Combustion Devices Research Articles

Air distribution to reduce nitrogen oxide emissions in the furnace of a boiler with tangential combustion scheme

Relevance. Determined by the importance of analyzing the combustion characteristics and factors affecting the emission of nitrogen oxides (NOx) in the furnace chamber of pulverized coal-fired power boiler unit when installing tertiary air nozzles and changing the air flow rate through them, to ensure the reduction of negative anthropogenic impact on the environment. It should be noted that the use of tertiary nozzles is the most cost-effective technology of the internal furnace measures of NOx emission reduction. Aim. To analyze the effect of oxidant redistribution between secondary air and tertiary blast in the range of 40% on the burnout and NOx emission in the furnace chamber of a boiler unit with tangential burner arrangement. Objects. Power pulverized coal-fired boiler unit with natural circulation. Straight-flow burner devices are arranged according to the tangential scheme, tertiary blast nozzles are installed above the burners. Methods. Computational fluid dynamics modeling methods. For the simulation study of furnace processes the tested software FIRE-3D was used. The averaged equations of conservation of mass, momentum and enthalpy were solved to predict the velocity, temperature and concentration of components of the furnace medium in the furnace volume. The turbulent flow was modeled by the standard k-ε model. Coal particle transport was modeled using a discrete-phase model. The P-1 model was used for radiant heat transfer. Results. The authors have carried out the analysis of O₂, CO, and NOₓ concentrations, as well as velocity fields and temperature to examine the effects of varying the ratio of secondary and tertiary air in the furnace volume of a boiler unit with tangentially arranged burners and tertiary blast nozzles. Numerical modeling results revealed that with tertiary blast nozzles, the active combustion zone shifts upward. However, at higher proportions of tertiary blast, NOₓ reduction is not achieved due to increased temperatures at the furnace outlet. Taking into account NOx emissions and completeness of fuel burnout, the most optimal for modernization of the investigated boiler is the value of tertiary blast fraction equal to 0.2.

Assessment of the influence of thermal characteristics of non-design lignite on their suitability for combustion in the furnace of an energy boiler

The possibility of burning various brown coals (LC) in the furnace of the BKZ-420-140-6 boiler was studied using numerical simulation. To check the compiled numerical model of solid fuel combustion, it was previously validated in relation to the combustion of BU of a certain composition by comparing the calculation results with data obtained during the operation of a real boiler. A method was developed for averaging the characteristics of coal and dependencies were obtained that determine the composition of coal based on an analysis of the composition of 14 types of coal in the range of values of the lower calorific valueQnr from 7.5 to 16 MJ/kg. Based on the obtained dependencies for 4 values of Qnr, the theoretical average composition (ATC) of coals was determined. For one of the TUSs, the humidity varied by 10% up and down. As indicators of the efficiency of the combustion chamber of the boiler, the temperature of the gases at the exit from the combustion chamber, mechanical underburning and the concentration of nitrogen oxides in the flue gases are taken. The results of numerical modeling show that with the calorific value of the fuelQnr ≤10 MJ/kg, the mechanical underburning q4 exceeds the permissible standards. The highest concentration of nitrogen oxides at the level of 800–900 mg/nm3 is observed for brown coals with highQnr and the highest carbon content. It is also shown that the use of direct-flow burners with the organization of staged fuel combustion makes it possible to reduce the formation of nitrogen oxides in the furnace by 3.25 times compared to the original combustion scheme using existing vortex burner devices. The influence of the lower calorific value of fuel on the gas temperature at the exit from the combustion chamber in the rangeQnr from 11.75 to 16.45 MJ/kg is insignificant. The effect of increasing fuel humidity on the gas temperature at the outlet of the combustion chamber and on mechanical underburning is insignificant up to a working fuel humidity of approximately 45%. In general, studies have shown that the firebox under consideration allows the combustion of various brown coals with changes in the physico-chemical composition and thermal characteristics within a wide range.

Development of a Stringent Ex Vivo-Burned Porcine Skin Wound Model to Screen Topical Antimicrobial Agents.

Background: Due to rising antibiotic-resistant microorganisms, there is a pressing need to screen approved drugs for repurposing and to develop new antibiotics for controlling infections. Current in vitro and ex vivo models have mostly been unsuccessful in establishing in vivo relevance. In this study, we developed a stringent ex vivo-burned porcine skin model with high in vivo relevance to screen topical antimicrobials. Methods: A 3 cm-diameter thermal injury was created on non-sterilized porcine skin using a pressure-monitored and temperature-controlled burn device. Commensals were determined pre- and post-burn. The burn wound was inoculated with a target pathogen, and efficacies of Silvadene, Flammacerium, Sulfamylon, and Mupirocin were determined. The in vivo relevance of this platform was evaluated by comparing the ex vivo treatment effects to available in vivo treatment outcomes (from our laboratory and published reports) against selective burn pathogens. Results: Approximately 1% of the commensals survived the skin burn, and these commensals in the burn wounds affected the treatment outcomes in the ex vivo screening platform. When tested against six pathogens, both Silvadene and Flammacerium treatment exhibited ~1-3 log reduction in viable counts. Sulfamylon and Mupirocin exhibited higher efficacy than both Silvadene and Flammacerium against Pseudomonas and Staphylococcus, respectively. The ex vivo treatment outcomes of Silvadene and Flammacerium against Pseudomonas were highly comparable to the outcomes of the in vivo (rats). Conclusions: The ex vivo model developed in our lab is a stringent and effective platform for antimicrobial activity screening. The outcome obtained from this ex vivo model is highly relevant to in vivo.

Stochastic modelling for the effects of micromixing on soot in turbulent non-premixed flames

The impact of micromixing and scalar dissipation rate (SDR) fluctuations on the evolution of soot particle size distribution (PSD) dynamics in non-premixed turbulent flames is investigated within the context of Large Eddy Simulation (LES). To accomplish this, a stochastic approach based on a physicochemical sectional soot model, spatially homogeneous Conditional Moment Closure (CMC) with first-order moment closure, and a stochastic differential equation for the SDR are employed, mimicking the function of an LES sub-grid scale (SGS) combustion model. After linking the magnitude of SDR fluctuations to the LES filter size, it was found that the standard deviation of observables, such as the soot volume fraction and soot number density, decreases rapidly with increasing LES filter size, which can significantly influence the intermittency of soot reproduced by models. Mean conditional expectations are also affected, indicating a closure problem, with the mean soot volume fraction, in particular, showing a decrease of up to 20% in the case considered compared to a direct numerical simulation resolution even when the Pope criterion is satisfied. The influence of SDR fluctuations on the dynamic behaviour of the soot PSD is further discussed under different working assumptions for particle transport, and the role of differential diffusion in soot oxidation- and formation-dominant regions is particularly highlighted. The findings underline the shortcomings of LES-CMC and similar mixture-fraction-based combustion models in capturing the dynamic evolution of soot at the SGS, which has significant implications for modelling the emissions of practical combustion devices. However, this study also offers a framework for better evaluating the effectiveness of these models using numerical experimentation and a stochastic model for the turbulent fluctuations of the micromixing rate.

Effect of hydrogen addition on the methane-air combustion under normal and reversed gravity

The existing scientific and technical results in the field of methane-air flames stabilization by adding hydrogen under various gravitational conditions were analyzed in relation to ensuring the safe operation of rocket and space power plants. Experimental setup was created for the purpose of studying the combustion stability of such fuel compositions under the various gravity conditions. The extended ranges of stable combustion rates were obtained experimentally at different percentages of hydrogen added to methane-air mixture under the both normal and reverse gravity conditions. The existence of a lean methane-hydrogen-air flame under reverse gravity in contrast to normal gravity conditions was observed. The influence of hydrogen addition on the dynamics and flame shape at such conditions was studied. The greater influence of hydrogen in ensuring stable combustion of a lean flame under reverse gravity than under normal gravity was demonstrated. Furthermore, the findings substantiated the rationale for utilizing hydrogen to expand the operational stability of burner devices.

Impact of boiler unit load changes on furnace processes

Relevance. The need to assess the stability of combustion, thermal stress and nitrogen oxides (NOx) emission during load reduction of a steam boiler. Since renewable energy sources and nuclear power plants will receive great attention in the future, coal-fired thermal power plants are to operate at reduced loads, so it is important to investigate the reliability of operation and environmental parameters of the boiler unit in case of load adjustment. Aim. To investigate pulverized coal fuel burnout, temperature parameters and NOx emission at 50 % reduction of boiler unit load in the base configuration and taking into account installation of tertiary blast nozzles. Objects. Furnace chamber of a natural circulation boiler unit with steam capacity of 220 t/h in the baseline layout and with tertiary air nozzles. Methods. The package of application programs FIRE-3D for numerical study was applied. Combustion of pulverized coal fuel is a complex physical and chemical process, therefore the interaction of gas flow and solid particles was modeled using Eulerian and Lagrangian schemes, respectively. In the gas phase, the combustion of volatiles and CO with further combustion of carbon residue are modeled. NOx emission is modeled using post-treatment models including formation of fast, fuel and thermal nitrogen oxides. Results. Temperature fields, flow characteristics, NOx emissions for different loads of the furnace chamber of the boiler unit with steam capacity of 220 t/hour are obtained on the basics of numerical modeling. The authors have obtained quantitative estimations of furnace environment parameters corresponding to several levels of boiler load reduction up to 50% of the nominal one. Installation of four tertiary blast nozzles allows reducing NOx emissions by 12.75% at theoretically required amount of air in burner devices (α=1.0).

Heat transfer characteristics of methane-air premixed jet flames with flat/hemispherical walls

Abstract The in-depth study of the mutual coupling between the flame and the wall can significantly enhance the efficiency of actual combustion devices. A two-dimensional numerical model of the heat transfer characteristics of methane-air premixed jet flames with flat/hemispherical walls has been developed. An examination of the effects of wall shape on the heat transfer characteristics of methane/air flames was conducted as a function of the equivalence ratio (ϕ=0.9-1.5), the mixture Reynolds number (Re=300-800), and the burner-to-plate distance (H/d=1-6). As the equivalence ratio and Reynolds number increase, the flame temperature increases on the surface near the wall, and the temperature near the flame centerline is higher under the influence of a hemispherical wall than it is under the influence of a plate. In addition, the wall's heat flux increases as both the equivalence ratio and the Reynolds number increase. It is observed that the heat flux of the hemispherical wall is greater than that of the flat plate near the stagnation point, whereas it is smaller at a distance from the stagnation point. Due to the burner-to-plate distance, thermal efficiency is maximized when the flame premixed cone contacts the impact surface, which is the desired condition for optimal performance. Due to different operating conditions, the efficiency of heat transfer is always higher under the action of a flat plate than under the action of a hemispherical wall.

Thermodynamic investigation of the efficiency of ammonia-powered marine solid oxide fuel cells with gas turbine

Improvement of marine power plants includes increasing their efficiency and drastically reducing emissions of pollutants, which involves transitioning to carbon-free fuels. This article discusses the evolution of a marine power system designed for decarbonization, utilizing ammonia and comprising solid oxide fuel cells with a gas turbine. To enhance efficiency, the system incorporates a steam supply into the fuel burning device of a gas turbine. By adding superheated steam, the system's performance improves significantly. Thermodynamic calculations identified optimal parameters for the gas turbine, which enhances efficiency by utilizing off-gases from the fuel cells. Key parameters include compressor and exhauster pressure ratios, which can be used to increase the specific power. A combined energy system with an overexpansion turbine achieves a total efficiency of 60.62 % at a fuel cell temperature of 1190 K during operation, with the SOFC efficiency being 43.76 %. These findings highlight potential improvements for marine power systems using ammonia and provide insights into the energy conversion processes within such hybrid systems.

FGM modeling considering preferential diffusion, flame stretch, and non-adiabatic effects for hydrogen-air premixed flame wall flashback

Preferential diffusion plays an important role especially in hydrogen flames. Flame stretch significantly affects the flame structure and induces preferential diffusion. A problematic phenomenon occurring in real combustion devices is flashback, which is influenced by non-adiabatic effects, such as wall heat loss. In this paper, an extended flamelet-generated manifold (FGM) method that explicitly considers the preferential diffusion, flame stretch, and non-adiabatic effects is proposed. In this method, the diffusion terms in the transport equations of scalars, viz. the progress variable, mixture fraction, and enthalpy, are formulated employing non-unity Lewis numbers that are variable in space and different for each chemical species. The applicability of the extended FGM method to hydrogen flames is investigated using two- and three-dimensional numerical simulations of hydrogen-air flame flashback in channel flows. The results of the extended FGM method are compared with those of detailed calculations and other FGM methods. The two-dimensional numerical simulations show that considering both preferential diffusion and flame stretch improves the prediction accuracy of the mixture fraction distribution and flashback speed. The three-dimensional numerical simulations show that the prediction accuracy of the flashback speed, backflow region, and distributions of physical quantities near the flame front is improved by employing the extended FGM method, compared with the FGM method that considers only the heat loss effect. In particular, the extended FGM method successfully reproduced the relationship between the reaction rate and curvature. These results demonstrate the effectiveness of the extended FGM method.Novelty and Significance Statement The novelty of this research is the development of a flamelet-generated mani-fold (FGM) method that explicitly considers preferential diffusion, flame stretch, and non-adiabatic effects. To the best of the authors’ knowledge, no studies have performed numerical simulations of pure hydrogen flames using such an FGM method. The developed FGM method was applied to numerical simula- tions of hydrogen-air premixed flame flashback at an equivalence ratio of 0.5 and reproduced the flashback speed of lean hydrogen-air premixed flame. The applicability of the FGM method to the numerical simulation of hydrogen-air flashback is reported first. This research is significant because the FGM method is one of the most widely used combustion models for premixed combustion, and the development of an accurate FGM method will contribute to the engineering field. The accurate prediction of the flame flashback attempted in this study is particularly important for the development of hydrogen-fueled combustion devices.

DEVELOPMENT OF NEW DESIGNS OF GP TYPE OIL-GAS BURNER DEVICES WITH NITROGEN OXIDES LOW EMISSION FOR OIL REFINERY FURNACES

The article discusses variants for new designs of GP type combined burner devices developed by the authors for oil refinery furnaces in order to reduce harmful emissions of toxic nitrogen oxides as one of the most dangerous chemicals. The main factors influencing the formation of nitrogen oxides during fuel combustion have been established. A classification of burner devices used in tube furnaces at oil refineries is given. Original designs of low-toxicity burner devices of the GP type, protected by patents for utility models differing from existing analogues in novelty and high environmental efficiency are recommended for implementation.

Determination of Major Heavy Metal Levels in Pepper Gas Used as Chemical Agents in CBRN Field

Introduction: The pepper gases used in the study are in the group of riot control agents. In this study; It was aimed to determine the main heavy metal levels of pepper gases obtained from capsaicin, the active ingredient of peppers grown in soil that we think may contain heavy metals. Material and Method: The presence of a total of 7 heavy metals, including iron, chromium, cobalt, copper, cadmium, lead and nickel, was investigated. The pepper gas sprays were subjected to sample preparation with a microwave combustion device. Then, heavy metal analyzes of the prepared samples were carried out with the ICP OES device. Results: According to this; As a result of the measurements, Chromium (Cr), Nickel (Ni), Cadmium (Cd) and Cobalt (Co) elements were not found in any of the samples. The amount of Lead (Pb) was determined as 0.653±0.064 mg/kg, and the amount of Iron (Fe) was 5.246±0.000 mg/kg. Finally, the Copper (Cu) element detected in a single sample was measured as 0.815 mg/kg. Discussion and Conclusion: We also think that the necessary sensitivity should be shown in laboratory and clinical studies to examine the content of pepper spray and to determine the optimum ratios of its active ingredients. We foresee that preparing promotional brochures, informative public service ads on topics such as what pepper gas is in what situations pepper gas should be used, and informing the public about the health problems it may cause will increase the conscious use of pepper gas.

Isolating effects of large and small scale turbulence on thermodiffusively unstable premixed hydrogen flames

Lean turbulent premixed hydrogen/air flames have substantially increased flame speeds, commonly attributed to differential diffusion effects. In this work, the effect of turbulence on lean hydrogen combustion is studied through Direct Numerical Simulation using detailed chemistry and detailed transport. Simulations are conducted at six Karlovitz numbers and four integral length scales. A general expression for the burning efficiency is proposed which depends on the conditional mean chemical source term and gradient of a progress variable, and the amount of superadiabatic burning. At a fixed Karlovitz number, the normalized turbulent flame speed and area both increase almost linearly with the integral length scale ratio. The effect on the mean source term profile is minimal, indicating that the increase in flame speed can solely be attributed to the increase in flame area. At a fixed integral length scale, both the flame speed and area first increase with Karlovitz number before decreasing. The qualitative observations and trends do not change when Soret effects are neglected. Specifically, neglecting Soret diffusion is shown to reduce the flame speed, area, and burning efficiency. At higher Karlovitz numbers, the diffusivity is enhanced due to penetration of turbulence into the reaction zone, significantly dampening differential diffusion effects.Novelty and SignificanceLean premixed hydrogen flames are subject to thermodiffusive instabilities, which can lead to system level instabilities such as flashback. A comprehensive study of the thermodiffusively unstable turbulent flames with detailed chemistry and detailed transport (including Soret diffusion) was conducted across a wide range of turbulent intensities and integral length scales. Varying these two parameters independently was necessary to isolate the effects of large- and small-scale turbulence. Using these results, we propose a general expression for the burning efficiency to explain the relationship between the turbulence intensity, flame speed, and flame area. This work is an important step in developing predictive models which can aid in the design of practical combustion devices.

Ignition and flow combustion characteristics of hydroxylammonium nitrate - based liquid propellant based on electric method

Hydroxylamine nitrate (HAN)-based liquid propellants are characterized by high energy content and low toxicity ionic monopropellants, offering great potential application prospects in the space engine. This paper proposes an experimental system for electric ignition and combustion of HAN-based liquid propellant. The energy for decomposition, current, and combustion image of HAN-based liquid propellant are measured under different initial voltage, propellant volume, and the initial pressures in Ar and air atmospheres. The results show that the energy for decomposition decreased with an increase in initial voltage, propellant volume, and initial pressure. An increase in the initial voltage accelerates the reaction speed of HAN-based liquid propellant, and the energy for decomposition is approximately 3 J. An increase in the propellant volume decreases the equivalent resistance in the circuit, increasing the current and enhancing the thermal effect of the propellant. The accumulation of decomposition products and heat under high pressure reduced the energy for decomposition; the energy for decomposition is less than 1.5 J for the pressurization condition. Compared with the Ar atmosphere, the energy for decomposition of HAN-based liquid propellant shows better stability in the air atmosphere. The flow combustion device of HAN-based liquid propellant for the flow state is designed. The HAN-based liquid propellant is successfully ignited for different initial voltages and flow rates. The research results provide a reference for the design of space engine for HAN-based liquid propellant based on the electric ignition method.

Early detection of combustion instabilities in liquid rocket engines: Assessment of statistical, recurrence, and fractal analyses for sub- and supercritical pressure conditions

Safe shutdown of a liquid rocket engine in ground testing prior to the onset of damaging combustion instabilities through reliable detection of instability precursors would translate to time and cost savings in engine development programmes. Methods derived from statistical, recurrence, and fractal analysis have been successfully applied in the literature to detect precursors in unsteady pressure signals from canonical combustion experiments, gas-turbine combustion experiments, and sub-scale rocket combustion experiments operated at low pressures. In the present work, several such methods were applied to data from two cryogenic oxygen-natural gas rocket experiments operated at higher pressures than previously reported; both sub- and supercritical with respect to oxygen. The goal was to identify methods that can discern limit-cycle instabilities from intermittently unstable operation and are sufficiently responsive to be applied as emergency shut-down criteria in engine tests. Among the methods applied were the standard deviation, variance of the auto-correlation, the second spectral moment, the ratio between determinism and recurrence rate, the Hurst-exponent, and the multifractal range. The second spectral moment, the Hurst-exponent, and a measure derived from the multifractal spectrum all have short detection delays for instability onset and short-lived could be discerned from self-sustaining instabilities with an appropriate choice of threshold value. They also have moderate computation cost which makes them of interest for potential real-time implementation. The Hurst-exponent has the additional advantage of a common threshold value for all test cases addressed, demonstrating its potential for broader application independent of combustion device or operating conditions.

Study on the key parameters of closed-loop free-piston Brayton generator

The escalating energy crisis and environmental issues have compelled countries worldwide to advance energy transitions and vigorously develop green and clean energy technologies. External combustion devices can effectively utilize hydrogen-based fuels, biomass, solar energy, and other sources, playing a critical role in achieving carbon reduction. As a type of external combustion device, the free-piston Brayton generator, with its strong energy adaptability, stands as a highly promising power generation device. Therefore, the design optimization methods and operational characteristics of the free-piston Brayton generator system are worthy of thorough investigation. This paper introduces a novel method for constructing and structurally optimizing a free-piston Brayton generator model. Sage software is utilized to construct a closed-loop free-piston Brayton generator system in the time domain, followed by numerical analysis. The results show that within the frequency range of 35 ∼ 80 Hz, the system maintains a thermal-to-electric efficiency exceeding 40.00 %, demonstrating the system’s ability to operate efficiently at high frequencies. The piston diameter influences the dynamic behavior of the system’s piston, emphasizing the necessity of gradually adjusting the piston diameter to achieve optimal efficiency during structural parameter optimization. System efficiency and power can be balanced by adjusting the compressor inlet pressure. Under design conditions, the system can output 2046 W of electrical power. The thermal efficiency and thermal-to-electric efficiency are 46.84 % and 41.36 %, respectively. Additionally, an innovative analysis of the temperature distribution of the system and the distribution of exergy losses in major components under conditions of high frequency and small piston displacement is conducted. This work provides a new method and guidance for the subsequent design and optimization of free-piston Brayton generators.

Numerical simulation of non-stationary regime of a submerged combustion setup operation

Relevance. The need to evaporate large quantities of brines at potash industry enterprises. Evaporation of brines in surface evaporators is difficult due to the encrustation of heat exchange surfaces by salt deposits. Therefore, such evaporation is most expedient to be carried out in submerged combustion apparatuses, since they do not contain heat-transmitting surfaces. However, in this type of apparatus, malfunctions may occur due to uncontrolled solid phase deposition. At the moment, the dynamics of the solid phase in submerged combustion devices is poorly studied. This study is part of a scientific program aimed at a comprehensive review of the laws of motion of solid particles in submerged combustion apparatuses. Aim. To study the hydrodynamic processes in the submerged combustion setup in the time interval corresponding to the beginning of its operation; describe the patterns of solid phase motion as a function of time. Object. Laboratory setup of submerged combustion. A simplified model of the thermal mode of operation without the subsequent transition of the liquid phase to steam is analyzed. Methods. The study was conducted by numerical experiment. The hybrid finite volume method was used in simulation in combination with the technology of the finite element method. The multiphase system was considered as two coexisting subsystems: gas–liquid and liquid–solid. Results. The paper considers the final time interval of the setup operation. It is found that during the time under consideration, a stationary mode of solid particle deposition is achieved. The authors have detected liquid flow velocity oscillations, leading to fluctuations in the mass flow rate of solid particles at the bottom of the setup. It was found that the velocity at the tip of the flue gas jet escaping from the burner nozzle, as well as the pressure at the nozzle section, have a similar form of oscillation. The authors substantiated the hypothesis about the determining influence of the instability of the jet movement of flue gases on the oscillatory behavior of the entire hydrodynamic system.

Transition to oscillatory instability of lean methane–air flames in microchannels

Micro-flow reactors and porous media burners are prone to instability in the form of flame with repetitive extinction and ignition (FREI) near the region of a temperature gradient. Although significant progress has been made in this field, there is still a lack of understanding regarding the nature of such stable-to-unstable transition. Some studies reported a smooth and continuous instability transition in microchannels, interpreting this phenomenon as a supercritical bifurcation. Meanwhile, others have observed that the transition is subcritical, with rapid, stepwise evolution of the stable flame to FREI. This study aims to rigorously determine the transition character theoretically and numerically using a two-dimensional model with a precise resolution of the bifurcation parameter (flow velocity) near the critical point and gradual wall temperature ramp. The results indicate that the transition is a subcritical bifurcation with strong hysteresis. However, the amplitude of the limit cycle born in the bifurcation point could be small compared to FREI-like oscillations. Moreover, further development of the instability depends significantly on the channel width. In relatively wide channels, the stable regime transforms immediately into the extinction/ignition one with rapid growth in amplitude. In moderate channels, complex transitional dynamics were observed with an evident pulsating regime, which evolved into FREI through mixed-mode oscillations. In narrow channels, the amplitude of transitional pulsating flame becomes comparable to the fully-developed extinction/ignition cycles, and the bifurcation diagram has a continuous character with no significant jumps or discontinuities. Such qualitative variation of transition character provides a possible explanation for contradictory interpretations presented in the literature.Novelty and Significance StatementCurrently, there is no consensus about the character of transition between the stable and extinction/ignition regimes in microchannels. Some studies have reported a continuous instability transition with an evident pulsating regime, interpreting it as a supercritical bifurcation, while others have observed subcritical bifurcation with a rapid transition. The novelty of this research lies in the rigorous determination of the bifurcation type and further instability development in microchannels of different widths based on an accurate simulation of flame dynamics near the critical point with very small steps of the flow velocity variation, along with the bifurcation theory. The study contributes to understanding the instability transition nature and may explain the different interpretations of the transitional phenomenon found in the literature. The results offer valuable insights for designing micro-scale and porous media combustion devices.

Mechanistic study of OH radical-promoted decomposition of polycyclic aromatic hydrocarbon oxyradicals

Polycyclic aromatic hydrocarbons (PAHs) are widely recognized for their detrimental impact on human health and the atmospheric environment. One of the primary challenges in mitigating these effects is the efficient and rapid decomposition of PAH oxyradicals, a byproduct of PAH oxidation. This study aims to elucidate the promoting mechanism of hydroxyl (OH) radicals on the decomposition of PAH oxyradicals. Employing density functional theory and transition state theory, we systematically investigate the decomposition mechanism of PAH oxyradicals under the influence of OH radicals. Our findings indicate that the interaction between OH radicals and PAH oxyradicals constitutes a pivotal oxidation process within flames. Notably, PAH oxyradicals are more inclined to undergo pyrolysis subsequent to their reaction with OH radicals within a flame, as opposed to direct pyrolysis. This observation underscores the role of OH radicals in facilitating the decomposition of PAH oxyradicals in flame conditions. Further analysis reveals that OH radicals can effectively react with various forms of PAH oxyradicals—zigzag, free, and armchair—leading to the formation of OH-OPAH compounds characterized by HO-C-O functional groups. Importantly, this addition reaction between OH radicals and PAH oxyradicals is found to be independent of the edge types of the PAH oxyradicals. The pyrolysis of OH-OPAH predominantly yields products such as COOH, CO2, and PAHs with five-membered rings, with COOH emerging as the primary oxidation product. This pyrolysis predominantly proceeds via a ring-opening reaction, rather than through cyclization. This study also reveals that the likelihood of PAH oxyradical pyrolysis increases with the number of free edges surrounding the oxygen-containing functional groups. Furthermore, the oxidation activity of the HO-C-O functional group is demonstrated to be more potent than that of the C-O functional group, significantly enhancing the reactivity of PAH oxyradical degradation in the presence of OH radicals. These insights not only contribute to refining the combustion model of PAH oxidation but also advance our understanding of PAH oxyradical decomposition. Such knowledge is crucial for the development and optimization of control technologies targeting PAH pollutants emanating from combustion devices.

Genetic programing control of self-excited thermoacoustic oscillations

In this experimental study, we use a data-driven machine learning framework based on genetic programing (GP) to discover model-free control laws (individuals) for suppressing self-excited thermoacoustic oscillations in a prototypical laminar combustor. This GP framework relies on an evolutionary algorithm to make decisions based on natural selection. Starting from an initial generation of individuals, we rank their performance based on a cost function that accounts for the trade-off between the state cost (thermoacoustic amplitude) and the input cost (actuator power). We then breed subsequent generations of individuals via a tournament in which the direct forwarding of elite individuals occurs alongside genetic operations such as mutation, replication, and crossover. We implement this GP control framework in both closed-loop and open-loop forms, followed by benchmarking against conventional open-loop control based on time-periodic forcing. We find that while all three control strategies can achieve similarly large reductions in thermoacoustic amplitude, GP closed-loop control consumes the least actuator power, making it the most efficient. It achieves this efficiency by learning an actuation mechanism that exploits the strong heat-release-rate amplification of the open flame at its preferred mode, even though the GP algorithm has never seen the open flame itself. This study demonstrates the feasibility of using GP to discover new and more efficient model-free individuals for suppressing self-excited thermoacoustic oscillations, providing a promising approach to data-driven feedback control of combustion devices.

Effect of elevated crossflow temperature on jet primary atomization

Liquid jet in hot gas crossflow is widely employed in industrial combustion devices, especially in the propulsion systems. In this work, the effect of elevated crossflow temperature on the primary atomization of a liquid jet is experimentally investigated, including breakup regime, surface wavelength, column breakup height, and near-field trajectory. The experiments are conducted at crossflow temperatures of 300 K and 500 K, with gas Weber number ranging from 8.82 to 67.55 and momentum flux ratio from 10 to 50. The gas Weber number and momentum flux ratio are kept constant via increasing the crossflow velocity when crossflow temperature increases. The results show that the elevated crossflow temperature weakens surface breakup, particularly at high momentum flux ratios, which makes the transition of breakup regime from column breakup to surface breakup require higher gas Weber number or momentum flux ratio. Besides, the elevated crossflow temperature leads to a slight increase in surface wavelength, column breakup height and near-field trajectory, with the increase in trajectory being more pronounced at lower gas Weber numbers. Finally, an empirical correlation for the near-field trajectory is obtained, including the effects of crossflow temperature, gas Weber number, and momentum flux ratio.