Flame Stability Research Articles

Fiber-based high-speed 3D schlieren imaging.

This Letter reports the first demonstration of a high-speed three-dimensional (3D) schlieren technique based on the combination of fiber imaging, Toepler's lens-type schlieren, and computed tomography (CT). The technique uses a single high-speed camera, two xenon lamps, and a series of fiber bundles to simultaneously capture the schlieren images of turbulent flames from seven orientations with a framerate beyond tens of kHz. The presented method complements the existing technique with advantages of being flexible, high speed, and low cost. The 3D schlieren technique is first demonstrated and validated on the turbulent premixed flame and stable laminar premixed flame, respectively. Then, the 3D schlieren technique is used to measure the transient, dynamic ignition process. The results show that time-resolved 3D fundamental properties of ignition kernels (i.e., structure and edge speed) can be obtained by the technique.

The State of the Art of Laminar Burning Velocities of H2-Enriched n-C4H10–Air Mixtures

Currently, hydrogen-enriched n-butane blends present a real interest due to their potential to reduce emissions and increase the efficiency of combustion processes, as an alternative fuel for internal combustion engines. This paper summarises the recent research on laminar burning velocities of hydrogen-enriched n-C4H10–air mixtures. The laminar burning velocity is a significative parameter that characterises the combustion process of any fuel–air mixture. Accurately measured or computed laminar burning velocities have an important role in the design, testing, and performance of n-C4H10–H2 fuelled devices. With this perspective, a brief review on the influence of hydrogen amount, initial pressure and temperature, and equivalence ratio on the laminar burning velocity of hydrogen-enriched n-C4H10–air mixtures is presented. Hydrogen has a strong influence on the combustion of butane–air mixtures. It was observed that a parabola with a maximum at a value slightly higher than the stoichiometric ratio describes the variation in the laminar burning velocity of hydrogen-enriched n-butane–air mixtures with the equivalence ratio. An increase in initial pressure or hydrogen amount led to an increase in this important combustion parameter, while an increase in initial pressure led to a decrease in laminar burning velocity. Overall, these studies demonstrate that hydrogen addition to n-C4H10–air mixtures can increase the laminar burning velocity and flame temperature and improve flame stability. These findings could be useful for the optimisation of combustion processes, particularly in internal combustion engines and gas turbines. However, the literature shows a paucity of investigations on the laminar burning velocities of hydrogen-enriched n-C4H10–air mixtures at initial temperatures and pressures differing from those in ambient conditions. This suggests that experimental and theoretical investigations of these flames at sub-atmospheric and elevated pressures and temperatures are necessary.

Study on Oxy-Methane Flame Stability in a Cylindrical Porous Medium Burner

Combustion in a porous medium can be beneficial for enhancing reaction rate and temperature uniformity. Therefore, considering the combination with oxy-fuel combustion can address some shortcomings in oxy-fuel burners, a cylindrical two-layer porous burner model is established based on OpenFOAM in this paper. A two-temperature equation model is adopted for the simulation of the heat transfer process. The CH4 skeletal kinetic mechanism is adopted for complex chemistry integration based on OpenSMOKE++. Corresponding experimental methods were used for complementary studies. The walls of the burner are wrapped with three types of thermal insulation materials to present different levels of heat loss. The results show that considering the convection and radiative heat loss of the burner wall, the temperature near the wall is reduced by more than 300 K compared to the adiabatic condition. As a result, the flame propagation speed and CO oxidation rate slowed down. The stable range will be destructively narrowed by more than 50%, and CO emissions will increase by more than 10 times. These defects will be aggravated by increasing the diameter of the burner. It is observed that when the diameter of the burner increases from the initial 5 cm to 10 cm, the effect of heat loss on the stable range is insignificant.

Effects of fuel-hydrogen levels on combustion, operability, and emission parameters of CH4/H2/O2/CO2 stratified flames in a dual-swirl gas turbine burner

Towards the development of emission-free hydrogen gas turbines with wider operability limits, the combined effects of lean premixed combustion, dual combustion (combustion staging), hydrogen enrichment, and oxy-fuel combustion on the combustion and emission characteristics of CH4/H2/O2/CO2 flames are investigated in a dual annular counter-rotating swirl burner. The central stream is of a higher equivalence ratio (∅) of 0.9 for stable flame ignition, whereas the annular stream is of a lower ∅ of 0.65 to maintain the burner’s environmental performance. The stratified flame reactions are modelled using the partially premixed combustion model, which has been validated using the available experimental data. The inner recirculation zone becomes smaller as the hydrogen fraction (HF) becomes higher until it vanishes and there exists only an outer recirculation zone when the secondary hydrogen fraction approaches 100%. For 100% HF in both primary and secondary flow, the maximum combustion temperature within the combustor was observed to be as high as 2350 K. Increasing HF makes the flame more compact and anchored to the burner centre body with faster kinetics, less ignition delay time, and, accordingly, with increased potential for flashback. The product formation rate was found to increase in the inner shear layer at higher HF, implying a higher flame intensity and a more compact flame. The hydrogen fraction of the primary stream (maintained at higher equivalence ratio) is observed to be dominant in regulating CO mole fractions. Stable well-anchored flames are obtained over the considered fuel mixtures, indicating the role of stratified combustion in widening the operability of gas turbine combustors under stratified combustion conditions.

Modeling of H2/air flame stabilization regime above coaxial dual swirl injectors

The prediction of the stabilization regime of partially premixed H2/air flames above an injector is a main subject of interest for the development of gas turbines powered by hydrogen. The way the flame is stabilized has an impact on NOx emissions, thermal stress on the injector, and combustion stability. A model based on the triple flame speed is revisited and improved to predict flame stabilization using information gathered at cold flow conditions. The model is called TFUP for Triple Flame Upstream Propagation. According to this model, the flame can anchor to the injector only if a zone with a flammable mixture and a sufficiently low local flow velocity exists continuously from the lifted flame to the injector lips. The TFUP model is applied to the case of a H2/air coaxial, dual swirl injector in which both the hydrogen and air streams are swirled. Fuel is injected through the central channel and oxidizer through the external channel. Particle image velocimetry and one dimensional Raman scattering measurements made at strategic locations are used to predict the edge flame speed for which a lifted flame should re-anchor to the injector. These predictions made in isothermal conditions are compared to observations of the flame stabilization regime. The central flow of hydrogen can be mixed with methane and helium and the external flow of air with nitrogen in order to prescribe different theoretical values for the triple flame speed. Predictions agree well with the observations made for all swirl levels conferred to the central fuel stream, fuel injection velocities, hydrogen contents in the fuel mixture injected in the central channel, and nitrogen contents in the oxidizer annular channel tested in the study. Validity limits are also discussed. The methodology presented in this study provides a simple framework to predict flame stabilization on coaxial injectors that can optionally be equipped with swirl vanes.

Experimental and simulation study of NH3–H2-Air flame dynamics at elevated temperature in a closed duct

Ammonia allows for long-distance and large-scale storage transport of renewable energy sources. The reactivity of NH3-Air flame can be enhanced by doping with hydrogen and by increasing the initial temperature. The flame combustion characteristics of NH3–H2-Air are studied in a rectangular closed pipe with a mole fraction of 45% H2, an equivalence ratio (φ) of 0.8–1.2 and 0.1 MPa. The initial temperatures (Tu) of the mixture are 300 K, 375 K and 450 K. Experimental studies of flame front speed, flame tip structure evolution, and overpressure dynamics are performed. Laminar burning velocity (SL), peak mole fraction of key radicals, and flame instability of NH3–H2-Air flames are simulated in Chemkin Pro-software using the Mathieu and Petersen mechanism. The results show that the relatively low equivalence ratio and initial temperature promote the structural changes of the flame tip, forming a “tulip” flame and a distorted “tulip”. As the equivalence ratio increases, the flame front speed and overpressure increase and the flame oscillates slightly. The distortion in the lip of the “tulip” is more pronounced. The increase in temperature promotes flame propagation in the early stages and the pressure decreases. Reactive activity increases with increasing temperature, the peak mole fraction of NH2 and H radicals increases, and SL increases. At normal temperature, the flame is affected by thermo-diffusive instability only when lean mixture. The Darrieus-Landau instability increases as the equivalence ratio increases. Conversely, the Darrieus-Landau instability decreases with increasing temperature. Therefore, the current experimental and simulation results help to understand the laminar combustion characteristics of NH3–H2-Air at high temperatures.

Spherically expanding flame simulations using Cantera coupled to an unsteady Lagrangian formulation

A Lagrangian-based one-dimensional approach has been applied, using Cantera libraries, to study the dynamics of spherically expanding flames. Detailed reaction models have been employed to examine spherical flame propagation in hydrogen-air mixtures. In the first part, our approach was validated against laminar flame speed and Markstein number data from the literature. It was shown that the laminar flame speed was predicted within 5% on average but that significant discrepancies were observed for the Markstein number, especially for rich mixtures. This suggests that even application of detailed reaction models can lead to inaccurate prediction of the stability of rich flames, which can be detrimental to the prediction of flame acceleration and the deflagration to detonation transition process. In the second part, a detailed analysis of the thermo-chemical dynamics along the path of Lagrangian particles propagating in stretched flames was performed. For mixtures with negative Markstein lengths, it was found that at high stretch rates, the mixture entering the reaction-dominated period is less lean with respect to the initial mixture than at low stretch rates. This induces faster rates of chemical heat release and active radical production which results in higher flame propagation speed. Opposite effects were observed for mixtures with positive Markstein lengths, for which slower flame propagation was observed at high stretch rates compared to low stretch rates.

Numerical investigation of H2/Air fueled micro combustion characteristics with trapezoidal ribs for micro thermophotovoltaic applications: Effect of rib height

In this numerical study, the impact of equidistant trapezoidal ribs on the characteristics of premixed H2-air micro-combustion was investigated, with a specific focus on the rib height. The study comprehensively examined flame structure, flame front position, flame speed, and combustion efficiency. A comparative analysis was performed between a backward-facing step micro combustor (MCSD) and micro combustors with varying rib heights: MCRD1 (0.5 mm), MCRD2 (0.6 mm), and MCRD3 (0.7 mm). The incorporation of trapezoidal ribs resulted in the creation of elongated and evenly distributed recirculation zones, significantly enhancing mixing and promoting flame stability, particularly at higher rib heights. The recirculation zones played a critical role in influencing the chemical reaction rate and the species distribution, leading to higher flame speed and greater combustion intensity. The findings highlight the outcomes of incorporating ribs in terms of combustion efficiency. The combustion efficiency values for MCRD1, MCRD2, and MCRD3 were recorded as 96.95%, 96.75%, and 96.61%, respectively, while the MCSD had a combustion efficiency of 97.14%. Hence, the recommended range of rib height is considered advantageous in ensuring an optimal balance between improved flame stabilization and maintaining a satisfactory level of combustion efficiency. The findings provide valuable insights for optimizing micro-thermophotovoltaic systems.

Structure and extinction of methane and propane nonpremixed counterflow flames at extremely low stretch rates in buoyant flowfields

This study experimentally investigated the structures and extinction behaviors of methane-air and propane-air counterflow nonpremixed flames influenced by individually controlled fuel injection velocity uF at extremely low stretch rates, which were induced by natural convection alone. A sooting limit fuel injection velocity (SLFIV) index was introduced, which increased with stretch rate in the low stretch rate region. Methane had higher SLFIV than propane. Quasi-one-dimensional blue flames appeared for both methane and propane within a certain uF range. The range for methane was higher than that for propane. The steady flame standoff distances yf had a relationship with uF and low stretch rate ab of yf∝uFabλ∞1/2, which differed from high-stretch flames. Different flame instabilities occurred for methane and propane by changing uF only without fuel or oxidizer dilutions. At low uF near extinction limit of methane, periodic flame holes appeared, which was due to large heat loss. This instability finally caused flame extinction by keeping retreating edges. At high uF, simultaneous cellular and wave flame instability appeared for propane, which differed from cellular flames occurred alone in highly diluted oxy-fuel flames. By modulating uF alone in this experiment, the cellular/wave flame was due to Rayleigh-Taylor and hydrodynamic instabilities. Flame extinction at the lowest uF for propane was through outer edge shrinking, where the displacement speed increased with stretch rate. Flammability maps of limit uF versus low ab were established for both methane and propane. Both maps were divided into four regions by sooting, instability and extinction limits. The three limits uF all increased with ab. This changing tendency was opposite with high-stretch flames reported previously. For propane, the instability limit uF approached infinity at higher ab. This work provided knowledge of extremely low stretch flame combustion, which can be applied to microgravity flames, because the combustion characteristics of microgravity forced low stretch flames were identical to those of low stretch flame induced by natural convection alone.

Air-blast atomization and ignition of a kerosene spray in hot vitiated crossflow

Increasingly stringent regulations of pollutant emissions from aviation require rapid implementation of novel combustion technologies. Promising concepts based on moderate or intense low-oxygen dilution (MILD) combustion have been investigated in academia and industry. This MILD regime can be obtained from the recirculation of the hot vitiated combustion products to raise the temperature of the reactants, resulting in distributed reaction regions and lower flame temperatures. In the present work, we consider the air-blast atomization of a kerosene spray in crossflow, which enables efficient mixing between fuel and oxidizer. We investigate experimentally and numerically the effect of the spray air-to-liquid mass-flow ratio (ALR) variation on the reaction front and flame topology of a kerosene spray flame. The spray is injected transversely into a turbulent vitiated crossflow composed of the products of a lean CH4-H2 flame. The spray flame thermal power is varied between 2.5 and 5 kW, along with the atomizer ALR between 2 and 6. The experimental characterization of the reaction zone is performed using OH* chemiluminescence and OH and fuel planar laser-induced fluorescence (PLIF). The Large Eddy Simulations (LES) of the multiphase reactive flow provide good agreement with the experimental observations. Experiments and simulations show that the ALR governs mixing, resulting in different flame stabilization mechanisms and combustion regimes. Low ALR results in a relatively small jet-to-crossflow momentum ratio and a large spray Sauter mean diameter (SMD). A thick windward reaction region is formed due to inefficient shear layer mixing between the fuel spray and the crossflow. Meanwhile, the correspondingly large spray SMD leads to isolated penetration and localized combustion of fuel clusters. At high ALR, the higher penetration and the faster droplet evaporation due to the lower spray SMD result in an efficient entrainment-induced mixing between the two streams, forming more distributed reaction regions.

Experimental and numerical study of flame stability in a rotary flow reversal reactor with ultra-lean toluene/air mixture

A rotary flow reversal reactor with a fixed packed bed and rotary intake is established to study the combustion stability with ultra-lean toluene/air mixture. The experimental results show that the stable temperature distribution in the packed bed is vertically graded and circumferentially uniform with bottom rotary intake. The three-dimensional numerical simulation using sliding grid technology on the combustion process is presented. The effects of intake toluene concentration, axial velocity and circumferential rotation speed on the combustion stability are analyzed numerically. High intake toluene concentration or feed axial velocity will decrease the propagation velocity of the flame front and raise the peak temperature and the maximum reaction rate to achieve a quasi-steady state finally. However, the intake rotation speed has little effect on the combustion process. The work will enrich the investigation of flame stability during the filtration combustion of ultra low-calorific value gas.

A waste textiles-based multilayer composite fabric with superior electromagnetic shielding, infrared stealth and flame retardance for military applications

The challenge of developing multifunctional military tent fabrics with low-cost, stable electromagnetic shielding and infrared stealth in complicated environments is enormous. Herein, a novel aluminium-flammability carbonized waste cotton-carbon felt (A-FCWCF) composite fabric consisting of hydrophobic flame-retardant fabric, carbonized waste cotton felt and carbon fiber felt was prepared via a simple yet low-cost stitching process. The resulting composite fabric was extensively investigated for its stability and applicability in a complicated environment. Due to the significant conductivity and impedance mismatches, the A-FCWCF composite fabric displayed exceptional EMI-shielding properties, plus an X-band EMI rating of 82.63 dB. Moreover, the A-FCWCF composite fabric presented excellent infrared stealth performance at room and extremely cold outdoor temperatures, owing to its high porosity and low emissivity. The functional finishing on the A-FCWCF composite fabric also made it stable flame retardate and hydrophobic. Furthermore, the EMI-shielding and infrared stealth performances of A-FCWCF composite fabric were almost unaffected even if subjected to environmental stability, mechanical properties and weather resistance tests, making it a promising candidate for tough natural environments. Overall, this low-cost and simple preparation strategy of multi-layer stitched multifunctional fabric can revolutionize the production of military materials, improving functionality even in the most complex environments.

Experimental study of stability and pollutant emissions of turbulent biogas flames under microsecond pulsed plasma

Biogas is a mixture of methane (50–80%.vol), carbon dioxide (20–45%.vol), and in smaller proportions of N2, O2, H2S, and NH3. For a high rate of CO2 in the biogas, the combustion can generate strong instabilities and the flame can be very lifted from the burner and even blown. To overcome these problems of instabilities and flame extinction, the use of a microsecond pulsed plasma is considered for this study. This study aims to characterize the effects of microsecond pulsed discharges on the stabilization and emission characteristics of non-premixed swirling biogas/air flames at different operating conditions of combustion (CO2 rate, power, and equivalence ratio) and plasma (pulse repetition frequency and input electrical voltage). The plasma is performed above a coaxial burner with a swirler of the oxidizer. It is generated by a DC-pulsed power generator with a high voltage (HV) pulse duration of a few microseconds. The burner consists of two concentric tubes and a central metal rod used as a cathode for the plasma. The outer tube contains a swirler and supplies the oxidant flow, and the inner tube delivers the fuel (methane, biogas) around the central metal rod. High voltage pulses are applied on two anode tungsten wires that are symmetrically arranged from either side of the cathode to obtain two plasma zones in the stabilization region of the flame. The burner is placed in a parallelepiped combustion chamber with a volume of 100x48x48 cm3. Mass flow rates of air and fuel are regulated by thermal mass flow controllers (Brooks SLA5851S). OH* chemiluminescence measurements are done to describe the structure and the flame stability throughout the lift-off heights (Hf) variation. Results showed that plasma generated by microsecond HV pulses can improve flame stability by decreasing its Hf especially at low fuel velocities. In this regard, the optical emission spectra (OES) were visualized. The results reveal that the pulsed plasma generates chemically active species such as excited N2*, CH*, OH* molecules, and H* atoms, leading to improved flame stability. Their intensities' dependence on the pulse repetition frequency, the sight position of the OES, the concentration of CO2 in the fuel, and the fuel velocities were investigated in detail. Furthermore, the NOx and CO emissions in the burned gas are investigated over a wide range of repetition pulse frequencies. It is seen that the microsecond pulsed plasma slightly increases the NOx emission and reduces CO concentration in the flue gases.

Self-Acceleration and global pulsation of unstable laminar Hydrogen-Air flames

The self-acceleration and global pulsation of spherically expanding laminar hydrogen-air flames were studied in a spherical explosion vessel, over a wide range of equivalence ratios (0.4–2.0), initial temperatures (300–400 K), and initial pressures (0.1–0.7 MPa). Comprehensive quantitative data on the self-acceleration propagation of unstable laminar hydrogen-air flames were obtained. The results show that the self-similarity appears after the onset of instability, and the previous assertion of acceleration exponent α = 1.5 is not valid for hydrogen-air flames. The derived values of α for such flames vary from 1.125 and 1.39 in the present work. For the self-accelerating flame, the temporal evolution of the flame radius is described by ru=Atα, and temporal flame propagation speed is described by Sn=Aαtα-1. A modified theoretical expression of the constant A is proposed and validated against the present experimentally derived results. The acceleration phase after flame instability presents a global pulsating acceleration pattern. The frequency of global pulsation rises as the pressure or temperature of the flame increases. A pulsing flame speed equation can be used to reliably estimate flame speed after the onset of cellular instability. Self-acceleration observed in small laboratory explosions can serve as a predictive indicator for flame behaviour on a larger atmospheric scale.

Role of hydrogen on aviation sector: A review on hydrogen storage, fuel flexibility, flame stability, and emissions reduction on gas turbines engines

Hydrogen is being recognized as a versatile energy carrier that can aid in the transition towards a decarbonized energy future due to its high specific energy density. Gas turbine engines are primary source of energy for the modern aircraft. To make the air transportation economical it is essential to achieve higher thermal efficiencies and the capability to operate on carbon–neutral fuels such as hydrogen. The current review article examines the effect of hydrogen in the gas turbine engines. Furthermore, the green routes for the hydrogen production and storage were discussed in detail. Additionally, the implementation of the conceptual design for the onboard hydrogen utilization in the long-range endurance aircrafts was considered. The utilization of hydrogen as a fuel in gas turbines is subject to the influence of different factors, including the functioning of the turbine components, ambient conditions, combustion stability, flame characteristics and operating parameters. This study explores the use of hydrogen as a fuel in aircraft gas turbine engines, which has the potential to reduce NOx emissions, improve fuel efficiency, and increase range while significantly decreasing pollutants such as carbon monoxide. However, modifications to the engine system are necessary, and careful consideration of factors such as fuel storage, combustion, and emissions is essential. While there are significant challenges to storing hydrogen for use as a fuel in aircraft, finding an optimized solution is crucial for reducing carbon emissions and moving towards a more sustainable future. Overall, hydrogen has the potential to be a viable alternative to conventional fuels in aircraft engines.

Numerical investigation on thermal performance improvement of hydrogen-fueled tube with rib/fins for micro thermophotovoltaic

Combustion characteristics and thermal performance of pre-mixed hydrogen/air in micro tubes for micro-thermophotovoltaic are conducted under various operating conditions. To improve flame stability and energy efficiency, a cylindrical stepped tube with rib/fins is proposed. Effects of burner optimization on hydrogen-fueled flame regime, heat transmission and thermal performance are numerically conducted, which is verified by experimental tests. The results illustrate that the inserted rib and its location strongly modify the anchoring and propagation of premixed H2/air flame, resulting in the improvement of heat transfer and radiation temperature. Furthermore, the employment of outlet-fins modifies the flow field and enhances heat recirculation by utilization of hot exhausted gases. There exists a balance between the radiation temperature reduction and the radiation surface augment via the increment of chamber length. Meanwhile, the influence grades of chamber length, flow rate, outlet-fins number and rib position are ranked in turn based on the grey relational analysis. So, electrical power of combustor C33-7-10 is 0.77 W higher than that of stepped tube for a micro-TPV with InGaAsSb at mf = 5.10 × 10−5 kg/s, and it obtains a higher power output of 2.22 W at mf = 8.16 × 10−5 kg/s. It can also be further improved by the flow rate increment and employment of high efficiency PV cells.

Combustion Characteristics of a Novel Ammonia Combustor equipped with Stratified Injection for Low Emissions

Ammonia – a carbon-free fuel- is being considered by various research groups and organizations as a potential alternative fuel to tackle some global warming issues of the 21st century. However, ammonia faces some challenges as a fuel, particularly, the production of unwanted NOx and unburned ammonia traces at the back end of combustion systems accompanied by unstable flame regimes. Therefore, new concepts are currently being developed to address these challenges whilst also providing stable, high-power operation. This work presents a newly designed stratified combustion system for gas turbines capable of operating with both premixed and stratified modes. The system has been initially evaluated using these two modes employing solely hydrogen and ammonia. Increase of hydrogen stratification resulted in the reduction of NO and NO2, while N2O slightly increased due to the reduction of fuel in the premixed system. Further tests were conducted to evaluate the potential of the systems to replace fossil fuels. Previous work done using methane as the fuel-to-be-replaced has shown interesting trends in terms of emissions and stability. Thus, to progress on the subject, the work conducted here attempted propane/ammonia/hydrogen using this innovative strategy. Results denoted how stratification has a strong impact on NOx production at the flame front, whilst the mechanism of flame stabilization of the new burner enables large operability regions under a great variety of fuel combinations. Strategic injection setups were identified, whilst the best propane/ammonia/hydrogen blends for stability and emissions reduction are also detailed for further implementation in larger systems.

The effect of hydrogen enrichment on thermoacoustic instabilities in laminar conical premixed methane/air flames

We perform a conceptual study about the linear stability of a hydrogen-enriched methane-air premixed laminar flame inside a cylindrical duct with both ends open. The flame and acoustic subsystems are coupled using the Flame Transfer Function (FTF). The power output is set to a constant value and the hydrogen molar fraction of the fuel mixture is varied. We show that adding hydrogen reduces the flame length and impacts the FTF in two ways: (i) gain drop-off shifts towards higher frequencies and (ii) phase lag reduces. Then, we compute the eigenvalues of the thermoacoustic system varying the hydrogen molar fraction. We show that adding hydrogen to the fuel mixture can stabilise linearly unstable systems and vice versa, depending on the duct natural frequencies. The results show that the hydrogen molar fraction can be a tunable parameter for controlling thermoacoustic instabilities.

Stabilization and soot/NOx emission of hydrogen-enriched methane flames in a turbulent jet with coaxial air under elevated pressures

Adding hydrogen to methane has gained interest as a means of reducing CO2 emissions from combustion systems. However, this process can lead to several issues, such as flashback and NOx emissions. Therefore, it is essential to determine the optimal conditions for flame stabilization and emission to design effective hydrogen burners. This study chose a turbulent fuel jet with coaxial air as the basic configuration, and its combustion characteristics were investigated under elevated pressures. Additionally, the depth of the fuel tube in the coaxial air tube was actively controlled to regulate flame stabilization, and its effects were examined for various pressures, velocities, and hydrogen concentrations. Three flame stabilization modes were observed clearly, i.e., inner-attached, outer-attached, and inner-lifted modes. Furthermore, the parameters that influenced the flame modes were identified, and the characteristics of soot and NOx emissions were studied. The inner-lifted mode demonstrated the best combustion performance, and the corresponding conditions were predicted to be extended through hydrogen enrichment and pressure elevation. Therefore, the inner-lifted flame is recommended as the conceptual design goal for hydrogen-enriched high-pressure burners.

Large-Eddy Simulation of swirled flame stabilisation using NRP discharges at atmospheric pressure

In this work, the use of Nanosecond Repetitively Pulsed (NRP) discharges is investigated numerically to improve the stabilisation of the flame in a swirl burner. The studied configuration is the PACCI burner test rig set up at KAUST, in which it was experimentally demonstrated that NRP discharges can effectively enhance flame stability. Simulations are performed with the reactive compressible Navier–Stokes solver AVBP, using a recently developed phenomenological model for plasma-assisted combustion of methane-air premixed flame. The ability of the numerical model to reproduce the main flow behaviours is first assessed by comparison with PIV measurements in cold flow conditions. Then, a lean atmospheric pressure case (ϕ=0.67) without plasma actuation is simulated and compared with OH* chemiluminescence image from experiment. Both numerical and experimental results reveal an unstable turbulent flame, with intermittent attachment to the burner exit. Finally, two operating conditions with NRP discharges are simulated. The NRP discharges correspond to 10 kV pulses applied at a frequency of 20 and 30 kHz, respectively corresponding to a discharge power 0.72 and 1.17% of the thermal flame power. First, a significant stabilisation effect is observed when applying the discharges, which efficiently mitigate the flame lifting. This effect is quantitatively demonstrated by tracking the flame centre of gravity, which shows that the flame moves upstream under the influence of the plasma, in good agreement with experimental observations. Results also show that a net power gain is obtained by applying NRP discharges to combustion by comparing the thermal flame power to the plasma power. In particular, an increase of plasma actuation efficiency from 4 to 6 as been observed in the cases studied by increasing the plasma power.