## 1 Introduction  
Flashing spray often occurs when high-pressure liquid is discharged into a low-pressure environment below its saturation pressure. The sudden pressure drop causes superheated liquid, which leads to a violent flashing spray and two-phase mixture flow. The fast expansion of vapor bubbles within the spray zone, particularly near the nozzle exit, shatters the liquid bulk in terms of explosion atomization. After that, under the influences of shear force due to the velocity difference between the liquid and gas phases, liquid viscosity, surface tension force, and flow instability, droplets continue to break up, experiencing the secondary atomization and creating a fine mist. Flashing spray has extensively existed in a variety of scientific and engineering fields, such as seawater desalination [1], flashing spray cooling in heat dissipation for high power chips [2], cryogen spray cooling in laser dermatology [3, 4], and fuel spray in combustion engines [5, 6]. It also holds a particular interest in the chemical or nuclear industry for accidental release of flammable and toxic high-pressure liquids or pressure-liquided gases to the environment from a vessel or pipe in the form of a small hole. This accidental release may easily result in an explosive flashing spray and a two-phase mixture due to the rapid pressure drop between the vessel/pipe and the environment. This violent flashing spray could cause catastrophic consequences, such as explosions, fires, injuries, and serious local pollution [7, 8, 9, 10]. Since the early 1990s, the European Union has been carrying out a series of research projects (STEP programs) aimed at understanding the two-phase flow behavior of chemical hazards. In the laboratory, various chemicals were injected from a straight tube nozzle with different operating conditions to simulate the flash spray that occurred in the leakage accidents. Various measurement technologies were also adopted to study the dynamic behavior rules of droplets in gas-liquid two-phase flow in flash atomization. For example, [PERSON] and [PERSON] [11] employed a Phase Doppler Particle Analyzer (PDPA) to evaluate droplet size and velocity within a jet at the exit of a discharge pipe of a liquid propane storage tank during a sudden blowdown. Their study indicated that strong evaporation took place along the jet direction and higher pressure caused more intense flashing and thus smaller droplets. [PERSON] [12,13] used an LDA system and a Malvern particle size to measure the centerline and lateral profiles of droplet velocity and size in a propane liquid flashing spray. He pointed out that careful and detailed data analysis was required to obtain valid data. [PERSON] et al. [14] and [PERSON] et al. [15] utilized a 2D Phase Doppler Anemometry to characterize droplet kinematics and spray quality in a nonflashing to flashing water spray. They developed droplet size correlations for different regimes of spray. Later, [PERSON] et al. [16] and [PERSON] et al. [17] conducted a further investigation of droplet size and dispersion in the scaled experiments for a range of volatilities (water, cyclohexane, butane, and propane) and superheats, in which the previous size correlations were improved. Some other researchers used more volatile mediums to investigate the spray and thermal characteristics of the flashing spray. Typically, [PERSON] et al. [18,19], [PERSON] et al. [20,21], and [PERSON] et al. [3,22,23] employed refrigerants of R134a, R404A, and R407C to generate a fully flashing spray. The spray morphology, droplet diameter, velocity, and temperature were comprehensively investigated using intrusive and nonintrusive techniques. [PERSON] et al. [24] employed a high-speed camera and the particle image velocimetry (PIV) method to capture the morphological characteristics and measure the velocity of an R134a leakage jet and develop an empirical two-phase mass outflow rate model to predict the possible leakage accidents.