Non-monotonic interminglement in impinging jets: Interplay between perturbation-driven enhancement and breakup-induced deterioration

Fuel-oxidizer jet impingement is the primary control means of hypergolic rocket propulsion systems for satellite orbit maneuvering and orientation control; however, its mixing process remains less understood than its fluid dynamics. In this experimental and computational study, we developed a new framework to characterize macroscopic liquid-phase interminglement performance in impinging jets and discovered a pronounced non-monotonic dependence of the defined interminglement index on the Weber number between 27 and 1400. This phenomenon arises from the competition between perturbation-induced enhancement of scalar interminglement and breakup-induced deterioration caused by the shortened coherent liquid-sheet interaction length. Specifically, increasing the Weber number from small values amplifies impingement-induced interfacial perturbations, which break the initial plane-symmetry and enhance interminglement, as manifested by the elevated helicity density within the liquid phase. Further increasing the Weber number to sufficiently high values, the liquid sheet breakup occurs substantially upstream, narrowing the effective region for interminglement, where the perturbation-driven interminglement enhancement is secondary to the breakup-induced deterioration. These findings provide a new physical insight into optimization strategies for liquid-phase interminglement in impinging jet atomization.