Experimental insights into thermal performance of a microtube precooler with drastic coolant properties variation and precooling impacts on turbojet engine operation

In theory, the operating Mach number of a turbojet engine could be broadened by using supercritical coolant precoolers. However, the thermal performance of microtube precoolers is drastically influenced by the variations of coolant properties, which may further affect the engine operation. In order to obtain the heat transfer characteristics and outlet temperature distortion of such precoolers, and to clarify the precooling impacts on turbojet engine thrust performance, an annular precooler with a tube diameter of 0.9 mm was designed and fabricated in this study. Besides, a comprehensive test platform for the precooler-turbojet engine coupling experiments was also built. On this basis, different Mach number simulating experiments were performed using supercritical cryogenic nitrogen as the coolant. For data reduction, a precooler segmented model was proposed to consider the effects of specific heat variation. The results indicate that the developed model is highly recommended for supercritical coolant precoolers operating with relatively high effectiveness. The operating Mach number of the turbojet engine was extended to Ma = 3.35 and the total thrust increased by 39.8% as the precooler outlet temperature decreased from 305.4 K to 248.6 K. Although there exists a temperature non-uniformity at the precooler outlet, it was confirmed to cause no damage to the turbojet engine operation during the experiments. Finally, a new Nusselt number correlation for the supercritical coolant heat transfer was developed based on the experimental data. The results of the present study verify the feasibility of using a supercritical coolant precooler to broaden the operating Mach number of turbojet engines and contribute to the preliminary thermal design of such precoolers.