Numerical analysis of mixing in transonic accelerated flow

To analyze the gasdynamical generation of nanoparticles, the turbulent mixing of the heavy gas tetraethoxysilane (TEOS - Si(C2H50)4) in a hot accelerated air co-flow is numerically investigated for two injector configurations, one with a plane geometry and one with a streamwise-ramp geometry. The precursor gas TEOS is premixed with nitrogen and ejected through circular holes in the base of an injector into the nozzle flow at a co-flow Mach number of Maco-flow ˜ 0.66 and a temperature of Tco-flow ˜ 1200K. Immediately downstream of the blunt trailing edge of the injector the flow field is accelerated to supersonic speed. The mixing is dominated by pronounced free shear layers generating trailing vortices, which transport the partially mixed precursor gas but do not enhance further mixing. After a certain mixing length a shock train decelerates the flow to subsonic speed and rapidly increases the temperature, which, after the elapse of an ignition lag, starts the reaction of the precursor with oxygen that generates the nanoparticles. The mixing process is determined by a multi-species large-eddy simulation (LES) that covers the mixing process in the injector wake and across the shock wave. The analysis of the mixing process upstream of the shock shows advantages for the injector with a streamwiseramp geometry over the plane injector configuration. Especially the temporal homogeneity is improved as a result of reduced vortex shedding at the injector base. Nevertheless, the mixture is sub optimal, even for the injector with streamwise ramps. The mixing process suffers from a rapid turbulence decay in the wake of the respective injector due to the acceleration of the flow. The remaining fluctuations are not strong enough to sufficiently drive the mixing process. The overall mixing quality is approximately 25% higher for the ramp configuration at a value of MQII ˜ 0.62 with MQ = 1 defining a perfect mixture. Then, the interaction of the injector wake flow with different shock configurations is investigated. In a first step, a shock without boundary layers interacts with the wake flow of the plane injector. The shock is almost normal and correspondingly the supersonic flow is immediately decelerated to subsonic flow and rapidly heated. It deforms the vortical structures in the wake flow and induces turbulence that supports the mixing process. Hence, the mixing homogeneity increases, especially regarding the temporal homogeneity. Finally, the boundary layers along the walls downstream of the nozzle are generated by synthetic turbulence such that the wake flows of both injector setups are encompassed not just by stream surfaces but by boundary layers. A further LES is conducted to investigate the interaction with a shock, or a shock system, respectively. The overall flow pattern of this problem is significantly more intriguing than that without boundary layers. The primary shock takes on a ?-shape, the boundary layer rapidly thickens at the interaction and downstream of the shock a reacceleration takes place, followed by further shock and acceleration cycles, forming a shock train. The turbulence intensity rises, the vortical structures are deformed, and the mixing quality rises, with the ramp injector remaining superior to the plane version.