Evaporation-condensation of levitated copper droplets

A detailed study of the evaporation of levitated copper droplets in presence of homogeneous condensation over a range of temperatures, from 1800 K to 2200 K, pressures, from 10 kPa to 101 kPa and inert gas flowrates, from 0.07 * 10-4 m3 s-1 to 7.40 * 10 -4 m3 s-1, was undertaken. Several calibration techniques to measure the temperature of a 5 mm copper droplet using a two-color pyrometer are described. The levitated droplets required a short time to reach steady temperature with the result that the average measured loss of copper by evaporation was less than steady state evaporation rate by as much as 1 to 15%. The experimental data were adjusted accordingly.The experiments found that the flux of evaporation at constant temperature, was directly proportional to the vapor pressure of copper. For all the levitation experiments in which a significant change in mass loss was observed, the surface area decreased linearly with time.A new technique to calculate the droplet temperature was obtained based on the measurements of mass loss and evaporation time. The estimated temperature values were 5% higher than those obtained with the two-color pyrometer.The mass transfer coefficient based on the evaporation data, varied from 0.10 m s-1 to 0.81 m s-1. The activation energy for copper evaporation was 299 kJ mol-1, similar to the latent heat of evaporation, 305 kJ mol-1. A new method to estimate the vapor pressure of copper was obtained. It was found that the flux of evaporation was inversely proportional to the total pressure. Since the diffusivity is inversely proportional to the pressure, the flux of evaporation close to the droplet-gas interface is mainly due to molecular diffusion.Four models were evaluated to predict the flux of evaporation. The empirical correlation of the Sherwood number was employed to estimate the mass transfer coefficient and the predicted flux of evaporation. The combination of factors which best predicted the experimental flux of evaporation were the diffusion and natural convection. The predicted fluxes using the empirical correlation were always higher than the experimental values and they varied from 2% to 107%. The Turkdogan model predicted four times higher fluxes than the measured values. When the reference temperature was higher than room temperature, the amended model also yielded higher figures than the evaporation data. Considering that the homogeneous condensation does not influence the flux of evaporation, Elenbass' values were lower than the measured fluxes, but were more accurate than the models which take into account homogeneous condensation.The main conclusion derived from the experimental data was the realization that the evaporation of copper follows a simple correlation and the cluster formation in the boundary layer has no influence on the flux of evaporation. (Abstract shortened by UMI.