|(Left) A traditional combustor mixes fuel and air before injection into the combustion chamber. (Right) Tech’s system injects the fuel and air separately into the combustor. Click to enlarge.|
Georgia Tech researchers have developed a new combustor (the combustion chamber where fuel is burned to power an engine or gas turbine) designed to burn fuel in a wide range of devices with ultra low emissions of nitrogen oxide (NOx) and carbon monoxide (CO).
The new Stagnation Point Reverse Flow Combustor, originally developed for NASA, burns fuel with NOx emissions of less than 1 parts per million (ppm) and CO emissions of less than 10 ppm, significantly lower than emissions produced by other combustors.
The device has a simpler design than existing state-of-the-art combustors and could be manufactured and maintained at a much lower cost, making it more affordable in everything from jet engines and power plants to home water heaters.
The project’s initial goal was to develop a low emissions combustor for aircraft engines and power-generating gas turbines that must stably burn large amounts of fuel in a small volume over a wide range of power settings (or fuel flow rates).
Existing combustors premix fuel with a large amount of swirling air flow prior to injection into the combustor to reduce emissions. This requires complex and expensive designs, and the combustion process often excites instabilities that damage the system.
The Georgia Tech combustor burns fuel in low temperature reactions that occur over a large portion of the combustor. By eliminating all high temperature pockets through better control of the flow of the reactants and combustion products within the combustor, the device produces far lower levels of NOx and CO and avoids acoustic instabilities that are problematic in current low emissions combustors.
The Georgia Tech design eliminates the complexity associated with premixing the fuel and air by injecting the fuel and air separately into the combustor while its geometry forces them to mix with one another and with combustion products before ignition occurs.
The reactants are injected at the center of the open end of the combustor, and flow towards its closed end, where the velocity must be zero—and hence, the velocity of the reactants must decrease as the flow approaches the closed end.
Click to enlarge.
This establishes a low velocity region in the vicinity of the closed end that can help to stabilize the combustion process [Stagnation Point]. Since the generated products and burning gas pockets can not leave the combustor through its closed end, they must reverse their flow direction and exit the system through the annular opening around the injection system [Reverse Flow].
As the stream of hot products laden with radicals flows out of the combustor, it must mix with the incoming reactants. Mixing with hot products increases the reactant temperature and the presence of radicals in the resulting mixture should lead to reduced ignition temperatures.
The combination of stagnation point and reverse flow allows the combustor to operate stably at lower inlet temperatures and/or lower fuel-air ratios, and, thus, produce lower NOx emissions.
The project was funded by the NASA University Research Engineering Technology Institute (URETI) Center on Aeropropulsion and Power and Georgia Tech.