Vent mixer can provide main flow directly into a recirculation region downstream of the mixer to enhance fuel-air mixing efficiency. Based on experimental results of three-dimensional velocity, vorticity and turbulent kinetic energy obtained by a stereoscopic PIV method, the performance of the vent mixer was compared with that of the step mixer which was used as a basic model. Thick shear layers of the vent mixer induced the increase of the penetration height. The turbulent kinetic energy mainly distributed along a boundary layer between the main flow and the jet plume. This turbulent field activates mass transfer in a mixing region, leading to the mixing enhancement.
An experiment was conducted to investigate the effect of injector/combustor geometry on combustion-induced peak wall pressure and associated upstream influence, as well as on mixing/combustion characteristics at an entrance Mach number of 2.5. The length of the constant area section downstream of injection orifices had a strong influence on the above-mentioned characteristics. However, the sweep of the rearward-facing steps on both side walls had little effect on these characteristics, nor did reversing them have any effect. The peak wall pressure and the length of the upstream influence agreed qualitatively with predictions of an analytical model and an empirical formula developed at Johns Hopkins University. Fuel jets injected from the model with the longest constant area section and the fuel equivalence ratio of unity, coalesced at a very early stage downstream of the fuel injection orifices. This coalescence led to a decrease in mixing rate downstream, despite the higher degree of mixing near the injection orifices. The combustion efficiencies were higher than those obtained at NASA Langley in the upstream region due to the higher mixing rate near the injection orifices.
A Relation between flow structure and local skin friction on the wall surface of the clustered linear aerospike nozzle models was investigated by conducting cold flow experiments using a skin friction sensor which has been developed in this study. The skin friction sensor is capable of measuring the local skin friction value on the wall. Measured skin friction value was compared to some existing prediction model and results found that the local skin friction value becomes drastically different from the one predicted by existing models. In cases of over-expansion condition skin friction was drastically increased by oblique shock wave. In cases of under-expansion condition skin friction was increased which was caused by interference of jet boundary of cell-nozzle flows jetted from adjoined cellnozzle.
The Mach 2 hydrogen–air supersonic combustor model without diffuser was designed and tested in atmospheric inflow condition. A new supersonic mixer, which was vent slot mixer (VSM), was developed and confirmed its performance as a mixing device. At low enthalpy inflow condition, plasma jet torch was used as the igniter and the flame-holder. The VSM showed that the interaction between the fuel and the air through the vent spread the fuel to the spanwise direction in the recirculation region. The shock train structure generally depends on the combustion pressure, and the combustor condition becomes the unstart condition with combustion pressure increase. Although the mixer loses its mixing efficiency under the shock train, the VSM shows that it stabilizes the combustion around the vent. Therefore, the VSM has distinct characteristics for the mixing efficiency and the combustion stabilization.
Single-time two-point spatial correlations of injectant concentrations in the supersonic mixing flowfield produced by a sonic transverse injection of air into a Mach 2.0 supersonic airstream were investigated using acetone planar laser-induced fluorescence data. Side-view and end-view contour maps were obtained in several planes to characterize the turbulent structure and three-dimensionality of the mixing flowfield. The correlation maps indicated an organized large-scale structure in the upper region of the jet. The side-view correlation maps revealed that the shape of the large dominating structure was elliptic and that its major axis turned from backward-leaning to forward-leaning as the reference point of correlation moved downstream. The end-view correlation maps showed that the instantaneous jet plume appeared by turns either in the top or in the lower sides of the time-averaged injectant plume in each cross section.
A UTOIGNITION characteristics of hydrogen fuel in a scramjet combustor were examined using a direct-connect test apparatus with particular reference to the effect of fuel injection patterns. Autoignition behavior fell into four distinct categories, separated by the three bounding curves. One of the boundaries was independent of fuel injection patterns, while the others significantly depended on them. In the case of fuel injection from a single wall or from a single orifice, local flame quenching caused by expansion wave emanating from the step on the opposite wall was observed. Compared with injection from a single orifice, injection from multiple orifices appreciably enhanced autoignition. The reason for this is that fuel jets from adjacent orifices and from the opposite wall tended to attenuate the local flame quenching caused by the expansion waves from the opposite wall. Ignition limit curves derived from the present experiment were compared with an autoignition criteria proposed by Huber et al. They agree well in the case of fuel injection from a single orifice. For the case of injection from multiple orifices, however, the agreement is poor.
Supersonic fuel injection with swirl is investigated to enhance the air-fuel mixing in scramjet combustors. We studied the effect of supersonic swirling jet for mixing enhancement with a PIV. In measurement of supersonic flow with a PTV, it is important for particles to track the flow, particularly when they pass through the shock wave. In this work, the size of particles was specified by gravitational separation. The response of the particles was examined by measuring velocity response downstream of a Mach disk. These results guaranteed the accuracy of the PIV system. Under-expansion jets in both cases with and without swirl were examined by the PIV. We got the result that the swirling jet mixed better than none swirling jet.
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