Experimental study of jet impingement boiling cooling with CO2 at subcritical pressures and comparisons with at supercritical pressures

Two-phase jet impingement cooling with carbon dioxide at subcritical pressures of 6.5 and 7.0 MPa (nearcritical pressure) on smooth surface was experimentally studied in this paper. Local heat transfer coefficients were measured with a homemade integrated thermal chip. The effects of heat flux, pressure and mass flow rate on heat transfer were studied. Coexistence of single-phase and two-phase heat transfer on the cooled surface was observed through flow visualization. The heat transfer coefficient increased with the increase of mass flow rate within the single-phase heat transfer region and then became insensitive to mass flow rate when boiling initiated on the cooled surface. No temperature overshoot was observed when boiling initiated due to the much-reduced latent heat of the fluid at near-critical pressures. Moreover, the heat transfer characteristics of jet impingement cooling at subcritical pressures were compared with that at supercritical pressures of the same experimental conditions both on smooth and micro-structured surfaces for a wide range of heat flux. The comparison showed that the heat transfer enhancement resulting from phase change at subcritical pressures was much higher than that resulting from large specific heat at supercritical pressures. However, CHF can be avoided for jet impingement cooling at supercritical pressure and the heat transfer rate of it was greatly enhanced on the surfaces with micropillar structures. (C) 2020 Elsevier Ltd. All rights reserved.