Numerical study on the effect of pyrolytic coke deposition of n-decane on flow and heat transfer behavior in porous media

Transpiration cooling method using endothermic hydrocarbon fuels offers high cooling efficiency for hypersonic vehicles but is faced with the challenge of coke deposition in porous media. A two-dimensional transpiration cooling model considering detailed pyrolysis mechanism and coking processes of n-decane is presented in this study. The influence of pyrolytic coke deposition on the porous media properties and transpiration cooling performance is investigated. Under the influence of inflow mainstream at 1600 K for 30 min, the specific coke deposition mass reaches 2.1 μg·cm⁻² in the porous zone. The local porosity and permeability of the porous media decrease by 60% and 91.8%, respectively. Moreover, the flow distribution non-uniformity coefficient of the coolant increases by 28%. Increasing the sintered particle diameter from 50 μm to 150 μm results in a decrease by 80% in local coke deposition and a decrease by 16.6% in the flow non-uniformity coefficient. Moreover, heat transfer in the boundary layer exhibits opposite variation trends with time for the proximal and the distal ends of the outlet surface of the porous media due to coking-induced coolant migration. The mechanistic analysis shows that the effect of particle diameter on coking rate is initially dominated by the permeability-induced temperature difference but then dominated by the increasing difference in coolant residence time as coking process becomes significant. Specifically, coking rate is more sensitive to local temperature and flow residence time when very small sintered particles are used. For coking time of 30 min, the average cooling efficiency with the sintered diameter of 50 μm and 150 μm decreases by 5% and 2%, respectively. The research findings contribute to a deeper understanding of the impact of pyrolytic coke deposition on flow and heat transfer behaviors in the transpiration cooling process using hydrocarbon coolants.