Exhaust port

 Aim: Perform a CHT analysis with the k-epsilon and k-omega SST model and explain which one is best refined & best possible mesh on an exhaust manifold.

Objective:-

Show the velocity, temperature, and heat transfer coefficient plots and explain it.

Exhaust manifold

In automotive engineering, an exhaust manifold collects the exhaust gases from multiple cylinders into one pipe. The word manifold comes from the Old English word manigfeald (from the Anglo-Saxon manig [many] and feald [fold]) and refers to the folding together of multiple inputs and outputs.

Exhaust manifolds are generally simple cast iron or stainless steel units which collect engine exhaust gas from multiple cylinders and deliver it to the exhaust pipe. For many engines, there are aftermarket tubular exhaust manifolds known as headers in American English, as extractor manifolds in British and Australian English,[1] and simply as "tubular manifolds" in British English. These consist of individual exhaust headpipes for each cylinder, which then usually converge into one tube called a collector. Headers that do not have collectors are called zoomie headers.

Description of why and where a CHT analysis is used.

Computational Fluid Dynamics (CFD) and more specifically conjugate heat transfer (CHT) analysis can accurately predict heat transfer by simultaneous solving all the relevant solid and flow field heat transfer processes, for example: conduction through solids, free and forced convection in the gases/fluids and thermal radiation.

The CHT approach has an advantage over FE thermal analyses in that wall heat transfer coefficients and their local variations on surfaces are directly calculated within the model rather than based on simplified empirical calculations. The CHT approach therefore has benefits for those applications where heat transfer is either non-uniform or difficult to calculate empirically.

The heat transfer projects performed by TriVista have covered a wide range of aerodynamic and hydrodynamic industrial applications but projects typically had objectives such as

• Temperature distribution analysis.
• Heat flux rate analysis – calculation of wall heat flux rates or heat transfer coefficients.
• Visualisation of heat transfer features for improved understanding of system flow phenomena.
• Identification of localised ‘hot spots’ or heat sinks in checking against specified requirements.
• Evaluating variation in temperature peaks for different operating conditions/ failure modes.
• Assessing insulation or heat shield effectiveness and optimising of material selections.
• Coupling CHT with FE analysis to evaluate thermally induced stresses, expansion or fatigue.
• Temperature field analysis
• CFD thermal analysis
• Heat transfer coefficient analysis
• Heat transfer rate analysis

Import the given exhaust manifold

Extract the volume from the given manifold

Enabe the share topology for merge the fluid volume and the solid volume, there after in workbench click on the share.

Base line mesh

Before meshing first mension the name of the exhaust manifold like inlet, outlet, outer-wall-heat convection then apply generate mesh whose element size is 150mm.

Cross-sectional view

Element quality

From the above element quality plot absorve that 88% mesh element are good quality.

Setup

Type-Pressure based

Velocity formulation -Absolute

Time steady -state

Energy based solver 

viscous model-k-epsilon 

Material

Fluid-Air

Solid-Aluminium

Velocity-5 m/s

Inlet temperature-700K

Residual plot

When number of iteration increases residual plot continiously decreases up to 300 iteration there after solution converged.

Velocity magnitude plot

From the above plot we can see that velocity is suddenly increases there after velocity is constant when number of iteration increases.

Temperature magnitue plot

In this plot also temperature is suddenly increases there after tempereture is constant when number of iteration increases.

Wall temperature contour

Velocity and stream line contour

Above velocity and stream line contour show the variation of velocity along the plane red region show the higher velocity, blue lower and green show the medium velocity. In the outlet cross sectional number of stream lines is more with respect to inlet, it shows the velocity of outlet is higher than inlet.

Heat transfer coefficient contour

Refined mesh line set-up

For k-epsilon and k-omega 

Refined mesh with inflation layer

Result compression of k-epsilon and k-omega for base line and refined mesh setup for velocity , temperature and heat transfer coefficient.

Residual

Temperature plot

Velocity plot

Temperature contour

Velocity contour

Stream line contour

Wall heat transfer coefficient contour

Conclusion

1. In base line set up heat transfer coefficient is lower for both the k- epsilon an k-omega sst model because no inflation layer is added in the base line set up.

2.From the above table we can say that the value of heat transfer coefficient is accurate for the refined mesh setup k-omega sst model. This variation is occurs for the finer and the addition of inflation layer ,for k-epsilon and k-omega value of yplus lies between 0.25 to 5 mm.

3. Both mesh line and the refined mesh setup temperature and velocity is slightly varying that means for temperature and velocity is independent to the mesh size.

4. After 25 iteration  velocity is get converged and  temperature is almost constant for base line setup after 50 iteration.

5. Refined mesh variation of temperature is occurs for both k-epsilon and k- omega model.