Effects of slit geometric parameters on spray characteristics of double-slit pintle injectors

Pintle injectors have garnered significant research attention in recent years, particularly for their applicability in reusable launch vehicles, owing to their wide thrust control range and excellent combustion stability. While research has explored the characteristics of pintle injectors in the context of developing these components for actual engine applications, studies focusing on the effects of design parameters on injector performance have been limited. This study investigated the effects of slit geometric parameters, specifically the blockage factor ($BF$) and slit area ratio ($\gamma$), on the spray characteristics of double-slit pintle injectors. Cold-flow tests were conducted using planar pintle injectors with water and ethanol as simulants. The spray angle and Sauter mean diameter (SMD) were measured using the shadowgraph technique, and the distribution of mass flow rate and mixture ratio was analyzed using a mechanical patternator. The experimental results revealed that two distinct streams were injected at different angles from each row of slits, resulting in a division of spray shape, SMD, and mass flow distribution into three regions based on the two streams. These spray angles, termed primary and secondary spray angles, were quantified as functions of the local momentum ratio, determined by $BF$ and $\gamma$. To correlate the spray characteristics with combustion performance, mixing quality and a representative droplet size metric, the integral Sauter mean diameter ($I{D}_{32}$), were presented. The study found that higher values of $\mathrm{BF}$ and $\gamma$ corresponded to improved mixing quality.