Numerical investigations of water droplet ingestion into ethylene flames in a model supersonic combustor

Water droplets are dispersed in the airstream at the entrance of a model supersonic combustor to mimic a scramjet flying through the cloud. Numerical simulations are performed to study the effects of water droplets on ethylene flames. The compressible two-phase reactive systems are solved using a hybrid Eulerian–Lagrangian approach. Vitiated air and preburned fuel enter the combustor parallelly at Mach numbers of 2.15 and 1.25, respectively. The experimental droplet-free condition is modeled as the base case, case 0. Eight cases (cases 1–8) are further simulated, in which the water droplet mass flow rate (ṁwd) is successively doubled while the droplet diameter is fixed at 20 µm. With the lowest ṁwd, fuel/air mixing and combustion are intensified by the large-scale vortex structures generated in the mixing layer in case 1. Compared with case 0, its total thrust and combustion efficiency increase 75.7% and 4.6%, respectively. With increased ṁwd, the formation of large-scale vortex structures is suppressed due to the increase in overall density, while the evaporative cooling effect of the droplets becomes more pronounced. Consequently, the combustion intensity, total thrust, and combustion efficiency gradually decrease until flame extinction occurs in case 8. Moreover, the evaporation rate, resultant force, and net energy exchange rate on individual droplets also generally decrease with increased ṁwd. In contrast, the increase in ṁwd results in higher magnitudes of the cell-summed two-phase mass, momentum, and energy exchange rates. It should be emphasized that the present results are constrained by the examined droplet diameter (20 µm) and the selected operating conditions, and are intended to elucidate qualitative trends rather than to furnish quantitative design data for practical scramjet applications. The reported quantitative values (e.g., heat release rate, total thrust, and combustion efficiency) apply specifically to the chosen droplet size and operating conditions.