Mitsuaki Tanabe

Two Stage Ignition of n-Heptane Isolated Droplets

ABSTRACT Experimental and theoretical studies on the spontaneous ignition process of isolated fuel droplets were carried out. Time dependent temperature fields around the igniting droplets were observed by interferometry so that two step temperature rise can be detected. Some experiments are performed under microgravity to obtain reference data. Induction times are examined as a function of ambient temperature. As a result, a zero temperature coefficient region is found, which is equivalent to the NTC (negative temperature coefficient) region for the ignition of premixed gas. A numerical model is developed applying a simplified chemical reaction model that includes the low and the high temperature reactions. The model is able to reproduce the two step temperature rise and the roles of the two kinds of reactions on the ignition process up to the establishment of a diffusion flame around the droplet are examined.

Spontaneous ignition of liquid droplets from a view of non-homogeneous mixture formation and transient chemical reactions

Spontaneous ignition of isolated single fuel droplets in air is investigated. Special attention is given to the transient behavior of chemical reactions. Single-stage and two-stage ignition or cool flame behavior is investigated. A fuel droplet of n -heptane, n -dodecane, or iso -octane is suddenly exposed in a high-temperature, high-pressure air to ignite. The succeeding process is observed by a Michelson interferometer that visualizes the instantaneous temperature field around a droplet. Experiments are done with air of 500–1100 K in temperature and 0.1–2.0 MPa in pressure. Ignition regions are mapped on a temperature-pressure plane and the roles of the low-and high-temperature branches of chemical reactions are described. Induction times and their temperature or pressure dependence are analyzed. The diameter dependence of the induction times shows influences of physical process only on first induction time. Pressure dependence of the second induction time indicates that the dominating factor for the induction time is the nature of a cool flame during the first stage. Mixture conditions at the occurrence of a cool flame and the relation between mixture condition and second induction times are examined. Fuel and temperature dependence of second induction time and TC (zero-temperature coefficient) behavior of total induction time are explained by the dependence of the mixture condition on those parameters through cool flame temperature.

Influence of standing sound waves on droplet combustion

The influence of acoustic fields on combustion of a single fuel droplet has been investigated using microgravity. The natural convection-free conditions allow the role of sound alone to be established by avoiding the coupling of sound-induced alternating convection and natural convection. Experiments were done with n -decane single droplets of about 1.5 mm in diameter. Sounds of 66.5 to 3353 Hz with sound pressure levels up to 135 dB were applied in a duct, which achieved maximum velocity amplitudes of about 1 m/s. As a result, a significantly deformed flame and a soot ring instead of soot shell were found in cases of large velocity amplitude. Burning rates increased with increasing velocity amplitude. The other parameters such as sound pressure level or frequency had little influence. Different influences of sound were found for droplets burning at different locations relative to the velocity antinode of the applied sound. All these observed phenomena are explained by the alternating convection due to sound and a unidirectional convection, which is a form of acoustic streaming, driven by the acoustic radiation force. A simple analysis has been made of the streaming mechanism and the increased burning rate. Differences with droplet location are explained by the enhancement in heat and mass transfer due to the two kinds of convection.

Effects of natural convection on two stage ignition of an n-dodecane droplet

The effects of natural convection on two-stage spontaneous ignition of a fuel droplet were studied experimentally. A suspended n -dodecane droplet of 0.7 mm in its initial diameter was employed. Ignition phenomena were observed by a fine thermocouple (0.025 mm) and by an interferometer so as to detect an invisible cool flame. A direct photographic method with a back light was also used to measure the droplet diameter. Ambient temperature was limited within the range where the cool flame can be observed (500 to around 800 K) and ambient pressure ranged between 0.1 and 1.0 MPa. Experiments were carried out under normal and microgravity conditions, and results were compared. Investigations were done mainly on the induction times at 0.2 and 1.0 MPa. Experiments showed longer first induction times (time to appearance of cool flame) under normal gravity than under microgravity in certain ambient conditions. Significant differences between critical temperatures, below which cool flame or hot flame does not appear, were also observed between the two gravity conditions. The induction times differed up to a factor of 3, and the critical temperature differed about 60 K. For the second stage, slightly increased induction times were observed under normal gravity conditions at 0.2 MPa. Differences observed between both gravity conditions can be explained by the effects of natural convection driven by the down-ward force due to the dense fuel vapor and by the buoyancy due to the cool flame.

Ignition Characteristics of Boron Particles in the Secondary Combustor of Ducted Rockets -Effects of Magnalium Particle Addition-

Boron has been considered most attractive fuel for ducted rockets because of its high combustion heat with oxygen. However, boron was found to be impractical as the fuel because of its poor combustion efficiency due to the meager ignition characteristics. Boron had to be ignited and combusted forcibly in the residence time of the recirculation zone. In this study, we focused on the case of another ignitable metal particles becoming the ignition source of boron. Magnalium was selected as the ignition source. In order to elucidate the effects of magnalium, three types of experiments were conducted. To obtain the basic properties of metals, thermogravimetry and differential thermo-analysis were conducted. The ignition delay time was measured with an electronic furnace, and combustion efficiency was obtained by using a connected-pipe ducted rocket. It was clarified that the ignition delay time of the mixture could be divided into two time regions, earlier and later regions than that of the single magnalium particle. The combustion efficiency could be distributed. The combustion efficiency when the gas generator contained boron can improve only if the earlier time region can be selected actively.

Combustion Characteristics of Boron Particles in The Secondary Combustor of Ducted Rockets

Ducted rockets are one kind of ramjet engines which are air-breathing propulsion system suited for supersonic flight. Fuel components used for ducted rockets are fuel-rich propellant. To obtain high specific impulse, it is desired that the fuel materials evolve high combustion heat per unit mass in the air. Boron is the most suitable material of the fuel of ducted rockets in terms of its high combustion heat. Obtaining the combustion characteristics of boron particles is the key to improve boron particles combustion. In this study, combustion times of single particles and flame areas around the particles were measured by direct photography and spectroscopic analysis, and flame structure of burning particles was observed by schlieren photography. It was obtained that boron particles burn in two stages separately and the combustion time decrease with increasing temperature. The flame areas are composed with boron dioxide.

Mass Transfer in the Recirculation Zone of Ducted Rocket

Ramjet induced compressed air to a secondary combustor and mixed with fuel. High thrust was needed to overcome air drug, therefore the diameter of nozzle throat must be large. The flow speed in the secondary combustor was higher than 200 m/s. The residue time in the secondary combustor was shorter than 10 ms, and the mixture gas burned at very short time. To increase combustion efficiency, the reaction rates of metal particles and mixture gas should be high, however mean temperature of mixture gas was 700 K and was impossible to ignite at short time. Local temperature of mixture gas in recirculation zone of the secondary combustor was an important factor to increase reaction rate of mixture gas. The surrounding gas temperature of solid particles in the recirculation zone was higher than mean temperature, and this was possible to become the ignition point. The combustion efficiency of the connected-pipe ducted rocket with the solid particles over 20 parts was larger than that without solid particles.

Roles of Baroclinic Torque by Acoustic Oscillation on Structure of Premixed Flame

Wrinkled and corrugated flames were observed at velocity anti-node in the standing acoustic field. It is thought that the deformation is cased by baroclinic torque. The detailed influence of the baroclinic torque on the structure of the flame front remains to be unclarified. In this study, numerical investigation has been carried out on the roles of the baroclinic torque on the rotation of fluid and that on the deformation of the flame. As a result, the rotation of the fluid could be quantitatively explained by the baroclinic torque by the combustion and the acoustic oscillation. Wrinkle depth is found to be influenced by the baroclinic torque, but it is not the only parameter. For relatively large laminar flame speed, the wrinkle depth is determined not only by the fluid rotaion due to the torque, but also by flame propagation within an acoustic cycle.

Characteriation of spherical hydrocarbon fuel flames: Laser diagnosis of the chemical structure through the oh radical

For the first time, spherical diffusion flames of n -decane, n -heptane, and methanol established around fuel-drenched porous spheres were investigated by applying the two-dimensional laser-induced predissociation fluorescence method to measure the OH radical under microgravity conditions. Focus of this work was laid on the development of a UV-laser diagnostic system attached to the Bremen drop tower for the characterization of combustion under buoyant-free conditions and on its applicability to the diffusion zone of hydrocarbon droplet flames. The experiments carried out on the previously mentioned fuels showed capabilities and limitations of this method applied on liquid fuels. While it is relatively straightforward to investigate the chemical structure of methanol droplet flames because of the absence of Mie scattering soot particles and of nonresonant fluorescing species, it is difficult for larger hydrocarbons such as n -hepatane and n -decane. It is considered that with the development of a luminous zone, strong broadband absorption by fuel vapor and intermediates larger than methane in connection with high laser pulserepetion rates may cause interactions of the input energy with the chemical kinetics of the combustion process.

Effect of Nitrogen Dilution on the characteristics of HCCI Combustion

Ignition delay timesand combustion times ofiso-octane/air mixture andnitrogendiluted/non-dilutedgasoline/air mixturewere measured by usinga rapid compression machine undertheconditionsequivalent to practical engines .The dilutionratioswere0,20 and 50% byexternalpercentage.For the case ofiso-octane,thepressurerangeis1.77 to 2.37 MPa, and temperaturerangeis732to1046K.Forthe case ofgasoline,the pressurerangeis 2.14 to2.45 MPa, and temperature rangeis659 to 1108 K.The results show the existence of two ignition regimes.In oneregime,mixture spontaneouslyignites at multiple points sequentiallyin the combustion chamber.Another regime yields reaction front formation and propagationinsidethe combustion chamber prior to multiple pointignition.Being diluted with nitrogen,HCCI combustion becomesslowand ignition delay time sbecome long inthe entire temperature range ,especiallyin NTC.For allthe nitrogen dilutionratios,the combustion time increasedin proportion to the ignition delay time .Combustion time can be related tothe difference of localignition delay time which is caused by local inhomogeneity of temperature.