| This model of the month demonstrates the power of RadTherm's CFD import function. An iterative method of analysis that utilizes RadTherm with supporting CFD analysis was used to predict the underbody surface temperatures of a moving automobile. RadTherm's 1-D fluid nodes were used to model the flowing exhaust gas and served as the main thermal input to the system. CFD results were imported for the external exhaust and underbody convection.
A simple underbody was created with common exhaust components. A vehicle body was fitted to provide reasonable air flow under the vehicle at cruising speeds.
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Isothermal Plumes of Flow
  
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Initial CFD Analysis - Develop Flow Convection Data
Although it is computationally advantageous to perform the first analytical step in RadTherm, the meshing for this model was executed in Fluent's Gambit, and thus we implemented an export to RadTherm using Fluent's built-in RadTherm Export function. This function exports a Patran Neutral file that contains surface mesh geometry and convection data for each element (convection coefficient and fluid temperature).
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The CFD model included the following flow parameters:
| Speed |
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120 km/h |
| Turbulence at Inlet |
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3% |
| Hydraulic Diameter |
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20m |
| Vehicle Angle of Attack |
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0° |
 
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Above: Flow ribbons predicted by Fluent 6.1 under the above operating conditions.
Below: Predicted convection coefficient distribution on the underbody in the first iteration.
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Above: Fluent 6.1's Built-in export to RadTherm.
RadTherm imported the patran neutral file directly and included the convection data. Surface conditions and materials were defined, and the one-dimensional flow down the exhaust interior was set up using the following inlet conditions:
| Flow Rate |
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2000 Liter per Minute |
| Inlet Temperature |
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537°C |
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Above: Temperature distribution over the exhaust exterior after convergence.
The library convection settings in RadTherm were set to Duct flow and the pipe diameter was input. The flow rate was taken from the advection link settings.
After the complete thermal analysis was performed in RadTherm using imported CFD for the exterior and 1-D fluid analysis for the flowing exhaust, the wall temperature distribution was exported from the RadTherm file using a stand-alone utility called TDF2Fluent. TDF2Fluent exports a point cloud of wall temperatures for use by Fluent as a BC profile. The TDF2Fluent utility is available for free download. Please download the file via the link and the end of this article.
Sample Results
Five to six Iterations were required to bring the model to full convergence. The first run in Fluent was far from accurate because there was not initial wall temperature data. Hence, the first RadTherm analysis had fluid temperatures that were unrealistically low. The animation below illustrates the convergence process.
Above: The convergence process. Click to open a larger version.
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Above: Temperature distribution along the exhaust line shown for iteration #1 (dark) and #6 (magenta).
Above: Plot of exhaust fluid temperatures.
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