Primary breakup regimes for cryogenic flash atomization

Flash boiling atomization can occur in rocket thrusters operating in the vacuum of space when the cryogenic liquid propellants are injected into the reaction chamber just before ignition. The sudden drop in pressure triggers the nucleation of microscopic vapour bubbles that grow in the superheated liquid, leading to extreme jet expansion and atomization. Direct numerical simulations are performed using the multiphase solver FS3D to bring insight into the primary atomization process occurring at the microscopic level. The code uses the volume of fluid method and PLIC reconstruction to track the liquid-vapour interface with high fidelity, fully resolving viscous and capillary effects. An incompressible scheme is used, yet all the relevant thermodynamic effects associated with flash boiling are considered by calibration of the evaporation rate and fluid properties based on exact solutions for bubble growth in superheat liquid. A series of test cases with regular bubble arrays demonstrates how the initial bubble spacing and liquid temperature can be correlated to a range of Weber and Ohnesorge numbers. This allows for the definition of very distinct breakup mechanisms and resulting droplet patterns. These extend beyond the common assumptions used in the literature to estimate the primary droplet size and indicate a range of possible droplet size distributions dependening on the breakup regime.