Experimental investigation of thermoacoustic coupling using blended hydrogen–methane fuels in a low swirl burner

In this study, experimental testing and analysis were performed to examine the combustion instability characteristics of hydrogen–methane blended fuels for a low-swirl lean premixed burner. The aim of this study is to determine the effect of hydrogen addition on combustion instability, and this is assessed by examining the flame response to a range of constant amplitude, single frequency chamber acoustic modes. Three different blends of hydrogen and methane (93% CH₄–7% H₂, 80% CH₄–20% H₂ and 70% CH₄–30% H₂ by volume) were employed as fuel at an equivalence ratio of 0.5, and with four different acoustic excitation frequencies (85, 125, 222 and 399 Hz). Planar laser induced fluorescence of the hydroxyl radical (OH-PLIF) was employed to measure the OH concentration at different phases of acoustic excitation and a Rayleigh Index was then calculated to determine the degree of thermoacoustic coupling. It was found, as has been previously reported, that the combustion characteristics are very sensitive to the fraction of hydrogen in the fuel mixture. The flame shows significant increases in flame base coupling and flame compaction with increasing hydrogen concentration for all conditions. While this effect enhances the flame response at non-resonant frequencies, it induces only minimal compaction and appears to decreases the coupling intensity at the resonant frequency.     

Combustion dynamics of a low-swirl combustor

The methodology for the measurement of dynamic combustor behavior has never been clearly established, due to the complexities associated with unsteady premixed flames and the difficulties in their measurement. The global and local distribution of Rayleigh index and the flame response functions are the main parameters normally employed to quantify and describe combustion dynamics. The Rayleigh index quantifies the thermoacoustic coupling, while the flame response function is a measure of the response of the system to outside disturbances. The primary objective of this work is to investigate the combustion dynamics of a commonly used low-swirl burner and to develop tools and methods for examining the dynamics of a combustion system. To this end, the effect of acoustic forcing at various frequencies on flame heat release behavior has been investigated. The current work uses OH-PLIF imaging of the flame region to produce phase-resolved measurements of flame behavior at each frequency. The response of the flame to the imposed acoustic field over the range of 22-400 Hz is then calculated from the processed images. This provides a starting point for an extension/extrapolation to practical acoustic ranges (∼5000 Hz). It was found that the thermoacoustic coupling was mainly evident in the shear mixing zone, producing a toroidal Rayleigh index distribution pattern. The phase shift of the flame fluctuation from the imposed acoustic wave seems to be very closely coupled to the vortices generated at the flame boundary due to shear mixing (Kelvin-Helmholtz instability), thus inducing the alternating toroidal structures. The peak value of the flame response function coincides with the peak absolute value of the Rayleigh index.     

Mixing and stabilization study of a partially premixed swirling flame using laser induced fluorescence

A laboratory-scale swirling burner, presenting many similarities with gas turbines combustors, has been studied experimentally using planar laser induced fluorescence (PLIF) on OH radical and acetone vapor in order to characterize the flame stabilization process. These diagnostics show that the stabilization point rotates in the combustion chamber and that air and fuel mixing is not complete at the end of the mixing tube. Fuel mass fraction decays exponentially along the mixing tube axis and transverse profiles show a gaussian shape. However, radial pressure gradients tend to trap the fuel in the core of the vortex that propagates axially in the mixing tube. As the mixing tube vortex enters the combustion chamber, vortex breakdown occurs through a precessing vortex core (PVC). The axially propagating vortex shows a helicoidal trajectory in the combustion chamber which trace is observed with transverse acetone PLIF. As a consequence, the stabilizing point of the flame in the combustion chamber rotates with the PVC structure. This phenomenon has been observed in the present study with a high speed camera recording spontaneous emission of the flame. The stabilization point rotation frequency tends to increase with mass flow rates. It was also shown that the coupling between the PVC and the flame stabilization occurs via mixing, explaining one possible coupling mechanism between acoustic waves in the flow and the reaction rate. This path may also be envisaged for flashback, an issue that will be more completely treated in a near future.     

Successive laser-induced breakdowns in atmospheric pressure air and premixed ethane–air mixtures

Two successive focused laser pulses are employed to experimentally simulate laser-induced breakdown plasmas at high repetition rates. We find that energy absorption of the second laser pulse by the plasma produced by the first laser pulse is enhanced slightly when the time interval between the pulses is shorter than several tens of nanoseconds but falls to almost zero when the time interval is between a few hundreds of nanoseconds and several tens of microseconds. This behavior is attributed to gas heating by the first breakdown event. In premixed ethane–air mixtures, we identify another strong reduction in the second laser pulse absorption when this pulse coincides with the heat released by combustion, typically milliseconds after the first laser pulse. The fuel–air equivalence ratio (?) and base flow speed are also varied in this study. The results show that the window of reduced absorption coinciding with heat release due to combustion is narrowed when the base flow speed is increased, and also under fuel lean and fuel rich conditions. These results suggest that the use of pulsed high frequency laser breakdowns for premixed combustion stabilization is optimized when laser pulse repetition rates below a certain frequency (e.g., 500 Hz at the conditions that ? is 1 and the base flow speed is 4.9 m/s) to maximize laser energy coupling and for improved anchoring of the flame base.     

Investigations of swirl flames in a gas turbine model combustor: II. Turbulence–chemistry interactions

The thermochemical states of three swirling CH₄/air diffusion flames, stabilized in a gas turbine model combustor, were investigated using laser Raman scattering. The flames were operated at different thermal powers and air/fuel ratios and exhibited different flame behavior with respect to flame instabilities. They had previously been characterized with respect to their flame structures, velocity fields, and mean values of temperature, major species concentrations, and mixture fraction. The single-pulse multispecies measurements presented in this article revealed very rapid mixing of fuel and air, accompanied by strong effects of turbulence–chemistry interactions in the form of local flame extinction and ignition delay. Flame stabilization is accomplished mainly by hot and relatively fuel-rich combustion products, which are transported back to the flame root within an inner recirculation zone. The flames are not attached to the fuel nozzle, and are stabilized approximately 10 mm above the fuel nozzle, where fuel and air are partially premixed before ignition. The mixing and reaction progress in this area are discussed in detail. The flames are short (<50 mm), especially that exhibiting thermoacoustic oscillations, and reach a thermochemical state close to adiabatic equilibrium at the flame tip. The main goals of this article are to outline results that yield deeper insight into the combustion of gas turbine flames and to establish an experimental database for the validation of numerical models.     

Droplet combustion in the presence of acoustic excitation

This experimental study focused on droplet combustion characteristics for various liquid fuels during exposure to external acoustical perturbations generated within an acoustic waveguide. The alternative liquid fuels include alcohols, aviation fuel (JP-8), and liquid synthetic fuel derived via the Fischer–Tropsch process. The study examined combustion during excitation conditions in which the droplet was situated in the vicinity of a pressure node (PN). In response to such acoustic excitation, the flame surrounding the droplet was observed to be deflected, on average, with an orientation depending on the droplet’s relative position with respect to the PN. Flame orientation was always found to be consistent with the sign of a theoretical bulk acoustic acceleration, analogous to a gravitational acceleration, acting on the burning system. Yet experimentally measured acoustic accelerations based on mean flame deflection differed quantitatively from that predicted by the theory. Phase-locked OH^∗ chemiluminescence imaging revealed temporal oscillations in flame standoff distance from the droplet as well as chemiluminescent intensity; these oscillations were especially pronounced when the droplets were situated close to the PN. Simultaneous imaging and pressure measurements enabled quantification of combustion-acoustic coupling via the Rayleigh index, and hence a more detailed understanding of dynamical phenomena associated with acoustically coupled condensed phase combustion processes.     

Premixed flame response to acoustic waves in a porous-walled chamber with surface mass injection

In order to study the internal coupling between flame dynamics and vortico-acoustic waves formed during solid propellant combustion, a numerical simulation of an idealized rocket combustion chamber is carried out. The chamber is modeled as a rectangular enclosure along whose porous walls a laminar mixture of premixed reactants is uniformly injected. The mathematical model is based on the conservation equations in two space dimensions. Full account is taken of variable thermo-physical properties and finite-rate chemical kinetics. Boundary conditions are specified using the method of characteristics that accommodates the transport of entropy, vorticity, and acoustic waves. The governing equations and associated boundary conditions are solved numerically using a preconditioning technique and a dual time-stepping integration procedure. For illustrative purposes, a propane-air mixture is injected through the chamber walls in an effort to emulate the flame evolution inside a solid-propellant rocket motor under laminar conditions. First, detailed flame structures under steady-state conditions are realized. Subsequently, traveling acoustic waves are imposed at the head end. Simulation results indicate that the oscillatory velocity exhibits a multidimensional structure caused by unsteady vorticity, pressure, and flame oscillations. Accordingly, the effects of laminar premixed flame oscillations are limited to a thin region above the burning surface. This region becomes thinner as the frequency of oscillations is increased. The flowfield outside the flame zone bears a striking resemblance to recent analytical solutions obtained in a geometrically similar chamber.     

Buoyancy effects on hydrogen diffusion flames confined in a small tube

In this work, buoyancy effects on hydrogen jet flames confined in a small tube without air co-flow were numerically investigated. The results show that the extinction limit of fuel velocity under buoyant condition is much lower than that without buoyancy. Moreover, hydrogen flames under buoyant condition attatch the nozzle exit for all fuel velocities investigated; however, the flames without buoyancy surround the lower wall at low fuel velocity. In addition, combustion is nearly complete in the presence of buoyancy, whereas the combustion efficiency under non-buoyant condition is below 45%. Furthermore, flame temperature under buoyant condition is much higher compared to the counterpart under non-buoyant condition at low and moderate fuel velocities. Analysis reveals that in the case without buoyancy, the negative gauge pressure in the annular space is unable to entrain sufficient air from the ambient. Hence, hydrogen has to diffuse downwards to sustain the flame and complete combustion is unrealizable.     

LIMIT OF EQUIVALENCE RATIO ON MIXING ENHANCEMENT NEAR THE TUBE EXIT IN A TONE EXCITED RICH FLAME

An experimental investigation has been made with the objective of studying the limit of equivalent ratio (ϕ) on mixing enhancement in a tone excited jet rich flame. The jet is pulsed by means of a loudspeaker-driven cavity and experiments are limited to very rich flames (ϕ>1⋅5). The excitation frequency is chosen for the resonant frequency identified as a pipe resonance due to acoustic excitation. Methane, propane and butane are used to examine the effect of mixture property on the limit of equivalence ratio. Mixing is always enhanced in a methane/air flame as the excitation intensity increases. In the case of propane/air and butane/air flames, mixing enhancement can be obtained only when the equivalence ratio lies in the range from a certain value (the equivalence ratio limit) to infinity (non-premixed flame), irrespective of mean mixture velocities. It is also found that the equivalence ratio limit is related to flame instability; the lower the Lewis number, the higher the equivalence ratio limit. As the excitation intensity increases, flame separation occurs below the equivalence ratio limit; an inner (premixed) flame is transformed into a cellular flame which then moves upstream, but the height of an outer (non-premixed) flame is not decreased. Acoustic pressure measurements using a microphone are made to quantify the oscillating velocity. The oscillating velocity amplitude at the cellular flame position is proportional only to mean mixture velocity regardless of fuel type. © 1997 by John Wiley & Sons, Ltd.     

Effect of flow field for colorless distributed combustion (CDC) for gas turbine combustion

Colorless distributed combustion (CDC) investigated here is focused on gas turbine combustion applications due to its significant benefits for, much reduced NOx emissions and noise reduction, and significantly improved pattern factor. CDC is characterized by distributed reaction zone of combustion which leads to uniform thermal field and avoidance of hot spot regions to provide significant improvement in pattern factor, lower sound levels and reduced NOx emission. Mixing between the combustion air and product gases to form hot and diluted oxidant prior to its mixing with the fuel is critical so that one must determine the most suitable mixing conditions to minimize the ignition delay. Spontaneous ignition of the fuel occurs to provide distributed reaction combustion conditions. The above requirements can be met with different configuration of fuel and air injections with carefully characterized flow field distribution within the combustion zone. This study examines four different sample configurations to achieve colorless distributed combustion conditions that reveal no visible color of the flame. They include a baseline diffusion flame configuration and three other configurations that provide conditions close to distributed combustion conditions. For all four modes same fuel and air injection diameters are used to examine the effect of flow field configuration on combustion characteristics. The results are compared from the four different configurations on flow field and fuel/air mixing using numerical simulations and with experiments using global flame signatures, exhaust emissions, acoustic signatures, and thermal field. Both numerical simulations and experiments are performed at a constant heat load of 25 kW, using methane as the fuel at atmospheric pressure using normal temperature air and fuel. Lower NOx and CO emissions, better thermal field uniformity, and lower acoustic levels have been observed when the flame approached CDC mode as compared to the baseline case of a diffusion flame. The reaction zone is observed to be uniformly distributed over the entire combustor volume when the visible flame signatures approached CDC mode.     

Experimental study of industrial gas turbine flames including quantification of pressure influence on flow field,fuel/air premixing and flame shape

A commercial swirl burner for industrial gas turbine combustors was equipped with an optically accessible combustion chamber and installed in a high-pressure test-rig. Several premixed natural gas/air flames at pressures between 3 and 6 bar and thermal powers of up to 1 MW were studied by using a variety of measurement techniques. These include particle image velocimetry (PIV) for the investigation of the flow field, one-dimensional laser Raman scattering for the determination of the joint probability density functions of major species concentrations, mixture fraction and temperature, planar laser induced fluorescence (PLIF) of OH for the visualization of the flame front, chemiluminescence measurements of OH^* for determining the lift-off height and size of the flame and acoustic recordings. The results give insights into important flame properties like the flow field structure, the premixing quality and the turbulence–flame interaction as well as their dependency on operating parameters like pressure, inflow velocity and equivalence ratio. The 1D Raman measurements yielded information about the gradients and variation of the mixture fraction and the quality of the fuel/air mixing, as well as the reaction progress. The OH PLIF images showed that the flame was located between the inflow of fresh gas and the recirculated combustion products. The flame front structures varied significantly with Reynolds number from wrinkled flame fronts to fragmented and strongly corrugated flame fronts. All results are combined in one database that can be used for the validation of numerical simulations.     

Numerical investigation of combustion characteristics for hydrogen mixed fuel in a can-type model of the gas turbine combustor

Numerical simulations are performed to analyze the combustion characteristics of propane fuel mixed with different amounts of hydrogen in a can-type combustor. The volume fraction of the hydrogen fuel varies from 0% to 100% in the fuel mixture. The results indicate that the hydrogen enrichment of the fuel significantly affects the flow structure, mixture fraction, and combustion characteristics. An increase in the volume fraction of hydrogen significantly affects the mean mixture fraction distribution, promotes combustion, and increases the flame temperature and the width of the flammable range within the combustor. Therefore, the degree of temperature uniformity at the outlet of the combustor increases with hydrogen enrichment, corresponding to an increase of 49.64% in the uniformity factor. The hydrogen enriched fuel can also reduce the emissions of CO and CO₂, owing to the reduced amount of carbonaceous fuel.     

Flame-vortex interaction and mixing behaviors of turbulent non-premixed jet flames under acoustic forcing

This study examines the effect of acoustic excitation using forced coaxial air on the flame characteristics of turbulent hydrogen non-premixed flames. A resonance frequency was selected to acoustically excite the coaxial air jet due to its ability to effectively amplify the acoustic amplitude and reduce flame length and NO_x emissions. Acoustic excitation causes the flame length to decrease by 15% and consequently, a 25% reduction in EINO_x is achieved, compared to coaxial air flames without acoustic excitation at the same coaxial air to fuel velocity ratio. Moreover, acoustic excitation induces periodical fluctuation of the coaxial air velocity, thus resulting in slight fluctuation of the fuel velocity. From phase-lock PIV and OH PLIF measurement, the local flow properties at the flame surface were investigated under acoustic forcing. During flame-vortex interaction in the near field region, the entrainment velocity and the flame surface area increased locally near the vortex. This increase in flame surface area and entrainment velocity is believed to be a crucial factor in reducing flame length and NO_x emission in coaxial jet flames with acoustic excitation. Local flame extinction occurred frequently when subjected to an excessive strain rate, indicating that intense mass transfer of fuel and air occurs radially inward at the flame surface.     

甲烷/空气典型混合模式在约束空间内的燃烧特性研究

燃料质量浓度分布在一定程度上影响混合气体的燃烧效率,能使燃气充分混合的同轴射流、旋片同轴、轴切结合、切向旋流等典型混合模式在航空发动机、燃气轮机及火箭发动机等先进燃烧技术应用中较为常见。因此,设计了甲烷/空气部分预混的燃烧实验装置,较为系统地实验研究了旋流数和轴向流速对混合气体在约束空间燃烧室内燃烧特性的影响。结果表明：对于有中心射流的混合结构,燃气轴向流速较低时产生黄色火焰,增大轴向流速,黄色火焰转为蓝色湍流火焰,且温度分布趋于均匀;纯切向旋流燃烧器的掺混效果较好,受燃气轴向流速的影响小,火焰结构稳定,均为蓝色火焰,温度轴/径向分布均匀且趋势一致,同当量比下燃烧产物中的污染物体积分数最小。     

Numerical study on supersonic combustion with cavity-based fuel injection

The present study describes the numerical investigations concerning the combustion enhancement when a cavity is used for the hydrogen fuel injection through a transverse slot nozzle into a supersonic hot air stream. The cavity is of interest because recirculation flow in cavity would provide a stable flame holding while enhancing the rate of mixing or combustion. Several inclined cavities with various aft wall angle, offset ratio and length are evaluated for reactive flow characteristics. The cavity effect is discussed from a viewpoint of total pressure loss and combustion efficiency. The combustor with cavity is found to enhance mixing and combustion while increasing the pressure loss, compared with the case without cavity. But it is noted that there exists an appropriate length of cavity regarding the combustion efficiency and total pressure loss.     

启喷压力对直喷式柴油机燃烧的影响

作者介绍了启喷压力对采用低惯量喷油器的直喷式柴油机性能影响的研究结果。通过采用激光全息和高速摄影等技术对燃油喷雾与气缸内燃烧过程的观察与分析，揭示启喷压力与燃油喷雾，混合气形成，扩散燃烧间的重要关系。根据研究结果，提出了直喷式柴油机采用低惯量喷油器启喷压力的一般方法。     

Influence of coaxial air jet on mass diffusion of hydrogen jet injected through lobe-injector at supersonic flow

The technique of fuel injection in the combustion chamber is crucial for increasing the performance of hypersonic vehicles. This study tries to investigate the mechanism of fuel injection and distribution when fuel and air are injected through coaxial lobe injectors. The main attention of this work is to present the mechanism of fuel mixing of transverse jet injected from various lobe injectors. Comparison of coaxial gets (air and fuel jet) with equivalent simple jet (fuel without air jet) is done to achieve an efficient model for the combustion chamber. In this work, finite-volume is used to simulate and study of fuel injection performance of a transverse hydrogen jet in different lobe injectors. 3-D flow visualizations are done to reveal the mechanism of the fuel penetration and streamline pattern for introduced models. Strength of circulation and fuel mixing efficiency are also investigated in the present work for 2-, 3-, and 4-lobe nozzles. Our outcomes indicate that the mixing performance of coaxial air and fuel jet injected through the 3-lobe nozzle is about 25% better than other nozzle types. Our findings confirm that injection of air jet through the core of the lobe nozzle increases fuel mixing up to 200% at the combustion chamber.     

Interaction Effects During Combustion of Linear Arrays of Gaseous Fuel Pockets

The numerical solution for the combustion of an infinite linear array of dense fuel gas pockets in a quiescent ambient is discussed in the present work. Fuel mass, flame, and gas pocket shape behaviors are analyzed in order to quantify the interference effects. The combustion process is considered isobaric. The model is based on mass, momentum, energy, and species conservation equations and considers the simple chemical reaction mechanism and ideal gas behavior. Coupling potentials are used in the solution of the energy and species conservations equations. The thermophysical properties, except the density, are assumed constant. The finite-volume method is used for the numerical solution, using a generalized system of coordinates and a nonstaggered grid. The SIMPLEC algorithm is applied to solve the modified pressure-velocity coupling. The flame and gas pocket morphological evolutions as well as the transient fuel consumption and the flame position behaviors are presented. The effects of the initial fuel and oxidant temperatures, of the interpocket distance, of the stoichiometric condition, of the heat of reaction, and of the mixture thermophysical properties on the combustion process are considered. Results show that the interactive effects on gas pocket linear array combustion are relevant only for small interpocket distances. Results also show flame shape temporal evolutions that indicate an oscillatory behavior dependent on the gas pocket burning conditions.     

Tracer-LIF diagnostics: quantitative measurement of fuel concentration,temperature and fuel/air ratio in practical combustion systems

The safe, clean, and reliable operation of combustion devices depends to a large degree on the exact control of the fuel/air mixing process prior to ignition. Therefore, quantitative measurement techniques that characterize the state of the fresh gas mixture are crucial in modern combustion science and engineering. This paper presents the fundamental concepts for how to devise and apply quantitative measurement techniques for studies of fuel concentration, temperature, and fuel/air ratio in practical combustion systems, with some emphasis on internal combustion engines. The paper does not attempt to provide a full literature review of quantitative imaging diagnostics for practical combustion devices; rather it focuses on explaining the concepts and illustrating these with selected examples. These examples focus on application to primarily gaseous situations.The photophysics of organic molecules is presented in an overview followed by discussions on specific details of the temperature-, pressure-, and mixture-dependence of the laser-induced fluorescence strength of aliphatic ketones, like acetone and 3-pentanone, and toluene. Models that describe the fluorescence are discussed and evaluated with respect to their functionality. Examples for quantitative applications are categorized in order of increased complexity. These examples include simple mixing experiments under isothermal and isobaric conditions, fuel/air mixing in engines, temperature measurements, and mixing studies where fuel and oxygen concentrations vary.A brief summary is given on measurements of fuel concentrations in multiphase systems, such as laser‐induced exciplex spectroscopy. Potentially adverse effects that added tracers might have on mixture formation, combustion, and the faithful representation of the base fuel distribution are discussed. Finally, a brief section describes alternative techniques to tracer-based measurements that allow studies of fuel/air mixing processes in practical devices.The paper concludes with a section that addresses key issues that remain as challenges for continued research towards the improvement of quantitative, tracer-based LIF measurements.     

用光学纹影摄影术观察分析定容燃烧室内氢燃料的燃烧过程

论述了采用纹影摄影术和高速摄影法观察分析氢气和空气预混合燃料在定容燃烧室内的火花点火燃烧过程,定性地分析了预混合氢气燃料的火焰形态和变化过程,以及燃烧室内的初始压力和空燃比对火焰传播速度及其燃烧压力的影响,通过采用纹影摄影术方法,初步揭示了预混合氢气燃料在定容燃烧室内燃烧时火焰初期紊流产生的机理,以及由开始的层流状火焰发展到湍流状火焰的过程,研究结果表明,预混合氢气燃料燃烧的火焰传播速度及燃烧压力明显地受燃烧室内的初始压力和空燃比的影响。