Drop evaporation; ventilation coefficient; evaporation-effect of electrical forces
A laboratory experiment has been performed to study the effect of ventilation on the rate of evaporation of the millimeter sized charged and uncharged water drops suspended in a vertical wind tunnel. The linear relationship,fu = 0.907 + 0.282X, observed between the mean ventilation coefficient, fu, and a non-dimensional parameterX, (X =NSc,v1/3NRe1/2whereNSc,uis Schmidt number andNReis Reynold’s number) is in agreement with the results of earlier investigations for uncharged water drops. However, in case of charged drops carrying 10-10C of charge, this relationship gets modified tofu = 0.4877 + 0.149X. Thus, the rate of evaporation of charged drops is slower than that of uncharged drops of the same size. Oscillations of the drop and the change in airflow around drops are suggested to contribute to lowering of the ventilation coefficients for charged drops. Applicability of the results to a small fraction of highly charged raindrops falling through the sub-cloud layer below thunderstorm is discussed. The relaxation time required for a ventilated drop to reach its equilibrium temperature increases with the drop size and is higher for the charged than for the uncharged drops. It is concluded that in a given distance, charged drops will evaporate less than that of uncharged drops.