Disruptive Burning Research Articles

Some Theoretical Considerations on the Combustion and Disruption of Free Slurry Droplets

Abstract The combustion and disruption or free slurry droplets are studied theoretically to elucidate a mechanism for the overall disruptive burning process. The disruptive burning process consists of three stages; a d2-law stage, a shell formation stage, and a disruption stage. It is proposed that the agglomeration of particles near the droplet surface due to capillary forces causes the shell formation and that the pyrolysis of additives due to the temperature rise in the outer surface of the shell produces an impermeable shell immediately and eventually disruption activity. A simple mathematical model is formulated to predict the shell formation and disruption time, and the fragmented shell thickness. The model simulation results for the disruptive burning of boron/JP-IO slurry droplets show good agreements with Takahashi er al. (1988a, b)'s experimental results. The disruption time increases with increasing initial slurry droplet diameter or increasing initial boron loading. The thickness of the fragmented shell increases as the initial diameter or the initial boron loading is increased. Finally, results show that the disruption time increases significantly whereas the thickness of the fragmented shell increases slightly as the environmental oxygen concentration is increased.

Disruptive Burning Mechanism of Free Slurry Droplets

Abstract A stream of free, monodisperse droplets of boron/JP-10 slurries, prepared in the laboratory, was projected downward into a high-temperature, atmospheric-pressure, oxidizing gas stream. High molecular-weight organic dispersing agents were found to be responsible for the violent disruption of the primary slurry droplets burning with an envelope diffusion flame. The time to disruption increased as (1) the concentration of the dispersant was decreased, (2) a dispersant with higher initial pyrolysis temperature was used, and (3) the size of the suspended solid particles in the slurry was increased. The quiescent vapor-phase combustion of unstabilized slurry droplets generated spherical “solid” boron agglomerates, while the disruptive burning of stabilized slurry droplets resulted in quasi-spherical hollow shells along with their fragments. In the light of the combined role of dispersing agents and solid particles in the formation of the agglomerate shells, a physical mechanism of the process leading to disruption is proposed herein. This work clearly demonstrates that the choice of dispersants for slurries not only affects the slurry stabilization but may significantly modify its combustion properties and those of the agglomerates formed.

Spray combustion of diesel-oil/ethanol solution and diesel-oil/water emulsion fuels.

In order to elucidate the combustion characteristics of diesel-oil/ethanol solution and diesel-oil/water emulsion sprays, the distributions of temperature, gas flow velocity and gas composition were measured on co-axial spray flames. This was accomplished using the same heat release rate under the condition of the same flow rates of combustion-air and atomizing-air. The following results were obtained : (1) Though the ethanol flames is longer than the diesel-oil flame, mixing ethanol with diesel-oil shortens the flame length. The flames of diesel-oil / ethanol become shorter than the diesel-oil flame at a diesel-oil content of 5070%. (2) Addition of water to diesel-oil decreases the flame length with the increase of water content. (3) Preheat of diesel-oil/water emulsion decreases the flame length even more. Cracking sound and violent flame fluctuation were observed in this flame, which may indicate the appearance of disruptive burning of fuel droplets.

Ignition and burning of single liquid droplets

The spherically symmetrical combustion of a fuel droplet in an oxidizing atmosphere is considered. Theoretical results are given concerning the ignition, gas-phase unsteadiness, liquid-phase unsteadiness, quasisteady burning, disruptive burning and extinction phenomena encountered. Experimental methods for testing theoretical predictions are presented.

Disruptive and Micro-Explosive Combustion of Free Droplets in Highly Convective Environments

Abstract Experinlental evidence is presented showing the existence of disruptive burning for two-component fuel droplets and micro-explosive combustion for water-in-singlc fuel component emulsificd drops subjected to high relative velocities ranging from 16 to 19 m/sec. Reynolds numbers, based on the high temperature (1200-1400 K) convective freestream conditions and droplet diameter, exceedcd 40. Calculations using the one-third averaging rule suggested by other investigators for evaluating the relevant properties lead to Reynolds numbers exceeding 135. The fuels considered in this study were a solution of 50 vol.% 11-hexane-50 vol.% n-hexadecane and water-in-11-hexadecane cniulsions, with 9 and 18 percent (volume) of water. The substantial decrease i n time from ignition-to-disruption (with increased relative gas-droplet velocity) in combination with the observcd "droplet-shoulder-orientcd disruptions" for. Ihe multi-component solulion supports the existence of a toroidal vortex-like structure within thc liquid phase. Furthermore, the observed micro-explosive behavior of the water-in-n-hexadecane emulsified fuel drop-lets under highly convecrivc conditions indicates that water and fuel are not vaporizing independently of each other. In addition, the intensity of the micro-explosive event for the emulsified fuels considered does not appear to be inhibited by the high relative gas-droplet velocilies.

Burning of Distillate Fuel Droplets Containing Alcohol or Water: Effect of Additive Concentration

Previous work on the addition of methanol, ethanol and water to pure n-paraffins is extended to a conventional No. 2 distillate fuel oil. Both the effect of additive content in the fuel oil and oxidizer concentration in the environmental flow on the existence and character of the disruptive behavior of these solutions and emulsions are investigated. For the ethanol in No. 2 oil solution cases it is shown that there is a maximum and a minimum ethanol con-centiation for which disruptive burning occurs, viz. 5% and 90% by volume. The results show an optimal ethanol concentration range within 15% and 55% by volume for which the disruption occurs prior to the consumption of 40% of the initial volume of the solution droplet. It is also shown that emulsions of water-in-No. 2 oil undergo micro-explosive vaporization and combustion and that the role of emulsion water content in promoting the dispersion of mass through micro-explosive phenomena is not highly significant.

High Pressure Homogeneous Nucleation of Bubbles within Superheated Binary Liquid Mixtures

The limits of superheat of some binary normal paraffin mixtures were measured at pressures up to 2128 kPa using the floating droplet method. The variation of nucleation temperature with liquid phase mole fraction was found to be nearly linear for these n-paraffin solutions over the whole pressure range in which the experiments were performed. Homogeneous nucleation theory was used to predict the limits of superheat of the solutions tested. The vapor pressures of the mixtures were estimated by using the Peng-Robinson equation of state to evaluate the liquid and vapor phase fugacities, and the mixture surface tensions were calculated using an empirical adaptation of the van der Waals expression for the surface tension of a pure liquid near the critical point. The predicted and measured limits of superheat were found to be in good agreement over the entire pressure range for all liquid phase compositions. The results of the present work could be useful for predicting liquid phase temperatures and compositions at which the microexplosive or disruptive burning of droplets of fuel blends which are mixtures of volatile and nonvolatile liquids will be initiated during droplet combustion at high ambient pressures.

On the disruptive burning of free droplets of alcohol/n-paraffin solutions and emulsions

A systematic experimental study of the low Reynolds number, atmospheric pressure combustion of free droplets of binary n-propanol/n-paraffin solutions and ethanol/n-paraffin solutions and emulsions was performed. Special attention was given to characterizing the disruptive burning phenomena for these fuel systems. The onset of disruptive burning was shown to be a function of both the difference between the normal boiling points and relative concentration of the solution components. Disruptive burning was found to be the result of homogeneous bubble nucleation within the interior of the droplet followed by the expansion and bursting of the gaseous nucleus. All ethanol/n-paraffin emulsions studied exhibited disruptive combustion behavior typical of solutions. This was attributed to dissolution of the dispersed phase during the transient heating period. In addition, similar disruptive behavior was demonstrated for solutions of ethanol in diesel fuel within the concentration range typical of that considered in fuel substitution applications. A simplified theory is proposed to predict the minimum concentration of alcohol required to produce the disruptive burning.

Experimental Observations on the Disruptive Combustion of Free Droplets of Multicomponent Fuels

Abstract The disruptive burning characteristics of isolated free droplets of binary n-paraffin mixtures have been observed experimentally. For disruptions to occur a minimum difference in the normal boiling points of the components as well as a certain initial concentration of the more volatile component must exist. The initial concentration of the volatile component must be within a limited range defined by the relation of the homogeneous superheat limit of the mixture to the normal boiling point of the less volatile material. Disruptive burning is a result of homogeneous nucleation of the mixture somewhere within the interior of the droplet. Mass diffusion is the limiting liquid transport process which results in superheating of the droplet interior. Comparison with previous studies of micro-explosive atomization of water-in-fuel emulsions shows that the disruptive burning of a binary fuel mixture is a slower and less violent process. It is also concluded that through relieving the requirement for homog...