Experimental study of the effects of droplet number density on turbulence-driven polydisperse droplet size growth

摘要

Interaction of polydisperse droplets in a turbulent air flow features prominently in a wide range of phenomena, such as warm rain initiation as an example. In the current study, we present an experimental investigation on the effects of initial droplet field characteristics on the maximum droplet size growth. By performing experiments in a vertically oriented air flow facility, the air flow turbulence was able to be controlled through the mean flow velocity and an active turbulence generator. The initial droplet field characteristics (droplet diameter range of 0-120 mu m) were varied using spray nozzles of different flow numbers. Based on quantitative measurements of the droplet size distribution at various spatial locations using phase Doppler interferometry (PDI), we estimated the droplet size growth rate R as a function of turbulence intensity I, initial droplet number density.N and initial mean droplet size (D) over bar. For each (rho(N), (D) over bar), we observed the occurrence of an optimum turbulence intensity I*, with the corresponding maximum droplet size growth rate being R*. Two different trends were observed. When rho(N) and (D) over bar were simultaneously increased and decreased, respectively, their competing influences resulted in small variations in R*. In contrast, when (D) over bar was held constant with a corresponding Stokes number St smaller than unity, there existed a threshold rho(N) above which R* increased rapidly with.N. These trends were then understood through long-distance microscopy (LDM) measurements. Beyond the aforementioned threshold rho(N), the fraction of uncorrelated small-sized (St < 1) droplet pairs was found to rapidly increase with rho(N). Further detailed analysis of droplet tracking in the LDM images identified that the velocity fluctuations in the small-sized droplet pairs being induced by close encounters with inertial droplets was the underlying mechanism for the rapid increase of R* with.N. This mechanism potentially explains how droplet collisions can be enhanced in small droplets if the droplet field is sufficiently polydisperse.