Marangoni Effect in Bipropellant Droplet Mixing during Hypergolic Ignition

Hypergolic ignition of a bipropellant is an intrinsically nonpremixed physicochemical process that involves both chemistry and fluidic mixing. While the ignition delay time (IDT) of various hypergolic propellants has been extensively measured by using the prevalent droplet test, the effect of fluidic mixing on the hypergolic ignition process has rarely been studied. Compared with the well-understood droplet mixing within the same liquid, a prominent feature of bipropellant droplet mixing is the substantial surface tension difference, which induces a Marangoni effect upon droplet coalescence; however, it has not been addressed by previous hypergolic ignition studies and still remains inadequately understood. In this work, we numerically study the internal mixing of colliding droplets of different surface tensions with implications for hypergolic propellant ignition. The results show that the Marangoni effect substantially enhances droplet mixing compared with the situation without surface tension difference, and indicate the Marangoni effect could be a pivotal physical mechanism in hypergolic ignition. In particular, we identify an interesting phenomenon in that the Marangoni effect yields a nonmonotonic variation of internal mixing with increasing impact inertia, and this provides a likely interpretation of the nonmonotonic variation of the IDT experimentally observed by Zhang et al. [Combust. Flame 173, 276--287 (2016)]. Although binary droplet collision is employed as the specific object of the study, the present results could also provide insight into the hypergolic ignition of a bipropellant using various mixing methodologies, such as impinging jet and droplet-pool impact.