In 20 12, astronauts analyzed the flame burning on the international space station. Through this "fire extinguishing experiment" (FLEX for short), scientists verified the deeper theoretical knowledge that hexane droplets can be ignited in the presence of oxygen to form a blue spherical cold flame. But how can a flame be cold? Why go to space to observe the cold flame first? Let's make a comprehensive analysis!
Chemical principle of cold flame formation
When an object is ignited, the gas around it will become overheated and start to glow to produce flames. The formula of fire is very simple, because only three raw materials are needed: oxygen, fuel and heat. This basic relationship is also called "fire triangle".
On earth, we don't have to worry about the lack of oxygen in the flame. At any time, the earth contains about 65.438+0.2 trillion tons of oxygen. In addition to maintaining life, this oxygen-enriched environment also provides perfect conditions for making a fire. Next, let's talk about fuel, which is a substance that burns in the presence of any oxygen and releases energy in the process. Technically, everything around us is fuel. If it reaches a high enough temperature, it will catch fire. However, we prefer to use flammable or low-burning materials as fuel, including coal, oil or hexane.
Flame combustion involves a simple chemical process, which is called combustion. In this process, fuel combines with oxygen, performs some chemical reactions, and releases energy in the form of light and heat. However, fuel can only react with oxygen when it is above the ignition temperature. The extra energy needed to reach this temperature and start the combustion process is provided by an external heat source. For example, the heat source for igniting the furnace is an electric spark, while for the matchstick, it is the heat generated by the friction between the matchhead and the matchbox texture panel that ignites the fuel on the matchhead.
The formation of cold flame follows exactly the same chemical process. Fuel hydrocarbons are ignited and burned in the presence of oxygen. These flames do not freeze things, but melt them. They are called "cold flames" because the temperature of these flames is quite low. The flame produced by an ordinary stove is about 1700 degrees Celsius, while the temperature of the cold flame is between 400 and 600 degrees Celsius.
What's unique about cold flame?
The cold flame observed on the International Space Station is spherical, and it is almost impossible to reproduce on the earth under normal circumstances. Most of us may not realize it, but gravity plays an important role in the burning phenomenon of the earth's flame. When people light a flame, the surrounding gas will be heated. Through convection, the less dense hot gas will rise and suck in colder and fresher air, so as to keep the flame burning. This push-pull effect between light hot air and light cold air produces obvious teardrop flame. In the space environment, there is no gravity to produce density gradient, which explains why spherical flames are formed.
At the same time, the cold flame cannot get oxygen supply, so an external regulator, such as a fan, can be used to increase the flame. This controllable oxygen flow produces a faint blue flame, and the fuel burns completely to form carbon monoxide and formaldehyde without any residual soot. The shape of the cold flame is slightly different under the condition of adjustment.
If we carefully observe the flame of candles, we can find two types of flames: the blue flame outside and the yellow flame inside, because the oxygen content and temperature are different. The blue area outside the flame has the highest oxygen concentration due to the entrance of fresh air around it, and becomes the hottest area in the flame. The fuel here (mostly carbon-based fuel) is completely burned, so only carbon dioxide and water are produced as by-products.
On the other hand, the yellow area has low temperature and low oxygen content, which leads to incomplete fuel combustion, forming unburned carbon particles-"soot", as well as carbon dioxide and water. The soot carbon particles are then reheated and form a typical yellow flame.
Although it is not very common, it can produce a completely blue flame on the earth. All people have to do is introduce enough oxygen into the flame. Bunsen burner, welding torch and other equipment, through careful adjustment of oxygen and fuel flow, can almost produce a completely blue flame.
What causes the space flame to "cool down"?
First of all, the space flame becomes cold because it is ignited in the space environment. Secondly, the flame diffusion combustion process is slow.
In microgravity environment, oxygen contacts the flame through diffusion, instead of forming a density gradient to form suction like gravity on the earth. This slow oxygen flow greatly reduces the flame temperature, and the flame temperature is highly dependent on the amount of fuel and oxygen available in the flame. Due to the lack of radiant heat and ionizing chemicals that emit light, these flames do raise the surrounding temperature or present bright flames.
Slow and low-temperature flame seems to be a sign of safety, but on the contrary, the flame on the earth is a rapid burning process, which requires continuous and rapid oxygen flow to continue burning, and it is easy to start and stop. If the oxygen supply is cut off for a while, the flame will go out. However, this is not the case with cold flame. Even with limited oxygen flow, cold flame will last for a long time under the condition of combustion.
In fact, we know very little about low-temperature flames and flame burning outside the earth. Revealing the mysterious chemical properties of cold flame can not only make space travel safer, but also help us develop efficient smokeless internal combustion engines!