Toshikazu Kadota

Recent advances in the combustion of water fuel emulsion

Abstract Recent advances in the combustion of water fuel emulsion which consists of base fuel and water doped with or without a trace content of surfactant are reviewed. The focus is on the fundamental mechanism relevant to the micro-explosion phenomena leading to the secondary atomization which is not common to the combustion of pure fuel. Described at first are the kinetic model and the probability model for predicting the nucleation of vapor bubbles in the liquid phase, and the measured and predicted results of the superheat limit of hydrocarbons and water beyond which the liquid phase cannot exist. This is followed by the micro-explosion phenomena of emulsion confined in a glass capillary aiming to suppress the heterogeneity of temperature profile and the bulk flow inside it, and the evaporation at its surface during the period of time prior to the micro-explosion. The evaporation and the combustion of emulsion droplet are among the primary subjects in the present paper. Discussed are the phenomenological burning processes, the burning rate constant, the ignition process, the flame phenomena including soot concentration profile in the droplet flame and the spheroidal evaporation on a hot surface. Also mentioned are the in-droplet transfer processes including the phase separation, the micro-explosion phenomena, the conditions for the micro-explosion to occur and the empirical equation for the rate of micro-explosion based on the probability model. Finally stated is the effect of water emulsification on the flame structure, the combustion efficiency and the exhaust emissions in laboratory spray combustor and the various practical combustion applications.

Water-coalescence in an oil-in-water emulsion droplet burning under microgravity

An experimental study was performed to obtain the detailed information needed to provide a deep understanding of the combustion process and the secondary atomization of an oil-in-water emulsion droplet. The experiments were conducted by using the drop shaft of JAMIC (Japan Microgravity Center) at Hokkaido. The oil-in-water emulsion, which consisted of n -hexadecane as a base fuel, distilled water, and a trace of surfactant was tested. Photographic observation and temperature measurements were made of the suspended emulsion droplet during the heating-up and combustion processes under microgravity. The primary attention was toward the phase separation in the droplet, and the time histories of droplet temperature and the amount of water in the droplet, during the period of time prior to disruptive microexplosion. The results showed that the separation of the base fuel and water as well as their agglomeration and coalescence occurred with the lapse of time. The increase in the droplet temperature resulted in phase separation, and the formation of a single water droplet enveloped by a shell of the base fuel, prior to the microexplosion. The volumes of the base fuel and the water in the droplet were estimated from the obtained droplet images. After the phase separation, selective evaporation of the base fuel occurred and the volume of the base fuel decreased, while the water volume did not change. The effects of the emulsion properties on the onset rate of microexplosion were also revealed by using statistical analysis.

Effects of reduced gravity on methanol droplet combustion at high pressures

An experimental study was carried out on the burning of methanol droplets in high-pressure air under the normal gravity and microgravity conditions which are available both during the parabolic flights of the CNES Airbus (g≈10 −2 g 0 ) in France, and in the JAMIC drop shaft in Japan (g≈10 −4 g 0 ). Methanol was chosen as the test fuel both for its lower sooting tendency and because it is considered to be one of the most promising candidates as an alternative fuel. The well-known suspended droplet technique was adopted: a methanol droplet was suspended at the tip of a fine quartz fiber and was ignited with an electrically heated kanthal wire in high-pressure air at room temperature. The results showed that the d 2 law holds for all ambient pressures and gravitational accelerations, and that the burning rate constant increases monotonically with the ambient pressure up to 1.4 times the critical pressure of pure methanol. Under microgravity, the dependence of the burning rate constant on the ambient pressure becomes weaker with increasing ambient pressure above the critical pressure. The burning rate constant decreases with a decrease in gravitational acceleration. The gravitational acceleration of the order of 10 −2 g 0 is found to induce an appreciable influence on the burning rate constant. The variation of the burning rate constant as Gr 1/4 was confirmed for the first time both by varying the gravitational acceleration and the ambient pressure. An experimental data set was therefore established on the effects of high pressure on droplet burning rates free from natural convection effects.

Effect of gravity on onset of microexplosion for an oil-in-water emulsion droplet

Microgravity combustion experiments have been carried out for an emulsion droplet suspended at a quartz fiber by free-fall method. Attention was mainly paid to the effect of gravity on the occurrence of microexplosion that may be caused by the bubble nucleation at temperatures below the superheat limit. The oil-in-water emulsion consisted of the base fuel and water was employed after degasification. The base fuel employed was n -dodecane and n -tetradecane. The water content was 0.2 in volume. The waiting time for the onset of microexplosion was measured for about 30 runs under normal gravity and microgravity. The onset probability of microexplosion was discussed from the statistical point of view by using the weakest link destruction model. The results showed that the distribution function of the waiting time correlated with the mixed Weibull distribution for both gravity conditions. That is, the distribution function is classified to the wear-out type at the initial heating period, and it is classified to the chance failure type at constanttemperature period. The change in the type of Weibull distribution occurs slightly earlier under normal gravity than microgravity for both base fuels. The Weibull distribution of the waiting time at constant-temperature period for normal gravity is almost the same as that for microgravity. The dependence of the onset rate of microexplosion on the superheating of water is independent of the gravity condition.

Ignition of binary mixture droplets by a propagating laminar flame

An experimental study was made of the ignition process of single droplets of binary mixtures subjected to a laminar flat flame propagating through a lean homogeneous propane/air mixture at constant pressure. Binary fuels consisting of n -hexane and n -hexadecane or n -dodecane, and benzene and n -hexadecane were tested. Water-emulsified liquids consisting of n -dodecane, and pure fuels like n -hexane, n -octane, n -dodecane, and n -hexadecane, were used as well. A combustion chamber made of a transparent duct was installed with spark electrodes, fine quartz fibers to suspend fuel droplets, and a shutter to allow the burned gas to escape at the top end. A high-speed video camera was provided for photographic observation of each droplet ignited by the propagating flame. The ignition delay of pure fuel droplets increased monotonically with increase of the initial droplet diameter and decrease of fuel volatility. The influence of the initial droplet diameter on the ignition delay decreased with increasing fuel volatility. Ignition delay of the binary fuel droplets decreased with an increase of the volume concentration of the higher-volatility fuel. Ignition delay of the binary fuels showed a minimum and a maximum as a function of the initial droplet diameter. The diameters at the minimum and the maximum of ignition delay decreased with increasing higher-volatility fuel concentration. For the emulsified fuel, on the other hand, ignition delay increased with increasing initial droplet diameter and increasing water content. Part of the propagating flame behind the droplet became convex toward the unburned gas as it passed a droplet of higher-volatility fuels, while the flame remained flat for the lower-volatility fuels. The deformation length of the propagating flame behind the droplet increased with increasing higher-volatility fuel concentration.

Autoignition and combustion of a fuel droplet in supercritical gaseous environments under microgravity

An experimental study has been made of the evaporation, autoignition, and combustion of a fuel droplet in supercritical gaseous environments for the deep understanding of the combustion processes in gas turbine, diesel engine, and rocket motor in which the operating pressure often exceeds the critical pressure of the liquid fuel of frequent utilization. The parabolic flight of aircraft provided the long period of time for the experiments that allowed observation of the processes during the entire lifetime of a fairly large droplet from the start of evaporation to the end of burning in quiescent supercritical gaseous environments under microgravity. A fuel droplet suspended at the tip of a fine quartz fiber in the cold section of the high-pressure combustion chamber was translated quickly to be subjected to a hot gas in an electric furnace. This resulted in the evaporation, autoignition, and combustion of the fuel droplet in supercritical gaseous environments. A video camera was provided to observe the behavior of the fuel droplet as well as the flame around the droplet. The fuel tested is octadecanol, which solidifies at 331 K and the critical conditions of which are 1.4 MPa and 747 K. The experiments were done in the quiescent gaseous environments at low oxygen concentration to reduce the soot produced in the flame and to make it possible to observe the backlighted image of the droplet during its entire lifetime. The ignition delay, the burning time, and the lifetime of the droplet showed minima at the ambient pressure approximately equal to the critical pressure of the liquid fuel. The burning rate constant that was determined from the time histories of the droplet diameter showed a peak at the ambient pressure 1.5 times as high as the critical pressure of the liquid fuel.

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.

Effects of ambient pressure on autoignition of a fuel droplet in supercritical and microgravity environment

An experimental study has been performed under microgravity conditions to obtain the detailed information needed to understand the combustion phenomena of a fuel droplet which autoignites in supercritical gaseous environment. A fuel droplet suspended at the tip of a fine quartz fiber in the cold section of the high-pressure combustion chamber was transferred quickly to be subjected to a hot gas in an electric furnace. This resulted in the evaporation, autoignition, and combustion of the fuel droplet in a supercritical gaseous environment. A high-speed video camera was provided to observe thebehavior of the fuel droplet and the droplet flame. In the present study, primary attention was focused on the ignition delay and the flame diameter at the initial stage of droplet burning. 1-Octadecanol and n-octadecane were selected as the test fuels. Mixtures of oxygen and carbon dioxide and of oxygen and nitrogen were used as the ambient gas. The ambient pressure was extended up to pressures around two times the critical pressure of the fuels tested. The ignition delay was found to have a minimum around the ambient pressure equal to the critical pressure of the fuel, and a maximum in the range from 1.5 to 2 times the critical pressure. Higher oxygen concentration of the ambient gas caused shorter ignition delay at ambient pressures equal to or above 1.5 times the critical pressure. Higher ambient pressure caused a smaller droplet flame at the onset of autoignition and higher flame growth rate and a larger flame diameter afterward. The flame diameter decreased with an increase in the oxygen concentration of the ambient gas.

Laser-induced fluorescence for the non-intrusive diagnostics of a fuel droplet burning under microgravity in a drop shaft

The laser-induced-fluorescence method has been employed for remote, non-intrusive and instantaneous measurements of a fuel droplet burning under microgravity. A fuel droplet was doped with naphthalene and TMPD. The fluorescence emission spectra from a droplet subjected to the incident nitrogen laser beam were measured with an image-intensifying optical multichannel analyser. The microgravity was generated in a capsule of a 100 m drop shaft. The results showed that the newly developed diagnostic system could be applied successfully for the simultaneous measurements of droplet temperature and diameters of the droplet, flame and soot shell under microgravity. The droplet temperature was determined from the measured ratio of fluorescence emission intensities at two different wavelengths. The soot shell was located in the vicinity of the droplet surface deep inside the flame during the early stage of the burning and moved away from the droplet with the elapse of time.

Temperature and velocity fluctuations of a jet diffusion flame in a cross-flow

An experimental study was made of the temperature and velocity fluctuations of a hydrogen/methanejet diffusion flame discharged normal to a free stream of air with a uniform velocity profile. Time-resolved measurements of temperature in the flame were carried out by laser Rayleight thermometry. The mean temperature, root-mean-square (rms) of temperature fluctuation, and probability density function (PDF) of fluctuating temperature were obtained in the various transverse cross sections normal to the jet axis of the flame. The mean temperature profiles obtained from Rayleigh scattering thermometry were compared with thermocouple measurements. Laser Doppler velocimetry was used to measure the profiles of mean jet-axial velocity and rms of velocity fluctuation in the flame. The results showed that the high-temperature region is located below the jet axis, where the velocity fluctuation is small. Two counter-rotating vortices enhance mixing of fuel and air, and chemical reaction proceeds actively in the lower region of the flame. It was also found that the maximum rms of temperature fluctuation was located at the upper edge of the flame, where the PDF of fluctuation temperature had two peaks. The rms of axial-velocity fluctuation has a maximum in the same location. This indicates that the cross-flow air is entrained into the upper edge of the flame by coherent structures with large vortices.