1 Transient heat transfer modelling in automotive exhaust systems P A Konstantinidis, G C Koltsakis and A M Stamatelos Laboratory of Applied Thermodynamics, Mechanical Engineering Department, Aristotle University of Thessaloniki, Greece Abstract: Transient heat transfer computations in automotive exhaust systems are increasingly employed in the design and optimization phases. The complex geometry of the exhaust line and the special flow conditions complicate the problem of accurately estimating several important heat transfer parameters. This paper initially summarizes the current status of knowledge regarding heat transfer phenomena in automotive exhaust systems. A comprehensive transient computer model covering all exhaust piping configurations (single wall, double wall with air gap or insulation) is presented. A novel solution procedure is proposed, resulting in significant savings in processing time. Two-dimen- sional heat transfer in connecting flanges is also accounted for. The model is validated with the help of full-scale measurements on vehicles. Examples are presented, illustrating the application of the model in the comparative assessment of different exhaust configurations. In conjunction with existing models, which simulate the operation of three-way catalytic converters and of other exhaust gas after- treatment devices, the model can be integrated in a CAE (computer aided engineering) package for the support of exhaust system design optimization. Keywords: heat transfer, exhaust systems, mathematical modelling, automobile NOTATION s thickness t time T temperature Bi Biot number =h Dr/l u exhaust gas velocity CAF convective augmentation factor V volume c p specific heat capacity x axial distance from entrance d diameter f pipe friction factor Fo Fourier number =a Dt/Dx2 Greek letters F v view factor g gravitational acceleration a thermal diffusivity h convective heat transfer coefficient b thermal expansion coefficient l length e emissivity factor (radiation) m mass transfer rate l heat conduction coefficient n vertical direction L l ins /l p NEDC new European driving cycle n kinematic viscosity Nu Nusselt number j shape factor =Dr/Dx NTU number of transfer units r density Pr Prandtl number s Stefan–Boltzmann constant q heat transfer rate r radius R heat transfer resistance Subscripts Ra Rayleigh number Re Reynolds number amb ambient cv convection The MS was received on 29 September 1995 and was accepted for publication on 11 June 1996. eff effective C11395 © IMechE 1997 Proc Instn Mech Engrs Vol 211 Part C