Thermal and dynamic structure of an ethanol flame in the boundary layer near an obstacle

The distribution of the dynamic and thermal parameters of a gas in the immediate vicinity of a flame stabilizer (behind a rib and a backward-facing step 3 mm high) are studied. Data on the longitudinal and transverse velocity components, velocity pulsations, and correlations of pulsations in the boundary layer with combustion and without combustion are obtained using the PIV equipment. Temperature is measured with a thermocouple. Flame is visualized in the visible and ultraviolet regions. It is shown that in the flow attachment region near the leading edge of the flame, the gas velocity is close to the normal burning velocity. Heat release is estimated from the results of measurements. The presence of regions of volume and frontal combustion and kinetic and diffusion reaction is established. With increasing distance from the stabilizer, the heat-release rate decreases by more than an order of magnitude.     

Stabilization of an inverted propane-air flame on a string stretched along the flow

The stabilization conditions for an inverted flame on a long thin string stretched along the cylindrical burner axis were studied experimentally. Gas temperature distributions in the inverted flame are obtained. The boundaries of stable combustion that are simultaneously the condition of excitation of acoustic self-oscillations of the flame are found. Inverted flame blow-off velocities are measured, and the variation of its geometrical characteristics are studied. It is found that during overturning of the flame relative to the vector of the acceleration due to gravity, stabilization of the inverted flame in the open atmosphere is impossible. The physical mechanism involved in the formation of the inverted flame in flow parallel to the stabilizer surface is considered. The role of the hydrodynamic stretching of the flame in flame blow-off and extinction is determined. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 2, pp. 3–11, March–April, 2008.     

基于火焰稳定特性浸没燃烧器的设计与试验研究

采用传统设计方法对浸没燃烧技术中的燃烧器进行设计,在实际应用过程中总出现回火、脱火、燃烧不稳定问题,本文根据火焰稳定特性知识构建了天然气火焰稳定性图,提供了一种对于燃烧气体燃料浸没燃烧器燃烧室的设计思路,并对该思路下设计的浸没燃烧器燃烧室进行了具体设计;融入旋流稳焰原理,一次空气与燃料旋流预混,二次空气冷却燃烧室外壁并被预热然后在燃烧室头部进入燃烧室助燃,进行了浸没燃烧器的总体设计,并对设计的燃烧器在自行设计的增压浸没燃烧试验台进行了初步试验研究,试验侧重燃烧稳定性与污染物排放。结果表明该浸没燃烧器燃烧稳定性相对较好,且由于良好的结构设计,节能减排效果显著。     

固体材料对微型钝体燃烧器吹熄极限的影响

通过数值模拟研究了固体材料（石英、不锈钢和SiC）对微型钝体燃烧器内氢气/空气预混火焰的吹熄极限的影响。结果表明：计量比为0.5时,3个燃烧器对应的吹熄极限分别为36、25和21 m·s^-1。理论分析揭示了微型钝体燃烧器中火焰稳定性与流动和传热之间的相互作用非常密切。当热导率较小时,通过壁面向上游传导的热量较少,壁面对未燃预混气的预热效果较差,燃烧后的气体膨胀相对较弱,从而钝体后的低速区面积较大,稳燃效果较好。对于不锈钢和SiC燃烧器,由于SiC的发射率更大,通过壁面的散热损失较大,从而使得其吹熄极限较小。为获得良好稳燃性能的微型钝体燃烧器选择合适的材料提供了指导。     

Experimental Study of a Cellular Ethanol Flame Evaporating “On the Ceiling”

This paper describes the study of evaporation and combustion of ethanol under a horizontal wall in a stratified shear gas layer in the case of the Rayleigh–Taylor instability. Data on the nature of flow are obtained with the use of particle image velocimetry (PIV), and temperature profiles are recorded by a thermocouple. It is shown that cells are formed in a narrow range of air velocity of 0.6 ± 0.05 m/s and does not depend on the height of the obstacle (backward ledge or an edge is 0–7 mm in height). The flow between the wall and flame front is an alternation of mushroom-shaped structures moving from one wall to another. In the cellular flame, the flow of substance (with respect to the air flow) exceeds its level in a standard laminar boundary layer three times. The averaged transverse velocity is directed away from the wall in the boundary layer with combustion without cells, and it is reduced and directed toward the wall in the cellular flame between the wall and flame front.     

Effect of the Wave Structure of the Flow in a Supersonic Combustor on Ignition and Flame Stabilization

Results of numerical and experimental investigations of a high-velocity flow in a plane channel with sudden expansion in the form of a backward-facing step, which is used for flame stabilization in a supersonic flow, are presented. The experiments are performed in the IT-302M high-enthalpy short-duration wind tunnel under the following test conditions: Mach number at the combustor entrance 2.8, Reynolds number 30 · 10⁶ m^?1, and total temperature T₀ = 2000 K, i.e., close to flight conditions at M = 6. The numerical simulations are performed by solving full unsteady Reynolds-averaged Navier–Stokes equations supplemented with the k–ω SST turbulence model and a system of chemical kinetics including 38 forward and backward reactions of combustion of a hydrogen–air mixture. Three configurations of the backward-facing step are considered: straight step without preliminary actions on the flow, with preliminary compression, and with preliminary expansion of the flow. It is demonstrated that the backward-facing step configuration exerts a significant effect on the separation region size, pressure distribution, and temperature in the channel behind the step, which are the parameters determining self-ignition of the mixture. The computed results show that preliminary compression of the flow creates conditions for effective ignition of the mixture. As a result, it is possible to obtain ignition of a premixed hydrogen–air mixture and its stable combustion over the entire channel height.     

Experimental study of characteristics of a laminar boundary layer with hydrogen combustion

Results of an experimental study of a laminar boundary layer with combustion of a hydrogen-nitrogen fuel mixture uniformly injected through a porous wall into an air flow are presented. Data characterizing the ignition conditions are obtained. Based on the recorded temperature distributions, streamwise changes in the location and temperature of the flame front are analyzed as functions of the free-stream velocity (1–4 m/s), injection intensity, and fuel composition. It is demonstrated that heat transfer can be adequately described by a “standard” dependence for the boundary layer with boundary conditions of the second kind.     

Heat and mass transfer in a boundary layer with the evaporation and combustion of ethanol

Experimental data on heat and mass transfer in a boundary layer upon ethanol evaporation from a porous surface and its combustion in an air flow are reported. It has been established that variations in the flow velocity in the flow core weakly affect the temperature and concentration of substances on the reactor wall. The flame temperature and the distribution of mass flows over the wall depend essentially on the flow velocity. It has been observed that heat-and mass-transfer coefficients decrease in combustion. The representation of experimental data using overall enthalpies and generalized concentrations as transfer potentials suggests an analogy between the processes of heat and mass transfer in a reacting boundary layer.Novosibirsk. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 1, pp. 8–15, January–February, 1994.     

Experimental studies of bluff-body stabilized LPG diffusion flames

Bluff-body stabilized turbulent jet diffusion flame has received renewed attention in recent years due to its practical applications. An experimental study is carried out to investigate the effect of coaxial air velocity, U_a, and lip-thickness, δ of the bluff-body on the flame stability limits and emission levels. The stability limits of a typical diffusion flame can be characterized in terms of two parameters namely flame lift-off height and blow-off velocity. It is experimentally observed that lift-off height is not linearly dependent on the fuel exit velocity, U_f, as compared to the simple jet. The flame stability is found to be improved for larger lip-thickness bluff-body because of the presence of lower pressure in the wake region behind the bluff-body. Flame length is observed to be dominated by buoyancy and momentum regimes. The transition from buoyancy to momentum regime is found to be extended with increase in lip-thickness. It is also observed that the blow-off limit is also extended further by 10% as compared to simple jet diffusion flames under similar conditions. The emissions data are reported in terms of mass based emission index, EINO_x (g [NO_x]/kg [fuel]) for a wide range of flow conditions. It is concluded that the addition of coaxial air in the larger lip-thickness bluff-body flames causes a marginal reduction in emission levels relative to smaller lip-thickness bluff-body.     

后台阶反应流概率密度函数-雷诺应力法模拟

后台阶流动是计算流体力学领域十分重要的课题,但是后台阶流动与化学反应耦合的数值研究则比较少见.文中使用雷诺应力模型模拟后台阶流动的速度场,使用标量联合概率密度函数(PDF)方程模拟化学反应项,并将化学反应项的结果作为源项引入雷诺应力模型,模拟了后台阶流动中的甲烷与氧气的缓慢氧化反应.结果发现:雷诺应力模型能够很好地模拟...     

Wärme-, Stoff- und Impulsübertragung in abgelösten Strömungen

Heat, mass and momentum transfer in separated flows . The simple model of a backward-facing step in flat plate boundary layers provides basic information on heat, mass and momentum transfer in local separated regions. These new investigations cover nearly the whole range of existence of incompressible separated flows with laminar as well as turbulent boundary layers at separation, including the two main parameters Re_δ1 and Re_s: The Reynolds number Re_δ1 represents the boundary layer at separation, the Reynolds number Re_s the shape of the body. It is demonstrated that independently of the state of the boundary layer at separation, there exist three types of local separated regions. Therefore a new general valid classification of local separated flows is introduced based on the actual state of the separated boundary (shear) layer from separation to reattachement. The new results presented are limited to the case where no temperature or concentration boundary layers exist at separation, i.e. to the case of unheated or inert starting length before separation: Only then can the analogy of heat, mass and momentum transfer give useful results over the whole range of existence of local separated regions. A new shear layer model is introduced to facilitate transfer of the results obtained for backward-facing steps to arbitrary shaped bodies. A compilation and comparison of available literature values shows that this shear layer model for the step also permits the calculation of heat and mass transfer in separated areas on arbitrarily shaped bodies.     

Nitrogen dilution effect on flame stability in a lifted non-premixed turbulent hydrogen jet with coaxial air

The nitrogen dilution effect on flame stability was experimentally investigated in a lifted non-premixed turbulent hydrogen jet with coaxial air. Hydrogen gas was used as the fuel and coaxial air was injected to initiate flame liftoff. Hydrogen was injected into an axisymmetric inner nozzle (d_F = 3.65 mm) and coaxial air jetted from an axisymmetric outer nozzle (d_A = 14.1 mm). The fuel jet and coaxial air velocities were fixed at u_F = 200 m/s and u_A = 16 m/s, while the mole fraction of the nitrogen diluent gas varied from 0.0 to 0.2 with a 0.1 step. For the analysis of the flame structure and the flame stabilization mechanism, the simultaneous measurement of PIV/OH PLIF was performed. The stabilization point was in the region of the flame base with the most upstream region and was defined as the point where the turbulent flame propagation velocity was found to be balanced with the axial component of the local flow velocity. The turbulent flame propagation velocity increased as the nitrogen mixture fraction decreased. The nitrogen dilution makes the flame structure more premixed. That is, the stabilization mechanism shifts from edge flame propagation based mechanism toward premixed flame propagation based mechanism. We concluded that the turbulent flame propagation velocity was expressed as a function of the turbulent intensity and the axial strain rate, even though the mole fraction of the nitrogen diluent varied.     

Flame stability and emission characteristics of turbulent LPG IDF in a backstep burner

The stability characteristics and emissions from turbulent LPG inverse diffusion flame (IDF) in a backstep burner are reported in this paper. The blow-off velocity of turbulent LPG IDF is observed to increase monotonically with fuel jet velocity. In contrast to normal diffusion flames (NDF), the flame in the present IDF burner gets blown out without getting lifted-off from the burner surface. The soot free length fraction, SFLF, defined as the ratio of visible premixing length, H_p, to visible flame length, H_f, is used for qualitative estimation of soot reduction in this IDF burner. The SFLF is found to increase with central air jet velocity indicating the occurrence of extended premixing zone in the vicinity of flame base. Interestingly, the soot free length fraction (SFLF) is found to be correlated well with the newly devised parameter, global momentum ratio. The peak value of EINO_X happens to occur closer to stoichiometric overall equivalence ratio.     

Multi-scale simulation of oblique collisions of a droplet on a surface in the Leidenfrost regime

This work illustrates the phenomena of oblique impact of a water droplet on a hot solid surface in the Leidenfrost regime using a multi-scale model. The flow and heat transfer behavior on the droplet scale as described by the macroscopic model is solved using the finite volume method, together with the level set and the immersed boundary methods which quantify the variations of the gas-liquid and fluid-solid interfaces. A micro-scale vapor layer model is used to account for the resistance effect of the vapor layer generated by the film-boiling evaporation. These two models are coupled in each time step, and solved concurrently. Based on this multi-scale model, the effect of the impact velocity is investigated numerically. The droplet shape and impact parameters such as momentum loss and contact time calculated from the present model are compared with the experimental results obtained from the literature. A good agreement is seen between the simulated and the experimental results.     

Boundary-Layer Structure with Hydrogen Combustion with Different Injection Intensities

Results of numerical simulation of the influence of intensity of hydrogen injection through a porous surface in the case of hydrogen burning in the boundary layer are presented. Turbulent characteristics of the flow were simulated using the k–epsiv; turbulence model with Chien's modification for low Reynolds numbers. The diffusion model (infinitely large burning rate) was used to describe the chemical reaction process, but the difference in diffusion coefficients of different substances was taken into account. A comparison of injection with and without combustion shows that the presence of a heat-release front delays the laminar–turbulent transition and significantly deforms the profiles of density and viscosity of the gas mixture. As the injection velocity increases, the flame front is shifted from the porous surface toward the outer edge of the boundary layer. The contributions of injection itself and combustion to reduction of skin friction are analyzed. Key wrds: boundary layer, combustion, porous injection, heat and mass transfer, friction.     

Exit of a Heterogeneous Detonation Wave into a Channel with Linear Expansion. II. Critical Propagation Condition

Propagation of a detonation wave in monodisperse suspensions of reacting particles (based on the model of the suspension of aluminum particles in oxygen) in channels with linear expansion is studied within the framework of mechanics of heterogeneous reacting media. Reduced kinetics is described with allowance for the transitional (from diffusion to kinetic) regime of combustion of micron-sized and submicron-sized spherical aluminum particles. The effects of the channel width, particle diameter, and expansion angle on propagation conditions and detonation regimes are determined. The critical channel width is found to be a nonmonotonic function of the expansion angle, which is associated with qualitatively different wave patterns behind an oblique step. Flow charts are constructed, and the results are compared with solutions of problems of heterogeneous detonation wave propagation in channels with a backward-facing step and with sudden expansion.     

Flow Structure and Heat and Mass Transfer in Boundary Layers with Injection of Chemically Reacting Substances (Review)

The uptodate status of experimental and theoretical studies of aerodynamics and heat and mass transfer with injection of chemically reacting substances into the boundary layer and their evaporation and combustion is reviewed. Laminar and turbulent flow regimes with subsonic flow velocities are considered. The effect of intensity of fuel injection, fuel type, streamwise pressure gradient, and external turbulence, and also the laminarizing effect of heat release in the flame front on the boundarylayer structure and heat and mass transfer are analyzed.     

Determination of schmidt number of mixed fuels by the characteristics of laminar lifted jet flames

A method to determine the Schmidt number of fuel is proposed from the behavior of laminar lifted jet flames. Based on the observation of a laminar lifted flame edge, the flame stabilization point is located along the stoichiometric contour in the mixing layer of fuel and air in laminar jets, since a tribrachial (triple) flame structure exists which is composed of a diffusion flame, a rich premixed flame and a lean premixed flame, all extending from a single location. For the flame edge to be stationary, the axial velocity at the edge should balance with the propagation speed of the tribrachial flame. Since the region between the flame stabilization point and the nozzle exit can be treated as a cold jet, the jet theory of momentum and species can be applied to obtain the correlation of liftoff height with jet velocity and nozzle diameter of . Using this relation, the mixture of fuels having Sc<1 and Sc>1 are tested. The dependence of liftoff height on jet velocity is curve-fitted to extract the effective Schmidt number of mixed fuels. Experimentally determined Schmidt numbers agree satisfactorily with the theoretical predictions from the kinetic theory.     

Dynamics of concurrent flame spread over a thin charring solid in microgravity

The dynamics of microgravity concurrent flame spread over thin cellulosic sheets are theoretically investigated. The mathematical model is based on the laminar, reactive Navier–Stokes equations coupled to solid-phase enthalpy and mass conservation equations. Simulations have been made for forced flow velocities in the range 0.25–15 cm/s, by decreasing the oxygen mass fraction of the concurrent flow below the ambient value and by increasing the solid charring rate (fire-retarded cellulose). For air, non-retarded cellulose and flow velocites larger than 5 cm/s, the dynamics of concurrent flame spread are qualitatively similar to those of normal gravity. As the concurrent flow is decreased below 5 cm/s, after short transients, a transition from fast flame spread to slow solid burning and then to flame quenching is predicted. Flame quenching is also observed, for relatively high flow velocities, in vitiated air or for fire-retarded cellulose. Finally, blow-off at the highest velocity considered (15 cm/s) is predicted only for sufficiently low oxygen concentrations. © 1998 John Wiley & Sons, Ltd.     

Validation of the Lagrangian Approach for Predicting Turbulent Dispersion and Evaporation of Droplets within a Spray

The accuracy of the Lagrangian approach for predicting droplet trajectories and evaporation rates within a simple spray has been addressed. The turbulent dispersion and overall evaporation rates of droplets are modeled reasonably well, although the downstream velocity decay of the larger droplets is underpredicted, which is attributed to a poor estimate of the radial fluctuating velocity of these droplets at the inlet boundary. Qualitative agreement is found between the predicted and experimental evolution of the droplet size distribution downstream of the nozzle. These results show that smaller droplets evaporate preferentially to larger droplets, because they disperse more quickly toward the edge of the jet, where the entrainment of fresh air from the surroundings produces a significant evaporative driving force. Droplet dispersion and evaporation rates are highly influenced by the rate of turbulence generation within the shear layer. This work demonstrates the potential of the Lagrangian approach for analyzing particle trajectories and drying within the more complex spray dryer system.