Exhaust Port Challenge
Aim: To simulate flow through an exhaust port/manifold and analyze the heat transfer characteristics.
Introduction and Theory: CHT(Conjugate Heat Transfer) corresponds to the heat transfer in solids and heat transfer in fluids. In solids, conduction often dominates and in fluids, convection dominates.CHT analysis allows us to analyze heat transfer between a solid domain and a fluid domain. We can simulate the flow of fluids of any velocity, temperature, physical and chemical property through or around a solid of any shape, size, physical, and chemical property. This, allows us to analyze the heat transfer characteristics at the interface between both the domains.
Running a CHT requires a multi-region mesh with shared topology between solid and fluid domains, along with a body-fitted mesh at the fluid domain. This is to ensure the smooth transfer of information(heat transfer) at the interface.
CHT analysis is beneficial in accurately predicting solid temperatures and heat transfer capabilities in engineering devices. Using CHT analysis, engineers can optimize the thermal characteristics of the product.
Applications: HVAC, Internal Combustion Engine, Gas Turbines, Heat Exchangers
Modeling Approach:
Pre-Processing:
Step 1: Preparation of geometry
The geometry is prepared by first repairing the geometry and removing certain unnecessary features such as extra edges, split edges, etc. Then the fluid domain is obtained using the Volume extract option. The parts are categorized into a single component and then the share topology option is selected. Under workbench, the share coincident topology is also selected.
Step 2: Meshing
Named Selections:
2.Case I. Baseline Mesh
Mesh properties:
Global Element size=150 mm
Statistics:
The mesh created is depicted below:
Mesh Quality:
1. Element Quality
2. Aspect Ratio
3. Skewness
4. Orthogonal Quality
2.Case II. Refined Mesh
Mesh properties:
Global Element size=10 mm
Capture Curvature Option turned ON.
Smoothing - High
Inflation Layer properties:
First cell height is selected such that to be coupled with the K-omega SST viscous model with a Y+ value of 0.85.
Statistics:
The mesh created is depicted below:
Inflation layer at the interface:
Mesh Quality:
1. Element Quality
2. Aspect Ratio
3. Skewness
4. Orthogonal Quality
Step 3: Solving using Fluent:
Solver-Pressure Based, steady-state solver with Absolute velocity formulation
Viscous Model - K-omega SST
Material properties:
Solid Domain:
Fluid domain:
Case I:
Residual Plot:
Temperature Contours:
Velocity Streamlines:
Velocity at a cut plane section at outlet:
Wall HTC at a cut plane section at outlet:
Case II:
Residual Plot:
Temperature Contours:
Velocity Streamlines:
Velocity at a cut plane section at outlet:
Wall HTC at a cut plane section at outlet:
Note: Since the inflation layer is quite thin, we are not able to observe the HTC at the interface
Result and Conclusion:
From the above results, we can conclude that refined mesh produces accurate results compared to baseline mesh. Especially, since there were no inflation layers in baseline mesh, it wasn't able to capture the heat transfer coefficient at the interface properly. (HTC values were calculated using function calculator and using AreaAvg option at the interface in CFD-Post.)
The velocity at the outlet region is more or less the cumulative effect of the four inlet velocities. The velocity at the inner corner is generally greater than outside as can be observed from the velocity contours. Wherever the fluid velocity is high, the heat transfer is pronounced, thus a high heat transfer coefficient.
Verification of HTC result:
We can verify HTC results by performing a grid independence test, where we will run the simulation for various mesh sizes, ranging from coarse to fine.
OR, by calculating Nusselts number by using the formula(Dittus–Boelter equation)
`Nu=0.023*Re^(4/5)*Pr^0.4`
and equating it to
Nu= hL/K
to find the heat transfer coefficient "h".
Factors affecting the accuracy of the simulation:
1. The number of elements in the mesh - finer mesh generally gives accurate results. Also, curvature and other geometric properties need to be properly captured.
2. Inflation layers - Body fitted mesh around geometry gives an accurate analysis of geometry.
3. The fluid properties around the solid domain along with the turbulence model.
4. Mesh should be conformal at the interface of both the domains.