Flashing spray of volatile liquids is a common phenomenon observed in many industrial applications such as fuel injection of engines, accidental release of flammable and toxic pressure-liquefied gases, failure of a vessel or pipe in the form of a small hole in chemical industry, and cryogenic spray cooling in laser dermatology. In flashing spray, the volatile liquid is depressurized rapidly at the exit of a nozzle (or a hole in a vessel) and becomes superheated. Such superheated liquid (in the form of either a jet or droplets) leads to explosive atomization, leading to fine droplet sizes and a short spray distance. This paper presents an experimental investigation of flashing spray of cryogen R404a. A photographic study of the spray is first conducted, providing visualization of spray formation and showing the dynamic characteristics of the spray. Then the R404a short spray is characterized by the phase Doppler particle analyzer (PDPA). The PDPA measurements provide the distributions of the diameter and velocities of liquid droplets in the spray., showing the dramatic dynamic variation of the liquid droplets due to explosive atomization of large droplets in the region near the exit of nozzle. The data finds that the average droplet axial velocity increases first to a maximum, followed by a gradual decrease, a typical variation expected for flashing spray. During the same time period, the average droplet diameter shows a quick decrease, from early large droplets of about 30 microns in diameter to fine droplet with about 10 microns within about 40 mm spray distance. This study provides quantitative data on droplet velocity and diameter in flashing spray, useful for model validation. The qualitative results help to have a better understanding of the flashing spray atomization mechanisms for volatile cryogens.

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