Kind of a follow up to yesterday’s post, also sourced from Aviation Week (Paid Subscription Required)
Basically, these technologies involve unsteady combustion, specifically pulse detonation and wave rotors, which can, in whole or in part, use meticulously controlled combustion to replace a conventional compressor:
As with PDEs and pulse combustors, the wave rotor concept hinges on the idea of increasing the thermodynamic efficiency of the engine by producing a pressure rise during the combustion process. In conventional gas turbines, the pressure reduces as the gas is burned in the combustor, resulting in an entropy gain or a reduction in efficiency. In a wave rotor, combustion takes place via pressure waves in a confined volume within a series of tubes or channels. This means the pressure of the gas rises because combustion occurs inside a limited or constant volume, thereby increasing the overall efficiency of the engine.
Although wave rotors and PDEs both rely on detonation shock waves traveling down the length of the tube to transfer energy, the two devices use this energy in slightly different ways. The PDE allows the blast wave to exit from the device as part of the propulsive effect, while the wave rotor merges successive waves to raise the pressure of the gas before it exits to the turbine stage.
The wave rotor consists of a series of tubes or passages arranged around the axis of a cylindrical drum which rotates between two fixed end plates. The plates contain the ports, or manifolds, which control the flow of the gas through the tubes. As the drum rotates, the ends of the channels are intermittently exposed to the ports which are at different pressures. This phase shift sets up the compression and expansion waves within the tubes as the mixture is ignited. The result is an expansion wave that travels into the gas and flows out of the wave rotor combustor into the turbine inlet at a total pressure more than 20% higher than that of the air delivered by the compressor.
As a result of the pressure gain from the wave rotor, more work can theoretically be extracted from the downstream turbine, increasing overall engine thermal efficiency. A key factor is that, as the combustion is contained within the tubes of the wave rotor, the pressure rise of the gas is not accompanied by a major increase in temperature, thereby keeping levels within the limits of the standard turbine materials.
Furthermore, experiments at NASA, as part of its ultra-efficient engine technology (UEET) study, indicate that because of the rapid expansion of the pressure wave, the gas temperature spends only a short time at or near stoichiometric conditions. As these represent ideal burning conditions for the fuel-air mix, they also coincide with the greatest production of nitrogen oxides (NOx), a major pollutant and focus for aviation emissions reduction efforts.
The wave rotor has the advantage that it smooths out the pulses before feeding it into a turbine.
Additionally, NASA is looking at pulsed propulsion for launch systems.
They are looking at a variable cycle engine where at slow speeds the afterburner would be fed by core air, and and at high mach numbers, would be fed off the fan, which is vaguely similar to how the J-58 used on the SR-71 operated.
It has been suggested that a conventional afterburner be replaced with a PDE.
