Itsuro Kimura

The use of a plasma jet for flame stabilization and promotion of combustion in supersonic air flows

Abstract The feasibility of the use of a plasma jet was investigated for the improvement of flame stabilization and the promotion of combustion of a fuel jet injected into supersonic air streams. Fuel gas (hydrogen) was injected transversely into supersonic air streams of M = 2.1 or 2.7 ( P s = 1.0 atm), with the pressure ratio P t,i P s = 16 , and the plasma (hydrogen, nitrogen, or argon) produced by a plasma jet was injected at 25 mm downstream from the fuel injector in most cases. The onset and spread of combustion and its influence on the flow field were investigated by the schlieren method and direct photography, and also by measurements of temperature and pressure in the reacting gas stream. It was shown that the injection of plasma, which is produced with relatively small electrical power, is very effective for flame stabilization and promotion of combustion, when the position of injection is adequate. For example, in the supersonic air stream of M = 2.1 with very low static temperature ( T s = 154°K), the progress of combustion of more than 68% was observed with the aid of hydrogen plasma jet operated by 4.7-kW electrical-power expenditure (∼2% of chemical energy throughput), at the cross section 10 cm downstream from the injection port of the plasma jet. It was also shown that the effectiveness of nitrogen plasma on the promotion of combustion is nearly equal to that of hydrogen plasma, although, in the valuation based on input electrical power, the nitrogen plasma jet is superior to the hydrogen plasma jet.

Stability of laminar-jet flames

A self-excited flame oscillation of low frequency appears often, when laminar fuel jets burn in open air. The flame oscillation is axially symmetrical and has no connection with the size of combustion chambers and the length of fuel pipes. The present paper reports the results of the study of this phenomenon on the basis of the hypothesis that the flame oscillation is caused by the instability of the laminar-jet flows. The instability of axially symmetrical parallel flows of inviscid gases is studied in the case of axially symmetrical disturbances, and the criteria for the instability are derived. A detailed study based on the instability theory is conducted on an oscillating city-gas laminar-jet flame, measuring its mean-velocity and mean-temperature fields. As the results of the study, it is shown that the observed facts, such as the occurrence of the oscillation during combustion, the aspects of streamlines of the flame, and the frequency of the oscillation, can be explained reasonably with the instability theory. Thus, it is concluded that the hypothesis that the flame oscillation of this type is caused by the instability of the laminar jet flows is reasonable.

Control of soot emission of a turbulent diffusion flame by DC or AC corona discharges

The effects of DC or AC (14 kHz) corona discharges, formed between tips of opposed needle electrodes, on soot emission of a propane turbulent diffusion flame were investigated experimentally. It is shown that when a DC corona discharge (e.g., 3.6 W; 0.06% of the combustion energy released by the flame) or a discharge system composed of three AC coronas (e.g., 25.5 W in total; 0.43% of the combustion energy) is applied across the lower part of the flame, with a gap width such that the electrode tips are located outside the reaction zone, a marked reduction in soot emission is observed, without noticeable change in the shape of flame luminous region. When corona discharges are applied, increases of the density of charged species and/or charged soot particles are observed in the flame over the whole length downstream of the corona application. It is suggested that, in the case of DC corona application, additional air and inorganic charged species and electrons, produced in the air near the tip of the positive electrode, are carried into the flame mainly by corona winds, whereas in the case of the AC corona application the inorganic charged species and electrons are carried into the flame by diffusion processes. The charged species and electrons carried into the flame may influence the state of charging of incipient soot particles and also reduce the concentration of growing ions, i.e., soot precursors, which directly relate to the soot emission of the flame. In TEM photographs it was found that separate soot particles, or those forming chains in the flame, decrease in mean size with the application of corona discharges. Smaller size soot particles burn faster than larger size particles in the high-temperature oxidizing atmosphere at the flame top region.

Mechanism of Explosion Induced by Contact of Hypergolic Liquids

The nature and mechanisms of explosions caused by the contact of hypergolic liquid propellants were investigated in detail for several combinations of fuels and oxidizers. It is shown that the explosion phenomena observed can be classified into three categories. 1) In the case of N2H4/NTO, sudden gasification of a superheated liquid layer formed at the boundary of two liquids occurs spontaneously and a detonation-like reaction proceeds in the reactive mixture produced. 2) In the cases of MMH/NTO and UDMH/NTO, the sudden gasification is caused by the shock of a local ignition, and a turbulent-combustion reaction proceeds in the reactive mixture produced. 3) In the cases of hydrazine type fuels/FNA, the sudden gasification occurs spontaneously as in the case of N2H4/NTO, but it is not augmented by chemical reaction, and in these cases the observed explosion is weak. Information on the vapor layer, which is formed between reactive fuel droplet and pool liquid plays an important role for the occurrence of explosion, is also given, that is based on the high-speed motion picture records.

Fundamental Study on Laser Plasma Accelerator for Propulsion Applications

Recent advances of laser plasma accelerators that promise very high gradients of the accelerating fields and enable small sized accelerators were reviewed, and a feasibility study of the utilization of these accelerators for space propulsion was conducted. Forward plasma acceleration through a foil target with a low-power laser pulse was observed, and resultant impulse was measured for various targets. The significant merit of this thruster was discussed in the case of operation of this type of thruster on earth and/or solar orbit, in which solar power is available. Related issues on this point were also discussed.

Influence of constrictor size on thrust performance of a very low power arcjet

An experimental study was performed to evaluate the feasibility of arcjet operation at very low power levels ranging 5 W 35 W. The very low power arcjet was run using nozzles with different material and geometry. Nitrogen gas was used as the propellant. Both propulsive performances and thermal characteristics at the constrictor exit were investigated for conventional nozzles which consist of an assemble of tungsten nozzle parts, and modified nozzles, consisting of an assemble, an insulator and a tungsten anode. In the modified nozzles, a ceramic material or an insulator was used as a part of a constrictor to allow an arc column penetrate further downstream of the constrictor or to maintain the high-voltage mode discharges and to reduce the electrode losses. Stable operations with the specific impulse levels of ~ 270 sec at very low power levels ranging about 5 W 35 W with the constrictor diameter of 0.3 mm or 0.5 mm were confirmed at efficiencies between 30 and 40 percent, except a singular case, glow discharge, in which little effect in propulsive performance was observed with the expense of electrical power. At higher specific powers the specific impulse was relatively independent of mass flow rate. At lower specific powers, the specific impulse for the lower mass flow rate was slightly above that for the higher mass flow rate. The constrictor diameter was found to have significant effect on the thermal characteristics (heavy particle temperature and thermal efficiency) of the internal gas flow and the performance of the device. With partially insulated nozzles the specific impulse and thrust efficiency were significantly increased compared to conventional nozzles.

OPTIMIZATION OF ARC CONSTRICTOR SIZES IN LOW POWER ARCJET THRUSTERS

It is known that the performance of a low power arcjet is influenced by the geometry of the constrictor. The merging of the arc with the propellant gas in the constrictor is a complex process involving electrical and non-equilibrium chemical phenomena with high energy transport rate to the wall. An experimental investigation was performed to investigate the effect of the constrictor geometry on the characteristics of arc discharges and the heat transport rate to the wall, using quartz glass constrictors with different geometries. Following results were obtained. 1) The discharge voltage of the arc increases with the constrictor length (/con) almost linearly, though no distinct effect of the constrictor diameter (cfcon) is observed on it. 2) The thermal efficiency, defined as the ratio of the power of gas ejected from the constrictor (Peject) to the input electrical power (Pin), increases with increasing gas pressure (Peon) and with decreasing /con generally, while with decreasing rfcon the efficiency takes a maximum value at a middle length of dcon. 3) The diameter of arc column decreases with increasing Peon, with increasing /con, and with decreasing rfcon by the thermal pinch effect. 4) The main feature of the variation of constrictor gas temperature with /con and fifcon coincides with that of ejected power and thermal efficiency. It is suggested that the existence of the maximum in the thermal efficiency for the change of cfcon, at a fixed Peon and a fixed mass flow rate of propellant, is due to the coexistence of two type processes, one suppresses heat transport to the constrictor wall and another promotes it.

Multi-Jet Effects of Micro-Nozzle Array in Very Low-Power DC Micro-Arcjets

Microfabrication of a 3 x 3 rectangular micro-nozzle array with each exit height of 500 μm using ultra-violet lasers and its operational tests were conducted. Slightly higher thrust and specific impulse were obtained at higher background pressure of 53 Pa. To evaluate thrust characteristics of the array-nozzle, thrust performance was compared with singlenozzles. Significant increases of the thrust and specific impulse with mass flow could be obtained with the array-nozzle case even in the lower background pressure of 4 Pa. A preliminary DSMC computation on internal and exhaust nozzle-flow characteristics of micro-nozzles of multi-nozzle array was also conducted. It was shown that the use of the multi-nozzle array was effective in suppressing expansion of each under-expanding jet, and in inducing axially confined jets, through the interactions of the jet boundaries, or multi-jet effects.

Microfabrication of Quartz Micro-Arcjet Nozzles with a Fifth-HG Nd:YAG Laser

Microfabrication of micro-arcjet nozzles with fifth-harmonic Nd:YAG pulses (wavelength 213 nm) and their operational tests were conducted. Micro-arcjet nozzles were machined in a 1.2 mm thick quartz plate. Sizes of the nozzle exit were 0.44 mm in height and constrictor height of 0.1 mm. For an anode, a thin film of Au (~ 100 nm thick) was coated by DC discharge PVD in vacuum on divergent part of the nozzle. As for a cathode, an Au film was also coated on inner wall surface. In operational tests, a stable discharge was observed for mass flow of 0.4 mg/sec, input power of 6 W. In this case, plenum pressure of the discharge chamber was 50 kPa. With 6 W input power, thrust obtained was 1.2 mN giving specific impulse of 147 sec with thrust efficiency of 7 %.

The mechanisms of explosions induced by contact of hypergolic liquid propellants, hydrazine and nitrogen tetroxide

Detailed falling droplet experiments (droplet: N2H4, pool: N2O4) were made, employing high-speed photography, pressure transducers and photodiodes to obtain information on the nature and the mechanisms of explosions. The probability and the strength of explosions increase with droplet size and impact velocity; and the strength has a tendency to saturate, when the impact velocity is increased beyond ≈4.0 m/sec. The magnitude of explosion overpressures observed and its dependency on droplet size could be explained based on the blast wave theory, using the source energy evaluated from high-speed motion picture records. The explosions of N2H4/N2O4 are always accompanied by simultaneous light emission. The time lag of the explosion fluctuates statistically for a fixed experimental condition. Introducing the concept of explosion probability in function of unit time, μ(t), it was shown that the time lag is composed of two parts: a first part of length to, where μ(t)=0, and a second part of length t1, where μ(t) is kept nearly constant (μ(t)=-μ). The length t0 was insensitive to droplet size and impact velocity (≈0.11 msec), although the value of -μ was influenced by them. High-speed motion picture records of ≈105 frames/second clearly showed a dispersed layer at the depressed N2O4 surface (the more volatile component), immediately before the beginning of the explosion. It is suggested, from considerations of the abrupt appearance of this layer and also the nature of the first period of explosion time lag, that the most likely explanation for the layer, which plays the role of a trigger for the explosion, is the sudden gasification of a thin surface layer of liquid N2O4, which was heated above the boiling point.