Heat transfer, condensation and fog formation in crossflow plastic heat exchangers

In this paper heat transfer of air-water-vapour mixtures in plastic crossflow heat exchangers is studied theoretically and experimentally. First, a model for heat transfer without condensation is derived, resulting in a set of classical differential equations. Subsequently, heat transfer with wall condensation and fog formation are considered in some detail. Separate attention is paid to the heat transfer and condensation of pure steam in the heat exchanger. Finally, the experiments performed are reported and the results compared with the models presented. From this comparison it can be learnt that the models are well able to predict the rate of heat transfer and phenomena such as condensation and fog formation

The mean condensate heat resistance of dropwise condensation with flowing inert gases

The quantification of the condensate heat resistance is\ud studied for dropwise condensation from flowing air-steam\ud mixtures. Flows are essentially laminar and stable with gas\ud Reynolds numbers around 900 and 2000. The condensate shaping\ud up as hemispheres on a plastic plane wall and the presence\ud of inert gases make it possible that thermocapillary convection\ud occurs making the resistance less than the mean condensate\ud thickness (ca. 0.185 mm) divided by the heat conduction coefficient.\ud The analysis of experiments shows that the effective\ud mean condensate resistance might indeed be less, by a factor of\ud 0.8+0.2. The analysis takes account of the sensible heat transfer\ud which may be as large as 35% of the total heat transfer if inlet\ud vapor concentration, cin, is low (ca. 0.07). A method is presented\ud to determine the gas-condensate interface temperature,\ud ti, that is needed in the analysis of the heat resistance. The\ud highest temperature differences (t i- tw), t w being the mean\ud temperature of the condenser plate at the gas side, have been\ud found to occur for relatively high values of Cin (ca. 0.3)