Buoyant Diffusion Flames Research Articles

Experimental research on the flow field and flame geometry of free buoyant diffusion flames with low dimensionless heat release rates

The flame geometry of turbulent diffusion flames is dominated by the turbulent mixing between the gaseous fuel and the entrained air. The significant variations of the power law for the mean flame height regarding the dimensionless heat release rate (Q˙*) in different Q˙* ranges implied that the turbulent mixing characteristics would change greatly among these Q˙* regimes. This paper presents a combined experimental and analytical study to reveal the turbulent transport properties and flame shape of free turbulent buoyant diffusion flames of relatively low Q˙* prevalent in large-scale fires. The instantaneous velocity fields of free buoyant methane diffusion flames (burner diameter: d = 0.30 m, Q˙*= 0.22–0.40) were measured by the stereo particle image velocimetry (SPIV) technique. The results indicate that the radial profiles of the time-averaged axial velocity, axial normal stress, radial normal stress, shear stress, and turbulent kinetic energy exhibited self-similarity at different heights under all the heat release rates. The turbulent flow in the buoyant flame was found to be anisotropic. Based on the gradient transport approximation, the mean turbulent viscosity within the mean flame height (νt=) was scaled by g1/2d3/2, where g is the gravitational acceleration. The semi-physical correlations for the mean flame height and width were derived based on the concepts of turbulent mixing and equal axial convection and radial diffusion times, respectively. The two correlations agreed reasonably with the experimental data of this work.

Synchronization modes of triple flickering buoyant diffusion flames: Experimental identification and model interpretation.

The synchronization modes of a nonlinear oscillator system consisting of three identical flickering buoyant diffusion flames in isosceles triangles were studied experimentally and theoretically. Five synchronization modes, such as the in-phase, flickering death, partially flickering death, partially in-phase, and rotation modes, were experimentally observed and identified by systematically adjusting the flame distance and fuel flow rates. Two toy models were adopted to interpret the experimentally identified dynamical modes: one is the classical Kuramoto model, and the other is a complexified Stuart-Landau model, which was proposed through the introduction of the complex coupling term. The theoretical results show that the Kuramoto model successfully interpreted the dynamical modes except for those associated with amplitude death, and the complexified Stuart-Landau model well interpreted all the dynamical modes identified in our experiment. Remarkably, the proposed complexified Stuart-Landau model breaks a new path in the investigation of globally coupled nonlinear dynamical systems with identical oscillators, especially for the study of amplitude death mode.

Joint OH-PLIF and Mie scattering imaging of enhanced water mist suppression of buoyant fires

This study presents the first near-field measurements of interactions between reaction layers and water mist droplets suppressing a buoyant turbulent diffusion flame that is representative of a fire. Joint Mie scattering and OH-PLIF are employed at a high repetition rate of 400 Hz to image the droplet field and OH structure, which are deemed here to approximate the high-temperature reaction zone. Compressed natural gas (CNG) buoyant diffusion flames are stabilised on the Sydney Turbulent Buoyant Fire Burner, which is surrounded by a low-velocity air co-flow that is seeded with fine water mist droplets (d32 ∼ 50 µm). The water mist is doped with varying concentrations of metal alkali salts (NaHCO3, KHCO3) that release radical scavenging molecules, providing enhanced suppression capabilities through chemical inhibition in addition to the quenching effect from the water droplets. The spray boundary condition across the wind tunnel exit plane is quantified using Phase Doppler Interferometry. Flame extinction limits are measured by defining the critical condition at which the flame destabilises and lifts off the burner exit plane. A stability map is developed to quantify suppression efficacy in terms of the water mist flow rate and inhibitor concentration. Joint Mie-OH images of selected cases at varying extinction levels show that the addition of inhibitor introduces frequent breakages in the OH layers, marking regions of local extinction and reignition. Large mass fractions of inhibitor in the water, which, once evaporated, produces excess salt crystals in the local extinction zones that causes streaks of salt clouds that line both the lean and rich side of the flame. The chemical scavenging effects of the OH radical by NaHCO3 and KHCO3 is quantified by evaluating the reduction of the mean OH fluorescence signal in the flame at varying locations. Probability density functions for the OH area characterise local extinction in the flame due to the radical scavengers. The order of OH radical depletion and localised extinction in the flame due to the water mist follows: KHCO3 > NaHCO3 > H2O.

Two-zone subgrid combustion model for large eddy simulations of buoyant diffusion flames

This work presents the development of a novel two-zone subgrid combustion model (SCM) recently designed to incorporate the finite-rate kinetics in the large eddy simulations of buoyant turbulent diffusion flames in which turbulence might not be fully developed. The reaction rate is evaluated as the mass-weighted average of contributions by the higher-temperature flamelet zone and the lower-temperature surrounding. The volume fraction of the flamelet zone and the time scale responsible for the inter-zone transfer are determined by the expected thickness of the laminar diffusion flamelet existing in the grid cell, and by wrinkling its surface by the unresolved turbulent eddies. This approach is applicable to an arbitrary level of turbulence (quantified by the subgrid Reynolds number) thereby overcoming the limitation of the conventional eddy dissipation concept formulated for the high-Re flames. Advancing the previous study [1], the flamelet status at every time step is evaluated by solving transient equations of heat and mass transfer between the subgrid zones. Effective kinetic parameters for the simplified global reaction of fuel oxidation are derived from experimental data for autoignition delay in methane-air and n-heptane-air mixtures and are shown to capture transition from the low- to high-temperature measurement datasets including the NTC (negative temperature coefficient) interval. With these kinetic parameters, the SCM is validated against the measurements in the unconfined methane-air flame produced by the UMD line burner and then applied to predict the development and extinction of the heptane-air flame in an under-ventilated enclosure. In the first case, flame extinction is predicted at the oxygen concentrations consistent with the experimental values. In the second scenario, global low-frequency oscillations of the heat release rate are predicted prior to flame extinction.

Mini but Mighty: Oscillations in diffusion flames at sub-atmospheric conditions

The oscillatory behaviour of buoyant diffusion flames plays a vital role in determining how flames evolve. Although previous studies have established an inverse square root relationship between oscillatory frequency and pool diameter, f ∼ D-1/2, few have explored the synergistic effect of diameter and reduced pressure on the pulsation phenomenon. This study mainly investigates the characteristics of convection-controlled pool fires (D < 20 cm) at varying sub-atmospheric pressures, with a particular focus on the influence of eddy effects. Four burner sizes (7 cm, 10 cm, 13 cm, and 16 cm) are selected, and pressure levels are set at 95 kPa, 80 kPa, 60 kPa, and 40 kPa, respectively. A pressure chamber, maintaining a nearly constant pressure during combustion, is employed. The morphology of diffusion flames is recorded and subsequently analysed at different pressures. The study finds that flame shapes change from turbulent to laminar as pressure decreases, which is attributed to the smaller vortex size and corresponds to the varicose and sinuous models. Flame height and oscillatory frequency show a slight increase with decreasing pressure. A new correlation based on the dimensionless Richardson number and Strouhal number is proposed to correlate the oscillation and diameter at different pressures, St=0.58(1/Ri)0.54. This study provides interesting insights into the behaviour of buoyant diffusion flames under sub-atmospheric conditions, highlighting the transition from turbulent to laminar flame shapes, changes in flame height, and pulsation frequency trends. This research could potentially contribute to the comprehension of buoyant diffusion flames at sub-atmospheric conditions in future.

A simpler tractable contour technique to model thermal radiation from buoyant diffusion flames

This work presents an original technique based on parametric interpolation to estimate the view factor between unsteady flames and a small target. The technique was tested on a buoyant diffusion flame generated in an ethylene-fueled porous burner that emulates a pool fire. Flame images were processed to determine the flame contours in terms of parametric polynomials. A mathematical formulation based on the Stokes’ theorem was developed to estimate the view factor between the flame and a radiometer, whose contour integrals were solved numerically. These results were compared to ones obtained with the point-source method and the solid flame model, assuming the flame as a cylinder and a cone. The proposed technique captures the influence of geometry on the radiant flux field more robustly than these methods, providing estimations of thermal radiation between the point and solid flame models.

Faster flicker of buoyant diffusion flames by weakly rotatory flows

Faster flicker of buoyant diffusion flames by weakly rotatory flows

Turbulent cylindrical fire whirl

The cylindrical fire whirl, characterized by nearly constant flame width in the axial direction and open flame tip, is a unique flame pattern of the buoyant diffusion flame under strong imposed rotation. This paper presents a combined experimental and analytical study to investigate the formation and flame geometry of turbulent cylindrical fire whirls. A medium-scale rotating screen facility (diameter: 2.0 m, height: 10.0 m) was utilized for experiments with independently controlled and finely regulated imposed circulation (Γ) and heat release rate (Q˙=15–250 kW), by using propane as the fuel. It was found that the formation of cylindrical fire whirl is accompanied by the downward movement of the unstable flame bulge, and the propagation velocity agreed well with the predictions of the classical vortex breakdown theory. The physical analysis showed that the onsets of cylindrical fire whirls would occur at specific critical values of Re/Bm, where Re and B are the dimensionless imposed circulation and the dimensionless fire buoyancy flux, respectively. The analytical results agree well with data from small to medium-scale experiments and field observation, with m = 1/4 and 1/3 for laminar and turbulent fire whirls, respectively. Contrary to the general fire whirl, the mean flame height decreases, and the mean flame width increases steadily with Γ in the cylindrical fire whirl. A flame height correlation is derived from the radially-integrated momentum and mixture fraction equations, and a flame width correlation is developed based on the concept of equal axial convection and radial diffusion times for the turbulent cylindrical fire whirl. The two models both couple the heat release rate and the imposed circulation, and agree well with the experimental data of this work. The enhanced turbulence level induced by the flow instability of vortex breakdown is suggested to be the dominant mechanism for flame height reduction and flame width increase in the turbulent cylindrical fire whirl.

Dynamical mode recognition of triple flickering buoyant diffusion flames in Wasserstein space

Triple flickering buoyant diffusion flames in an isosceles triangle arrangement, as a nonlinear dynamical system of coupled oscillators, were experimentally studied. The focus of the study is two-fold: we established a well-controlled gas-fuel diffusion flame experiment, which well remedies the deficiencies of prevalent candle-flame experiments, and we developed a Wasserstein-space-based methodology for dynamical mode recognition, which is validated in the present triple-flame systems but can be readily generalized to the dynamical systems consisting of an arbitrary finite number of flames. By use of the present experiment and methodology, seven distinct stable dynamical modes were recognized, such as the in-phase mode, the flickering death mode, the partially flickering death mode, the partially in-phase mode, the rotation mode, the partially decoupled mode, and the decoupled mode. These modes unify the literature results for the triple flickering flame system in the straight-line and equal-lateral triangle arrangements. Compared with the mode recognitions in physical space and phase space, the Wasserstein-space-based methodology avoids personal subjectivity and is more applicable in high-dimensional systems, as it is based on the concept of distance between distribution functions of phase points. Consequently, the identification or discrimination of two dynamical modes can be quantified as the small or large Wasserstein distance, respectively.

Experimental investigation of soot and radiation characteristics in ethylene/propane buoyant diffusion flames

Experimental study was carried out on the soot and radiation characteristics of ethylene/propane buoyant diffusion flames. Characteristics reported include flame natural luminosity, average soot distribution, maximum soot volume fraction, probability density function, soot yield, radiation flux and flame radiation fraction. Results showed that for a constant burning rate of 1.0 kW, different soot and radiation levels of flames produced can be achieved by adjusting gas composition of fuel mixtures. Propane exhibited a synergistic effect on the soot formation, resulting in nonlinear relationship between the maximum soot volume fractions and blending ratios. Based on the analysis of characteristic lengths and characteristic mixture fractions, a correlation of characteristic soot volume fraction with blending ratio was proposed to predict the maximum soot volume fraction of buoyant diffusion flames. The skewed distribution of probability density function elucidates differences in the effect of blending ratio on soot intermittence in the flames. Flame radiation fraction was proportional related to maximum soot volume fraction and linear correlated with soot yield. The analysis provided new comprehensive understanding of relationship between soot formation and fuel mixture, and could further apply to establish flame radiation equivalence principles of real fire emulation in fire research, fire safety testing and fire-fighting training.

Vortex interaction in triple flickering buoyant diffusion flames

Triple flickering buoyant diffusion flames of methane gas in equilateral triangle arrangement, as a nonlinear dynamical system of coupled oscillators, were computationally investigated. The four distinct dynamical modes (in-phase, death, rotation, and partially in-phase), that were originally observed in candle-flame experiments, were computationally reproduced for the first time. The four modes were interpreted from the perspective of vortex interaction and particularly of vorticity reconnection and vortex-induced flow. Specifically, the in-phase mode is caused by the periodic shedding of the trefoil vortex formed by the reconnection of three toroidal vortices; the death mode is due to the suppression of vortex shedding at small Reynolds numbers; the rotation mode appears as three toroidal vortices alternatively shed off with a constant phase difference; the partially in-phase model is caused by the vorticity reconnection of two toroidal vortices leaving another one shedding off in anti-phase. This work well establishes a bridge between the vortex dynamics and the nonlinear dynamics of the triple-flame system, which is believed to play a key role in understanding larger dynamical systems of multiple flickering flames.

Large-eddy simulation of lab-scale ethylene buoyant diffusion flames: Effects of subgrid turbulence/soot production interaction and radiation models

Large eddy simulation of the 15-kW ethylene buoyant diffusion flames burning under O2/N2 oxidizers with oxygen concentrations, XO2, of 0.209, 0.168 and 0.152, investigated experimentally at FM Global, are performed by using a flamelet/presumed filtered density function (FDF) combustion model, an acetylene/benzene-based soot production model and the rank-correlated full-spectrum k (RCFSK) with non-grey soot as radiative property model. The presumed FDF considers the spatial intermittent nature of soot and the coupling between soot quantities and mixture fraction. Subgrid-scale (SGS) absorption turbulence/radiation interaction (TRI) is neglected whereas the filtered emission term is modelled from the presumed FDF approach. Model predictions reproduce with fidelity the flame structure, the soot production dynamics and the radiative loss for the three flames. Model results show that soot and radiating gas have a similar contribution to radiation for the flame burning in air and the contribution of radiating gas becomes increasingly dominant with reducing XO2 due to a decrease in soot volume fraction (SVF) throughout the overall flames. A sensitivity study is performed on the influence of the modelling approach of SGS turbulence/soot production interaction (TSI) and radiative heat transfer. Model predictions show that neglecting the coupling between soot quantities and mixture fraction overestimates substantially soot oxidation, resulting in significantly lower soot production and radiative loss. For the present LES where more than 80% of temperature variance is resolved, disregarding the SGS emission TRI has also a significant impact with an overprediction of mean and rms SVF by a factor of two. On the other hand, the approximation of grey soot is valid for these lab-scale flames characterized by moderate optical thicknesses and the use of a non-grey weighted-sum-of-grey-gases (WSGG) model coupled to the grey soot approximation represents an alternative to the full-spectrum k-distribution models.

Insight into flickering/shedding in buoyant droplet-diffusion flame during interaction with vortex

The interaction of incident vortex with buoyant diffusion flame of a pendant fuel droplet (n-dodecane) is investigated in the present study. The flame stretching/shedding events occur depending on vortex strengths, which are characterized using vortex Reynolds number (Rev). Different flame response regimes have been identified for different vortex strengths. The velocity scale associated with the vortices is higher when compared to natural convection velocity leading to forward extinction, followed by flame lift-off. The flame lift-off increases with Rev, and partial extinction occurs at very high Rev. In all the cases, the flame oscillates/sheds during the vortex interaction, which causes a momentary increase in the flame heat release rate. As the vortex moves past the flame, the oscillations dampen gradually, and the flame regains the natural buoyant behavior. The high-velocity scales imposed during vortex interaction make the flow momentum-dominant, resulting in a low Richardson number (Ri). Since, the flickering frequency scaling valid for Ri>>1 is not obeyed, a new scaling has been proposed to incorporate the vortex momentum effect by using vortex circulation instead of gravity. Only a portion of the incident vortex circulation contributes to the shedding as some of it is lost to forward extinction and lift-off events. Thus, the reduced vortex velocity scale is used to estimate the critical circulation. The flame shedding height calculated using new scaling in the critical circulation equation agrees with experimental observations.

Cellular flame structures and thermal characteristics of axi-symmetric ceiling fires: An experimental study and scaling analysis

The present study provides an experimental characterization of cellular flame structures and thermal characteristics of axi-symmetric ceiling fires. Experiments were carried out employing circular gaseous burners of various diameters at the ceiling facing downward using methane and propane as fuel. For this configuration, a hot buoyant diffusion flame burning beneath the cold fuel gas issuing from the ceiling above was naturally resulted in. A cellular flame structure, due to the inherent “Rayleigh-Taylor’’ instability of this buoyant flow configuration, was formed beneath the ceiling. The flame length (radius), temperature and heat flux distribution along the ceiling as well as characteristic inner cell size evolution of this special cellular flame were quantified experimentally for such ceiling fire of various conditions. Scaling analysis was performed to analyze the obtained parameters of the cellular flame structure and thermal characteristics of the ceiling fire. Results showed that the overall length (radius) of the cellular flame has a 2/5 power dependence on heat release rate and is independent of burner sizes and fuel types. Temperature profile and heat flux distribution for various conditions can be universally expressed as a function of L/Rf, the distance from the burner center normalized by the flame length. The characteristic cell size of the cellular flame was related to temperature, which can be well represented by a 2.33 power function based on the “Rayleigh-Taylor’’ instability scaling analysis. The cell size spatial evolution inside the cellular flame was self-similar corresponding to the dependence of temperature on relative position, and a non-dimensional function was obtained. This study provides fundamental data and a scaling analysis into the basic physics of the cellular flame behavior and thermal characteristics of ceiling fires.

Experimental study of buoyancy-induced instability in the DME and LPG jet diffusion flame

An experimental study is conducted on the buoyant jet diffusion flames to explore the interaction between the fuel jet and the outer vortex and their impact on the flame structure. Dimethyl ether (DME) and liquefied petroleum gas (LPG) diffusion flames are investigated. A laminar diffusion burner setup is used to study the flame dynamics of three different tube diameters (6, 8, and 10 mm) over a wide range of Reynolds numbers (250–2000). Flame flicker frequencies are analyzed using chemiluminescence signals and image processing techniques, and high-speed Schlieren images visualized flame dynamics and structure. The spatiotemporal structure of jet diffusion flames is analyzed using the dynamic mode decomposition (DMD) tool. Analysis of the Schlieren images revealed that the outer vortex structure interacts with the fuel jet, which eventually leads to convective feeding to the outer vortical structure. The outcome is the radial expansion of the leading vortex, which allows the trailing vortex to merge and produce a subharmonic frequency. However, all test conditions, regardless of the presence of subharmonic frequency in the buoyant diffusion flame, follow the established relationship between Strouhal-Froude numbers. The dominant mode of DMD captured the spatial features of the outer vortex at associated temporal frequencies. The large-scale coherent structure is observed in the region of subharmonic oscillation due to the merging of successive vortices. This process occurs at the alternate flame bulge formed near the tube exit.

Numerical modeling of soot radiation in optically-thin, buoyant diffusion flames at varying oxygen concentrations

A flamelet-based subgrid soot-radiation model, which applies the laminar smoke point concept for soot formation, is further developed to account for varying oxygen concentration (χO2) and applied in radiation modeling of buoyant, turbulent diffusion flames. The model is verified against experimental data for laminar flames. A lookup table of radiant emission fractions is developed by parametrizing the flamelet model with a range of strain rates, enthalpy defect (prior heat loss) and χO2. The three-dimensional lookup table is then used in the large eddy simulations (LES) to predict the turbulent flame-sheet radiant emission. LES of optically-thin, buoyant, turbulent ethylene diffusion flames of 10 and 15 kW heat release rates (HRR) are conducted for χO2 in the range of 0.13 - 0.21. With decreasing χO2, the overall radiant fraction and the peak radiant power output from the flames are shown to decrease, as would be expected. Predicted results for radiant power distributions, overall radiant fraction and temperatures show reasonably good comparison against experimental data.

Experimental study on flame pulsation behavior of external venting facade fire ejected from opening of a compartment

The present study investigated experimentally the flame pulsation magnitude (ratio of maximum flame height to mean flame height; ratio of mean flame height to continuous flame height) and flame pulsation frequency of the external venting facade fire (EVF) ejected from opening of a compartment. Experiments were conducted by employing three model-scale fire compartments with an opening and a vertical facade wall. Fifteen different opening geometries were considered. For the flame pulsation magnitude of EVF, results showed that the ratio of maximum flame height to the mean flame height was about 1.30, while the ratio of mean flame height to continuous flame height was about 1.60; both of them were independent of opening geometries and heat release rates. The flame pulsation frequency of EVF increased with increasing of heat release rate, meanwhile decreased with increasing of opening size (or namely opening characteristic lengths: ℓ1= (AH1/2)2/5; ℓ2= (AH2)1/4). The measured flame pulsation frequency of EVF showed to be much lower than the predicted values by previous classic models developed for free buoyant diffusion flames, which is due to physically the inherent special exit condition (with initial horizontal convection flow of heat) at the opening for EVF. A new formula was proposed for the flame pulsation frequency of EVF in terms of a non-dimensional function between Strouhal number (St) and Froude number (Fr), using Vf∝gZf as the characteristic flame uprising velocity and ℓ1= (AH1/2)2/5 as the characteristic length of the source exit convection flow condition at the opening. The flame pulsation frequency showed to be well represented by the proposed formula. These new observations and the proposed formula provide essential knowledge for the characterization of flame pulsation behavior of external venting facade fire (EVF) ejected through the opening of a compartment.

Dynamic self-tuning, flickering and shedding in buoyant droplet diffusion flames under acoustic excitation

We report the mechanism of acoustic-flame coupling for buoyant diffusion flame in droplets. The flame under acoustic excitation exhibits differential varicose and sinuous modes including partial extinction and pinch offs at certain shedding heights. Contrary to traditional buoyant flames, the diameter (d) regression of a droplet results in self-tuning of the instability modes with progression in burning times. In essence, we show that a droplet exhibits multi frequency response with advent of new oscillations modes at different points of the burning lifetime. The acoustic perturbations modulate the flame surface or heat release only at lower frequency bands by feeding energy into the natural vortical stability modes of a buoyant plume. Interestingly, we unearth that a flame is naturally unstable at all frequencies lower than g/d. Acoustic pumping at any such frequency (<g/d) corresponds to a unique convective length scale h such that fflame=g/h. We therefore establish that the flame exhibits multiple convective length scales throughout the burning cycle. We subsequently established that the shedding length scale of the droplet flame shortens by more than 25% with increase in acoustic pressure. We theoretically proved that using critical circulation argument, one can uniquely determine the shedding length scale at any acoustic loading. However, it is interesting to note that while the acoustic pressure amplitude is crucial for determining the convective length at which a flame rolls up, the frequency of excitation has no role to play. In other words, the velocity field introduced by the acoustic pressure is responsible for the shedding of the flame. The acoustic perturbations also lead to bulk oscillation of the flame surface near the forward stagnation zone. This is attributed to the acoustic induced velocity that leads to transient shifting of the stoichiometric surface.

Large eddy simulations of the UMD line burner with the conditional moment closure method

The conditional moment closure (CMC) method is applied in combination with large eddy simulations (LES-CMC) to the UMD line burner configuration. This corresponds to a low-strain, buoyant diffusion flame. The low strain rate conditions are representative of fire conditions and the aim of the study is to evaluate LES-CMC results, including radiative effects. Only conditions far from extinction are studied. The weighted sum of grey gases model (WSGGM) for radiation is applied in mixture fraction space. Results are compared to what is obtained when radiation is taken into account as a local loss term in the conditional heat release rate, with prescribed radiative fraction. It is shown that the conditional scalar dissipation rate (CSDR) profile in mixture fraction space and the spatial terms in the transport equations for the conditional quantities have a major impact with the non-local WSGGM approach, while this is not the case in adiabatic simulations, nor in the radiative fraction approach. In the latter, only the value of the CSDR at stoichiometric is important, making this approach less sensitive. However, the latter are not well suited for extinction simulations.

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.