Phase-Change Radiation: Enhanced Infrared Absorption in Vapor-Liquid Water Mixtures

Phase-change radiation is studied in water clouds undergoing liquid vapor phase transitions. Radiant energy transmission in the midinfrared (3.1-4.95 mu m) spectrum is measured in vapor liquid mixtures of water at 1 atm and 60 degrees C. Monte Carlo analysis of radiative heat transfer is performed to assess the contribution of infrared absorption associated with evaporative phase transition, for which a theoretical description is developed. The probability of absorptive, phase-change radiative relaxation is found to be on the order of 5 x 10(-8) per collision between a water vapor molecule and a liquid water droplet, which leads to a surface absorption efficiency of order 0.02 to 0.06 in the 3.1-4.95 mu m spectrum. Thereby, absorption of midinfrared radiation is slightly enhanced by evaporative absorption in water clouds.