Supercritical heat transfer deterioration and pseudo boiling

In many systems, from rocket engines to concentrated solar power, pressures are increased to increase efficiency, thus resulting in these systems operating at supercritical pressures. At subcritical pressures, heat transfer deterioration (HTD) is a sudden reduction of heat flux when certain temperature thresholds are exceeded. It is well known and linked to the liquid-vapor phase transition. However, a HTD is also observed at supercritical pressures, despite no classical phase transition. To explore this phenomenon, we performed simulations of heat transfer in laminar boundary layers and observed supercritical HTD (scHTD). We found that the onset and strength of scHTD are linked to pseudo boiling, a nonequilibrium higher-order phase transition between supercritical liquid-like and gas-like states. Results agree well with theoretical predictions of the onset temperature and the maximum pressure at which scHDT occurs. Thus, pseudo boiling is physically what causes scHDT. Pseudo boiling is coupled to drastic changes in fluid properties, and thus accurate modeling is essential; the popular Peng Robinson equation of state leads to erroneous results.