Saburo Yuasa

Ignition and combustion of aluminum in oxygen/nitrogen mixture streams

Abstract The ignition and combustion of a solid cylinder of Al was studied experimentally by using the stagnation region of impinging O 2 N 2 ( 20 80 ) mixture streams over a wide range of pressure and velocity of the streams. When an original oxide coating did not exist on the Al surface, the critical spontaneous ignition temperatures were lower than the melting point of Al2O3, decreasing with reduction in pressure and velocity of the streams. Whether surface reactions initially formed a protective Al2O3 film on the surface or not decided the ignition, leading to the development of a luminous diffusion flame. When the Al surface was covered with an original oxide coating, it was observed that ignition occurred in the gas phase at the instant of the breaking of the coating. During combustion, the Al surface remained clean or covered with porous deposits. AlO was produced in the gas phase through the reactions of Al vapor with O2. The AlO had a peak concentration in the gas phase away from the surface. Al2O3 also condensed in the gas phase. N2 in the mixtures was found not to affect the ignition and combustion process due to much less reactivity with Al. The combustion mechanism for the Al O 2 N 2 system has been postulated, including the surface and gas-phase reactions producing Al vapor, AlO, Al2O, AlO2, O, and condensed Al2O3. The ignition mechanism is also discussed.

Ignition and combustion of metals in a carbon dioxide stream

In the prospect of using metals as fuel of breathing combustion engines in carbon dioxide rich planet atmospheres without oxygen such as those of Mars and Venus, a fundamental study was performed experimentally on the ignition and combustion of metals in an impinging pure CO2 gas stream. Metals selected were lithium, magnesium, boron and aluminum, because of their high heats of reaction with CO2. To help the understanding of combustion processes of the metals in CO2, the chemical equilibrium compositions of reaction products and flame temperatures were calculated, and compared with the experimental results. Li and Mg could ignite spontaneously in the CO2 stream. The ignition process of Li in the CO2 stream had two distinct steps, first a surface reaction and then a gas phase reaction. This process was found to be similar to those of Mg and Na in air streams, but was different from that of Li in an air stream. The spontaneous ignition temperature of Li in the CO2 stream was much lower than that in the air stream. It was also found that Li burned in vapor-phase at first and then on the metal surface. At the ignition in the CO2 stream, the Mg surface was covered with a protective film against further oxidation, resulting a higher spontaneous ignition temperature than that in the air stream. Mg could burn in vapor-phase. B could not ignite in the CO2 stream, but the reactions occurred appreciably on the surface. The reaction rate was quite slow, compared with that in an oxygen stream, and the molten boron oxide appeared on its surface. Al could not ignite within the temperature limits of our apparatus. However, the possibility of its ignition and combustion in the CO2 stream was demonstrated in a different experiment allowing the attainment of somewhat higher temperatures.

Effects of Pressure and Oxygen Concentration on Ignition and Combustion of Boron in Oxygen/Nitrogen Mixture Streams

Abstract An experimental study on the spontaneous ignition and combustion of small lumps of boron was performed by using the stagnation region of impinging O 2 /N 2 mixture streams over a wide range of ambient pressure and oxygen concentration of the streams. Two types of ignition processes, the gas-phase ignition with a green–white flame and the surface ignition without a flame, were observed. The critical ignition temperature for gas-phase ignition decreased with decreasing pressure. With decreasing oxygen concentration, the ignition temperature slightly decreased until the surface ignition occurred, and then increased abruptly. During combustion with a flame, the sample appearance, the sample temperature, emission intensity distributions of specified species for boron combustion, and the mass burning rates were measured. It was found that BO, which was generated heterogeneously on the boron surface, and BO 2 , which was generated in the gas phase, reacted in the flame close to the surface to finally form gaseous B 2 O 3 , which condensed at the periphery away from the flame. The qualitative characteristics of the flame structure did not depend on pressure and oxygen concentration. The thickness of the flame decreased with decreasing oxygen concentration. The burning rates of boron were found to decrease with decreasing pressure and oxygen concentration. The ignition and combustion mechanism is discussed: BO produced from heterogeneous reaction with combustion products plays a crucial role in determining the gas-phase combustion or the surface combustion. The dependence of the ignition temperatures on pressure and oxygen concentration is explained in terms of the balance between the rates of production and evaporation of B 2 O 3 .

Effects of oxygen concentration on combustion of aluminum in oxygen/nitrogen mixture streams

The authors studied the ignition temperature of a solid cylinder of aluminum with an original oxide coating placed in a stagnation region of oxygen/nitrogen mixture streams over a wide range of oxygen concentration of the streams. The sample temperature during combustion and the flame structure of burning aluminum were measured. The burning rates of aluminum based on the experimental results were also estimated. The study showed, firstly, the ignition temperature of the aluminum sample was almost constant irrespective of the oxygen concentration when the sample was heated at a constant rate up to its ignition. Secondly, with an increase of the oxygen concentration, the peak positions of Al and AlO emissions shifted to the aluminum surface in the flame zone. Thirdly, the burning rates of aluminum depended on the sample temperature and slightly on the oxygen concentration. The mass transfer number of aluminum was much smaller than those of various hydrocarbon fuels in air.

Effects of Swirling Liquid Oxygen Flow on Combustion of a Hybrid Rocket Engine

In order to investigate combustion processes of directly exposed PMMA to LOX, tests of a small hybrid combustor with a single port filled with LOX or GOX without swirling were carried out. It was found that combustion processes of LOX/PMMA are determined by evaporation of LOX. Experiments of combustion in a hybrid rocket engine with a swirling LOX flow were conducted. Axial profile of local fuel regression rates showed that evaporation of LOX made the effects of swirl of the injected LOX small. Fuel regression rates, C* efficiency and specific impulse of the hybrid rocket engine with the swirling LOX flow were smaller than those with the swirling GOX flow. The performance of the engine was lower than that with GOX. The existence of LOX in the combustion chamber decreased the combustion characteristics due to the loss of the angular momentum, the decrease of the net Go and the longer combustion time. Combustion oscillation also occurred at a certain condition and this combustion oscillation was confirmed to be a “Chugging” mode due to combustion time lag of LOX.

Combustion characteristics of Mg−CO2 counterflow diffusion flames

To examine the details of the Mg−CO2 combustion consisting of the gas-phase reactions and the surface reactions, we tried to separate the Mg−CO2 flame from the surface reactions. For this purpose, we formed a stable Mg−CO2 counterflow diffusion flame between the Mg vapor and a CO2 stream by using a Mg vaporizer with many small ejection holes. The Mg−CO2 counterflow diffusion flames contained two types of flames: the luminous flame and the dark flame. In the luminous flame, the homogeneous reaction of Mg with CO2 forming gaseous MgO and CO occurred, as well as the condensation of MgO. The dark flame was observed when the Mg ejection velocity was small. The heterogeneous reaction of Mg with CO2 forming condensed MgO and C occurred. The change from the luminous flame to the dark flame was caused by the heat loss to the Mg vaporizer. The flame stability limits diagram of Mg−CO2 counterflow diffusion flame was obtained, in which the Mg ejection velocity is plotted against the stagnation stream velocity gradients. There exists a critical value of the Mg ejection velocities, below which the flame can never be stabilized. This limit is due to the thermal quenching of the flame near the Mg vaporizer. Also, the critical value of the stagnation stream velocity gradients exists and is due to the chemical limitations on the combustion rate in the flame zone. This flame stability limits diagram was similar to those of the ordinary gaseous fuel-oxidizer counterflow diffusion flames.

Ignition and combustion of ammonium perchlorate in a hydrogen atmosphere

In the view to using ammonium perchlorate (AP) as an oxidizer in a hydrogen-breathing combustion engine in the Jupiter atmosphere, a fundamental study was performed experimentally on the ignition and combustion of a lump of AP in a hydrogen atmosphere. AP was found to ignite just after decomposition. The burning AP had a decomposition flame adjacent to the AP surface, and a flame of light-violet and orange color attributed to OH, ClO, and H 2 O emissions developed in the gas phase surrounding the decomposition flame. During combustion, the burning rate followed the same d 2 law as that for liquid fuel droplets. The measured emission intensity distributions of OH, ClO and H 2 O showed that OH had a peak concentration away from the AP surface, and ClO was generated near the surface. The flame location was much closer to the AP surface in comparison with those of various hydrocarbon fuels in air, which was explained in terms of the large mass ratio of the stoichiometric oxidizer to fuel. To elucidate the combustion process of AP in an H 2 atmosphere, the flame structure in the gas phase was calculated assuming that the oxidizing gases produced by an AP decomposition flame react with hydrogen and using a detailed gas-phase kinetic mechanism. The calculation results were compared with the measured emission intensity distributions.

Prediction of Time Variation of Ballistic Parameters For a Swirling-Oxidizer-Flow-Type Hybrid Rocket Engine using Burning Data

For hybrid rocket engines, there are some unique relationships between the parameters, such as the fuel regression rate, oxidizer mass flow rate, equivalence ratio and so on, which determine the burning properties. These relationships are strongly related to the engine performance. Our final goal of this study is to establish the optimum design method which considers the engine performance of hybrid rocket engines for practical hybrid rockets varying with time. In this paper, we focused on the time variation of ballistic parameters. First, we established the calculation method which well simulates the burning condition of the swirling-oxidizer-flow-type hybrid rocket engine. Next, we predicted the time variation of the burning properties and ballistics parameters using this calculation method and some experimental burning data. The simulation results showed little difference in the time variations of the combustion pressure and thrust from the experiment. The simulated values of the engine efficiencies of Isp, C* and CF obtained well agreed with those experimentally. The simulation method proposed here is appropriate for predication of time variation of ballistic parameters and may be useful as a design tool for hybrid rocket engines.

Combustion Characteristics of Methane-CO2 Mixture and a Microturbine Cogeneration System Utilized Sewage Digester Gas

This paper describes an approach to utilize sewage digester gas as a fuel for gas turbines. Sewage digester gas is composed of about 60% methane and 39% carbon dioxide. To apply it as a gas turbine fuel requires optimizing the combustion system to improve the combustion efficiency, flame-holding characteristics, etc. This paper presents an approach whereby a mass-produced microturbine and its peripheral equipment can be converted for such application with a minimum of modification and without the use of extraordinary combustors. The approach is described whereby a recuperative cycle microturbine having rich-burn, quick-mix, lean-burn (RQL) combustor is started up with a high-Btu fuel and the fuel is switched to digester gas when the inlet-air has been preheated to 600K or higher. This approach has proven that reliable starting, stable operation from idling to the rated power output, and efficiency equivalent to that obtained with a high-Btu fuel, can be achieved by the microturbine utilizing sewage digester gas.Copyright

Hybrid Propulsion Technology Development in Japan for Economic Space Launch

The demand for the economic and dedicated space launchers for vast amount of lightweight, so-called nano-/microsatellites, is now growing rapidly. There is a strong rationale for the usage of the hybrid propulsion for economic space launch as suggested by the assessment conducted here. A typical concept of development of such an economic three-stage launcher, in which clustering unit hybrid rocket engines are employed, is described with a development scenario. Thanks to the benefits of hybrid rocket propulsion, assuring and safe, economic launcher dedicated to lightweight satellites can be developed with a reasonable amount of quality assurance and quality control actions being taken in all aspects of development such as raw material, production, transportation, storage, and operation. By applying a multi-objective optimization technique for such a launch system, examples of possible launch systems are obtained for a typical mission scenario for the launch of lightweight satellites. Furthermore, some important technologies that contribute strongly to economic space launch by hybrid propulsion are described. They are the behavior of fuel regression rate, the swirling-oxidizer-flow-type hybrid rocket, the liquid oxygen vaporization, the multi-section swirl injection, the low-temperature melting point thermoplastic fuel, the thrust and O/F simultaneous control by altering-intensity swirl-oxidizer-flow-type (A-SOFT) hybrid, the numerical simulations of the internal ballistics, and so on.