Strategy to diagnose ultra-lean (ϕ < 0.6) premixed flames by acetone-OH simultaneous PLIF with one-laser and one-detector combination

Abstract  

A strategy to diagnose ultra-lean flames utilizing the “acetone-OH simultaneous PLIF” concept via a one-laser and one-detector combination system is presented. The main aim of the present work is to overcome difficulties encountered in our previous studies; namely, that the seeding amount of acetone used for visualization purposes must be sufficiently small in order to avoid its effect on flame structure (at least <5% of fuel), while clear imaging also must be accomplished under such conditions. For this purpose, several important revisions have been made: (1) the 266-nm excitation line has been added to improve the fluorescence from acetone, (2) a dual-peak band-pass filter has been introduced instead of conventional blue filter and (3) controllability of fine acetone seeding has been improved. The effects of these changes on flame imaging are also investigated. Clear visualization of the flame zone of a very lean premixed flame, of which the limiting equivalence ratio is below 0.6, has been successfully achieved for the first time with one-laser and one-detector system. The time-dependent, near-extinction flame behavior is also clearly imaged, suggesting that this method could utilize to investigate the flame extinction study.     

A structural study of premixed hydrogen-air cellular tubular flames

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

Temperature measurement of a premixed radially symmetric methane flame jet using the Mach-Zehnder Interferometry

The temperature field of a premixed methane symmetric laminar flame jet is visualized by studying the interferograms of the flame, using the Mach-Zehnder Interferometry. Two kinds of oxidizers are chosen for combustion: industrially pure oxygen and oxygen-enriched air. The flame is chosen to be both lean, and rich. For the lean oxygen-enriched combustion (OEC), the equivalence ratio was held constant at 0.5, and the oxygen enrichment was adjusted to 0.5 and 0.6, and for rich OEC, equivalence ratio is chosen to be 1.2 while the oxygen enrichment was 0.7 and 0.8. For methane/oxygen combustion, the equivalence ratio varied from 0.35 to 0.55 for the lean flame, and 1.3 and 1.7 for the rich flame. Attempt was made to keep the Reynolds number unchanged at 500, for OEC, and 1000, for methane/oxygen flame. In the present study a non-contact method is successfully developed to measure the temperature field of a premixed radially symmetric laminar methane flame jet. The effect of oxygen enrichment and equivalence ratio on temperature field is also investigated and depicted.     

Vortical dynamics in the wake of water strider locomotion

Abstract  

Previous studies reported that the hydrodynamic propulsion of the water strider also results from transferring momentum to the underlying fluid through hemispherical dipolar vortices shed by its driving legs. However, there are no accuracy experimental measurements of these vortical structures to prove the mechanics of vortical propulsion. Here, we reveal the vortical structures by reporting the simultaneous measurements of the water strider’s motion and the fluid velocity field with the high-speed PIV, and proposing a new method of calculating the vortex kinetic energy and vortex momentum. We found that the asymmetrical vortical structure in each dipolar vortex, generated by one driving stroke, propels the water strider forward, and the outer elliptic vortex is weaker than the inner circular vortex. The movement of the dipolar vortex is divided into two stages: (1) translating backward and (2) return curving. In this way, the water strider obtains the maximum velocity with minimal consumption of energy. The fluid vortical momentum, generated by the driving stroke, accounts for about 64–90% of the water strider’s momentum.     

Laser induced plasma spectroscopy for local equivalence ratio measurements in an oscillating combustion environment

Equivalence ratios measured with a laser induced plasma spectroscopy (LIPS, also referred as LIBS) are reported in two different setups. First, a small premixed turbulent burner is used to address fundamental issues concerning the LIPS technique. It is shown that hydrogen excitation within the created plasma is the key parameter to measure in order to retrieve correctly equivalence ratio measurements. Results compared with a spark energy classification strategy show better results with excitation classification, as variations in ratio between the different lines come not only from gaseous concentration but also from plasma’s characteristics. Using spectra from 450 to 800 nm allows the determination of two independent emission ratios to improve single shot accuracy. The developed approach is afterwards applied to phase-locked measurements of equivalence ratio in a lean premixed combustor, for which strong thermo-acoustics oscillations exist. This combustor runs with methane-air, preheated at 700 K and with a typical equivalence ratio of 0.50, for which the sound pressure levels of the oscillations are 170 dB. Measurements at the inlet of the combustor reveal strong correlations between fluctuations of the incoming stoichiometry and pressure fluctuations. It is shown that stoichiometry changes within one oscillating cycle of about 3%. Those changes are crucial for the flame dynamics as dealing with very lean mixtures.     

The effect of hydrogen addition on flammability limit and NOx emission in ultra-lean counterflow CH4/air premixed flames

The effect of hydrogen addition to ultra lean counterflow CH₄/air premixed flames on the extinction limits and the characteristics of NO_x emission was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. The results show that the addition of hydrogen can significantly enlarge the flammable region and extend the flammability limit to lower equivalence ratios. If the equivalence ratio is kept constant, the addition of hydrogen increases the emission of NO in a flame due to the enhancement in the rate of the NNH or N₂O intermediate NO formation routes. The addition of hydrogen causes a monotonic decrease in the formation of NO₂ and N₂O, except flames near the extinction limits, where the emission of NO₂ and N₂O first increases, and then decreases with the increase in the fraction of hydrogen. Overall, hydrogen enrichment technology allows stable combustion under ultra lean conditions, resulting in significant CO₂ and NO emission reduction.     

A numerical study on the effect of CO addition on extinction limits and NO x formation in lean counterflow CH4/air premixed flames

The effect of CO addition on extinction and NO _x formation in lean premixed counterflow CH₄/air flames was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. A method that gradually switched off the initial reactions of NO formation from different routes was used to analyse the variation of NO formation mechanism. The results indicate that the addition of certain amount of CO increases the strain extinction limits and reduces the radiation extinction limits. As a result, the lean flammability limit of CH₄/air premixed flame is extended to leaner side by the addition of CO. The formation of NO in a flame is increased with the addition of CO at a constant equivalence ratio. For an ultra-lean flame, the increase in the formation of NO is mainly because of the increase in the contribution from the NNH intermediate route, while for a near stoichiometric flame, this increase is mainly attributable to the rise in the contribution from the thermal route. With the fraction of added CO being gradually increased, the formation of NO₂ in a flame first decreases and then increases at a given equivalence ratio. The addition of CO reduces the formation N₂O in an ultra-lean flame, while affects little on the formation of N₂O in a near stoichiometric flame.     

Dynamic properties of combustion instability in a lean premixed gas-turbine combustor

We experimentally investigate the dynamic behavior of the combustion instability in a lean premixed gas-turbine combustor from the viewpoint of nonlinear dynamics. A nonlinear time series analysis in combination with a surrogate data method clearly reveals that as the equivalence ratio increases, the dynamic behavior of the combustion instability undergoes a significant transition from stochastic fluctuation to periodic oscillation through low-dimensional chaotic oscillation. We also show that a nonlinear forecasting method is useful for predicting the short-term dynamic behavior of the combustion instability in a lean premixed gas-turbine combustor, which has not been addressed in the fields of combustion science and physics.     

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

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

Flame front analysis of high-pressure turbulent lean premixed methane–air flames

An experimental study on lean turbulent premixed methane–air flames at high pressure is conducted by using a turbulent Bunsen flame configuration. A single equivalence ratio flame at Φ = 0.6 is explored for pressures ranging from atmospheric pressure to 0.9 MPa. LDA measurements of the cold flow indicate that turbulence intensities and the integral length scale are not sensitive to pressure. Due to the decreased kinematic viscosity with increasing pressure, the turbulent Reynolds numbers increase, and isotropic turbulence scaling relations indicate a large decrease of the smallest turbulence scales. Available experimental results and PREMIX code computations indicate a decrease in laminar flame propagation velocities with increasing pressure, essentially between the atmospheric pressure and 0.5 MPa. The u′/S_L ratio increases therefore accordingly. Instantaneous flame images are obtained by Mie scattering tomography. The images and their analysis show that pressure increase generates small scale flame structures. In an attempt to generalize these results, the variance of the flamelet curvatures, the standard deviation of the flamelet orientation angle, and the flamelet crossing lengths have been plotted against which is proportional to the ratio between the integral and Taylor length scales, and which increases with pressure. These three parameters vary linearly with the ratio between large and small turbulence scales and clearly indicate the strong effect of this parameter on premixed turbulent flame dynamics and structure. An obvious consequence is the increase in flame surface density and hence burning rate with pressure, as confirmed by its direct determination from 2D tomographic images.     

Temperature measurement of axisymmetric partially premixed methane/air flame in a co-annular burner using Mach–Zehnder interferometry

In this paper partially premixed laminar methane/air co-flow flame is studied experimentally. Methane–air flame is established on an axisymmetric co-annular burner. The fuel-air jet flows from the central tube while the secondary air flows from the region between the inner and the outer tube. The aim is to investigate the flame characteristics for methane/air axisymmetric partially premixed flame using Mach–Zehnder interferometry. Different equivalence ratios (φ=1.4–2.2) and Reynolds numbers (Re=100–1200) are considered in the study. Flame generic visible appearance and the corresponding fringe map structures are also investigated. It is seen that the fringe maps are poorly influenced by equivalence ratio variations at constant Reynolds number but are significantly affected by Reynolds number variations in constant equivalence ratio. Temperatures obtained from optical techniques are compared with those obtained from thermocouples and good agreement is observed. It is concluded that the effect of Reynolds number increment on maximum flame temperature is negligible while equivalence ratio reduction increases maximum flame temperature substantially.     

Premixed flame response to equivalence ratio perturbations

This paper studies the heat-release oscillation response of premixed flames to oscillations in reactant stream fuel/air ratio. Prior analyses have studied this problem in the linear regime and have shown that heat release dynamics are controlled by the superposition of three processes: flame speed, heat of reaction, and flame surface area oscillations. Each contribution has somewhat different dynamics, leading to complex frequency and mean fuel/air ratio dependencies. The present work extends these analyses to include stretch and non quasi-steady effects on the linear flame dynamics, as well as analysis of nonlinearities in flame response characteristics. Because the flame response is controlled by a superposition of multiple processes, each with a highly nonlinear dependence upon fuel/air ratio, the results are quite rich and the key nonlinearity mechanism varies with mean fuel/air ratio, frequency, and amplitude of excitation. In the quasi-steady framework, two key mechanisms leading to heat-release saturation have been identified. The first of these is the flame-kinematic mechanism, previously studied in the context of premixed flame response to flow oscillations and recently highlighted by Birbaud et al. (Combustion and Flame 154 (2008), 356–367). This mechanism arises due to fluctuations in flame position associated with the oscillations in flame speed. The second mechanism is due to the intrinsically nonlinear dependence of flame speed and mixture heat of reaction upon fuel/air ratio oscillations. This second mechanism is particularly dominant at perturbation amplitudes that cause the instantaneous stoichiometry to oscillate between lean and rich values, thereby causing non-monotonic variation of local flame speed and heat of reaction with equivalence ratio.     

A DNS study on the flame structures and flame stabilization mechanism of a laboratory-scale lean premixed jet flame in crossflow

In the present work, direct numerical simulation (DNS) of a laboratory-scale lean premixed reacting jet flame in crossflow was performed to understand the flame structures and the flame stabilization mechanism. In the DNS, an ethylene-air jet with an equivalence ratio of 0.6 was injected into a hot vitiated crossflow. The jet Reynolds number reaches 6161. The DNS results were compared with those of the experiment with a good agreement. It was found that the windward and leeward branches of the flame show significantly different behaviors. The windward flame branch, appearing lifted and discontinuous, is located in the shear layer regions with high temperature, low vorticity and low scalar dissipation rate. The location of the peak heat release rate shifts to a higher mixture fraction with increasing distance from the jet exit. The leeward branch of the flame anchors in the shear layer near the jet exit. The recirculation zone in the wake of the jet facilitates the stabilization of the leeward flame. The chemical explosive mode analysis (CEMA) and species budget analysis were employed to characterize the local combustion mode. Auto-ignition plays a key role in the stabilization of the windward flame where a large range of extinction is also found due to the high strain rate. In contrast, premixed flame propagation is dominant on the leeward side.     

Measurements of local mixture fraction of reacting mixture in swirl-stabilised natural gas-fuelled burners

Local, time-dependent measurements of mixture fraction of the reacting mixture were obtained in a swirl-stabilised natural gas-fuelled, nominally non-premixed burner using the intensity of chemiluminescence from OH^∗ and CH^∗ radicals. The measurements quantified the mean, rms of fluctuations and probability density functions of local mixture fraction at the stabilisation region of the flame. In addition, the probability of flame presence and the degree of lean or rich versus stoichiometric reaction is reported. The burner was operated for three air flow Reynolds numbers (Re=18970, 29100 and 57600), at an overall equivalence ratio of 0.32, without and with imposed oscillations to the air flow of the burner at the resonance frequency of 350 Hz. Results show that combustion occurred in a partially premixed mode for all flow conditions, although fuel and air were injected separately in the reaction zone. The mean local mixture fraction was nearly stoichiometric at the base of the flame without imposed air oscillations, but with large fluctuations leading to around 80% of lean or rich reaction. The degree of non-stoichiometric reaction increased with axial distance from the burner exit and Reynolds number and lean reaction dominated. Imposed air oscillations led to lifted flames and increased the degree of non-stoichiometric reaction for Re=18970 and 29100, whereas the flame remained attached onto the injector for Re=57600 and little modification of the mixture fraction was observed.     

Computational study on lean and rich combustion of flame ball,counterflow flame and planar flame: Their limits and stoichiometries

To investigate (fuel-)lean/rich limits and essential stoichiometries, i.e., the borders of lean/rich combustion, one-dimensional steady computations with detailed chemistry for flame balls, counterflow flames, and stretch-free planar flames were conducted using a CH₄/O₂/Xe mixture that has been used in microgravity experiments. As continuous converged solutions were obtained under lean/rich conditions, it was suggested that the existence of flame ball not only under lean but also under rich condition. Flame radii and temperatures of flame balls decreased and increased toward the lean/rich limits from their maximum and minimum values, respectively. The lean limits were wider in the order of the flame ball, counterflow flame, and stretch-free planar flame. Therefore, the lean flammability limit corresponded to the lean limit of the flame ball in the mixture. Conversely, the rich limits were wider in the order of the counterflow flame, stretch-free planar flame, and flame ball. Thus, the rich flammability limit corresponded to the rich limit of the counterflow flame in the mixture. Essential stoichiometry, which represents the actual stoichiometry depending on the dominant transport in near-flame front, was not uniquely determined as conventional stoichiometry (ϕ = 1); it was located between the equivalence ratio of ϕ = 1 and ϕ_c, where ϕ _c denotes the critical equivalence ratio is evaluated using the fuel and oxidizer Lewis number of a target mixture. The results indicated that the essential stoichiometry of the stretch-free planar flame corresponded to ϕ = 1, that of the flame ball corresponded to ϕ = ϕ _c, and that of the stretched flame was located between ϕ = 1 and ϕ _c depending on the stretch rate.     

Cellular and sporadic flame regimes of low-Lewis-number stretched premixed flames

3D structure and dynamical behavior of low-Lewis-number stretched premixed flames are numerically simulated within the framework of a thermo-diffusive model with one-step chemical reaction. The results are compared with microgravity experiments at qualitative level. The influence of Lewis number, equivalence ratio, and heat loss intensity on flame structure is investigated. It is experimentally and numerically found that lean counterflow flames can appear as a set of separate ball-like flames in a state of chaotic motion. It is shown that the time averaged flame balls coordinate may be considered as important characteristic similar to coordinate of continuous flame front. Numerical simulations reveal essential incompleteness of combustion at high level of heat losses. This incompleteness occurs in the process of lean mixtures combustion and is caused by fuel leakage through the gaps among ball-like flames.     

Experimental and numerical study of product gas and N2O emission characteristics of ammonia/hydrogen/air premixed laminar flames stabilized in a stagnation flow

In order to achieve carbon neutrality, the use of ammonia as a fuel for power generation is highly anticipated. The utilization of a binary fuel consisting of ammonia and hydrogen can address the weak flame characteristics of ammonia. In this study, the product gas characteristics of ammonia/hydrogen/air premixed laminar flames stabilized in a stagnation flow were experimentally and numerically investigated for various equivalence ratios for the first time. A trade-off relationship between NO and unburnt ammonia was observed at slightly rich conditions. At lean conditions, NO reached a maximum value of 8,700 ppm, which was larger than that of pure ammonia/air flames. The mole fraction of nitrous oxide (N₂O) which has large global warming potential rapidly increased around the equivalence ratio of 0.6, which was attributed to the effect of a decrease in flame temperature downstream of the reaction zone owing to heat loss to the stagnation wall. To understand this effect further, numerical simulations of ammonia/hydrogen/air flames were conducted using the stagnation flame model for various equivalence ratios and stagnation wall temperatures. The results show that the important reactions for N₂O production and reductions are NH +NO = N₂O + H, N₂O + H = N₂ + OH, and N₂O (+M) = N₂ + O (+M). A decrease in flame temperature in the post flame region inhibited N₂O reduction through N₂O (+M) = N₂ + O (+M) because this reaction has a large temperature dependence, and thus N₂O was detected as a product gas. N₂O is reduced through N₂O (+M) = N₂ + O (+M) in the post flame region if the stagnation wall temperature is sufficiently high. On the other hand, it was clarified that an increase in equivalence ratio enhances H radical production and promotes N₂O reduction by H radical through the reaction of N₂O + H = N₂ + OH.     

Visible chemiluminescence of ammonia premixed flames and its application for flame diagnostics

We report a spatially resolved spectroscopic study of the visible chemiluminescence emission from different premixed ammonia-air-oxygen flames stabilized on a laminar flat flame burner, with equivalence ratio ranging from 0.7 to 1.35 and an O₂/N₂ ratio of 0.4. In the reaction zone of the observed flames, the visible emission was recognized to be the chemiluminescence of excited NH₂* radicals, while in the post-flame zone, two types of chemiluminescence were observed: NO₂* chemiluminescence dominated in the fuel-lean flames and NH₂* chemiluminescence dominated in the fuel-rich flames. The high-resolution spectra of the NO₂* and NH₂* chemiluminescence in the visible region (400-700 nm) were recorded. The intensity of both spectra increased gradually with wavelength. However, the NO₂*-chemiluminescence spectrum appeared to be continuous and unstructured, while the NH₂*-chemiluminescence spectrum consisted of groups of distinct emission lines. Based on the spectral feature, the ratios of the integrated chemiluminescence intensities over the 598-603 nm wavelength range to the intensities over the 586-592 nm range and 447-453 nm range were used to sense equivalence ratio. In addition, slightly different colors of the fuel-lean and fuel-rich flames were observed, due to the fact that NO₂* chemiluminescence had a relatively stronger signal in the blue region than NH₂* chemiluminescence. The difference was used to infer flame equivalence ratio using the flame images recorded by a RGB digital camera, where the ratio of the signal from the red channel to the signal from the blue channel was calculated.     

Stability and characteristics of NH3/CH4/air flames in a combustor fired by a double swirl stabilized burner

Compared to hydrocarbons, ammonia's low reactivity and higher NO_x emissions limit its practical application. Consequently, its implementation in combustion systems requires a different combustor geometry, by adapting existing systems or developing new ones. This study investigates the flame stability, NO emissions, and flame structure of NH₃/CH₄/air premixed flames in a novel combustor comprising a double swirl burner. A lean premixed CH₄/air mixture of equivalence ratio, Φ_out, was supplied to the outer swirl, while a NH₃/CH₄/Air mixture fed the inner swirl. The molar fraction of NH₃ in the inner fuel blend, x_NH3, was varied from 0 (pure CH₄) to 1 (pure NH₃) over far-lean to far-rich inner stream equivalence ratio, Φ_in. This new burner's stability map was established in terms of Φ_in versus x_NH3 for different Φ_out. Then, NO emissions were measured versus Φ_in for various x_NH3 and Φ_out. Finally, based on the NO emissions, eight flames were down-selected for in-flame measurements, which included temperature and OH-PLIF. The stability measurements revealed that increasing x_NH3 modifies the stability map by increasing the lean blowout limits and narrowing the flashback region. At Φ_out ≥ 0.6, a stable flame was achieved for a pure inner NH₃/air mixture. Low NO emissions were achieved in this burner configuration at x_NH3=1 by either enriching or far-leaning Φ_in. Enriching Φ_in led to a steep decrease in NO concentrations. However, to achieve low NO concentrations, precise control of Φ_out was needed. At Φ_in=1.4, 220 ppm NO at Φ_out=0.7 versus 690 at Φ_out=0.6 was measured. Moreover, substantially enriching Φ_in>1.2 led to a slight decrease in measured NO. Generally, the OH-PLIF images revealed a conical OH-layer at the burner exit. Certain flame conditions created OH-pockets inside the conical structure or formed a V-shaped OH-layer far downstream. This change in flame structure was found to impact NO emissions strongly.