Week 4 - CHT Analysis on Exhaust port

Aim : To run simulation to anlyse Conjugate Heat transfer in Exhaust port.

Objective :

• To understand why and where a CHT analysis is used.
• Run the CHT simulation using different turbulence model and justify results.

Introduction :

The Conjugate Heat Transfer (CHT) analysis type allows for the simulation of heat transfer between solid and fluid domains by exchanging thermal energy at the interfaces between them.

Heat transfer coefficient :

The heat transfer coefficient , in Thermodynamics is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat.

`Q=hA(T_2-T_1)`

where

`A= Surface are`

`T_2` = Temperature of surrounding fluid

`T_1` = Temperature of solid surface

CHT analysis is used in :

• Internal Combustion Engines

The internal combustion engine industry is moving toward simulating the entire system rather than independent components. The accuracy of the predicted combustion in the cylinder is dependent on the temperature boundary conditions in the cylinder. By considering heat transfer in the metal components (e.g., the cylinder head, liner, piston, etc.), the cylinder wall no longer has a user-specified temperature, but instead has temperatures predicted as part of the complete-system simulation. This makes the overall simulation results more predictive by reducing the dependence on assumed boundary conditions.

• Pumps and Compressors

Resolving the spatially varying surface temperature distributions in pumps and compressors improves the accuracy of the simulation and provides useful temperature data. For example, in a twin screw compressor, thermal expansion affects the clearance between the rotors. If that effect is not captured, the simulations will incorrectly predict leakage flows and lower the accuracy of your results.

• Electromobility

Effective cooling strategies are critical for various components of emobility systems. Electric motors have high power densities and thus generate a significant amount of heat. If a motor is not properly cooled, high temperatures in the motor can cause the demagnetization of the permanent magnets and the breakdown of the winding insulation.

• Aftertreatment

Predicting and mitigating urea deposit formation is a key challenge in Urea/SCR aftertreatment systems. When urea-water solution is sprayed into the engine exhaust system, it impinges on the wall as a liquid film. The decomposition of urea and the formation of undesirable deposits are highly dependent on wall temperature, and the heat transfer between the film and the wall is important to capture. Predicting the wall temperature helps you identify areas that are vulnerable to wall deposition.

Geometry setup :

• Load the CAD model of Exhaust manifold in Spaceclaim and check for any extra edges, remove them if any.
• Go to Repair and open Volume extract, and select the edges to separate inner and outer volume. Name the outer volume as Solid volume and inner ass Fluid volume and move bothe the volume to new component and delete empty components.
• Go to workbench and open Shareprep, to share the physics between the fluid and solid volume and conformal mesh can be achieved.pink circle are seen after completing the procces.

Meshing :

• Open the mesh tab and select the geometry and named the parts.
• Ansys fluent automaticaly add unnamed part as wall and adiabatic boundary condition is applied.
• Select the mesh and generate base mesh of default size as 150 mm.
• Take mesh sizing as default and body sizing as 12 mm.
• Then generate the mesh.

• Now hide the outer body and select the inner face and named it as inflation layer.Go to inflation layer and select inflation layer and maximum thickness of 5 mm.

• Then take the reference values accordingly.

• Go to viscous model and select K-epsilon standard
• Go to velocity inlet and take inlet velocity magnitude as 5 m/s.

• And temperature as 700K.

CASE 1 :

Boundary conditions :

Inlet velocity = 5m/s

Temperature = 700k

Outlet pressure = 0 pa

Thermal condition = convection

Heat transfer coefficient = 20 w/m2k

Viscous model = K-eplison

Results :

Residual plot :

Temperature contour static pressure :

Velocity streamlines:

Velocity contour :

Temperature contour :

Wall heat transfer coefficient :

CASE 2 :

Boundary conditions :

Inlet velocity = 5m/s

Temperature = 700k

Outlet pressure = 0 pa

Thermal condition = convection

Heat transfer coefficient = 20 w/m2k

Viscous model = K-omega

Results :

Residual plot :

Temperature contour static pressure :

Velocity streamlines :

Velocity contour :

Temperature contour :

Wall heat transfer coefficient :

• How would you verify if the HTC predictions from the simulations are right? On what factors does the accuracy of the prediction depend on?

• The typical flow conditions in automotive exhaust systems produce Re numbers in the range of 10^3 to 5*10^4 . The exhaust flow often enters the region Re< 2300, especially in the exhaust manifold runners. In spite of that, the flow remains actually turbulent, since it has flowed through a substantial restriction. The exhaust valves and the persisting, unsteady, flow pulsation effects do not favour the transition to the laminar region

• According to the Sieder Tate relation which correlates the Nu number with Re and Pr

• `Nu=0.027Re^0.8(Pr)^(1/3)`

• It means Nusselt no. is the function of Reynolds no. and Prandtl no.

• As the turbulent boundary layer in the vicinity of bend at outlet port is thin maximum velocity of flow is observed at bend and hence higher Reynold no.

• Now, `Nu=(h*L)/K`

• As Length of pipe and thermal conductivity are constant, value of h will increases as Nu increases and Nu increases as Re increases.

• As per our results, heat transfer coefficient is high for higher value of velocity and hence from above discussion we think that our results are correct.

Conclusion :

• More refined mesh leads to better results
• Selection of turbulance model should accourding to y+.
• Inflation layers helps to refine the mesh and to get the proper results.