This paper describes the further development of an exhaust system model based on the experimental characterization of heat transfer in a series of different pipe sections. Building on previous work published in this journal by the present authors, this study was undertaken to improve the operating range, accuracy, and usability of the original model as well as to introduce the ability to model twin-skin exhaust sections with an air gap.

Convective heat transfer relationships for nine stainless steel exhaust bend sections of various wall thicknesses and radii were experimentally characterized over a range of steady state conditions. In each case a correlation between the observed Reynolds number Re and the Nusselt number Nu was developed. Based on measured experimental data, a generic model was built using MATLAB/Simulink; this model is capable of predicting the relationship between the Nusselt number and the Reynolds number for previously unseen pipe geometries falling within the experimental design range. To develop the usefulness of the model further, 15 twin-skin test sections, intended to represent a range of geometries applicable to production automotive gasoline exhaust systems, were also fabricated and characterized. Within the model, both skins of each pipe section were split into five axial elements and five radial elements with the inner and outer skins linked via the modelling of free convection and radiation between them.

The predicted Reynolds–Nusselt relationships for each bend section and twin-skin configuration were validated using transient experimental data over a portion of the US06 drive cycle. The final model demonstrated an improved accuracy of exhaust gas temperature predictions, compared with the previous model iterations, with typical errors of less than ±1 per cent and a mean error over the US06 cycle of +0.2 per cent.

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An empirical approach to predicting heat transfer within single- and twin-skin automotive exhaust systems