Flow boiling of refrigerant R141B in small tubes

Small circular and non-circular tubes are widely encountered in compact evaporators and condensers. This paper presents an experimental study of two-phase flow and heat transfer of refrigerant R141b, in small tubes. Four circular tubes with diameters of 1.1, 1.8, 2.8, 3.6 mm and one square tube of 2 × 2 mm² were used in the test programme. The parameter ranges were mass flux 50 ~ 3500 kg m^-2 s^-1, heat flux 1 ~ 300 kW m^-2, and inlet pressure 1 ~ 3 bar resulting in mean boiling heat transfer coefficients of 0.1 ~ 10 kW m^-2 C^-1. It was found that local heat transfer coefficients are not only a strong function of heat flux but also a function of vapour quality and a weaker function of mass flux for the all the small tubes tested, showing that both nucleate boiling and convective evaporation occur in small tubes. The mean heat transfer coefficient was found to be is primarily a function of the heat flux, rather than the mass flux. Comparison with data in the literature shows that a general flow map developed from adiabatic two-phase flow tests can provide guidance for prediction of flow regimes in heated tubes. This study provides useful design data for two-phase flow and heat transfer in small tubes and channels.