The influence of buoyant convection on the operation of the upward thermal diffusion cloud nucleation chamber

Abstract

Recently, the stable operation of the upward thermal diffusion cloud chamber with respect to buoyancy-induced convection has become a concern in obtaining reliable nucleation data. During chamber operation, evidence of strong convective currents are clearly visible due to the curved trajectories of entrained droplets. A potential problem exists when these flows are much smaller in magnitude; there is no visible evidence of convection, yet these minute flows may result in systematic errors in the nucleation data calculated via 1D diffusive models of the transport mechanisms within the chamber. To examine whether such flows are possible and the characteristics of these flows we have developed an extension to recent 2D modeling of the nucleation chamber which includes buoyancy-induced, convective motion. We have examined both wet and dry chamber operation with an example case of 1-propanol in helium at a pressure of 1.18 bar. In addition, for the dry wall case we examined the effect of overheating the chamber wall and varying the chamber diameter. Results indicate that, for the representative cases investigated, very subtle convective flows can exist and that these minute flows can affect the maximum attainable supersaturation along the chamber centerline. Finally, a list of general recommendations are given for minimizing the possibility of such flows within the cloud chamber.

Condensation of Supersaturated Water Vapor on Charged/Neutral Nanoparticles of Glucose and Monosodium Glutamate

“…Recently, the transport in the static diffusion cloud chamber has been reevaluated by a two-dimensional treatment to examine the wall effects and convection on the supersaturation profiles (44)(45)(46)(47). They confirmed that one-dimensional treatment led to essentially the same peak supersaturation under proper conditions (such as a large enough ratio of diameter to height, usually >5, no overheating of the side wall, no convection, etc.).…”

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

“…Ferguson and Nuth used a numerical model of the TDCC to show that buoyant convection could result in a reduction of the maximum supersaturation by several percent depending upon a variety of chamber conditions. 14 Schaeffer et al performed a similar analysis, but with higher molecular weight gases. 15 Their results indicated that the TDCC behaves like a classical Rayleigh-Bénard cell, and when conditions exceed a critical value, vigorous convection can occur within the chamber.…”

Buoyancy-driven convective motion in a thermal diffusion cloud chamber using a water/helium mixture

“…At the upper and lower plates, the temperatures and the mass fraction were kept uniform and constant. Although these two plates are covered with a liquid pool of the condensable species, it is assumed that the velocity components are zero because the liquid layer is so thin that the velocities at the interface can be neglected [10]. Indeed the calculation of evaporating velocity at the lower plate leads to the value of 1.4 · 10 −3 mm/s.…”

“…The nucleation rate is calculated using the classical BeckerDöring-Zeldovitch theory of homogeneous nucleation [15]. The homogeneous rate J can be specified as: (10) where G is the Gibbs free energy of cluster formation, i.e. the elevation of the free energy required for a transition from a monomer to a cluster.…”

“…Indeed, Heist et al [8,9] have shown that the presence of a non-condensable component (carrier gas) influences the clustering process, that the use of heavy carrier gases in a TDCC may influence the stable stratification of the vapor gas mixture with the non-condensable gas and may therefore lead to convection. Moreover, the heated chamber wall (the wall is heated to be kept clean of condensate) may cause a buoyancy driven convection that can propagate towards the center of the chamber and cause a motion of the gas mixture (Ferguson and Nuth [10]). These effects have been examined numerically by Schaeffer et al [11] who used a 2D mass, heat and momentum numerical model to show that the choice of the carrier gas within the TDCC is of importance since an inappropriate choice can alter the nucleation processes.…”