Ice crystals produced by expansion: experiments and application to aircraft-produced ice

Abstract

The production of ice crystals as a result of the expansion and cooling of moist air was investigated by laboratory experiment. In particular, the warmest expanded air temperature that produces crystals was sought as a function of the initial temperature. The results fit the standard theory of homogeneous nucleation of water droplets, as long as the droplets remain at a cold enough temperature for sufficient time to freeze by homogeneous nucleation. Experiments were also carried out with "dry' air (dewpoint less than -40°C). Quantitatively different results were obtained, namely, that much colder expanded air temperatures were required to produce crystals with essentially no variation in numbers of crystals produced as the initial temperature varied. These results are also consistent with the same homogeneous nucleation theory. These ideas were applied to the production of aircraft-produced ice particles by means of the adiabatic expansion and cooling that occur near the propeller blades of the aircraft. -from Authors