Towards Full Resolution of Spray Breakup in Flash Atomization Conditions Using DNS

Ignition of rocket thrusters in orbit requires injection of cryogenic propellants into the combustion chamber. The chamber’s initial very low pressure leads to flash boiling that will then determine the dynamics of the spray breakup, the mixing of fuel and oxidizer, the reliability of the ignition and the subsequent combustion process. As details of the spray breakup process of cryogenic liquids under flash boiling conditions are not yet well understood, we use direct numerical simulations (DNS) to simulate the growth, coalescence and bursting of vapour bubbles in the superheated liquid that leads to the primary breakup of the liquid oxygen jet. Considering the main breakup patterns and droplet formation mechanisms for a range of conditions, we evaluate the effectiveness of the volume of fluid (VOF) method together with the continuum surface stress (CSS) model to capture the breakup of thin lamellae formed at high Weber numbers between the merging bubbles. A grid refinement study indicates convergence of the mass averaged droplet sizes towards an a priori estimated droplet diameter. The order of magnitude of this diameter can be estimated based on thermodynamic conditions.