Evaporation of high-velocity particles in free-molecule flow.

Abstract

An effective heat-transfer coefficient X has been determined for microscopic iron particles traversing rarefied target gases at velocities between 25 and 40 km/sec. X is defined as the ratio of the increase in internal energy of the particles to the energy incident on the surface of the particles from the colliding gas molecules. The particles were completely vaporized in the target gas and the increase in internal energy was taken as the total vaporization energy. The total energy incident on the particle surface from the colliding gas molecules was determined experimentally. For an air target gas, X decreased more or less linearly from a value of 0.68 d= 0.07 at 25 km/sec to a value of 0.27 ± 0.07 at 4Q km/sec. Results for oxygen and argon target gases were similar. other than completely elastic molecule-surface collisions, the passage of a high-velocity object through a rarefied gas results in an increase in the internal energy of the object. The rate of energy input is a function of both the velocity and the gas density. If the rate of energy input is small, radiation cooling may be sufficient to establish thermal equilibrium. For very high rates, however, evaporative cooling may be required. Obviously evaporation reduces the mass of the object and, under certain conditions, it may be completely consumed in the process. This behavior is typical of high-velocity photographic and visual meteors as well as many man-made objects that re-enter the earth's atmosphere. The objective of the experiment discussed in this paper was to determine the efficiency with which the kinetic energy of molecules impinging on the surface of high-velocity particles was converted into internal energy of the particles. The experiment was accomplished by injecting high-velocity (25 < v < 40 km/sec) microscopic iron particles of measured velocity and mass into target gases of known density. The gas pressure was such that free-molecule flow conditions prevailed and, in all cases, the particles were completely vaporized within the target gas volume. Both the total energy required to vaporize the particles and the total energy flux incident on them from the colliding gas molecules were determined. The ratio of these two quantities yields what may be called an effective heat-transfer coefficient A which, in turn, is related to details of the molecule-surface interactions. Although the experiment was conducted with microscopic particles under idealized conditions, the results are quite fundamental in nature and may be applied to problems involving much more massive objects such as meteors or other objects entering the earth's atmosphere. 2. Experimental Approach