Thermodynamic Limitations of Ice Accretion Instruments

Abstract

A number of instruments used for determining the supercooled water content of an icing cloud depend on measuring the rate of accretion of ice on an unheated body. It has been shown by Ludlam that, since only part of the supercooled water freezes on impact, and the rest cannot always be frozen by the limited convective and evaporative heat losses which are available, there are limiting water contents beyond which such methods of measurement are unreliable. These limits have been calculated for a rotating cylinder at normal and high flight speeds, and the results show that rotating cylinder measurements are of limited usefulness and may, in fact, prove entirely misleading as regards both water content and droplet size. Experimental results show agreement with the calculated values. The limits for a conventional rotating-disc rate-of-icing meter and for an NAE-Smith ice detector head have also been obtained experimentally. It is concluded that apart from the possibility of using refrigerated ice-accretion instruments, it is necessary to use a thermal method of measuring high values of water concentration, but that an ice-accretion method must also be employed to distinguish between warm water, supercooled water and ice crystals.