Interphase matter transfer: an experimental study of condensation of mercury

Abstract

Comparisons between interphase matter-transfer theory and measurements have, in the past, been hindered by uncertainties about the 'condensation coefficient'. Large experimental errors have often been misinterpreted as indicating low values of the condensation coefficient. Condensation experiments with metals are convenient for the study of interphase matter transfer since, owing to the high thermal conductivity of the liquid, the temperature drop across the condensate film is small and, particularly at low pressures and high condensation rates, the temperature discontinuity at the vapour-liquid interface is of measurable magnitude. Condensation rate, and vapour and condenser surface temperature measurements have been made during film condensation of mercury on a vertical, plane, square (side 40 mm), nickel-plated, copper surface. Thermocouples, accurately located and spaced through the copper condenser block, served to measure, by extrapolation, the temperature of the copper-nickel interface and, from the temperature gradient, the heat flux from which the condensation mass flux was determined. Special care was taken to ensure that the results were not vitiated by the presence in the vapour of noncondensing gases. The observations cover wider ranges of vapour pressure (temperature) and condensation rate (heat flux) than hitherto studied, i.e. 50-4300 Pa (378-493 K) and 0.2-3.6 kg m$^{-2}$ s$^{-1}$ (56-1062 kW m$^{-2}$) respectively. The results are considered to have enhanced accuracy. In particular, after the accuracy of calibration and positioning of the thermocouples, and that of the thermoelectric measurements has been considered, it is estimated that the condenser surface temperature was measured to within around $\pm$ 0.1 K. Interface temperature discontinuities up to around 70 K have been observed at low vapour pressure and high condensation rate. The results lend support to recent theoretical studies and indicate that the condensation coefficient exceeds 0.9.