Exhaust Port Challenge

AIM

The aim of this project is to simulate the conjugate heat transfer simulation inside an exhaust manifold.

OBJECTIVES

The objectives of the simulation are

• Need of simulating Conjugate heat transfer on exhaust manifold
• Calculation of wall heat transfer coefficient on internal solid surface
• Maintaining appropriate y+ value for the turbulence model

CONJUGATE HEAT TRANSFER

The contemporary conjugate heat transfer model was developed after computers came into wide use in order to substitute the empirical relation of proportionality of heat flux to temperature difference with heat transfer coefficient which was the only tool in theoretical heat convection since the times of Newton. This model, based on a strictly mathematically stated problem, describes the heat transfer between a body and a fluid flowing over or inside it as a result of the interaction of two objects. The physical processes and solutions of the governing equations are considered separately for each object in two subdomains. Matching conditions for these solutions at the interface provide the distributions of temperature and heat flux along the body–flow interface, eliminating the need for a heat transfer coefficient. Moreover, it may be calculated using these data.

APPLICATIONS OF CHT

Starting from simple examples in the 1960s, the conjugate heat transfer methods have become a more powerful tool for modeling and investigating nature phenomena and engineering systems in different areas ranging from aerospace and nuclear reactors to thermal goods treatment and food processing, from complex procedures in medicine to atmosphere/ocean thermal interaction in meteorology, and from relatively simple units to multistage, nonlinear processes. The applications in specific areas of conjugate heat transfer at periodic boundary conditions and in exchanger ducts.

EXHAUST PORT SIMULATION(BASE LINE MESH)

The stages involved in the simulation are as follows

• Geometry
• Mesh
• Setup
• Solution
• Results

Geometry

The first step is to import the geometry into spaceclaim and extract the fluid volume where the fluid interacts with the exhaust port for 4 cylinder inline engine.

Now we show the image where we have created the fluid volume and now we have to enable the share topology.

Mesh

The base line mesh was created to start the simulation with the element size of 0.4136m and this takes the number of elements count to 137850. 

Setup

The setup involves the following changes into the solver. 

1. Solver - pressure based solver
2. solver state - steady state
3. Turbulence Model - K-epsilon with standard wall function
4. boundary conditions

• Inlet velocity = 5 m/s
• Heat transfer coefficient = 20 W/m^2K
• pressure outlet = 0 pascals

Here we see that the simulation was run for 500 iterations and it is observed that solution has converged for 250 iterations.

Results

Temperature Contour

Velocity Contour

Heat Transfer Coefficient contour

EXHAUST PORT SIMULATION(FINE MESH)

The stages involved in the simulation are as follows

• Geometry
• Mesh
• Setup
• Solution
• Results

Geometry

The first step is to import the geometry into spaceclaim and extract the fluid volume where the fluid interacts with the exhaust port for 4 cylinder inline engine.

Now we show the image where we have created the fluid volume and now we have to enable the share topology.

Mesh

The finer mesh was created to start the simulation with the element size of 0.4136m and then we are adding inflation layers to the outside wall by taking the total thickness of 0.005m. We also define the sizing to decrease the the element size to 

Setup

The setup involves the following changes into the solver. 

1. Solver - pressure based solver
2. solver state - steady state
3. Turbulence Model - K-epsilon with standard wall function
4. boundary conditions

• Inlet velocity = 5 m/s
• Heat transfer coefficient = 20 W/m^2K
• pressure outlet = 0 pascals

Here we see that the simulation was run for 500 iterations and it is observed that solution has converged for 250 iterations.

For the calculation of the surface/wall heat transfer coefficient the area weighted average plot was created when the solution for the simulation was run for 500 iterations. The value of heat transfer coefficient is 37.2944 W/m^2K. 

Results

Temperature Contour

Velocity Contour

Heat Transfer Coefficient contour

HEAT TRANSFER COEFFICIENT VERIFICATION

The heat transfer coefficient is calculated from the following formula of the nusselt number

`Nu= 0.023Re^(4/5).Pr^(0.4)`

Also nusselt number is given by

`Nu=(hL)/k`

As the reynolds number increases the heat transfer coefficient also increases 

By the area weighted average plot for the inflation layers we can estimate that the value of heat transfer coefficient is 37.29 W/m^2K.

The prediction of the heat transfer coefficient can depend upon many factors like

1. Mesh inflation layers
2. Sharing of the topology should be done in an accurate manner
3. Mesh should capture the curvature effect 
4. It also depends on the mesh refinement.

CONCLUSION

1. The simulation of the conjugate heat transfer analysis is done
2. The heat transfer coefficient is calculated at the wall surface.
3. As we increase the mesh count the CHT analysis, the accuracy of the heat transfer increases.