Muneo Izumikawa

Load Oscillations Caused by Unstart of Hypersonic Wind Tunnels and Engines

Large-amplitude load oscillations were observed during the tests of a hypersonic engine model in a freejet-type wind tunnel. To clarify the cause of the oscillations and their characteristics, oscillating wall pressures and loads on a drag model and engine models were investigated. Flowe eld was observed by shadowgraph to determine the cause of the large starting loads. Power spectral density functions and probability functions of wall pressures and loads were derived by the fast Fourier transform. The amplitude of the unsteady frontal pressure was correlated with the dynamic pressure. The magnitude of the starting load was related to the drag coefe cient of the models, and the expected maximum peak loads of a large-scale ramjet engine test facility were evaluated. Engine unstart loads were also simulated by means of secondary e ow injection into a small-scale model of a ramjet engine. With these methods, characteristics of engine unstart loads and the possibility of sensing engine unstart in its early phase were studied. Engine unstart could be sensed with pressure measurement around the engine throat before it became severe. Furthermore, engine unstart loads associated with scramjet engine combustion were related to the drag coefe cient of the engine.

Combustion Efficiencies of Aluminum and Boron in Solid Propellants

A closed-type strand bomb was used to trap all species participating in solid propellant combustion to measure combustion efficiencies of aluminum (Al) and boron (B) using titration techniques. Variations of combustion efficiencies (17) of Al and B with pressure and oxidizers of propellants were investigated. In order to improve B combustion, effects of the addition of Al, granulated propellants, and oxidizers were also investigated. It was found that burning of B particles could be enhanced by using fine powders. Boron particles granulated with potassium nitrate had the highest r/B (40%) in the strand burner. Burning of Al was found to be hindered by B burning when Al and B particles coexisted. These observations could be interpreted by considering the agglomeration and ignition processes of metal particles.

Vitiation Effects on Scramjet Engine Performance in Mach 6 Flight Conditions

Quasi-one-dimensional analyses with either chemical equilibrium or finite-rate reaction were conducted to evaluate the effects of contamination due to flow vitiation on engine performances, namely, thrust production. The incoming flow state was calculated based on measurements, and a higher total enthalpy for the freestream through a vitiation heater compared to that through a storage heater was found, the difference in the flow condition being responsible for two-thirds of the reduction in the thrust production with vitiation observed in the engine tests. With the finite-rate reaction calculation, the possibility of a further reduction due to the contamination effect on the combustion mechanism was found, which was responsible for one-third of the measured reduction. The one-dimensional analyses were further pursued to find matched test conditions with the storage heater to that with the vitiation heater in view of the thrust production and pressure distribution within the engine, and both freestream enthalpy and fuel equivalence ratio should be adjusted to attain the matched conditions.

Criteria for flame holding in H2-fueled scramjet engines

Although self-sustaining combustion has often been termed auto ignition in studies on scramjet combustors, it should be interpreted as a flame-holding phenomenon rather than ignition. Criteria for flame holding in H 2 -fueled scramjet engines have been discussed by approximating the recirculation zone in the combustor with a perfectly stirred reactor. The influence of mass flow rates and reactor volumes on flame holding can be summarized by a critical reaction time in the recirculation zone. It was found that the reaction time was proportional to pressure, with P −13 for H 2 reactions, and the recirculation zone should be occupied with reactants having equivalence ratios in the range 0.4

Supersonic combustion with Supersonic injection through diamond-shaped orifices

Supersonic injections through diamond-shaped injectors were applied to a supersonic combustor with a diverging section, with a Mach 2.4 incoming vitiated airflow at a total temperature of 2000 K, and combustion characteristics were experimentally investigated and compared to those with sonic injection through circular injectors. Without any flame-holding devices installed, scramjet-mode combustion was attained in wide range of fuel flow rate, in which the supersonic injections resulted in a higher pressure-rise in far-field than the sonic injections. Regardless of the injection scheme (i.e, orifice shape, orifice number, and injection conditions), transition to dualmode combustion enhanced the thrust production, and the supersonic injection through the diamond-shaped orifice had lesser ability to cause the transition than the sonic injection through the circular orifice. The ability was enhanced by installation of a recess and introduction of recess fueling, and resulting combustor performance was slightly better than that with the sonic injection, in the case with single orifice. Both the number of the orifice and the injection pressure (by changing the total orifice throat area) were varied for the supersonic injection through the diamond-shaped orifice for fixed fuel flow rates, and the change in combustion characteristics were also reported.

Matched Pressure Injections into a Supersonic Crossflow through Diamond-Shaped Orifices

Matched pressure injections through diamond-shaped injectors were applied to a Mach 2.5 supersonic crossflow, and penetration and mixing characteristics of the injected plume were experimentally investigated. In determining injection conditions, the effective backpressure to the injectant plume was assumed to be equal to pressure on a solid-wedge surface with the identical wedge angle to the injector orifice at a designed flow rate. Both subsonic and supersonic injections were introduced to attain the required low plume pressure at a high supply pressure, ensuring a stable injectant flow rate in reacting flows with high backpressures. The matched pressure injections through the diamond-shaped orifices resulted in little jet-airflow interaction. With the supersonic injection, the plume floated from the injection wall, and the best penetration height was attained, whereas the benefit of matched pressure supersonic injection over the matched pressure sonic injection was not as remarkable as the circular injector case. The penetration height increased at an overexpanded condition, while the maximum mass fraction decay was insensitive to the injection pressure. In the case with the subsonic injection, the plume shape was similar to a pillar, and a certain fraction of the injectant was left within the boundary layer region. The penetration height as well as the maximum mass fraction decay was found to be insensitive to the injection pressure.

Gas-phase ignition of a solid fuel in a hot stagnation-point flow

Experimental ignition times of a solid fuel in a hot oxidant stagnation-point flow are obtained. It is found that by allowing flow velocity to change, at constant flow temperature, the ignition time takes on a minimum value at a certain velocity. The lower velocity range up to the minimum point corresponds to a pyrolysis-controled region. Ignition times, which are nearly equal to gasification times, decrease as flow velocity increases. At high velocities above the minimum point, the pyrolysis-control region changes to a reaction-control region and ignition can not occur rapidly even if gasification is completed. A counterflow field of vaporized fuel gas and hot oxidant gas is formed after rapid vaporization, and in this stretched-flow field the exothermic reaction time in the gas phase lengthens as the flow velocity increases. Experimental ignition times, measured by changing either the oxygen concentration or the gas temperature of a flow, demonstrage the transition from a pyrolysis-control to a reaction-control region. Measured surface temperature histories verify a slow gas-phase reaction period.

Ignition of double-base propellant in a hot stagnation-point flow

Abstract Ignition times of double-base propellant in a hot stagnation-point flow are measured experimentally. External flow temperatures are 568°K, 639°K, and 727°K, flow velocities 4–32 m/sec, and starting heat fluxes 0.5–3.5 cal/cm 2 sec −1 . Ignition times are detected by variation of surface temperature and infrared (IR) emission with time. Experimental results are compared with a theoretical formula for condensed-phase ignition, derived earlier by the asymptotic method in the limit of large activation energy. The resulting comparison shows excellent agreement; reasonable overall activation energy is found to be 30 kcal/mole for ignition.

Ram and Ejector-Jet Modes Experiments of the Combined Cycle Engine in Mach 4 Flight Conditions

A combustion-capable combined cycle engine model which was constructed based on the rocket and ramjet technology was tested in Mach 4 flight condition. At this speed, engine is designed to shift its operation mode from an ejector-jet to a ramjet. Both modes were simulated by changing the rocket combustion pressure. Even with full rocket exhaust, no effect to the air flow could be observed. The injection point of the secondary fuel affected thrust performance. In the ramjet mode, the pressure rise due to the fuel combustion traveled to the entrance of the combustor, but it stayed near the injection point in the ejector-jet mode.

Experimental study on cellular flame propagation of blend fuels

An experimental study on cellular flame propagation of air and mixed fuel in a tube is presented. Methane and decane are used as light and heavy hydrocarbon, respectively. Propagation velocity, flame temperature, and cell size were measured. Cellular structure was observed for equivalence ratios below 0.85 as well as above 0.97, when nitrogen-diluted air (17% oxygen) was used as an oxidant. A phenomenological explanation is presented for the complex cellular flame propagation under consideration here, with attention being paid to the variation of the reduced Lewis number with fuel composition.