Effect of various key factors on the law of droplet evaporation on the heated horizontal wall

Abstract

Water evaporation in a wide range of initial droplet diameters and at different wall temperatures on structured surface was studied experimentally. With an increase in the wall temperature from 31 to 72°С and an increase in the initial droplet diameter, exponent n in the evaporation law increases from 1 to 1.37. Under the transitional regime, exponent n=1.6 reaches its maximum. Usually, while simulating droplet evaporation, a linear relationship of the evaporation rate of water vapor and droplet radius R is considered (dm/dt∼Rn, n=1). In this paper, it is shown that exponent n increases with a growth of the wall temperature. In generalization of droplet evaporation rate, the exponent for the Rayleigh number (Ra) is 0.457 due to the predominant role of gas convection. For large water drops (for air–vapor mixture over the droplet surface) high Reynolds numbers are achieved (Ra=2×105). A diffusion vapor layer on the droplet surface and boundary layer of air on the surface of the heated cylinder, whose diameter exceeds the droplet diameter, are formed. A neglect of free convection understates simulation results in comparison with experimental data more than by the factor of 10. The sequence of key factors, taking into account their influence on the rate of droplet evaporation, is as follows: 1) convection in a vapor–gas medium; 2) effect of wall roughness, wettability, and convection in a liquid; 3) thermal inertia of the metal wall. The calculation methodology for a sessile drop enables a qualitative analysis for a high-temperature gas-droplet flow.