Seiichiro Kumagai

Propagation velocity and structure of flames in droplet--vapor--air mixtures

Abstract Abstract-Flame speeds of ethanol and n-octane mixtures of mono-sized droplets, vapor and air have been measured to study the fundamental aspects of spray combustion. The state of the mixtures, which were prepared in an apparatus similar to the Wilson cloud chamber, was experimentally verified in detail. A rugged, undulated and thickened flame front with the cellular structure is peculiar to the flame propagation in the mixtures containing large droplets and is in sharp contrast to the smooth and continuous flame front observed with premixed gas mixtures or mixtures containing droplets of much smaller size. This difference in the flame structure, caused by the heterogeneous nature of the unburned mixture, can explain the observation that the burning velocity in a droplet-vapor-air mixture is larger than in a homogeneous mixture of the same overall fuel-air ratio, even on the lean side of the stoichiometric point, provided that the droplet diameter is large enough. Another aspect of the flame propa...

Thermodynamic and experimental determinations of knock intensity by using a spark-ignited rapid compression machine

Abstract By use of thermodynamic theory, the intensity of engine knock was calculated in tems of the energy of gas vibration which occurs when the end gas ignites spontaneously. In the theory, it is assumed that the gas vibration energy originates from the difference between displacement works of two types of burned gases which are produced by the flame propagation and the spontaneous ignition processes. Results of the theory were successfully verified by the experiment by using a spark-ignited rapid compression machine. In the experiment, the energy of gas vibration accompanied by knock was determined by application of the FFT method to the pressure record. The agreement between theoretical and experimental results suggests that in order to give a general validity to the knock intensity, it is reasonable to use the vibration energy as a quantitative measure.

The optimum condition for ignition of gases by composite sparks

To determine the optimum ignition condition for sparks consisting of a capacitance spark followed by a dc- (glow) or ac-discharge (1 MHz), the effects of gap width, electrode configuration, mixture strength, spark duration, and energy distribution between the two components on the minimum ignition energy were investigated, using a quiescent propane-air mixture. The condition in question is conveniently characterized by the optimum spark duration for which the minimum ignition energy is lowest and the corresponding energy value. For a dc-discharge spark, the well-defined optimum spark duration varies from about 50 to 300 μ sec and the minimum ignition energy for spark durations larger than the optimum increases in different modes, depending on the mixture strength and the quenching effect of spark electrodes. For an ac-discharge spark, the optimum condition for ignition is much the same as for a dc-discharge spark, but the minimum ignition energy and the spark duration are always proportional to each other above the optimum, and therefore the optimum spark duration is easily obtained up to about 5 msec. Flash-schlieren photographic observations of the initial behavior of the spark kernel confirmed that such differences in the mode of minimum ignition energy are related to electrostatic attraction by the negative electrode.

Ignition of Gases by Two Successive Sparks with Reference to Frequency Effect of Capacitance Sparks

Abstract The fact that the igniting ability of a capacitance spark increases as the frequency of an oscillatory discharge decreases has been previously ascertained down to about 260 kHz. The present study was undertaken to investigate how matters stand at lower frequencies. In the experiments using lean propaneair mixtures, the igniting ability of a capacitance spark is found to reach a maximum at a frequency of about 100 kHz. In order to elucidate the mechanism of such a frequency effect, two successive sparks of very short duration, which pass across the gap at a controllable time interval, are used as substitutes for halves of the first cycle of an oscillatory discharge. The experimental results show that the igniting ability of two successive sparks is highest at an interval of 10 to 50 μsec, which corresponds to a frequency of 50 to 10 kHz for the oseillatory discharge. This fact suggests that the discharge frequency rather than the spark duration is the predominant factor in ignition by a capacitance spark. A simplified thermal theory taking into consideration the shape and behavior of a spark kernel is given to explain the effect of frequency on the igniting ability of capacitance sparks.

Limiting factor of flame propagation in low-volatility fuel clouds

The limit of flame propagation, in terms of the number density or spacing of droplets and fuel concentration, in low-volatility fuel clouds has been determined, varying the droplet size. The stream of mono-sized droplets, produced by a rotating-disk atomizer, enters downward into the test section. The dropled size and number density are controlled by adjusting the rotating speed of the atomizer and the fuel feed rate, respectively. With increasing number density for a given droplet size, the following occur in sequence: droplet ignition with mispropagation; upward propagation of isolated diffusion flames surrounding the droplets; flame spread over the stream cross section. As the droplet size is increased, the critical number density for propagation decreases rapidly, whereas the droplet spacing increases at a very small rate according to a mathematical relation. The critical fuel concentration decreases sharply with increasing droplet size in the range from a few μm to about 40 μm, where transition from ignition of the premixed gases to relay ignition of the neighboring droplets occurs, and then decreases moderately toward less than half of the lower flammability limit. This creates a serious explosion hazard, since a fuel-air mixture which is not flammable in the premixed state, may become flammable through condensation of the fuel into droplets.

The effect of dc to 10 MHz electric field on flame luminosity and carbon formation

The effect of electric fields, dc and ac at frequencies up to 10 MHz and the maximum strength of about 3 kV/cm, on the luminosity and soot emission of flames was investigated by using a counter-flow diffusion burner and a nozzle burner. The luminosity, with both diffusion and premixed flames of acetylene and air, was found to be a complicated function of the frequency, electric field strength and gas flow rate. Under certain conditions, the luminosity could be either increased or decreased by up to a factor of about two. There was no effect on the soot emission from the counter-flow diffusion flame at frequencies above 500 kHz. The observations can be successfully explained by adopting a simple model for the growth of carbon particles.

Calorimetry of Spark Energy

Abstract A new calorimeter of balanced heat transfer type for the spark energy measurement has been designed and tested, and the calorimeter has been proved to exhibit a satisfactory performance, in accuracy and reproducibility of the measurements, as compared with conventional calorimeters of transient heat transfer type. The construction, experimental procedure and some results of this new type of calorimeter are described. Theoretical relations, which explain the results and will be helpful in the design of this type of calorimeter for different heat input and size, are also introduced.

Ignition of gases by high-energy sparks

For an ultra-high-speed spark-ignition engine, with a maximum speed approaching 20,000 rpm, it becomes very important to reduce the prepropagation period and the initial, or accelerating, period of flame development that follows the passage of a spark in a combustible mixture. This means producing as large an initial flame or flame kernel as possible. Even with the spark energy released by an ordinary ignition coil or magneto, which is only about 0.1 J, the time required for the initial flame to develop can be reduced considerably by using the multi-gap spark plug designed for an ultra-high-speed spark-ignition engine, because with this arrangement the original flame kernels merge into one flame in the ignition process. Two ignition sources for producing larger flame kernels have been investigated: (1) metal wires exploded by a condenser discharge, and (2) triggered capacity sparks, which can release energies that are a few to several orders of magnitude higher than the minimum ignition energy. High-speed schlieren motion pictures of the developing flame reveal that the pre-propagation period is reduced and subsequent flame propagation is promoted by the turbulence of the combustible mixture generated by the rapid ejection of the fine particles of the exploded material, and by the hot gas produced primarily by a triggered capacity spark which acts as a massive ignition source. A triggered capacity spark with 0.5 to 1.0 joule energy is sufficient for engine applications in the ultra-high-speed range.

Power and energy measurements of transient electric discharge

A simple and inexpensive calculating device for electrical power and energy of transient electric discharges is described. The calculation is made by multiplication of the voltage and current of the discharge by use of wideband integrated circuits.

ELUCIDATION OF COMBUSTION INSTABILITY IN SOLID PROPELLANT ROCKETS

With special reference to combustion instability the behaviour of solid propellants is studied to such a periodic heating as results from the gas oscillation in rocket motors. The present paper describes one of two methods devised to perform periodic heating of solid propellants. A small piece of solid propellant is subjected to radio heating in a capacitor of a VHF (40 MC) oscillator. The piece of solid propellant can be heated periodically by modulating the VHF oscillation with an audio- or ultra-audio frequency. The time required for ignition is measured for various frequencies of periodic heating. The first part of the time required for ignition, solid state, is not affected by heating frequency. On the contrary, it seems that the later stage of heating, solid-liquid state, is increased selectively at about 19 KC.