Burning Flames in Microgravity
Have you ever considered how flames burn in a microgravity environment? How they appear, or how properties such as size, soot formation, and temperature change?
The subject of microgravity combustion has become a prominent branch of combustion science and research. Microgravity combustion research is essential for gaining an in-depth understanding of combustion processes on earth. Without gravitational effects, novel flame behaviors are revealed. Some of the advantages of microgravity combustion research are:
1. Understanding the physical phenomena necessary to improve spacecraft safety.
2. Understanding fundamental combustion processes on earth.
3. Understanding combustion at very small scales.
The number of combustion-related experiments in microgravity has increased significantly from the first conducted by Kumagai and Isoda in 1957 regarding the burning rates of spherical droplets, to the implementation of combustion experiments aboard past space shuttle missions such as the Laminar Soot Processes (LSP 1 and LSP 2) experiments. The entire space shuttle mission STS-107 was dedicated to microgravity research, with the microgravity glovebox remaining aboard the International Space Station.
Combustion studies in microgravity environments have included droplet combustion, flame spread over liquid and solid fuels, smoldering combustion, premixed flames, and diffusion flames. Additionally, both laminar and turbulent diffusion flame regimes have been studied, and understanding laminar flames is a prerequisite to understanding the more complex turbulent combustion processes.
Laminar diffusion flames involve the mixing of both fuel and oxidizer flow streams upon reaction. Diffusion flames are characterized by the initial separation of the fuel and oxidizer. A reaction only occurs once the reactants come into contact with each other during ignition.
Generally, the fuel molecules diffuse outwards in a co-flow laminar diffusion flame, while the oxidizer molecules diffuse toward the flame from the opposite direction.
Microgravity refers to the weightlessness of an object; a state that is achievable through free-fall against the effects of gravity. The gravitational force acts as a single downwards non-contact force on an object. Any object has weight when there is a reaction force opposing the gravitational force. However, when an object is free-falling, no force opposes gravity and the object appears to be weightless. These effects can also be translated into buoyant forces on an object.
The most significant difference between combustion processes in normal gravity (1-g) and in microgravity is due to the absence of buoyancy in a microgravity environment. Hydrostatic pressure differences exerted by the surrounding fluid (usually air) develop a buoyant force during combustion at 1-g. More specifically, the hottest location of the flame is inside the reaction zone. Therefore the temperature gradient is in the opposite direction to gravity on the oxidant side of the reaction zone. The net motion of the high temperature, low density combustion products is a result of the difference between the buoyant and gravitational forces. The overall motion results in the recognizable teardrop flame shape.
Under normal gravity conditions, the convective buoyant force adds increasing complexity to many combustion processes. Since convection is minimized in microgravity, the products of combustion reactions tend to stay trapped within the reaction zone and therefore prevent fresh oxidizer from reaching the reaction zone. Instead of convection, molecular diffusion operating on a much slower time frame is the dominant means of transporting the oxidizer to the reaction zone.
Furthermore, at 1-g, gas velocities increase with increasing flame size and with increasing distance from the jet exit. With higher velocities, the laminar flow regime transitions to the turbulent regime over a smaller range of flame lengths. There is a larger range of laminar flames sizes that can be studied in microgravity before buoyancy interferes with the results.
The formation and emission of soot in flames has been a long-standing research topic among scientists and engineers. The processes by which soot forms within flames are still not completely understood and various theories are described extensively in literature. Soot formation is a result of incomplete combustion within flames and exemplifies both inefficiency and a loss of usable energy during combustion. While soot formation is a health and environmental issue, soot is an unavoidable by-product in most practical applications of combustion.
Under normal gravity conditions, soot forms in aggregates that are roughly spherical particles. The soot particles have nearly uniform diameters on the order of 20 nm and soot formation begins with the pyrolysis of fuel molecules and the formation of Polycyclic Aromatic Hydrocarbons (PAH).
Soot formation has been studied extensively at both atmospheric pressures and at high pressures. Experiments at high pressures are necessary in order to simulate most practical combustion technology that operates at high pressures. Results have shown that increases in pressure cause an increase in the concentration of soot formed in the flames, although this could change under reduced gravity conditions due to the lack of buoyancy.
In a 1-g environment, buoyancy increases the axial velocities of the combustion species with increasing distance from the jet exit. The particles at the flame tip have the highest velocities because buoyancy has accelerated the particles over the largest distance. Contrary to normal gravity, the absence of buoyancy in a microgravity environment tends to slow down the axial velocities of combustion species, which increases the combustion time scale or residence time (the length of time the particles spend between the burner rim and the flame tip).
In addition to affecting soot formation, an increase in the reactant residence time in microgravity significantly alters the thermochemical flame environment. This indicates that radiation is an important mode of heat transfer and heat loss for non-buoyant diffusion flames.
Increased radiative heat losses decrease the flame temperature, which decreases luminous flame lengths, eventually leading to flame extinction. Most current analytical studies under normal gravity conditions neglect heat loss caused by radiation. However, it is evident from experimental results that this assumption is not valid for microgravity combustion, where the combustion rate is much slower resulting from the dominance of the diffusive transport mechanism.
More specifically, heat loss due to radiation causes a decrease in the fuel burning rate, eventually producing a low-power flame. This low-power flame now has a longer time period for heat loss to occur. The higher radiative heat losses experienced in microgravity indicate lower temperatures in the soot production regions and lower maximum flame temperatures for diffusion flames. Eventually, radiative heat transfer cools diffusion flames to the point of visible soot luminosity disappearance.
Overall, eliminating buoyancy simplifies combustion processes. The dominance of a diffusional transport mechanism in microgravity produces flames that are rounder, thicker, sootier, more stable, and cooler than normal gravity flames.
Naturally, studying the effects of combustion without the interference of gravity requires facilities free from gravitational influences that seek innovation in apparatus design to accommodate various user constraints. There are many methods for simulating a microgravity environment, including drop towers (tall shafts provide free-fall environments), sounding rockets (launching a payload), flying in aircraft with parabolic trajectories (‘vomit comet’), flying in spacecraft (flying experiments on-orbit), or simulating microgravity using sub-atmospheric pressures (vacuum).
In a microgravity environment, luminous flame height decreases with decreasing pressure to the point of visible luminosity disappearance, resulting in blue flames. Flame width increases with decreasing pressure until the flame is almost spherical. Soot formation decreases with decreasing pressure to negligible concentrations in a near vacuum.