Lean Blow-off Limit Research Articles

Experimental characterization and 3D simulations of turbulent flames assisted by nanosecond plasma discharges

This paper presents quantitative experimental data generated for the validation of plasma-assisted combustion (PAC) simulations. These data are then used to validate the phenomenological model of Castela et al. They are also useful to test other PAC models. In the experiment presented here, Nanosecond Repetitively Pulsed (NRP) discharges are applied to a lean-premixed turbulent methane-air flame initially near the lean blow-off limit. The discharges significantly enhance the combustion and stabilize the flame after a few pulses. Electrical and optical diagnostics are employed to extensively quantify the transient and steady state of the plasma-stabilization process. The flame shape is characterized by OH* chemiluminescence imaging. In the discharge region, OH density profiles are obtained by 1D laser-induced fluorescence, and the gas temperature is measured by optical emission spectroscopy measurements. The local gas temperature increases by 1250 K, and the OH number density rises sevenfold when NRP discharges are applied. These results evidence the cumulative thermal and chemical effects of NRP discharges, which are especially challenging to replicate numerically. A Large Eddy Simulation (LES) of the experiment is performed. Combustion chemistry is modeled by an analytically reduced mechanism, while the plasma discharge is described by the low-CPU cost phenomenological model of Castela et al., which aims to capture the main thermal and chemical effects induced by the discharges. The model of Castela et al. is validated in the burnt gases by the remarkable agreement between the simulations and the experiments regarding the flame shape, the local gas temperature, and the OH number density. More generally, this work demonstrates the relevance of simplified plasma models in LES solvers to simulate complex plasma-assisted burners.

Role of secondary hydrogen injection on flame stabilization of ammonia/air swirling flames

Ammonia is widely recognized as one of the most advanced hydrogen carriers and can be utilized as a fuel in gas turbines. Premixing hydrogen into the ammonia carbon free fuel system can substantially enhance stable limits, but it significantly promotes cross-reactions, leading to increased NO production. Recent findings related to other fuels suggest that the alternative approach for mitigating combustion instabilities involves introducing minimal pure hydrogen at the chamber inlet in a non-premixed mode. This may introduce a novel approach to stabilize ammonia/air swirl flames by extending the stability through a minimal hydrogen via secondary injection, with minimal impact on increasing nitrogen oxide emissions. In the present study, the flame stabilization mechanism and nitrogen oxide emission behaviors of swirl ammonia/air flames by a secondary hydrogen injection were compared with the premixed ammonia/ hydrogen/air flames. OH-/NO-PLIF and PIV techniques were applied to reveal the lean blow-off characteristics and the reacting flow features. The NOx emissions were measured by the FTIR gas analyzer. The large eddy simulation method with a developed dynamic thickened flame model was employed to further reveal the experimental findings. Experimental results show that the flame stabilization limits are largely depended on the way in which hydrogen is introduced. The secondary hydrogen injection exhibits the stronger enhancement ability for the lean/rich blow-off limits, primarily due to the local diffusion hydrogen flame at the flame root providing more active radicals and higher temperature gases. The NO emission shows an increase with the addition of premixed hydrogen, while in the secondary hydrogen injection, NO emission deteriorates due to higher temperatures, with the NO emission increasing by less than 10% compared with the ammonia flame. In the large eddy simulation analyses, the physical effects of the secondary hydrogen injection enhance the flame stability by increasing the resistance of the flame root to extinction. The heat release rate and the mass fractions of NH and H near the flame root are significantly increased by 2% secondary hydrogen injection. The enhancement of the ammonia decomposition process is stronger with secondary hydrogen injection than with fully premixed hydrogen addition. For 2% secondary hydrogen injection case, the local hydrogen injection to form a non-premixed combustion mode can provide much higher capability to increase the local heat release rate compared to the fully premixed mode.Novelty and significance statement: Introducing small amount of hydrogen to ammonia flame can effectively improving the flame stabilization in a swirl combustor. In this study, it is found that the flame stabilization limits are largely depended on the way in which hydrogen is introduced. Comparing the way of premixing hydrogen in the unburned mixture, a larger effect on blow-off limits extension was found when introducing a separate non-premixed hydrogen at the chamber inlet, which is mentioned as the secondary hydrogen injection. The lean blow-off limits can be extended from about 0.67 to 0.59 when only introducing 2% secondary hydrogen injection by heat value. The NO emission shows in the secondary hydrogen injection, NO emission deteriorates by less than 10% compared with the ammonia flame. A thickened flame model for non-premixed combustion was established and verified adequate with velocity field and flame structure. The mechanisms of enhancing flame stabilization by hydrogen introduction including physical and chemical effects for premixing and injection cases were revealed. Furthermore, the difference in the mechanisms of the two cases was emphasized.

Effect of rotating gliding discharges on the lean blow-off limit of biogas flames

This study investigates the effect of a rotating gliding discharge on synthetic biogas combustion at atmospheric pressure. Synthetic biogas was produced by mixing methane and carbon dioxide. Three mixtures were considered: 100%/0%, 70%/30%, and 50%/50% of methane and carbon dioxide, respectively. The plasma effect was investigated in a low-swirl-number burner equipped with a high-voltage electrode to produce gliding discharges. The effect of plasma on the stability limits of the flame is reported for several electrical powers. During plasma-assisted combustion, the lean blow-off limits of biogas-air flames were significantly improved, which agrees with what can be found in the literature for other fuels. The electrical parameters of the discharge and the plasma emissions were measured using electric probes and emission spectroscopy, respectively. The mixture with the CO2 dilution was associated with a higher reduced electric field and higher ion production. A better understanding of the excited-species concentration evolution during plasma is necessary and will be investigated in future work.

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.

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.

Combustion enhancement subjected to the inlet distortion in a cavity-based supersonic combustor

The present study experimentally investigates the effect of inlet distortion on the combustion process in a direct-connect test environment. A blockage by introducing physical intrusions with three heights (H = 2.5 mm, 3.2 mm, and 4 mm) was mounted at the entrance of the isolator to mimic the inlet distortion. Cases without blockage were used as a comparison. Schlieren visualization synchronized with high-speed CH* chemiluminescence imaging and pressure measurements were employed to characterize the flowfields. The results show that the blockage mounted at the entrance of the isolator could reasonably mimic distortion generated at the inlet. The combustion enhancement is achieved in all cases with blockage, and the lean blowoff limits could be widened in the combustor with blockage (H = 2.5 mm and 3.2 mm). In the cavity stabilized combustion mode and transition mode (Φ=0.159∼0.689), the stronger oblique shock wave emanating from the cavity leading edge and the additional background shock wave created by the inlet distortion are the chief causes of combustion enhancement in cases with blockage. The combustion enhancement gradually recedes with the increase of blockage height. It could be speculated that the relatively concentrated impinging of the incoming shock waves induces a more remarkable combustion enhancement. In the jet-wake stabilized combustion mode (Φ=0.848∼1.325), the effect of blockage height on combustion enhancement is significantly suppressed. As the equivalence ratio rises to 1.06, the regularity that the effect of blockage height on combustion enhancement is completely reversed. That is, the combustion enhancement attenuates with the decrease in blockage height. It could be inferred that in these cases, the dominance of combustion enhancement is the incoming flow velocity upstream of the reaction zone instead of the impinging sparsity of the incoming shock waves.

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.

Combustion performance of plasma-stabilized lean flames in a gas turbine model combustor

This work presents an experimental study of the stabilization of lean methane-air flames by nanosecond repetitively pulsed (NRP) discharges. The experimental facility consists of a gas turbine model combustor with a Lean-Premixed-Prevaporized injector. The working pressure is 1 atm. The fuel is injected through two stages, each stabilized by swirl. The main stage consists of a multipoint annular injection. This facility is representative of a single sector of a gas turbine combustion chamber. The NRP discharges significantly extend the lean blow-off limit for a wide range of operating conditions, down to an equivalence ratio of 0.16. Lean flame stabilization is demonstrated for flame thermal powers up to 100 kW, with an electric power of less than 0.2% of the flame thermal power. We also observe plasma-assisted lean flames emiting less NOX than the leanest stable flames without plasma. Finally, by exploring the application of various pulse patterns instead of applying the discharges continuously at a constant repetition frequency, the plasma-to-flame power ratio required to stabilize a lean flame is decreased to 0.06% and the pollutant emissions can be further decreased.

Experimental study on burning velocity, structure, and NOx emission of premixed laminar and swirl NH3/H2/air flames assisted by non-thermal plasma

This paper investigates the effect of dielectric barrier discharge (DBD) plasma on premixed laminar and swirl NH3/H2 flames by applying a coaxial DBD to the unburned gas mixture. We focused on analyzing the flame structure, burning velocity, lean blow-off limit, and NOx emission characteristics of premixed NH3/H2 flames in the presence and absence of plasma. The mechanism of plasma-enhanced burning velocity was revealed through kinetic simulations based on the CHEMKIN software. The results indicate that DBD plasma can reduce the laminar flame height and increase the laminar burning velocity (LBV). The chemical kinetic effect is dominant for the enhancement of LBV, while the effect of hydrogen generated from ammonia dissociation and ozone is relatively weak. DBD plasma can significantly change the structure of the swirl flames, while has a slight effect on extending the lean blow-off limits of the swirl flames. Finally, emission measurements indicate that DBD plasma increases the NOx emissions for both the premixed laminar and swirl NH3/H2 flames. This effect can be attributed to the reaction pathways involving of H and OH radicals for NOx formation. It is assumed that the effect of hydrogen and DBD plasma on stability and burning velocity is synergistic, while the effect of hydrogen and plasma on NOx emissions is competitive.

On the Stability and Characteristics of Biogas/Methane/Air Flames Fired by a Double Swirl Burner

CO2-diluted methane fuel is relevant to biogas combustion applications. Despite its poor heating value and low reactivity, which limit its practical applicability, biogas gains popularity as a renewable fuel. However, implementing it in combustion systems requires either modifying or replacing the existing burners. This study investigates the stability, temperature field, and pollutant emissions of CH4/CO2/air-premixed flames fired by a double-swirl burner. A CH4/air mixture of equivalence ratio, Φout, was used in the outer stream, while a CH4/CO2/air mixture was supplied to the inner stream. The CO2 mole fraction, $${x}_{{\mathrm{CO}}_{2}}$$ , in the inner fuel blend varied from 0 to 0.4 for various inner stream equivalence ratios, Φin. The stability diagram of these flames was mapped in terms of Φin verses $${x}_{{\mathrm{CO}}_{2}}$$ for a fixed Φout. Based on the stability map, the inflame temperature field was investigated for six flames. Increasing the %CO2 in the biogas modifies the stability map by increasing the inner stream lean blow-off limits. However, increasing Φout sustains the flame stability, while reducing the CO2 decreases the overall flame blow-off equivalence ratio. Flame size growth with increasing $${x}_{{\mathrm{CO}}_{2}}$$ requires a longer residence time for efficient combustion. The addition of CO2 physically and chemically affects the thermal flame structure, and hence the pollutant emissions. In this burner, ultra-low NOX emission was reported, while an increase in the CO and unburned hydrocarbons (UHC), with increasing $${x}_{{\mathrm{CO}}_{2}}$$ was observed. However, the results show that, for a given $${x}_{{\mathrm{CO}}_{2}}$$ , controlling Φin and Φout could reduce CO and UHC emissions.

Effect of microsecond repetitively pulsed discharges on lean blow-off limit and emission of rapidly-mixed ammonia/air swirling flames

This work aims to implement an actuator integrating a swirl burner with electrodes to explore stable and low-emission combustion of ammonia/air. The cavity structure holds a plasma torch driven by low-power microsecond repetitively pulsed discharges (µRPDs) in the nozzle, which serves as a pretreatment for ammonia/air gas before it enters the combustion region. The plasma generates strong excited N2, OH, and atomic H* and O* signals, and fitting N2 spectra indicates a rotational temperature of ∼3000 K and vibrational temperature of ∼4500 K. The flame without plasma is easier to detach from the wall and operate as a cone-shaped structure in the confined quartz tube, while plasma can help attach the flame and extend the lean blow-off limit from ∼0.6 to 0.4∼0.5 in a Reynolds number range of ∼3000 to 7500. Then, optical diagnostics for OH radical and NH2* chemiluminescence are performed to enable analysis of intermediate chemistry. The NH2* signals are distributed at the edge of the OH profile in both attachment and detachment cases. Finally, flue gas analyzers are used to find an optimal lean equivalence ratio where plasma anchors a stable ammonia/air flame with a relatively low emission of approximately 200 ppm NO and zero NH3 and H2.

Transfer Functions of Ammonia and Partly Cracked Ammonia Swirl Flames

The replacement of hydrocarbon fuels by ammonia in industrial systems is challenging due to its low burning velocity, its narrow flammability range, and a large production of nitric oxide and nitrogen dioxide when burned close to stoichiometric conditions. Cracking a fraction of ammonia into hydrogen and nitrogen prior to injection in the combustion chamber is considered a promising strategy to overcome these issues. This paper focuses on evaluating how different levels of ammonia cracking affect the overall burning velocity, the lean blow-off limit, the concentration of nitric oxide and nitrogen dioxide, and the flame response to acoustic perturbations. Swirl stabilized premixed flames of pure ammonia–air and ammonia–hydrogen–nitrogen–air mixtures mimicking 10%, 20%, and 28% of cracking are experimentally investigated. The results show that even though ammonia cracking is beneficial for enhancing the lean blow-off limit and the overall burning velocity, its impact on pollutant emissions and flame stability is detrimental for a percentage of cracking as low as 20%. Based on an analysis of the flame dynamics, reasons for these results are proposed.

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.

Influence of key geometrical features on the non-reacting flow of a Lean Direct Injection (LDI) combustor through Large-Eddy Simulation and a Design of Experiments

Lean Direct Injection (LDI) emerged as an interesting concept to limit NOx emissions in aero engines at the cost of operating close to the flame lean blow-off limit. In this technology, fuel is injected into a swirled airstream that generates recirculating flow structures that stabilize the flame. It is then of paramount importance at the design stage to understand the effect of various features on these structures. The present investigation makes use of Eulerian-Lagrangian Large-Eddy Simulations (LES) previously validated against existing experimental data for a reference condition to study the liquid non-reacting flow inside the CORIA Spray LDI burner with the help of Adaptive Mesh Refinement (AMR). A Design of Experiments (DoE) is proposed to analyze the significance of several geometrical features on the flow field, namely the combustor width, the air swirler vane angle, the number of swirler vanes and the axial location of the fuel injector tip. The study covers the qualitative appearance of the flow and the quantitative characterization of the spray dispersion and fuel-air mixing process. In this way, the chosen response variables include the size of the relevant coherent flow structures (Central Toroidal Recirculation Zone induced by the Vortex Breakdown Bubble, Corner Recirculation Zone and Swirled Jet) and their associated velocities, spray features (global drop sizes and spray penetration), pressure drop across the swirler and induced swirl number. Besides, the Precessing Vortex Core (PVC) relevance and frequency content is studied through Proper Orthogonal Decomposition (POD). Results from the statistical analysis show that the number of swirler vanes and their angle are the geometrical parameters that most importantly influence the flow features: stronger recirculation zones leading to an improved atomization and mixing have been found both when decreasing the number of swirler blades and increasing the blade angle. However, both solutions also increase the pressure losses across the swirler. As far as the spectral analysis is concerned, the number of swirler vanes is the most influencing factor on both the frequency and intensity of the PVC modes, being crucial for the possible activation and the energetic content of a double-helix PVC mode.

Effect of confinement ratio on flame structure and blow-off characteristics of swirl flames

Blow-off of swirl flames as well as its mechanism are fundamental issues and attract continuous attention due to its wide application in modern combustors. This study experimentally investigated the blow-off and flame macrostructure transition behavior of unconfined and confined premixed CH4/air flames stabilized on a swirl and bluff-body burner. PIV and OH-PLIF techniques were used to measure the flow field and instantaneous OH profile, respectively. It was observed that the combustor confinement could significantly broaden the blow-off limits, which was further widened with the increase of confinement ratio (CR) of the confined combustor. The influence of CR on flame stabilization was revealed by analyzing flame structures and flow fields. For the unconfined combustor, when the flames are far away and close to the blow-off, the shear layer and inner recirculation zone (IRZ) play the critical role in stabilizing the flame front, respectively. For the confined combustor, when the flame was far from blow-off, both the shear layer and the outer recirculation zone (ORZ) contributed to the stabilization. With the decrease of equivalence ratio, the stabilization effect of ORZ started to decrease and gradually weakened. When near lean blow-off limit, for CR = 2 (Φ= 0.48), the flame was mainly stabilized by the IRZ, while for CR = 3 (Φ = 0.46), the flame stabilization depended on the upstream IRZ/inner shear layer (ISL) and the downstream IRZ/ISL and ORZ/outer shear layer (OSL). Furthermore, the ISL vortexes would further accelerate the blow-off near the lean limit for confined and unconfined flames. The transition from stable to unstable was extended by increasing the CR in the confined combustor.

Experimental study on structure and blow-off characteristics of NH3/CH4 co-firing flames in a swirl combustor

Experimental study on the unconfined lean premixed NH3/CH4/air flames with three CH4 fractions of 0, 50%, and 100% stabilized on a bluff-body and swirl burner was investigated. The flow fields and instantaneous OH distributions were captured by synchronous PIV/OH-PLIF measurement. The macrostructure and critical mechanism of blow-off for the compact NH3/air flame at conditions far away and near blow-off, and the effect of CH4 addition on the topology and flame stabilization was discussed. Results show that the NH3/air flame possesses a poor lean blow-off limit Φb, and the NH3/air mixture can not be ignited when Uin is higher than 5 m/s. After adding 50% CH4 fuel, the laminar flame speed SL and the extinction strain rate Kext are approximately 3 times and 7 times that of the NH3/air flame with equivalence ratio Φ = 0.8, resulting in a more stable flame. And the NH3/air flame Φb = 0.78 with inlet bulk velocity Uin = 4 m/s can be extended wider by 16.6% to Φb = 0.65. The NH3 flame front has been locally extinguished due to excessive stretching, which is the key factor of triggering the overall blow-off. Stretching is mainly caused by shear strain. CH4 addition makes the flame front more wrinkled. Furthermore, the flame is strengthened due to the enhanced resistance to stretching by blending CH4 with NH3. But the root of the NH3/CH4/air flames with methane fractions of 50% and 100% also have excessive straining near the blow-off conditions, which induces flame blow-off. The increase of burning velocity and temperature by adding CH4 is also conducive to improving flame stability. The Inner Shear Layer (ISL) vortexes promote the uniform mixing of combustion products in the Inner Recirculation Zone (IRZ) and downstream combustion gas, which is conducive to the stability of CH4/air flame. For NH3/air flame, the ISL vortexes accelerate the flame blow-off by entraining the cold reactants into the IRZ.

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.

Lean blow-off characteristics of a tangential entry type dual swirling free and impinging flame

An experimental study has been carried out to investigate the lean blow-off (LBO) characteristics of a tangential entry type dual swirling flame under different operating and geometrical parameters. The amount of swirl is controlled by changing the ratio of tangential to axial in-flows (T:A). The qualitative flame structure is accessed by taking the direct flame images and the radial distribution of static impingement pressure. The flame structure has also been illustrated using commercial CFD code ANSYS Fluent®. The results showed that the flame behavior and the mixing between the reactants are strongly influenced by the outer flame Reynolds number (Re(o)) and the intensity of swirl (S). Significant transitions in flame shapes were observed owing to these flow alterations. These transitions in flame shapes strongly govern the stability near blow-off conditions. It has been seen that LBO limits are considerably improved with the presence of an inner supporting flame and also in presence of the impingement surface. The inner conical flame significantly supports the outer swirling flames for stability at high flow rates and under fuel-lean operating conditions. The outer flame could be sustained stably up to ϕ(o) of 0.1 with inner stoichiometric flame and both inner and outer flames could be stabilized for simultaneous variation of equivalence ratio up to ϕ of 0.6 (S = 0.86). Geometrical design parameters of the burner showed a major impact on improving the LBO limits. Thus, the dual flame presents significant improvements to lean blow-off limits as compared to the single outer swirling flame.