Droplet Gasification Rate Research Articles

Numerical modeling of isolated n-alkane droplet flames: initial comparisons with ground and space-based microgravity experiments

Transient, spherically symmetric, combustion of single and multi-component liquid n-alkane droplets is numerically simulated with a model that includes gas phase detailed, multi-component molecular transport and complex chemical kinetics. A compact semi-detailed kinetic mechanism for n-heptane and n-hexadecane oxidation consisting of 51 species (including He, Ar, and N 2) and 282 reactions is used to describe the gas phase. Non-luminous, gas phase radiative heat transfer and conservation of energy and species within the liquid droplet interior are also considered. Computed quasi-steady flame structure for pure n-heptane droplets is compared with that produced using the kinetic mechanism of Warnatz (frequently used in the past for modeling both premixed and diffusion flame properties). Transient calculations are also compared with the numerical results of King, which consider infinite rate chemical kinetics, but temperature dependent molecular diffusion. Modeling results are in reasonable agreement with small-diameter, drop tower experiments, though slow convective effects and droplet sooting effects exist in the experimental data. Comparisons with isolated large-diameter free droplet data (1 atm, He/O 2 mixtures and air) from recent space experiments are reasonable for droplet gasification rate, flame position, and flame extinction. Very small extinction diameters are predicted for small initial diameter droplets (<1 mm). As droplet size is increased, or oxygen index is decreased, the model predicts decreasing gasification rates and for an appropriate range of parameters, radiative flame extinction. Bi-component droplet combustion of n-heptane and n-hexadecane is also considered. Modeling results qualitatively reproduce experimentally observed, multi-stage burning, resulting from the volatility differential and diffusional resistance of the liquid components. Internal liquid convection effects are examined by parametrically varying an effective liquid mass diffusivity. Flame extinction can occur either in the initial or the secondary droplet heating period, with subsequent, continuing vaporization of the more volatile component from the residual heat within the liquid phase.

An Experimental Investigation On the Vaporization and Combustion of Methanol and Ethanol Droplets

Abstract The vaporization and combustion of freely-falling methanol and ethanol droplets in dry and humid environments has been studied experimentally. From time-resolved data of the droplet size and composition, it is demonstrated that water vapor, either from the ambience or generated at the flame, can freely condense at the droplet surface and subsequently dissolve into the droplet interior. The associated condensation heat release and dilution of the droplet alcohol concentration can significantly modify the gasification behavior from that of the d2-law. Specifically, droplet gasification is characterized by an initial period during which the gasification rates of methanol and the entire droplet are enhanced due to the condensation heat release, an intermediate period during which water vapor still condenses although the overall droplet gasification rate is retarded due to water build-up, and a final, slow-gasification period during which the initially condensed water is gasified. The stucy further demonstrates that the alcohol droplets can sustain diffusional combustion even with substantial amounts of water dilution, and identifies a neutral state of water condensation/gasification such that the associated data can be advantageously used for numerical modeling of alcohol droplet combustion.

Combustion of liquid-fuel droplets in supercritical conditions

A comprehensive analysis of liquid-fuel droplet combustion in both subcritical and supercritical environments has been conducted. The formulation is based on the complete conservation equations for both gas and liquid phases, and accommodates variable thermophysical properties, finite-rate chemical kinetics, and a full treatment of liquid-vapor phase equilibrium at the droplet surface. The governing equations and associated interfacial boundary conditions are solved numerically using a fully coupled, implicit scheme with the dual time-stepping integration technique. The model is capable of treating the entire droplet history, including the transition from the subcritical to supercritical state. As a specific example, the combustion of n-pentane fuel droplets in air is studied for pressures in the range of 5–140 atm. Results indicate that the ambient gas pressure exerts significant control of droplet gasification and burning processes through its influence on fluid transport, gas-liquid interfacial thermodynamics, and chemical reactions. The droplet gasification rate increases progressively with pressure. However, the data for the overall burnout time exhibit a considerable change in the combustion mechanism at the critical pressure, mainly as a result of reduced mass diffusivity and latent heat of vaporization with increased pressure. The influence of droplet size on the burning characteristics is also noted.

Burning-rate enhancement of organic diazide propellants - Dihalide addition and pressure elevation

Following the recent development of a class of organic diazides as high-energy-density, high-burning-rate fuels, an experimental investigation on the combustion, vaporization, and microexplosion of droplets of these organic diazides and their mixtures with alkyl halides at atmospheric and elevated pressures has been conducted in an effort to further enhance their high-burning-rate characteristics. Results show that the addition of small quantities of alkyl dihalides leads to significant advancements in the state of droplet microexplosion, regardless of the halogen substituent and its relative volatility, that increasing pressure dramatically increases the droplet gasification rate

Vaporization behavior of fuel droplets in a hot air stream

Tm GASIFICATION behavior of a liquid droplet provides a fundamental input for the modeling of many spray systems, and has been studied extensively [l-4]. Theoretical analysis for the multicomponent fuel droplet presents several complexities absent in a similar analysis for the single component droplet. First, the phase-change process at the droplet surface and the transport of a fuel vapor mixture in the gas phase need to be properly described. Second, the evaporation process is inherently time varying due to the continuous change in the composition and temperature of the droplet as vaporization proceeds. Another difference between the two cases is due to the phenomenon of microexplosion [2]. The earlier viewpoint [2] of multicomponent droplet vaporization assumes that the composition and temperature within the droplet are spatially uniform but time varying. Such theories predict that the gasification process is similar to batch distillation in that the sequence of gasification is controlled by the volatility differentials among the different components. However, Sirignano [4] showed that even in the limit of high vortex strength, the internal liquid circulation can only reduce the characteristic length scale for diffusion by a factor of three. Landis and Mills [S] investigated the vaporization of a bi-component fuel droplet in a stagnant atmosphere. A quasi-steady gas-phase model was used and the equations governing the unsteady mass and heat diffusion within the droplet were solved numerically. Results when compared with rapid mixing behavior showed significant differences. A unique feature of the diffusion-dominated droplet gasification mechanism is the possible attainment of approximately steady-state temperature and concentration profiles within the droplet, which then leads to a steady-state gasification rate. Based on this concept, Law and Law [6] formulated a d2-law model for multicomponent droplet vaporization and combustion. As indicated above, the literature on the gasification behavior of an isolated droplet is extensive. However, most of these studies deal with droplet combustion or evaporation under high-temperature conditions. Not much information is available on the behavior of evaporating droplets in relatively low-temperature air streams. Under such conditions, the possibility of an envelope flame is precluded and the droplet gasification rate is low. The droplet heat-up time may not be negligibly small compared to its lifetime, although the latter is relatively large and the liquid-phase transient processes may still be important. In this paper, the vaporization behavior of pure and multicomponent fuel droplets flowing in a well-characterized laminar flow is studied. The predictions of three vaporization models are compared with the experimental data.

Aerothermochemical studies of energetic liquid materials. 2. Combustion and microexplosion of droplets of organic azides

An experimental investigation has been conducted on the vaporization, combustion, and microexplosion characteristics of freely falling droplets of organic azides in hot oxidizing or inert environments. The droplet gasification rate and the state of microexplosion have been measured. Results show that, compared with the conventional hydrocarbon fuels, droplets of azido fuels gasify faster and microexplode earlier in the droplet lifetime. The effect is especially strong for n-alkyl diazides whose gasification rates can exceed those of the corresponding n-alkanes by several hundred percent. Such strong responses are believed to be caused by the exothermic decomposition of the azides either in the liquid phase or upon gasification. Other important and interesting effects studied include molecular branching, OH-substitution, the proximity and number of the azide groups, and alcohol and halide blending.

Time-resolved gasification and sooting characteristics of droplets of alcohol/oil blends and water/oil emulsions

The gasification mechanisms and sooting characteristics of freely-falling droplets of alcohol/oil blends and water/oil emulsions were experimentally studied by using microphotography to time-resolve the droplet gasification rate and phase-discriminating sampling to determine the instantaneous droplet composition and the gas-phase soot level. Results on the gasification studies show that while liquid-phase diffusional resistance is only moderately effective for a miscible mixture, it is sufficiently strong for water/oil emulsions such that the droplet composition remains basically frozen during combustion. Results on soot studies demonstrate that the primary causes for soot reduction with alcohol blending or water emulsification are the dilution of the fuel composition by non-sooty components and the reduction of the flame temperature because of reduced heats of combustion. The results do not seem to suggest any significant catalytic/oxidative effects of the hydroxyl radicals from the fuel mixtures.

Alcohol droplet vaporization in humid air

Experimental and theoretical investigations were conducted for the vaporization of a single alcohol droplet in air with various degrees of humidity. Experimental results show that the vaporization of a volatile alcohol, such as methanol and ethanol, is accompanied by the simultaneous condensation of water vapor on the droplet surface and its subsequent diffusion into the droplet interior such that the associated condensation heat release greatly facilitates the initial gasification rate of alcohol. However, for alcohols which are less volatile, or for liquids which are not miscible with water, atmospheric moisture has practically no effect on the droplet gasification rate. Theoretical results substantiate the above observation.

Interactive vaporization and combustion of binary droplet systems

The interactive vaporization and combustion of two horizontally-aligned droplets were experimentally studied using an automated data acquisition and reduction system. Factors investigated include the droplet spacing and size, ambient oxygen concentration, fuel type, droplets of dissimilar fuels, intensity of buoyancy, and pure vaporization. Results show that much of the burning results can be interpreted on the basis of the flame size and oxygen competition, that the existing theory over-predicts the intensity and persistency of interaction, that the gasification rate correction factor depends on the initial as well as the aerothermochemical parameters of the system in addition to the scaled instantaneous droplet separation distance, which is the only (geometrical) parameter present in the theory, that interactive buoyant convection can substantially augment the droplet gasification rate to exceed the isolated value, and that mass exchange between droplets of different volatilities can be quite significant. It is further suggested that interactive effects can be considered to be shorrranged for the droplet gasification rate but long-ranged for the mergedness of the flame.

Simulation of Fuel Droplet Gasification in SI Engines

The heating and gasification of a fuel droplet during the intake and compression strokes of an SI engine are modeled. Results show that the simultaneous increases in the gas temperature and pressure during compression tend to have compensatory effects on the droplet gasification rate such that it remains somewhat insensitive to changes in the cylinder environment. Generalized results are presented allowing for the assessment of the lower and upper bounds in the initial size of the droplet that can achieve complete gasification prior to the end of the compression stroke.