Takashi Niioka

Extinction phenomenon of premixed flames with alkali metal compounds

Abstract In order to investigate the flame suppression mechanism of alkali metal salts, ultrafine powders of these salts with diameters less than 1 μm were introduced into premixed flames in the form of mists of their water solutions. Premixed flames inhibited by the mist suddenly blew offif the mist density was increased beyond a critical value. This sudden extinction of flames was peculiar to alkali metal salts and it was not observed for other inhibitors. From the existence of the extinction condition, the time necessary for the salts to have a chemical inhibition effect was found to be 10 ms at 1200K and 0.5 ms at 1800K. This critical time was found to correspond to the decomposition time of the salts. The result shows that alkali metal compounds must stay in the flames longer than this critical time in order to work as chemical flame suppressants. If the residence time in flames is less than the critical time, the inhibition effect of the powders will be purely thermal, especially for powders with larger diameters. This model for flame suppression of alkali metal compounds can explain many contradictory results in experimental studies reported previously.

An experimental study of droplet ignition characteristics near the ignitable limit

Abstract The present paper reports the experimental results for ignition characterisitcs of a fuel droplet near the ignitable limit. It is found experimentally that the ignition time increases as the droplet diameter decreases at the region near the ignition limits, and that, over the broad range, ignition occurs more rapidly as the diameter increases if the initial droplet temperature is high. The reason why such pehnomena occur is examined. A comparison between the results of Faeth and Olson and the present ones is made and the discrepancies discussed. The effect of initial droplet temperature on ignition is examined. The general trends agree with the results of computation obtained previously.

Ignition of a reactive solid in a hot stagnation-point flow☆

Abstract A theory is developed to describe ignition of a solid body that is quickly immersed in the flow of a hot gas. The homogeneous solid, semi-infinite in extent, experiences a one-step reaction of the Arrhenius type without reactant depletion. This exothermic process, which produces ignition, is treated by asymptotic methods based on large values of the ratio of the overall activation energy to the thermal energy of the solid. Simplification is provided by the usually justifiable approximation of quasi-steady gas flow. Parametric results for the ignition time are developed and compared with the few experimental results that are available. It is reasoned that the configuration analyzed herein is attractive for well-controlled experiments that simulate conditions encountered in practice.

Combustion and microexplosion behavior of miscible fuel droplets under high pressure

Experiments have been performed on single droplets of miscible fuel under high pressure. Filament-supported droplets with relatively large diameters 1 to 1.9 mm are burnt in high pressure environments up to 2 MPa. Mixtures having n-heptane as the more volatile component and n-hexadecane as the less volatile one are used. A three-staged combustion behavior in d 2 versus time plots are observed by shadowgraph measurement of droplet diameter, in accordance with a former experiment under atmospheric pressure, but it is found that this staged-burning diminishes with increasing pressure. The pressure ranges responsible for microexplosion are also examined by changing the initial fuel composition. Two types of droplet microexplosion are observed, one of which occurs abruptly and intensively for high concentrations of the volatile component, and the other takes place in a slow-moving manner for low concentrations of the volatile component. In the former case a high-speed camera suggests a rapid gas evolution from inside a droplet, and therefore microexplosion must be caused by heterogeneous nucleation based on the boiling point in the later process of combustion. In the latter case, separation of the crescent part from a droplet is observed, and droplet disruption is considered to correspond to homogeneous nucleation due to the temperature within the droplet over the limit of superheat. It should be noticed that both microexplosions diminish due to increasing pressure. The phenomenological consideration of pressure limits for microexplosion is discussed.

Experimental study on inhibited diffusion and premixed flames in a counterflow system

Utilizing a counterflow flame system, the inhibition effect by CF3Br was examined. The extinction limits of the counterflow diffusion flame, which depend on fuel and oxygen concentration, flow velocity, and which side (fuel or air) the inhibitor is added, are measured by increasing the amount of inhibitor. As a result, it was found that all the data could be rearranged onto one curve using the mass flux fraction of inhibitor in the diffusion flame. Temperature measurements suggest that the ratio of oxygen and fuel mass flux was stoichiometric even when the flame was inhibited and that the reaction zone widened. Taking advantage of the systems stability and adiabaticity, the fundamental behavior of inhibited premixed flames was observed in twin counterflow premixed flames formed on the sides of the stagnation plane. This system readily confirmed that the final burned gas temperature did not change with the addition of a small amount of inhibitor but did show that the flame speed was altered. Also, extinction tests of the premixed flame showed that the inhibition effect was stronger in rich flames, as has been indicated in the earlier measurements elsewhere.

Ignition process of fuel spray injected into high pressure high temperature atmosphere

The ignition process of fuel sprays injected into high pressure high temperature atmosphere has been studied experimentally and theoretically, and a new concept for the ignition process has been proposed. Experiments were conducted using a large high pressure combustion chamber and a high pressure fuel injection system. Ignition and fuel spray behavior were observed with high speed photography. From these experiments, it may be inferred that the ignition of fuel spray occurs at the stagnation region of the fuel spray tip. The color of the first flame observed was blue. The stagnation velocity gradient at the fuel spray tip, proportional to the fuel spray tip speed and inversely proportional to the fuel spray tip width, decreases rapidly with time from the start of fuel injection or with distance from the fuel nozzle tip. Based on experimental results, a new concept for fuel spray ignition, derived from the knowledge of ignition in a stagnation flow field, has been proposed. By using the equations governing ignition phenomena in the stagnation flow field solved by the asymptotic method, the ignitable limit and ignition time in the stagnation region at the fuel spray tip have been analyzed. These studies show that the ignition behavior of the fuel spray can be well explained by considering the effects of the stagnation velocity gradient at the fuel spray tip on the ignition time of the fuel-air system. The ignition delay of a fuel spray is divided into two parts: one is the time spent for reducing the velocity gradient at the spray tip below the critical velocity gradient for ignition; the other part is the time for an ignition reaction at the given velocity gradient. Since the latter is much smaller than the former, most of the ignition delay is time for reducing the velocity gradient at the fuel spray tip below the critical velocity gradient for ignition. Experimentally obtained ignition delay data can be predicted fairly well by using the above concept.

Ignition time in the stretched-flow field

Ignition times in a typical stretched-flow field counterflow system are calculated for both initially unmixed and premixed gases. To simplify the calculation, ideal gases with equal velocities are assumed, and asymptotic analysis for the limit of large activation energy is developed in order to reduce the six parameters which control the field. The solution of the present unsteady problem is uniquely determined in terms of initial conditions. Mixing and heat transfer should increase as the flow velocity increases, and therefore the ignition time is expected to decrease. However, in both cases of unmixed and premixed gases, the ignition time increases as the stretch rate, which is proportional to the flow velocity, increases. This indicates that a long induction time is required to reach a reaction intense enough for ignition. At a certain large flow stretch rate, the ignition time becomes infinite and ignition can no longer take place. Previously, such an ignitable limit was discussed as a function of flame temperature variation with flow velocity, but in this analysis it is derived for ignition time variation.

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.

An Analytical and Experimental Study for Solid Propellant Combustion in an Acceleration Field

Abstract The objectives of this paper are twofold. The first objective is to better define the theoretical burning rate augmentation of solid propellants in the acceleration by using the presented fundamental concept on the pitting of the combustion surface and the agglomeration of metal at the bottom of a pit, particularly the dependence upon the aluminum content and its distribution is investigated. The second objective is lo show the comparison of the calculated results with the experimental ones of the CMDB (Composite Modified Double-Base) propellants at acceleration levels from 0 g to 350 g.