANFER COFFECIENT PLOT:

VELOCITY PLOTS:

The velocity of the outlet is increased as compared to the baseline mesh Because of the inflation layer the region which gave the more accurate result.

The bending of the outlet area produced more Temperature and wall heat transfer coefficient because of high velocity at that region.

Conclusion: From the above plots it can be concluded that the baseline simulation shows similar results in comparison with the simulation with fine mesh grid. However the inflation layer at the interface has been crucial in obtaining a smoother plot of wall heat transfer coefficient as can be seen in above images.

Heat Transfer Coefficient verification :

Heat Transfer coefficient at boundary surface is often calculated using Nusselt number. Hence to verify the HTC obtained from the CHT analysis we first need to calculate the theoretical value of Nusselt number.

For a turbulent flow inside a smooth pipe, Nu number is given by Dittus-Boelter equation.

(Nu)D=0.023(Re)45(Pr)n\">`( N u ) D = 0.023 ( R e ) ^frac{4}{ 5} ( P r )^ n`

Nu=hL/K

Hence, using value of Nu from first equation we can find the value of heat transfer coefficient, h from the 2nd equation and verify it with the actual results obtained from the above simulations.

Factors affecting the accuracy of prediction from simulations are

1. Grid size and shape – The finer the mesh the accurate the results. Also, it should be ensured that the curvature and other geometrical features are properly captured.
2. Inflation layer – It increases the quality of mesh near the boundary interface and it can capture smooth plots of heat transfer coefficient as we have seen from our above example.
3. Properties of fluid surrounding the system,and its movement/motion.
4. It also depends on the time step size. However its not applicable in above case since it’s a steady state simulation.