Significance: Experimental conditions in microgravity differ considerably from those in Earth's normal gravity. Combustion experiments conducted in microgravity eliminate the effects of natural convection and simplify the complex factors of combustion processes. Combustion experiments can reveal many physical and chemical phenomena only under normal gravity conditions, providing significant insights for fundamental scientific research. Meanwhile, microgravity combustion experiments allow a deeper investigation into the fundamental physical phenomena of advanced combustion issues, serving as a crucial means for basic research. This research supports China's energy and power industries in addressing the needs related to energy conservation, emission reduction, and green energy transition, as well as those related to fire prevention on the ground and in space. Progress: The China Space Station (CSS) is planned to support combustion science experiments using multiple fuel types, including gaseous, liquid, and solid fuels, in orbit. The first series of CSS combustion experiments consisted of gaseous combustion experiments, a few of which were conducted in the combustion science rack (CSR). This article reviews the progress of microgravity jet flame research and introduces types of scientific research that can potentially be supported by the combustion science application system and gaseous combustion experiment insert (GCEI) in the CSR. The combustion science experiment system provides the GCEI with the necessary resources, such as water cooling, electricity, and gas emissions. The GCEI supports gas-flow regulation functions, allowing the adjustment of the gas type, flow rate, and ignition power based on the project's scientific objectives. The GCEI features a universal burner platform and can adjust the gas composition, flow rate, and ignition energy. Various types of flames can be generated by replacing the project burners. Optical diagnostics conducted outside the optical windows of the combustion chamber provide data on the flame dynamics, flow fields, and spatial distributions of OH and CH. Currently, astronauts aboard the CSS have installed an igniter in the gas experiment module and mounted the GCEI in the CSR combustion chamber. The GCEI automatically completes a series of actions, including configuring the combustion environment gas, ejecting the fuel gas, heating the igniter, determining parameters, performing optical diagnostics, filtering and circulating, and exhausting waste gases. Because of the lack of buoyancy effects, microgravity flames exhibit considerable differences compared to normal gravity flames. After transmitting the experimental data to the ground operation control center, the control and monitoring of the experimental conditions are performed to confirm the normal operation of each subsystem. The fuel, oxidizer, and inert-gas flow rates are set according to predetermined delays and settings, demonstrating the normal operation of key modules, such as the GCEI's fuel gas cylinder module, gas-distribution solenoid valve, igniter, and oxidizer and diluent subsystems of the CSR. The image intensifier camera of the combustion diagnostic subsystem captures corresponding OH and CH emission images, demonstrating an increase in the flame width and a rapid decrease in the flame height until localized extinction occurs at the end of the non-premixed flame. Conclusions and Prospects: The present study verifies that the GCEI can effectively realize microgravity flames for gaseous experiments in orbit and provide a support and design basis for subsequent diversified combustion science experiments. The GCEI is expected to provide valuable data and platform support for subsequent microgravity experiments aboard the CSS.
Significance: With the accelerating pace of human aerospace endeavors and broadening horizons of space exploration, investigating combustion phenomena in variable-gravity environments, particularly microgravity and supergravity, has emerged as a research frontier in aerospace science. Comparing to diffusion flames, premixed flames makes more adequate combustion, which improves energy efficiency and generates much less pollutants. Thus it is essential to elucidate how gravity influences these flames to optimize combustion processes in space. This review aims to consolidate current knowledge on gravity's effect on premixed jet flames, fostering advancements in aerospace combustion technology. Progress: This article systematically surveys domestic and international research advancements on how gravity affects premixed jet flames. It encompasses various experimental setups designed to simulate microgravity and supergravity conditions, including tower drop experiments, centrifuge facilities, and aircraft weightlessness simulations. The focus is on various flame configurations, such as conical, rod-stabilized, and polyhedral jet flames. The effects of gravity on flame morphology, stability, flickering behavior are examined using multiple techniques such as laser imaging for flame visualization, particle image velocimetry for flow field measurements, planar-laser-induced fluorescence for species-concentration mapping, and computational fluid dynamics simulations for detailed mechanistic insights. These methodologies help explore the complex interactions among fluid dynamics, heat transfer, and chemical kinetics under altered-gravity conditions. Furthermore, the review examines different control parameters, such as the equivalence ratio, Reynolds number, and initial pressure, which influence the behavior of premixed flames. Attention is also given to experimental conditions that affect the reproducibility and generalizability of the findings, such as fuel type, combustion-chamber geometry, and specific flame initiation and stabilization procedures. The findings reveal that gravity significantly affects the characteristics of premixed jet flames, influencing their shape, stability, and dynamic behavior. In terms of flame shape, the review demonstrates how gravity significantly influences the geometric structure. Microgravity causes flames to become more spherical owing to reduced buoyancy effects, while supergravity elongates and distorts flames owing to intensified buoyancy-driven flows. In terms of flame stability, the stability boundaries of premixed flames are found to be highly dependent on the magnitude of gravity. Microgravity allows for a broader range of stable operations, while supergravity narrows these boundaries, leading to increased instability and flame extinguishment. The review also addresses buoyancy-induced flame flickering, highlighting that the flickering frequency is directly related to the magnitude of gravity. Lower gravity magnitudes lead to less frequent flickering owing to diminished buoyancy-induced shear-layer oscillations. Conversely, increased gravity magnitudes intensify these oscillations, increasing the flickering frequency. Conclusions and Prospects: This comprehensive review consolidates state-of-the-art knowledge on the effect of gravity on premixed jet flames, offering valuable insights for researchers. It underscores the significance of ongoing exploration in the aerospace domain, driven by advancements in experimental techniques and computational modeling. By providing theoretical underpinnings and practical guidance, this review aims to stimulate further research, driving the development of more efficient and environmentally friendly space propulsion systems.
Significance: Research on soot formation in gas flames under microgravity conditions is a key area in combustion science. Studying soot production in microgravity environments not only elucidates fire behavior in space stations but also eliminates the influence of natural convection, creating a more controlled flame environment for detailed exploration of soot formation processes. The importance of microgravity research lies in its ability to provide essential data for advancing theoretical models and clarifying soot formation mechanisms. This research holds valuable implications for improving combustion technologies, benefiting applications on Earth and in space. Progress: This paper presents a comprehensive review of recent advances in the study of soot formation in gas flames under microgravity conditions. The review systematically summarizes research progress, emphasizing soot formation pathways, smoke point studies, and primary factors influencing soot formation in microgravity. Key methods covered include both qualitative and quantitative analyses, with a focus on advanced diagnostic techniques such as flame emission spectroscopy and laser-induced incandescence, which provide detailed data on soot concentration, particle size, and distribution. The findings indicate that most current research is centered on qualitative descriptions of soot formation, with a marked gap in quantitative analysis and detailed mechanistic insights. The necessity of multifactor coupling studies under microgravity is also highlighted to clarify the combined effects of variables such as fuel type, oxygen concentration, pressure, flow rate, and preheating on soot formation. Advanced diagnostic techniques are increasingly becoming essential tools for measuring soot concentration in space experiments. Conclusions and Prospects: The review concludes that although substantial progress has been achieved, future research should prioritize more detailed quantitative analyses and the development of comprehensive models to uncover fundamental soot formation mechanisms. Continued advancements and application of diagnostic techniques in space experiments are essential. Potential research directions include exploring novel diagnostic methods, improving measurement accuracy and reliability, and examining the effects of various external conditions on soot formation. As space research facilities, such as the Chinese Space Station, continue to advance, these developments will support more comprehensive experimental designs, multifactor coupling studies, and the integration of advanced diagnostic techniques with numerical simulations. These efforts will be critical for devising effective soot control strategies, thereby advancing combustion science and promoting cleaner, more efficient combustion technologies for both space and terrestrial applications. The review calls for collaborative efforts within the scientific community to leverage the advancements in microgravity research to further elucidate soot formation processes.
Objective: The flame spread rate, burning rate, and heat release rate are the key aspects of flammability, which determines the fire development process and the intensity of the heat release. The burning characteristics of a solid fuel strongly depend on the environmental conditions, such as the oxygen concentration, flow rate, and ambient pressure. Most studies have focused on the flame spread rate, and only a few have focused on the burning rate, heat release rate, and soot generation characteristics. When the burning rate of solid materials exceeds the smoke point, the distribution of soot within the flame and the volume fraction of soot undergo a large transformation, thus affecting the heat release rate and changing the flame propagation process. In addition, the generation and transport of soot are crucial for fire safety. An urgent need exists to understand the combustion and soot behavior during flame propagation in real fire scenarios. Methods: In this study, flame spread phenomena over a cylindrical polymethylmethacrylate (PMMA) at different airflow velocities have been experimentally studied under microgravity and normal gravity conditions. Microgravity experiments were performed in a drop tower. In microgravity experiments, flame spread in purely opposed flow was observed, and in normal gravity experiments, downward flame spread behaviors in the mixed flow with buoyancy-induced and forced flows were investigated. The airflow velocities used in both experiments were 1-35 cm/s, and the diameter of the solid sample was 2—10 mm. In the normal gravity environment, the variation in the sample mass during the flame spread process was recorded using an electronic balance, and the soot volume fraction inside the flame was tested using the light extinction method. In both sets of experiments, the luminescent flame and the stoichiometric flame contour photographed with the CH filter were recorded. Results: The flame area, which is estimated from the stoichiometric contour of the CH radicals of the flame, shows a good linear correlation with the measured mass burning rate. Meanwhile, the flame area decreases with increasing flow rate in a normal gravity environment, while in a microgravity environment, the flame area increases to a maximum value and then decreases with increasing opposed flow velocity, indicating a nonmonotonic variation trend. The soot formation of PMMA specimens depends on the diameter of the specimen and the flow conditions, and the experiments in normal gravity show that larger specimen diameters and lower flow rates favor soot formation. However, the flow velocities corresponding to the smoke points of PMMA specimens in different gravity environments are quite different. The flow velocities corresponding to the smoke points of specimens in microgravity environments are even lower. In normal gravity, the soot concentration in the flame decreases with increasing flow velocity. In contrast, in microgravity, solid materials have different smoke points, and the soot concentration increases with the convection velocity. Conclusions: The fuel burning rate and soot formation depend on the airflow velocity. The relationship between the flame area and the burning rate is independent of the fuel smoke point. Because of the variation in the flow condition, the resident time and oxidization time become different, resulting in variation in the soot formation characteristics.
Significance: By eliminating buoyancy-driven convection and flow instabilities, microgravity jet flame experiments provide a unique platform to study fluid-chemistry interaction. When the characteristic chemical time scale is sufficiently long and comparable to the fluid dynamic time scale, the structure and transient behavior of microgravity jet flames offer valuable insights into fundamental combustion physics under near-limit conditions. These experimental data are crucial for validating theoretical models. Progress: This paper reviews key microgravity jet flame experiments conducted worldwide, including both ground-based and space-based studies. The topics covered include experimental methods for investigating microgravity jet flames, simulated experiments, flame structure, soot formation, radiative heat loss and extinction, limit phenomena, flame transition into turbulence, effects of varying physical fields, flame-based particle synthesis, and diagnostic techniques for microgravity flames. Despite the progress, many dynamic phenomena associated with microgravity gas flame are out of the scope of this paper. These phenomena often stem from the balance between combustion-generated heat and radiative heat loss or interactions involving diffusion and fluid dynamics. Microgravity provides an ideal environment with controllable flow fields, allowing researchers to study these weak interactions, especially in the context of weak reaction systems operating far from the mixing ratio of equivalent ratios. The study of flame dynamics under microgravity remains an important way to develop corresponding theories. Conclusions and Prospects: Looking ahead, the study of microgravity jet diffusion flames, as reviewed in this paper, identifies several key research areas. From the perspective of near-limit chemical reactions, there is a need for more experiments involving weak flames under microgravity conditions. From the perspective of fluid and combustion transition, understanding the shift from laminar to turbulent flow is critical, as this fluid transition directly affects flame behavior. From the perspective of soot and radiation, the reaction kinetics of soot precursors and the physical processes that follow soot nucleation require more concise and accurate models. Current radiation heat transfer models face challenges in accurately predicting the behavior of macromolecular fuels and their derivatives, especially in high-pressure microgravity flame experiments where experimental data are more scarce. Improved radiation models must account for the unique radiation characteristics of fuel components, even at a high computational cost. Regarding the interaction between sound fields and microgravity flames, further research should explore the relationship between near-limit flames and fluid. Existing studies on microgravity premixed flames have used sound fields as a source of fluid disturbance. For near-limit diffusion flames, it is necessary to essential to evaluate the theoretical and modeling implications of traditional experimental approaches, such as standing waves and fluid instabilities. With ongoing investigations, including microgravity jet flame experiments aboard the China Space Station, this paper can be used to further consolidate scientific and challenging problems in the area.
Objective: Currently, multi-element diffusion flames find applications in flame synthesis, combustion mechanism studies, and aerospace engine design. Therefore, investigating the characteristics of multi-element diffusion flames under both terrestrial gravity and microgravity conditions is crucial. In this study, numerical simulation methods are used to investigate the structural characteristics of ethylene-oxygen multielement laminar diffusion flames and the effects of pressure and oxygen volume fraction on the flame structure under both terrestrial gravity and microgravity conditions. Methods: This study was conducted using ANSYS Fluent software. First, a geometric model of the multi-element combustion chamber was constructed. The selected computational model included solving flow using a laminar model, diffusion using Fick's law, chemical reactions using a finite rate model, and radiation using a discrete ordinates model. Grid independence verification was performed, with 600, 000 grids chosen for calculations eventually. After the computational model was established, three sets of operating conditions were designed to study variations in flame behavior under different gravitational accelerations (0-9.8 m/s2), pressures (50-500 kPa), and oxygen volume fractions (0.25-1.00). The flame height obtained from the numerical simulation differed by less than 10% from the experimental results; thus, our method was considered to provide reliable results. Results: The results indicated that the flame had a double-layer structure. With decreasing gravity, because of the inhibition of buoyancy, the flame height increased from 7.3 to 12.8 mm, whereas the flame temperature decreased by 300 K. With increasing pressure, both the outer flame height and width decreased. At normal gravity, the temperature increased by 590 K, whereas it increased by only 80 K at microgravity. At 500 kPa pressure, the normal gravity fire separated, changing from a closed-tip flame to an open-tip flame at microgravity. As the volume fraction of oxygen decreased from 1.00, the flame height gradually increased. When it reached 0.50, the flame changed from a single flame to a double-layer one. Under normal gravity conditions, the flame temperature decreased by about 250 K, whereas it decreased by 600 K under microgravity conditions. Conclusions: The multi-element diffusion flame exhibited a double-layer structure under atmospheric pressure and fuel-rich conditions, with the inner and outer flames generated by the combustion of ethylene and CO, respectively. Meanwhile, modifications in pressure or oxygen volume fractions could change the shape from double-layer fire to separate flames or open-tip flame. The microgravity conditions enhanced the role of radiative heat transfer, leading to a significant decrease in flame temperature and eliminating convective mass transfer caused by buoyancy, thus increasing the flame height and width. Increasing pressure accelerated the reaction rate, increased the flame temperature, and reduced the flame height and width. Under microgravity conditions, increasing pressure enhanced the radiative heat transfer and lowered the flame tip temperature. Reducing the oxygen volume fraction reduced the flame temperature, increased the flame height, and converted the flame from separate flames to a double-layer flame, which was more susceptible to radiative effects and had a particularly low flame temperature in microgravity.
Objective: Ammonia fuel is a promising alternative fuel. Research on the near-limit combustion characteristics of ammonia is the basis for the efficient use of ammonia fuel and provides guidance for the construction and optimization of the detailed and comprehensive chemical reaction kinetic models of ammonia. Herein, the near-limit combustion characteristics of ammonia gas were investigated experimentally and computationally. Methods: The constant volume combustion bomb method was used to experimentally measure the laminar burning velocity (SL) of the near-limit premixed ammonia/air/dilution mixture. A high-speed camera with a frame rate of 10 000 fps was employed to record images of the outwardly propagating spherical flame. MATLAB code was used to process the images and extract the flame radius. In addition, a corresponding computational study, including the prediction of the SL of the near-limit ammonia/air/dilution premix and analysis of chemical reaction kinetics, was conducted with the CHEMKIN code package. During the simulation, CURV and GRAD values were set to 0.02 and at least 500 grid points were used. Results: The SL values and flame images of NH3/air, 95% (NH3/air)/5% N2, and 95% (NH3/air)/5% Ar with an equivalence ratio (ϕ) range of 0.8—1.2 were obtained. A buoyancy effect on the flame of NH3/air plus dilution gas (N2/Ar) was observed, and N2 suppressed flame propagation more than Ar. As ϕ increased, SL values first increased and then decreased, and the SL peak value (maximum) was at ϕ=1.1. For the NH3/air premixed mixture, the experimental values were in overall good agreement with nine chemical reaction kinetic mechanisms of NH3, and the predicted values of the Han 2020 mechanism were in the best agreement with the experimental values. For NH3/air/dilution(N2/Ar), the predicted values of the Mei 2021 mechanism were in the best agreement. Sensitivity and reaction pathway and flux analyses were performed under different dilution conditions by using the Mei 2021 mechanism. The reaction H+O2$\rightleftharpoons$O+OH had a strong promoting effect on SL, whereas the reaction NH2+NO$\rightleftharpoons$N2+H2O had a strong inhibiting effect. The sensitivity coefficients, as well as the fluxes of each reaction branch, were slightly different under the two dilution conditions. By comparing adiabatic reaction temperatures, the increase in SL for Ar dilution compared with that for N2 dilution was found to be mainly due to the difference in thermophysical properties between Ar and N2. Conclusions: N2 has a stronger inhibitory effect on flame propagation than Ar, a buoyancy effect on the flame of the NH3/air/dilution (N2/Ar) premix exists, and a minimum SL of approximately 2.9 cm/s is obtained. Numerical simulation results indicate that the experimental and simulated values are generally in good agreement. The Mei 2021 mechanism predicts the SL of NH3/air/dilution(N2/Ar) well. A slight difference exists between the reaction pathways and sensitivity reaction coefficients for N2 and Ar dilution, and the concentrations of the active free radicals O, OH, and H significantly affect laminar burning velocities.
Objective: Microgravity environment on the space station decouples buoyancy from other limit effects on flame instability. The decoupling facilitates the study dynamical response and associated theories of edge flame under vortical and acoustic excitation. Such research can contribute significantly to the theory development for flame instability control and prevention in energy and power systems and fire suppression mechanisms under microgravity conditions within spacecraft. Methods: The paper introduces the design and initial testing of an experimental apparatus aboard the China Space Station (CSS) for generating and studying acoustic or vortices disturbed edge flames. The apparatus comprises an acoustic slot burner and an optical module, installed on the gaseous combustion experiment insert within the Combustion Science Rack (CSR) aboard the CSS. Compared to traditional co-flow structures, the slot design assures a better control of shear effects and flow field uniformity, allowing more precise control of flame characteristics. Diagnostic methods are introduced to create and capture oscillating edge flames in orbit. The optical module and high-speed CCDs in the CSR are used for two-dimensional temperature inversion of flames. Structural optimization and unique optical beam-splitting design improve diagnostic accuracy and flame visibility. This setup provides a controlled environment to study the effects of vortical structures and acoustic disturbances on flame oscillations. Results: A set of ground testing experiments were conducted to verify the response of edge flames to acoustic disturbances across different acoustic frequencies and vortical disturbances generated by shear layers. At low vortex intensities, the edge flames exhibit low-frequency vertical oscillation patterns, while at high vortex intensities, the flames display high-frequency horizontal oscillation patterns. Under extreme stretching conditions, edge flames can even extinguish. Based on this analysis, future experiments are planned to refine the stability and extinction diagram boundaries of edge flame oscillation. Additional ground experiments and microgravity data will be collected to provide a comprehensive understanding of edge flame behavior under different shear layer strengths and acoustic frequencies. These experiments aim to develop a robust theoretical framework for predicting and controlling flame oscillations and instabilities, contributing to safer and more efficient energy and power systems. Conclusions: The design of the experimental apparatus for the CSS represents a significant advancement in the study of edge flame dynamics under microgravity. The initial results from ground tests demonstrate the complex interaction between flame behavior and external disturbances, which has direct implications for flame stability control in various applications. The stable operating conditions identified through ground experiments will serve as a reference for future experiments conducted in microgravity, where key parameters such as flame structure, response frequency, oscillation modes, and temperature field distribution will be further analyzed. Continued research in this field promises to enhance our understanding of combustion processes in both terrestrial and space environments, ultimately contributing to safer and more efficient energy systems.
Objective: Jet diffusion flames are widely used in industry, energy, and power, including industry kiln stoves, power station boilers, and gas turbines. Flame stabilization is an important problem in jet diffusion flames because of its involvement in the safety of combustion equipment, combustion efficiency, and pollutant emissions. Investigating the characteristics of flame stabilization and the related fire safety issues is important for the design of practical combustion equipment. Many investigations into the stabilization characteristics of laminar jet diffusion flames have been conducted. Our understanding of the characteristics of the flame shape and stabilization behavior of laminar flames is clear. However, for transitional and turbulent flames, especially for lifted flames, under normal gravity conditions, the coupling of buoyancy, jet flow, and Kelvin-Helmholtz (K-H) instability at the flame edge makes the problem more complex. Methods: Experiments were conducted on laminar-to-turbulent lifted jet diffusion flames under normal gravity and microgravity conditions. Variations in the flame lift-off height and length during the transitional process and the stabilization behavior of "non-buoyant flames" were observed. This study analyzes and discusses the stabilization characteristics of the lifted jet diffusion flames under microgravity conditions based on the experimental data of flame lift-off height and flame length. Compared with the results obtained under normal gravity conditions, the influences of buoyancy on the characteristics of flame stabilization were further analyzed. Results: Results showed that lifted flames under microgravity and normal gravity conditions yielded similar critical Reynolds numbers corresponding to the start and end of the transitional stage, respectively. During the transitional process, the flame lengths under microgravity conditions are approximately twice those under normal gravity conditions. The flame lift-off heights under microgravity conditions are always lower than those under normal gravity conditions; however, the differences between normal gravity and microgravity decrease as the jet flow velocity increases. The flame lift-off height is significantly influenced by the jet upstream of the flame base. In the transitional stage, the jet flow exhibits intermittent breakup, causing flame lift-off heights to oscillate. The transitional and turbulent flames all exhibit severe separation phenomena, resulting in the variation of the flame length with time. Because the flame lengths under microgravity conditions are longer than those under normal gravity conditions, the development of the K-H instability at the flame edge toward the downstream region is greater, and flame length varies over a wider range. Conclusions: Although flame lift-off heights under microgravity conditions were relatively low, the flame lift-off height fluctuations under the two gravity conditions had a similar control mechanism. Flame splitting is somewhat random. As Re=2 460 (transitional regime), the mean separation frequencies under the two gravity conditions have only a slight difference. In the turbulent stage, as Re also increases, the mean separation frequency under microgravity conditions increases. However, in the turbulent regime, the flame separation frequencies under microgravity conditions are relatively lower, indicating that buoyancy can promote flame splitting. Moreover, the relationship between the Strouhal and Froude numbers for the flame-splitting phenomenon indicates that the jet Froude number cannot correlate with the separation/oscillation frequency of the flames under microgravity conditions.
Objective: Flame spread is one of the key aspects of flammability, and the flammability of solid materials in low-velocity environments is crucial for fire prevention in manned spacecraft. Under microgravity environments, buoyant flow is greatly reduced or even disappears, and the material combustion characteristics are obviously different from those in normal gravity environments. In confined spaces under normal gravity, buoyant flow is inhibited. Until now, a control mechanism of flame propagation in horizontal channels has not been fully understood, mainly due to the lack of systematic understanding of the influences of residual buoyancy convection and heat loss on the flame-propagation characteristics when the channel height changes, which is directly related to the selection of channel height in experiments. Therefore, in this work, the applicable conditions for the narrow passage are first studied, and the upper and lower critical heights are proposed and verified from the perspectives of buoyancy convection and flame heat loss. Based on previous studies, the channel height intervals for different flame-propagation characteristics are identified, and several critical channel heights are proposed. Methods: The flame-spread behavior of thermally thin solid materials in horizontally confined space is studied by experiments and theoretical analysis based on heat-transfer mechanisms. The effects of buoyancy and heat loss on flame-spread behavior are discussed. The controlling mechanisms and the significance of multiple critical heights were analyzed together with the flame characteristics. Results: When the channel height is too great, the buoyant flow will not be completely suppressed; this is the maximum critical height of the narrow channel in a simulated microgravity environment. When the oxygen concentration is 21%, the upper critical height Lcr, h is 9 mm, and when the oxygen concentration is 18%, the corresponding Lcr, h is 7 mm. The experimental results confirmed the existence of the upper critical height Lcr, h, which is in good agreement with the theoretical prediction. When the channel height is low, excessive heat loss will extinguish the flame and make combustion unsustainable. This is the lower critical height, Lcr, l, for a simulated microgravity environment in a narrow channel. Conclusions: Two critical heights were identified for the narrow channel that can reproduce the flame characteristics in microgravity, Lcr, h and Lcr, l. When the channel height is greater than Lcr, h, buoyancy has a noticeable effect on flame spread, and heat loss becomes important when the channel height is less than Lcr, l. Between the two heights, heat conduction is greater than the buoyancy convective heat transfer, and it has a controlling effect on flame propagation. The two limiting heights are verified by theoretical analysis and experiments. Combined with the literature results, three regimes were identified, namely the quenching-height regime in the area where the channel height is between Lcr, l and Lcr, h, in which flame propagation is affected by excessive heat loss and oxygen supply conditions (forced flow), the weak buoyant flow regime when the channel height is between Lcr, h and Lo, and the regime in which the buoyant flow is fully developed when the channel height is higher than Lo. These results revealed the flame-propagation and extinguishing behaviors in confined spaces and will contribute to a deeper understanding of the influence of confined spaces on flame propagation in materials.
Significance: Single droplet combustion in a microgravity environment is an important model for understanding spray combustion. This study aims to enrich the theory of droplet combustion, providing crucial insights for practical applications such as engine design of aerospace and other spray combustion systems. Progress: By combining single droplet combustion experiments in microgravity with numerical simulations, this study discusses unique phenomena and analyzes the influence of various uncertainties, such as experimental methods and environmental conditions, on combustion characteristics. This study begins by explaining the D2 law, a fundamental theory of single droplet combustion, and its influencing factors. Then, it focuses on the suspending fiber wire technique, analyzing how it affects droplet combustion characteristics. This study examines soot shell formation, flame extinction phenomena, and cool flames during droplet combustion, discussing the mechanisms behind soot shell generation and its influence on the combustion process. The single-droplet flame, a typical diffusion flame, is affected by radiation extinction and diffusion extinction. The cool flame is controlled by the low-temperature oxidation reaction of hydrocarbon fuel, leading to a complex multistage ignition process in droplet combustion. In addition, this study reviews how high-pressure environments affect combustion characteristics and explores phenomena such as preferential evaporation and possible microexplosions during multicomponent droplet combustion. Finally, research on alternative fuels and biofuels reveals that biofuels produce considerably lower soot emissions than conventional hydrocarbon fuels. Conclusions and Prospects: By combining experiments and numerical simulations, this study expanded basic combustion theory through new phenomena observed in microgravity experiments, offering new ideas for developing microgravity experiments and improving numerical models. These experiments on single-droplet combustion in a microgravity environment made several important contributions: using new phenomena to address gaps in droplet combustion theory; revealing fundamental characteristics of autoignition, quasi-steady-state combustion, and extinction of different liquid fuels through experiments under reduced buoyancy convection conditions; and establishing a novel theoretical framework for droplet combustion based on multistage reaction flame structures. However, the experimental and theoretical aspects of single-droplet combustion in the microgravity environment still face several challenges: deficiencies in optical diagnostics for high-pressure combustion experiments, the lack of a large amount of experimental data to support relevant theories in high-pressure environments, the controversy of the pressure effect in microexplosions, insufficient experimental data for practical fuel surrogates, the difficulty in accurately using simple models with few components to develop representations of complex surrogates for practical fuels, and the lack of research data on new liquid fuels (e.g., biodiesel). Addressing these challenges can provide theoretical support for developing new combustion technologies and facilitate the transition to green and low-carbon energy solutions.
Significance: The combustion of hydrocarbon fuels is a significant element in energy conversion and utilization, and it involves complex chemical reactions and physical phenomena. The formation of soot is a critical phenomenon in this process. In addition to environmental pollution, the formation of soot in engines may induce safety risks. An excessive amount of soot may accumulate and block the nozzle of an aerospace engine, resulting in flight accidents. Therefore, it is critically important to control soot formation to ensure flight safety and thus reduce environmental pollution. To effectively control soot, an extensive analysis of the formation mechanism of soot is required. Under normal gravitational conditions, the combustion process may be significantly affected by natural convection, which intensifies the complexity and instability of combustion. This further constrains the analysis of soot formation. However, under microgravity conditions, the intrinsic nature of the combustion phenomena is more pronounced, and the combustion and flow problems are simplified. Furthermore, compared with normal gravity conditions, the flame structure is more stable and symmetrical, which can be attributed to the reduction or even elimination of buoyant convection. Additionally, factors such as residence time, concentration, and particle size exhibit obvious increases, facilitating the investigation of the soot formation process. However, due to the current limitations of microgravity facilities in terms of time and space, existing research on soot formation under microgravity conditions is not comprehensive. Therefore, it is important to summarize the progress made in the current research on soot formation under microgravity conditions and evaluate the limitations of these experiments in these conditions. This may promote the development of soot-formation research under microgravity combustion. Progress: In terms of soot models, soot growth models primarily involve acetylene single-equation and acetylene/benzene two-equation soot models, which are optimized and improved in accordance with experimental measurements for soot nucleation, growth, and oxidation, in addition to radiation models (optical thin radiation model). These models can help to analyze the variation in soot formation location, particle size, and nucleation and oxidation processes to some extent. However, there are still significant discrepancies between the numerical and experimental microgravity results. In terms of diagnostic techniques, soot diagnostic methods operating under microgravity conditions include intrusive and non-intrusive techniques, which can be used to measure parameters such as soot morphology, structural size, and concentration distribution. Because of the significant disturbances caused by intrusive techniques in the combustion flow field, nonintrusive optical diagnostic techniques have garnered more attention for use in experiments. With the improvement and development of microgravity facilities, experimental detection techniques have evolved from one-dimensional to multi-dimensional measurements, and comprehensive results are obtained. However, existing research on multi-dimensional measurements under microgravity conditions is limited. In accordance with the impact of gravity on soot formation, numerical and experimental methods are often combined to explore soot formation characteristics in drop towers, space stations, and parabolic flights by considering small-molecule hydrocarbons as fuel sources. The main purpose of these investigations was to analyze the impact of buoyant convection on soot formation pathways, distribution areas, and soot morphology and structural characteristics. Conclusions and Prospects: Although numerous experiments and numerical simulations have been conducted to study the formation of soot under microgravity conditions, there is a lack of extensive research. Future research directions for microgravity soot studies may focus on multi-dimensional measurements of experimental parameters under microgravity conditions to obtain more precise experimental results, which may help optimize soot models, improve predictive accuracy, and deeply explore the intrinsic mechanisms of soot formation.
Objective: The study of fire spread behavior in wires holds significant importance for guiding fire safety measures in variable pressure environments such as nuclear power plants, aerospace applications, hyperbaric oxygen chambers, and high-altitude areas. Currently, there is a lack of comprehensive research focused on variable pressure environments widely applied in spacecraft, high-altitude regions, nuclear power plants, and civilian hyperbaric oxygen chambers. Therefore, this study investigates the fire spread and molten droplet behavior of polyethylene (PE) wires under variable pressure conditions ranging from 40 to 500kPa using a self-built experimental platform. Methods: This study selected typical thermoplastic PE wires as the research subject and constructed a fire spread experimental platform to investigate the effects of variable pressure conditions (40-500kPa) and oxygen volume fractions of 21% and 30% on fire spread behavior. Simulations were conducted using the solidification/melting model in Fluent software to analyze the melting and dynamic motion of molten material suspended on a metal plate. Results: 1) Within the 40-100kPa range, a low-luminosity blue flame appears at the base of the flame and fades as pressure increases. For an oxygen volume fraction of 21%, the bottom blue flame disappears after 100kPa. For a 30% oxygen volume fraction, it vanishes at 60kPa. At pressures from 100kPa to 500 kPa, the blue region at the bottom of the flame disappears, flame brightness intensifies. The top flame color changes from bright yellow to orange, and soot production increases. Increasing the oxygen volume fraction from 21% to 30% reduces the orange region at the flame's top and decreases black soot in the upper section, reshaping the flame into a more triangular form. During this stage, the fire spread rate and mass loss rate increase significantly; 2) As pressure increases, the flame width of the PE wire decreases. At 500kPa, the flame width measures 2.0 and 2.2cm for oxygen volume fractions of 21% and 30%, respectively. Flame height increases with pressure, peaking at 500kPa. At oxygen volume fractions of 21% and 30%, the maximum flame heights are 3.7 and 4.8cm, respectively; 3) At an oxygen volume fraction of 21%, molten dripping occurs in the 40-80kPa range. However, at an oxygen volume fraction of 30%, molten dripping ceases above 60kPa. Simulations reveal that molten droplets form 4.4s after PE is heated and separate from the main body under gravity with a maximum velocity of approximately 22cm/s. Surrounding airflow exhibits a spiral motion during droplet detachment. Conclusions: This study primarily reveals the fire spread and molten droplet behavior of PE wires under different pressure conditions, providing a foundation for predicting and preventing fire development in PE wires found in variable pressure environments.
Significance: The frequent occurrence of fire accidents poses a serious threat to the safety of people's lives and properties. Thus it is vital to improve the accuracy and efficiency of fire warning systems. Currently, fire alarm sensors or detector systems can be used to provide an early warning of potential fire hazards. Existing conventional fire alarm detectors include infrared (IR) and smoke detectors, that trigger alarms by detecting heat radiation or smoke particles. However, these systems are susceptible to interference from environmental factors, resulting in false alarms or delayed warnings (>100 s). Progress: Unlike traditional smoke alarms, new fire warning sensors can provide timely responses in the early stages of a fire, providing a stronger guarantee for fire safety. Therefore, there has been increasing interest in smart fire warning materials and sensors that combine traditional passive fire retardant strategies with active fire alarm response. Carbon-based two-dimensional (2D) nanomaterial graphene oxide (GO), a typical representative of smart fire warning materials, is characterized by its positive feedback between electrical conductivity and temperature. This paper reviews local and international research progress in the field of GO-based fire early warning sensors. The working principle of GO-based sensors is first summarized, followed by detailed descriptions of current research on GO body materials and GO coating materials. Furthermore, we analyze in depth the practical applications of such sensors in a variety of application scenarios and requirements, demonstrating their wide range of application prospects. We also categorize GO-based fire warning sensor warning signals into traditional and remote and IoT-based alarm signals and then elaborate on these. Finally, we provide a comprehensive summary of the research on GO-based fire early warning sensors, which shows that GO-based fire alarm materials can provide sensitive fire alarm signals within < 10s, making them more sensitive than conventional fire alarm systems. Based on such updated information, we summarize the future research directions in this field. Conclusions and Prospects: Future research should focus on several aspects. First, the fire warning and fire protection performance of the GO coating can be further optimized by developing new coating materials and improving the structural design, while ensuring that it can quickly respond to fire and effectively stop it from spreading. Second, in optimizing the design of the response of organics to GO, researchers should consider the thermal response sensitivity of the functional groups and the properties of the organics themselves. In particular, quantifying the number of functional groups and the effect of pyrolysis of organics on fire warning can help establish a synergistic quantitative relationship between them. Such a relationship helps to precisely regulate the properties of the materials, thus achieving accurate and efficient fire warning functions. Third, a more reasonable preparation method must be proposed to realize the precise control of the number and type of functional groups on the GO surface. This can be achieved by precisely controlling the conditions of the chemical reaction, including temperature, time, and pH levels. Finally, the GO fire warning and fireproof coating technology must be integrated with the Internet of Things to realize real-time data monitoring, as well as remote control and automated response systems to improve their level of intelligence.
Objective: In the context of advanced globalization, the transmission of infectious diseases, coupled with multiple viruses, has become a key issue in the field of public health. Traditional models for infectious disease transmission, such as susceptible-exposed-infected-covered (SEIR), typically only consider the transmission of a single virus, limiting its capability to capture the complex interactions of multiple viruses within the same social environment. Multi-virus coupling may demonstrate either a mutually promotional effect or a mutually inhibitory effect between different viruses, affecting the dynamics of transmission and the associated infection risks. Methods: This article enhances the SEIR model by incorporating additional states, such as asymptomatic individuals, divided individuals, and mortal individuals, to accurately characterize the complex population state transitions during the transmission of a single virus. On this basis, a multi-virus coupled transmission model is proposed. This model not only considers the impact of various factors, such as mutually promotional effects, mutually inhibitory effects, and immune escape effects, on coupled infections but also integrates the influence of medical resources, changes in personnel behavior, and prevention and control interventions on epidemic transmission. The multi-virus coupled transmission model can quantify the strength of mutual reinforcement or inhibition between viruses by adjusting the interaction coefficient, facilitating the simulation and analysis of the coupling effect between viruses. Results: Results reveal that the mutually promotional effect between viruses can increase the scale of infection, extend the transmission cycle, and lead to an earlier peak infection time. Additionally, this effect is more pronounced for viruses with larger infection scales than for those with smaller infection scales. In contrast, the mutually inhibitory effect can reduce the infection scale, slow down the transmission rate, and delay the peak infection time. The inhibitory effect is more substantial for larger viruses than for smaller ones. The increase in the immune escape rate can significantly increase the scale of infection and prolong the transmission cycle of the virus. When multiple viruses are transmitted together, changes in the transmission rate of viruses with larger infection scales have a considerable impact on the peak infection rate. In addition, comparing the simulation results of the coupled transmission model with real data shows that the trends are consistent, demonstrating a bimodal phenomenon. The simulation results align well with real data in terms of macro indicators, such as peak time and peak duration. However, in terms of the number of infections, the simulated infection rate is relatively lower compared to the detection-positive rate. Conclusions: The research results have effectively explained the infection patterns of social populations under the coexistence of multiple viruses, emphasizing the crucial role of complex interaction mechanisms between different viruses in real-world infectious disease monitoring and prediction. This article presents a new theoretical framework for the transmission of infectious diseases in a multi-virus coupled environment, offering more accurate and applicable model support for epidemic risk analysis and the development of intervention strategies.
Significance: Amid the rapid development of new productivity tools, the active thermal management system of power lithium-ion batteries is facing significant challenges, such as improving charge and discharge ratios and adapting to harsh application scenarios. To maintain stable operations of the power system in the best state, the technical bottleneck of efficient and long-term heat dissipation needs to be overcome. At the same time, in the consumer market, the cost factors of engineering products, including design, materials, space volume, cooling refrigerants, and plumbing systems, need to be carefully considered. Therefore, the active thermal management system of power lithium-ion batteries, which is widely used and has great potential, needs to be systematically summarized. Progress: This paper comprehensively reviews research progress on the active thermal management of power lithium-ion batteries in recent years. First, we summarize the research status of single-phase thermal management methods, including forced air cooling, natural air cooling, immersion liquid cooling, and microchannel liquid cooling. In the context of low charge and discharge ratios and lightweight engineering, air cooling still plays an important role. The main factors affecting battery temperature include air flow rate, air flow velocity, battery layout, and flow channel design. The air cooling system has unique engineering advantages because of its low cost. With an increase in charge and discharge ratios, the effect of microchannel and immersion liquid cooling is significantly enhanced, which is beneficial in controlling the battery's temperature and temperature uniformity. Several factors, such as liquid flow rate and channel design, have notable effects on the battery's heat dissipation; however, corresponding costs also increase. Second, we discuss advanced cooling techniques based on gas/liquid two-phase flow, such as submerged boiling cooling and spray-integrated cooling. In the context of increasing demand for batteries with high charge and discharge ratios, these technologies provide efficient, flexible, and adaptable solutions to thermal management challenges. The cooling medium, the flow rate, and the nozzle arrangement all have different effects on the temperature of the battery, along with the size of the droplets. The feasibility of the comprehensive and market recovery costs to maintain profits and long-term development of the enterprise also needs to be considered. Conclusions and Prospects: Based on the literature review, this paper forecasts the progress trend of active thermal management technology from multiple application scenarios to meet the development needs of lithium electric power in sea, land, and air. We believe that the development of the active thermal management technology of the new generation of power lithium-ion batteries should fully consider practical engineering requirements, such as charge and discharge ratios and harsh application scenarios. Future research and development should focus on improving heat transfer efficiency, system integration, and intelligent control capabilities while overcoming the challenges of reliability, cost, adaptability to extreme operating conditions, and energy consumption optimization.
