Heat transfer characteristics and flow distribution behavior of pyrolytic ammonia in scramjet regenerative cooling channels

Regenerative cooling technology is crucial for the thermal protection of hypersonic vehicles. However, traditional hydrocarbon fuels face challenges of insufficient cooling capacity and coking issues at high Mach flight. Ammonia, with its high heat sink, hydrogen-rich, and non-coking characteristics, shows promise as an alternative fuel to meet the thermal protection demands of scramjets. Therefore, this paper investigates the multiphysics coupling mechanisms of ammonia pyrolysis-flow-heat transfer and flow distribution characteristics in a regenerative cooling system for scramjets, using ammonia as the cooling fluid. The performance of ammonia and hydrocarbon fuels within the regenerative cooling channel is compared. Then, the multiphysics coupling characteristics of ammonia pyrolysis-flow-heat transfer under Robin boundary conditions are revealed. And the flow distribution and flow non-uniformity of ammonia in parallel channels with manifolds is clarified. The results show that under the given working conditions, the heat sink capacity of ammonia at the outlet (776.8 K) is 22.6 % higher, and the temperature non-uniformity coefficient is 17.7 % lower than that of n-decane (659.1 K), demonstrating significantly better overall performance compared to hydrocarbon fuels. Furthermore, compared to conditions without pyrolysis, ammonia pyrolysis can further reduce the wall temperature by approximately 18.7 % (from 1583.7 K to 1288.2 K). The occurrence of the pyrolysis reaction, the increase in the external gas temperature and convective heat transfer coefficient all exacerbate flow non-uniformity in the parallel channel. This study provides a theoretical foundation for the application of ammonia in regenerative cooling technology for hypersonic scramjets.