Supercritical “boiling” number, a new parameter to distinguish two regimes of carbon dioxide heat transfer in tubes

The objective of this paper is to develop a criterion to predict the onset of heat transfer deterioration (HTD) for supercritical CO2 heat transfer. A new mechanism is proposed by assuming supercritical pseudo-boiling. Before bulk fluid reaches pseudo-critical temperature (Tpc), the tube cross-section contains a vapor layer and a liquid-like fluid. The saturation temperature interface is defined at T = Tpc, inside and outside which are a vapor layer (T > Tpc) and a subcooled liquid (T < Tpc). The subcritical boiling number is extended to supercritical “boiling” number, defined as SBO = qw/(Gipc), where qw, G and ipc are heat flux, mass flux, and CO2 enthalpy at Tpc, respectively. SBO, by coupling the density ratio between “liquid” and “vapor”, represents the competition between vapor expansion induced momentum force and inertia force. The experiment of supercritical CO2 heat transfer is performed in a 10.0 mm inner diameter tube, covering ranges of P = 7.5–21.1 MPa, G = 488–1600 kg/m2s and qw = 74–413 kW/m2. Surprisingly, the onset of HTD is found to occur at a critical SBO which is 5.126×10−4 and the critical heat flux is expressed as qCHF=5.126×10−4Gipc. It is shown that our new criterion is also suitable for other experiments reported in literature, providing a general guidance to design and operate SCO2 heaters to avoid HTD.