Droplet Gasification Research Articles

Hydrogen peroxide droplet gasification and decomposition: Classic evaporation model vs. conjugate model

The purpose of the present study is to investigate the evaporation of hydrogen peroxide droplet and the impacts of gas phase thermal decomposition on this process. Two different flow solvers are developed for analysis where one uses the classic evaporation model, and the other solves the conjugate heat transfer problem considering temperature distribution within both gas and liquid phases accurately. The finite volume method is utilized to solve the governing equations of the reacting two-phase flow. Single droplet evaporation in the hot open surrounding is analyzed first to illustrate the effect of gas phase decomposition on droplet regression rate. It is shown that the decomposition changes the temperature distribution around droplet that leads to higher regression rate by increasing the heat transfer from the gas phase into the liquid phase. This cannot be captured using the classic model and must be studied by conjugate model. In addition, the effects of droplet radius, ambient pressure, and ambient temperature on the regression rate augmentation due to decomposition are investigated for single droplet. Finally, evaporation and decomposition of hydrogen peroxide droplet cloud in a closed vessel is simulated and it is demonstrated that conjugate model has more accurate prediction than the classic model and must be used in practical studies especially when the transient processes are drastic.

Jetting Dynamics of Burning Gel Fuel Droplets.

Jetting in burning gel fuel droplets is an important process which, in addition to pure vaporization, enables the convective transport of unreacted fuel vapors from the droplet interior to the flame envelope. This aids in accelerating the fuel efflux and enhancing the mixing of the gas phase, which improves the droplet burn rates. In this study, Schlieren imaging was used to characterize different jetting dynamics that govern the combustion behavior of organic-gellant-laden ethanol gel fuel droplets. To initiate jetting, the gellant shell of the burning gel fuel droplet was subjected to either oscillatory bursting or isolated bursting, or both. However, irrespective of the jetting mode, the jets interacted with the flame envelope in one of three possible ways. Based on the velocity and the degree to which a jet disrupts the flame envelope, it is classified as either a flame distortion, a fire ball outside the flame or a pin hole jet (localized flame extinction), where the pin hole jets have the highest velocity (1000-1550 mm/s), while the flame distortion events have the lowest velocity (500-870 mm/s). Subsequently, the relative number of the three types of jetting events during the droplet lifetime was analyzed as a function of the type of organic gellant. It was demonstrated that the combustion behavior of gel fuels (hydroxypropyl methylcellulose: HPMC at 3 wt.%) that tend to form thin-weak-flexible shells is dominated by low-velocity flame distortion events, while the gel fuels (methylcellulose: MC at 9 wt.%) that facilitate the formation of thick-strong-rigid shells are governed by high-velocity fire ball and pin hole jets. Overall, this study provides critical insights into the jetting behavior and its characterization, which can help us to tune the droplet gasification and the gas phase mixing to achieve an effective combustion control strategy for gel fuels.

Effects of temporally varying liquid-phase mass diffusivity in multicomponent droplet gasification

The relative roles of liquid-phase diffusional resistance and volatility differential in multicomponent droplet gasification are revisited, recognizing that liquid-phase mass diffusivities can be substantially increased as the droplet is progressively heated upon initiation of gasification, leading to a corresponding substantial weakening of the diffusional resistance. Calculations performed using realistic and temperature-dependent thermal and mass diffusivities indeed substantiate this influence. In particular, the calculated results agree with the literature experimental data, indicating that the gasification mechanism of multicomponent fuels is intermediate between diffusion and distillation limits. Investigation was also performed on gasification at elevated pressures, recognizing that the liquid boiling point and hence the attainable droplet temperature would increase with increasing pressure, causing further weakening of the liquid-phase diffusional resistance. This possibility was again verified through calculated results, suggesting further departure from diffusion limit toward distillation limit behavior for gasification at high pressures. The study also found that diffusional resistance is stronger for the lighter, gasoline-like fuels as compared to the heavier, diesel-like fuels because the former have overall lower boiling points, lower attainable droplet temperatures, and hence lower mass diffusivities in spite of their lower molecular weights.

Experimental Investigation of Black Liquor Pyrolysis Using Single Droplet TGA

Black liquor pyrolysis is studied in a single droplet gasification reactor. The reactor is equipped with online analytical instruments to measure mass, temperature, and concentration of released gases during the process. It is also possible to follow visually and to record droplet geometric changes through a glass window. The pyrolysis parameters measured in this study are: the droplet swelling profile, pyrolysis gas released, and temperature inside the droplet. The gas detect-ed by the analytical instrument can be deconvoluted to obtain the gas released at the actual time of droplet pyrolysis, thus minimizing the effect of transportation time. These data are important for studying the kinetics of black liquor droplet pyrolysis.

Effects of internal heat transfer and preferential diffusion on stretched spray flames

The extinction of dilute spray flames propagating in a stagnation-point flow under the influence of flow stretch, preferential diffusion, and internal heat transfer is analyzed using activation energy asymptotics. A completely prevaporized mode and a partially prevaporized mode of flame propagation are identified. The internal heat transfer, associated with the liquid fuel loading and the initial droplet size of the spray, provides heat loss and heat gain for rich and lean sprays, respectively. It is found that the flow stretch coupled with Lewis number ( Le) reduces and enhances the burning intensity of the lean methanol-spray flame ( Le>1) and rich methanol-spray flame ( Le<1), respectively; and that the flame extinction characterized by a C-shaped curve for the Le>1 flame is dominated by the flow stretch, while the S-shaped extinction curve for the Le<1 flame is mainly influenced by the internal heat loss associated with the droplet gasification process.

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.

Dynamics of ignition transience and gasification partition of a droplet

Thermochemical evolution of a droplet suddenly exposed to a hot environment is studied to assess the transient characteristics of the ignition, flame bifurcation and scavenging combustion, transition of premixed flame to nonpremixed combustion, and the ultimate burnout of an isolated droplet. Canonical theory of droplet gasification gives a general criteria of ignition and serves to identify all the gasification submechanisms of an arbitrary geometry in a stationary or convective environment. The theory is used in conjunction with numerical analysis for prediction of the transient flow-field structures and the gasification rates of all the submechanisms of gasification. The results reveal that the ignition transience exhibits flame bifurcation in a broad range of the environmental temperature, which lies between 930 K and 1700 K, for an n -heptane droplet with the simulated reaction rate model. At temperature higher than 1700 K, flame splitting does not occur. There are, in general, seven gasification submechanisms for a droplet: however, the net gasification rate during the ignition, flame bifurcation, and scavenging combustion is primarily controlled by the exothermic reaction and thermal energy accumulation, each of which has the effective gasification rate of nearly 10 to 40 times of that of the conventional Godsave-Spalding gasification rate: whereas the other submechanisms contribute at nearly the same order of magnitude of the Godsave-Spalding gasification rate. The droplet combustion is also classified into fully evolved and partially evolved combustion depending on the state of the droplet at the burnout. The predicted ignition delay time for n -heptane droplets in the size range of 600–2200μm are in good qualitative agreement with available experimental data. Areas of future research are also discussed.

FUEL VAPOR ACCUMULATION EFFECT ON THE COMBUSTION CHARACTERISTICS OF MULTICOMPONENT FUEL DROPLETS

The effect of fuel vapor accumulation on the gasification behavior of a multicomponent fuel droplet has been investigated. Expressions for the droplet vaporization rate, flame front standoff ratio and other quantities of interest have been obtained by using a formulation that considers quasi-steady, diffusive-convective transport in the gas phase, and transient diffusive transport within the droplet. Results are presented for the combustion of a bicomponent heptane-decane fuel droplet. It is demonstrated that a significant amount of fuel vapor can be accumulated in the inner zone to the flame, which has a noticeable effect on the droplet gasification behavior since the fuel consumption rate is reduced compared to the vaporization rate. This has important implication for spray flames using multicomponent fuel droplets, since the fuel vapor accumulation can substantially modify the rates of interphase energy and mass transport rates, as well as the fuel vapor distribution in the flow field. Results also indicate that the fuel accumulation effect, combined with the effect of high liquid mass diffusional resistance (large Lewis number), leads to a self-sustained oscillation of the vaporization rate. The oscillation is unique to the multicomponent fuel droplet in that the accumulation effect does not cause any oscillation for a pure fuel droplet. The oscillatory behavior is also suppressed for a multicomponent fuel droplet for moderate values of liquid Lewis number with the accumulation effect, and for any values of liquid Lewis number without the accumulation effect.

Energetic Fuel Droplet Gasification with Liquid-Phase Reaction

Abstract An exploratory analytical and computational study of gasifying energetic liquid fuel droplets is presented. The liquid-fuel reaction, heating, and breakup are modelled and characterized. A spherically-symmetric geometry is considered, and the problem is described by considering a general formulation capable of resolving the solution domain consisting of a liquid-phase region, a bubbly two-phase region, and a gas-phase region. The transient, two-phase, governing equations are solved numerically for various values of the nondimensional rate constant, heat of decomposition, activation energy, number of bubbles per unit mass, and ratio of gas-phase to liquid-phase thermal conductivities.

Transcritical liquid oxygen droplet vaporization - Effect on rocket combustion instability

The response of a liquid oxygen droplet to oscillatory ambient conditions consistent with liquid-rocket engines over a wide range of frequencies is computed. Two configurations are considered: 1) isolated droplets and 2) droplets in an array. The consequences of nonuniformities introduced in the gas phase by neighboring droplets on the droplet response are evaluated. The potential of gasification as the ratecontrolling mechanism was evaluated through the computation of a response factor derived from the Rayleigh criterion. Computations show that the peak frequency for the computed response factor is mainly correlated to the droplet lifetime and also depends on the type of flow that the droplet is experiencing (with or without reversal). Consequently, droplet secondary atomization, which causes a substantial droplet lifetime reduction, induces a significant (one order of magnitude) shift in the peak frequency. As a result, the frequency of the maximum response factor is too high to correspond to the acoustic frequencies of the typical modes for standard cryogenic rocket engine chambers. Since droplets are likely to undergo secondary atomization in the stripping regime for most of their lifetime in these engines, this phenomenon explains the observed better stability of such engines compared to storable propellant engines. It was also shown that the droplet gasification process, whether undergoing stripping or not, can drive combustion instabilities for the longitudinal mode, under certain simplifying assumptions. The effects of mean pressure and pressure fluctuations on the droplet response were also evaluated.

Characteristics of supercritical droplet gasification

A liquid-fuel droplet exposed to a supercritical ambient gas (its thermodynamic state exceeding the fuel-critical point) may experience the transition to continuous phase change during its lifetime. This transition can be analyzed in numerical simulation by using a fuel-related diffusion coefficient that vanishes at the droplet surface when the surface reaches the critical state of the mixture involved. Based on the numerical simulation of spherically symmetric gasification process, a droplet gasification regime map is constructed that predicts which of the following three gasification regimes is realized for the given values of system pressure, ambient gas temperature, and initial droplet temperature: (1) subcritical gasification regime in which the droplet maintains its surface throughout its lifetime, (2) transitional regime in which the droplet experiences the transition to continuous phase change during its lifetime, and (3) supercritical gasification regime in which continuous phase change occurs from the onset of gasification. The characteristic physics underlying each gasification regime are revealed. The pressure dependence of gasification lifetime for fixed ambient gas temperature and initial droplet temperature shows that there is a pressure corresponding to minimum gasification lifetime for which the droplet surface reaches the mixture-critical state as soon as the droplet is exposed to the ambient gas. The gasification lifetime is reduced significantly by increasing initial droplet temperature. The effects of initial droplet temperature are discussed in detail. Implication of the gasification regime map when applied to sprays is discussed to address future investigations.

96/00720 A numerical investigation of multiple flame configurations in convective droplet gasification

Hydroliquefaction of several coals, Taiheiyo (Japanese), Mi-ike (Japanese), Wandoan (Australian), and Illinois No.6 (American), was carried out using iron pentacarbonyl(Fe(CO)5) at 425–460°C under a hydrogen pressure of 4.9MPa in a non-hydrogen donating solvent, 1-methylnaphthalene. With the addition of iron pentacarbonyl coal conversion increased substantially for all of the coals used. Lighter fraction (oil) also increased, by ≈ 10–17 wt%, in the presence of the catalyst. The addition of Fe(CO)5 suppressed coking, resulting in high values of coal conversion and oil fraction even at 460°C. The amounts of hydrogen transferred from the gas phase increased by 2–4 times with Fe(CO)5. A process involving direct hydrogen transfer to coal fragment radicals is proposed.

High-pressure droplet burning experiments in microgravity

High-pressure droplet burning characteristics of five fuels are investigated under normal and reduced gravity conditions. The reduced gravity experiments have been conducted by using the parabolic flights of the CNES Caravelle and the NASA KC-135 aircrafts. A fully automatic high-pressure droplet gasification facility has been developed for these experiments. Rapid videography is used to determine the time histories of burning droplets, from which average droplet burning rates are determined. For all experiments, the suspended droplet technique is used. Initial droplet diameters are about 1.5 mm. Subcritical and supercritical droplet burning regimes are explored. Droplet time histories are only determined for weakly sooting fuels, such as methanol. These results show that the D 2 law holds even under very high pressures and allows the estimation of an average droplet burning rate. For the heavily sooting n -alkane droplets, this result is used to deduce the average burning rate by measuring the droplet burning life-time. The experimental results for all fuels show that the droplet burning lifetime decrease strongly with increasing pressure in the subcritical regime. When the pressure is increased above the critical pressure of the pure liquid, the droplet burning life-time remain constant on the average. Therefore, the minimum burning time observed in the literature is not confirmed here. To take into account the residual gravity-enhanced natural convection effects, a correction based on the Grashof number is applied. This correction allows a comparison of various experiments under different gravity and pressure conditions and an estimate of the burning time without any convection effect. In the subcritical regime, this burning time varies as p −0.37 .

A numerical investigation of multiple flame configurations in convective droplet gasification

Multiple flame configurations and gasification rates of a convective fuel droplet are numerically investigated at various Damköhler and Reynolds numbers. The gas flow field is predicted by solving the quasi-steady conservation equations of mass, momentum, and energy, in which gas-phase combustion is modeled by a one-step global finite-rate chemical reaction. Droplet internal motion is predicted by solving the momentum equations. Both gas- and liquid-phase solutions are obtained through an iterative process that satisfied the interfacial conditions. Numerical results reveal that plots of droplet gasification rates versus the Damköhler number exhibit multivalued curves for Reynolds numbers smaller than 80. Similarly, plots of droplet gasification rates versus the Reynolds number also exhibit multivalued curves for low Damköhler numbers. However, monotonically increasing curves are obtained for high Damköhler numbers. Multiple flame configurations are obtained under the present quasi-steady predictions, depending upon the branch solution examined. As the Damköhler number increases or the Reynolds number decreases, flame structure first exists as pure vaporization, then as a wake flame, a transition flame, and finally as an envelope flame for the lower branch solution, while for the upper branch solution, flame structure exists as a wake flame and then an envelope flame. Both lower and upper branch solutions are in qualitative agreement with previous unsteady calculation (Dwyer and Sanders 1988) and experimental studies (Gollahalli and Brzustowski 1973 and 1975), respectively. Based on whether the gasified fuel vapor acts as an oxidizer sink or as a fuel vapor source, convective droplet ignition and extinction points are suggested according to the flame structure rather than the gasification rate.

Transient numerical modeling of carbon particle ignition and oxidation

The chemically reacting flow field on and around the surface of an isolated carbon particle has been simulated numerically using a fully transient formulation. Computations were performed for a spherically symmetric system utilizing detailed transport properties, a comprehensive gas-phase reaction mechanism, and a five-step heterogeneous surface mechanism. The effects of surface regression on the combustion characteristics of carbon particles are investigated for two size ranges (for small particles, the diameters ranged from 100 to 300 μm and for large particles, the diameter is typified by 2000 μm). Particles were ignited by placing them in various gaseous oxidizing media at a temperature sufficiently high (approximately 1400 K) to initiate gas-phase oxidation of CO evolved from the particle surface. Results show the typical behavior commonly associated with diffusion-limited and chemical kinetically limited burning of “large” and “small” particles. However, the range of diameters over which both diffusion and kinetics affect the burning characteristics is large. The regression of the particle surface introduces an additional characteristic time to the problem. The order of this time for small particles is approximately that necessary to establish a reactive gas-phase boundary layer and causes burning predictions to depart substantially from those found for quasi-steady burning. For large particles, other transients result from effects similar to those of “accumulation” for liquid droplet gasification and burning. Finally, transient behavior departures from quasi-steadiness are obtained for a large range of surface kinetic rates that encompass modifications that could result if the particles were porous.

The influence of external heat transfer on flame extinction of dilute sprays

The extinction of a dilute spray flame burning in a steady, one-dimensional, low-speed, sufficiently off-stoichiometric, two-phase flow, and experiencing the external heat transfer from the spray to a tube wall upstream is further analyzed. The external heat transfer results in globally external heat loss, excess enthalpy burning and external heat gain, respectively, to the spray system with increasing the wall temperature. However, the droplet gasification provides the overall internal heat loss and heat gain for rich and lean sprays, respectively. Therefore, the burning and extinction of the dilute spray flame can be fully described by the interaction between external and internal heat transfers in two spray models which were identified to be the completely and partially prevaporized burnings. The C-shaped and S-shaped extinction curves are clearly classified and mapped with parameters of the wall temperature, the overall external heat transfer and the initial droplet size. Variations of the extinction curves under the influence of transition from overall external heat loss to heat gain, and the jump between the completely and partially prevaporized burnings on flame extinction, are reported and discussed for both lean and rich sprays.

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.

Observations of Chlorinated Hydrocarbon Droplet Gasification

Abstract Burning of liquid chlorinated waste is often accomplished by spray combustion, leading to diffusion flames surrounding an individual or a cloud of droplets. We have previously reported on the gasification and flame extinction characteristics of individual droplets of a series of mono-chlorinated alkanes and mixtures of nonane and 1,1,2,2-tetrachloroethane. These studies have been extended to include octane/tetrachloroethene mixtures and several chlorinated benzenes, compounds whose complete combustion was anticipated to be more difficult. Effects of mixture composition and changes to the ambient environment surrounding the droplets, i.e., H2O and O2, concentrations, were observed in order to produce a data set that can be used for future modeling studies. The burning rate data obtained indicate that equal volatility mixtures of octane/tetrachloroethene burn readily. However, as the Cl/H ratio of a mixture increases and approaches unity, burning rates diminish toward the vaporization rate as was p...

Theory of Azeotropic Gasification of Miscible Multicomponent Droplets

Abstract Analytical studies of quasi-steady and spherically-symmetric gasification of miscible multi-component droplets in a quiescent ambient are presented. Emphasis is placed upon identifying conditions where azeotropic gasification (defined as gasification with time-invariant droplet compositions) may occur. It is shown that if differences between gas-phase species diffusivities are negligible, azeotropic gasification states correspond to thermodynamic azeotropic states. These thermodynamic azeotropic states are at the partial pressure of the droplet components in the gas phase at the liquid surface; ignoring insoluble gas-phase components, the gas and liquid compositions at the gas-liquid interface are the same. When gas-phase preferential species diffusion is significant, azeotropic gasification states are shifted away from thermodynamic azeotropic states. Theory suggests that under special circumstances, gas-phase preferential diffusion may induce azeotropic gasification or volatility reversal for d...

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.