Tomoyuki Komuro

Ignition and Combustion Performance of Scramjet Combustors with Fuel Injection Struts

Ignition and combustion performance of a scramjet combustor with a fuel injection strut was experimentally investigated with Mach 2.5 vitiated air. Five strut models with different leading-edge geometry were tested without fuel injection to select the less flow-disturbing configuration. The nonreacting flowfields were also investigated by computation with a two-dimensional Navier—Stokes code. Using the selected strut, combustion and ignition tests were conducted. A pitot pressure and gas composition survey was carried out to deduce mixing and combustion efficiencies. It was found that mixing and combustion with a less flow-disturbing strut was considerably worse than those with a more flow-disturbing strut. Autoignition and forced ignition with plasma torches were tested for hydrogen. Ignition characteristics of parallel and perpendicular injection were quite different. The plasma igniters could successfully ignite both parallel and perpendicular fuel jets without a noticeable time delay between both sides of the strut.

Effectiveness of plasma torches for ignition and flameholding in scramjet

A newly developed plasma torch with a feed stock of air or oxygen was investigated experimentally in order to determine its effectiveness on ignition and flameholding in a scramjet combustor. This design comes from the viewpoint of total system design of scramjet engine and vehicle because it is preferable to utilize incoming air or onboard propellants as a feed stock. Three patterns of fuel injection were tested 1) from one orifice: 2) from four orifices on one wall; and 3) from all nine orifices on both walls. Ignition and flameholding phenomena were examined through direct photographs of internal and exit flames of the combustor and by wall temperature measurements. The specially devised plasma torch with air or oxygen was able to operate stably without any support gas, for example, argon. Ignition limit curves, with and without the plasma torch, were obtained on a plane relating the air total temperature to the fuel equivalence ratio for the three patterns of fuel injection, and then compared to each other. For a wide range of experimental conditions, it was shown that the effectiveness of an air or oxygen plasma torch was comparable to that of a nitrogen or argon-hydrogen plasma torch. For single-wall injection, it was observed that the plasma torch ignited the fuel jet located directly downstream, and the flame thus formed ignited adjacent fuel jets. In double-wall injection, however, ignition of the fuel injected from the wall opposite the plasma torch was unsuccessful. It was also found that, under some conditions, flameholding can be continued even after the plasma torch is turned off, most notably in the case of single-wall injection. The occurrence or nonoccurrence of this phenomenon is also shown in the ignition limit curves diagram.

Effects of injector geometry on scramjet combustor performance

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.

Mach 4 testing of scramjet inlet models

Six scramjet inlet models were tested in a Mach 4 wind tunnel. Wall and pitot pressure were measured and schlieren photographs were taken. Parameters of these models are side-plate sweep angle, contraction ratio, and cowl geometry. The shock pattern inside one of the models, as shown by schlieren photographs, coincides with the calculations. Both mass-capture ratio and total pressure recovery are 50-70%. There seems to be a better sweep angle and a better cowl length for maximum total pressure recovery.

Experimental study on autoignition in a scramjet combustor

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.

Experiment on multiple fuel supplies to air breathing rocket combustors

An experimental study on multiple fuel supplies to cylindrical subsonic mode combustors of air breathing rockets was made for the purpose of reducing combustor length. The experiment consisted of two parts, one in which all the fuel was supplied through a rocket with multiple nozzles and the other in which a portion of the fuel was directly fed into a secondary combustor with the rest being supplied through a rocket with a single nozzle. The results are compared with each other and with those of a reference experiment in which a rocket with a single nozzle without fuel injection was used. In the case with multiple nozzles, mixing and combustion efficiencies were higher than those in the reference experiment for the same combustor length. They collapse to a single curve against the combustor length nondimensionalized by the rocket nozzle exit diameter, except for the combustion efficiencies for unstable combustion. Increase of the injection mass flow rate makes mixing and combustion efficiencies rise, but it soon becomes less effective beyond a certain limit. Combustion instability observed in the reference case for long combustors is suppressed in both cases.

Performance Evaluation of Scramjet Combustors Using Kinetic Energy and Combustion Efficiencies

A performance parameter of scramjet combustors was introduced on the basis of thrust. It was written in terms of the kinetic energy and combustion efe ciencies. To evaluate this parameter from available data of direct connect combustor experiments, ideal nozzle calculation was carried out for each stream tube, starting from the data at the combustor exit, and the results were integrated across the nozzle exit to obtain the efe ciencies and performance parameter. The relative importance of heat release and aerothermodynamic losses were compared for various cone gurations of the fuel injectors and combustor ducts. For e ight Mach numbers less than 7, the heat release had a stronger ine uence on the performance than the losses. The optimum length, where the effects of both effects canceled each other, was 15 ‐20 times the e ow section width for the rectangular section combustor with perpendicular wall injectors.