Stable stratification alteration in a thermal diffusion cloud chamber

The formation of clouds resulting from the homogeneous condensation of vapo r phase diluted in a background or carrier gas was studied numerically. The effect of the background gas on the nucleation process in a cloud chamber heated from below is discussed. The computations were performed using 1-pro panol as the condensable gas and helium, hydrogen, nitrogen, and argon, res pectively, as carrier gases. Results of the simulation conducted show that large differences appear in the cloud formation when operating with hydroge n or helium, and with argon or nitrogen for which the onset of convective m otions are predicted. Therefore, the isothermal patterns and streamlines ar e similar to those obtained in the case of Rayleigh-Benard instabilities. T he influence of the thermal Rayleigh number on the nucleation process is al so considered, and it is shown that supersaturation isolines exhibit comple x distortions for supercritical thermal Rayleigh numbers. As a consequence, only small zones of important nucleation rate are observed. (C) 2000 Ameri can Institute of Physics. [S0021-9606(00)00742-X].