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Courtesy of Volvo Car Corporation
The Volvo Car Corporation in Göteborg, Sweden needs to be able to
predict the performance of new exhaust system designs before they are
built and tested. In particular, the ability to detect localized hot spots
on the pipework and predict the temperature distribution in rubber isolators
in the exhaust system during normal driving conditions is of interest.
Front and rear isolator RADTHERM temperature predictions.
Volvo exhaust system showing he exhaust isolators ear the silencer.
Exhaust duct CFD temperature predictions
To model the thermal characteristics of an automotive exhaust system,
a very large CFD simulation involving internal and external flow, along
with extensive radiative heat transfer calculations, would be required.
To circumvent this time-consuming approach, Volvo engineers have innovated
and opted for a coupled simulation using FLUENT and RADTHERM. FLUENT is
used for the exhaust system internal flow with assumed heat transfer coefficients
and planar conduction. The simulation typically uses large meshes of approximately
1.0 M cells, and provides surface temperatures for the exhaust system
elements. A separate RADTHERM calculation uses these temperatures as inputs
to model the isolator elements. Volvo engineers had to overcome some difficulties
with this approach, such as estimating the surrounding air temperatures,
heat transfer coefficients, and material data (emissivity and absorptivity)
on the surfaces of the exhaust system. While these uncertainties give
rise to inaccuracies in the absolute temperature predictions, the trends
in the temperature distribution and in particular the positions of local
hot spots are usually very reliable. This project is ongoing and Volvo
engineers intend to validate the technique against experimental measurements.
They also plan to evaluate the sensitivity of the RADTHERM calculations
to boundary conditions supplied from the CFD simulations.
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