The effect of carrier gas pressure and wall heating on the operation of the thermal diffusion cloud chamber

Experimental observations indicate that the nucleation behavior within the thermal diffusion cloud chamber (TDCC) changes with increasing carrier gas pressure and applied sidewall heating, even though such an effect is not pr edicted by typical nucleation theories and it is not seen in typical expans ion-based nucleation studies. In this work we present a model of the chambe r which shows that both of these effects are likely due to buoyancy-induced convection within the TDCC. As the chamber pressure is increased, the calc ulated critical supersaturation within the chamber decreases. Results from a simple model of the chamber wall heating are also presented. Previously, it was argued that unheated chamber walls result in a significant, radial c oncentration gradient which lowers the vapor concentration and condensation flux within the chamber center. In contrast, we show that this reduction i s due primarily to a convective flow induced by the sidewall concentration gradient. The model has been applied to recent experimental data for n-pent anol. Results indicate that, with respect to buoyancy-induced convection, t he typical 1D model should be regarded as an upper limit to the maximum att ainable supersaturation within the chamber. (C) 2001 American Institute of Physics.