Thermoacoustic oscillations in ducted domestic heating systems

Thermoacoustic oscillation is a significant problem in ducted domestic heating systems fitted with fully premixing burners as well as in combustion systems of industrial scale. This paper reports experiments using a burner closely representative of practical designs in which high-intensity free-field sound has been used to simulate the feedback which may be excited naturally in installations with different acoustic properties. This experimental approach facilitates study of the non-linear response of the burner/flame sub-system to feedback of different frequencies and direct measurement of the acoustic impedance of the burner under operational conditions. A function proportional to the Rayleigh integral has been evaluated directly from the experimental results. It is found that the head of this type of burner divides the typical installation into a quarter-wave tube and a Helmholtz resonator which must couple unstably for thermoacoustic oscillations to be autoexciting. It is further found that there is a strong interaction between the acoustic field and the mean flow which modifies significantly the behaviour of the burner as a Helmholtz resonator and thus the stability of the whole system.     

Analytic prediction of unconfined boundary layer flashback limits in premixed hydrogen–air flames

Flame flashback is a major challenge in premixed combustion. Hence, the prediction of the minimum flow velocity to prevent boundary layer flashback is of high technical interest. This paper presents an analytic approach to predicting boundary layer flashback limits for channel and tube burners. The model reflects the experimentally observed flashback mechanism and consists of a local and global analysis. Based on the local analysis, the flow velocity at flashback initiation is obtained depending on flame angle and local turbulent burning velocity. The local turbulent burning velocity is calculated in accordance with a predictive model for boundary layer flashback limits of duct-confined flames presented by the authors in an earlier publication. This ensures consistency of both models. The flame angle of the stable flame near flashback conditions can be obtained by various methods. In this study, an approach based on global mass conservation is applied and is validated using Mie-scattering images from a channel burner test rig at ambient conditions. The predicted flashback limits are compared to experimental results and to literature data from preheated tube burner experiments. Finally, a method for including the effect of burner exit temperature is demonstrated and used to explain the discrepancies in flashback limits obtained from different burner configurations reported in the literature.     

Violent folding of a flame front in a flame-acoustic resonance

The first direct numerical simulations of violent flame folding because of the flame-acoustic resonance are performed. Flame propagates in a tube from an open end to a closed one. Acoustic amplitude becomes extremely large when the acoustic mode between the flame and the closed tube end comes in resonance with intrinsic flame oscillations. The acoustic oscillations produce an effective acceleration field at the flame front leading to a strong Rayleigh-Taylor instability during every second half period of the oscillations. The Rayleigh-Taylor instability makes the flame front strongly corrugated with elongated jets of heavy fuel mixture penetrating the burnt gas and even with pockets of unburned matter separated from the flame front.     

Influence of Burner Geometry on Heat Transfer Characteristics of Methane/Air Flame Impinging on Flat Surface

An experimental study has been conducted to find the heat transfer characteristics of methane/air flames impinging normally to a flat surface using different burner geometries. The burners used were of nozzle, tube, and orifice type each with a diameter of 10 mm. Due to different exit velocity profiles, the flame structures were different in each case. Because of nearly flat velocity profile, the flame spread was more in case of orifice and nozzle burners as compared to tube burner. Effects of varying the value of Reynolds number (600–2500), equivalence ratio (0.8–1.5) and dimensionless separation distance (0.7–8) on heat transfer characteristics on the flat plate have been investigated for the tube burner. Different flame shapes were observed for different impingement conditions. It has been observed that the heat transfer characteristics were intimately related to flame shapes. Heat transfer characteristics were discussed for the cases when the flame inner reaction cone was far away, just touched, and was intercepted by the plate. Negative heat fluxes at the stagnation point were observed when the inner reaction cone was intercepted by the plate due to impingement of cool un-burnt mixture directly on the surface. Different heat transfer characteristics were observed for different burner geometries with similar operating conditions. In case of tube burner, the maximum heat flux is around the stagnation point and decay is faster in the radial direction. In case of nozzle and orifice burner, the heat transfer distribution is more uniform over the surface.     

Combustion dynamics of inverted conical flames

An inverted conical flame anchored on a central bluff-body in an unconfined burner configuration features a distinctive acoustic response. This configuration typifies more complex situations in which the thermo-acoustic instability is driven by the interaction of a flame with a convective vorticity mode. The axisymmetric geometry investigated in this article features a shear region between the reactive jet and the surrounding atmosphere. It exhibits self-sustained oscillations for certain operating conditions involving a powerful flame collapse phenomenon with sudden annihilation of flame surface area. This is caused by a strong interaction between the flame and vortices created in the outer jet shear layer, a process which determines the amplitude of heat release fluctuation and its time delay with respect to incident velocity perturbations. This process also generates an acoustic field that excites the burner and synchronizes the vortex shedding mechanism. The transfer functions between the velocity signal at the burner outlet and heat release are obtained experimentally for a set of flow velocities fluctuations levels. It is found that heat release fluctuations are a strong function of the incoming velocity perturbation amplitude and that the time delay between these two quantities is mainly determined by the convection of the large scale vortices formed in the jet shear layer. A model is formulated, which suitably describes the observed instabilities.     

Effect of Le on criteria of transition to secondary acoustic instability of downward-propagating flame in a tube with controlled curvature induced by external laser

An experimental study is conducted to investigate the effect of Le on the transition to secondary acoustic instability when the curvature of the flame front in a tube is induced and controlled by using external laser irradiation. Once a downward-propagating flame in the primary acoustic instability region is exposed to a specific laser irradiation condition, the flame is transferred to the secondary acoustic instability region. The transition limit is decreased, that is, transition occurs is an easier manner, with increasing laser power input. While the flame propagates with increasing laser irradiation, the flame first exhibits a convex curvature owing to laser irradiation and then a concave structure is formed owing to buoyancy-induced flow. Two types of transition behavior caused by the concave structure and the convex structure are observed. The conflicting thermal-diffusive effect depending on Le leads to the differing transition behaviors. Based on an evaluation of the flame stretch effect attributed to the flame front curvature, it is confirmed that the Lewis number effect influences the transition criteria.     

Experimental investigation on burner lip thickness effect on the liftoff and blowout velocities of jet diffusion flame

This experimental study addresses issue on the effect of burner lip thickness on the liftoff and blowout velocities of jet diffusion flame discharging into quiescent air. Burner tubes of two kinds of inner diameter (2 and 3 mm) with a wide range of lip thicknesses (0.25–16.5 mm) are implemented with methane or propane jet diffusion flame, respectively. The results show that the burner lip thickness has a profound effect on flame liftoff velocities, especially the blowout velocities. With the increase of the lip thickness, the blowout velocities firstly increase, then decrease and lastly remain unchanged. Specifically, the blowout velocities of 2 and 3 mm inner diameters tubes reach the maximum values when the corresponding burner lip thicknesses are 2 and 1.5 mm, respectively. In addition, compared with free (unconfined) jet diffusion flame, the jet confinement results in slight reductions of flame liftoff and blowout velocities. The existence range of lifted flame issuing from larger diameter burner tube is wider than that of smaller diameter burner tube. The existence range of propane lifted flame is wider than that of methane.     

Three-dimensional visualization of diffusion flame shapes under acoustic excitation using stereoscopic imaging and reconstruction technique

Stereoscopic imaging and reconstruction technique is introduced to reconstruct the flame structures that are subject to acoustic excitation. The laminar diffusion flame under investigation was created in a cylindrical tube and excited by a loudspeaker. Stereo images were taken at a shutter speed of 1/10000^th second using a ‘stereo camera’ and the depth cue of the flame structures along the camera viewing direction were then computed using machine vision methodology. By visualizing the computed three-dimensional flame models, as well as judging the corresponding attribute such as surface gradient, the understanding of the flame and acoustic wave interaction could be improved.     

Exploring a critical diameter for thermo-acoustic instability of downward propagating flames in tubes

Thermo-acoustic oscillations are observed when a flame ignited at open end of a tube propagates towards the closed end due to interaction between unsteady heat release rate fluctuations from flame and acoustic fluctuations. In our past work, it was found that thermo-acoustic instability increases with decreasing diameter from 7.0 cm to 3.0 cm. A recent study in flame propagation in Hele–Shaw cells showed that thermo-acoustic instability is not observed for plate separation less than or equal to 0.4 cm. Thermoacoustic instabilities cannot be observed in very narrow tubes due to excessive damping from the wall. This opens up the possibility of a critical diameter where thermo-acoustic instability would be maximum. In this work we perform flame propagation experiments with diameter of combustion tube in the range 0.5 cm to 3 cm for a fixed length of 70.2 cm. It was found that thermo-acoustic parametric instability begins at lowest laminar burning velocity when the diameter is around 1.0 cm. This diameter is termed as critical diameter. Critical diameter is found to be independent of Lewis number of mixtures. Existence of a critical diameter is thus proved experimentally. Growth rates of primary instability increase with decreasing diameter and show a maximum around the critical diameter and decrease with further decrease in tube diameter. But, growth rates of secondary instability as well as maximum pressure fluctuation amplitude decreases continuously with decreasing diameter. Mechanisms responsible for these observations and existence of a critical diameter are clarified.     

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.     

Atomic Absorption Spectrophotometry with the Absorption Tube Technique

A device to increase the sensitivity of the atomic absorption spectrophotometric measurement by elongating the absorption path length has originally been developed by Fuwa and Vallee and the present authors called it “absorption tube technique”. Although the technique has been fully described in comparison with the flame photometry, and the behavior of the elements and the reactions in the tube were investigated for several elements, application of a newly designed ring burner to this technique and the detailed examination of the operating conditions have never beer, published.     

微重力环境下V型层流预混火焰锋面不稳定性分析

本章试图寻求描述火焰锋面动态特性的方法，以解释微重力环境下出现的V型火焰锋面的涟漪现象。采用线性稳定性理论从经典的G方程中导出了描述火焰锋面动态结构的一阶偏微分方程。采用该方程计算了声波扰动后，不同时刻的V型火焰锋面的动态结构．对于谐波扰动，其频率与波数的关系是分析固有火焰锋面不稳定性的基础。因此，微重力环境下V型火焰锋面的不稳定性可能是声波与谐波相耦合的结果。     

On the thermo-acoustic Fant equation

A ‘reduced complexity’ equation is derived to investigate combustion instabilities of a Rijke burner. The equation is nonlinear and furnishes limit cycle solutions for finite amplitude burner modes. It is a generalisation to combustion flows of the Fant equation used to investigate the production of voiced speech by unsteady throttling of flow by the vocal folds [G. Fant, Acoustic Theory of Speech Production. Mouton, The Hague, 1960]. In the thermo-acoustic problem the throttling occurs at the flame holder. The Fant equation governs the unsteady volume flow past the flame holder which, in turn, determines the acoustics of the entire system. The equation includes a fully determinate part that depends on the geometry of the flame holder and the thermo-acoustic system, and terms defined by integrals involving thermo-aerodynamic sources, such as a flame and vortex sound sources. These integrals provide a clear indication of what must be known about the flow to obtain a proper understanding of the dynamics of the thermo-acoustic system. Illustrative numerical results are presented for the linearised equation. This governs the growth rates of the natural acoustic modes, determined by system geometry, boundary conditions and mean temperature distribution, which are excited into instability by unsteady heat release from the flame and damped by large scale vorticity production and radiation losses into the environment. In addition, the equation supplies information about the ‘combustion modes’ excited by the local time-delay feedback dynamics of the flame.     

Experimental evaluation of DC electric field effect on the thermoacoustic behaviour of flat premixed flames

One promising approach to eliminate thermoacoustic instabilities in combustion appliances is the use of adaptive control of the flame/burner acoustic transfer function (TF). Application of a DC electric field (EF) as a spatially distributed, easily and quickly adjustable and low-energy method to affect the flame behaviour may be considered as a possible actuation method to control the flame TF. Experimental evaluation of such a possibility is the main goal of the present study. The effect of a DC EF on the acoustic TF of premixed flat burner-surface stabilized flames is studied systematically as a function of the following parameters: flow velocity, equivalence ratio, applied voltage and burner geometry. It is established that the response of the flame TF on the DC EF can be characterized as a TF shift towards higher frequencies. The mechanism of TF alteration is related to the decrease of the flame stand-off distance and the related increase of the burner deck surface temperature. From a practical point of view, the efficiency of the EF control of the flat flame TF is restricted to a relatively narrow frequency range around the position of the TF resonance peak. To get insight into the physics of the EF–flame interaction, the method of [1] to measure the EF effect on the adiabatic flame speed is improved and the measurement range is extended. The new measurements allow a revision of previous results and allow an explanation for the ambiguity in the old measurements.     

Flashback-induced flame shape transition in a two-stage LPP aeronautical combustor

Large-Eddy Simulations were performed to study the flashback-induced flame shape transition of a lean premixed M flame in a staged liquid-fuelled aeronautical lean-burner, as observed experimentally. The BIMER combustor is a Lean Premixed Prevapourised (LPP) burner composed of two stages, each with its own injector and swirler: the main outer stage, called multipoint, uses jet-in-crossflow injection to achieve the LPP regime, while the central stage, called pilot, uses a pressure swirl injector to create a hollow cone spray to stabilise the flame. During LPP operation, this M flame presents a strong acoustic activity, promoting a periodic flashback of its leading edge. When, aiming to stabilise the flame, the pilot injection is increased and the multipoint injection decreased, the oscillating leading edge (due to the longitudinal acoustic perturbations) attaches to the pilot spray, changing the flame into a Tulip shape. Two phenomena were identified as being the most relevant causes of this flame shape transition. First, the leading edge position and the thermoacoustic instability amplitude are directly linked to the combustion chamber final temperature. The higher the temperature in the chamber, the more upstream the leading edge stabilises, and the higher the acoustic oscillation amplitude, both increasing the risk of a successful flashback. Second, the injection regime with high pilot injection allows the leading edge to attach to the pilot spray, as the flame only transitions when the pilot spray is sufficiently high. The higher the pilot fuel flow, the higher the amount of fuel sprayed in the critical region where the flame might attach for a transition to the Tulip shape. Therefore, as the change in injection regime is the main mechanism lean staged burners use to reduce emissions while increasing operability, this works shows that an M flame is unsuitable to such burners with similar aerodynamic topology and properties.     

Numerical study of premixed flame dynamics in a closed tube: Effect of wall boundary condition

Numerical simulations were conducted to study the dynamics of premixed flames propagating in a closed tube by solving the fully compressible reactive Navier–Stokes equations using a high-order numerical method on a dynamically adapting grid. A simplified chemical-diffusive model was used to describe the reactions and energy release in a stoichiometric hydrogen-air mixture. The influence of wall boundary condition on the flame dynamics was explored by considering three different types of condition on the walls: adiabatic no-slip, adiabatic free-slip, and isothermal. The calculations show that the wall boundary condition has a significant effect on the generation and amplification of pressure waves and consequently on the flame dynamics. In the early stages of flame propagation, the flame behaves in a similar manner for different boundary conditions, that is, the flame develops a tulip shape that further evolves into a distorted tulip flame (DTF) through Rayleigh-Taylor instability arising from acoustic-flame interaction. Significant differences, however, arise after DTF formation in the late stages, especially when the primary acoustic wave is amplified to form a shock wave in the adiabatic free-slip and isothermal cases. The shock-flame interactions facilitate the formation of a series of increasingly corrugated flames by triggering the Richtmyer–Meshkov instabilities. The way how the lateral flame fronts touch the tube sidewalls to generate the primary acoustics and the heat conduction through the tube sidewalls play an important role in the generation and amplification of the pressure waves.     

On nonlinear model equations for the response of premixed flames to acoustic like accelerations

The response of a dynamical flame model to imposed acoustic accelerations is studied analytically and numerically. Through linear stability analyses, two analytical approximations for the primary and the parametric stability boundaries are found. The approximation for the primary instability boundary is accurate for any periodic accelerations, in the limit of large acoustic frequencies. The critical acoustic amplitude u _a for Landau–Darrieus instability suppression is identified and found to depend only on the density contrast and the shape of the periodic acoustic stimuli. The proposed model evolution equation is next integrated numerically with various imposed acoustic accelerations; the primary and parametric flame responses are identified. It is shown analytically and numerically that in the presence of a fully developed, yet weakened by acoustics, Landau–Darrieus (or primary) instability the wrinkle amplitude and the mean flame speed oscillate at the same frequency as the acoustic stimuli; the threshold for suppression of primary instability by acoustic forcing is determined exactly. Increasing the acoustic amplitude allows the flame to respond parametrically to the acoustics. This response is characterised by troughs and crests interchanging their roles while the mean flame speed again oscillates with the same frequency as the acoustic stimuli and at twice that of wrinkle amplitude oscillations.     

Combustion driven oscillations

Combustion driven oscillations can occur when a turbulent flame is enclosed in a tube or cavity. Interaction between heat fluctuations and the internal standing wave field at one of the natural frequencies of the air column produces strong organ pipe tones. The sound power emitted by this thermal-acoustic interaction depends on the impedance either side of the combustion zone and on a transfer function defining the response of the flame to sound wave disturbances. If this power exceeds the rate at which energy is dissipated at the cavity boundaries then there is a growth of the internal pressure field and an increase in the radiated sound. Plane wave theory is used to calculate the flame transfer function and adjacent impedances for a simple gas fired tube assembly. The predicted instability frequencies are then compared with experimental data. The results indicate that the flame transfer function plays a dominant role in determining the acoustic stability of the cavity and that insufficient data is available for accurately predicting unsteady flame front behaviour.     

Flame stabilization with a tubular flame

A new technique to stabilize a flame in a high-velocity stream with use of a tubular flame has been proposed. To elucidate the validity of this technique, an experiment has been conducted by mounting a tubular flame burner on the nozzle. Flame stability limits and temperature distributions around the burner port have been determined, and experiments have been extended to the ducted combustion to measure pressure fluctuations and to analyze the burned gases. Results show that the tubular flame can successfully stabilize the main flame up to 130 m/s, which is the upper limit of the present supply facility. The main flame is well anchored at the exit of the nozzle, and the tubular flame efficiently supplies heat and radicals to the main flame. In the ducted combustion, the pressure fluctuations are reduced significantly. The exhaust gas analyses, however, indicate that an almost chemical equilibrium condition can be achieved at 50 m/s, but not at 90 and 130 m/s. Since the energy input relative to the main flame is just 6.1% at 130 m/s, the present tubular burner is not enough to burn all the unburned gas completely at high velocities, although the main flame can be anchored. The slit length and/or the slit width of the tubular flame burner should be larger to overcome this shortage. From the above results, it is concluded that the tubular flame has a potential for stabilizing a flame in a high-speed stream.     

壁面附近火焰OH自由基行为的PLIF研究

利用OH-PLIF测试技术在狭缝燃烧器上考察了不同壁面条件对平行平板间甲烷/空气预混火焰壁面附近处OH浓度分布的影响.实验结果表明,随着壁面间距的减小,狭缝火焰出现不稳定传播现象,不同壁面温度下不稳定传播现象不同.壁面附近OH浓度越高,熄火间距越小.壁面附近的OH浓度是决定熄火间距的关键因素,而火焰的OH浓度峰值只表示...