Conjugate Heat Transfer (CHT) analysis of exhaust manifold

Skills or knowledge: Knowledge of heat transfer and fluid flow

Software or programs used: CFD, Fluent, CHT analysis, heat transfer analysis

• Project: Conjugate heat transfer analysis exhaust manifold

• Introduction :

• The term conjugate heat transfer (CHT) is used to describe processes that involve variations of temperature within solids and fluids, due to coupled thermal interaction. The exchange of thermal energy between the two physical bodies is called the study of Heat Transfer, the rate of transferred heat is directly proportional to the temperature difference between the bodies.

• Conjugate heat transfer corresponds with the combination of unsteady heat transfer in solids and unsteady fluid flow. In solids, conduction often dominates whereas in fluids, convection usually dominates. Efficiently combining heat transfer in fluids and solids is the key to designing effective coolers, heaters, or heat exchangers. Heat transfer in solids and in fluids are coupled in the majority of applications. The time scales of the physics are quite different and the selection of a time step plays a critical role in CHT analysis.

• Typical examples on which CHT analysis is performed are Exhaust manifolds, car radiators, heat exchangers, coolers, chemical reaction vessels, engine cooling and high-speed aerodynamics study, etc.

• Objective :

• 1. Performing CHT analysis on exhaust manifold with different sizes of mesh.

• 2. Evaluate velocity, temperature & HTC (heat transfer coefficient) produced in the exhaust manifold and explain the results.

• 3. Accuracy of HTC predictions in the exhaust manifold.

• Geometry Creation & Ansys space-claim:

• There are 4 inlets and 1 Exhaust pipe in the Geometry.

• After uploading it in Ansys space-claim, Fluid volume is extracted.

• Some edge repairs were done & topology was set to share, and share prep was run to ensure the transfer of information between the Fluid & Solid-body to be smooth & uniform.

• Meshing :

• A Grid independency study was carried out to determine the optimal grid size required for the analysis and the optimal grid having 3 million elements was chosen for the study.

• Physics models and boundary conditions which were used in the analysis are given below:

• 1. Solver: Pressure Based (incompressible flow)

• 2. Time: Un-Steady, (2nd order time accurate)

• 3. Energy: On (for calculation of temperature)

• 4. Turbulence model : K epsilon (2eqn)

• 5. Material of Fluid - Air

• 6. Material of Solid: Aluminium

• 7. Cell zone - Volume: Fluid Solid: Solid

• 8. B.C : inlet - 5 m/s Temp : 700k in all 4 inlets

• Outer-wall-convection (Solid-body) - Heat transfer Coefficient 20 w/m^2-k (Assumed)

• Outlet: Gauge pressure 0 pascal: 1 atm

• 9. Hybrid Initialization was used.

• 10. Time -step: 1e-5 s with 40 flow solver inner iterations.

• Initial steady flow analysis.

• An Initial steady solution for the fluid was obtained, which will be used as the initial solution for the transient analysis.

• Solvers and turbulence models used:

• The Navier- Strokes equation will be solved numerically by the finite volume method (FV) in the flow-solver, while the heat conduction equations are solved in the solid part considered. The K-Epsilon turbulence model was chosen for the study, as for the study considered, it is accurate enough. But for situations of high shear stresses and huge thermal gradients involved, the K-Omega SST model should be preferred for accuracy.

• Time -accurate CHT analysis and observations from the contours of the field variables :

• The fluid develops a boundary layer due to the velocity gradients created near the wall (no-slip boundary). Due to the convective boundary specified at the fluid-solid interface, the heat is transferred from the hot fluid to the wall, which can be observed from the temperature contours of the exhaust manifold.

• The velocity contours show the fluid entering the 4-inlets converges near the outlet thus increasing the velocity, which gets reflected in the increase of heat transfer and hence the exhaust manifold wall temperature. This is due to increase in the convective heat transfer coefficient with fluid velocity.

• Generally, the heat transfer coefficient will not be a known quantity, so a convective boundary with 0 (zero) convective heat transfer coefficient will be used and the coupled fluid and solid CHT analysis will be used to determine the coefficient and the increase of wall temperature due to heat transfer. But here a coefficient value is assumed for the simplification of the analysis. This value of the heat transfer coefficient should be determined by solving the convective heat transfer equations for the Nusselt number.

• HTC Predictions & its factors for accuracy :

• 1. HTC varies significantly for different applications, materials, conditions, geometry etc.

• 2. Htc predictions depend upon theoretical/analytical calculations, to begin with, giving us a certain baseline target to achieve. There are numerous correlations for internal, and external flows, Compositions to factor in conduction, and convection of geometry. Whether it is forced, natural, flow is laminar, turbulent, different fouling factors for different applications, etc

• 3. Then there is experimental data, which is compared with analytical values for specific conditions.

• 4. The convergence of the flow solver within each time step should be ensured and the determination of the time step for a CHT problem is really a challenge but is vital for the accuracy of the analysis.

• 5. As far as factors for htc predictions accuracy goes, it's the skill of the application engineer that comes in handy, selecting proper turbulence models, fine computational grid, resolving the thermal and velocity gradients, transient simulations to predict time accurate data and observe changing htc with time.