"Fighting fire with fire" space combustion experiment - Why is fire in space so exciting?

In February this year, the Combustion Science Experiment Cabinet of the Mengtian Experiment Module of the Chinese Space Station successfully carried out its first on-orbit ignition experiment. This experiment used methane as fuel, and the two ignition experiments lasted for about 30 seconds in total, verifying the completeness of the functions of the space station's combustion science experiment system and the accuracy and scientificity of the overall experimental process, laying a good foundation for subsequent space science combustion experiment projects. So, what are the characteristics of microgravity combustion in space? What is the purpose of conducting combustion experiments in a microgravity environment in space?

The image of methane combustion in the combustion science experiment cabinet on the space station (left) and the image of methane combustion under the same working conditions on the ground (right). Photo: Institute of Engineering Thermophysics, Chinese Academy of Sciences, Combustion Energy Center, Tsinghua University

Microgravity combustion experiments have been going on for half a century

The first to conduct microgravity combustion experiments was conducted by the University of Tokyo in Japan, where they used free-fall facilities such as drop towers to conduct droplet combustion experiments. Subsequently, the United States also conducted multiple microgravity combustion experiments using drop towers and weightlessness in aircraft.

In 1967, a fire broke out on the Apollo 1 spacecraft, killing three astronauts. This made the aerospace industry pay great attention to fire prevention of spacecraft, and the safety of microgravity combustion experiments has received more and more attention.

In 1974, the United States conducted the first microgravity combustion experiment in space on the Skylab space station. The United States continued to conduct microgravity combustion experiments during the space shuttle era, with the goal of studying the ignition, self-ignition, propagation and extinction of flames in space.

With the construction and completion of the International Space Station, the United States, ESA and Japan have conducted microgravity combustion experiments using the space station's combustion integrated cabinet and other equipment. They have carried out a series of experiments such as air purification combustion experiments, solid combustion extinction experiments, flame extinction experiments and experiment 2, flame structure and flame lift experiments, smoke measurement experiments and combustion rate, flame diffusion, spherical cold flames, etc. These studies have enhanced human knowledge and understanding of the basic combustion process of flames and achieved fruitful research results.

In 2006, my country launched the Shijian-8 space test satellite, which successfully completed two combustion experiments and became the first batch of microgravity space combustion experiments in my country. In 2016, my country launched the Shijian-10 returnable scientific experimental satellite, which also arranged microgravity combustion experiments, including the study of coal combustion and pollutant generation characteristics under microgravity and the study of ignition and combustion characteristics of non-metallic materials in microgravity environment.

After the launch of the Mengtian laboratory module of the Chinese space station, the independent combustion experiment cabinet has laid a solid foundation for my country to carry out microgravity combustion experiments in space.

Combustion experiment has distinct characteristics

The combustion reaction represented by flame is the cornerstone of modern civilization. A deeper understanding of the combustion process is of great significance to reducing fuel consumption and pollution emissions.

Flames look simple, but combustion is actually very complex, involving a complex process of chemistry and fluid mechanics. People have long recognized that gravity has a great influence on the combustion process. If the influence of gravity on the combustion process can be eliminated, the combustion process can be better understood and analyzed, and the model and theoretical research of combustion reactions can be improved.

The biggest feature of the microgravity combustion experiment is that the influence of the earth's gravity is basically eliminated in the free fall state. In a weightless or microgravity environment, flame combustion presents many completely different phenomena.

The products formed after combustion in microgravity are also different. For example, heptane burns on the ground to produce smoke, carbon dioxide and water, while in a microgravity environment there will be a unique cold flame combustion state. Although there is no open flame at this time, the low-temperature flame combustion will produce carbon monoxide and formaldehyde.

As we all know, gravity is the key to generating buoyancy convection during flame combustion, which directly affects the structure of the flame, combustion stability, flame propagation and extinction, and other combustion phenomena. Research on the nature of flame combustion under conventional gravity is like looking at flowers in the fog, and it is difficult to gain a deeper understanding.

In addition, flames have a center of gravity under the action of gravity and will appear droplet-shaped, but the combustion in a microgravity environment is completely different. The flames are shorter and rounder, and appear hemispherical or spherical. The disappearance of gravity causes the convection phenomenon to be severely weakened or disappear, and the microgravity environment also inhibits the sedimentation of droplets and particles. At the same time, some combustion phenomena that are masked by strong convection when the flame burns on the ground, such as the weak basic effects of flame combustion, such as thermal radiation, diffusion, capillary effect and electrostatic force, can also be clearly manifested in space, which provides a better environment for in-depth research on the basic combustion process of flames.

In addition, suspended droplets and particles in a microgravity environment can also burn, and the flames appear spherical. These are all wonders that flames cannot present on the ground, expanding people's understanding of the flame combustion process.

Thanks to the unique microgravity environment, microgravity combustion experiments provide a more effective way to verify the basic processes and theories of flame combustion and study the physical and chemical mechanisms of the combustion process.

Combustion experiments will have great uses in the future

In order to meet the needs of combustion science research, energy and environmental protection, many countries in the world have carried out microgravity combustion science experiments.

Since mankind entered the space age, especially the manned space age, human lives are at stake and no carelessness is tolerated. The Apollo 1 spacecraft fire occurred on the ground and the astronauts were unable to escape. If a spacecraft fire occurs in space, the success or failure of fire prevention and fire extinguishing is more directly related to the safety of the astronauts' lives.

The development of manned space flight has placed high demands on the fire prevention of spacecraft. One of the main purposes of scientific researchers conducting microgravity combustion experiments is to gain an in-depth understanding of the combustion process of flames in the microgravity environment of space, and to conduct theoretical research for the development of fire prevention and fire extinguishing technologies.

Astronauts on the International Space Station have discovered in flame extinguishing experiments that droplets of heptane and other liquid fuels continue to burn at low temperatures, i.e., cold flames, after visible flames are extinguished. Rapid vaporization of the droplets does not stop until the cold flames are extinguished, which means that the extinguishing of open flames in microgravity is not the end of the combustion process. Therefore, the fire extinguishing device on the space station must take into account the problem of secondary combustion of flames.

These new findings from the microgravity combustion experiment will help develop more complete spacecraft fire detection technology and safer space fire extinguishing technology; the understanding of the flame combustion process in the microgravity combustion experiment will also help improve the combustion efficiency of ground internal combustion engines and reduce pollution emissions from combustion exhaust gases, so it has important value in energy conservation and emission reduction.

The coal combustion and pollutant research experiment under microgravity conducted by my country's Shijian-10 scientific satellite aims to better discover the laws of coal combustion and pollutant generation through comparison of combustion in a microgravity environment, so as to develop better coal combustion theories and technologies and enable people to use coal resources more efficiently.

In addition, in-depth research on the combustion process of flames in a microgravity environment, represented by the FLEX-2 experiment on the International Space Station, will also help improve the combustion efficiency of rocket engines in interplanetary exploration missions, develop rocket engines with higher specific impulse and efficiency, and reduce launch mass and mission costs.

Therefore, flame microgravity combustion experiments are of great significance to the development of science and technology and the utilization of resources. (Author: Zhang Xuesong)