Impacting factors and operation thresholds of electron transpiration cooling-based thermal protection system

Electron transpiration cooling (ETC) is a spontaneous endothermic process that occurs during thermionic emission, which shows great potential in ultra-high-temperature thermal protection systems (TPS). Herein, we systematically analyze the main influencing factors on the cooling effect of ETC, such as incoming flow velocity, geometry, and material work function. A two-dimensional finite element model, based on Navier-Stokes equations coupled with the 11-species air reaction model and two-temperature model, is developed to solve the thermal interaction between ETC and the non-equilibrium flow field. Three operational thresholds for ETC are identified. Lower work functions enhance electron emission, thereby reducing wall temperature. With a 2.0 eV work function, ETC significantly outperforms blackbody radiation at 1360 K, and its cooling efficiency increases with temperature. For flow velocities above Mach 9.0, ETC is effective at the leading edge with a 2.4 eV work function and a 5 mm radius. However, it loses effectiveness with a 300 mm leading edge radius, even at Mach 16.0. Notably, at Mach 19.6, with a 2.0 eV work function and a 5 mm radius, ETC reduces surface temperature by up to 48.1 %. These findings highlight the considerable potential of ETC for applications in ultra-high-temperature TPS. These findings highlight the considerable potential of ETC for applications in ultra-high-temperature TPS.