Analysis of bifurcation, transient mass-flow paths, and heat transfer in the flow excursion of cracking hydrocarbon fuel in parallel cooling channels

Thermal protection is a major technical bottleneck in the development of scramjet engines. However, flow excursion may occur in regenerative cooling channels during the trans-critical and pyrolysis processes of hydrocarbon fuel. Flow excursion can cause significant deviations in mass flow rate and temperature, posing serious threats to the safety of cooling channels. Based on the one-dimensional flow heat-transfer model and the lumped-parameter transient model, this study theoretically derives the formation mechanism of the negative-resistance characteristic. Results show that the partial derivative of fuel density with respect to mass flow rate is an effective indicator of channel resistance: large values of the derivative near the outlet lead to negative-resistance behavior. Bifurcation analysis is employed to investigate system stability. Using heat flux as the bifurcation parameter, saddle-node and subcritical pitchfork bifurcations are observed in the trans-critical region, while the pyrolysis region is dominated by supercritical pitchfork bifurcations. With back pressure as the bifurcation parameter, the system mainly exhibits supercritical pitchfork bifurcation behavior. Further analysis of mass flow paths under varying operating conditions reveals the phase evolution of the working fluid and shows that coupling between saddle-node and subcritical pitchfork bifurcations induces significant hysteresis. Quantitative analysis of wall temperature deviations indicates smaller temperature differences in the channel midsection due to enhanced trans-critical heat transfer, whereas larger deviations concentrate at both ends. During pyrolysis, outlet temperature deviations are somewhat alleviated. As heat flux increases, the mass flow path may enter large-deviation regimes that cause extreme temperature deviations, with maximum wall temperature differences exceeding 700 K.