When combustion occurs in normal gravity, the hot gases rise due to buoyancy and affect fluid flow and flame shape. When combustion occurs in microgravity, buoyancy effects are eliminated so there is greater control of the fluid flow. Without buoyancy, it is actually possible to achieve spherically symmetric flames and a one-dimensional flow field, something that cannot be achieved in normal gravity. To do this, we utilize a small porous sphere burner inside a sealed combustion chamber. Spherical flames can be created when the fuel emanates from the porous sphere into a quiescent oxidizer (a “normal” flame) or when the oxidizer emanates from the sphere into a quiescent fuel (an “inverse” flame).
In non-premixed combustion, soot forms on the fuel side of the flame, and on Earth, where gravity is present, buoyancy can dramatically affect the motion of the soot particles after they are formed. Thus the effects of flow direction on soot formation are difficult to discern or control. But in microgravity, the direction of the fluid flow is governed by the gas stream exiting the porous sphere. Thus, for normal flames, soot particles form and are transported into regions of higher oxygen concentration, while in the inverse flames, soot particles are transported into a fuel-rich region. This level of control — not available in normal gravity — allows us to isolate the effects of convection on soot inception when the stoichiometric mixture fraction (Zst) is varied.
Two competing theories have been proposed for why the flame becomes non-sooty (blue) at high Zst (see blue flame on the left): One is based on convection direction, the other on flame structure. By performing experiments in microgravity, the effects of convection direction can be isolated.
We’ve been making progress on this microgravity research with terrestrial-based experiments, and are preparing for extraterrestrial experiments. Microgravity diffusion flame experiments have been conducted in the 2.2 Second Drop Tower at the NASA Glenn Research Center. Two flames taken during those experiments are shown. These studies have allowed us to prepare for microgravity experiments that will be conducted on the International Space Station in early 2019.