Exhaust Port Challenge

Objective:

To perform a steady state Conjugate Heat Transfer (CHT) analysis on an exhaust manifold using ANSYS Fluent.

Conjugate heat transfer:

Heat transfer in solids and heat transfer in fluids are combined in the majority of applications. This is because fluids flow around solids or between solid walls and because solids are usually immersed in a fluid.

An accurate description of heat transfer modes, material properties, flow regimes, and geometrical configurations enables the analysis of temperature fields and heat transfer. Such a description is also the starting point for a numerical simulation that can be used to predict conjugate heat transfer effects or to test different configurations in order, for example, to improve thermal performances of a given application.

The CHT approach has benefits for those applications where heat transfer is either non-uniform or difficult to calculate empirically. Not only that, but conjugate heat transfer analysis can accurately predict heat transfer by simultaneous solving all the relevant solid and flow field heat transfer processes.

The analysis type Conjugate heat transfer (CHT) allows the simulation of the heat transfer between Solid and Fluid domains by exchanging thermal energy at the interfaces between them.

In solids, conduction often dominates whereas, in fluids, convection usually dominates. Conjugate heat transfer is observed in many situations. For example, heat sinks are optimized to combine heat transfer by conduction in the heat sink with the convection in the surrounding fluid. Other examples include simulation of heat exchangers, and general-purpose cooling and heating systems.

Examples of conjugate heat transfer analysis:-

HVAC, Heat exchanger, Tank, Internal combustion engine, Heat sink, Boiler, Reactor, Heat pipe, Turbocharger.

The outcome of CHT:

• Temperature distribution analysis.
• Heat flux rate analysis – calculation of wall heat flux rates or heat transfer coefficients.
• Visualization of heat transfer features for improved understanding of system flow phenomena.
• Identification of localized ‘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 optimizing 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

Geometry:

The given model of exhaust manifold consists of 4 individual inlets converging into one outlet.

The fluid domain is extracted using the extract tool in the Spaceclaim. The image is as follows:

For proper meshing of the fluid domain and solid part of the manifold share topology is used in Spaceclaim. The image of the geometry of the manifold from Spaceclaim is as follows:

Baseline Mesh:

Baseline mesh is the mesh where no local setting has been done and mesh is completely system generated. The number of elements is 138379. The fluent setting for both the mesh is the same where the k-epsilon standard model is selected and parameters are solved by first order approximation.

Case - 1 K-Epsilon :-

Basic element size - 150 mm

Body sizing in solid zone with element size - 20 mm

Inflation Layer - No. of layers - 6

Total thickness with maximum thickness - 3.0 mm

Yplus value- 250

Streamline :

Wall heat transfer heat coefficient :

Temperature at throat :

Velocity at throat:

Heat transfer coefficient plot:

Scaled residuals:

y plus value :-

Case- 2 K-Omega :-

Yplus value- 2.5

Basic element size - 150 mm

Body sizing in solid zone with element size - 20 mm

Inflation Layer - No. of layers - 6

Total thickness with maximum thickness - 0.4 mm

Streamline :

Temperature at throat :

Velocity at throat:

Heat transfer coefficient plot:

Scaled residuals:

Y-plus plot :-

Factors that affect the analysis:

Mesh: Finer the mesh, more accurate the solution.

The conformal mesh i.e. the proper mesh connectivity of solid surface to fluid domain.

Inflation layer in mesh i.e. better mesh at boundary of solid and fluid surface gives better accuracy of the heat transfer calculations.

Conclusion :-

• As we compare the result K-omega will given the correct value for wall heat transfer coefficient.
• We have to maintain the y+ value.
• When we compare to k-epsilon value are lower so we have to k-omega for best result