Abstract
The strong shear induced by the injection of liquid sprays at high velocities induces turbulence in the surrounding medium. This, in turn, influences the motion of droplets as well as the mixing of air and vapor. Using fluorescence-based tracer particle image velocimetry, the velocity field surrounding 125–135 m/s sprays exiting a 200-upmum nozzle is analyzed. For the first time, the small- and large-scale turbulence characteristics of the gas phase surrounding a spray has been measured simultaneously, using a large eddy model to determine the sub-grid scales. This further allows the calculation of the Stokes numbers of droplets, which indicates the influence of turbulence on their motion. The measurements lead to an estimate of the dissipation rate epsilon approx 35 m^{2} s^{-3}, a microscale Reynolds number Re_{lambda } approx 170, and a Kolmogorov length scale of eta approx 10^{-4} m. Using these dissipation rates to convert a droplet size distribution to a distribution of Stokes numbers, we show that only the large scale motion of turbulence disperses the droplet in the current case, but the small scales will grow in importance with increasing levels of atomization and ambient pressures.
Highlights
When a high-speed liquid jet breaks up into a spray, it drags along the air surrounding it, generating strong shear forces in the ambient gas
The Stokes number quantifies the response of the droplets in the spray environment to the smallest timescale of the spray-induced turbulence, estimated from the measured dissipation rate to be ≈ 6 × 10−4 s
The estimate of the integral length scale leads to a large eddy turnover time L = L∕u ≈ 10−2 s, one order of magnitude larger than, corresponding to a turbulent diffusion rate Dturb ≈ Lu 2 ≈ 10−2m2∕ s
Summary
When a high-speed liquid jet breaks up into a spray, it drags along the air surrounding it, generating strong shear forces in the ambient gas. This shear can be the driving force for generating turbulence. This turbulence, in turn, can influence the mixing and dispersion of the droplets in the spray itself (Bharadwaj et al 2009; Bocanegra Evans et al 2016). Little information exists on spray-induced turbulence, due to the challenge of measuring the velocity field in such a complex environment, as well as obtaining the number of measurements required to obtain statistical information.
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