Microgravity combustion research is essential for understanding fundamental combustion phenomena and advancing combustion theory. However, conducting experiments in orbit involves significant technical challenges and resource demands. The Combustion Science Rack (CSR) aboard the China Space Station (CSS) has been operational since October 2022. To further support combustion science research aboard CSS, consolidate critical scientific questions in microgravity combustion, and validate the in-orbit experiment feasibility, a ground-based research platform has been established at the space experiment center in Huairou District, Beijing. This platform replicates the combustion environment and apparatus dimensions of the in-orbit CSR. Equipped with high-precision diagnostic tools and versatile experimental modules, the platform enables researchers to validate in-orbit experiment feasibility, conduct ground-based validation tests, and generate baseline control data for CSS experiments. The paper highlights the platform's design, operational principles, and preliminary test results. Together, these demonstrate its ability to meet the diverse requirements of current and future microgravity combustion research projects.

The platform consists of the experimental insert subsystem, the supporting facility subsystem, and the combustion diagnostic subsystem. Designed to match the experimental space and apparatus sizes of the in-orbit CSR, this ground-based platform takes full advantage of laboratory amenities, including gas supply, ventilation, power supply, and thermal control facilities. The platform comprises three subsystems: the experimental insert subsystem, the supporting facility subsystem, and the combustion diagnostic subsystem. The experimental insert subsystem supports a wide range of experiments with its gas, liquid, and solid combustion modules. The combustion diagnostic subsystem is equipped with high-precision measurement devices such as high-speed cameras, particle image velocimetry, and planar laser-induced fluorescence, enabling real-time measurements of flame morphology, flow velocity, and intermediate species distribution. Initial tests demonstrate that the platform can generate various types of gas flames, including premixed, diffusion, and partially premixed flames, by adjusting the fuel-to-oxidizer flow ratio. The liquid combustion module conducted suspension and ignition tests for single and multiple droplets, while the solid combustion module examined how planar, cylindrical, and linear materials combust under microgravity conditions. The system precision and reliability were validated by comparing diagnostic data with established data on flame oscillation. Additionally, the platform's modular design supports upgrades to both future software and hardware.

The ground-based research platform replicating on-orbit combustion environments plays a crucial role in supporting and complementing future combustion studies conducted aboard the space station. With its advanced experimental modules and diagnostic tools, the platform enables systematic and in-depth combustion experiments, advancing fundamental research in space combustion. Notably, the diagnostic system facilitates high-precision measurements across diverse combustion experiments. This ensures accurate analysis of flame structures, flow velocity fields, and chemical component distributions, providing critical ground-based validation and comparative data for addressing key scientific questions faced in space-based research. Overall, the platform is equipped with comprehensive facilities necessary for conducting combustion experiments. By supporting systematic experimentation, it helps optimize the design of space-based experiments, strengthening the cutting-edge and innovative aspects of space combustion studies. Furthermore, the extensive diagnostic resources and results from ground experiment results offer valuable data for in-orbit combustion experiments, driving the advancement of space combustion science theories and applied technologies.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.