Week 4 - CHT Analysis on Exhaust port

Conjugate Heat Transfer

The contemporary conjugate convective heat transfer model was developed after computers came into wide use in order to substitute the empirical relation of proportionality of heat flux to temperature difference with heat transfer coefficient which was the only tool in theoretical heat convection since the times of Newton. This model, based on a strictly mathematically stated problem, describes the heat transfer between a body and a fluid flowing over or inside it as a result of the interaction of two objects. The physical processes and solutions of the governing equations are considered separately for each object in two subdomains. Matching conditions for these solutions at the interface provide the distributions of temperature and heat flux along the body–flow interface, eliminating the need for a heat transfer coefficient. Moreover, it may be calculated using these data.

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.

Case 1

Base line mesh

No .of elements 136314

Boundary condition

Inlet velocity = 5m/s

Inlet temperature = 700k

Pressure at outlet = 1atm

Fluid - air

soild - aluminum

CHT analysis on the exhaust port

The figure represents the exhaust manifold of 4 cylinder engine

The geometry is prepared by using a volume extraction tool i.e, the fluid volume is extracted.

Now enable share topology in order to have conformal mesh between solid and fluid volume.

Geometry Setup:

• Load the CAD model of Exhaust manifold in Spaceclaim and check for any extra edges, remove them if any.
• Go to Repair > 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 > 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.

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.

Solver:

Presure based steady state solver.

Boundary Conditions:

Inlet:

Velocity = 5 m/s

Temperature = 700 K.

Outlet: Pressure = 0 Pa (Gauge pressure)

Outlet wall convection:

Thermal condition: Convection

Heat transfer coefficient = 20 W/m2K.

Simulation is ran for 150 iterations.

TURBULENT MODEL: K-epsilon

Result:

Residual Plot:

Temperature contour:

Velocity plane:

Pressure plane:

Velocity streamlines:

Wall Heat transfer coefficient:

Case 2

Refined mesh:

no .of elements 389641

5mm inlation layer is applied for fluid volume

no of layer is 5

Sizing>>body sizing >> 10mm

RESULTS:

Residuals:

Temperature contour:

Velocity plane:

Pressure plane:

Velocity Streamlines:

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

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.

Factors affects the quality of prediction:

• More refined mesh leads to the better results.
• Use inflation layers and apply proper Boundary condition.
• Proper selection of turbulence model according to y+ values.

Conclusion:

1. Correct choosing the value y+ which satisfies the turbulent model, by effectively capturing the velocity and temperature at the boundary wall interface.
2. Meshing at the junction of the solid and fluid domain. With a definite value of y+, the no of inflation layers and the growth rate determines the smooth flow of heat transfer between the two zones.