Masato Mikami

Combustion of Miscible Binary-Fuel Droplets at High Pressure Under Microgravity

Abstract The objective of this research is to study near-critical and super-critical combustion of droplets consisting of binary fuel mixtures. Experimental results are reported on the burning of fiber-supported droplets of mixtures of n-heptane and n-hexadecane, initially about 1 mm in diameter, under free-fall microgravity conditions. The ambient pressures range up to 3.0 MPa, extending above the critical pressure of both fuels, in room-temperature nitrogen-oxygen atmospheres having oxygen mole fractions of 0.12 and 0.13. Three-stage burning of the binary fuel droplets is observed, and the onset lime of the second stage is compared with the predictions of an existing theory. Experimental evidence of thermo-capillary and/or diffuso-capillary convection during the droplet burning is obtained. The results contribute to improving understanding of binary-fuel droplet-combustion processes at high pressures.

Interactive combustion of two droplets in microgravity

Interaction between two burning fuel droplets have been studied experimentally for a wide range of initial separation distance between two droplets. Experiments were performed in microgravity. Results show that for an initial separation distance within a certain range, the transition from two separated flames to a merged flame occurs initially, and then the contrary mode transition occurs due to the gas-phase unsteadiness. The burning lifetime has a minimum for a certain initial separation distance. In order to explore instantaneous interaction effects, the instantaneous burning rate correction parameter is introduced. The correction parameter includes effects of the liquid-phase unsteadiness on d 2 −t curves, both of the droplet in array and the single droplet, and is a function of the normalized droplet diameter. When the instantaneous burning rate correction parameter is positive, positive interaction occurs, which causes the burning rate larger than that of the single droplet. When the initial separation distance is larger than a certain value, in the earlier stage of combustion, the positive interaction occurs due to radiative heat transferred from the other flame, and in the later stage, the negative interaction occurs due to oxygen starvation between the flames. When the initial separation distance is smaller than the value, the negative interaction occurs for an entire period of combustion. The initial separation distance at which the burning lifetime has a minimum is almost consistent with that at which the positive interaction is a maximum in the earlier stage.

Occurrence probability of microexplosion in droplet combustion of miscible binary fuels

Occurrences of microexplosion in droplet combustion of miscible fuel mixtures were studied. Experiments were performed using unsupported droplets of n-alkane/n-hexadecane mixtures injected into atmospheric air in normal gravity. It was found that the occurrence of microexplosion is stochastic and cannot be predicted by the classical criterion for microexplosion occurrence using the limit of superheat and the droplet temperature. An occurrence model for the microexplosion based on the homogeneous nucleation theory is presented and shows that the occurrence probability of microexplosion is, in general, controlled by the ratio of the liquid-phase lifetime to the nucleation time. For binary fuel mixtures whose constitutent fuels have very different volatilities, the occurrence probability of microexplosion is controlled by the ratio of the liquid-phase lifetime to the nucleation time during the quasi-steady vaporization period after the transition period. The nucleation time is inversely proportional to the nucleation rate and superheated liquid volume. The droplet temperature and the limit of superheat affect the occurrence probability through the nucleation rate. The model shows that the occurrence probability reaches a maximum at a certain initial concentration of the fuel mixture. The effects of the initial droplet diameter on the occurrence of microexplosion were also investigated. The occurrence probability was found to depend largely on the initial droplet diameter. It decreases with the decrease in the initial droplet diameter.

An experimental and modeling study on stochastic aspects of microexplosion of binary-fuel droplets

Disruptive droplet burning is characterized by microexplosion occurrence during burning. The microexplosion phenomena of binary-fuel droplets are caused by internal boiling that is initiated by the homogeneous nucleation of bubbles in the liquid phase. Since homogeneous nucleation is a random process due to density fluctuation, some stochastic aspects appear in disruptive burning of the droplets. In order to investigate stochastic characteristics in disruptive burning, the microexplosion occurrence was studied experimentally and theoretically. In experiments, the burning droplet of an n -hexane/ n -hexadecanemixture was injected upward, and the induction time for microexplosion occurrence was obtained. Results show that microexplosion induction time is distributed over the quasi-steady vaporization period. The maximum frequency of microexplosion induction time and the occurrence probability of microexplosion increase with the initial droplet diameter. The microexplosion occurrence was modeled considering the homogeneous bubble nucleation rate. The present stochastic model well demonstrated the experimental results. The theory shows that these stochastic characteristics of microexplosion occurrence depend on the fifth power of the initial droplet diameter. The bubble nucleation leading to microexplosion depends on the timescale and the superheated liquid volume. The timescale in droplet combustion is proportional to the second power of the initial droplet diameter, and the volume of superheated liquid is proportional to the third power of the initial droplet diameter. For relatively large droplets, the microexplosion occurrence can be understood in a deterministic way. For smaller droplets, appearing in spray combustors, however, the microexplosion occurrence is more stochastic. The scatter in explosion behavior is also discussed considering the bubble nucleation location inside the droplet.

Counterflow diffusion flame with polydisperse sprays

An experimental study was performed on counterflow diffusion flames, in which polydisperse n-heptane sprays carried by a nitrogen stream from the lower duct, burn in oxidizer streams from the upper duct. The droplet-size distributions were varied with the nitrogen flow rate for atomization. The burning behaviors of the spray were observed and the oxygen concentration at extinction was determined for different droplet-size distributions. The flat blue diffusion flame was observed between two ducts, which was supported by vaporization of small droplets. Relatively large droplets penetrated the flat blue flame and continued to burn on the oxidizer side with an envelope flame surrounding each single droplet. The burning behaviors for different droplet-size distributions were discussed based on the Stokes number, St0, and the ratio, ψ, of the vaporization time to the characteristic flow time. The extinction limit was also found to be affected by the droplet-size distribution. The oxygen concentration at extinction had a minimum value for a specific volume mean droplet diameter irrespective of the fuel flow rate. This dependence was explained as a result of a competitive effect of St0 and ψ. The extinction limit of an n-octane spray was also examined to verify the effect of St0 and ψ on the extinction limit of the spray.

Strongly interacting combustion of two miscible binary-fuel droplets at high pressure in microgravity

The interactive combustion of two miscible binary-fuel droplets at elevated pressures, up to the values above the critical pressure of the fuel, was investigated experimentally. Heptane and hexadecane were selected as the fuel mixture, to study the occurrence of staged combustion, and a nitrogen-oxygen atmosphere with a relatively low oxygen concentration was employed to facilitate observation of droplets during combustion at elevated pressures. The effects of fuel composition, ambient pressure, and droplet spacing were investigated. Results show that staged combustion of binary-fuel droplets still exists for interacting droplets at high pressure and that burning-rate constants of the first and the third stages, as well as the transition droplet volume, are not strongly affected by interaction. The increase in droplet lifetime by strong interaction is demonstrated and explained. It is shown that the lifetime achieves a minimum value at a critical droplet spacing even for binary-fuel droplets that undergo staged combustion at high pressure without buoyancy, a result which is explained by influences of radiant energy transfer. It is also demonstrated experimentally that flame lifetimes exceed droplet lifetimes at high pressure both for single droplets and strongly interacting droplet pairs, which is explained by vapor presence after liquid disappearance and reduced tendencies toward flame extinction at elevated pressures.

Clarification of the flame structure of droplet burning based on temperature measurement in microgravity

In microgravity, soot accumulates inside the flame in droplet combustion. The yellow luminosity prevents the inner structure of the flame from being revealed. No experimental work has been conducted on the flame structure of the droplet burning in microgravity. Microgravity experiments on the droplet burning were performed for the purpose of clarifying the flame structure of spherically symmetrical burning of the droplet by using the drop tower. Gas temperature was measured by thermocouples with tip parts that are concentric with the spherical flame. This configuration reduces heat loss from the contact point of the thermocouple to the supporting wires. The gas-temperature distributions and temporal variations of the flame temperature were obtained and were examined for an ambient pressure ranging from 0.1 to 1.1 MPa. As results, it was found that the maximum temperature zone, that is, the primary reaction zone, exists about 0.8 mm outside the outer edge of the yellow luminous zone at 0.1 MPa for n -heptane droplets. The distance between the reaction zone and the outer edge of the yellow luminous zone decreases with increasing ambient pressure. The flame temperature is almost constant at the normalized time ranging from 0.3 to 0.6 and less dependent on the ambient pressure than on oxygen concentration of the ambient gas. In the later period of combustion, the flame temperature decreases with time because of the increase in the soot accumulation.

Pressure Effects in Droplet Combustion of Miscible Binary Fuels

The objective of this research is to improve understanding of the combustion of binary fuel mixtures in the vicinity of the critical point. Fiber-supported droplets of mixtures of w-heptane and n-hexadecane, initially 1 mm in diameter, were burned in room-temperature air at pressures from 1 M Pa to 6 M Pa under free-fall microgravity conditions. For most mixtures the total burning time was observed to achieve a minimum value at pressures well above the critical pressure of either of the pure fuels. This behavior is explained in terms of critical mixing conditions of a ternary system consisting of the two fuels and nitrogen. The importance of inert-gas dissolution in the liquid fuel near the critical point is thereby re-emphasized, and nonmonotonic dependence of dissolution on initial fuel composition is demonstrated. The results provide information that can be used to estimate high-pressure burning rates of fuel mixtures.

Interactive effects in two-droplet combustion of miscible binary fuels at high pressure

The objective of this research is to improve the understanding of droplet interaction in combustion of binary fuel droplets at high pressure. Combustion experiments were performed on two fiber-supported droplets of n -heptane/ n -hexadecane mixtures in room-tempperature air at pressures from 0.1 MPa to 6 MPa under free-fall microgravity conditions. The burning lifetime of the interacting droplets is compared with that of single droplet. The results show that the pressure dependence of the burning lifetime for the interacting droplets is similar to that of a single droplet. For most mixtures, the burning lifetime reaches a minimum value at pressures well above the critical pressure of either of the pure fuels. The interactive effects are evaluated using the total interaction coefficient, which is the ratio of the burning lifetime of the interacting droplets to that of a single droplet. As the pressure increases, the total interaction coefficient increases slightly and then decreases. The total interaction coefficient is a minimum for a certain initial liquid-phase mixture concentration. At relatively low pressure, the interactive effects are explained by the use of the quasi-steady interaction coefficient and temporal variation of the droplet diameter squared for a single droplet. At high pressures, the decrease in the influence distance of the droplet with increasing pressure causes the decrease in the total interaction.

Flexural component and extensional component of vibration energy in shell structure

In this research, the behavior of the flexural component and the extensional component of vibration intensity and their transmission in curved shells are presented. L‐shaped shell model was employed as an analysis model of FEM. As FEM analysis methods, both the frequency response analysis and the transitional response analysis were employed. The flexural component and the extensional component of vibration intensity (VI) were calculated by the results of FEM analysis. In the flexural component of the VI, the vibration energy supplied in the flat part decreased at the boundary from the flat part to the curved part and VI vectors flew in circumferential direction in the curved part. In the extensional component of the VI, the vibration energy appeared at the boundary from the flat part to the curved part and most VI vectors flew parallel to the shell axis in the curved part. The total vibration energy of the flexural component and the extensional component was conserved. So, the vibration energy transformed to each other between the flexural component and the extensional component in L‐shaped shell.