Instantaneous Mass Transfer From Moving Microdroplet Surfaces

Instantaneous mass transfer of gas-microdroplet two-phase flow is one of the most important procedures in the field of chemical engineering, colloid science and environment science. As the development of atomizing drying, chemical vapor deposit and plasma reactor and so on, the theoretical researches and applications have flourished. Since the characteristic size of microdroplet is in the order of micrometer, which belongs to the field of aerocolloid, the researches for mass transfer of those systems become a emerging area in the cross-field of chemical engineering and colloid science.The most obvious character of microdroplet is that there is much more surfaces in the same volume. In another words, gas-microdroplet system has much more surfaces of mass transfer than those in traditional system, so they will cause the enhancement of mass transfer. In order to illuminate the nature of mass transfer in the gas-microdroplet system, a relatively comprehensive theoretical research in the mass transfer of gas-microdroplet system is presented from two aspects: firstly the case of mass transfer from an oscillating microdroplet in the gaseous media, and the case of mass transfer from the microdroplets accelerated by high-speed gas secondly.The mass transfer from an oscillating microdroplet is enhanced by the oscillation of microdroplet especially in moderate oscillatory frequency and amplitude. The theoretical predictions for this phenomenon based on the past theories, whose precision is far away from the satisfaction in both theoretical research and engineering, however, have performed poorly. Thus a novel mass transfer model is presented based on the framework of pseudo-steady assumption. Applying the mass transfer model into the conservation equation of mass transfer in axis symmetric coordinates, a new mass transfer equation is deduced. Based on the mass transfer equation, correlation expressions for the rate of mass transfer and Sherwood number are obtained. The results of a mass transfer experiment, in which a dodecanol microdroplet trapped and oscillated in the electrodynamic balance (EDB) was evaporating while the microdroplet's diameter is measured by means of light scattering which has the precision of 10-5, verify the theoretical predictions. The present theoretical predictions for Sherwood number have about 39.5% error in comparison of experimental data, which is better than 50.3% error in the predictions based on the literature.Further, in order to show the instantaneous effect of mass transfer, an advanced version of instantaneous mass transfer model is presented. Based on the instantaneous mass transfer model, the mass transfer equation is deduced from two different paths of coordinate transformation and mass conservation in the control volume. The respondent correlation expression for Sherwood number and the rate of mass transfer are obtained. The simulation results show that the predictions of Sherwood number are much better, in which the average error is about 9.2%. It indicates that the instantaneous mass transfer model can explain the nature of mass transfer from an oscillating microdroplet.Further investigation to the simulation results of instantaneous mass transfer show that the instantaneous Sherwood numbers change with time periodically, which is regarded as the periodical changes of mass transfer inherit the periodical character of motion. It also indicates that the higher oscillatory frequency the microdroplet has, the more obvious periodical character the mass transfer has.Turbulent flow is the most important flows in engineering because most of flows in practice are in the status of turbulent flow. Highly dispersion systems of gas-microdroplet are concerned by many scholars and engineers for its excellent interphase transport properties. Especially, the system that microdroplets accelerate by high-speed gas which is jetting through the Laval nozzle is one of the highly dispersion systems, which can be applied into the new equilibrium plasma reactor. The excellent mas...