tufty wrote: What, exactly, is the difference between a Reynst pot and, for example, a Lockwood? If we take a Reynst pot (with or without internal diffuser, but with is easier to picture mentally) and add tuned exhaust and inlet as per patent no FR949103, page 7, Fig 5, I think we actually have a valveless pulsejet with two openings, very similar to a lockwood.
Simon,
Speaking very broadly, all valveless pulsejets are the same. Mark 'Thixis' has long preached that approach. We are only looking at different ways to arrange the parts. However, this approach is helpful only to a fresh student of pulsejet design and as a philosophical reminder. In practice, the various arrangements of chambers, pipes and openings operate in a great variety of modes.
In real pulsejets, it is sometimes a tiny detail that decides. In some of its iterations, the Reynst may resemble Lockwood or Ecrevisse, but one detail remains – exhaust is always located at least a tiny bit more downstream from the intake. If it isn't, it is not a Reynst. The order of parts is always chamber-intake-exhaust, rather than the more common intake-chamber-exhaust. A true Reynst combustor has a single opening on the chamber. If there is more than one, they have to be absolutely identical. Otherwise, it starts working like an ordinary pulsejet, or does not work at all.
The crucial difference between the Reynst and the others is truly fundamental, and it depends on having a single opening or, multiple identical openings.
Namely, ignition in a Reynst pot is different. It is not triggered by the wave reflected off the exhaust end, which – in the 'ordinary' pulsejets -- slows the propagation of fresh charge and enables combustion to spread from its initial kernels and pockets to the entire mixture. The Reynst combustor ignites its mixture
at speed. It does not stop its incoming mixture, but simply changes its direction. Instead of the mixture progressing in a more or less straight line, it starts swirling in an orderly vortex (or a train of orderly vortices). In other pulsejets, it turns into chaotic turbulence.
For this reason, a Reynst combustor
does not even need the exhaust pipe to work. Now, try making a Lockwood or an Ecrevisse, or any other pulsejet, work without the exhaust pipe! Unlike any other pulsejet that I know, the Reynst is not an acoustic resonator. It is not a pure Kadenacy breather, either. It is a beast completely unto its own. It can use an acoustic tailpipe, true, but it is an accessory that enhances its performance, rather than an essential part. It can be likened to the augmenter on an ordinary pulsejet – it can transform performance, but the basic combustor will work without it.
But, you are right in one detail. If equipped with a resonant exhaust, the acoustic length is effectively the distance between the bottom of the ‘pot’ and the end of the exhaust pipe. The bottom is the pressure node and the end is the speed node. The problem is that in the intake part of the cycle, the situation changes and a new speed node (pressure antinode) appears at the intake slit. The exhaust tube becomes an open-ended resonator, with a pressure node in the middle and antinodes at the ends. In practice it means that the thing simultaneously resonates in a basic frequency and its first odd harmonic. Those two must be dominant over everything else for the machine to function properly.
Without the exhaust resonator, acoustic behavior disappears and the pot works to its own specific frequency that has nothing to do with its acoustic properties. It is an order of magnitude lower than its acoustic frequency.