An analytical investigation into dropwise condensation of different steam‐air mixtures on substrates of various materials

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Abstract
A mathematical model has been developed from the kinetic theory of gases. The model accounts for the heat transfer performance during dropwise condensation of different steam‐air mixtures on surfaces of various materials. In the development of the analytical model, the droplet surface is assumed to be in equilibrium or very near to equilibrium conditions with the fluid in contact, whereas the steam at the liquid‐vapour interface is visualized to be in the saturated state. It is concluded that a minute fraction of the part of the substrate surface covered by liquid droplets (the minute fraction is referred to as the effective area) is most effective for the flow of heat and also it is concluded that the contribution to the heat transfer by the bare area is negligible. In the investigation it is deduced that the effective area may consist of: a) droplets with very small radii, and b) tiny portions of the liquid surfaces of large droplets. The tiny portion of the liquid surface can be visualized to lie in the vicinity of the droplet perimeter. The effective area is found to vary from one surface to another according to the substrate material. Concerning the influence of the presence of non‐condensable gases, the effective area is found to decrease rapidly with the increase of the air concentration.

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