In this study, a high-speed camera was used to investigate the effects of auxiliary atomization methods on the spray characteristics of a direct-injection nozzle operating under high-temperature and large-Mach-number conditions. Three configurations were examined: (1) adjusting the coflow pressure of circular holes, (2) varying the nozzle insertion depth, and (3) altering the relative position between the circular-corner rectangular coflow holes and the nozzle. The quantitative analysis focused on the breakup length, fuel concentration, droplet size distribution, and oscillation behavior. The results are summarized as follows: (1) The flow boundary layer is thinner than the thermal boundary layer; thus, increasing the nozzle insertion depth decreases the penetration breakup length, while the impinging breakup length remains unchanged with variations in coflow pressure or hole configuration. (2) Because the circular coflow air jet exhibits weak momentum, the coflow pressure hardly affects the breakup length, concentration boundary, droplet distribution, or main oscillation frequency. (3) For the circular-corner rectangular coflow, the relative position of the liquid and low-temperature air jets does not change the breakup length but affects the pulsation frequency and fuel concentration boundary. Moreover, the concentration boundary is maximized when the air jet is upstream and the fuel jet is downstream.
Li et al. (Thu,) studied this question.