Primary atomization study of a swirl coaxial liquid propellant rocket injector

The present work addresses swirl-induced injection and atomization phenomena for rocket combustion applications. An experimental approach is taken which focuses on the fluid mechanics of the injection process for non-combusting water sprays and then applies the knowledge to measure and describe liquid oxygen spray atomization in a research rocket chamber. Following industry practice, two swirl coaxial injectors are designed and fabricated for research; the smaller unit is representative of an upper stage propulsion engine injector, and a larger unit of identical design but twice the size is representative of a booster propulsion engine injector.; Despite the difference in scale, visualizations comparing the sprays produced by the two injectors reveal a remarkably similar pattern of spray breakup and ligament and drop formation when the injection Weber number is maintained. Atomization flowfield similitude is thus alluded in imaging studies, and is further explored in spatially-resolved drop measurement experiments. Guided by dimensional analysis, experimental conditions are chosen such that all non-dimensional spray parameters are maintained in comparing the sprays formed from the two injectors. It is found that a matching of injection Weber number, and liquid-to-gas momentum ratio, results in identical drop flowfields from the two injectors in terms of the normalized measurements of drop size and velocity. Experiments for liquid only flow and liquid/gas flows are consistent in demonstrating this basis for similitude.; A combusting spray of liquid oxygen and gaseous hydrogen (adiabatic flame temperature of 2450 K, and chamber pressure of 2.1 MPa) is then compared with its non-combusting counterpart at equivalent Weber number and momentum ratio. While the non-burning and burning drop flowfields are qualitatively similar, the burning oxygen drops are found to be significantly smaller, probably due to drop evaporation effects in the elevated temperature environment of the combustion chamber.; This progression of spray flowfield characterizations encompassing visualizations and drop measurements in non-combusting and combusting environments demonstrates a systematic approach for identifying the fundamental spray parameters and their roles in governing the fluid dynamic aspect of swirl atomization.