NUCLEATION LABORATORY

THE EFFECT OF CARRIER GAS PRESSURE AND WALL HEATING 

ON THE OPERATION OF THE THERMAL DIFFUSION CLOUD CHAMBER

accepted for publication in J. Chem. Phys.2001

F.T. Ferguson and R.H. Heist

RESEARCH SUMMARY

     In this paper we report new results in which existing nucleation data from the literature has been re-examined using a more refined model of cloud chamber operation.  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 predicted by typical nucleation theories or is it seen in typical low pressure, expansion-based nucleation studies.  In this work we present a model of the chamber that shows that both of these effects are likely due (in large part, at least) to buoyancy-induced convection within the TDCC.  Using this model, we are able to show that as the chamber total pressure is increased, the calculated critical supersaturation within the chamber actually decreases.  Supersaturation and temperature conditions for an experiment carried out in our laboratory involving the high pressure cloud chamber (HPCC) and 1-propanol in helium background gas as predicted by our current model for vapor and mass transport are shown below.  Also shown below are the radial and axial velocity components of the overall motion in the HPCC.  Positive velocity values denote motion upward (axial) and to the right (radial).  Wall heat was not used during this experiment.

     Additional results obtained by using a simple model describing the chamber wall heating are also presented.  Previously, it was argued that unheated chamber walls result in a significant, radial concentration gradient that lowers the vapor concentration and condensation flux within the chamber center.  In contrast, we show that this reduction is due primarily to a convective flow disturbance induced by the sidewall concentration gradient.  This model has been applied to recent experimental data for 1-pentanol.  Results indicate that, with respect to buoyancy-induced convection, the typical one-dimensional model used to describe vapor and energy transport in the TDCC should be regarded as an upper limit to the maximum attainable supersaturation within the chamber.

SUPERSATURATION	TEMPERATURE
RADIAL VELOCITY COMPONENT	AXIAL VELOCITY COMPONENT

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