Thermal radiative transfer characteristics of a single water droplet

Direct heating of water droplets by thermal radiation is the key process in incorporating solar energy to achieve green production in many industrial applications, such as desalination, carbon capture, and energy storage. The radiative transfer characteristics of a single water droplet under given irradiation, including reflection, absorption, and transmission were investigated through a calculation model built with the zone method. The ranges of the equivalent diameter and length-width ratio of the droplet were 0.1 - 1.0 mm, 1 - 10 respectively. The absorption coefficient was 10 - 15 000 m(-1) and the refractive index was 1.2 - 2.0. Results suggested that, (1) the reasonable irradiating angle between external irradiation and droplet's long axis was 30 degrees - 150 degrees, in which the energy allocation among reflection, absorption and transmission was almost fixed. (2) The absorption mode of the droplet could be classified into surface absorption (SA) and volumetric absorption (VA). In VA, the droplet could achieve a uniform and rapid temperature increase, but it required the optical thickness of droplet to be less than 1. (3) Scattering included reflection and transmission. Compared with incident irradiation, reflection was distributed mainly on the same side as droplet's long axis, whereas transmission was distributed mainly on the opposite side. The overall reflectivity increased depended mainly on refractive index, but was independent on droplet size. Transmissivity increased with increasing length-width ratio or decreasing equivalent diameter. (3) In VA, at least 40% of incident irradiation was scattered out. The total amount of scattering could be adjusted by changing the absorption coefficient in VA or the refractive index in SA. At last, an empirical formula of droplet's absorptivity was proposed when the irradiating angle was in the range of 30 degrees - 150 degrees