Combustion of ethanol fuel droplet in vertical direct current electric field

Flame is affected by an external electric field because it contains ions and electrons related to chemical reactions. On the other hand, the movement of ions and electrons affects the external electric field due to their charge. This paper reports the combustion experiments of ethanol droplets in vertical electric field with variable distance electrodes apparatus in order to discuss the change of the external electric field due to the existence of flame. From a one-dimensional steady-state analysis, if the electric field is changed spatially, its effect on combustion behavior is aligned with V²/L³ and not V/L, where V is the applied voltage between electrodes, and L is distance between the electrodes. The droplet is burned between the two horizontal parallel electrodes. The flame deformation and the electric current are characterized by various electrode distances, and respectively, applied voltages. The vertical electric field induces a body force downwards on the flame. The flame deforms downward in the electric field because the electric body force counters the natural buoyancy. The relation between the applied voltage and electrode distance is investigated when the flame becomes vertically symmetrical and the results show that the deformation is the function of V/L^1.5. This indicates that the change in the electric field should be considered to discuss the effect of an external electric field on combustion behavior. The experimental results are rearranged using εV²/L³ where ε is electric permittivity of air because its unit is N/m³ and it considered to be the representative electric body force. Although its application is limited, qualitatively it can help to explain the experimental results of a droplet combustion. In addition, the degree of electron attachment to neutral molecules is discussed to interpret our experimental results.     

Droplet combustion in standing sound waves

Interaction between droplet combustion and acoustic oscillation is clarified. As the simplest model, an isolated fuel droplet is combusted in a standing sound wave. Apart from the conventional idea that oscillatory component of flow influences heat and mass transfer and promotes combustion, a new model that a secondary flow dominates combustion promotion is examined. The secondary flow, found by the authors in the previous work, is driven by acoustic radiation force due to Reynolds normal stress, and named as thermo-acoustic streaming. Since the force is described by the same equation as buoyancy, i.e., F = ΔρVg, the nature of the streaming is thought to be the same as natural convection. The flow patterns of the streaming are analyzed and its influence on burning rate of a droplet is predicted. Experimental investigation was mainly done with burning droplets located in the middle of node and anti-node of standing sound waves. This location realizes the strongest streaming. By varying sound pressure level, ambient pressure, and acoustic frequency, the strength of the streaming was controlled. Flame configuration including soot and burning rate were examined. Microgravity conditions were employed to clarify the influence of acoustic field through the streaming, since it is similar to and must be distinguished from natural convection. Experiments using microgravity conditions confirmed the new combustion promotion model and the way to quantify it. By introducing a new non-dimensional number Gr_a, that is the ratio of acoustic radiation force to viscosity, burning rate constants for various ambient and sound conditions are rearranged. As a result, it was found that the excess burning rate (k/k₀ − 1) is proportional to or , for weak sound and for strong sound, respectively.     

Problems of predicting turbulent burning rates

Different approaches to the modelling of turbulent combustion first are reviewed briefly. A unified, stretched flamelet approach then is presented. With Reynolds stress modelling and a generalized probability density function (PDF) of strain rate, it enables a source term, in the form of a probability of burning function, P_b, to be expressed as a function of Markstein numbers and the Karlovitz stretch factor. When P_b is combined with some turbulent flame fractal considerations, an expression is obtained for the turbulent burning velocity. When it is combined with the profile of the unstretched laminar flame volumetric heat release rate plotted against the reaction progress variable and the PDF of the latter, an expression is obtained for the mean volumetric turbulent heat release rate. Through these relationships experimental values of turbulent burning velocity might be used to evaluate P_b and hence the CFD source term, the mean volumetric heat release rate.

Different theoretical expressions for the turbulent burning velocity, including the present one, are compared with experimental measurements. The differences between these are discussed and this is followed by a review of CFD applications of these flamelet concepts to premixed and non-premixed combustion. The various assumptions made in the course of the analyses are scrutinized in the light of recent direct numerical simulations of turbulent flames and the applications to the flames of laser diagnostics. Remaining problem areas include a sufficiently general combination of strain rate and flame curvature PDFs to give a single PDF of flame stretch rate, the nature of flame quenching under positive and negative stretch rates, flame responses to changing stretch rates and the effects of flame instabilities.     

大加速度场中熄火试验的研究

处于由离心力和科里奥利力构成的大加速度场中的燃烧过程，其熄火极限会发生较大的变化。作者通过对液化石油气和空气的预混火焰和扩散火焰在不同的空燃比（ＡＦＲ）下，在气流喷射方向与燃烧器旋转切线方向垂直、相同或相反三种情形时分别作了试验，得出了离心力和科里奥利力对预混和扩散火焰的熄火的影响规律．     

Stratified turbulent Bunsen flames: flame surface analysis and flame surface density modelling

In this paper it is investigated whether the Flame Surface Density (FSD) model, developed for turbulent premixed combustion, is also applicable to stratified flames. Direct Numerical Simulations (DNS) of turbulent stratified Bunsen flames have been carried out, using the Flamelet Generated Manifold (FGM) reduction method for reaction kinetics. Before examining the suitability of the FSD model, flame surfaces are characterized in terms of thickness, curvature and stratification.

All flames are in the Thin Reaction Zones regime, and the maximum equivalence ratio range covers 0.1?φ?1.3. For all flames, local flame thicknesses correspond very well to those observed in stretchless, steady premixed flamelets. Extracted curvature radii and mixing length scales are significantly larger than the flame thickness, implying that the stratified flames all burn in a premixed mode. The remaining challenge is accounting for the large variation in (subfilter) mass burning rate.

In this contribution, the FSD model is proven to be applicable for Large Eddy Simulations (LES) of stratified flames for the equivalence ratio range 0.1?φ?1.3. Subfilter mass burning rate variations are taken into account by a subfilter Probability Density Function (PDF) for the mixture fraction, on which the mass burning rate directly depends. A priori analysis point out that for small stratifications (0.4?φ?1.0), the replacement of the subfilter PDF (obtained from DNS data) by the corresponding Dirac function is appropriate. Integration of the Dirac function with the mass burning rate m=m(φ), can then adequately model the filtered mass burning rate obtained from filtered DNS data. For a larger stratification (0.1?φ?1.3), and filter widths up to ten flame thicknesses, a β-function for the subfilter PDF yields substantially better predictions than a Dirac function. Finally, inclusion of a simple algebraic model for the FSD resulted only in small additional deviations from DNS data, thereby rendering this approach promising for application in LES.     

Parameters influencing the burning rate of laminar flames propagating into a reacting mixture

The laminar flame speed is an important property of a reacting mixture and it is used extensively for the characterization of the combustion process in practical devices. However, under engine-relevant conditions, considerable reactivity may be present in the unburned mixture, introducing thus challenges due to couplings of auto-ignition and flame propagation phenomena. In this study, the propagation of transient, one-dimensional laminar flames into a reacting unburned mixture was investigated numerically in order to identify the parameters influencing the flame burning rate in the conduction-reaction controlled regime at constant pressure. It was found that the fuel chemical classification significantly influences the burning rate. More specifically, for hydrogen flames, the “evolution” of the burning rate does not depend on the initial unburned mixture temperature. On the other hand, for n-heptane flames that exhibit low temperature chemistry, the burning rate depends on the instantaneous temperature and composition of the unburned mixture in a coupled way. A new approach was developed allowing for the decoupling the flame chemistry from the ignition dynamics as well as for the decoupling of parameters influencing the burning rate, so that meaningful sensitivity analysis could be performed. It was determined that the burning rate is not directly affected by fuel specific reactions even in the presence of low temperature chemistry whose effect is indirect through the modification of the reactants composition entering the flame. The controlling parameters include but not limited to mixture conductivity, enthalpy, and the species composition evolution in the unburned mixture.     

Ps four-wave mixing measurements of electric field in a ns pulse discharge in a hydrogen diffusion flame

Time-resolved electric field in ns pulse discharge plasmas generated in room air and in an atmospheric pressure hydrogen diffusion flame has been measured by ps four-wave mixing, for plane-to-plane electrode geometry. Electric field is put on the absolute scale using the Laplacian field measured before breakdown. The results show that peak electric field during breakdown in the flame, approximately 40?kV/cm, is significantly lower compared to that in room air, 75?kV/cm, due to higher temperature of combustion products. In both cases, peak electric field is higher compared to DC breakdown field. Both in air and in the flame, the electric field follows the applied voltage before breakdown and decreases rapidly after breakdown, due to charge separation and plasma self-shielding. The electric field in air is compared with the predictions of an analytic model of ns pulse breakdown, showing good agreement between the predicted and the measured breakdown field. The model also predicts earlier breakdown as well as breakdown voltage reduction as the temperature is increased, in qualitative agreement with the experimental data. The use of the present ps four-wave mixing diagnostics for measurements of electric fields below ～20?kV/cm in atmospheric pressure flames is challenging, due to low signal-to-noise. The sensitivity of the present diagnostics is controlled by the high temperature and low N₂ fraction in the combustion product mixture, as well as by the limited bandwidth of the Stokes beam generated by the stimulated Raman cell, which provides access only to several rotational levels of nitrogen molecules. The present diagnostics will have much better sensitivity in high-pressure flames, since the four-wave mixing signal scales as the squared number density of nitrogen.     

Effects of body force on the statistical behaviour and modelling of scalar variance in turbulent premixed flames

The effects of body force/external pressure gradient on the statistical behaviours of the reaction progress variable variance and the terms of its transport equation have been investigated for different turbulence intensities using DNS data of statistically planar flames. Since the extent of flame wrinkling increases with the strengthening of body force promoting unstable stratification, the scalar variance has been found to decrease under strong body force promoting stability. This trend is particularly strong for low turbulence intensities where the probability density function of the reaction progress variable cannot be approximated by a bimodal distribution. Therefore, an algebraic relation for the reaction progress variable variance, derived based on a presumed bimodal probability density function of reaction progress variable, cannot be used for general flow conditions. The contributions of chemical reaction and scalar dissipation rates in the scalar variance transport equation remain leading order source and sink, respectively for all cases irrespective of the strength and direction of the body force. The counter-gradient type transport is found to weaken with increasing body force magnitude when the body force is directed from the heavier unburned gas to the lighter burned gas side of the flame brush, and vice versa. Although a scalar dissipation rate-based reaction rate closure can be utilised to model the reaction rate contribution to the scalar variance transport accurately, the dissipation rate contribution due to the gradient of the Favre-averaged reaction progress variable cannot be ignored and it plays a key role for large magnitudes of body force promoting stable stratification. An algebraic closure of the scalar dissipation rate, originally proposed for high Damköhler number combustion, has been modified for the thin reaction zones regime combustion by incorporating the effects of Froude number. This model has been shown to predict the scalar dissipation rate accurately for all cases considered here.     

An experimental study of partially premixed flame sound

The occurrence of oscillating combustion and combustion instability has led to resurgence of interest in the causes, mechanisms, suppression, and control of combustion noise. Noise generated by enclosed flames is of greater practical interest but is more complicated than that by open flames, which itself is not clearly understood. Studies have shown that different modes of combustion, premixed and non-premixed, differ in their sound generation characteristics. However, there is lack of understanding of the region bridging these two combustion modes. This study investigates sound generation by partially premixed flames. Starting from a non-premixed flame, air was gradually added to achieve partial premixing while maintaining the fuel flow rate constant. Methane, ethylene, and ethane partially premixed flames were studied with hydrogen added for flame stabilization. The sound pressure generated by methane partially premixed flames scales with M⁵ compared to M³ for turbulent non-premixed methane flames. Also, the sound pressure generated by partially premixed flames of ethane and ethylene scales as M^4.5. With progressive partial premixing, spectra level increases at all frequencies with a greater increase in the high-frequency region compared to the low-frequency region; flames develop a peak and later a constant level plateau in the low frequency region. The partially premixed flames of methane, ethylene, and ethane generate a similar SPL as a function of equivalence ratio when the fuel volume flow rate is matched. However, when fuel mass flow rate is matched, the ethane and ethylene flames produce a similar SPL, which is lower than that produced by the methane flame.     

A flamelet analysis of the burning velocity of premixed turbulent expanding flames

Direct numerical simulation is a very powerful tool to evaluate the validity of new models and theories for turbulent combustion. In this paper, direct numerical simulations of spherically expanding premixed turbulent flames in the corrugated flamelet regime are performed. The flamelet-generated manifold method is used to deal with detailed reaction kinetics. The numerical method is validated for both laminar and turbulent expanding flames. The computational results are analyzed by using an extended flame stretch theory. It is investigated whether this theory is able to describe the influence of flame stretch and curvature on the local burning velocity of the flame. If the full profiles of flame stretch and curvature through the flame front are included in the theory, the local mass burning rate is predicted accurately. The influence of several approximations, which are used in other existing theories, is studied. When flame stretch is assumed to be constant through the flame front or when curvature of the flame front is neglected, the theory fails to predict the local mass burning rate.     

An investigation of oscillating instabilities in deflagrations with edges

We examine the Lewis-number-greater-than-1 stability of a deflagration sitting on a porous-plug burner with an inert coflow. The flame edges generated by the coflow influence the stability, and this influence is examined. Very wide flames display the same stability characteristics as unbounded flames (flames sans edges), but for moderately wide flames the instability is suppressed. A new two-dimensional instability can occur for narrow flames. There is a range of mass fluxes for which a monotonic decrease in burner (flame) width generates a transition from unstable flames to stable flames, to unstable flames, to quenching. The insertion of a cold probe into the combustion field can stabilize an unstable flame or destabilize a stable flame, depending on the point of insertion.     

Structure of propagating edge diffusion flames in hydrocarbon fuel jets

Direct numerical simulations with a C₃-chemistry model have been performed to investigate the transient behavior and internal structure of flames propagating in an axisymmetric fuel jet of methane, ethane, ethylene, acetylene, or propane in normal earth gravity (1g) and zero gravity (0g). The fuel issued from a 3-mm-i.d. tube into quasi-quiescent air for a fixed mixing time of 0.3 s before it was ignited along the centerline where the fuel–air mixture was at stoichiometry. The edge of the flame formed a vigorously burning peak reactivity spot, i.e., reaction kernel, and propagated through a flammable mixture layer, leaving behind a trailing diffusion flame. The reaction kernel broadened laterally across the flammable mixture layer and possessed characteristics of premixed flames in the direction of propagation and unique flame structure in the transverse direction. The reaction kernel grew wings on both fuel and air sides to form a triple-flame-like structure, particularly for ethylene and acetylene, whereas for alkanes, the fuel-rich wing tended to merge with the main diffusion flame zone, particularly methane. The topology of edge diffusion flames depend on the properties of fuels, particularly the rich flammability limit, and the mechanistic oxidation pathways. The transit velocity of edge diffusion flames, determined from a time series of calculated temperature field, equaled to the measured laminar flame speed of the stoichiometric fuel–air mixtures, available in the literature, independent of the gravity level.     

Heat fluxes and flame heights in façades from fires in enclosures of varying geometry

Fire spread in high rise buildings from floor to floor occurs if flames emerge and extend on the façade of the building to cause ignition in floors above the floor of fire origin. Even though considerable effort has been exerted to address this issue, proposed relations for flame heights and heat fluxes are incomplete and contradictory because the relevant physics have been poorly clarified. By performing numerous experiments in small scale enclosures having various door-like openings and fire locations, the physics and new relations are underpinned for flames on façade emerging from (under-ventilated) ventilation controlled fires at the floor of fire origin. To limit the variables and uncertainties, propane and methane gas burners created controlled (theoretical) heat release rates at the source. Gas temperatures inside the enclosure and at the opening, heat fluxes on the façade wall, flame contours (by a CCD camera) and heat release rates (by oxygen calorimetry) inside and outside the enclosure have been measured. The gas temperatures inside the enclosure were uniform for aspect ratio (length to width) of the enclosure varying from one to three to one. Previous relations for the air inflow and heat release rate inside the enclosure were verified. These flames are highly radiative because soot can be formed at high temperatures inside the enclosure before the combustion gases and the unburned fuel exit the enclosure. Remarkably the efficiency of combustion is one for well over-ventilated and very under-ventilated fires by it dropped to 80% for burning conditions around stoichiometric. The flame height and heat fluxes have been well correlated by identifying new length scales related to the effective area of the outflow and the length after which the flow turns from horizontal to vertical due to buoyancy. The results can be used for engineering calculations for real fires and for validation of new large eddy scale simulation models.     

The two-dimensional structure of low strain rate counterflow nonpremixed-methane flames in normal and microgravity

The structure and extinction of low strain rate nonpremixed methane–air flames was studied numerically and experimentally. A time-dependent axisymmetric two-dimensional (2D) model considering buoyancy effects and radiative heat transfer was developed to capture the structure and extinction limits of normal gravity (1-g) and zero gravity (0-g) flames. For comparison with the 2D modelling results, a one-dimensional (1D) flamelet computation using a previously developed numerical code was exercised to provide information on the 0-g flames. A 3-step global reaction mechanism was used in both the 1D and 2D computations to predict the measured extinction limit and flame temperature. Photographic images of flames undergoing the process of extinction were compared with model calculations. The axisymmetric numerical model was validated by comparing flame shapes, temperature profiles, and extinction limits with experiments and with the 1D computational results. The 2D computations yielded insight into the extinction mode and flame structure. A specific maximum heat release rate was introduced to quantify the local flame strength and to elucidate the extinction mechanism. The contribution by each term in the energy equation to the heat release rate was evaluated to investigate the multi-dimensional structure and radiative extinction of the 1-g flames. Two combustion regimes depending on the extinction mode were identified. Lateral heat loss effects and multi-dimensional flame and flow structure were also found. At low strain rates in 1-g flames (‘regime A’), the flame is extinguished from the weak outer edge of the flame, which is attributed to a multi-dimensional flame structure and flow field. At high strain rates, (‘regime B’), the flame extinction initiates near the flame centreline owing to an increased diluent concentration in the reaction zone, similar to the extinction mode of 1D flames. These two extinction modes can be clearly explained by consideration of the specific maximum heat release rate.     

Reaction zone structures and mixing characteristics of partially premixed swirling CH4/air flames in a gas turbine model combustor

The mixing, reaction progress, and flame front structures of partially premixed flames have been investigated in a gas turbine model combustor using different laser techniques comprising laser Doppler velocimetry for the characterization of the flow field, Raman scattering for simultaneous multi-species and temperature measurements, and planar laser-induced fluorescence of CH for the visualization of the reaction zones. Swirling CH₄/air flames with Re numbers between 7500 and 60,000 have been studied to identify the influence of the turbulent flow field on the thermochemical state of the flames and the structures of the CH layers. Turbulence intensities and length scales, as well as the classification of these flames in regime diagrams of turbulent combustion, are addressed. The results indicate that the flames exhibit more characteristics of a diffusion flame (with connected flame zones) than of a uniformly premixed flame.     

Axis switching in turbulent buoyant diffusion flames

Dynamics of buoyant diffusion flames from rectangular, square, and round fuel sources were investigated using direct numerical simulation (DNS). Fully three-dimensional simulations were performed employing high-order numerical methods and boundary conditions to solve governing equations for variable-density flow and finite-rate Arrhenius chemistry. Significant differences among the different cases were revealed in the vortex dynamics, entrainment rate, small-scale mixing, and consequently flame structures. Mixing and entrainment enhancement in non-circular flames in comparison with circular ones was explained using the Biot–Savart instability theory, which relates vortex dynamics to the local azimuthal curvature. An extension of the theory elucidated why rectangular flames entrain more efficiently and spread wider than square ones, although both configurations have corners. It also provided an explanation for the aspect ratio effects in the near field. In the far field, nonlinear effects were dominant and the general transport equations for vorticity were analyzed in detail. The corner effects and aspect ratio effects were shown to be augmented by the intricate interactions among vortex dynamics, combustion, and buoyancy through the various terms in the equations. The presence of corners in non-circular flames led to concentrated regions of fine-scale mixing and intense reactions centered around the corners. Moreover, the rectangular flames exhibited a different dynamic behavior from even the square one, by creating discrepancies in entrainment, mixing, and combustion between the minor and major axis directions. Increasing the aspect ratio exacerbated such directional discrepancies, and ultimately led to axis switching. It was the first time that axis switching was observed by DNS in a rectangular flame of aspect ratio 3, which raised further questions in combustion prediction and control. Finally, a unified explanation for corner and aspect ratio effects was given on the basis of the Biot–Savart instability theory and the vorticity transport equations.     

Flame structure of HMX/GAP propellant at high pressure

The chemical and thermal structure of a HMX/GAP propellant flame was investigated at a pressure of 0.5 MPa using molecular beam mass spectrometry and a microthermocouple technique. The pressure dependence of the burning rate was measured in the pressure range of 0.5–2 MPa. The mass spectrometric probing technique developed for flames of energetic materials was updated to study the chemical structure of HMX/GAP flames at high pressures. Eleven species, including HMX vapor, were identified, and their concentrations were measured in a zone adjacent to the burning surface at pressures of 0.5 and 1 MPa. Temperature profiles in the propellant combustion wave were measured at pressures of 0.5 and 1 MPa. Species concentration profiles were measured at 0.5 MPa. Two main zones of chemical reactions in the flame were found. The data obtained can be used to develop and validate combustion models for HMX/GAP propellants.     

介质阻挡体放电对甲烷扩散火焰特性的影响

本文在高频交流激励模式下,采用同轴圆柱构型激励器,开展了介质阻挡体放电对空气/甲烷同轴剪切扩散火焰燃烧特性影响实验研究。激励器敷设在外喷嘴环缝以电离空气,采用纹影系统和B型热电偶分别获取流场形态和火焰温度,激励频率为8 kHz,通过改变气体流量和放电电压,分析了不同工况下射流流场、火焰结构和火焰温度在等离子体作用下的变化规律。结果表明:等离子体气动效应能有效增强射流湍流强度,强化空气/甲烷掺混,增大射流角,并随激励电压提高作用效果逐渐增强,实验中未形成明显扩张流动的初始射流在放电电压30 kV时其射流角最大为23.5°。贫燃条件下等离子体激励会改善火焰形态,增强燃烧稳定性,并在流量较低时缩短火焰长度。此外,富燃火焰下游温度会随着激励强度增大不断升高,而贫燃火焰下游温度变化受上游燃烧强度影响存在升高和降低两种情况。