CHT analysis of Exhaust Port

CHT ANALYSIS OF EXHAUST PORT

Why and Where CHT analysis is used:

Conjugate heat transfer (CHT) refers to the combination of conduction in solids with the convective and radiative heat transfer in the surrounding fluids. Whenever the solid and fluid interface appears and there is an exchange of thermal energy between them, the CHT analysis can be performed.

Conjugate heat transfer investigation helps in determining the surface temperatures and heat exchanging abilities of the devices. Through this analysis, engineers can optimize the thermal parameters for their design.

The CHT analysis is performed on electronics cooling in devices like Graphic Card, turbochargers, Heat-sinks, Boilers, HVAC, etc.

CHT analysis of Exhaust Manifold

ANALYSIS SETUP

There are two types of analysis is being performed, the first analysis is done using the baseline mesh and second analysis is performed using refined mesh with inflation layers.

Firstly fluid volume is extracted by using the volume extract tool in space-claim. The solid volume and fluid volume regions are combined using Share topology so that we can get the conformal meshing.

The solver used here is Steady Pressure-Based. The turbulence model used here is the k-epsilon model.

The material used for analysis is Aluminum and fluid flowing through the exhaust manifold is taken as Air.

Boundary Conditions:

The air at 5 m/s is allowed to flow at 700 K temperature from the 4 inlets and convection boundary condition (h=20 W/m^2 k) is applied on the outside walls of the exhaust manifold.

Geometry of Exhaust Manifold

CASE-1 ANALYSIS WITH BASE-LINE MESH

The number of elements generated is 137926

Residual Plots:

The solution is getting converged with-in the number of iterations provided.

Temperature Distribution:

• The maximum temperature on the solid region is approximately 600K at the outlet throat.
• Max temperature is 700K and least temperature is 464K.

Velocity Contour at the throat:

Streamline Distribution:

The maximum velocity is approximately 37.39 m/s at the throat region.

WALL HEAT TRANSFER COEFFICIENT

The maximum wall heat transfer coefficient produced is 105.7 W/m^2 K.

CASE-2 ANALYSIS WITH REFINED MESH

The number of elements generated is 373348

Meshing:

Inflation Layers:

Residual Plot:

The solution is getting converged within the specified number of iterations.

Temperature Distribution:

The maximum temperature on the solid region is approximately 620 K at the outlet throat. Max temperature is 700 K at the inlet and the least temperature observed is 506 K.

Velocity Contour at the throat:

The maximum velocity is 41 m/s at the throat.

Streamline Distribution:

Wall Heat Transfer Co-efficient:

Maximum wall heat transfer coefficient produced is 219.1W/m^2-k

Observations:

• There is a change in maximum and minimum temperature values as we refine the mesh.
• The temperature of the solid body at the inlet is at the lowest temperature in spite of the inlet temperature of the fluid at 700 K.
• The velocity is directly proportional to temperature, the temperature of the solid body is found highest in the throat section of the solid body as velocity is high at that point to maintain conservation of mass.
• The velocity is directly proportional to Reynolds number which is, in turn, proportional to convective heat transfer coefficient. Thus heat transfer in that area is highest between fluid and solid. Therefore, the highest Wall heat transfer Coefficient is obtained at that location.
• The Wall Heat Temperature Coefficient raises from 105.7 W/m^2-k to 219.1 W/m^2-k as the mesh number of elements increases.
• There is a change in maximum and minimum velocities from 37 m/s to 41 m/s at the throat section.

Verification of the predictions of HTC obtained from simulations

One can verify the HTC predictions by the results of PHYSICAL EXPERIMENTS and by ANALYTICAL CALCULATIONS.

For obtaining the value of HTC analytically we will need Reynolds number which can be found out by using velocity.

This Reynolds number will be used to calculate the Nusselt number by using the following formula :

where Re is Reynolds Number and Pr is Prandtl Number.

Prandtl Number will be determined by the following formula:

The Nusselt number is related to the heat transfer coefficient (h) as follows:

thus one can determine the value of HTC and verify the predictions.

Accuracy of the prediction will depend on the following:

• Number of elements
• Inflation layers