In aerospace Micro-Electro Mechanical Systems （MEMS）, the characteristic length scale of the flow approaches the molecular mean free path, thus invalidating the continuum description and enforcing the use of particl...In aerospace Micro-Electro Mechanical Systems （MEMS）, the characteristic length scale of the flow approaches the molecular mean free path, thus invalidating the continuum description and enforcing the use of particle methods, like the Direct Simulation Monte Carlo （DSMC）, to deal with the non-equilibrium regions. Within the slip-regime （0.01〈Kn〈-0.1） both approaches, continuum and particle-based, seem to behave well in terms of accuracy. The present study summarizes the implementation and results obtained with a 2nO-order slip boundary condition in a Navier-Stokes solver to address the rarefaction near the nozzle walls. Its assessment and application to a cold-gas micro-scale conical nozzle of 300μm throat diameter, discharging into the low-pressure freestream, constitutes the major aim of the work. The slip-model incorporates the velocity slip with thermal creep and temperature jump, thus permitting to deal with non-isothermal flows as well. Results show that the gas experiences an intense rarefaction in the lip vicinity, pointing to the limits of model validity. Furthermore, a strong Mach deceleration is observed, attributed to the rather thick subsonic boundary layer and supersonic bulk heating caused by the viscous dissipation, in contrast with the expansion to occur in large rocket nozzles during underexpanded operation.展开更多
文摘In aerospace Micro-Electro Mechanical Systems （MEMS）, the characteristic length scale of the flow approaches the molecular mean free path, thus invalidating the continuum description and enforcing the use of particle methods, like the Direct Simulation Monte Carlo （DSMC）, to deal with the non-equilibrium regions. Within the slip-regime （0.01〈Kn〈-0.1） both approaches, continuum and particle-based, seem to behave well in terms of accuracy. The present study summarizes the implementation and results obtained with a 2nO-order slip boundary condition in a Navier-Stokes solver to address the rarefaction near the nozzle walls. Its assessment and application to a cold-gas micro-scale conical nozzle of 300μm throat diameter, discharging into the low-pressure freestream, constitutes the major aim of the work. The slip-model incorporates the velocity slip with thermal creep and temperature jump, thus permitting to deal with non-isothermal flows as well. Results show that the gas experiences an intense rarefaction in the lip vicinity, pointing to the limits of model validity. Furthermore, a strong Mach deceleration is observed, attributed to the rather thick subsonic boundary layer and supersonic bulk heating caused by the viscous dissipation, in contrast with the expansion to occur in large rocket nozzles during underexpanded operation.
