Heat transfer augmentation, endothermic pyrolysis and surface coking of hydrocarbon fuel in manifold microchannels at a supercritical pressure

Regenerative cooling technology, which utilizes onboard fuel as a coolant, is widely employed in the thermal protection systems of airbreathing combined-cycle engines. However, current isoparallel straight cooling channels often encounter flow maldistribution and inefficient heat sink utilization. In this work, manifold microchannel (MMC) are proposed as an alternative cooling solution to address flow maldistribution and enhance local heat transfer. Three-dimensional numerical simulations, based on Reynolds Averaged Navier-Stokes (RANS) equations, were performed to investigate flow dynamics, heat transfer, pyrolysis, and surface coking of hydrocarbon fuel within MMC cooling panels operating at supercritical pressures, following thorough model validation. The results demonstrate significant improvements in heat transfer with MMC channel compared to conventional parallel cooling channels. These enhancements are attributed to flow reorganization and fuel impingement cooling effects, although such improvements are not evident in channels with only one MMC unit. Additionally, increasing the number of MMC units leads to lower and more uniform wall temperature distributions. Under the tested conditions, the average heated wall temperature is reduced by up to 150 K, and the heat transfer coefficient is increased by approximately 50 % compared to the isoparallel channel. The MMC structure can maintain its effectiveness across a wide range of operating conditions with the inlet Reynolds number ranging from 2394 to 7182 and heat flux ranging from 0.5 MW/m² to 2.0 MW/m², for both uniform and non-uniform heat flux boundary conditions. In regions with low temperatures, where momentum transport dominates heat transfer, MMC structure plays a critical role in enhancing heat transfer. However, in the high-temperature cracking zone, where thermal diffusion governs the heat transfer process, the effects of turbulence attenuation caused by the MMC structure becomes less pronounced, leading to reduced wall temperature differences. The manifold structure induces flow reorganization and impingement effects, redistributing high-temperature fluid from near the heated wall towards the tube center. This improves pyrolysis rates and reduces bulk fuel temperatures within the MMC structure. Consequently, overall surface coking is minimized due to significantly lower wall temperatures. These findings provide a solid foundation for the design of advanced regenerative cooling channels in high-performance propulsion systems.