An experimental and theoretical study on the concept of dropwise condensation

Hydrophobic coatings have been created through self-assembled mono layers (SAMs) of n-octadecyl mercaptan (SAM-1) and stearic acid (SAM-2) on copper alloy (99.9% Cu, 0.1% P) surfaces to enhance steam condensation through dropwise condensation. When compared to complete filmwise condensation, n-octadecyl mercaptan (SAM-1) coated surface increased the condensation heat transfer rate by a factor of 3 for copper alloy surfaces, under vacuum condition (33.86 kPa) and to about eight times when operated under atmospheric condition (101 kPa). A model using the population balance concept is used to derive a theoretical formula to predict the drop-size distribution of small drops which grow mainly by direct condensation. All the important resistances to heat transfer such as the heat conduction through the drop, vapor–liquid interface are considered in developing this model. By knowing the contact angle of the drop made with the condensing surface and the maximum drop radius the sweeping effect of large falling drops could be calculated which is also incorporated into the model. The effect of interfacial heat transfer coefficient on heat transfer rate is also considered in developing the theoretical model. This is combined with the well known size distribution for large drops proposed by Le Fevre and Rose [E.J. Le Ferve, J.W. Rose, A theory of heat transfer by dropwise condensation, in: Proceedings of 3rd International Heat Transfer Conference, vol. 2, Chicago, 1966, pp. 362–375] which grow mainly by coalescence. There has been a satisfactory agreement between our experimental data and the present theoretical model.