CONJUGATE HEAT TRANSFER ANALYSIS USING EXHAUST PORT

Aim

The aim of the project is to stimulate a conjygate heat transfer set stimulation in ansys fluent using an exhaust port and study its heat transfer and flow behaviours.

Conjugate Heat Transfer

Conjugate heat transfer is a behaviourial analysis where convective (transfer between fluid) and conductive (transfer between solids) heat transfer takes place in a system. This combination of heat transfer has forced or natural convection where fluid is dominated by convection and solid is dominated by conduction.

Heat transfer in solids

When there is only conduction taking place in solid heat transfer it is defined by fouriers law which is given by

Where q is the conductive heat flux proportional to Temperature gradient.

In terms of transient Analysis the Heat Equation is given by

Heat transfer in fluids

There are three main contibuting factors for the heat transfer in fluids due to its moments, which are the density of the fluid, the viscosity of the fluid and the transport energy of the fluid. The temperature field for fluid flow (transient) which involves the above mentioned parameters including the conductive term is given by

Applications of CHT

Some of the main applications where CHT is used prmarly is coolers, heaters or heat exchangers. These systems are advantageous when there is heat transfer in fluids and solids are effective. In most applications fluids are the main carrier of heat transfer but solids are also needed to sepearte the fluids in the heat exchangers. An example to uphold this is the shell and tube exchanger where the heat transfer between two fluids are seperated with the help of a metallic wall.

Figure 1: Shell and tube heat exchanger

Another great example is the radiators in the car, where the hot engine coolant is cooled by the incoming cold air, which intake cools the combustion engine.

Figure 2: Working of a radiator in automobiles

CHT study on the Exhaust mainfold using Mesh dependency method

The study is carried out using a base line mesh and a finer mesh. The resuts are then discussed and the efficiency of the meshing in the results are discussed.

Preparation of the model

Figure 3: Geomentry of the Exhaust port

Spaceclaim is used to create the solid and the fluid volumes in the exhaust port which is then imported into the mesh. The fluid and the solid volumes are show in the figure 4

Figure 4: Fluid volume(left) and Solid volume(right)

Case 1 : Base line mesh

The image below shows the baseline mesh and the number of elements created using the baseline mesh

Figure 5: The baseline mesh

The set up used for both the case are steady state and pressure based solver. The energy Equation is switched on with the K-epsilon relizable turbulence modelling. The input velocity was given was 5m/s with the inlet temperature being 700K. The outlet pressure was zero pessure with 300K. The Heat transfer coefficient for the outlet wall Convection was taken as 20(w/m2-k). The efficiency of the model is determined by the convergence of the residuals which is shown below and the convergence rate was achieved with 700 iterations.

Figure 6: Convergence of the residuals for baseline mesh

Baseline mesh results

Figure 7: Wall heat flux Plot

Figure 8: Velocity plot of the exhaust port

Figure 9: Temperature plot of the exhaust port

Case 2: Fine Mesh

In this case 5 inflation layer with total thickness of 5mm is given to the fluid volume to capture the near wall heat transfer. Also, body sizing for the solid volume is performed to increase the number of solving equations to obtain more accurate results. The pre-processing was carried out with the same settings in the base line mesh.One of the main changes in the finer mesh case is that the curvature proximity is captured by toggling on the option. The convergence plot is shown in Figure 11.

Figure 10: Fine mesh with inflation layers

Figure 11: Convergence of the residuals for finer mesh

Finer Mesh Results

Figure 12: Wall heat flux Plot

Figure 13: Velocity plot of the exhaust port

Figure 14: Temperature plot of the exhaust port

Discussion on the Results

1. Both the mesh results on the velocity and the temperature are same.
2. With the finer mesh the curvature is captured more precisely as show in the figure 12. Also the heat flux takes place in the first cell near the wall which is captured more accurately in the finer mesh.
3. The Near wall heat transfer is not captured in the baseline mesh.
4. The velocities in both the cases are same which makes contirubuting to a higher velocity in a region in the outlet which can be seen in both the velocities plot.
5. Due to higher velocity flow the temperature at the outlet is lower(595K-600K) than the inlet(700K).
6. Comparing both the mesg results we could conclude that near wall heat transfer is capured more accurately in the finer mesh, which is making us to conclude that a finer mesh is more result oriented than the baseline mesh.
7. With the finer mesh the body fitted mesh can be achieved where the mesh flow the geomentry curvature leading to more good results.

Factors on the which the HTC predictions and accuracy depend on

Nusselt Number

The Nusselt Number is a non-dimensional parameter which is used in the prediction of convective heat transfer at surfaces. It is the ratio of Convection to pure Conduction heat transfer.

The Accuracy of the stimulation depends on the following factors

Mesh dependency test

• With the mesg dependent test we can achieve more relevant results leading to consistency.

Residulas convergence less than 1e-6(widely accepted in industry)

• The solution has to reach a residual value which is to be less than 1e-6

Convergence of the Iterations

• Number of iteration to be guessed appropriately to obtain a converged plot

Consistency of the solution

• The mass flowing in and out should be same leading to mass conservation

Compare experimental and Computational results

• Compare the computational results with an experimental value or research papers or books.

Examining the model Uncertainties

• The uncertainties in the different models is to be tested for accuracy by solving the solution for one or more different models.