Exhaust manifold CHT analysis

Objective: To perform a conjugate heat transfer analysis on exhaust manifold.

CONJUGATE HEAT TRANSFER ANALYSIS

Conjugate heat transfer analysis is done to study the thermal interaction between solids and fluids, which is caused due to temperature differences between solids and fluids. In solids conduction is the mode of heat transfer and convection in fluids. In CHT analysis multiregion mesh is created to have a clear definition of the interfaces in the computational domain.

CHT analysis are carried out for some of the typical problems such as:-

• Heat Exchangers
• Cooling of electronic equipment
• HVAC systems
• Overall heat transfer coefficient, e.g. for a car radiator.
• Highest (or lowest) temperature in a system, e.g. in a gas turbine, chemical reaction vessels, food ovens.
• Temperature distribution (related to thermal stress), e.g. in the walls of a spacecraft.
• Temperature response in time dependent heating/cooling problems, e.g. engine cooling.

CHT analysis of Exhaust port:-

In this project conjugate heat transfer analysis on Exhaust manifold is carried out to find out the velocity, temperature and heat transfer coefficient. Two mesh configurations have been used, one is baseline mesh and another is refined mesh.

Geometry of Exhaust manifold:-

Fluid volume of exhaust manifold:-

1. Baseline Mesh

residual plot-

Outlet temperature plot-

Temperature contour-

Streamlines-

velocity contour-

Wall heat transfer coefficient-

2. Refined mesh

In this mesh configuration the elements size in the solid part has been reduced and inflation layer has been created in the fluid part near the wall to effectively capture the boundary layer. total no. of elements are 835940.

Residual plot-

Outlet temperature plot-

Temperature contour-

Streamlines-

Velocity contour at throat-

Wall heat transfer coefficient-

Comparison of K-ε and K-ω SST model-

convergence plot for both the models for refined mesh has been shown below:-

1) K-EPSILON MODEL

2) K- OMEGA SST MODEL

K- epsilon model is generally used for fully turbulent and non separated flows, whereas K- omega sst model is suitable for complex boundary layer flows with adverse pressure gradient and it is generally used for low reynolds number. In the present case the flow is fully turbulent and there is no separation of flow, hence the K- epsilon model is the best choice for solving the RANS equation. Moreover, from the residuals plot it can be observed that with the K-epsilon model the solution is getting converged at faster rate and the residuals value is also low as compared to the K-omega SST model, which signifies the low compuation time and stability of results with the K-epsilon model.

Observation table

mesh type = tetrahedral		h (W/m^2 K)
	no. of elements	outlet temp	surface (h)	wall (h) max
coarse	137994	680.9976	21.91513	106.43
medium	251717	678.016	21.35658	210.365
fine	835940	677.694	21.39792	215.758

The HTC values obtained from the numerical simulations can be verified by comparing it with some empirical formulas derived for the turbulent flow. One of the empirical relation is Dittus- boelter equation, which gives the heat transfer coefficient h for heat transfer from the fluid flowing through a pipe to the pipe wall. It is represented as :-

`Nu = 0.023 Re^0.8 Pr^n

the value of Nu can be equated to Nu=hl/k and h can be calculated.

Various factors on which the accuracy depends are:-

1) The mesh should be fine, as finer mesh gives more accurate results.

2) The inflation layer should be created with proper first cell height to effectively capture the boundary layer.

3) The mesh should be conformal as the conjugate heat transfer analysis involves both solid and fluid domain and it is must for better interpolation.

4) The selection of turbulence model also plays an important role in the accuracy of results.