Experimental study on the spray characteristics of a gas–liquid pintle injector element

As a suitable choice for throttling liquid rocket engines and reducing engine weight, the pintle injector with unique spray characteristics can not only adjust the mass flow rate of propellants, but also allow for a single injector system. In order to fully understand the spray characteristics of the gas–liquid pintle injector with discrete radial orifices, a pintle injector element using air and water as simulants was designed for spray experiments in the atmospheric environment. The variance in the range from 0.19 to 1.16 for the local momentum ratio (LMR) was achieved by changing the injection pressure drop of air. Backlight imaging and laser-scattering methods were respectively adopted for visualizing the near or far-field spray pattern and measuring Sauter mean diameter (D_SMD). The results indicate that once injected into the crossflow air film, the water jet is rapidly broken and atomized in the near-field. As the LMR decreases, the atomization process of water jet is gradually dominated by the Kelvin–Helmholtz (K–H) instability with the Rayleigh–Taylor (R–T) instability wavelength basically unchanged. And the droplet enrichment phenomenon is more obvious, which means poor gas–liquid mixing. An empirical correlation was formulated between the non-dimensional near-field windward trajectory and the LMR, and acceptable agreement is found at elevated LMRs. As for the far-field region, the divergence angle at the frontal view does not change under various operating conditions, while the change of the spray angle at the side view is positively correlated with the LMR. As the spray develops downstream, the D_SMD gradually decreases and its distribution tends to be uniform in the horizontal direction. Along with the decrease of the LMR, the D_SMD distribution changes from an inverted "V" shape to a hollow cone shape, and then to an inclined "N" shape. However, the D_SMD does not decrease significantly, mainly because the droplet enrichment weakens the aerodynamic effects and is not conducive to secondary atomization. The research work of this paper will provide a reference for optimal design of the gas–liquid pintle injector.