Experimental investigation on hydrogen/air shear and swirl coaxial injector diffusion combustion characteristics

Efficient and stable hydrogen/air combustion technology is crucial for enhancing the performance of the pre-cooling air turbo rocket (PATR) engine. The combustion behavior of shear and swirl coaxial injectors, which hold promise for application in PATR engines, remains unclear. This study experimentally investigates combustion characteristics of hydrogen/air shear and swirl coaxial injectors with the high-speed OH* chemiluminescence imaging method. The flame structure characteristics and transient combustion properties are analyzed and compared between different injectors. The results indicate that both shear and swirl coaxial injectors enable stable hydrogen/air combustion over a wide range of equivalence ratios. The shear-coaxial injector 1 exhibits a shear layer combustion regime, while the shear-coaxial injector 2, with a higher air-fuel momentum ratio, transitions into a lifted flame structure resembling partially premixed combustion mode. The swirl-coaxial injector demonstrates an expanding flame configuration. The swirl effect effectively enhances mixing and combustion while reducing the flame length. Compared with the shear-coaxial injector 1, the swirl-coaxial injector enlarges the flame area and markedly diminishes the flame perimeter by up to 47.3 %. Swirl shear increases the flame thickness and greatly promotes the radial mixing, yielding a more compact flame with more uniform heat release. Analysis of the flame dynamics reveals that the lifted flame fluctuation of shear-coaxial injector 2 presents stronger fluctuation than the attached flame of shear-coaxial injector 1 and swirl-coaxial injector, with the fluctuation intensity increasing with the equivalence ratio. The swirl-coaxial injector shows minimal heat release fluctuation, demonstrating superior potential for improving the combustion performance of PATR engines.