Molecular clustering physical model of steam condensation and the experimental study on the initial droplet size distribution

A molecular clustering physical model is proposed to describe the state of steam molecules in bulk steam phase before condensing on the cooled surface. Based on the general feature of nucleation processes, it is presumed that the steam molecules become clusters with the Lognormal size distribution in bulk steam phase firstly before condensing onto the cooled surface. In order to prove this physical model, the condensation of moist air on the surfaces with different wettability is observed using a high speed camera and microscope. The size distribution of the pre-coalescing droplets with radius of several microns is obtained, and for the primary droplets, about 3–10 nm, the size distribution is generated based on the experimental images reported in literature. It is concluded that the size distribution of the droplets from initial to the pre-coalescing size both satisfy the Lognormal distribution function, this proves the present physical model to be reasonable and consistence with the cluster physical theory. In addition, the observation that the initial condensation mode of moist air on the surfaces (either hydrophobic or hydrophilic) is always dropwise confirms the physical model further.