Fuel–gas mixing characteristics in premixing chamber for an air-assisted fuel injector

Air-assisted fuel injection (AAFI) effectively atomizes high-viscosity fuels like kerosene and diesel, yet the fuel–gas mixing process inside the premixing chamber is not fully understood. In this study, a full-scale transparent premixing chamber model combined with high-speed imaging techniques was employed to systematically investigate the influencing factors and mechanisms of fuel–gas mixing. The effects of fuel–gas pressure drop, fuel injection pressure, and fuel injection width on the macroscopic premixing process, two-phase flow evolution, fuel penetration distance, and fuel projection area were analyzed. Results show that the fuel–gas mixing process can be divided into an injection stage and a post-injection mixing stage, during which five distinct two-phase flow patterns were identified: falling film flow, slug flow, wavy flow, annular flow with ligaments, and mist flow, ranked in ascending order of mixing uniformity. The fuel–gas pressure drop was found to be the dominant factor influencing premixing, enhancing breakup and mixing during injection but causing fuel accumulation at the chamber bottom during prolonged mixing. Injection width primarily affects mixing by altering the falling film flow region, while the effect of injection pressure on mixing uniformity is negligible. Furthermore, an increase in the pressure drop led to the observation of three typical mixing modes: stratified-flow, transitional-flow, and dispersed-flow mixing. Overall, excessive fuel–gas pressure drop and injection width both lead to fuel accumulation and reduced mixing uniformity. These findings highlight the necessity of optimizing the interplay between fuel–gas pressure drop, injection width, and premixing time to achieve efficient fuel–gas mixing in practical AAFI applications.