Droplet distortion in accelerating flow

Several commercial instruments size particles based on their acceleration i n a high-velocity flow field. Previous work suggested that droplet distorti on in these instruments resulted in inaccurate sizing. Liquid aerosol dropl et shape distortion produced in an accelerating flow field was therefore co mputed through analytical solution of the Navier-Stokes equation for compar ison to experiment. A high-Reynolds-number empirical approximation to the p ressure external to the droplet was used in these calculations. Within the droplet, the longest-lived excitations correspond to a quadrupolar distorti on of shape. Droplet excitations were obtained in terms of aerosol diameter , viscosity, surface tension and density. At the largest viscosities consid ered (as in many oils), only a damped relaxation was found, whereas at lowe r viscosities and high surface tension (as in water) damped capillary oscil lations were predicted as possible, given rapid shifts in the surrounding a ir flow. In order to compute the effect of airflow varying in time, an appr oximate Green's function was used. The Green's function in the frequency do main was approximated using only a pair of poles, thereby accounting for on ly the longest-lived excitations. In application of the theory to compute a erosol distortion on passage through an aerodynamic particle sizer (APS) ac celeration nozzle, the change in air velocity was found to be so gradual th at no oscillations were induced for droplets as small as 20-mum diameter. M easurements of droplet undersizing in the APS compared favorably with the t heoretical predictions. The theoretical results were also consistent with p hotographs of distorted oleic acid and only slightly distorted water drople ts emerging from a nozzle. (C) 2000 Elsevier Science Ltd. All rights reserv ed.