Carbon dioxide flow boiling in a single microchannel - Part I: Pressure drops

Carbon dioxide two-phase flow pressure drops have been investigated in a single horizontal stainless-steel micro-tube having a 0.529 mm inner diameter. Experiments were carried out in adiabatic conditions for four saturation temperatures of −10; −5; 0; 5 °C and mass fluxes ranging from 200 to 1400 kg/m2 s, for inlet qualities up to unity. Measurements have been compared to the predictions of well-known methods. The Müller-Steinhagen and Heck correlation and the Friedel correlation gave the best fit as well as the homogeneous model when the liquid viscosity is used to represent the apparent two-phase viscosity. Further, an analysis based on the homogeneous model has not shown any clear appearance of the laminar or the transition regimes in any given range of Reynolds number. The apparent viscosity of the two-phase mixture was found larger than the liquid viscosity at low vapour qualities, namely at the lowest temperatures. Hence, a new expression to determine the equivalent viscosity was suggested as a function of the reduced pressure. Lastly, the Chisholm parameter from the Lockhart-Martinelli correlation was found lower than expected and also mainly dependent on the saturation temperature.