Lean Blowoff Research Articles

Quantitative comparison of the benefits of cracking and nanosecond repetitive pulsed discharges on the lean blow-off, emissions, and topology of ammonia premixed swirl flames

In the last decade, ammonia has gained interest as an alternative fuel for the energy sector. The main advantages include its carbon neutrality and ease of storage at industrial scale. However, to consider ammonia as an alternative carbon-free fuel it is necessary to solve two main issues, namely flame stability and pollutant emission. In this work we compared two promising technologies used to enhance ammonia combustion, specifically partial cracking and plasma assisted combustion (PAC). The experiments were carried out for different blends of ammonia–hydrogen–nitrogen–air that reproduce the conditions of cracking as well as for pure ammonia assisted by nanosecond repetitive pulsed (NRP) discharges for the PAC. The cracking and PAC were compared for the same added power ratio at atmospheric conditions. Different added power ratios, from 0.1% to 4% of the flame’s total thermal power, were tested. The lean blow-off limits were measured for a range of bulk flow velocities between of 4 and 12 m/s. For the pollutant emissions, NOx, NH3, and N2O were considered. The effect of both strategies on the Laminar burning velocity (LBV) was explored by numerical simulations. For the same added power ratio, partial cracking extended the lean blow-off limits further than NRP discharges, even with a high pulse repetition frequency of 30 kHz. This result indicated that cracking was more effective in stabilizing the flame than PAC. Regarding NOx emissions, NRP discharges induced only a third of the increase observed with cracking. However, the reduction in N2O associated with cracking was twice as much as that for NRP discharges. Finally, cracking proved more efficient in reducing the flame height, which was supported by the simulations of LBV. These results suggest that PAC strategies optimized for hydrocarbon flames are less efficient on ammonia combustion, and that more work is needed to develop plasma actuators for NH3 combustion.Novelty and Significance statement This work marks the first one-to-one quantitative comparison between plasma actuation and thermal cracking on ammonia swirl flames. It serves as a pivotal exploration to assess and understand the economic aspects of these strategies, providing valuable insights into their efficiency and potential for practical implementation.

A universal Karlovitz number to predict the lean blowoff limits of stabilized premixed flames

A Karlovitz number formulation is defined to predict the stability and lean blowoff (LBO) limits of premixed bluff-body flames. The Karlovitz number relies on a newly defined flame timescale as a representative chemical time, where the flame timescale is identified from experimental measurements of LBO of premixed, propane-air, bluff-body flames in the literature. The timescale is first defined as a function of the extinction strain rate across the premixed flame, and satisfies a criterion for LBO to occur when Ka ≥ 1. Theoretical analysis shows that the defined chemical timescale based on the extinction strain rate can be recast using unstretched laminar flame properties, and reveals a coupling between premixed flame (τpf) and extinction strain rate timescales (τesr) commonly used in literature. The new chemical timescale and the coupling between τpf and τesr is explored for a variety of premixed fuel-air mixtures including propane, methane, ethylene, ammonia, and hydrogen. The timescale definition is found to uphold for all fuels except for hydrogen, which required a Lewis number correction to account for the effects of enhanced mass diffusion. The timescale is also validated against a variety of LBO data within literature. When coupled with a flow timescale, defined as recirculation zone residence time, the chemical timescale is able to collapse the LBO data to a nominally uniform Karlovitz number near unity for a range of flameholder geometries and Reynolds numbers. However, adjusting the flow timescale to estimate the inverse of the flame strain rate at LBO shifted the compiled data to a collapsed value of Ka ≈ 1, thereby realizing a unified Karlovitz (or Damköhler) theory to predict LBO. The advantage of the new Karlovitz model is its ability to provide an a priori method to predict the stability and LBO limits of premixed flames based on appropriately defined timescales.

Lean blowoff dynamics in bluff body stabilized flames: unsupervised classification and balance analysis

Lean blow-out (LBO) is a critical phenomenon in gas turbines. It is enhanced by very to ultra-lean operating conditions which are considered today to decrease the environmental impact of combustion. Despite many studies on the subject, the physical mechanisms leading to global flame extinction are not fully understood. Recently, unsupervised classification has appeared in the literature as an efficient tool to identify key features in reactive flows. In this work, unsupervised classification relying on Principle Component Analysis (PCA) and K-means clustering algorithms is used to investigate the underlying mechanisms of a blowoff event in a bluff body configuration. The unsupervised classification allows to identify and track in time 4 distinct zones: fresh gases, burnt gases, fast reacting flame and preheat zone. To elucidate the blow-out processes, an analysis of mass and energy balances of the different zones is proposed. This analysis describes the temporal evolution of the zones as a result of their interactions, which is the main driver for flame stabilization or blowoff. For the considered blowoff event, the extinction is induced by an imbalance between the various contributions identified by the proposed analysis: while the decrease in fuel mass flow rate modifies both the conductive fluxes and chemistry source terms in the reactive zones, the convective fluxes remain constant over time as the total mass flow rate is kept constant. This work suggests that the proposed methodology is a useful tool to analyze unsteady configurations and understand the main mechanisms at work in such configurations.

Investigation of the influence of the bluff-body temperature on a lean premixed DME/air flame approaching blowoff

In flames stabilized by a two-dimensional (2D) bluff body, the lean blowoff (LBO) limit can be extended by increasing the bluff-body temperature, which is verified experimentally in premixed dimethyl ether (DME)/air flames in this work. Two bluff-body temperatures of 573 K and 773 K are tested, respectively. High-speed OH* chemiluminescence (CL) imaging is applied to record the spatial distribution of the heat release. Particle image velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) are performed simultaneously to explore the flow field and the combustion characteristics. Based on the results of OH*-CL, the heat release of the area near the bluff body is strengthened in the case of higher bluff-body temperature, and the enhanced chemical reactivity will further influence the downstream area. The turbulent flame speed obtained by OH-PLIF is increased in the entire area of interest. Due to the higher velocity gradient adjacent to the bluff body in the upstream region (y < 35 mm), the flame surface density (FSD), brush thickness, and flame wrinkling ratio are not sensitive to the bluff-body temperature. However, in the downstream region (y > 35 mm), a noticeable change can be found in these three parameters due to a higher turbulent flame speed and a much lower velocity gradient. Based on the PIV results, a limited effect on the velocity field by increasing the bluff-body temperature by 200 K. Furthermore, the probability density functions (PDF) of the vorticity and strain rate on the flame front are computed. The results indicate that the enhanced flame stability leads to a more pronounced overlapping between the flame front and the high-vorticity region, further increasing the values of these two parameters.

Impact of pilot flame and hydrogen enrichment on turbulent methane/hydrogen/air swirling premixed flames in a model gas turbine combustor

This work investigates the impact of pilot flame and fuel composition on the structures and stabilization of swirling turbulent premixed methane/hydrogen/air flames in a lab-scale gas turbine model combustor. Simultaneous measurements of the velocity field and OH radicals distribution in the combustor were conducted using particle imaging velocimetry (PIV) and planar laser-induced fluorescence (PLIF) methods, respectively. Flames under stable and close to lean blow-off (LBO) conditions were studied for two fuel mixtures, with a hydrogen mole ratio of 0 and 50 % in the hydrogen/methane mixture, respectively. The studied flames were at a constant Reynolds number of 20,000 with different equivalence ratios. Two pilot-to-global fuel ratios were investigated (2 % and 6 %) while keeping the pilot-to-global air ratio constant at 2 %. Data for non-piloted flames were also acquired for comparison. The pilot flames were shown to extend the operability range. The LBO equivalence ratio of the main flame decreased with increasing fuel mass flow rate in the pilot flames due to the increased amount of hot gases with high concentrations of OH radicals in the outer recirculation zone (ORZ), which significantly enhanced the stabilization of the main flame. The stable flame reaction zone was in the high-speed shear layer between the ORZ and the inner recirculation zone (IRZ). When approaching LBO, the reaction zone was pushed downstream to the IRZ and subsequently decreased the size of IRZ, indicating a strong flow/flame interaction. Hydrogen enrichment was shown to reduce the LBO equivalence ratio. When close to LBO, the OH radicals in the hydrogen-enriched flames were observed in isolated pockets due to differential diffusion, which enhanced resilience to LBO. The flame front curvature, mean progress variable, and flame surface density were calculated from the acquired OH-PLIF data to quantify the impact of fuel composition and pilot flames on the flame structures.

Effect of CO[formula omitted] dilution on structures of premixed syngas/air flames in a gas turbine model combustor

The impact of CO2 dilution on combustion of syngas (a mixture of H2, CO, and CH4) was investigated in a lab-scale gas turbine model combustor at atmospheric pressure conditions. Two mild dilution levels of CO2, corresponding to 15% and 34% of CO2 mole fraction in the syngas/CO2 mixtures, were experimentally investigated to evaluate the effects of CO2 dilution on the flame structures and the emissions of CO and NOx. All experiments were performed at a constant Reynolds number (Re = 10000). High-speed flame luminescence, simultaneous planar laser-induced fluorescence (PLIF) measurements of the OH radicals and particle image velocimetry (PIV) were employed for qualitative and quantitative assessment of the resulting flame and flow structures. The main findings are: (a) the operability range of the syngas flames is significantly affected by the CO2 dilution, with both the lean blowoff (LBO) limit and the flashback limit shifting towards fuel-richer conditions as the CO2 dilution increases; (b) syngas flames exhibit flame-pocket structures with chemical reactions taking place in isolated pockets surrounded by non-reacting fuel/air mixture; (c) the inner recirculation zone tends to move closer to the burner axis at high CO2 dilution, and (d) the NOx emission becomes significantly lower with increasing CO2 dilution while the CO emission exhibits the opposite trend. The flame-pocket structure is more significant with increased CO2 dilution level. The low NOx emissions and high CO emissions are the results of the flame-pocket structures.

Time-resolved multi-parameter detection in a kerosene-fueled combustor using tunable diode laser absorption spectroscopy

Non-contact and real-time measurements for species concentration and temperature with high-time resolution in practical combustion environments are desired. A laser absorption system was developed for simultaneous sensing of CO, H2O concentrations and temperature with 1 kHz in a centrally staged kerosene-fueled combustor. An absorption feature near 4.98 μm in the mid-infrared region was selected for CO detection to minimize spectral interference with other combustion species in the kerosene combustion environment. Two spectral lines of H2O near 1.4 μm were used for temperature and H2O detection using a direct scan absorption strategy. The method of the developed multi-parameter measurement was validated in a laboratory-scale flat flame under various equivalence ratios. Comparisons were made between the experimental and theoretical results within the error of 5%. Time-resolved and in-situ measurements of temperature, H2O and CO concentration in the practical kerosene-fueled staged combustor were demonstrated. The transient characteristics of CO emission in different conditions were captured experimentally. It is found that the fluctuation amplitude of CO concentration increases as the equivalence ratio decreases during the pilot stage work only. There is a dramatic rise in CO emissions during the transition of combustion conditions. Strong fluctuation occurs with a low frequency at 32 Hz near lean blowoff. The location of the flame surface and the dynamic pressure have a significant coupling relationship with CO emission by analyzing measured data with high time resolution simultaneously.

Investigation of the applicability of conditioned particle image velocimetry under conditions close to lean blow-off

In recent years, conditioned particle image velocimetry (CPIV) has attracted much attention for flame front extraction. It is an economical and straightforward technique because the flame front can be obtained simply from Mie-scattering images. In the current work, Mie-scattering and hydroxyl planar laser-induced fluorescence (OH-PLIF) measurements were taken simultaneously to study the applicability of CPIV under conditions with varying equivalence ratios, and the reliable working range of the CPIV method and the source of bias were clarified quantitatively. The premixed dimethyl ether (DME)/air flames stabilized on a two-dimensional bluff body were tested. It is found that the accuracy of the CPIV method worsens as the equivalence ratio decreases. The bias of CPIV is supposed to be related to the flame structure and aerodynamics. The extraction deviation in the upstream region for the stable flames is more significant than that in the downstream area due to the intensified interaction between the shear layer and flame near the bluff body. However, for the flames approaching the lean blow-off (LBO), the bias in the upstream part is smaller than that in the downstream area, which is related to the “back-support” of the hot recirculation zone (RZ). In addition, the difference between the results obtained by CPIV and OH-PLIF is usually the preheat layer of flame and concave-wrinkled location of OH-PLIF filled with CH2O radicals, especially under conditions close to the LBO, which can be used to estimate the spatial distribution of CH2O.Graphical

Pressure gradient effect on flame–vortex interaction in lean premixed bluff body stabilized flames

This investigation considers the effect of axial pressure gradient on the dynamics of flame–vortex interaction for a lean premixed bluff body stabilized flame. Large eddy simulations (LESs) of four different combustor geometries generated through combustor wall adjustments that resulted in mild to strong pressure gradients are studied. A bluff body stabilized combustor for a propane/air flame is analyzed first. The results are compared with all available experimental data with the purpose of validating the LES methodology used in OpenFOAM and obtaining a base solution for the study of the pressure gradient effect on flame–vortex interaction. The role of the pressure gradient on flame structure, emission characteristics, vortex dynamics, and flame stability is presented. The mild favorable pressure gradient due to the decelerated flow in diffuser configurations influences flame–vortex dynamics by suppressing flame-induced vorticity sources, baroclinic torque and dilatation, and hence resulting in augmented hydrodynamic instabilities. The sustained hydrodynamic instabilities maintain the large flame wrinkles and sinusoidal flame mode in the wake region. The nourished near-lean blowoff dynamics also affect the emission characteristics, and the emission of species increases. However, the accelerated flow in the nozzle configuration amplifies the flame-induced vorticity sources that preserve the flame core, resulting in a more organized, symmetric, and stable flame. Ultimately, the combustion performance and operation envelope in the lean premixed flames can be increased by maintaining the flame stability and suppressing the limiting lean blowoff dynamics and emissions with the help of a strong favorable pressure gradient generated through adjusting the combustor geometry.

Stability and structure of lean swirling spray flames with various degrees of prevaporization

Achieving full premixing and complete evaporation in lean prevaporized premixed combustors is challenging as it depends on the spray injector characteristics, prevaporisation strategy, and flow conditions. This article experimentally explores the stability and structure of a turbulent swirling n-heptane spray flame under various degrees of prevaporization. The results show that preheating the air to 343 K and 393 K has little effect on the lean blow-off velocity, while recessing the fuel injection significantly decreases the lean stability limit. To correlate these limits, various attempts to define a Damköhler number were made, but unlike previous studies with no prevaporisation, the difficulty in defining laminar flame speed in the present case does not allow a single correlation to work for all degrees of prevaporization. Four stable cases that differ in equivalence ratio, air preheat temperature, and fuel injection recess are investigated using one-dimensional PDA, OH* chemiluminescence and CH[Formula: see text]O-planar laser-induced fluorescence (PLIF). Cases without fuel injection recess or air preheat exhibit a conical-shaped heat release zone near the shear layers. Preheating the air to 393 K reduced the Sauter mean diameter, increased prevaporization, and enabled a second heat release zone downstream of the fuel injection. Recessing the fuel injection by 25 mm reduced droplet velocities and led to a semi-spherical instead of a conical heat release zone. The CH[Formula: see text]O-PLIF signal without injection recess was high along the central axis and its distribution resembled that observed for spray jet flames. In contrast, with recessed spray injection, CH[Formula: see text]O was mainly found outside the central recirculation zone and only appeared inside during lean blow-off; similar to previous work with premixed flames. These findings show that different methods of prevaporization, which only differ by subtle changes in droplet characteristics, strongly impact flame stability. The present data can be used for turbulent flame modelling focusing on sprays and finite-rate kinetics.

An experimental study of syngas combustion in a bidirectional swirling flow

The paper reports on the results of an experimental study of methane and syngas combustion as well as their co-firing in a bidirectional swirling flow. The results confirmed that the bidirectional flow structure provides a significant decrease in the lean blow-off equivalence ratio as well as that of emissions of main pollutants. The combustion intensification becomes more evident when using syngas is as fuel. The composition of the used syngas is as follows (by volume): H2 - 29.42%; CO - 14.32%; CH4 - 3.8%; N2 - 49.11%; H2O - 3.35%. In this case, the lean blow-off is achieved at ϕ < 0.1, NOx emission is halved, while CxHy and CO emissions become 20 times less compared to pure methane combustion. However, according to experimental results, the co-combustion of syngas (volume fraction Vsyn = 15%) and methane is the most appropriate fuel utilization mode. It provides blow-off and emission properties similar to those for combustion of pure syngas, whereas energy consumption for its production is much lower. Moreover, unlike hydrocarbon fuel combustion, that of syngas in a bidirectional swirling flow is characterized by the presence of density stratification. This is accompanied by the flame formation at significantly different locations in the combustion chamber at lean and “ultra-lean” modes of operation. Hydrogen combustion most likely to occur in the core region at near-blow-off modes ϕ < 0.1, whereas normal ‘operating modes in the range 0.2 = ϕ ≤ 0.4 result in the formation of a conical flame surface where CH4 and CO combustion occurs. These new results with respect to the flame structure as well as blow-off and emission properties make it possible to consider bidirectional vortex combustors for application in modern gas turbine power plants in order to meet the strict environmental and energy requirements.

Lean blow-off of premixed swirl-stabilised flames with vapourised kerosene

Lean blow-off (LBO) conditions, duration of the extinction transient, and flame structure were investigated for swirl-stabilised, fully-premixed flames operated with methane and vapourised kerosene. High-speed (5 kHz) OH* chemiluminescence, OH-, and Fuel-PLIF imaging were used. Low-speed (10Hz) CH2O-PLIF imaging was also implemented to visualize the structure of CH2O. OH* chemiluminescence imaging of the LBO transient suggested that the primary reaction regions were confined within the central recirculation zone (CRZ). Sharp temporal increases in OH* signal were observed in the CRZ for both fuels, possibly due to flame re-ignition events during LBO transients. Both fuels showed a decrease in flame length and an increase in fragmentation in downstream regions. In the OH-PLIF images, pockets void of OH entered the CRZ from downstream. The CH2O- and Fuel-PLIF images suggest that these pockets contain either partially burned or fresh reactants. Furthermore, the CH2O- and Fuel-PLIF images indicated that CH2O and Fuel exist in the outer recirculation zone (ORZ) during the early stages of the LBO transient. The blow-off duration was calculated by integrating the OH* signal and for kerosene was found to be 1.5 times longer than for methane. In addition, the equivalence ratio at which kerosene blew-off was consistently lower than that of methane for a range of bulk flow velocities. These results indicate that fuel chemistry effects are important for the LBO condition and duration.

Expanding swirl combustor operability on methane-ammonia-air mixtures using a distributed fuel injection technique and inlet air preheating

The necessity to reduce greenhouse gas emissions has prompted the search for carbon-free fuel alternatives. One such carbon-free fuel source that can be used for power generation in a gas turbine-based system is ammonia. Ammonia combustion poses challenges due to reduced flame speed and reactivity, which can be alleviated by addition of methane or natural gas. However, NOx emissions continue to be an issue, which needs resolution before practical consideration of ammonia as a fuel source. One of the potential solutions that has been proposed is a two-stage, rich-lean combustion process to minimize NOx while ensuring complete reactant consumption. This work evaluates the use of two strategies to widen stability limits for swirl combustors operating on premixed methane-ammonia-air mixtures, which would facilitate the two-stage combustion approach. The first strategy involves the use of a distributed fuel injection approach utilizing a novel micro fuel injection swirler to facilitate homogeneous mixing in a highly compact manner while preventing flashback concerns. The second strategy involves use of inlet air preheating to increase flame stability and delay blow-off. The effectiveness of these strategies in expanding combustor operability limits is studied using experiments conducted in a model swirl-combustor setup designed to measure blow-off limits and evaluate exhaust NOx emissions. Complementary numerical simulations, particularly aimed at understanding NOx production pathways, are carried out using a reactor network model, wherein key inputs pertaining to recirculation volume and mass fraction are generated through detailed reacting flow simulations. The influence of the proposed strategies on NOx emissions are studied and underlying reaction pathways leading to NOx production are analyzed. Results of the study indicate that a distributed fuel injection strategy is able to significantly expand stability limits of a swirl combustor operating on methane-ammonia-air mixtures. Inlet air preheating provides additional expansion of stability limits, ≈ 7% for rich and lean blow-off, however, this is accompanied by increased NOx production, up to 14%, which is undesirable from an emissions standpoint. NOx is found to be significantly lower for rich fuel-air mixtures primarily through the effect of NHi reaction pathways responsible for NO consumption facilitated by the presence of excess NH3.

Effect of Fuel-Injection Distance and Cavity Rear-Wall Height on the Flameholding Characteristics in a Mach 2.52 Supersonic Flow

The ethylene-fueled flameholding characteristics of a cavity-based scramjet combustor are experimentally and numerically investigated. The test facility used the air heater, which heats air from room temperature to total temperature 1477 K. A nozzle is installed behind the heater outlet to increase the air speed to Mach 2.52. Two cavity geometries with different rear-wall heights of 8 mm and 10 mm and two injection distances upstream of the cavities of 10 mm and 40 mm are compared to show the effect of these parameters. The CH* spontaneous emission images obtained by dual-camera synchronous shooting and the wall-pressure distribution obtained by a pressure-scan system are used to capture the flame dynamics. The global equivalence ratio range for different combination schemes is controlled from 0.14 to 0.27 in this paper. The results show that the conventional cavity (the rear-wall height is 10 mm) and the shorter injection distance can effectively decrease the lean blowoff limit of the combustor, while the rear-wall-expansion cavity (the rear-wall height is 8 mm) and the longer injection distance can effectively increase the rich blowoff limit. Compared with the injection distance, the rear-wall height of the cavity has little effect on the oscillation distribution of the shear layer-stabilized flame. However, the fuel-injection distance and cavity rear-wall height both have great influence on the spatial distribution of the flame.

Blowoff of bluff body flames: Transient dynamics and three dimensional effects

This work examines the dynamics of confined bluff body flames in the process of lean blowoff (LBO) using simultaneous stereo-PIV (particle image velocimetry), OH PLIF (planar laser induced fluorescence) and formaldehyde (CH2O) PLIF. Flames at high density ratios blow off in at least two distinct stages: stage 1, where intermittent extinction occurs along the flame front, but the flame and flow remain qualitatively similar to stable conditions, and stage 2, where there is permanent downstream flame extinction and large-scale changes in dynamic flow characteristics. This paper particularly focuses on stage 2 processes, with the goal of understanding what ultimately leads to irrecoverable flame blowoff. A test facility was developed with the flexibility to achieve two goals: (1) approach LBO by keeping the parameters that influence its hydrodynamic stability approximately constant, particularly flow velocity (ubulk) and gas expansion ratio (σ), and (2) compare near-LBO dynamics under conditions where, well away from blowoff, the flame is globally stable (high σ case) and globally unstable (low σ case, where the Bénard-Von Karman, BVK, instability of the flow is present). The latter case was of particular interest as most prior detailed diagnostic studies of LBO have been performed at high σ, BVK-suppressed conditions. We find that the transient blowoff process remains largely unchanged in the high and low σ cases, apparently due to the fact that the BVK instability reappears in either case under conditions very close to LBO. In all cases, blowoff is preceded by permanent downstream extinction that moves progressively closer to the bluff body as LBO is approached. We also find that near-LBO dynamics are intrinsically three dimensional, due to both secondary instabilities of the shear layer and large out-of-plane motions believed to be due to confinement effects associated with bluff body-wall interactions. These three-dimensional structures often manifest themselves as burning reactant fingers which are caught in the backflow of the recirculation zone; under very near LBO conditions they impinge on the back of the bluff body and extinguish as well. At the very edge of blowoff, the recirculation zone is no longer composed of hot products and is unable to autoignite the oncoming reactant flow, leading to global extinction. The characteristic time associated with this feedback between downstream extinction and wake structure alteration that leads to blowoff is about two orders of magnitude larger than the characteristic flow time, D/ubulk. We also discuss several implications of these results on computations of LBO - in particular, LBO's intrinsically three-dimensionality and the need for many flow through times to capture it.

Lean blow-off investigation in a linear multi-burner combustor operated in premixed and non-premixed modes

The flame behaviour and the lean blow-off (LBO) mechanism of a multi-burner linear arrangement of five identical burners each fitted with a central bluff body and a swirler up- stream, operated in either premixed or non-premixed mode, was studied experimentally. The flame stability was assessed over a wide range of velocities. High-speed (5 kHz) OH∗ chemiluminescence and low-speed (10 Hz) CH2O-PLIF were employed. As the velocity in- creased, the premixed flame exhibited a shape change with significant CH2O build up in the region between adjacent burners, suggesting the presence of localised quenching where the flames interact. Close to LBO, the flame showed a pattern of lift-off and reattachment over individual bluff bodies, with movement of OH∗ and CH2O pockets across the burners. The non-premixed flame was shorter and showed less visible interaction between adjacent burners. When the fuel velocity was reduced, the flame showed intermittent blow-off/reignition prior to the complete extinction. The flame shapes and stability regions were compiled in a regime diagram and compared to a full annular combustor that used the same individual burners. Finally, the LBO conditions were correlated through a Damkh¨oler number (Da) based on laminar flame speed and compared to the annular combustor and a scaled-up single burner. In the premixed flame, the linear multi-burner LBO conditions agreed with those in the annular geometry when expressed in terms of Da, but the single burner did not. For the non-premixed flame, the correlation proved less successful. The results reported here improve the understanding of the LBO process in practical systems and can assist the development of turbulent flame models.

Measurement of Lean Blowoff Limits in Swirl-Stabilized Distributed Combustion With Varying Heat Release Intensities

Abstract Lean blowoff in distributed combustion was investigated at moderate heat release intensities of 5.72, 7.63, and 9.53 MW/m3-atm to characterize the blowoff phenomenon. Distributed combustion conditions were established from a conventional swirl flame at an equivalence ratio of 0.9 using carbon dioxide as the diluent to the inlet airstream. A gradual increase in the air flowrate provided a reduction of equivalence ratio that eventually resulted in the lean blowoff limit. Blowoff occurred at relatively higher equivalence ratios for higher heat release intensities, which was attributed to higher inlet turbulence leading to the early introduction of flame instabilities and blowoff. High-speed chemiluminescence imaging (at 500 frames/second) performed near blowoff moments demonstrated the transition of distributed reaction zone to a near V-shape zone due to quenching of flame surface along the sides. A closer examination of the reduction in equivalence ratio in small steps near the global blowoff showed the presence of a very thin thread-like rotating reaction zone. The observations of blowoff were further supported by the analysis of chemiluminescence signals in each case. The effect of inlet air preheats on blowoff was also investigated. Air preheats broadened the lean blowoff to a lower equivalence ratio which was attributed to enhanced flame speed, providing additional flame stability and reduction of flowfield instabilities. The laminar flame speeds obtained at each preheats case using Chemkin-Pro© simulation with GRI-Mech 3.0 reaction mechanisms supported such a hypothesis of gradually enhanced flame speed, providing additional flame stability.

Effect of Rotating Gliding Arc Plasma on Lean Blow-Off Limit and Flame Structure of Bluff Body and Swirl-Stabilized Premixed Flames

The effect of rotating gliding arc plasma on flame stabilization in a laboratory scale swirl-stabilized plasma-assisted combustor was experimentally investigated. The characteristics of a gliding arc plasma were studied using high-speed camera and simultaneous measurements of current and voltage waveforms. Effects of plasma on the extension of lean blow-off (LBO) limit and the swirl flames structures as well as OH radical distribution have been studied systematically. Results show that the discharge and combustion are coupled together effectively due to the dynamic processes of discharge. When the plasma is activated, the flame structures are drastically changed; oscillating flame and lifted flame convert to stable columnar flame tending to be attached to the plasma column. Besides, the plasma columns can promote OH formation and can produce much more energetic radicals due to the reactions between the discharge, methane and oxygen. The gliding arc plasma can stabilize the flame, provide an additional anchoring mechanism, and significantly extend the LBO limit. The plasma column can provide active radicals and continuous ignition to sustain the flame, and the thermal effect and kinetic effect may occupy the dominant role.

Hydrogen enhancement on a mesoscale swirl stabilized burner array

This manuscript presents a study of hydrogen-enhanced methane flames in a compact mesoscale burner array, where even a minute addition of hydrogen can significantly improve flame stability with reduced combustion length scale thereby enabling volumetric flexibility in small-scale combustion systems. The effect of hydrogen enhancement is demonstrated through combustion reaction length scale and operation limit analysis using methane fuel. The added hydrogen lowered lean blow off equivalence ratios, increased flame temperatures, and shorten the flame length scale, ultimately producing a denser flame array. 10 kHz OH∗ chemiluminescence and OH planar laser induced fluorescence imaging techniques were used to estimate heat release rate and visualize flame structures. Hydrogen addition increased the OH intensity and decreased global heat release rate fluctuation, and also showed more stable operation under acoustic perturbations. Hydrogen enhancement can be a promising solution for reducing geometric constraints and improving operating capabilities for compact propulsion and power generation systems.

Extension of flammability and stability limits of swirling premixed flames by AC powered gliding arc discharges

This work seeks to achieve stable combustion of swirling premixed flows using gliding arc discharges and understand how plasma alters flame dynamics near blow-off. Gliding arc discharges powered by 7 kHz and 25 kHz alternating current (AC) supplies are demonstrated to significantly extend the flammability and stability limits of swirling premixed flames either confined or unconfined within a quartz tube, with time-averaged discharge power close to 1% of the power typically released by a stable flame. Side visualization of OH profiles using the planar laser-induced fluorescence (PLIF) technique illustrates the unsteady evolution of flames approaching lean blow-off, confirming the positive effect of gliding arcs on the ultra-lean residual flames. Contrary to the unconfined flame anchored near the nozzle exit or the bluff body, the confined flame detached from the wall progressively presents different unstable morphologies, e.g., oscillating flames and lifted helicon flames. The activation of gliding arcs provides the additional anchoring mechanism for the lean swirling flame and particularly suppresses the natural flame oscillation with heat-release rate fluctuations of above 30%. The mechanism controlling flame responses to gliding arc includes a nonlocal effect through the externally applied AC electric field displacing flame and a local effect of spark discharge triggering flame kernels by producing heat and active species. The external sub-breakdown electric field can enlarge the spreading angle of OH-PLIF layers in the unconfined flame, but demonstrates few positive effects on the blow-off limit. Regarding the local effects within or near the arc column, top infrared imaging provides visible evidence for flame kernels initiated upstream of the inner recirculation zone, yielding repetitive ignition in the fresh reactants.