The concept of rivulet degree was introduced to elucidate the evolution characteristics of transition condensation on a condensing surface

such that the heat flux through the surface could be expressed as the summation of the flux values through the dropwise region and rivulet regions respectively.For the heat flux in the dropwise region

droplets with the maximum diameter were correlated with the rivulet degree

on the basis of a time conformation model of droplet size distribution

to establish the heat transfer model in the dropwise region related to surface effect. In the rivulet region

the liquid film thickness and the area fraction covered by the rivulet region would be calculated by using the model of condensation heat transfer through a liquid film with certain thickness. The heat flux through the whole surface was the weighted mean of the values of the two regions mentioned above.The present model also explained the gradual change of heat transfer coefficient of transition condensation with surface free energy difference.The experiments of transition condensation heat transfer were conducted for a specially designed surface.The present model predictions agreed well with the experimental data and those reported in the literature.It could be useful in modeling the transition of condensation pattern from dropwise to filmwise

and establishing a unified model for the whole process of condensation heat transfer.