Goro Masuya

Effects of Radiative Emission and Absorption on the Propagation and Extinction of Premixed Gas Flames

Premixed gas flames in mixtures of CH 4 , O 2 , N 2 and CO 2 were studied numerically using detailed chemical and radiative emission-absorption models to establish the conditions for which radiatively-induced extinction limits may exist independent of the system dimensions. It was found that reabsorption of emitted radiation led to substantially higher burning velocities and wider extinction limits than calculations using optically-thin radiation models, particularly when CO 2 , a strong absorber, is present in the unburned gas. Two heat loss mechanisms that lead to flammability limits even with reabsorption were identified. One is that for dry hydrocarbon-air mixtures, because of the differences in the absorption spectra of H 2 O and CO 2 , most of the radiation from product H 2 O that is emitted in the upstream direction cannot be absorbed by the reactants. The second is that the emission spectrum of CO 2 is broader at flame temperatures than ambient temperature, thus some radiation emitted near the flame front cannot be absorbed by the reactants even when they are seeded with CO 2 . Via both mechanisms some net upstream heat loss due to radiation will always occur, leading to extinction of sufficiently weak mixtures. Downstream loss has practically no influence. Comparison to experiment demonstrates the importance of reabsorption in CO 2 -diluted mixtures. It is concluded that fundamental flammability limits can exist due to radiative heat loss, but these limits are strongly dependent on the emissionabsorption spectra of the reactant and product gases and their temperature dependence, and cannot be predicted using gray-gas or optically-thin model parameters. Applications to practical flames at high pressure, in large combustion chambers and with exhaust-gas or flue-gas recirculation are discussed. Published in the Proceedings of the Twenty-Seventh International Symposium on Combustion , Combustion Institute, Pittsburgh, 1998, pp. 2619-2626.

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.

Ignition Characteristics of Methane and Hydrogen Using a Plasma Torch in Supersonic Flow

Ignition and flame-holding characteristics of methane and hydrogen using a plasma torch igniter were experimentally investigated in a supersonic airflow. The main airflow Mach number was 2.3, and the stagnation pressure and temperature corresponded to atmospheric conditions. Nitrogen, oxygen, a hydrogen/nitrogen mixture, and a methane/nitrogen mixture were used as feedstocks for the torch. The fuel was vertically injected from the wall where the plasma torch was attached. The wall pressure and the total temperature at the exit of the test section were measured. Ignition was confirmed for hydrogen injected both upstream and downstream of the torch. No strong dependence on the kind of feedstock of the torch for effectiveness of ignition of the hydrogen fuel was evident in a supersonic flow. By contrast, ignition of the methane fuel was confirmed only when it was injected upstream of the torch. In addition, the wall-pressure increase of the methane fuel was about half that of the hydrogen fuel. An important result for methane fuel was that using oxygen as a feedstock resulted in the most remarkable increase of the total temperature and the wall pressure.

Ignition Characteristics of Plasma Torch for Hydrogen Jet in an Airstream

The ignition characteristics of a plasma torch for a hydrogen jet injected parallel to a subsonic aire ow were experimentally studied. Theregion of the injector position whereignition occurred became gradually narrow with an increase in the distance between the fuel injector and the plasma torch and steeply narrow with an increase in the aire ow velocity. This suggests that the ignition limit strongly depends on the penetration height of the plasma torch, which is in inverse proportion to aire ow velocity. Nitrogen and oxygen were compared as feed stocks. Results obtained showed no difference in the behavior of the plasma jet itself, the ignition limit, and the e ame shape. Calculations of ignition delay time for an H 2/air mixture with the addition of N and O radicals showed the sameeffectivenessand werefoundto besuperiorto theHandtheOHradicals.However,thedegreeofdeterioration of an anode nozzle made of copper was more severe for oxygen plasma than for nitrogen.

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.

Effect of Film Cooling/Regenerative Cooling on Scramjet Engine Performances

Film cooling was modeled to allow performance prediction of scramjet engine design. The model was based on experimental results of the compressible mixing layer for the vicinity of the injection slot, and on analytical results of the turbulent boundary layer in the region far from the slot. The film cooling model was integrated with a quasi-one-dimensional scramjet performance prediction model. In an engine employing a combination of film cooling and regenerative cooling, coolant flow rate of the engine slightly exceeded the stoichiometric flow rate, even at high flight Mach numbers, and had the best specific impulse and system pressure performances. These advantages were achieved by an increase in the volume flow rate and a decrease in the velocity difference between the main flow and the coolant, both due to an increase in the film coolant temperature. The effective cooling system with a combination of film cooling and regenerative cooling was also advantageous with regard to avoidance of excess cooling of the engine wall.

Combined effects of radiation, flame curvature, and stretch on the extinction and bifurcations of cylindrical CH4/Air premixed flame

Abstract The combined effects of flame radiation, stretch, and curvature on the extinction and flame bifurcations of the cylindrical premixed CH 4 /air flames are numerically investigated with a detailed chemistry. The interaction between radiation heat loss and flame curvature is emphasized. The results show that a mixture below the standard limit can burn in the cylindrical flame configuration by imposing a moderate stretch rate. This flame is quenched either by radiation heat loss at low stretch rate or by incomplete combustion with an excessive stretch. As the fuel concentration increases, it is found that two kinds of flames, normal flame and weak flame, can exist at the same boundary conditions. A G-shaped extinction curve showing the flammable regions of these two flame regimes is obtained. The relation between the flammability limit of the cylindrical flame and the standard limit is discussed. Furthermore, comparisons between the cylindrical flame and the counterflow flame are made. The results show that the interaction of flame curvature and radiation heat loss greatly affects the flame strength and extinction. It is shown that flame curvature extends the radiation extinction limit and accelerates the opening up of the sublimit flame branch. Comparisons between the predicted results with the experimental data show good agreements both in the extinction limit and in the extinction flame diameter.

Ignition and flame-holding of H2 and CH4 in high temperature airflow by a plasma torch

Abstract The effect of airflow temperature on ignition characteristics of a plasma torch was experimentally investigated. Various combinations of fuels (CH 4 and H 2 ) and plasma jets (PJs) (N 2 , O 2 , and N 2 /H 2 ) were tested for a wide range of airflow temperature from 300K to 700K and the PJ power required for ignition was investigated. Ignition by the PJ occurred more easily in a high temperature airflow than in an atmospheric temperature airflow. The main reason for this was considered to be the increase in the reactivity of the fuel at high temperature rather than the effect of radicals in the PJ, because there was no difference in the spectroscopic measurement of the PJ between T air = 300K and 700K. The addition of H 2 to the N 2 feedstock was also effective for ignition enhancement of both fuels (H 2 and CH 4 ). In particular, the H 2 30%/N 2 70% PJ was able to ignite both fuels even at atmospheric temperature and the lowest electric power input for the stable operation. One of the reasons for this advantage of the H 2 30%/N 2 70% PJ was the heat release from the diffusion flame of the H 2 included in the feedstock with the airflow after injection. Moreover, the conditions around the local ignition site such as the local fuel concentration or the size of the contact area of the PJ and the fuel jet were found to be important factors for the success of ignition.

Mechanism and Control of Combustion-Mode Transition in a Scramjet Engine

A sidewall compression scramjet engine operated in two combustion modes underMach 6 flight condition, weakand intensive-combustionmodes.Theweakmode occurredbelow the overall fuel equivalence ratio ( ) of around0.4. Transition from the weak mode to the intensive mode occurred at 0:4, accompanied by a sudden increase in thrust. Mechanisms of the transition were numerically investigated in this study. Simulations captured the sudden increase in thrust at the mode transition. In the weak mode, combustion occurred in only a region near the topwall where an igniter was installed. The combustion region expanded toward the cowl with boundary-layer separation at the mode transition. Simulations demonstrated that low ignition capability resulted in the weak mode. This study demonstrated that the presence of additional igniters on the sidewalls improved the ignition capability and achieved the intensive mode in the entire range.