View this example from presentation slides converted to PDF. Transient thermal analysis of complex systems with significant radiation heat transfer is extremely time consuming using commercial CFD codes. However, the accurate convection and advection capabilities of CFD can be leveraged together with RadTherm software to efficiently produce an accurate transient solution. This sample underbody thermal analysis was conducted using RadTherm thermal analysis software and Fluent CFD software. The computation is complex because it involves cruising, stopping for a hot soak, and moving the vehicle back to cruising speed again.		Figure 1. Partial underbody Thermal Mesh used in RadTherm.
		The model was developed as follows: Preliminary thermal analyses were performed in RadTherm to develop an approximate surface temperature distribution for cruising and hot soak conditions. The convection settings for this preliminary analysis were set using RadTherm's built-in convection library. The surface temperatures were exported to Fluent using File>Export, which can also be set up automatically using MPCCI. Fluent imports these wall temperatures as boundary condition profiles.
Figure 2. RadTherm-predicted surface temperatures imported to Fluent CFD software and displayed as boundary conditions for flow analysis.		Relatively rapid steady-state CFD analysis was carried out to develop accurate forced (cruising) and buoyant (hot idle soak) convection data. H and Tfluid values were then exported from Fluent using (File>Export>RadTherm).

Figure 3. CFD Flow fields in cruising and hot soak conditions. Flow vectors colored by velocity magnitude.

Figure 4. Convection Coefficient Values Mapped onto Geometry. Left: Original CFD Mesh colored by H-values. Right: Thermal Mesh in RadTherm with H values mapped. The mapping is automatically handled in RadTherm.		The convection data was then imported to RadTherm for use at specific times throughout the transient timeline. RadTherm carried out a final high-temporal resolution thermal analysis.

Figure 5. Timeline plot of vehicle speed used for transient analysis. The two CFD runs were imported as shown in Figure 6 below.

By using only two steady state CFD runs, computation time was kept to a minimum. RadTherm interpolated the convection values between specific import times above. The complete transient analysis for 10 minutes of model time at 6-second timesteps was executed by RadTherm in 9 minutes on an AMD 1.9 GHz processor running Windows XP OS.

Figure 6. Timeline showing CFD results import to RadTherm. Only two CFD data sets were used, but were imported to five different points in the timeline.

Results

The conclusion of the analysis show the expected result. The temperature of the system rises and approaches steady state within about five minutes. The hot soak temperature climbs steadily until restarting movement at 7.75 minutes, and by ten minutes, the system is again nearing steady-state cruising values.