After Combustion: Detonation!

The vehicle Dean plans to use to reach his goal looms over the rest of the test cell, looking for all the world like a chunk of water main. The GE research group calls it simply "the big rig"-a heavily instrumented length of pipe roughly 16 inches in diameter. The one part of the test cell that's off-limits to visitors is the area just in front of the big rig's mouth-the only place that would give a view down its throat, presumably revealing details that GE would prefer to keep to itself. Those details, one guesses, have something to do with what Dean refers to as "valveless" operations, which could be the key to generating detonation frequencies as high as hundreds or even thousands of cycles per second in a single tube.




Dean is reserved on this subject, noting in a later e-mail that he's "not ready to say much about this." But Maclin, GE's marketing manager, is more expansive. "We're looking at an order of magnitude higher frequency than anybody else in the industry," Maclin says. "I like to think of it as an aerodynamic valve as opposed to mechanical valves, and that's what allows us to get to the much higher frequency, because there's a limit to what you can do with mechanical valving." In such a design, the air-fuel mixture and timing would be controlled by aerodynamic forces created by the shape of the detonation chamber itself. This "aerodynamic valve," Maclin says, would "allow air in before detonation, but the pressure from detonation will be high enough to prevent the second charge of air and fuel from entering until the detonation wave moves downstream."




When asked what kind of results he's getting so far with the big rig, Dean again speaks carefully. "We're getting fast flames and all sorts of interesting behaviors, but I would not characterize them at the moment as detonations," he says. "I do think we'll be successful-we've got the right measurements, the right people, the right computational tools-but I can't claim that we've gotten there yet."




It's an odd mix-a healthy skepticism combined with faith unswayed by the presence of so many unknowns. In a way, the skepticism is part of Dean's job description. Maclin, by contrast, is unreservedly bullish on GE's prospects for taking full advantage of PDEs. For him, as for his counterparts at Pratt & Whitney, focusing only on incremental improvements to conventional gas turbines at the dawn of the 21st century runs the risk of going the way of a buggy whip manufacturer at the beginning of the 20th. "People are always discovering new things," he says. "You can't be fixated on the buggy whip, and you can't be fixated on the turbine engine."

Jim Kelly lives in Galveston, Texas, where he writes about
science for the University of Texas Medical Branch.