Exhaust Port Challenge

Objective:

1. Disscuss importance of Conjugate Heat Transfer Analysis.
2. Simulate Exhaust port model in fluent while maintaining the proper Y+ value according to the turbulence model.
3. Calculating the wall/surface heat transfer coefficient on the internal solid surface & show the velocity & temperature contours in appropriate areas.

Introduction:

Conjugate heat transfer is defined as the heat transfer between two domains by exchange of thermal energy. For a system the thermal energy available is defined by its temperature and the movement of thermal energy is defined by its heat flux through the outer walls. Heat transfer in solids happens through conduction and through the walls by convection and in liquid phase through convection.

CHT provides the temperature prediction and the hotspots regions at the solid liquidinterface and we can also predict the heat transfer accurately for example conduction through solids, convection through fluid and thermal radiation. It also provides the velocity and pressure distribution of the fluids moving inside in the solid. We can also use CHT in design optimisation for improvement for heat transfer and cooliing capacity.

CHT can be used in turbochargers, engine equipments, HVAC, exhaust port and heat ejection though electronic devices etc.

Exhaust Port

In exhaust port the hot exhaust fumes come out of the engine head and it transfers the heat from hot fumes to solid wall. Thus it transfers the heat from the solid surface to the ambient air. The process of heat transfer is known as Conjugate Heat Transfer.

Methodology

1. Modeling
2. Meshing
3. Pre-processing
4. Setting up Physics
5. Post Processing

The model can be prepared in any CAD software and can further be imported in ANSYS fluent. The model can then be imported in ANSYS Spaceclaim and can be repaired for the fluent solver and the fluid volume can be extracted. Further in ANSYS Mesher the mesh for the model can be created and in ANSYS Fluent the physics can be set up and further results can be calculated in ANSYS Post-processing.

1.)Modeling:

The 3D model can be created in any CAD Software in my case i just imported the file in stp format.

Further after creating the mesh all the extra edges will be removed and the fluid volume for the port will be generated and can be seen below:

Once fluid volume has been created the topology will be set to share such that the data could be shared between the different volumes of fluid and solid body of exhaust port.

Case 1: Baseline Mesh

Here we will create the mesh with the default settings provided by the ANSYS Fluent and we will further provide the named selection to the different boundaries of the exhaust port.

The different named selection can be seen below:

In this named selection the wall d cannot be seen as it is the wall sharing the solid fluid volume and thus is below the solid volume.

Further the baseline mesh can be seen below:

Here the default element size for this mesh is 150mm and the total number of the elements are 137475 and the number of nodes are 27421. Further the mesh quality was also good with the maximum number of elements with the 88% quality. The mesh quality can be seen below:

Setting Up the physics

Once the mesh has beeen created without any errors we will proceed for setting up the physics. First we will start with checking the mesh and then we will start with definng the solver. We will define the solver as the k-epsilon mode along with the energy equation. After that we will define the materials as the solid exhaust port as aluminium and the fluid as air. Further the boundaries conditions will be defined as shown below:

Inlet = velocity-inlet 5m/s and thermal conditions as 700K

Outlet = pressure-outlet with 0 gauge pressure

Coupled wall conditions at fluid and solid interface

Heat Transfer Coefficient at outer wall convection = 20w/m^2-K

After that we will start the simulation with the hybrid initialisation and then we will start with 200 iterations.

The residual plot can be seen below:

The temperature contour of the fluid volume can be seen below:

The area weighted average for the heat flux for convection for the liquid solid interface can be seen below:

Temperature streamline at exhaust port

Vector velocity at the mid plane

Pressure at the mid plane:

Heat transfer at the outer body convection wall

During the velocity distribution at the mid plane we can notice that the velocity at the bend is maximum near the turning and apart from that the pressure decreases and the velocity increases as the fluid moves towards the outlet. Also the maximum pressure can be see at the bend as seen in the contour.

In an exhaust port analysis the hot air enters from the inlet and then it transfers it heat from the convection to the exhaust port body and then it transfer it heat through conduction all along the body of the port. Here in base line mesh the results are seen as expected because the temperature of exhaust port should rise when all the inlets combine with each other near the outlet pipe. In the heat transfer along the convection plot it can be seen irregular near the walls due to the coarser mesh along the walls and also the heat transfer coefficient along the walls and also the heat flux is about 25 in the plot shown in area average. This can be due to the coarser mesh and the turbulence model being used is k epsilon which is generally used in between the Y+ of 30 to 300. Further we will now proceeding with the refined mesh and reduing the mesh size and near the walls to maintain the accurate y+.

Case 2: Redefined Mesh

In the refined mesh the whole geometry and setup will be same. Here we will change the mesh and we will redefine the mesh by giving the proper boundary layer mesh sizes by providing the inflation layer size according to the turbulence model. If we want to capture the flow away from the boundary where the turbulence conditions are quiet high. Here we will be using k-epsilon with standard wall function turbulence model where the y+ will be in the range between 30-300. Further if we want to capture the flow near the walls where the flow has reynolds number we will be using the k-omega boundary condition with the y+ value between 1-5.

For the refinement for the case we will provide the inflation layer by giving the total thickness of the inflation layer. Apart from that we will also provide the edge sizing for 1 degree such that the edges of the are almost taken as circles. Also we will provide the y+ by the initial guess and we will increase and decrease the value until we get the desired answer.

For the meshing the edge sizing was setup as 7.5 mm that is about the 0.5 degree from the centre. Apart from that we will setup the body sizing for the extracted fluid volume with the size of 17mm. Apart from that we will also give the mesh sizing of 17mm for the exhaust port body. Apart from that we will provide the inflation layer to the wall named inflation layer. In the inflation layer sizing we will be providing the total inflation thickness of 6mm and with the number of layers to be 5. The total number of mesh elements are 447750 and the number of nodes are 151723.

The named selection can be seen below:

The mesh can be seen below:

The inflation layer can be seen below:

While setting up the physics we will be make using the same boundary conditions with the k-epsilon turbulence model with the standard wall function and also we will be using the energy equation. Further we will be using the

Inlet velocity=5m/s

Temperature=700K

Heat transfer coefficient at outer wall = 20W/m^2-K

After that we will be intialising the solution with hybrid initialisation and then run for 400 iterations.

The residuals plot can be seen below:

Here it can be seen that the solution has fully converged and here we can check the plot for the area weighted average for the heat flux for the heat transfer coefficient at the interface of the solid and fluid volume.

The temperature contour for the exhaust port can be seen below:

Further in post processing the streamline flow can be seen below:

A plane in the mid surface for the exhaust port has been made and the vector plot was created for the velocity plot. Plot can be seen below:

The pressure at the outlet mid plane can be seen below:

The mid surface plot can be seen below for the heat transfer coefficients near the walls:

Results

Here we performed the simulation for both the cases and we thus get the velocity, temperature and pressure contours . Here we can easily find that the refined mesh give more accurate results and there is large difference between the heat transfer coefficient and the results for the refined mesh are much more closer to the experimental data. Further the inflation layers have been added accordingly along the boundary of the fluid and solid volume thus capturing the accurate boundary layers according to the k epsilon turbulence model.

Observation Table:

	Baseline Mesh	Refined Mesh
No of Nodes	27421	137475
Elements	151723	447750
Heat transfer Coefficient between the wall and fuid	30	74
Max Velocity	37.0938	40.9301
Max Temperature	700	700
Min Temperature	463	497

Simulation Result Validation

The results for the simulation can be validated either by some one research paper who has done the similar work or by setting up the experiment and then manually calculating out the data for result validation. Apart from that there are several factors that affect the results of teh simulation such as :

1. Mesh: The proper grid size is required for capturing the boundary layer phenomenon thus a proper size of inflation layer and edge sizing or body sizing according to the problem is highly required for solving the problem. Further, if the mesh size is chosen randomly and is very fine without any proper explaination, that could lead to the inaccurate results and may require much more computational time and resources.
2. Turbulence model: The proper turbulence model should be chosen wisely for the problem to capture the different required phenomenon and according to those turbulence model the y+ value should be setup.

Further for capturing the phenomenon near the walls where the reynolds number is low or where ther is formation of boundary layers the k-omega method should be used. For k-omega (Y+= 1 to 5).

And for capturing the flow away from the walls where the reynolds number is high and the boundary layers disrupts and flow is uneven the k-epsilon model should be used. Fork-epsilon (Y+ = 30 to 300).

It should be avoidede to use the y+ value in between the range of 5 to 30 as the flow in betwnn this range is in transition period thus no proper theory could be defined or further no proper explaination for the results could be provided.

The heat transfer coefficient could also be calculated theoritically with the help of nusselts number,

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

`Nu=frac{hl}{k}`

Conclusion

We learned to use the conjugate heat transfer ananlysis to study the transfer of heat between the solid and the fluid surfaces of an exhaust port. The CHT is heavily used for a large number of applications such as heat exchangers, turbines, pumps, automobiles, heat exchanger. for solving teh solution we forst started with the baseline mesh to check teh setup. Further we refined the mesh by checking the baseline mesh. Further it was also observed that the mesh quality and the proper turbulence model played an vital role in solving these type of analysis accurately.