Effect of nozzle diameter on spray and thermal characteristics of R134a flashing spray

Two-phase flashing spray holds a special interest in the safety area. High pressure liquids or liquefied gases accidentally released from small hole in chemical or nuclear industry can form two-phase mixtures due to the rapid decrease of pressure, resulting in injuries, fatalities, serious local pollution, etc. Besides, flashing spray has been successfully used in laser dermatology to prevent thermal injuries during recent decade. In this work, an experimental study is conducted to characterize the two-phase flashing spray in the close field of flashing occurrence and quantify the effect of nozzle diameter on the spray and thermal characteristics, using the volatile and low boiling point cryogen of R134a. From the axial and radial distributions of droplet diameter, velocity and average temperature, it is found that droplet diameter and average temperature experience rapid decline along the axial spray distance, and the velocity has acceleration near the nozzle exit due to the explosive atomization. Droplet diameter keeps almost constant except at the spray periphery, and droplet velocity decreases linearly in the radial direction. Droplet average temperature has complex variation along the radial distance depended on the axial distance. Nozzle diameter has significant effect on the spray and thermal characteristics. Larger diameter nozzle results in longer acceleration distance of droplet velocity, whereas smaller diameter nozzle causes higher decrease rate of droplet diameter and temperature near the nozzle exit. Nevertheless, nozzle diameter has little effect on the minimum droplet temperate with the same value of -60^oC. The dimensional analysis indicates that the non-dimensional lateral velocity profile and the non-dimensional droplet average temperature in the spray centerline all show self-similarity. New correlations of non-dimensional droplet velocity and average temperature are proposed.