Combustion of partially premixed spray jets

Gas turbines, liquid rocket motors, and oil-fired furnaces utilize the spray combustion of continuously injected liquid fuels. In most cases, the liquid spray is mixed with an oxidizer prior to combustion, and further oxidizer is supplied from the outside of the spray to complete diffusion combustion. This rich premixed spray is called “partially premixed spray.” Partially premixed sprays have not been studied systematically although they are of practical importance. In the present study, the burning behavior of partially premixed sprays was experimentally studied with a newly developed spray burner. A fuel spray and an oxidizer, diluted with nitrogen, was injected into the air. The overall equivalence ratio of the spray jet was set larger than unity to establish partially premixed spray combustion. In the present burner, the mean droplet diameter of the atomized liquid fuel could be varied without varying the overall equivalence ratio of the spray jet. Two combustion modes with and without an internal flame were observed. As the mean droplet diameter was increased or the overall equivalence ratio of the spray jet was decreased, the transition from spray combustion only with an external group flame to that with the internal premixed flame occurred. The results suggest that the internal flame was supported by flammable mixture through the vaporization of fine droplets, and the passage of droplet clusters deformed the internal flame and caused internal flame oscillation. The existence of the internal premixed flame enhanced the vaporization of droplets in the post-premixed-flame zone within the external diffusion flame.     

Microgravity experiments of flame spreading along a fuel droplet array in fuel vapor-air mixture

Combustion experiments of fuel droplet array in fuel vapor-air mixture were performed at microgravities to investigate growth mechanism of group combustion of fuel droplets. A 10-droplet array was inserted into the test section filled with a saturated fuel vapor-air mixture as a simple model of prevaporized sprays. Gas equivalence ratio of the fuel vapor-air mixture was regulated by the test section temperature. n-Decane droplets of 0.8 mm in the initial diameter were suspended at the crossing points of 10 sets of X-shaped suspenders. The first droplet was ignited by a hot wire to initiate flame spread along a fuel droplet array. Flame spread speed was obtained from the history of the leading edge position of a spreading flame. Effects of droplet spacing and gas equivalence ratio on the flame spreading behavior and the flame spread speed were examined. The droplet spacing and the gas equivalence ratio were varied from 1.6 to 10.2 mm and from 0.2 to 0.7, respectively. The gas equivalence ratio has little effect on the relationship between the flame spreading behavior and the droplet spacing. The flame spread speed increases as the increase in the gas equivalence ratio at all droplet spacings. The influence of the gas equivalence ratio on the flame spread speed becomes strong as the increase in the droplet spacings. The flame spread speed increases as the increase in the droplet spacing, and then decreases. The maximum flame spread speed appears in the range from 2.4 to 3 mm at all gas equivalence ratios.     

Identification of droplet burning modes in lean, partially prevaporized swirl-stabilized spray flames

Experimental and numerical investigations of single droplet burning modes in a lean, partially prevaporized swirl-stabilized spray flame are reported. In the experiment single droplet flames have been visualized by CH-PLIF and simultaneous recording of the Mie signal. Two single droplet burning modes were identified: the envelope flame is a spherical diffusion flame burning at near-stoichiometric conditions. The wake flame is a potentially lean, partially premixed flame located downstream of the droplet. The droplet burning mode is of practical relevance, since it has significant impact on NO formation due to incomplete prevaporization.The droplet burning mode is determined by the ratio of chemical and convective time scales. The convective time scale is related to the droplet slip velocity. The impact of turbulent gas phase velocity fluctuations on droplet mechanics and droplet burning is discussed, based on a previous numerical investigation. In the present study the droplet slip velocity was measured with the 3D Phase Doppler (3D-PD) technique. For the measured slip velocities and ambient conditions in the hot gas region of the spray flame, simulations of single droplet burning were performed utilizing detailed models for chemical reaction, diffusive transport and vaporization. An agreement between the droplet burning modes predicted by the simulation and the droplet burning modes observed in the experiments was found.     

Effect of non-zero relative velocity on the flame speed of two-phase laminar flames

A numerical study of one-dimensional n-heptane/air spray flames is presented. The objective is to evaluate the flame propagation speed in the case where droplets evaporate inside the reaction zone with possibly non-zero relative velocity. A Direct Numerical Simulation approach for the gaseous phase is coupled to a discrete particle Lagrangian formalism for the dispersed phase. A global two-step n-heptane/air chemical mechanism is used. The effects of initial droplet diameter, overall equivalence ratio, liquid loading and relative velocity between gaseous and liquid phases on the laminar spray flame speed and structure are studied. For lean premixed cases, it is found that the laminar flame speed decreases with increasing initial droplet diameter and relative velocity. On the contrary, rich premixed cases show a range of diameters for which the flame speed is enhanced compared to the corresponding purely gaseous flame. Finally, spray flames controlled by evaporation always have lower flame speeds. To highlight the controlling parameters of spray flame speed, approximate analytical expressions are proposed, which give the correct trends of the spray flame propagation speed behavior for both lean and rich mixtures.     

Influence of micro-explosions on the stability of laminar premixed water-in-fuel emulsion spray flames

A model is presented for a one-dimensional laminar premixed flame, propagating into a rich, off-stoichiometric, fresh homogenous mixture of water-in-fuel emulsion spray, air and inert gas. Due to its relatively large latent heat of vaporisation, the water vapour acts to cool the flame that is sustained by the prior release of fuel vapour. To simplify the inherent complexity that characterises the analytic solution of multi-phase combustion processes, the analysis is restricted to fuel-rich laminar premixed water-in-fuel flames, and assumes a single-step global chemical reaction mechanism. The main purpose is to investigate the steady-state burning velocity and burnt temperature as functions of parameters such as initial water content in the emulsified droplet and total liquid droplet loading. In particular, the influence of micro-explosion of the spray’s droplets on the flame’s characteristics is highlighted for the first time. Steady-state analytical solutions are obtained and the sensitivity of the flame temperature and the flame propagating velocity to the initial water content of the micro-exploding emulsion droplets is established. A linear stability analysis is also performed and reveals the manner in which the micro-explosions influence the neutral stability boundaries of both cellular and pulsating instabilities.     

Effects of pressure on flame propagation in a premixture containing fine fuel droplets

Combustion experiments on fuel droplet–vapor–air mixtures have been performed with a rapid expansion apparatus which generates monodispersed droplet clouds with narrow diameter distribution using the condensation method. The effects of fine fuel droplets on flame propagation were investigated for ethanol droplet–vapor–air mixtures at various pressures from 0.2 to 1.0 MPa. A stagnant fuel droplet–vapor–air mixture, generated in a rapid expansion chamber, was ignited at the center of the chamber using an ignition wire. Spherical flame propagation under constant-pressure conditions was observed with a high-speed video camera and flame speed was measured. Total equivalence ratio, and the ratio of liquid fuel mass to total fuel mass, was varied from 0.6 to 1.4 and from zero to 56%, respectively. The mean droplet diameter of fuel droplet–vapor–air mixtures was set at 8.5 and 11 μm. It was found that the flame speed of droplet–vapor–air mixtures less than 0.9 in the total equivalence ratio exceeds that of premixed gases of the same total equivalence ratio at all pressures. The flame speed of fuel droplet–vapor–air mixtures decreases as the pressure increases in all total equivalence ratios. At large ratios of liquid fuel mass to total fuel mass, the normalized flame speed (the flame speed of droplet–vapor–air mixtures divided by the flame speed of the premixed gas with the same total equivalence ratio), increases with the increase in pressure for fuel-lean mixtures, and it decreases for fuel-rich mixtures. The outcome is reversed at small ratios of liquid fuel mass to total fuel mass; the normalized flame speed decreases with the increase in pressure for fuel-lean mixtures, and increases for fuel-rich mixtures. The results suggest that the increase in pressure promotes droplet evaporation in the preheat zone.     

Map of growth rate of diamond film synthesized by use of flame CVD

Optimum conditions for the flame synthesis of diamond films have been studied by examining effects of the equivalence ratio, ejection velocity, and velocity gradient on the growth rates and morphologies of diamond films. Important factors that can affect growth rates and morphologies of diamond films deposited in the flame are confirmed to be temperature, flow, and species concentration fields. By use of a flat flame burner, these influences are well understood because the flat acetylene/hydrogen/oxygen flame is stabilized in a well-defined stagnation flow field, which can be regarded as one-dimensional field. It is found that the maximum growth rate can be obtained when the equivalence ratio is from 2.45 to 2.50. It has also been confirmed that the growth rate is nearly the same when the velocity gradient is kept constant. This result indicates that the velocity gradient is one of the important parameters that can govern the growth rate of diamond film. Furthermore, in order to obtain universal, optimum conditions for the flame synthesis of diamond films, an attempt has been conducted to make a map of the growth rates, as functions of equivalence ratio and velocity gradient. Although growth rates increase with increasing velocity gradient, excessively high velocity gradients cause decrease in growth rates. It is found that the maximum growth rate can be obtained when the equivalence ratio is around 2.50 and velocity gradient is 4000 s⁻¹.     

Investigation of droplet combustion in strained counterflow diffusion flames using planar laser-induced fluorescence

Experimental investigation of an isolated droplet burning in a convective flow is reported. Acetone droplets were injected in a steady laminar diffusion counterflow flame operating with methane. Planar laser-induced fluorescence measurements applied to OH radical and acetone was used to measure the spatial distribution of fuel vapour and the structure of the flame front around the droplet. High-magnification optics was used in order to image flow areas with a ratio of 1:1.2. The different combustion regimes of an isolated droplet could be observed from the configuration of the envelope flame to that of the boundary-layer flame, and occurrence of these regimes was found to depend on the droplet Reynolds number. Experimental results were compared with 1D numerical simulations using detailed chemistry for the configuration of the envelope flame. Good agreement was obtained for the radial profile of both OH radical and fuel vapour. Influence of droplet dynamics on the counterflow flame front was also investigated. Results show that the flame front could be strongly distorted by the droplet crossing. In particular, droplets with high velocity led to local extinction of the flame front whereas droplets with low velocity could ignite within the flame front and burn on the oxidiser side. PACS 33.50.-j; 42.62.-b; 47.55.D-; 47.70.Pq; 47.80.Jk     

Numerical investigation of the effects of polydisperse water sprays on extinction conditions of counterflow methane non-premixed flames

Water, sprayed in the form of tiny droplets, has emerged as a potential fire suppressant after the halon compounds such as trifluorobromomethane (CF₃Br, Halon 1301) were banned by the Montreal protocol. The size distribution of the water droplet plays a crucial role in the effectiveness of the water spray in fire suppression. A numerical investigation of the influence of size distribution of a polydisperse water spray on extinction of counterflow diffusion flames is presented in this paper. This study uses laminar finite rate model with reduced CHEMKIN chemistry for numerical simulations. The discrete phase, namely the water spray, is simulated using Lagrangian Discrete Phase Modelling approach. In this work, the polydispersity of water spray is taken into account in the numerical simulation by a suitable Rosin–Rammler distribution. Results obtained from numerical simulation are validated with the experimental results reported in the literature. This study demonstrates that the representation of the polydisperse spray by a monodisperse spray (with droplet diameter same as the SMD of the polydisperse spray) in numerical simulations is not always justified and it leads to deviation from the experimental results. The effects of number mean diameter and spread parameter on the efficacy of flame suppression are investigated for polydisperse sprays. A comprehensive comparison is done between the effectiveness of monodisperse and polydisperse water sprays. An optimum droplet diameter is obtained for monodisperse sprays for which the effectiveness of the spray is maximum. The effects of evaporation Damköhler number and Stokes number of water droplets on flame suppression have also been explained.     

Spatiotemporal measurements of flame stretch and propagation rates for lean and rich CH4/air premixed flames interacting with a turbulent-wake

A 1.5 m long turbulent-wake combustion vessel with a 0.15 m × 0.15 m cross-sectional area is proposed for spatiotemporal measurements of curvature, strain, dilatation and burning rates along a freely downward-propagating premixed flame interacting with a parallel row of staggered vortex pairs having both compression (negative) and extension (positive) strains simultaneously. The wanted wake is generated by rapidly withdrawing an electrically-controlled, horizontally-oriented sliding plate of 5 mm thickness for flame–wake interactions. Both rich and lean CH₄/air flames at the equivalence ratios  = 1.4 and  = 0.7 with nearly the same laminar burning velocity are studied, where flame–wake interactions and their time-dependent velocity fields are obtained by high-speed, high-resolution DPIV and laser-tomography. Correlations among curvature, strain, stretch, and dilatation rates along wrinkled flame fronts at different times are measured and thus their influences on front propagation rates can be analyzed. It is found that strain-related effects have significant influence on front propagation rates of rich CH₄/air (diffusionally stable) flames even when the curvature weights more in the total stretch than the strain rate does. The local propagation rates along the wrinkled flame front are more intense at negative strain rates corresponding to positive peak dilatation rates but the global propagation rate averaged along the rich flame front remains constant during all period of flame–wake interaction. For lean CH₄/air (diffusionally unstable) flames, the curvature becomes a dominant parameter influencing the structure and propagation of the wrinkled flame front, where both local and global propagation rates increase significantly with time, showing unsteady flame propagation. These experimental results suggest that the theory of laminar flame stretch can be applicable to a more complex flame–wake interaction involving unsteadiness and multitudinous interactions between vortices.     

Microgravity experiments of single droplet combustion in oscillatory flow at elevated pressure

An experimental study for 1-butanol single droplet flames in constant and oscillatory flow fields was conducted under microgravity conditions at elevated pressure. In the constant flow experiments, flow velocities from 0 to 40 cm/s were tested. Using obtained data of d², the burning rate constants were evaluated. The burning rate constant in the quiescent condition was also calculated successfully at high pressure by the extrapolation method based on the Frössling relation. In the oscillatory flow experiments, the flow velocities were varied from 0 to 40 cm/s at the frequencies of 2–40 Hz. Results showed that the burning rate constant during the droplet lifetime varied following the quasi-steady relation at 0.1 MPa; however, in the conditions with higher frequencies at 0.4 MPa, the average burning velocity became larger than that for the constant flow case with the velocity equivalent to the maximum velocity in the oscillatory flow. Under the condition where the burning rate constant increased, it was observed that the flame did not sufficiently move back upstream, leading to enhancement of the heat transfer from the flame to the droplet surface. Therefore, the instantaneous burning rate constant increased. To investigate the mechanism of such flame behavior, the ratio of two characteristic times, τ_f/τ_D (τ_f: flow oscillation characteristic time, τ_D: diffusion characteristic time), were compared. As the flow oscillatory frequency increased, τ_f/τ_D becomes smaller. τ_f/τ_D also became smaller at high pressure. If τ_f/τ_D is small due to the small mass diffusion rate, the droplet flame could not move back to the appropriate position for the minimum velocity in steady flow, leading to an increase of the burning rate constant, especially in the case of higher frequency at high pressure.     

Burning velocity of triple flames with gentle concentration gradient

We investigated the local flame speed of a two-dimensional, methane-air triple flame in a rectangular burner. The velocity fields and the concentration profiles were measured with particle image velocimetry and the Rayleigh scattering method, respectively. There was a requisite combination of initial velocity and initial concentration gradient for consistency of the local concentration gradient at the leading edge of the flame. In these cases, the flame curvatures were also consistent. Accordingly, the burning velocity, defined as local flow velocity at the triple point, was determined by the flame curvature. The burning velocity increased with increasing flame curvature, when the curvature was near zero. After that, the burning velocity decreased with increasing curvature. The peak value thus exceeded the adiabatic one-dimensional laminar burning velocity. Comparing the effects of the measured flame stretch rate on the flow strain κ_s and flame curvature κ_c, κ_s is larger and increases more rapidly than κ_c for flame curvatures satisfying 1/R_f < 250 m⁻¹ and then becomes constant while κ_c still increases for 250 m⁻¹ < 1/R_f, so that κ_c becomes much larger than κ_s. There is also a peak in burning velocity at roughly the transition in flame curvature specified above. Therefore, the burning velocity for a low concentration gradient correlates with the flame stretch rate.     

A structural study of premixed hydrogen-air cellular tubular flames

Two dimensionally spatially resolved structural measurements are reported for cellular phenomena in lean laminar premixed hydrogen-air tubular flames. Laser-induced Raman scattering and chemiluminescence imaging are combined to investigate low Lewis number lean hydrogen-air flames. The strong effect of thermal-diffusive imbalance is observed in radial profiles interpolated through the centers of reaction and extinction zones. In the flame cell, the equivalence ratio is ～80% higher than the inlet mixture, resulting in a peak flame temperature of 1600 K that is 550 K above the adiabatic flame temperature of the inlet mixture (1055 K). In the adjacent extinction zone, the temperatures are ～900 K lower than the peak flame temperature and the equivalence ratio is similar to the inlet mixture. Despite doubling the global stretch rate from 200 s^?1 to 400 s^?1, the enhancement of local equivalence ratio and peak temperature in the flame cell remain similar. This enhancement seems dependent on the local cellular flame curvature, that is similar between both cases. With strong preferential diffusion effects, cellular flames offer unique validation data to improve the accuracy of current molecular transport modeling techniques.     

Flame structure and stabilization of lean-premixed sprays in a counterflow with low-volatility fuel

An experimental study was performed on the combustion of lean-premixed spays in a counterflow. n-Decane was used as a liquid fuel with low volatility. The flame structure and stabilization were discussed based on the flame-spread mechanism of a droplet array with a low-volatility fuel. The spray flame consisted of a blue region and a yellow luminous region. The flame spread among droplets and group-flame formation through the droplet interaction were observed on the premixed spray side, while envelope flames were also observed on the opposing airflow side. The blue-flame region consisted of premixed flames propagating in the mixture layer around each droplet, the envelope diffusion flames around each droplet, the lower parts of the group diffusion flame surrounding each droplet cluster, and the envelope flame around droplets passing through the group flame. The flame was stabilized within a specific range of the mean droplet diameter via a balance between the droplet velocity and the flame-spread rate of the premixed spray.     

Conditional statistics of nonreacting and reacting sprays in turbulent flows by direct numerical simulation

Conditional statistics concerning evaporation and combustion of a spray are investigated in homogeneous, isotropic, and decaying two-dimensional (2D) turbulence. Randomly distributed, polydisperse droplets of n-heptane go through single-step combustion chemistry. Attention is focused on parametric effects of initial Sauter mean radius (SMR), turbulence level and droplet velocity in both reacting and nonreacting cases. A simple linear model for the conditional evaporation rate is proposed and validated against DNS data. A conventional β-probability density function (pdf) is shown to be valid with no peak occurring on the fuel side. The amplitude mapping closure (AMC) model works well for the conditional scalar dissipation rate with evaporating and reacting sprays. Parametric study shows that initial SMR and droplet velocity are major factors affecting conditional flame structures, whereas the effect of reaction is not significant except during autoignition.     

Premixed turbulent flame front structure investigation by Rayleigh scattering in the thin reaction zone regime

Premixed turbulent flames of methane–air and propane–air stabilized on a bunsen type burner were studied using planar Rayleigh scattering and particle image velocimetry. The fuel–air equivalence ratio range was from lean 0.6 to stoichiometric for methane flames, and from 0.7 to stoichiometric for propane flames. The non-dimensional turbulence rms velocity, u′/S_L, covered a range from 3 to 24, corresponding to conditions of corrugated flamelets and thin reaction zones regimes. Flame front thickness increased slightly with increasing non-dimensional turbulence rms velocity in both methane and propane flames, although the flame thickening was more prominent in propane flames. The probability density function of curvature showed a Gaussian-like distribution at all turbulence intensities in both methane and propane flames, at all sections of the flame.The value of the term Dκ, the product of molecular diffusivity evaluated at reaction zone conditions and the flame front curvature, has been shown to be smaller than the magnitude of the laminar burning velocity. This finding questions the validity of extending the level set formulation, developed for corrugated flames region, into the thin reaction zone regime by increasing the local flame propagation by adding the term Dκ to laminar burning velocity.     

Parametric study on a compound-drop spray flame

The introduction of compound-drop spray in a combustion system is a new concept. These droplets bear two gasification stages to cause an integral positive or negative effect on a premixed flame to raise or lower the local temperature of the gasification region. In this paper, we adopt a compound drop which contains a water core encased by a layer of shell fuel. A one-dimensional homogeneous lean or rich premixed flame with the dilute compound-drop spray was investigated by using large activation energy asymptotic analysis. The compound-drop spray burning mode was defined and divided into completely pre-vaporised burning (CPB), shell pre-vaporised burning (SPB) and shell partially pre-vaporised (SPP) burning modes by way of the gasification zones of the shell fuel and the core water relative to the flame position. The influences of the initial droplet radius, the shell-fuel mass fraction and the liquid loading of the compound-drop spray on the lean and rich flames were analysed. By means of the normalisation parameter of flame propagation mass flux (), enhancement, suppression or extinction of the compound-drop spray flame can be represented clearly. Furthermore, from the observation of extinction, the necessary conditions of extinction of a lean spray flame by the internal heat transfer are that the spray is a negative effect and causes a sufficient heat loss rate at flame sheet downstream side. For a rich spray flame, three extinction patterns were observed; they occur in SPP, SPB or at the critical SPB mode, but do not in CPB. The extinction maps of the compound-drop spray demarcate the patterns and also indicate the limitations and corresponding conditions of the flame extinction.     

Design and performance of a high-resolution dual-channel heterodyne laser velocimeter

We describe a dual-channel laser velocimeter based on a single laser source and a single set of carrier wave generation optics. The apparatus is intended for simultaneous vibration measurements on several points of instable objects, such as biological specimens or micro electronic mechanical systems, so that instantaneous phase relationships and amplitude ratios can be determined. Our instrument presently allows measurements on two points of interest which can be arbitrarily chosen. The optical design allows expansion to at least four independent channels. At a maximal velocity amplitude of 52 mm s⁻¹, the velocity resolution and the detection limit equal 2.6 μm s⁻¹ Hz^−1/2. Even with object points less than 0.4 mm apart, channel cross-talk is less than −78 dB at all frequencies.     

Influence of water mist on propagation and suppression of laminar premixed flame

The combustion of premixed gas mixtures containing micro droplets of water was studied using one-dimensional approximation. The dependencies of the burning velocity and flammability limits on the initial conditions and on the properties of liquid droplets were analyzed. Effects of droplet size and concentration of added liquid were studied. It was demonstrated that the droplets with smaller diameters are more effective in reducing the flame velocity. For droplets vaporizing in the reaction zone, the burning velocity is independent of droplet size, and it depends only on the concentration of added liquid. With further increase of the droplet diameter the droplets are passing through the reaction zone with completion of vaporization in the combustion products. It was demonstrated that for droplets above a certain size there are two stable stationary modes of flame propagation with transition of hysteresis type. The critical conditions of the transition are due to the appearance of the temperature maximum at the flame front and the temperature gradient with heat losses from the reaction zone to the products, as a result of droplet vaporization passing through the reaction zone. The critical conditions are similar to the critical conditions of the classical flammability limits of flame with the thermal mechanism of flame propagation. The maximum decrease in the burning velocity and decrease in the combustion temperature at the critical turning point corresponds to predictions of the classical theories of flammability limits of Zel'dovich and Spalding. The stability analysis of stationary modes of flame propagation in the presence of water mist showed the lack of oscillatory processes in the frames of the assumed model.