Coflow Air Research Articles

Co-flow jets application for occupant targeted ventilation: Focus on thermal comfort and air quality

Co-flow air jets can be used in occupant-targeted ventilation, such as personalized ventilation, stratum ventilation, aiming at improving air quality in the breathing zone of occupants. However, these have not been studied sufficiently. The velocity field and cleanness of co-flow jets comprising inner jet of clean air encircled by outer jet of room air were studied under isothermal conditions by physical measurements and CFD simulations at numerous combinations of initial diameter and velocity of the inner and outer jets appropriate for practical application. The co-flow jet had a lower axial velocity decay along the jet direction than that of the single jet. The magnitude (turbulence intensity) and frequency of the velocity fluctuations in co-flow jets were lower compared to those in a single jet. The results revealed that with regard to thermal comfort co-flow jets may provide more cooling compared to single jets, especially when supplied toward to the user. The level of cleanness depended on the initial velocity of the inner and outer jets as well as their diameters. The air on the jet axis of the co-flow was up to 40 % cleaner compared to the single jet. The increased area ratio of outer nozzle to inner nozzle from 1.0 to 3.0 improved the cleanness of the air by at least 10 % to the distance from 8 to 14 initial diameters compared to a single jet. The co-flow jet with velocity ratio of 1.5 had longest concentration core length, Lu, (up to 6 initial diameters to the inner jet) with clean air. The results can be used for design of occupant targeted ventilation.

Soot and Flame Structures in Turbulent Partially Premixed Jet Flames of Pre-Evaporated Diesel Surrogates with Admixture of OMEn

In this study, the soot formation and oxidation processes in different turbulent, pre-evaporated and partially premixed diesel surrogate flames are experimentally investigated. For this purpose, a piloted jet flame surrounded by an air co-flow is used. Starting from a defined diesel surrogate mixture, different fuel blends with increasing blending ratios of poly(oxymethylene) dimethyl ether (OME) are studied. The Reynolds number, equivalence ratio, and vaporization temperature are kept constant to ensure the comparability of the different fuel mixtures. The effects of OME addition on flame structures, soot precursors, and soot are investigated, showing soot reduction when OME is added to the diesel surrogate. Using chemiluminescence images of C2 radicals (line of sight) and subsequent Abel-inversion, flame lengths and global flame structure are analyzed. The flame structure is visualized by means of planar laser-induced fluorescence (PLIF) of hydroxyl radicals (OH). The spatial distribution of soot precursors, such as polycyclic aromatic hydrocarbons (PAHs), is simultaneously measured by PLIF using the same excitation wavelength. In particular, aromatic compounds with several benzene rings (e.g., naphthalene or pyrene), which are known to be actively involved in soot formation and growth, have been visualized. Spatially distributed soot particles are detected by using laser-induced incandescence (LII), which allows us to study the onset of soot clouds and its structures qualitatively. Evident soot formation is observed in the pure diesel surrogate flame, whereas a significant soot reduction with changing PAH and soot structures can be identified with increasing OME addition.

The effect of sodium chloride on the charge state of soot particles in a laminar diffusion flame

This study investigates the effect of sodium chloride on the charge state of soot nanoparticles formed in a laminar diffusion flame by measuring the soot particle size distribution, average charge per particle, and charge fraction at various heights within the flame. The well-studied Santoro burner with methane as the fuel (at 0.35 L/min) and co-flow air (at 70 L/min) was used, which produced a stable laminar diffusion flame with flame height of 61 mm. Samples of soot nanoparticles were extracted via a 0.3 mm orifice in a 3-mm stainless steel tubular probe at various heights above the burner and were immediately diluted by a factor of a few thousand for aerosol measurement. The effect of sodium chloride on a methane flame was investigated by comparing the experimentally measured data for methane-only and methane-NaCl flames. The addition of NaCl particles to the laminar diffusion flame did not have a significant effect on the particles in the nucleation region of the flame. The majority of the incipient soot particles in both flames are uncharged, and their sizes are nearly the same, with diameters of approximately 5 nm or less. However, the size of soot particles differs by approximately 10 % to 25 % between methane-only and methane-NaCl flames in the coagulation-dominated region of the flame. The net charge on soot particles within the coagulation region of the methane-only flames is negative, while it is positive with NaCl addition. The fraction of charged particles and ion concentration decreases with NaCl addition within the coagulation region. The study indicates that the smaller particle size observed in methane-NaCl flames may be attributed to reduced coagulation via altered particle charge states. These factors could be the major contributor to the variations in soot formation between methane-only and methane-NaCl flames. Novelty and Significance StatementThis research addresses a notable knowledge gap concerning the influence of NaCl, a prevalent component of hydraulic fracturing fluids, on soot particle behaviour in flames. By explaining the impact of NaCl on soot formation and charge states, the findings contribute to a deeper understanding of combustion processes in environments influenced by hydraulic fracturing operations, thereby informing strategies for reducing emissions and improving environmental sustainability in energy production. Overall, this article represents a significant advancement in the field of combustion science, with implications for environmental stewardship in energy production.This study provides valuable insights into how the introduction of NaCl alters the electrostatic properties of soot particles. This investigation expands upon existing knowledge by shedding light on the intricate mechanisms underlying soot formation and evolution in the presence of NaCl, elucidating its potential implications for combustion processes involving fossil fuels and industrial flaring.

Soot modeling of oxygen-depleted and highly sooty turbulent buoyant flames using the In Situ Adaptive Tabulation (ISAT) method

In this paper, a newly developed LSP-based soot (LSPTwoEq) modeling is systematically assessed in the Large Eddy Simulation (LES) of various turbulent buoyant flames against a previously developed LSP-based soot (LSPOneEq) model. Two turbulence-soot interaction (TSI) closure schemes are formulated based on the Eddy Dissipation Concept (EDC). Accordingly, two soot modeling frameworks consisting of the mentioned soot and their corresponding TSI models are applied in the simulations. An in situ Adaptive Tabulation (ISAT) implementation is also proposed to speed up the simulation of the TSI model formulated for the LSPTwoEq model. The results indicate that the proposed soot modeling framework is significantly more accurate for ethylene flames in reduced oxygen concentrations and a highly sooty toluene flame. The accuracy of both frameworks is comparable for the ethylene flame with an air co-flow. The speed-up gained by using the ISAT scheme is found to be a factor of four for the ethylene flame with an air co-flow and the toluene flame. It is larger than nine for the ethylene flames with reduced oxygen co-flow.

Investigation of hydrogen/air co-flow jet flame propagation mechanism in supersonic crossflow

The design of fuel injection schemes is crucial for improving the combustion performance of high-Mach number scramjet. To clarify the feasibility of the coaxial jet injection scheme, high fidelity Large Eddy Simulation of the supersonic coaxial jet flame is conducted. The simulations are in good agreement with the experimental results in terms of time-averaged velocity, temperature, and species distribution. Auto-ignition phenomenon and the characteristics of partially premixed flame are well captured. The introduction of co-flow air increases the vorticity magnitude close to the injection port and downstream near-wall region, which results in a 400 K rise in the time-averaged temperature on the downstream near-wall region and a 4% increase in the proportion of premixed combustion near the injection port. Moreover, the instantaneous distribution of hydroxy radical indicates that the spanwise width of the windward reaction shear layer is reduced utilizing the coaxial jet scheme. Chemical kinetic analysis is applied to reveal the propagation mechanism of partially premixed flames. Thermal explosion is the chemical explosion mode for all coaxial jet flame front, which are dominated by a high-temperature reaction path. The low-temperature reaction path mainly exists in the transverse jet injection port, downstream near-wall region of the single transverse jet and co-flow lifted flame base. These significant findings provide valuable insights for the potential engineering application of the coaxial jet injection scheme to a high Mach number scramjet.

Effects of O2 concentration of O2/CO2 co-flow on the flame stability of non-premixed coaxial jet flame

Greenhouse gas emissions should be reduced to stop the advance of global warming. Oxy-fuel combustion is one of the methods to reduce CO2 emissions. This paper aims to study the stabilization of CO2-diluted oxy-methane non-premixed jet flames and compare this with the results of methane-air flames. The lower the co-flow velocity and the higher the oxygen concentration in the co-flow, the higher the stability. If the flame stability is strong enough to withstand flame shape changes from the over-ventilated flame to the under-ventilated flame, the flammable range without blowout increases significantly under the under-ventilated flame. Otherwise, by decreasing the liftoff flame stability, the over-ventilated flame cannot change to the under-ventilated flame and blows out near the globally stoichiometric condition. The flame stability of a CO2-diluted oxy-fuel flame can differ from that of methane-air flames due to the properties of CO2 and N2. The effective velocity proposed by Guiberti et al. shows a good collapse of the dimensionless liftoff height among the O2/CO2 co-flow, but the dimensionless liftoff height under the air co-flow is not on the same line with the O2/CO2 co-flow. This study suggests a modified dimensionless liftoff height equation considering the fuel and co-flow mixture kinematic viscosity instead of the fuel kinematic viscosity and the ratio of the thermal diffusivity of the mixture. The thermal diffusivity ratio does not affect the result of the air co-flow. Additionally, the new coefficient β and turbulent Schmidt number are suggested because these numbers are factors depending on the burner geometry. The modified effective velocity using new coefficients and the modified dimensionless liftoff height show a good collapse for not only O2/CO2 co-flows but also air co-flow. Also, the modified effective velocity shows the possibility of the critical liftoff velocity prediction.

Numerical investigation of methane-air jet flame with hydrogen addition in industrial kiln burners

Emission reduction and decarbonisation in the industry are crucial for low-carbon industry and energy transition at the global level. Replacing traditional fossil fuel with clean energy is an effective approach to reduce carbon emissions and optimize energy efficiency in manufacturing processes. Industrial kiln, which requires high natural gas fuel consumption, typically releases amounts of harmful combustion products. In this paper, the objective is to study the influence of hydrogen addition into methane-air jet flame in industrial kiln burners. The industrial burner analysed in this study is a cylinder vessel with axial orifices and swirl turbulent co-flow air jets in the fuel-air inlet structure. A more recent reduced chemical kinetic mechanism for methane-hydrogen combustion is utilized in the present flame simulation and validated in benchmark flames. The chemical mechanism involves 45 reactions and 18 species. Various methane-hydrogen blending fuels are studied in the jet flame, where flame structure and flame characteristics including chemical species, temperature, and velocity are predicted.

Experimental investigation of the effects of inlet high-swirl air preheating and dilution on kerosene flame

This study investigated the effects of co-flow air swirling on the combustion of Kerosene, as relatively heavy fuel, under preheated and diluted air conditions. Various factors affecting the flame structure, such as preheating temperature, dilution, swirl number, and airflow rate were studied using the upgraded MILD (Moderate or Intense Low-Oxygen Dilution) combustion tester of Sharif University of Technology (SMSTR). This tester, which is symmetrically axial, provides co-flow air with the desired temperature, swirl number, and dilution for a liquid fuel spray burner. The flame characteristics was investigated through direct photography and chemiluminescence. The results show that the stability range significantly increases with the air swirling, while the flame length decreases. Enhancement of combustion quality and more complete combustion of fuel were observed with preheating. Preheating resulted in geometric changes in the appearance of the flame, specially reducing flame lift-off and the front view area in high airflow. A decrease in the air oxygen content causes a general change in the structure of the flame, resulting in a shorter and more lifted one compared to non-diluted condition. The chemiluminescence images of the flame also revealed weaker and more uniform emission pattern with the inlet high-swirl air preheating and dilution. By utilizing around 15 % oxygen content dilution, preheating at around 656 K, and a swirl number of 0.8, the combustion zone exhibited similarities to the MILD combustion mode, suggesting the achievement of MILD combustion. The high swirl concept allowed higher dilution limits (about 12 %) and reduced preheating requirements (approximately 11 %) in achieving the MILD combustion regime compared to the previous study conducted without swirl conditions. In contrast to the non-swirl mode flame, the flame in this study transitioned completely to a MILD-like condition.

Impact of electric body force on lifted flame displacement speed: Numerical insights

Recent advancements in experimental capabilities have unveiled that externally applied electric fields primarily affect flames through flow modification induced by an electric body force. Specifically, the displacement speed of a flame could be significantly increased (decreased) by a parallel (orthogonal) electric field to the flame's propagation direction, while the flame's propagation speed remained negligibly influenced by an electric field. Here we present numerical findings attributing a modified upstream velocity to an electric body force created by redistributed space charges and a resulted electric field. Our investigation involved both simulation and experiment on propagating edge-flames from a stationary lifted flame exposed to the parallel external electric field. Nitrogen diluted methane jet in a coflow air was employed, and seven charged species (electron, H3O+, O2–, O–, OH–, CHO3–, and CO3–) were included in a reduced methane kinetic mechanism. Both the experiment and simulation consistently demonstrated that enhanced flame displacement speeds resulted from a reverse flow in the upstream, created by the electric body force. A stronger reverse flow, and thus more enhanced flame displacement, was achieved when the electric field direction was aligned with the flame propagation direction compared to the opposite direction. This difference in the reverse flow was attributed to variations in the electric field, caused by the different electric field screening zone within the flame. Surprisingly, space charge densities in the upstream were similar in both polarities, challenging previous hypothesis. The rapid transformation of electrons into O2–, CHO3–, and CO3– imposed a substantial number of charges in the upstream. This insight enhanced our understanding of how flame displacement can be controlled by applying electric fields. Importantly, we suggested that the use of a simple ionic kinetic model may fail to predict the flow field and, consequently, cannot adequately investigate modified flame characteristics.

NO emission characteristics of air coflowed non-premixed ammonia jet flame at elevated ambient temperatures and with N2 dilution

Ammonia is promising carbon-free alternative fuel, while the poor combustion performance and severe NO emission hinder its application. In this work, a strategy combined with preheating and dilution was discussed, and NO emission characteristics of air coflowed non-premixed ammonia jet flame were experimentally investigated over 550–700 °C and with N2 dilution, using a novel preheating facility. Results show that the general excess air coefficient is significant for NO emission, but not the unique factor. NO emission monotonically increases with the general excess air coefficient, while the slope decreases. Ambient temperature is also of a remarkable effect, that NO emission decreases by about 100 ppm (15% O2, dry) when the ambient temperature rises from 550 °C to 700 °C. Besides, effects of dilution were considered with three specific strategies. Air coflow dilution is of a negative influence on NO emission, while, NH3 jet dilution promotes NO emission in contrast. The interaction between the two single-dilution strategies is limited, and the effect of double dilution seems to be a simple sum of that of the other two single-dilution strategies. In addition, flame liftoff was observed in experiments and a considerable decline of NO emission was detected with the flame regime transition. According to the experimental results, the combination of elevated ambient temperature and air dilution is evaluated to be a promising low NO emission combustion method of ammonia.

Joint OH-PLIF and Mie scattering imaging of enhanced water mist suppression of buoyant fires

This study presents the first near-field measurements of interactions between reaction layers and water mist droplets suppressing a buoyant turbulent diffusion flame that is representative of a fire. Joint Mie scattering and OH-PLIF are employed at a high repetition rate of 400 Hz to image the droplet field and OH structure, which are deemed here to approximate the high-temperature reaction zone. Compressed natural gas (CNG) buoyant diffusion flames are stabilised on the Sydney Turbulent Buoyant Fire Burner, which is surrounded by a low-velocity air co-flow that is seeded with fine water mist droplets (d32 ∼ 50 µm). The water mist is doped with varying concentrations of metal alkali salts (NaHCO3, KHCO3) that release radical scavenging molecules, providing enhanced suppression capabilities through chemical inhibition in addition to the quenching effect from the water droplets. The spray boundary condition across the wind tunnel exit plane is quantified using Phase Doppler Interferometry. Flame extinction limits are measured by defining the critical condition at which the flame destabilises and lifts off the burner exit plane. A stability map is developed to quantify suppression efficacy in terms of the water mist flow rate and inhibitor concentration. Joint Mie-OH images of selected cases at varying extinction levels show that the addition of inhibitor introduces frequent breakages in the OH layers, marking regions of local extinction and reignition. Large mass fractions of inhibitor in the water, which, once evaporated, produces excess salt crystals in the local extinction zones that causes streaks of salt clouds that line both the lean and rich side of the flame. The chemical scavenging effects of the OH radical by NaHCO3 and KHCO3 is quantified by evaluating the reduction of the mean OH fluorescence signal in the flame at varying locations. Probability density functions for the OH area characterise local extinction in the flame due to the radical scavengers. The order of OH radical depletion and localised extinction in the flame due to the water mist follows: KHCO3 > NaHCO3 > H2O.

Iron particles ignition in different hot coflow temperatures

Understanding the ignition of iron particle combustion in hot environments is critical for harnessing the use of metal fuels in clean energy production. In this research work, a new single particle burner is implemented to disperse and burn iron particles using hot air coflow. The burner disperses iron particles through Coulomb forces between two air-spaced capacitor electrodes. The oxidizer coflow is heated up as it passes through a newly designed induction gas heater. Electromagnetic induction is used to heat up multiple metallic and porous heat discs by Joule heating. When the coflow gas passes through the pores of the discs, it carries the heat generated within the porous discs and delivers it to the travelling iron particles which provides ultimate heat transfer effectiveness. Coflow temperature homogeneity is examined using Schlieren imaging and confirmed by thermocouple measurements at the outlet of the coflow tube. Particle ignition and combustion occur in open space, providing excellent conditions for optical diagnostics. Two synchronized high-speed cameras were used to simultaneously determine the particle size using green laser shadowgraphy and detect iron particles ignition. Iron particles ignition experiments were conducted for a sieved patch (45–53 μm) at different hot coflow temperatures starting at 700℃ up to 900℃. The burner showed extensive capabilities to maintains a stable and homogeneous hot coflow for a wide range of temperatures up to 900℃. Ignition experiments showed 5.67 % of ejected particles are burnt at 700 ℃, while full ignition (99.33 %) can be achieved at 900 ℃.

Comparative study of combustion and thermal performance of a meso-scale combustor under co- and counter-rotating fuel and oxidizer swirling flows for micro power generators

Direct energy conversion systems, such as thermophotovoltaic and thermoelectric generators, have received increasing attention in micro power generation. Micro and meso-scale combustors are one of the most core components in these systems. So, developing combustion stabilization technologies for micro or meso-scale combustors is of great importance. In these systems, a hydrocarbon fuel with high energy density is burned in a micro or meso-scale combustor. Many studies have been conducted to explore various combustion stabilization techniques, but as a novelty, in this work, we study the combustion and thermal performance of a meso-scale micro power generator powered by a swirling fuel jet discharging into a co- or counter-rotating air coflow. To do so, we used 3D-printed axial swirlers with double annulus to form the swirling co- and counter-rotating fuel (methane) jet-air coflow configurations at various air and fuel flow rates. Blow-out limit, flame characteristics, combustor mean outer wall temperature, pollutant emissions, emitter efficiency, and normalized temperature standard deviation are investigated in this study. The results show that swirl addition enhances the blow-out limit significantly and co-rotating swirling flows generally enhances the flame blow-out limit when compared with the counter-rotating swirling flows mode at high fuel flow rates. Moreover, the combustor with co-rotating swirling flows has shorter lift-off height and longer flame length. The sensitivity of the flame lift-off height to an increment of the fuel mass flow rate is smaller in co-rotating than in counter-rotating swirling flows (more than 40%). Furthermore, it is observed that under the same operating conditions, co-rotating swirling flows exhibit lower values of CO and NOx in the flue gas and higher values of mean outer wall temperature, combustion efficiency, emitter efficiency, and normalized temperature standard deviation. The enhancement of the emitter efficiency by implementing co-swirl configuration is about 35%, 26%, and 8% for the methane flow rates of 0.050, 0.100, and 0.150slpm, respectively when compared with the counter-swirl mode. The results of this work can provide useful information to choose between co- and counter-rotating flows for combustors of combustion-based micro power generators.

Quantitative studies of NO emissions from various reaction pathways in swirling combustion of hydrogen-enriched methane

Hydrogen-enriched methane is a promising alternative fuel for carbon dioxide reduction. Large eddy simulations of turbulent combustion and NO emission are carried out using a new numerical method, which incorporates an improved NO prediction model and a recently-developed NO reaction pathway separation approach. Quantitative analyses of NO formations from all reaction pathways are conducted in an open swirl burner (combustion in the open environment) with hydrogen-enriched methane at a constant thermal power of 10.85 kW at the atmospheric pressure. Results indicate that different NO reaction pathways response very differently with hydrogen addition. In the open burner with hydrogen enrichment up to 60 %, the prompt NO makes the largest (around 50 %) contribution to the total NO emission, but its relative contribution decreases with hydrogen addition. The thermal NO strongly increases with hydrogen addition, but it accounts for only around 25 % of the total NO emission in hydrogen-enriched cases, because of the reduced high-temperature zone by co-flow air dilution. NO formation from the NNH pathway largely increases and contributes around 20 % to the total NO emission with hydrogen enrichment. Results would provide quantitative understanding of NO emission in the bluff-body stabilized swirling combustion of hydrogen-enriched methane in an open burner.

Simulation of reacting flows in packed beds using flamelet generated manifolds

The propagation speed is a key characteristic of a premixed flame since it determines its location and shape. Its correct prediction is particularly important when the flame propagates in a packed bed due to the acceleration of the inflow mixture as it passes through the interstices and the potential interaction with the walls surrounding the interstices. Since the kinetic mechanism has an effect on the propagation speed, the use of detailed chemical kinetics is desired. To limit the cost of chemistry resolution, the feasibility of using the flamelet generated manifolds method (FGM) is investigated. A 2D laminar premixed methane/air flame with a co-flow of air, which propagates into a packed bed, is set up as a test case. The packed bed is either represented by a porous medium or by a resolved approach where particle shapes are fully resolved. The solutions using chemistry tables and finite rate chemistry with a detailed mechanism (GRI3.0) under adiabatic conditions and with heat exchange between the two phases are in good agreement. In addition, the computational effort using FGM is about 13 times lower. In a next step, the employed FGM approach is validated against experiments. For this purpose, an experimentally investigated 2D premixed flame confined by three isothermal cylinders is simulated using flamelet generated manifolds for three operating conditions. Comparison of simulated and measured flame shape and location shows reasonable agreement. This proves that the flamelet generated manifolds method is suitable and cost-effective to simulate reacting flows within packed beds.

Confinement Effects on Coaxial Jet Diffusion Flame

ABSTRACT A combined experimental and numerical study has been conducted to investigate the characteristics of confined coaxial jet diffusion flames in air coflow, using methane as fuel. The main concerns were focused on the unequal fuel velocity (0.1–72 m/s)/air velocities (0.045–0.53 m/s) and confinement ratio (δ*, defined as the ratio of the confinement tube diameter and the jet diameter), and most of the flow remains laminar while the ranges of δ* and gas velocities vary widely. The flame height linearly increases with the central fuel jet velocity, until the flame tip-opening or liftoff occurs, which is similar to that of the free jet flame. The critical fuel jet velocity for flame liftoff decreases with the ratio of air velocities to δ * . The critical fuel jet velocity of flame tip-opening increases with δ*, and the critical volumetric flow rate of air increases to a maximum, then decreases, indicating a combined effect of the oxygen supplement and the mixing with the fuel. Under smaller δ* with high jet flow velocity, a circular plane flame is observed before flame extinction. The formation of a recirculation zone in a confinement with combustion reaction has a significant influence on flame behaviors. A critical nondimensional Craya-Curtet number is used to predict the onset of flow recirculation with combustion reaction. At a certain coflow air velocity, the critical number initially increases and then remains constant with δ*, while at a certain δ*, the critical number decreases with air velocity. When the Craya-Curtet number is higher than the critical point, recirculation cannot be established, resulting in the formation of a tip-closed flame. With the decrease of the Craya-Curter number, the recirculation evolves to the upstream of the flame, then, the flame may be lifted off, along with the recirculation zone moving downstream. With the effect of recirculation, the stoichiometric mixture fraction contour shrinks toward the central axis where there is a higher local velocity. Therefore, the flame will be lifted off under a lower jet velocity in a smaller δ * . With the increase of the jet velocity, the Craya-Curtet number further decreases, and the recirculation becomes much stronger than the upstream inflow, resulting in the flame being compressed into a circular plane flame at a small δ* with a relatively high jet flow velocity or blowout directly at a relatively large δ * .

Effects of coflow velocity on the lift-off characteristics of autoignited jet flame in hot air coflow

This paper aims to understand the effects of coflow velocity on the lift-off characteristics of autoignited jet flame and to develop a lift-off height model that considers such effects. A jet-in-coflow burner was used to conduct autoignition experiments. As the jet velocity increased, three distinctive flame types were observed: attached flame (AF), random spots (RS), and lifted flame (LF). The coflow velocity was found to have an interesting impact on the transition from one type to another. As the coflow velocity increases, the jet velocity needed to transition from AF to RS monotonically decreases, which aligns with our expectation. However, as the coflow velocity increases, the jet velocity required for RS to LF transition initially increases and then decreases. Such a shift implies that the dominant factor of the RS flame stabilization changes from the mixing to small-scale strain. The lift-off characteristics of RS under different coflow velocities were analyzed. With the increase of coflow velocity, the spots number increases non-monotonic, and the average lifetime decreases monotonically, further indicating that mixing and small-scale strain jointly determine the generation and extinction of RS. Moreover, the sensitivity of lift-off height to jet velocity is not only greater than that of coflow velocity but also affected by the coflow velocity. Therefore, considering the coflow velocity's effect, a modified mixing-strain model of RS was developed by improving the calculation of both strain rate and mixing time.

BPNN model based AI for the estimation of soot data from flame luminosity emissions in H2/N2 diluted ethylene laminar diffusion flames

Advanced combustion devices and alternative fuels like hydrogen require an estimation of the impact of such changes on engine exhaust emissions of regulated pollutants like soot. Huge experimental data are then required to be collected and post-processed, both in academic and industrial setups. Machine learning and artificial intelligence can be employed to reduce cost and time. This study presents a Back Propagation Neural Network (BPNN) model that has been validated and optimized for the simultaneous prediction of soot volume fraction, temperature and particle sizes from flame luminosity measurements. The compatibility, robustness and reliability of neural network models in terms of fuel composition and flow rate variations was analyzed , by varying both fuel flow rate and fuel stream dilution in axisymmetric diffusion flame burning in a coflow of air. BPNN model predicted results were contrasted against those obtained through laser diagnostics. A detailed analysis of the performance of BPNN model, based on an extensive experimental study revealed indifference to N2/H2 dilution and fuel flow rate variations. Furthermore, an original BPNN model is optimized to improve learning rate and to reduce computation cost, by applying a Bayesian algorithm which continuously monitors and minimizes an error function. It was concluded that a training set size of three was sufficient to predict soot field parameters with a fair prediction accuracy. These results have direct implication for the application of proposed technique in practical combustion equipment where operating parameter, e.g. Air–fuel ratio and EGR, show both gradual and instantaneous variations and affect both performance and emissions. These results clearly demonstrate the robustness of this technique which offers opportunities for real-time, in situ, monitoring of practical combustion devices. Hence, Real-time data of sooting characteristics of flames can be used to both monitor and control pollutant emissions by integrating a response strategy in the system.

Lift-off height model of hydrogen autoignited flame in turbulent hot air coflow

This study aims to develop a lift-off height model suitable for hydrogen autoignited flames in turbulent hot coflow. The differences in flame structure and lift-off characteristics between hydrogen and acetylene flames are compared through experiments. Due to hydrogen's high flame speed, the autoignition spots of hydrogen are undetectable, and the flame zone becomes regular and continuous. Therefore, the flame speed can be used to improve the calculation of the chemical time scale to incorporate the flame propagation factor into the lift-off height model. A novel model is established to consider the combined effects of flame propagation and autoignition on flame stabilization through the Karlovitz number and Damkohler number, respectively. Positive parameters β and 1-β represent the relative contributions of flame propagation and autoignition. The Ka/Da model successfully predicts the variations in the lift-off height of hydrogen flames at various coflow temperatures and fuel mole fractions. In addition, the good correlations based on the Ka/Da model to the lift-off heights of hydrogen, methane, and ethylene flames in the literature prove the model's generality. Unlike hydrocarbon flames (β < 0.5), hydrogen flames' β is bigger than 0.5, indicating that flame propagation plays a more significant role than autoignition in stabilizing autoignited flames.

Stabilization Characteristics of Transitional and Weakly Turbulent Lifted Jet Diffusion Flames

ABSTRACT This work reports experimental results from laminar-to-turbulent lifted jet diffusion flames. Characteristics of flame liftoff height, flame base structure, cold and reacting jet flow fields, and velocity fields near the flame base during the transitional process were obtained. The flame transition and stabilization characteristics are the focus of the present study. Results show that the lifted flame transition is mainly caused by the interaction between turbulent jets and flame base. As the transition begins, folding and distortion of the flame base occur. With the increase in Reynolds number (Re), the extent to which flame base is distorted increases constantly due to influences of more vortices. Simultaneously, at this transitional stage, the liftoff height decreases with Re. At different co-flow air velocities (U air ), the mean liftoff height with Re are qualitatively the same. While as U air is increased, the critical transitional Reynolds number (in which the flame transition begins) decreases. In the later transitional regime within a range of Re, both the cold and reacting jets exhibit intermittent breakup. After jet breakup, flame base immediately becomes turbulent. When the intermittent flow in the transitional stage gradually becomes less turbulent, flame base still remains turbulent for some time, within which flame and flow can be evidently decoupled. By statistical analysis, the integral length scale and the root mean square (r.m.s.) velocity fluctuation near the lifted flame base were obtained, and then the turbulent flame speed can be estimated. Experimental evidence provided for stabilization mechanism of the original transitional flames was confirmed by the balance between the local flow velocity and the flame speed. However, the difference between the two velocities gradually increases with the Reynolds number, suggesting some new stabilization characteristics. When the upstream eddy contains flammable fuel/air mixture, the flame base responds with moving upstream after ignition. On the contrary, when the mixture in the eddy is nonflammable, the flame base would move downstream.