The influence of coking on heat transfer in turbulent reacting flow of supercritical hydrocarbon fuels

Significant influence from coking (aggregation and formation of a non-desirable carbonaceous porous layer on a solid metal surface) on the heat transfer in a cooling system of hypersonic aircraft with hydrocarbon fuel as the coolant is observed. In the present work, an explanation was proposed to understand the influence mechanism of coking on the heat transfer in cooling channel. In addition, a numerical model was further established to verify the explanation by comparing the heat transfer in cooling channel with different thicknesses of coke layer. The results indicated that critical thicknesses of coke layer with respect to effective thermal conductivity (delta(c)(cl,lambda)) and heat transfer (delta(c)(cl,h)) existed. More specifically, the heat transfer was noticeably enhanced when delta(cl) < delta(c)(cl,h), while the heat transfer is deteriorated as delta(cl) > delta(c)(cl,h). A transition thickness with length of (delta(c)(cl,h)-delta(c)(cl,lambda)) existed between delta(c)(cl,lambda) and delta(c)(cl,h) under the combined influences of convection of core flow and coke layer. By comparing the heat transfer in cooling channel after coke deposit at different inlet Reynolds number, it was further found that delta(c)(cl,lambda) and delta(c)(cl,h) decrease with increasing the turbulent intensity. This implied that the influence of inlet Reynolds number on the coke deposition rate and turbulent heat transfer were dissimilar. This study is expected to provide further insight into the optimization of cooling channel for the purpose of reducing the heat transfer deterioration caused by coke. (C) 2019 Elsevier Ltd. All rights reserved.