Numerical analysis of heat transfer and flow characteristics of methanol surface cracking reaction in microtubes

The benefits of chemical precooling technology in reducing fuel consumption and improving the performance of turbojet engines make its application in precooling airbreathing engines of great significance. Revealing the heat transfer and flow characteristics of methanol surface cracking reaction in microtubes plays an important guiding role in the design and optimization of precoolers with chemical reactions. This paper focuses on the numerical simulation of the flow and heat transfer of methanol surface cracking reaction in microtubes. The results indicate that under low heat flux of the precooler, the use of surface cracking reaction in microtubes can significantly improve the heat sink and convective heat transfer capacity of the fuel compared to thermal cracking reactions. Reducing the tube diameter can increase the contact area between the tube and methanol per mass flow rate, which helps promote the reaction. Unlike endothermic hydrocarbon fuels, the changes in composition and temperature caused by the cracking of methanol can cancel out the impact on density, without affecting the flow field and pressure loss along the flow direction. Reducing the mass flow rate is beneficial for the reaction. The higher the pressure, the more unfavorable the reaction is, but it will increase the total heat absorption capacity of the fuel and help reduce the fuel temperature. The increase in heat flux can improve the methanol cracking rate and enhance the convective heat transfer capacity of the fuel. Under the same boundary conditions and reaction parameters in the design condition Ma4, the cracking rate of the methanol surface cracking reaction is 19.17 %. The air can be precooled to 698 K, which is 66 K lower than that of thermal cracking with only a 1.56 % cracking rate. The compact structure of the precooler provides it with sufficient internal surface area, making it more suitable for using surface reaction types to achieve an extremely high fuel cracking rate. At the same time, increasing the heat transfer temperature difference and total heat transfer coefficient can bring a dual improvement to the air precooling effect of the precooler while expanding the upper limit of the Mach number for the airbreathing engine, and achieving the goal of reducing fuel consumption.