Week 4 - CHT Analysis on Exhaust port

Aim:

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

Objective:

1.To understand why and where a CHT analysis is used.

2.Run the CHT simulation using different turbulence model and justify results.

Introduction:

Conjugate Heat Transfer (CHT):

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.

where a CHT analysis is used

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.

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.

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.

Heat transfer coefficient (h):

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

the heat transfer rate is:

Setup:

geometry:

Mesh:

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.

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.

Solver:

Solver: Presure based steady state solver.

Turn on energy equation for cht problem and also setup proper boundary condition and viscous model

Results:

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

Nusselt number is given by:

where:

D is the inside diameter of the circular duct

Pr is the Prandtl number

n=0.4 for the fluid being heated, and n=0.3 for the fluid being cooled.
Nu= Nusselt's Number
Re = Renold's number

We know that the Nusselt's number is a function of Reynold's number, which in turn is a function of flow velocity.
Therefore, HTC is directly proportional to flow velocity.
We can see from the images above that both the HTC and velocity are maximum near the outlet.
Hence, the HTC predictions from the simulations are correct.

Factors affecting the prediction are as follows:
Element Size and Quality = The finer the mesh, the accurate the results.
Inflation Layer = It increases the quality of mesh near the boundary interface and helps to capture smooth plots of different quantities.
Mesh at the solid-fluid interface = The mesh at the solid-fluid interface must be conformal.