An experimental investigation of sootshell formation in microgravity droplet combustion

Spherically symmetric droplet combustion experiments were performed at the NASA Glenn Research Center (GRC) 2.2 second drop tower in Cleveland, OH in an effort to better understand the mechanism leading to sootshell formation. Rapid insertion of a blunt plunger was used to remove the symmetric sootshell that formed during the period of quasi-steady burning. This allowed for the observation of sootshell re-formation. Soot particles were formed near the flame front and migrated towards the droplet to ultimately reside at the sootshell location. These experiments helped to bring about a better understanding of soot transport in microgravity droplet combustion.     

Sooting characteristics of isolated droplet burning in heated ambients under microgravity

This paper presents an investigation into the sooting characteristics of isolated droplets (for fuel n-decane) burning in heated ambients in microgravity. A backlit video view of the droplet was taken to determine the soot shell size and to judge the transient soot generation according to qualitative amount of soot. The independent experiment variables were the ambient temperature and initial droplet diameter. Soot generation was higher for initially larger droplets when compared at the same burning time normalized with the initial droplet diameter squared (called normalized burning time). At the same absolute burning time there existed an obvious initial transient period after ignition in which the stated relationship was not satisfied. This transient time increased with increasing the ambient temperature. There was a peak in the soot generation at about 1000 K throughout the lifetime of the droplet. The soot shell size was generally larger for an initially bigger droplet at the same instantaneous droplet diameter or normalized burning time. At the same absolute burning time, however, an initially smaller droplet exhibited larger relative soot shell sizes (the soot shell size normalized with the initial droplet diameter). The soot shell size increased monotonically with increasing ambient temperature. This is due to the increase in the Stefan flow drag with the larger burning rate at the higher temperature. The consequent result is that the soot shell sizes are much larger for droplets burning in heated ambients than for droplets burning in room-temperature ambients.     

Effects of ammonia borane on the combustion of an ethanol droplet at atmospheric pressure

Adding compounds rich in hydrogen to liquid fuels has the potential to change combustion behavior and enhance performance. One potential additive is ammonia borane (AB), which contains 19.6 wt.% hydrogen and can be dissolved in anhydrous ethanol (up to 6.5 wt.%). The particular system studied here would have limited use due to energy density and stability but is studied as a model system. Single droplet combustion experiments were performed with AB concentrations in ethanol varying from 0 to 6 wt.%. Measurements performed using high speed (5 kHz) planar laser-induced fluorescence (PLIF) indicate that hydrogen gas addition from the decomposition of AB continues throughout the droplet burning process. The hydrogen addition leads to an increase in the D² law rate constant, k₀, of up to 16%. While AB (and residual material) participates throughout the combustion process, it dramatically impacts the combustion behavior at the end of the droplet lifetime as the concentration of AB residual grows within the droplet. This results in droplet shattering, causing fine atomization and rapid combustion of the remaining fuel. Boron is also oxidized in this short period of time, increasing the energy released. In combustors, droplet shattering could enhance mixing and increase combustion efficiency. Thus, the approach of adding compounds rich in hydrogen is a promising method to introduce H₂ gas to practical combustion systems, while enhancing performance.     

On droplet combustion of biodiesel fuel mixed with diesel/alkanes in microgravity condition

The burning characteristics of a biodiesel droplet mixed with diesel or alkanes such as dodecane and hexadecane were experimentally studied in a reduced-gravity environment so as to create a spherically symmetrical flame without the influence of natural convection due to buoyancy. Small droplets on the order of 500 μm in diameter were initially injected via a piezoelectric technique onto the cross point intersected by two thin carbon fibers; these were prepared inside a combustion chamber that was housed in a drag shield, which was freely dropped onto a foam cushion. It was found that, for single component droplets, the tendency to form a rigid soot shell was relatively small for biodiesel fuel as compared to that exhibited by the other tested fuels. The soot created drifted away readily, showing a puffing phenomenon; this could be related to the distinct molecular structure of biodiesel leading to unique soot layers that were more vulnerable to oxidative reactivity as compared to the soot generated by diesel or alkanes. The addition of biodiesel to these more traditional fuels also presented better performance with respect to annihilating the soot shell, particularly for diesel. The burning rate generally follows that of multi-component fuels, by some means in terms of a lever rule, whereas the mixture of biodiesel and dodecane exhibits a somewhat nonlinear relation with the added fraction of dodecane. This might be related to the formation of a soot shell.     

Morphology of soot collected in microgravity droplet flames

Measurements of the primary particle size, radius of gyration, fractal dimension, and the mass fractal prefactor were measured for soot sampled in n-heptane droplet flames under microgravity conditions. These represent the first such measurements obtained for spherically symmetric droplet flames. Experiments were performed in the NASA-Glenn Research Center in Cleveland, OH. Soot was sampled at 0.2 and 0.5 s after ignition for a 2.1 mm droplet burning in atmospheric pressure air. The measured primary particle sizes were significantly larger under microgravity conditions compared to normal gravity and were found to increase with residence time. The average radius of gyrations were also larger for soot collected in microgravity conditions. Differences in the particle and agglomerate dimensions are believed to be caused by the longer residence times. The fractal dimensions were found to be nearly constant for all experiments while the prefactor term was found to increase slightly with residence time.     

Methanol droplet extinction in carbon-dioxide-enriched environments in microgravity

Diffusive extinction of methanol droplets with initial diameters between 1.25 mm and 1.72 mm, burning in a quiescent microgravity environment at one atmosphere pressure, was obtained experimentally for varying levels of ambient carbon-dioxide concentrations with a fixed oxygen concentration of 21% and a balance of nitrogen. These experiments serve as precursors to those which are beginning to be performed on the International Space Station and are motivated by the need to understand the effectiveness of carbon-dioxide as a fire suppressant in low-gravity environments. In these experiments, the flame standoff distance, droplet diameter, and flame radiation are measured as functions of time. The results show that the droplet extinction diameter depends on both the initial droplet diameter and the ambient concentration of carbon dioxide. Increasing the initial droplet diameter leads to an increased extinction diameter, while increasing the carbon-dioxide concentration leads to a slight decrease in the extinction diameter. These results are interpreted using a critical Damköhler number for extinction as predicted by an earlier theory, which is extended here to be applicable in the presence of effects of heat conduction along the droplet support fibers and of the volume occupied by the support beads.     

On the validity of quasi-steady assumption in transient droplet combustion

A study on droplet combustion in unsteady force convection at high pressure under microgravity conditions was performed. The hysteresis loop of the instantaneous burning rate of a single suspended 1-butanol droplet was observed for the first time. Results showed that the classical quasi-steady film model cannot describe droplet combustion in an unsteady flow. Based on precise experimental observation and by utilizing dimensional analysis of the energy conservation equation, a new criterion is herein proposed for the condition in which the quasi-steady assumption is valid and for that in which it is not. The dimensional analysis led to formulation of a new time scale. Based on the time scale which we call the response-time-scale, a new Damköhler number, termed the response-Damköhler-number was formulated. Using the definition of the new time scale and that of the Damköhler number, unsteady behaviors of droplet combustion under conditions of various pressures and varying force convection were examined. Finally, using the response-Damköhler-number and the deviation factor between the actual instantaneous burning rate and the burning rate predicted by the quasi-steady theory, droplet combustion was categorized into four specific regimes. This study is also of fundamental interest in terms of the effects of turbulence on droplet evaporation and combustion in spray flames.     

Detailed numerical simulation of syngas combustion under partially premixed combustion engine conditions

Two-dimensional detailed numerical simulation is performed to study syngas/air combustion under partially premixed combustion (PPC) engine conditions. Detailed chemical kinetics and transport properties are employed in the study. The fuel, a mixture of CO and H₂ with a 1:1 molar ratio, is introduced to the domain at two different instances of time, corresponding to the multiple injection strategy of fuel used in PPC engines. It is found that the ratio of the fuel mass between the second injection and the first injection affects the combustion and emission process greatly; there is a tradeoff between NO emission and CO emission when varying the fuel mass ratio. The ignition zone structures under various fuel mass ratios are examined. A premixed burn region and a diffusion burn region are identified. The premixed burn region ignites first, followed by the ignition of mixtures at the diffusion burn region, and finally a thin diffusion flame is formed to burn out the remaining fuel. NO is produced mainly in the premixed burn region, and later from the diffusion burn region in mixtures close to stoichiometry, whereas unburned CO emission is mainly from the diffusion burn region. An optimization of the fuel mass in the two regions can offer a better tradeoff between NO emission and CO emission. The effects of initial temperature and turbulence on the premixed burn and diffusion burn regions are investigated.     

Activation energy asymptotics for methanol droplet extinction in microgravity

An activation energy asymptotic theory for methanol droplet combustion in microgravity is presented by extending earlier models to account for time-dependent water dissolution or evaporation from the liquid droplet. The model predictions for droplet extinction diameter as a function of its initial diameter are shown to compare favorably with experimental results for methanol burning in air.     

Tethered methanol droplet combustion in carbon-dioxide enriched environment under microgravity conditions

Tethered methanol droplet combustion in carbon dioxide enriched environment is simulated using a transient one-dimensional spherosymmetric droplet combustion model that includes the effects of tethering. A priori numerical predictions are compared against recent experimental data. The numerical predictions compare favorably with the experimental results and show significant effects of tethering on the experimental observations. The presence of a relatively large quartz fiber tether increases the burning rate significantly and hence decreases the extinction diameter. The simulations further show that the extinction diameter depends on both the initial droplet diameter and the ambient concentration of carbon dioxide. Increasing the droplet diameter and ambient carbon dioxide concentration both of them lead to a decrease in the burning rate and increase in the extinction diameter. The influence of ambient carbon dioxide concentration on extinction shows a sharp transition in extinction for larger size droplets (d_o > 1.5 mm) due to a change in the mode of extinction from diffusive to radiative control. In addition predictions from the numerical model is compared against a recently developed simplified theoretical model for predicting extinction diameter for methanol droplets, where the presence and heat transfer contribution of the tether is not taken into account implicitly. The numerical results suggest some limitation in the theoretical modeling assumptions for favorable comparisons with the experimental data.     

Advances in droplet array combustion theory and modeling

A review of research on the subject of the vaporization and burning of fuel droplets configured in a prescribed array is presented, including both classical works and research over the past decade or two. Droplet arrays and groups and the relation to sprays are discussed. The classical works are reviewed. Recent research on transient burning and vaporization of finite arrays with Stefan convection but without forced convection is presented, including extensions to non-unitary Lewis number and multi-component, liquid fuels. Recent results on transient, convective burning of droplets in arrays are also examined. In particular, transient convective burning of infinite (single-layer periodic and double-layer periodic) and finite droplet arrays are discussed; attention is given to the effects of droplet deceleration due to aerodynamic drag, diameter decrease due to vaporization, internal liquid circulation, and arrays with moving droplets in tandem and staggered configurations. Flame structure is examined as a function of spacing between neighboring droplets and Damköhler number: individual droplet flames versus group flames and wake flames versus envelope flames. Based on existing knowledge of laminar droplet array and spray combustion theory, experimental evidence, and turbulent studies for non-vaporizing and non-reacting two-phase flows, comments are made on the needs and implications for the study of turbulent spray and array combustion.     

Simplified model for droplet combustion in a slow convective flow

An idealized model for droplet vaporization or combustion in the Burke-Schumann reaction-sheet approximation is analyzed in terms of a Peclet number based on the Stefan velocity, taken to be of order unity, for Lewis numbers of unity and for small values of a parameter ?, defined as the ratio of the convective velocity far from the droplet to the Stefan velocity at its surface. Asymptotic solutions for the velocity, pressure, and mixture-fraction fields are obtained through second order in ?. The results are employed to calculate the effects of convection on the burning rate and on the flame shape. The prediction that the burning-rate constant increases linearly with ? for small values of ? is shown to be consistent with available experimental data. It is demonstrated that reasonable values of diffusivities provide approximate agreement of predicted burning rates and flame shapes with results of measurements.     

Effects of ambient pressure,gas temperature and combustion reaction on droplet evaporation

The effects of ambient pressure, initial gas temperature and combustion reaction on the evaporation of a single fuel droplet and multiple fuel droplets are investigated by means of three-dimensional numerical simulation. The ambient pressure, initial gas temperature and droplets’ mass loading ratio, ML, are varied in the ranges of 0.1–2.0 MPa, 1000–2000 K and 0.027–0.36, respectively, under the condition with or without combustion reaction. The results show that both for the conditions with and without combustion reaction, droplet lifetime increases with increasing the ambient pressure at low initial gas temperature of 1000 K, but decreases at high initial gas temperatures of 1500 K and 2000 K, although the droplet lifetime becomes shorter due to combustion reaction. The increase of ML and the inhomogeneity of droplet distribution due to turbulence generally make the droplet lifetime longer, since the high droplets’ mass loading ratio at local locations causes the decrease of gas temperature and the increase of the evaporated fuel mass fraction towards the vapor surface mass fraction.     

Surface tension effects during low-Reynolds-number methanol droplet combustion

A numerical investigation of methanol droplet combustion in a zero-gravity, low-pressure, and low-temperature environment is presented. Simulations have been carried out using a predictive, transient, and axisymmetric model, which includes droplet heating, liquid-phase circulation, and water absorption. A low initial Reynolds number (Re₀=0.01) is used to impose a weak gas-phase convective flow, introducing a deviation from spherical symmetry. The resulting weak liquid-phase circulation is greatly enhanced due to surface tension effects, which create a complex, time-varying, multicellular flow pattern within the liquid droplet. The complex flow pattern, which results in nearly perfect mixing, causes increased water absorption within the droplet, leading to larger extinction diameters. It is shown that, for combustion of a 0.43-mm droplet in a nearly quiescent environment (Re₀=0.01) composed of dry air, the extinction diameter is 0.11 mm when surface tension effects are included, and 0.054 mm when surface tension effects are neglected. Experimental work available in the literature for a 0.43-mm droplet reported extinction diameters in the range of 0.16 to 0.19 mm. Results for combustion in a nearly quiescent environment (Re₀=0.01) with varying initial droplet diameters (0.16 to 1.72 mm) show that including the effect of surface tension results in approximately linear variation of the extinction diameter with the initial droplet diameter, which is in agreement with theoretical predictions and experimental measurements. In addition, surface tension effects are shown to be important even at initial Reynolds numbers as high as 5.     

Experimental results of hydrogen enrichment of ethanol in an ultra-lean internal combustion engine

An investigation was made to determine the effects of hydrogen enrichment of ethanol at ultra-lean operating regimes utilizing an experimental method. A 0.745 L 2-cylinder SI engine was modified to operate on both hydrogen and ethanol fuels. The study looked at part throttle, fixed RPM operation of 0%, 15%, and 30% hydrogen fuel mixtures operating in ultra-lean operating regimes. Data was collected to calculate NO and HC emissions, power, exhaust gas temperature, thermal efficiency, volumetric efficiency, brake-specific fuel consumption, and Wiebe burn fraction curves.     

Inverse influence of initial diameter on droplet burning rate in cold and hot ambiences: a thermal action of flame in balance with heat loss

Isolated droplet burning were conducted in microgravity ambiences of different temperatures to test the initial diameter influence on droplet burning rate that shows a flame scale effect and represents an overall thermal action of flame in balance with heat loss. The coldest ambience examined was room air, which utilized a heater wire to ignite the droplet. All other ambiences hotter than 633 K were acquired through an electrically heated air chamber in a stainless steel can. An inverse influence of initial droplet diameter on burning rate was demonstrated for the cold and hot ambiences. That is, the burning rate respectively decreased and increased in the former and latter cases with raising the initial droplet diameter. The reversion between the two influences appeared gradual. In the hot ambiences the burning rate increase with increasing the initial droplet diameter was larger at higher temperatures. A “net heat” of flame that denotes the difference between “heat gain” by the droplet and “heat loss” to the flame surrounding was suggested responsible for the results. In low-temperature ambiences there is a negative net heat, and it turns gradually positive as the ambience temperature gets higher and the heat loss becomes less. Relating to luminous flame sizes and soot generation of differently sized droplets clarified that the flame radiation, both non-luminous and luminous, is determinative to the net heat in microgravity conditions. In addition, the work identified two peak values of soot generation during burning, which appeared respectively at the room temperature and at about 1000 K. The increase in ambience temperature made also bigger soot shells. The heat contribution of flame by both radiation and conduction was demonstrated hardly over 40% in the total heat required for droplet vaporization during burning in a hot ambience of 773 K.     

High-pressure combustion of submillimeter-sized nonane droplets in a low convection environment

An experimental study of droplet combustion of nonane (C₉H₂₀) at elevated pressures burning in air is reported using low gravity and small droplets to promote spherical gas-phase symmetry at pressures up to 30 atm (absolute). The initial droplet diameters range from 0.57 to 0.63 mm and they were ignited by two electrically heated hot wires positioned horizontally on opposite sides of the droplet. The droplet and flame characteristics were recorded by a 16-mm high-speed movie and a high-resolution video camera, respectively. A photodiode is used to measure broadband gray-body emission from the droplet flames and to track its dependence on pressure. Increasing the pressure significantly influences the ability to make quantitative measurements of droplet, soot cloud, and luminous zone diameters. At pressures as low as 2 atm, soot aggregates surrounding the droplet show significant coagulation and agglomeration and at higher pressures the soot cloud completely obscures the droplet, with the result being that the droplet could not be measured. Above 10 atm radiant emissions from hot soot particles are extensive and the resulting flame luminosity further obscures the droplet. Photographs of the luminous zone in subcritical pressures show qualitatively that increasing pressure produces more soot, and the mean photodiode voltage output increases monotonically with pressure. The maximum flame and soot shell diameters shift to later times as pressure increases and the soot shell is located closer to the flame at higher pressure. The soot shell and flame diameter data are correlated by a functional relationship of reduced pressure derived from scaling the drag and thermophoretic forces on aggregates that consolidates all of the data onto a single curve.     

Transport processes and associated irreversibilities in droplet combustion in a convective medium

A mathematical model of the combustion of a droplet surrounded by hot gas with a uniform free stream motion is made from the numerical solution of conservation equations of heat, mass and momentum in both the carrier and the droplet phases. The gas-phase chemical reaction between the fuel vapour and the oxidizer is assumed to be single-step and irreversible. The phenomenon of ignition is recognised by the sudden rise of temperature in the temperature/time histories at different locations in the carrier phase. To ascertain the process irreversibilities, the instantaneous rate of entropy production and its variation with time have been determined from the simultaneous numerical solution of the entropy conservation equations for both the gas and liquid phases. The relative influences of pertinent input parameters, namely the initial Reynolds number Re_i, the ratio of the free stream to initial temperature T_∞ and the ambient pressure on (i) the local and average Nusselt numbers, (ii) the life histories of burning fuel drops, and (iii) the entropy generation rate in the process of droplet combustion have been established.     

Droplet combustion in the presence of acoustic excitation

This experimental study focused on droplet combustion characteristics for various liquid fuels during exposure to external acoustical perturbations generated within an acoustic waveguide. The alternative liquid fuels include alcohols, aviation fuel (JP-8), and liquid synthetic fuel derived via the Fischer–Tropsch process. The study examined combustion during excitation conditions in which the droplet was situated in the vicinity of a pressure node (PN). In response to such acoustic excitation, the flame surrounding the droplet was observed to be deflected, on average, with an orientation depending on the droplet’s relative position with respect to the PN. Flame orientation was always found to be consistent with the sign of a theoretical bulk acoustic acceleration, analogous to a gravitational acceleration, acting on the burning system. Yet experimentally measured acoustic accelerations based on mean flame deflection differed quantitatively from that predicted by the theory. Phase-locked OH^∗ chemiluminescence imaging revealed temporal oscillations in flame standoff distance from the droplet as well as chemiluminescent intensity; these oscillations were especially pronounced when the droplets were situated close to the PN. Simultaneous imaging and pressure measurements enabled quantification of combustion-acoustic coupling via the Rayleigh index, and hence a more detailed understanding of dynamical phenomena associated with acoustically coupled condensed phase combustion processes.     

Microexplosion and ignition of biodiesel/ethanol blends droplets in oxygenated hot co-flow

Microexplosion and ignition characteristics of biodiesel/ethanol blends were studied by suspending single droplets on fibers in a tubular oven at different ambient conditions. The range of ethanol content in initial fuel, as an important parameter, was changed from 0% to 100%.Heating temperature and gas flow were also reference variables, which were respectively changed from 300 °C to 500 °C and from 0 L/min to 10 L/min. Behaviors of droplet microexplosion and ignition were recorded by high speed camera. Temporal variation of droplet size, time to microexplosion and ignition delay time were all analyzed for every cases. The results showed that microexplosion could be found in most cases due to high volatility distinction of biodiesel and ethanol and be expedited and intensified at fuel for nearly equivalent volume mixing, or at enough high temperature and in a certain gas flow. However, ignition was achieved only in some situations which biodiesel content in initial fuel and temperature was relatively high and gas flow rate was moderate. Further, under the condition that the temperature and flow rate remain unchanged, when the ethanol content reached 50%, the micro-explosion intensity which was named the normalized diameter reaches the maximum (1.3). At the same time, the delay time of micro-explosion is also reduced to the minimum (0.23 s). Under the condition that the ratio and flow rate remained the same, when the ambient temperature rose to 400 °C, the micro-explosion intensity reached the maximum. At this time, the normalized diameter of the mixed droplets approached 1.5. Under the condition that the temperature and ratio remained the same, when the flow rate was 5 L/min, the ignition delay time was the lowest (about 2.8 s), when the flow rate was 10 L/min, the micro-explosion intensity was the largest, and the normalized diameter reached 1.8. Beyond this, it was found that raising the ratio of ethanol in the blended fuel could increase the burning rate but lower the ignitability, and the ignition delay time could be shortened when the droplet exhibited microexplosion and fuel of near equi-volume blends, experiencing the most violent microexplosion, optimized the improvement, which could be found more obviously at high temperature and high speed airflow.