Experimental study on axial self-propagation of heat transfer deterioration of near-critical carbon dioxide at low mass fluxes

Near-critical CO2 exhibits unique boiling heat transfer behavior due to its high saturated vapor density, low latent heat, and extremely low surface tension. In this study, an experimental investigation was conducted to study the heat transfer deterioration of near-critical CO2 inside a vertical circular channel at low mass fluxes. The experimental results show a rapid rise in wall temperature at the outlet side with minimal power increase (Delta power = 33 W), and the wall temperature rise continuously self-propagates toward the inlet side. The heat transfer deterioration mechanism responsible for the wall temperature rise was considered to be a departure from nucleate boiling type boiling crisis based on the low vapor quality of the bulk fluid. In addition, the evolution of the axial heat transfer deterioration was analyzed in detail. And a phenomenological model was developed to explain the axial self-propagation of heat transfer deterioration based on the low mass fluxes and unique bubble behavior resulting from the low surface tension, high vapor density and low latent heat of the near-critical CO2. The model was validated using different dry/wet interface movement speeds.