Low mass flux upward vertical forced flow boiling of HFE7000

Forced convective boiling experiments with HFE7000 as the working fluid were conducted for upward flow in a vertical tube at 122 kPa (1.2 bar), inlet subcooling of 2 K and mass flux ranging between 50 and 150 kg/m2 s. The experimental facility consisted of a smooth 8 mm inside diameter optically transparent sapphire tube with overall length of 120 mm. A central 80 mm long section was coated with optically transparent and electrically conductive Indium Tin Oxide (ITO) functioning as a heater to facilitate boiling whilst allowing for visual observation of the two-phase flow. Tests were performed by fixing the flow rate and progressively increasing the heat flux. The results show that after the onset of nucleate boiling, the flow regime is bubbly and increasing the heat flux acts to increase the boiling heat transfer coefficient due to increased nucleation site density, bubble frequency and size. This results in a linear increase in the boiling curve. However, once slug flow is established, the slope of the boiling curve increases indicating that this flow regime augments the heat transfer positively. Overall, the boiling curves were insensitive to mass flux for the range tested. However, the inflection on the boiling curve, associated with flow regime transition from bubbly to slug, was observed to be sensitive to mass flux. The Chen convective boiling heat transfer correlation was compared with the measured data showing reasonable agreement. A modified Chen correlation has been proposed for low mass flux convective boiling which predicts the measured data more accurately.