Hiroshi Yamasaki

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.

Variables affecting the reactivity of acid-catalyzed transesterification of vegetable oil with methanol

The dominant factors affecting the reactivity of acid-catalyzed transesterification of vegetable oil with methanol have been investigated. Effects of varying the acid catalyst species, surface active agent type and content were studied. Also, the effect of the type of oil was examined. Biodiesel fuel yields increased with the addition of sodium dodecylsulfonate as surface active agent because the mass transfer rates of protons and methanol to the oil phase through the oil-methanol interface were increased with increasing interfacial area. Evaluation of the reaction kinetics, based on changes in parent oils containing triglyceride or diglyceride showed that the activation energies were almost no change, but the frequency factors were 2210 and 9827mol(-1)min(-1), respectively. Therefore, it was concluded that a lower reactivity during acid-catalyzed transesterification with methanol was caused by a lower contact probability for oil, methanol and acid catalyst in the presence of an oil-methanol interface during the reaction.

Observation of sooting behavior in an emulsion droplet flame by planar laser light scattering in microgravity

Two-dimensional visualization of the soot concentration profile in a flame formed around a suspended droplet was carried out under microgravity conditions using the planar laser light scattering technique. The 500-in drop shaft in Japan was used for microgravity experiments, which offers the effective time of the microgravity of 10 s and the relatively large space of the falling assembly for constructing the optical measurement system. The soot concentration and the instantaneous amount of soot were estimated approximately from the intensity of the scattered light using the image analysis system. The fuels employed were water-in-oil emulsions composed of base fuel, water, and surfactant. The base fuel was n -dodecane. The sooting region can be observed spherically around the droplet by using the apparatus developed in the present work. The results showed the unsteadiness of the sooting behavior and the flame behavior. Most of the soot concentration is located in the vicinity of the inner edge of the sooting region, which corresponds to the soot shell observed previously. The maximum of soot concentration does not vary with the water content, whereas the maximum amount of soot decreases significantly with water emulsification.

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.

Statistical analysis of onset of microexplosion for an emulsion droplet

A experimental study has been carried out on the burning behavior of an emulsion droplet suspended on a quartz fiber. Attention was paid mainly to the coccurrence of microexplosion, which may be caused by the bubble nucleation at temperatures below the superheat limit. The oil-in-water emulsion consisting of the base fuel and water was employed after degasification. The base fuels used were n -tetradecane and n -hevadecane. The water content varied from 0.1 to 0.3 by volume. The waiting time for the onset of microexplosion was measured for about 30 runs. The microexplosion is assumed to be a random process, and the onset probability of microexplosion was discussed from the statistical point of view. The Weibull distribution, which is derived mathematically from the weakest link destruction model, was adopted for this purpose. The results showed that the distribution function of the waiting time correlated with the Weibull distribution. The distribution function is classified the wearout type at the initial heating period and as the chance failure type at the constant-temperature period. Thus, the type of Weibull distribution of the waiting time is dependent on the variation of the droplet temperature with time. The onset rate of microexplosion increases exponentially as the superheating of water increases. The reliance of the onset rate on the superheating of water is independent of the base fuel.

Heat transfer characteristics of fuel based emulsions with alcohol additives

Experimental study has been conducted to study the heat transfer characteristics of the hydrocarbons as the base fuel and water emulsions. The focus is mainly paid on the effect of the ambient emulsion temperature and alcohol additives on the boiling and its inception of fine water droplet dispersed in the emulsions. Heat transfer was measured using an electrically heated horizontal thin Pt wire. It is concluded that there is the transition point in the heat transfer coefficient, which is just above the boiling point of emulsified component of water. The heat transfer of water in fuel emulsions and effect of alcohol additives on it were also revealed.Copyright

Effects of Fuel Properties on the Evaporation of a Binary Miscible Fuel Droplet on a Heated Wall

An experimental study has been made of the evaporation of a fuel droplet of miscible binary mixture on a heated wall. Primary attention is toward the phenomena which are characteristic of the evaporation of a binary fuel droplet as distinguished from the phenomena for a pure fuel droplet. Fuels tested are the mixture of various couples of normal paraffine hydrocarbons. The results show that two minima and two maxima appear in the lifetime curve. The preferential evaporation of the highly volatile component of the fuel is responsible for the first maximum in the film evaporation region. The increase in the content of the highly volatile component results in the first minimum and maximum shifted toward higher wall temperature, as well as the second minimum and maximum toward lower wall temperature. Four different modes of the droplet evaporation are observed.

Visualization of Sooting Field in an Emulsion Droplet Flame by using Microgravity Environment.

Sooting field in a flame formed around a suspended droplet was visualized under microgravity using the planar laser light scattering technique. The soot concentration and the instantaneous soot amount were approximately estimated from the intensity of the scattered light using the image analysis system. The fuels employed were water in oil emulsions composed of n-dodecane, water and surfactant. The water content was varied from 0 to 0.2 in volume. A soot layer which was concentric with the droplet was observed inside the luminous flame. The results show the unsteadiness of the sooting behavior as well as the flame behavior. The maximum of the soot concentration is located near the inner edge of the soot layer. The time history of the instantaneous soot amount is similar to that of the instantaneous flame radius. The maximum of the soot concentration does not vary with the water content, while the soot amount decreases significantly with the water emulsification.

Three-Dimensional Numerical Analysis of Mixture Formation Process in the Cylinder of a Spark-Ignition Engine.

A three-dimensional numerical analysis was applied to the prediction of the fuel vapor concentration profile in the cylinder of a typical four -cycle reciprocating spark ignition engine with an off center valve. The conservation equations of mass, momentum and energy were solved on the basis of the finite volume method. The ordinary two-equation model was employed as the turbulence model. It was assumed that the fuel injected in the intake port flows into the cylinder as a vapor. Calculation was done from the TDC of the intake stroke to the TDC of the compression stroke every 10 degrees of the crank angle. The temporal variations of velocity and fuel mass fraction profiles were predicted during both intake and compression strokes. The results showed that the fuel mass fraction distribution was not uniform at the last stage of the compression stroke. The effect of the inflow velocity distribution on the mixture formation process was discussed.