Modelling of the gas to fuel droplets radiative exchange

The results of the implementation of the thermal radiation transfer model into the commercial computational fluid dynamics (CFD) code VECTIS of Ricardo Consulting Engineers and its application to modelling the fuel droplets radiative exchange with gas in a Diesel engine cylinder are reported. The P-1 model with Marshak boundary conditions at the droplets' surfaces is shown to be the most suitable for modelling the thermal radiation transfer in a Diesel engine where the contribution of soot allows the combustible charge to be approximated as an optically thick medium. The results of the implementation of this model were tested for the idealised case where droplet evaporation and burning are ignored, and the gas temperature is kept constant. In this case the equation for droplet heating in gaseous media, taking into account the effects of convection and thermal radiation, is resolved analytically and numerically (based on the VECTIS CFD code). Analytical results are obtained in two limiting cases where the effects of radiation dominate over or are dominated by the effects of convection. Possible time dependence of gas temperature, radiation temperature, and/or convective heat transfer coefficient is accounted for. Solutions obtained for spherical droplets are generalized for the case of droplets having the forms of prolate and oblate spheroids. Good agreement between the analytical and numerical results endorses the approximations on which the analytical solutions are based and the VECTIS numerical results. It is shown that thermal radiation noticeably accelerates the droplet evaporation, which is reflected in a more rapid decrease in the droplet diameter when compared to the case, when thermal radiation is ignored. The asymptotic values of droplet surface temperature are shown to be independent of thermal radiation. (C) 2000 Elsevier Science Ltd. All rights reserved.