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Laminar flow combustion chamber
What does the combustion chamber actually do? It's a place for the
propellants to mix and burn subsonicly. The mixing typically requires ferocious turbulence. The problem I have with mixing and burning simultaneously is that a lot of the mixing is happening at high temperatures, where the gases have large volumes and require an enormous amount of turbulence to mix. My guess is that this turbulence is a way to lose energy. An order of magnitude less turbulence is required to mix the propellants before burning. Suppose the propellants are not hypergolic. Why not mix them, then burn them, more like a bunsen burner? My understanding is generally that any gas acceleration that happens while heat is added to the gas (example: bunsen burner) increases entropy, but that acceleration that happens with no heat added (example, rocket nozzle) does not. This means you want the burning to happen at the lowest gas velocities possible. Deflagration in premixed gas phase propellants is pretty slow. It gets faster as the initial pressure increases, and as the initial temperature increases. I'm going to guess that methane-oxygen is around 16 m/s at 10 atm. The burned gas velocity is going to be around 160 m/s. Detonation pressures for methane are 20-40 atm, which is why these operating pressures seem low. To capture the flame, I'd use a tube that flares, perhaps with wires crossing it to form a flameholder (and to provide initial ignition). The flaring section needs have a large enough area ratio that the flame is still trapped for any expected variation in flame speed due to startup temperature and pressure transients. After the flaring tube, I'd have the flow reconverge in a de Laval nozzle. So the engine ends up looking basically like all rocket engines do, except instead of a flat injector plate I have a flaring tube, and the propellants injected at the end of that tube are injected gas-phase. I might have other (not hot) wire grids in the throat to increase the low-temperature turbulence and mixing. The area ratios of injector throat to combustion chamber to nozzle throat might be 1:10:1. Now consider start-up. Begin running 100% fuel (gas phase somehow) at full operational mass rate. Run current through the ignition wires to get them hot, then add oxygen progressively until you get to operating mixture ratio. The flame starts at the wide end of the flare, then moves down near the throat as chamber pressure builds. Advantages: - Less noisy - Potentially better Isp across smaller chamber/ambient pressure drops, from lower turbulence - More complete burning - Short "combustion" chamber - More laminar flow near walls means - less heat transport to walls - film cooling works better, with less wasted fuel - less scouring action - rocket engine scales down better (same Isp at smaller size) - Reliable, nonexplosive startup Disadvantages: - Extra drag from larger nozzle convergence ratio - Need to boil propellants before injection - More constraints on cooling system (film cooling less usable) - Smaller input/output temperature ratio will lower Isp - Larger effect at higher pressures How thermodynamically efficient is accelerating a boiling two-phase mixture? My guess is that boiling the propellant in the combustion chamber is not critical to efficiency, since the SSME injects gas-phase hydrogen. Perhaps higher-temperature methane, propane, and oxygen are a different story. If you haven't figured it out already, this idea is targetted to a restartable upper-stage engine that is simple enough for amateur work. |
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