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NUCLEATION LABORATORY
DIFFUSION CLOUD CHAMBER OPERATION AND THE BACKGROUND GAS EFFECT
Atmospheric
Research, 46, 195 (1998)
Anne Bertelsmann and Richard H. Heist
RESEARCH SUMMARY
Key design and operational aspects for thermal diffusion cloud chamber
applications are discussed in the context of a two-dimensional solution to the
mass and energy balances describing diffusion through a stagnant background gas.
The important issue of buoyancy-driven convective disturbances and their
impact upon nucleation measurements made using a diffusion cloud chamber are
discussed. A new derivation of the
relation that predicts the upper limit of total pressure allowed for stable (the
absence of buoyancy- driven convective disturbances) operation of the diffusion
cloud chamber is presented. For the first time, this limit of stable operation
can be predicted prior to making experimental measurements.
Nucleation data obtained in our laboratory are examined in the context of
this predicted limit of stable operation. New
nucleation data are presented for 1-pentanol with helium as a background gas.
Only data corresponding to stable operation in the cloud chamber is used
in the analysis (see below). The
effect of background gas on nucleation we have reported previously is confirmed
for 1-pentanol, as well as for all the other alcohols that have been
investigated in our laboratory.
The
equation that defines the upper limit for the total pressure for which the
density gradient will be stable throughout the chamber can be written as:
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All
of the terminology is defined in J. Chem. Phys. 106, 624 (1997).
As
a result of this investigation, we obtained
results of a two-dimensional (z, r) treatment of the mass and energy
transfer processes that occur during the operation of a thermal diffusion cloud
chamber. The location of the wall
is considered in solving the mass and energy transport equations, in addition to
the vertical distance, z, between the upper and lower plate surfaces.
We
examine the effects on diffusion cloud chamber operation of:
aspect (diameter to height) ratio;
operation with either a dry or a wet interior chamber wall and the effects on
temperature, supersaturation, nucleation rate, and total density profiles in the
chamber;
overheating the interior of the chamber wall on temperature, supersaturation,
nucleation rate, and total density profiles in the chamber;
using different density background gases.
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