The influence of nucleation and droplet growth on the onset of condensation in supersonic nozzle expansions

Abstract

Unlike most other techniques used to study nucleation, supersonic nozzles do not yield nucleation rates directly because the length of time over which nucleation contributes significantly to particle formation is not easy to determine or control. Nevertheless, experiments in nozzles are extremely important because they provide higher rates of cooling, higher supersaturations and higher nucleation rates than any of the other techniques. Their operating conditions are more typical of important industrial conditions found in aerodynamic and turbomechanical flows where homogeneous condensation can have serious consequences for the gas flow behavior. Because the fluid mechanics of nozzles is well-understand, condensation experiments in the nozzle are amenable to sophisticated modeling efforts, and much useful insight can be gained regarding the nucleation and droplet growth processes under these severe cooling conditions. This paper summarizes our recent experimental work using a gently diverging supersonic Laval nozzle to investigate the variation of onset temperature and pressure for varying amounts of condensible vapor in an excess of carrier gas. Many similar studies have been carried out previously, but the results of these studies are usually not sufficiently well documented to enable us to do modeling studies that permit assessment of the condensate characteristics at onset. By carrying out modeling of the particle size distributions for our own experiments, we can avoid this difficulty. In modeling our experimental results, we have found that the mechanism for producing observable condensate varies considerably with conditions. Nucleation of small droplets can dominate at one extreme, but droplet growth can also be found to play a dominant role at other conditions.