CHT Analysis on the Exhaust Port of an Engine

Objective:

To conduct CHT Analysis on the exhaust port of an IC Engine using ANSYS Fluent.

Introduction:

Conjugate Heat Transfer describes processes where there are temperature variations in both solids and fluids. There is thermal interaction between the solid and fluid. For CHT analysis, solid material conduction and fluid convection is analyzed simultaneously. The conductive heat transfer in solid region is coupled with the convective heat transfer in the neighboring fluid.

CHT analysis provides Temperature distribution in the solid and the velocity distribution and mechanism of heat transfer in the fluid. The output of a CHT analysis is often used as an input to structural simulation as thermal load.

The simplest example where CHT analysis is used is when flowing air is used to cool a hot plate.

CHT analysis has the following applications:

• It is performed to improve cooling performance of the water jacket and increase engine life
• Cooling applications for gas turbine components.
• Motor and Battery CHT analysis is performed to get optimized design for high voltage systems.
• Cooling of electronic components and optimization of thermal loads.

Exhaust Manifold:

The Exhaust manifold is mounted on the cylinder head in an engine. The exhaust gases are collected from all cylinders and combined into one pipe. The temperature of the gases entering the exhaust manifold are 800-degree C. Exhaust manifold is usually made up of grey cast iron.

Independent flow or thermal analysis is insufficient to understand this case. Hence a coupled approach is adapted to understand the energy exchange between the solid and fluid domains.

Analytical Method to compute Heat Transfer Coefficient:

The HTC can be computed from the Reynolds number analytically.

It is given by the following formula:

`h=(Nu times k)/L`

where Nu is the Nusselt number

k is the thermal conductivity of the fluid.

L is the characteristic dimension.

`Nu=0.023Re^0.8Pr^n`

where n depends on the type of flow.

Nusselt number is a dimensionless parameter that characterises the convective heat transfer. It is mainly a ratio od the heat transfer by convection to heat transfer by conduction. It can be represented as a function of Reynolds number and Prandtl Number.

Prandtl Number is the ratio of diffusion of momentum to diffusion of heat. It usually gives the relation between the velocity boundary layer and thermal boundary layer. It is given by

`Pr=(mu C_p)/k`

These are fluid properties, viscosity, specific heat anf thermal conductivity.

The Reynolds number of the fluid can be calculated by

`Re=(rhoVD)/mu`

Thus it can be concluded that the HTC depends on the properties of the fluid such as the thermal conductivity, Specific heat, viscosity and the density. it also depends on the characterictic dimension of the flow geometry and the velocity of flow. For example turbulent flow had higher heat transfer coefficient.

Thus the HTC can be analytically computed anf used to verify the numerically obtained value.

Computational Domain:

The computational domain consists of the external solid piping and the fluid region is created using the Extract Volume option from the Prepare tab. The solid and fluid regions can be seen in the figure below.

The next step is to share the topology in order to get a conformal mesh. The Solid and the fluid component are added into the same component and the share option is used from the workbench tab. The pink edges in the below figure indicate the shared surfaces.

Meshing:

A base mesh is generated and the simulation is carried out. In the next step the mesh is refined. Inflation is applied on the fluid solid interface and the solid mesh id further refined using Body Sizing.

The details of both variations in the mesh are tabulated below.

	No of Elements	Element size
Base	138163	0.15m
Refined	373438	0.15 m Body Size:0.01m

Base Mesh:

Refined Mesh:

The named selections used are:

1. Inlets
2. Outlets
3. Convection Walls
4. Adiabatic wall
5. Inflation Layer.

These can be seen in the images below.

Case Setup:

The steady state analysis is performed here. The materials are selected from the physics tab. Aluminum is chosen as the solid and Air is the fluid in the base case. The cell zones are examined.

The boundary conditions are provided as follows:

Inlets	Velocity Inlet	V= 5m/s T=700K
Outlet	Pressure Outlet	0Pa gauge
Convection Wall	No Slip wall with Convection	h=20W/m2K
Adiabatic wall	No slip wall

Monitors are set up to monitor the outlet temperature. The animation is setup to observe the temperature contours on the external convection wall.

Next different materials are chosen for the solid and liquid and the outlet temperatures are compared in various cases. The fluids used for the simulation are Air, CO2 and Hydrogen. The solids used for simulation are Aluminum and Copper.

(The materials chosen are merely chosen for simulation purpose and are not corresponding to the real gases that flow out through the exhaust port)

The thermal properties used are in the table below:

Material	Specific Heat	Conductivity
CO2	840.37	0.0145
Hydrogen	Piecewise Polynomial	0.1672
Air	1006.43	0.0242
Aluminum	381	387.6
Copper	871	202.4

Results:

The base case and the refined case of the mesh are compared.

The Temperature contours are compared along with the HTC along the walls.

The temperature contours on the convection wall can be seen to be the same for both the base case in the first figure and the refined case in the second case.The contours however are more diffusive in the base case due to the large mesh size.

The above figure shows the temperature contours in a plane that cuts the exhaust port in the z direction. Smoother and less diffusive contours near the fluid wall are observed in the second image of the refined mesh due to the addition of the inflation layer.

The following cases show the temperature contours at the inlet and the outlet.

From both the above figures it is evident that the temperature is less diffusive in the refined case than the base case due to the application of inflaton layers in the refinement.

The line plot below shows the comparison between the outlet temperatures in both the cases.The Area weighted average of the outlet temperatures is taken.

The Wall heat transfer coefficient can be seen in the figues below. The wall heat transfer coefficient is obtained only on the first cell near the wall. This ca be observed in the figures below.

The zoomed in image of the HTC contours can be seen below.

As inflation is applied in the refined mesh the Wall HTC is seen very narrow as the first cell thickness is comparitively low.

The maximum HTC coefficient obtained is 212.3 W/m^2K for the refined case and 106.1 for base case.

The refined mesh is used for all further analysis.

The figure below shows the pressure variation in the fluid region.

The back pressure is the pressure exerted on a moving fluid by obstructions against the direction of the flow. In Exhaust systems it restricts the flow rate of the exhaust gases and reduces the engine performance.

The back pressure is at the bend on the exhaust port as seen in the pressure comtours above.

The streamlines of the flow are seen in the figure below:

The velocity contour in the z plane is seen below:

The maximumm velocity is obtained at the bend on the pipe. The value is 39.52m/s.

Comparison of outlet temperatures of different materials:

The fluid flowing through the exhaust port is changed and the effect of the fluid on the Outlet Temperature is exhamined.

In the following image, the solid material is Aluminum.

In the following plot the solid material is Copper

Now it can be observed that the outlet temperature of CO2 is least and Hydrogen is highest in both cases.The thermal conductivity of CO2 is least and that of hydrogen is higher and as discussed earlier the HTC is directly proportional to the fluid conductivity. Thus it can be explained that the heat tranfer is better for the fluid that has the higher thermal conductivity.

The following plots draw a comparison between the outlet temperatures for different solid for the same flowing fluid.

The following plot shows the outlet temperature for Air as the fluid.

The following plot shows the variation of outlet temperature for the different solid for CO2 fluid.

It can be noted in both cases that the outlet temperature for copper is slightly lesser than Aluminum. This is because copper being a better conductor than Aluminum has better heat transfer.Thus the outlet temperature for the fluid coming out of the copper solid is lower.

Conclusion:

• The simulation is carried out for two different grids. the results obtained by the refined grids show better accuracy at the outlet temperature and less diffusive contours.
• The HTC coefficient is compared for both the cases and the max value of HTC obtained for the refined grid is higher.
• The factors influencing the heat tranfer coefficient are discussed.
• The maximum velocity obtained is noted and it is found at the bend of the outlet pipe.
• The back pressure formed at the bend of the outlet pipe is also noted.
• The outlet temperature for various materials is compared.