Collapse dynamics of cavitation bubble cluster and its effect on heat and mass transfer

It is extremely challenging for reliable ignition in scramjet combustor due to the long ignition delay time of hydrocarbon fuel, inefficient fuel–air mixing, and limited fuel residence time. The laser ignition technology has many advantages for the lean fuel ignition because of the larger initial flame kernel, higher flame kernel propagation velocity, and more flexible ignition position control compared with the traditional spark ignition. During laser ignition of liquid fuels, cavitation bubbles are induced when the laser is focused onto the liquid medium. Particularly, cavitation bubble clusters are readily induced in multi-channel ignition, potentially affecting laser ignition performance. Additionally, string cavitation phenomena often accompany fuel atomization during injection. The collapse dynamics of cavitation bubble clusters significantly influence fuel atomization quality. The experiments compare the evolution of a single cavitation bubble with that of two different cavitation bubble clusters: single-bubble dominated clusters and synchronous double-bubble dominated clusters. The study reveals that single-bubble dominated clusters generate a pronounced lateral jet upon collapse, while synchronous double-bubble dominated clusters tend toward longitudinal expansion. Furthermore, numerical simulations indicate that mass transfer processes induced by bubble cluster collapse are much weaker compared to those in single bubble.