Avenger wrote:Would it be a question of compression and expansion?
Basically yes. The Kadenacy oscillation is just that -- a slowly decaying cycle of compression of expansion. But, that is only the beginning. Forgive me if you already know the following, but I can see that many participants in the forum fail to appreciate the basic mechanisms behind the functioning of pulsejets.
In a pulsejet, the Kadenacy cycle is started by a combustion event. The cycle does not decay because the combustion events generate each other and repeat endlessly, in a kind of relaxational oscillation. You can see it in its purest form in the Reynst combustor (or a simple jam jar with burning alcohol on the bottom).
However, the cycle is very similar in many ways to a sound wave. If you measure the changes in gas pressure over time at any point in the combustor and plot the values against time, you get a sine curve. The same goes for gas speeds. This is the same kind of behavior you see in sound.
For that reason, you can superimpose acoustic resonance on the pulsating process by adding a vessel which will allow the formation of a gas column long enough to support a standing wave. Presto! You get an acoustic combustor. The reason to do it is that resonance amplifies the process greatly and imposes on it a frequency determined by the length of the gas column. The pressure swings are greater, which gives you greater combustion efficiency. The frequency is higher, which packs more bangs into a second, giving you greater power.
This, however, is where the real complication starts. In the simple kind of generation of sound, in which you have some kind of a mechanical starting impulse (e.g. a strike of a mallet on a wire or a hammer blow on a bell) the sound you have generated does not affect the impulse. Generally speaking, there is no feedback.
In pulsating combustion, however, the sound-like pressure waves generated by one combustion event greatly affect the next combustion event. They determine (or at least affect greatly) the extent to which the engine will purge itself of the remains of combustion â€“ burnt gases. They determine the amount of fresh charge the engine will manage to suck in. They determine the amount of pre-compression the charge will receive before ignition and the amount of mechanical compression that will be added to the pressure of the burning gas.
In practice, the acoustic oscillation governs almost the entire behavior of your engine. It is one of the most important factors of all for engine performance, together with the fuel delivery (quality of mixing of fuel and air), ignition and thrust augmentation.
That is why all those little things are so important â€“ the exact lengths, the tapers, the orifice sizes, the openings and closures and branchings â€“ because they determine the exact shape of the wave train that will be generated and the exact kind of resonance. That is why it is not at all the same thing whether your intake or exhaust are here or there or anywhere, and whether there is one of five or none of each.