Critical supercritical-boiling-number to determine the onset of heat transfer deterioration for supercritical fluids

Supercritical fluids such as CO2, water and organic fluids are frequently applied in power systems. The accurate prediction of heat transfer deterioration (HTD) is important to keep the safe operation of advanced power systems such as solar driven supercritical carbon dioxide Brayton cycle. As described in textbooks, it is impossible to identify liquid from gas beyond the critical point, thus supercritical fluid is assumed to have homogeneous structure with a single-phase. The single-phase assumption cannot explain and predict supercritical heat transfer (SHT). Instead, we investigate SHT by the pseudo-boiling concept. Heat transfer is analogized between supercritical pressure and subcritical pressure to create a new non-dimensional supercritical-boiling-number SBO, representing the bubble expansion induced momentum force against the inertia force when it is coupled with the ratio of liquid density with respect to vapor density. Our study reveals sudden changes from normal heat transfer (NHT) to heat transfer deterioration (HTD) with obvious temperature peak when crossing a critical SBO, which is 5.126 x 10(-4), 2.018 x 10(-4), 1.653 x 10(-4) and 1.358 x 10(-4) for CO2, H2O, R134a and R22, determined by a large quantity of database. Our work paves a new way to understand the SHT mechanism and supports the heterogeneous structure of liquid-like fluid and vapor-like fluid for supercritical fluids.