The experiment, carried out aboard the International Space Station, gave unexpected results - the open flame behaved quite differently than scientists expected.
As some scientists like to say, fire is humanity's oldest and most successful chemical experiment. Indeed, fire has always gone with humanity: from the first bonfires, on which meat was fried, to the flame of a rocket engine that brought a person to the moon. By and large, fire is a symbol and instrument of the progress of our civilization.
Dr. Forman A. Williams, professor of physics at the University of California, San Diego, has a long history of researching flames. Fire is usually a complex process of thousands of interconnected chemical reactions. For example, in a candle flame, hydrocarbon molecules evaporate from the wick, decompose when exposed to heat, and combine with oxygen to produce light, heat, CO2, and water. Some of the hydrocarbon moieties in the form of ring-shaped molecules, called polycyclic aromatic hydrocarbons, form soot, which can also burn or turn into smoke. The familiar teardrop shape of the candle light is given by gravity and convection: hot air rises upward and draws fresh cold air into the flame, thereby pulling the flame upward.
But it turns out that in zero gravity everything happens differently. In an experiment called FLEX, scientists studied fire aboard the ISS to develop technologies to extinguish fires in zero gravity. The researchers ignited small bubbles of heptane inside a special chamber and watched how the flames behaved.
Scientists are faced with a strange phenomenon. In microgravity, the flame burns differently; it forms small balls. This phenomenon was expected because, unlike a flame on Earth, in zero gravity, oxygen and fuel meet in a thin layer on the surface of a sphere. This is a simple scheme that differs from earthly fire. However, an oddity was discovered: scientists observed the continued burning of fireballs even after, according to all calculations, the combustion should have stopped. At the same time, the fire passed into the so-called cold phase - it burned very weakly, so much so that the flame could not be seen. However, it was burning, and the flame could instantly burst forth with great force upon contact with fuel and oxygen.
Usually visible fire burns at high temperatures between 1227 and 1727 degrees Celsius. The heptane bubbles on the ISS also burned brightly at this temperature, but as the fuel depleted and cooled, a completely different combustion began - cold. It takes place at a relatively low temperature of 227-527 degrees Celsius and does not produce soot, CO2 and water, but the more toxic carbon monoxide and formaldehyde.
Similar types of cold flames have been reproduced in laboratories on Earth, but under gravitational conditions such a fire itself is unstable and always quickly dies out. On the ISS, however, a cold flame can burn steadily for several minutes. This is not a very pleasant discovery, since cold fire poses an increased danger: it ignites more easily, including spontaneously, it is more difficult to detect it and, moreover, it releases more toxic substances. On the other hand, the discovery can find practical application, for example, in the HCCI technology, which involves igniting fuel in gasoline engines not from candles, but from a cold flame.
Promotional video:
This picture was taken during an experiment to study the physics of combustion in a special 30-meter tower (2.2-Second Drop Tower) of the John Glenn Research Center (Glenn Research Center), created to simulate the conditions of microgravity in free fall. Many experiments that were then performed on spacecraft were preliminary tested in this tower, which is why it is called “a gateway to space”.
The spherical shape of the flame is explained by the fact that under zero gravity conditions there is no ascending air movement and convection of its warm and cold layers does not occur, which on Earth "pulls" the flame into a drop shape. The flame for combustion does not have enough fresh air containing oxygen, and it turns out to be smaller and not as hot. The yellow-orange color of the flame familiar to us on Earth is caused by the glow of soot particles that rise upward with a hot stream of air. In zero gravity, the flame acquires a blue color, because little soot is formed (this requires a temperature of more than 1000 ° C), and the soot that is, due to the lower temperature, will glow only in the infrared range. In the top photo, the yellow-orange color is still present in the flame, since the early stage of the ignition is captured when there is still enough oxygen.
Investigations of combustion in zero gravity are especially important for ensuring the safety of spacecraft. For several years now, Flame Extinguishment Experiment (FLEX) experiments have been carried out in a special compartment on board the ISS. Researchers ignite small droplets of fuel (such as heptane and methanol) in a controlled atmosphere. A small ball of fuel burns for about 20 seconds, surrounded by a sphere of fire with a diameter of 2.5–4 mm, after which the drop decreases until either the flame goes out or the fuel runs out. The most unexpected result was that a drop of heptane, after visible combustion, passed into the so-called "cold phase" - the flame became so weak that it was impossible to see it. And yet it was burning: fire could instantly break out when interacting with oxygen or fuel.
As the researchers explain, during normal combustion, the flame temperature fluctuates between 1227 ° C and 1727 ° C - at this temperature in the experiment there was a visible fire. As the fuel burned, “cold combustion” began: the flame cooled to 227–527 ° C and produced not soot, carbon dioxide and water, but more toxic materials - formaldehyde and carbon monoxide. The FLEX experiment also selected the least flammable atmosphere based on carbon dioxide and helium, which will help reduce the risk of spacecraft fires in the future.