Shinji Nakaya

Numerical Analysis on Flame Kernel in Spark Ignition Methane/Air Mixtures

A flame kernel initiation of methane/air combustible mixtures in the spark ignition process was investigated using a two-dimensional theoretical model including a detailed description of gas-phase chemical kinetics, shock capturing scheme and diffusive molecular transport. Interactions of chemical reactions and diffusive transports of radicals in the process of the flame kernel initiation were investigated. Although the model of plasma might be oversimplified, the qualitative behavior of OH for hydrogen/air mixture agreed well with experimental one. The influences of diffusive molecular transport and ignition energy on the flame kernel initiation were discussed. As a result, in the early stage of the flame kernel development for methane/air mixture, the hot gas expansion was dominated by a flow whichwas inducedby the blastwave and the thermal gas at the electrode gapwas self-sustainedwith anapplication of minimum ignition energy. The induction time of the flame kernel initiation strongly depended on the ignition energy and effects of preferential diffusion of lighter molecules in the early phase of the flame kernel development are outstanding especially in the case of low ignition energy near the minimum.

Optical Measurements of Ethanol/Liquid Oxygen Rocket Engine Combustor with Planar Pintle Injector

The pintle injector is a promising candidate of the propellant injection systems for a rocket engine with deep throttling capability which is essential for future space transportation missions. However, studies focusing on combustion phenomena in a rocket engine with a pintle injector is rather limited. In this study, optical measurements inside an ethanol/liquid oxygen rocket engine combustor with a planar pintle injector are conducted to clarify spray and flame structures in the pintle injector. Combustion tests where the chamber pressure is 0.36 to 0.40MPa and O/F is 1.15 to 1.40 are conducted. Effects of the injection configuration on spray structures are also evaluated. High speed imaging techniques are used to observe the flame and spray structures under hot fire conditions. Strong chemiluminescence of CH is observed in the vicinity of the impinging point of two propellants. Luminous flame is observed intermittently in the vicinity of the faceplate and the upper wall of the combustor with the direct observation of the flame. Characteristic exhaust velocity efficiency with oxidizercentered configuration is lower than fuel-centered configuration due to the large amount of propellant impinging on the upper wall of the combustor. Periodical phenomena with the frequency of approximately 300Hz which can be related to atomization processes are also observed.

Spontaneous Ignition of Fuel Droplets in Lean Fuel-Air Mixtures

Experiments have been carried out on the spontaneous ignition of single fuel droplets in lean fuel-air mixtures. The residence time, which is defined as the elapsed time from the introduction of the mixture into the furnace to the insertion of the droplet, is 15 s. The change in the chemical species concentration in fuel-air mixtures before the insertion of the droplet is predicted by the calculation of chemical reaction. In the case of lower ambient temperatures, the methane-air mixture does not react so much and most of methane and oxygen in the ambience survives for the residence time of 15 sec. In this case, the ignition delay time in mixtures is almost the same as that in air, which indicates that the existence of fuel in the ambience has little effect on the spontaneous ignition behavior. On the other hand, the ignition delay time in mixtures becomes larger than that in air for higher ambient temperature. This is mainly due to the decrease in the oxygen concentration in the ambience caused by chemical reaction of fuel-air mixtures during the residence time. However, the quantitative comparison suggests that the increase in the ignition delay time in fuel-air mixtures is not explained by only reduced oxygen concentration.

Effects of spacing and arrangement of droplet on combustion characteristics of monodispersed suspended-droplet cluster model under microgravity

For bridging between knowledge on droplet combustion and spray combustion, an experimental study was performed on autoignition and combustion of multiple droplet clusters. The monodispersed suspended-droplet cluster (MSDC) model with which arrangement, spacing and initial diameter of the droplet are well controlled has been developed. The effects of spacing and arrangement of droplet on combustion characteristics of the MSDC model in a high-temperature air were examined using microgravity environment in a drop shaft. The ignition delay and the burning time increased with decreasing the droplet spacing, regardless of the droplet number and the model dimensions. Larger droplet number with three-dimensional (3D) hexagonal closest packing (HCP) structure model resulted in longer ignition delay and longer burning time. 3D cubic closest packing (CCP) structure model showed rather longer ignition delay and much shorter burning time than 2D model. For 3D HCP model, an individual flame which enveloped each droplet was formed whole in the combustion duration with larger droplet spacing, while the group flame was formed whole in the combustion duration with smaller droplet spacing. When the droplet spacing was in the intermediate range, each droplet was ignited to form the individual flame, and each flame merged into the group flame. The diameter of the burning sphere decreased at the beginning of combustion, and turned to increase afterward. The transition from the individual flame to the group flame occurred around the time when the burning sphere diameter reached its minimum. The burning sphere diameter relative to the model diameter increased with decreasing the droplet spacing in the middle stage of combustion.

EFFECTS OF FUEL VAPOR IN AMBIENCE ON SPONTANEOUS IGNITION OF ISOLATED FUEL DROPLET

Experiments have been carried out on the effects of pre-vaporized fuel in the ambience on spontaneous ignition behavior of isolated fuel droplets. An isolated droplet of n-heptane or n-dodecane, which is sustained by a quartz fiber, is inserted into a high-temperature gas field including fuel in the electric furnace. Methane and n-heptane are selected as fuels of the ambience. The ambient pressure and the temperature are set to be 0.3 MPa and 700 K, respectively, in order to prevent the consumption of methane and oxygen in the ambience. The condition of the ambience including fuel is elucidated using a numerical analysis of chemical reactions. The results indicate that the ignition delay time increases due to methane included in the ambience. In the case of the ambience where n-heptane vapor is included, both n-heptane and oxygen are consumed by chemical reactions prior to the droplet insertion. The ignition delay time of n-heptane droplet in the reacted n-heptane and air mixture is larger than that in air and chemical reactions at the reduced oxygen concentration seems to be slow. The ignition delay time is shorter in the reacted n-heptane and air mixture than in an air with the same amount of oxygen as the reacted mixture.

Sooting Flame Behavior for a Droplet Combustion with Electrical Field under Microgravity

The sooting droplet combustion behavior under electrical fields is investigated with n octane fuel. The numerical simulation which includes the combustion gas, liquid fuel and soot particle is carried out and compared with experimental results obtained in the microgravity environmental. Flame de formation and burning rate constant are predicted and enhancement of evaporation is shown. The mechanism that n -octane sooting flame under electrical fields is evaporated than none electrical fields case is considered.

Evaporation of a Binary Component Miscible Fuel Droplet on a Hot Surface in High Pressure Gaseous Environments

An experimental study has been made of the evaporation of a droplet of miscible binary component fuel on a hot surface in high pressure gaseous environments. Photographic observation is made to elucidate how a fuel droplet or film behaves on a hot surface and to obtain the characteristic lifetime curve at various surface temperatures. Primary attention is toward the effect of ambient pressure on the evaporation of a binary component fuel droplet as distinguished from the phenomena of a droplet of single component fuel. The results show that a maximum in the film evaporation region of the lifetime curve appears as well as a Leidenfrost point. The increase in the ambient pressure causes the lifetime curve shifted toward higher surface temperature, and the decrease in the lifetime at these points which finally tend to disappear. The increase in the initial concentration of the highly volatile base fuel results in the maximum shifted toward higher surface temperature and the Leidenfrost point toward lower surface temperature. The lifetime curve shifts toward the higher surface temperature with a decrease in the volatility of low volatile base fuel.