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 (T-pc), the tube cross-section contains a vapor layer and a liquid-like fluid. The saturation temperature interface is defined at T = T-pc inside and outside which are a vapor layer (T > T-pc) and a subcooled liquid (T < T-pc). The subcritical boiling number is extended to supercritical boiling number, defined as SBO = q(w)/(Gi(pc)), where q(w), G and i(pc) are heat flux, mass flux, and CO2 enthalpy at T-pc, 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/m(2)s and q(w) = 74-413 kW/m(2). Surprisingly, the onset of HTD is found to occur at a critical SBO which is 5.126 x 10(-4) and the critical heat flux is expressed as q(CHF) = 5.126 x 10(-4)Gi(pc). It is shown that our new criterion is also suitable for other experiments reported in literature, providing a general guidance to design and operate S-CO2 heaters to avoid HTD.