# Experimental Study of the Formation and Evolution of Gas Jets in Supersonic Combustion Chambers
###### Abstract  
A simple and efficient flow field visualization method (based on shadow imaging) was applied in a direct-connect test to explore the influence of the momentum flux ratio and the jet angle on the formation and evolution of nitrogen jets in supersonic combustion chambers. The test setup adopts a rectangular flow passage to simulate a flight condition with Mach number of 6 and altitude of 25 km. The experimental results showed that (a) the flow field visualization method adopted in this paper can clearly register the formation and evolution of the shock wave structure in the flow field and the windward shear vortex on the jet surface. (b) The evolution process of the windward shear vortex is significantly affected by the jet angle. In particular, the stretching position of the windward shear vortex changed when the jet angle was obtuse. (c) The bow shocks showed local distortion due to the periodic generation of large-scale shear vortexes. (d) Under the working conditions of the test, the largest instability of the flow field was found for a jet angle of 120\({}^{\circ}\). This work provides, on one hand, the experimental basis for clarifying the formation and evolution mechanism of transverse gas jets, and on the other, valuable data that can be used to validate numerical simulations. supersonic combustion chambers; flow field visualization; windward shear vortex
With the development of power systems, there are nowadays many types of transverse jets, such as the pure gas-liquid fuel jet (liquid scramjet), gas-liquid concomitant fuel jet (liquid scramjet) and gas-solid fuel jet (solid-powder scramjet) [15; 16]. In all these, the gas jet plays an important role. The flow field vortex structure generated by the gas jet, especially the windward shear vortex on the jet surface, plays an important role in the fuel mixing process. Current research on the formation and evolution of gas jets mainly focuses on numerical calculations [6; 17; 18]. Many scholars have carried out numerical simulations to illustrate the jet flow characteristics. For example, [PERSON] et al. used a direct numerical simulation to study the transverse jet structure in a supersonic flow field and introduced the formation and evolution mechanism of various rotating vortex pairs in detail, including their influence on flow characteristics [19; 20]. Schlieren and other related technologies based on trace particle imaging have been widely used in experiments. [PERSON] et al. used an experiment to show that a large-scale shear vortex structure is periodically generated at the shear layer formed between the jet and inflow near the nozzle [21; 22]. The shear vortex evolution in the flow field has three main steps: formation, stretching, and tearing. [PERSON] et al. used the Nano-based Planar Laser Scattering (NPLS) technique to observe the gas-phase jet wake [23]. Their experiment clearly showed the bow shock in the jet upstream, the turbulent boundary layer of the inlet flow, and the large-scale turbulent structure in the downstream area, including the K-H vortex and the high-turbulence wake structure. However, [PERSON] et al. argued that the generation of shear vortexes is considerably random and not periodic. The transverse jet in supersonic flow fields has been studied based on many factors, such as jet fuel type, jet momentum flux ratio, jet nozzle parameters (number, shape), etc. [7; 13; 14; 24]. Regardless of this, there are only a few experimental studies on the formation and evolution of gas jets, including the effects of the jet injection angle [25; 26; 27]. Moreover, the observation techniques used in current experiments have various shortcomings, such as a complex optical path and image interference. Schlieren usually requires a Z-type optical path, and the imaging quality of NPLS depends on the concentration and location of the tracer particles [23; 28; 29; 30].