Droplet Combustion Research Articles

Apparatus to investigate liquid oxygen droplet combustion in hydrogen under microgravity conditions.

To investigate the fundamentals of liquid oxygen droplet combustion in hydrogen under microgravity conditions, a drop tower apparatus has been developed. In a cryogenic combustion chamber cooled with liquid nitrogen, single oxygen droplets with a diameter of 1 mm are suspended at the tip of a thin suspender. Ignition is accomplished after microgravity conditions are reached by a near-infrared laser, which generates a plasma spark positioned in the immediate vicinity of the oxygen droplet. The subsequent combustion is investigated with various optical diagnostics. Shadowgraph imaging is used to determine the initial droplet size and the droplet diameter regression with high temporal resolution. In addition, the position and diameter of the flame are determined by OH chemiluminescence imaging. The oxygen/hydrogen combustion at two different pressure levels is reported. At a pressure of 0.1 MPa, irregular structures appear to form on the droplet surface during the combustion process, which lasts 137 ms before the droplet is completely consumed. The formation of these irregular structures is consistent with the condensation and freezing of water vapor in the cold-temperature region near the droplet surface. At a higher pressure of 4 MPa (reduced pressure pr,O2 =0.79), the water ice formation is significantly reduced due to the shorter burning time of 66 ms and the closer proximity of the flame to the droplet surface. The measured burning constant is k0.1 = 5.5 mm2/s at 0.1 MPa and k4 = 7.1 mm2/s at 4 MPa, respectively. This increase with increasing pressure is consistent with the smaller flame standoff distance.

Flame Extinction of Spherical PMMA in Microgravity: Effect of Fuel Diameter and Conduction

A series of experiments were conducted in the 3.6-s microgravity drop tower and normal gravity to investigate the effect of solid fuel curvature, conduction, and reradiation on the flame extinction of spherical polymethyl methacrylate (PMMA). In the semi-quiescent microgravity environment, flame extinction was observed if the PMMA diameter was larger than 40 mm, because of a smaller flame conductive heating in larger diameter (i.e., the curvature effect). Compared to the droplet combustion with a low evaporation point and fast heat convection in the liquid phase, the solid fuel has a high pyrolysis point and large transient heat conduction. Thus, the large surface reradiation effectively cools down the fuel surface to promote extinction. Also, as the initial burning duration increases, the conductive cooling into the solid fuel decreases, which delays or prevents the flame extinction in microgravity. The extinction criterion for microgravity flame is explained by the critical mass flux and mass-transfer number. This work helps to understand the curvature effect of solid fuel on flame extinction and the material fire safety in the microgravity spacecraft environment.

Ignition, puffing and sooting characteristics of kerosene droplet combustion under sub-atmospheric pressure

The ignition, puffing and sooting characteristics of Chinese RP-3 kerosene droplet burning have been studied using high-speed, OH* chemiluminescence and soot thermal radiation imaging. The experiments were conducted in air at standard temperature and sub-atmospheric pressures ranging from 0.2 bar to 1 bar. The kerosene droplet was supported by a thermocouple tip and ignited by a retractable coiled heating wire. The results showed that the ignition delay time increased with a decrease of the ambient pressure, due to an increased distance between kerosene and oxygen molecules. Steady burning and disruptive burning were identified following the ignition. OH* chemiluminescence images showed a spherical flame and a longer flame standoff distance under a lower pressure. The puffing intensity was observed to be enhanced with a reduction of the ambient pressure, and a decreased pressure was found to lower the sooting emission.

Comparison of the burning of a single diesel droplet with volume and surface contamination of soot particles

Following an examination on the effect of soot contamination on the surface of a diesel droplet to the combustion characteristics, an investigation on a uniform suspension of diesel soot particles within a diesel droplet was conducted experimentally for the first time. The aim is to determine the effect of uniformly suspended soot to the evaporation characteristics of a diesel droplet. Soot formed by the combustion of diesel was collected and suspended uniformly within a neat diesel. The particle loadings were varied from 0.1% to 0.5% by mass and ignited in a single isolated droplet, normal gravity and ambient condition. It is found that the burning rate of contaminated droplets is lower than the neat condition in all particle loading. A critical loading was identified to be at 0.2% which has shown a slight improvement of the burning rate despite having a reduced rate compared to its neat counterpart. It is found that within the critical loading, the particle briefly improves the thermal conductivity towards the core of the droplet which in turn slightly enhances the evaporation rate. Once the shear stress exerted by the external gas flow is subjected to the droplet, the internal circulation is induced and moves the particle towards the surface of the droplet. Agglomeration of particles on the surface of the droplet inhibit the liquid diffusion from the core to the surface of the droplet thus suppressing the evaporation rate. High particle loading would accelerate the agglomeration process whereas a low and critical particle loading briefly improves the evaporation in a reduced condition. These insights are of significance for determining the main cause of the detrimental effect from soot contamination in a liquid fuel; providing a base approach upon identifying the role of nanoparticles within the fuel droplet combustion study.

Sub-millimeter sized multi-component jet fuel surrogate droplet combustion: Physicochemical preferential vaporization effects

Isolated droplet burning behaviors of fuel mixtures that all share the fully pre-vaporized combustion behaviors of the same “global” Jet-A real fuel are investigated numerically to elucidate the effects of preferential vaporization. Predictions are generated using three such multi-component hydrocarbon mixtures (Mixture-1: n-decane/iso-octane/toluene 42.7/33.0/24.3, Mixture-2: n-dodecane/iso-octane/1,3,5 trimethyl benzene 49.0/21.0/30.0 and Mixture-3: n-hexadecane/iso-octane/1,3,5 trimethyl benzene 36.5/31.0/32.5 molar ratios), each having very different distillation properties. Simulations are performed using a transient one-dimensional sphero-symmetric model, involving numerically reduced detailed chemical kinetics, and multi-component gas-phase diffusive transport. The interactions among the different liquid-phase components are modeled using UNIFAC activity coefficient methodology. The predictions using Mixture-1 are found to be in good agreement with previously published drop-tower experiments for 550 µm droplet combustion in air. Stagnant and internally mixed liquid-phase behaviors are both considered. Near fully mixed internal conditions and including sooting effects (observed in the experiments) result in predictions that are in good agreement with experimental drop diameter and flame-standoff ratio histories. By comparing predictions for the three mixtures, preferential vaporization effects exhibit strong non-linear dependences on initial droplet diameter and ambient pressure. At low pressures and for droplet sizes typical of those found in multi-phase gas turbine combustors, comparable diffusion and vaporization characteristic times favor frozen limit vaporization. However, at pressures typical of those found in applications, batch distillation dominates and preferential vaporization chemical property differences are found to become as significant as physical property effects in influencing combustion behaviors.

Flame spread and combustion characteristics of two adjacent jatropha oil droplets

The present study investigated the flame spread and combustion characteristics of two jatropha oil droplets, droplet A and B. Specifically, the pyrolysis characteristics of jatropha oil were studied using thermogravimetric and differential scanning calorimetry (DSC) analysis. Liquid chromatography-mass spectrometry (LC-MS) was used to analyze the components in jatropha oil. Then, the flame spread from droplet A to droplet B and combustion dynamics of two droplets were studied in a constant volume chamber under atmospheric pressure and room temperature (300 K). The effects of normalized droplets spacing S/d0 (2.7–4.9) on flame spread were also studied. The results showed that the combustion of jatropha oil droplet containing three stages: initial expansion stage, quasi-steady combustion stage and micro-explosive combustion stage. Periodic puffing and micro-explosion occurred during micro-explosive combustion stage. This is due to the superheating of low boiling-point components produced by pyrolysis of long-chain fatty acids in jatropha oil. The duration of initial expansion stage increases significantly with the increase of droplet spacing. When the normalized droplet spacing reaches 4.9, the second droplet cannot ignite and reached flame spread limit. Interestingly, the flame spread velocity increase at first and then decrease with increase droplet spacing. This is because the diffusion flame is cooled by unburned droplet when the spacing is relatively small. However, when the spacing is relatively large, the flame spread velocity mainly depends on the duration of second droplet in first stage.

Reference data set for three-dimensional measurements of double droplet combustion of p-xylene

In order to provide fundamental information for droplet interactions in the versatile and promising flame spray pyrolysis (FSP) technique, double droplet combustion experiments of p-xylene have been conducted via an improved piezoelectric droplet-on-demand generator. Using a steel nozzle plate with two laser-drilled nozzles, the generator can simultaneously generate two upwards-flying droplets with initial diameters of around 100 µm. A three-dimensional (3D) measurement technique using two time-synchronized high-speed cameras has been developed to detect double droplet combustion processes. Droplet diameters, flame diameters, droplet 3D trajectories, droplet 3D velocities, and the center distances between the two burning droplets are highly time-resolved measured, and then visualized via 3D videos and images. Flame spacing, as a new definition, is proposed to describe the interactions between the two flame fronts. The influences of the flame spacing and the oxygen concentration in the ambience on double droplet combustion were experimentally investigated. The obtained 3D data can serve as an ideal validation case for modeling and simulating droplet interactions during combustion as well as a highly time-resolved reference data set in the future.

Sharp-interface calculations of the vaporization rate of reacting aluminum droplets in shocked flows

The vaporization rate of reacting aluminum droplets in high-speed flows plays a crucial role in determining the energy released during the combustion of aluminized solid propellants and explosives. This work develops a model for the vaporization rate of burning aluminum droplets in shocked flows using data from high-resolution simulations. A levelset-based sharp-interface method is used to resolve the dynamics at the droplet-air interface. Several reactive calculations of shock-droplet interaction are performed in the parameter space of Mach number (1.1 - 3.5) and Reynolds number (100 – 1000) to understand the influence of local flow parameters on the flame structure around the reacting aluminum droplet. The results show a transition from diffusion limited to kinetically limited combustion of the aluminum droplets when the Mach number is increased and the droplet size or the Reynolds number is decreased. The transition from diffusion-limited combustion to kinetically limited combustion is found to significantly affect the flame dynamics and vaporization rate of the reacting droplets. Therefore, empirical models for the vaporization rate of droplets, developed for diffusion limited combustion regimes, do not represent the kinetically limited combustion in high Mach number conditions. A new model for the Sherwood number spanning diffusion and kinetically limited regimes of the reacting aluminum droplets is developed using an economical multi-fidelity surrogate modeling technique. The current model for the Sherwood number for reacting aluminum droplets will be useful in predicting the energy released from the combustion of aluminized energetic materials in shocked flows. Although the current method is developed in the context of aluminum droplet combustion, it can be extended to develop surrogate models of Sherwood number for general liquid fuel droplets in compressible flows.

Microgravity combustion of polyethylene droplet in drop tower

Microgravity experiments of polyethylene (PE) droplet combustion were conducted by a 3.6-s drop tower with the gravity level of 10−3~10−4 g to investigate the burning behaviors and fire hazards of molten thermoplastics in the spacecraft. Pre-ignited droplets with a diameter of about 3 mm were continually generated and detached from burning PE tubes. Once the drop capsule started free-fall, droplets entered the microgravity environment with an initial velocity of 10–35 cm/s (Stage I). A comet-shape flame with an intense bubbling and ejecting process of the moving droplet was observed, and the burning-rate constant (K) was found around 2.6 ± 0.3 mm2/s. After the droplet landed on the floor, it could rebound with a near-zero velocity, showing as a spherical flame (Stage II). The combustion of PE droplet followed the classical d-square law with K = 1.3 ± 0.1 mm2/s. The measured large burning-rate constant (or the volume shrinkage rate) of the moving droplet was caused by the robust bubbling process, which reduced the bulk density of molten PE and ejected unburnt fuel (about 25% of total mass loss). However, the actual mass burning rate of the PE droplet should be smaller than most hydrocarbon liquids because of a smaller mass-transfer number (B ≈ 2). The flame burning rate of PE droplet is 4 ± 1 g/m2-s per unit flame-sheet area that may be used to estimate the fuel mass-loss rate and fire heat release rate in microgravity. This novel microgravity combustion experiment on the thermoplastic droplet could expand the physical understanding of fire risk and hazard of plastic material in the spacecraft environment.

The role of composition in the combustion of n-heptane/iso-butanol mixtures: experiments and detailed modelling

Experimental data and detailed numerical modelling are presented on the burning characteristics of a model gasoline/biofuel mixture consisting of n-heptane and iso-butanol. A droplet burning in an environment that minimises the influence of buoyant and forced convective flows in the standard atmosphere is used to promote one-dimensional gas transport to facilitate numerical modelling of the droplet burning process. The numerical model includes a detailed combustion kinetic mechanism, unsteady gas and liquid transport, multicomponent diffusion inside the droplet, variable properties, and non-luminous radiative heat transfer from the flame. The numerical simulation was validated by experimental measurements in the standard atmosphere which showed good agreement with the evolutions of droplet and flame diameters. The iso-butanol concentration had a strong effect on formation of particulates. Above ~20% (volume) iso-butanol, flame luminosity was significantly diminished anddecreased with increasing iso-butanol concentration, while CO2 emissions as a representative greenhouse gas were not strongly influenced by the iso-butanol loading. The soot shell was located near a 1350 K isotherm for concentrations up to 20% (volume) iso-butanol, suggesting this value as a possible soot inception temperature for the mixture droplet. The combustion rate decreased with increasing iso-butanol concentration which was attributed to iso-butanol's higher liquid density. No evidence of a low temperature burning regime, or of extinction, was found (in experiments and simulations) for the small droplet sizes investigated.

Numerical study of a polydisperse spray counterflow diffusion flame

A counterflow two-phase diffusion flame with polydisperse spray is numerically studied in a 2D configuration, using a Lagrangian formalism for the liquid phase and accurate combustion chemistry. Results exhibit a very complex double flame structure with diffusion and premixed flames, as well as group and individual droplet burning. Monodisperse two-phase counterflow flames are also computed to help analyse the polydisperse flame, and confirm the strong link between droplet diameter and flame regime. For small droplet diameters, the flame has the same structure than a gaseous flame at a different equivalence ratio, and the total heat release of the flame increases with the droplet size. For larger droplets, the premixed mode becomes dominant and the total heat release of the flame exceeds the maximum gaseous diffusion one.

Droplet combustion studies on novel cage hydrocarbons using color-ratio pyrometry

The temperature, luminosity, and relative soot volume fraction profiles of three novel energetic cage hydrocarbons–bis(nitratomethyl)-1–3-bishomocubane (DNMBHC), diazido-dimethyl-bishomocubane (DADMBHC), 1,3 bishomocubane dimer (BHCD), an RP-1 surrogate fuel (RP-1-s), and its constituents were estimated using color-ratio pyrometry (CRP) with a consumer-grade DSLR camera. For temperature measurements, data acquisition rates of 50 fps were obtained by enabling the RAW video recording feature in the camera. In combination with the temperature profiles of the combusting droplets, luminosity measurements allowed the estimation of the relative soot volume fractions, and thus sooting propensities of various fuels were established. Additional temperature measurements were also made using calibrated bare wire B-Type thermocouples, which were found to corroborate well with CRP results for RP-1-s and its constituent fuels. However, in scenarios where the sooting tendencies of the fuel were high, as was the case for the novel energetic fuels, soot deposition over the thermocouple bead was a problem, and thermocouples were found to be ineffective in predicting the maximum flame temperatures. Though the novel energetic fuels generated more soot than RP-1-s, CRP results revealed that their maximum flame temperatures were also higher than RP-1-s, which is advantageous from a combustion standpoint. The secondary atomization as a result of the microexplosions in the novel fuels was also found to be beneficial in increasing the combustion temperatures. Overall, the BHC compounds were assessed to hold considerable promise as liquid propellants, with DNMBHC emerging as the most promising.

Energy analysis of secondary droplet atomization schemes

This paper presents the energy analysis of the secondary atomization of droplets with various component compositions. We use the following four schemes together or separately: initial droplets colliding with each other, interaction with a solid wall, droplet transformation due to the oncoming air flow, and micro-explosive breakup of a droplet being heated. We consider the kinetic energies of primary (pre-atomization) and secondary (post-atomization) droplets and determine the heat spent on atomization. To show the need to atomize secondary droplets, we establish the differences between the heat released per unit time during the combustion of primary and secondary droplets. We also show that the least energy-consuming scheme is the one involving droplets colliding with each other and the most effective one in terms of fine aerosol production is the scheme with the micro-explosive droplet breakup. The latter may increase the liquid surface area more than tenfold. The research findings show that the energy spent on the atomization of fuel droplets is less than 1% of the energy released from their combustion. A combination of several atomization schemes becomes more effective with larger initial liquid droplets and their complex component composition.

Aluminum combustion in CO2-CO-N2 mixtures

This paper considers a new experimental set-up to image and study the combustion of a single burning aluminum droplet, which is levitated electrostatically. This work focuses on CO2, CO and mixtures thereof, that have not or scarcely been studied. Aluminum is found to burn in pure CO2 in a diffusion mode (with a D01.9 scaling, where D0 is the initial particle diameter) whereas combustion in pure CO is kinetically limited and halts swiftly, irrespective of pressure in the range 1–15 atm. Mixtures of CO2 and CO suggest a major driving effect from CO2 compared to CO. Overall, CO and N2 behave as inert species. All the burning time data are processed to propose a new empirical correlation for CO2-CO-N2 mixtures that significantly improves the widely used Beckstead’s correlation while including the role of CO and N2.

Transient combustion of a multi-component fuel droplet with gas radiation

The transient modeling of radiation effects on combustion of an isolated nonane and hexanol droplet in standard atmospheric conditions with finite rate chemistry has been performed. Transient effects have been included in a comprehensive manner by including initial droplet heating, variable liquid and gas phase properties and non-luminous gas radiations. Results show that gas radiations are important for determining various parameters in droplet combustion. Estimation of Planck's mean absorption coefficient is critical for non-luminous gas radiation modeling. Gas radiation suppresses gas, liquid and flame temperatures. The burning rate decreases in the presence of radiative losses with increasing initial droplet diameter.

Data driven forecast of droplet combustion

The characteristics of a diffusion flame resulting from the gasification of a condensed fuel are predicted from the synthesis of simple models and data. Combustion of a droplet in microgravity is used as a canonical configuration to illustrate the methodology. The simplicity of the spherical configuration and the detail of the measurements make the available experimental data ideal for this study. The approach followed combines the classical analytical solution first proposed by Spalding to describe the condensed phase gasification with a numerical method that describes the gas phase. Available data on flame geometry and regression rates are used to initialize the model and produce adequate predictions of the time evolution of all relevant variables. The method was shown to make proper predictions under numerous configurations and with very small computational cost.

Self-tuning and topological transitions in a free-falling nanofuel droplet flame

In the present study, flame dynamics of free-falling contactless burning fuel droplets (pure and nanofuel) is investigated. A droplet under free fall undergoes acceleration resulting in progressive increase of its velocity (increase in Reynolds Number, Re) along the path. The consequent dynamic external relative flow induced along the drop trajectory allows self-tuning of the flame through a series of transitions. At low Re, the droplet flame transitions from an initial fully enveloped buoyant diffusion flame to a premixed wake flame structure. It is shown that this transition occurs due to flame extinction at the droplet forward stagnation point when critical strain rate is exceeded due to the external flow. Subsequently the wake flame topology undergoes significant variations with further increase in Reynolds number. The flow conditions necessary for wake-flame stabilization are characterized. Using a round-jet analogy, a linear relationship between the flame height and Reynolds number is established which shows that topological transitions are predominantly hydrodynamic in nature. Droplet burning rate for all functional droplets exhibits minimal change due to reduced energy input from the wake flame. However, for nanofuel droplets, the global heat release is lowered due to the reduction in gasification rate brought about by the in-situ formation of porous structure by the agglomeration of nanoparticles. Furthermore, for particle laden droplets, absence of sufficient energy input from the flame suppresses internal heterogeneous boiling as found in most pendant droplet experiments.

Initial diameter effects on combustion of unsupported equi-volume n-heptane/iso-octane mixture droplets and the transition to cool flame behavior: Experimental observations and detailed numerical modeling

This study reports an experimental and numerical investigation of droplet combustion of a miscible n-heptane/iso-octane mixture at a fixed mixture fraction (equi-volume) for initial diameters (Do) in the range of 0.8 mm ≤ Do < 5 mm. This range encompasses burning transitions from hot flame (HF) combustion to the cool flame (CF) regime where radiative extinction can occur. The simulations assume spherically symmetric gas transport which was promoted in the experiments by a low gravity environment and relatively stationary droplets. Unsupported or free-floating droplets were deployed and ignited in a sealed chamber on the International Space Station to provide a low gravity condition and to accommodate the anticipated long droplet burning times (tens of seconds) for the droplet sizes investigated. The simulations incorporated multistep combustion kinetics with an embedded low temperature kinetic mechanism, non-luminous flame radiation, a model for phase equilibrium of the mixture, variable properties, unsteady gas and liquid transport, and species diffusion in the liquid. The results showed no evidence of preferential vaporization because of the close boiling points of n-heptane and iso-octane. For Do < 3 mm, the mixture droplets remained in the initial HF burning regime. For larger Do, a transition to extinction-like behavior occurred. Measured flame radiances confirmed the importance of radiation as a controlling mechanism for driving radiative extinction and transitioning to CF burning. Radiative extinction diameters exhibited a linear relationship with Do which agreed very well with simulations. Mixture radiative extinction diameters were also consistent with literature values for n-decane, n-heptane, and iso-octane. Simulated droplet and flame diameters, burning rates, and flame radiances were also in good agreement with experiments.

A scalable approach of using biomass derived glycerol to synthesize cathode materials for lithium-ion batteries

Glycerol, a main byproduct of biodiesel, was used as the fuel agent in a scalable flame-assisted spray pyrolysis approach to synthesize micron-size cathode materials. Single droplet combustion, particle morphology, and electrochemical performance were examined considering the effects of salt concentration, water addition, and post-annealing time on synthesizing Li(Ni1/3Co1/3Mn1/3)O2 (NCM111) cathode materials. Single droplet tests showed that a sufficient amount of water in the glycerol-based precursor could delay droplet inflation and ignition. Moreover, using a two-fluid atomization nozzle, spray tests demonstrated that adding water would result in less wrinkled and more uniform particles. Nevertheless, increasing water proportion in the precursor could reduce the reaction heat, resulting in that samples prepared with water addition needed longer post-annealing time to improve their electrochemical performance. Using the precursor without water, the best performed NCM111 cathode material was synthesized with only 2 h of post-annealing. It has a discharge capacity of 163.3 mAh/g at 0.1C and 85.5% capacity retention at 1C after 100 cycles. The current work confirmed the capability of using glycerol as a sustainable fuel agent to synthesize NCM111 cathode materials, and also provided basic conditions for the development of large-scale production in the future.

The role of rhodium sulfate on the bond angles of triglyceride molecules and their effect on the combustion characteristics of crude jatropha oil droplets

The alternative fuels were developed from a blended fuel with crude jatropha oil (CJO) and rhodium sulfate as a homogeneous combustion catalyst. In this study, the performances of blended fuel droplets were investigated experimentally under normal gravity conditions. Combustion characteristics were observed by igniting droplets at the thermocouple junction, and the droplet combustion was observed using a high-speed camera. To this end, understanding how the molecular structure (bonding and angular properties of triglyceride carbon chains) of the fuel affects the combustion characteristics is crucial. The results showed that CJO had 53 rotatable and 111 nonrotatable bonds. This makes CJO more rigid and nonpolar. The interaction of the catalyst with triglycerides changes the molecular chain bond angle so that the atoms get extra space to move, thereby increasing the electron mobility. This makes the fuel molecule more reactive so that the viscosity and flash point are reduced and the fuel is easily ignited. Moreover, the results also showed that the catalyst improved combustion performance, where droplet ignition times were shorter at higher temperatures, and fuel droplets had higher burning rates and combustion temperatures.