Experimental study on the droplet size distribution of radial orifice gas–liquid pintle injector at ambient pressure

•The droplet spatial distribution of pintle injector is stated.•The central reflux zone brings more small droplets close to pintle injector tip.•Influences of working parameters on droplet size are stated.•An empirical formula for predicting axial cross-sectional droplet size is developed. Pintle injector can achieve reliable and stable combustion of propellant, thereby greatly reducing the design difficulties of the combustion chamber. Droplet size, which has a great influence on the subsequent combustion process with high ambient pressure, is an important atomization characteristic. However, the influence of ambient pressure on the droplet size distribution of radial orifice gas–liquid pintle injector has not been extensively investigated. At the ambient pressures of 0.4–0.6 MPa, the tests were carried out in the ambient pressure chamber with the liquid mass flow rates of 10–15 g/s and the gas mass flow rates of 7.2–15 g/s. Droplet size was measured by Spraylink High-speed Spray Particle Size Analyzer. The results indicate that droplets Sauter mean diameter (SMD) increases with axial and radial distances. A dimensionless parameter N, which represents the ratio of the vertical component of droplet aerodynamic force to gravity, is defined to analyze the reasons for the change trend of axial droplet size in the central reflux zone of the spray field where droplet size increases with axial distance. Droplets SMD increases with ambient pressure and liquid mass flow rate, and decreases with increasing gas mass flow rate. Furthermore, droplets SMD is positively correlated with local momentum ratio (LMR), whereas negatively correlated with Weber number (We). The empirical formula for axial cross-sectional droplets SMD was fitted with a determination coefficient of 0.993. The results can help elucidate droplet spatial distribution and the influences of several working parameters on the spray characteristics, thereby facilitating the design of pintle injector.