Local condensation heat transfer rates in fine passages

Abstract

A novel apparatus capable of quantitative control of local heat flows to single fine passages through thermoelectric coolers has been developed. Local heat transfer coefficients for flow condensation of HCFC-123 and of R11 have been measured for a wide range of mass fluxes (70–600 kg m −2 s −1), heat fluxes (15–110 kW m −2), vapour qualities (superheated to fully condensed), and pressures (120–410 kPa) in tubes with internal diameters of 0.92 and 1.95 mm. The data show a strong influence of mass flux and local quality on the heat transfer coefficient, with a weaker influence of system pressure. The data also show a clear enhancing effect of the condensation rate (heat flux) on the heat transfer coefficient. There is very little influence of tube diameter over the range investigated. A simple model of gas–liquid shear-driven annular flows, with a turbulent liquid film and allowance for the influence of condensation rates on the gas–liquid interfacial shear force, accounts quantitatively for the major effects observed. An average overprediction of 5% is obtained for the data set of more than 2000 points, with a standard deviation of 25%. However, the model does not predict the full extent of the observed effect of condensation rate on the heat transfer coefficients and some improvement in the description of the interfacial shear force is apparently needed.