Molecular simulation on viscous effects for microscale combustion in reactive shock-bubble interaction

Abstract

Molecular simulations of reacting shock-bubble interaction were performed to study viscous effects on the supersonic premixed combustion, using the Direct Simulation Monte Carlo (DSMC) method. Different bubble diameters that are comparable to the thickness of the normal shock wave (Mach=2) were taken into account. The evolution of bubble morphology indicates that viscous effects (diffusion and dissipation) leads to the disappearance of some typical phenomena, including the shock focusing, positive viscosity deposition and wake-like cross-section. Meanwhile, the viscous dissipation is mainly responsible for the enstrophy degeneration, which is enhanced by reducing the bubble scale and heat release in combustion. The observed combustion process, which is prominent near the bubble core, is more evident with a larger bubble diameter and unlikely to occur with the smallest bubble diameter. The corresponding reason is that the strong viscous diffusion reduces the collision events between fuel and oxygen molecules. In addition, a time-scale analysis was conducted to illustrate the viscous diffusion in the hydrodynamic and chemical reaction time-scales with different bubble diameters. As a consequence, a changing dimensionless Damkohler number was presented to understand the dominant diffusion effect in microscale supersonic premixed combustion.