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Pulsejet expels pulse of hot gas (unlike turbojet engine) which create some thrust(Not good enough for ). These gases are hot, fast and pulsed. Is it possible to convert kinetic energy of these gases to mechanical energy using Turbines? (Assuming hot gases are not too hot for turbine blades to melt down). My concern is about Pulsed gases. What kind of problems/challenges it may cause in an engine perspective?

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  • $\begingroup$ Why do you say a turbojet engine is not good enough to create thrust? $\endgroup$ – Solar Mike May 23 '18 at 8:30
  • $\begingroup$ @SRD: Are you proposing to install the turbine behind the jet engines of an aircraft? $\endgroup$ – Transistor May 23 '18 at 17:09
  • $\begingroup$ @Transistor - if that is the plan, then I hope they realize that partially blocking the flow will cause a signicant reduction in the kinetic energy and therefore thrust. $\endgroup$ – Mark May 23 '18 at 18:35
  • $\begingroup$ @Solar Mike : Apologizes, if my first sentence is confusing. I was saying Pulse Jet is not good enough to make thrust - This is very debatable point. It expels little mass at high speed which causes two problems - 1. Low propulsion efficiency 2. Noise. I'm looking at feasability of adding a turbine which solve both these two problems. This is my analysis so far -> $\endgroup$ – SRD May 26 '18 at 2:17
  • $\begingroup$ If turbine is added and we turn a ducted fan just like in Turbofan engine. (1). Noise may decrease because no fast moving air and duct can be designed to absorb some noise (2). Fan acts like a fly wheel. Once it gets into study state most of thrust is generated by fan and its speed almost constant , it may give continuous thrust force resulting in low vibrational levels. (3). Down side is PulseJet simplicity is compromised to some extent because of turbine (4). Combustion may be inefficient compared to Pure PulseJet. This idea has both Pros and Cons. $\endgroup$ – SRD May 26 '18 at 2:38
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Two challenges (not necessarily "problems" so long as the device is designed to deal with them) are:

  1. Vibration. If the pulse frequency matches a resonant frequency of the turbine, the vibrations may reach a high enough amplitude to break something, or significantly reduce the life of the components. The waveform of the pulse will probably contain a large number of harmonics that are multiples of its fundamental frequency, and a typical turbine also has a large number of vibration modes - not just the modes of a single blade on its own, but coupled modes where all the blades and the turbine disk vibrate as a unit.

  2. Putting a turbine in the exhaust jet stream will change the aerodynamics of the jet itself. If the speed of the jet is subsonic, some energy will be "reflected" from the turbine and travel back upstream through the jet, in effect partially blocking the flow of the jet compared with the original pulse-jet design.

Even apparently small issues here can be catastrophic. True story of an anecdote when testing a conventional turbojet engine in a different test bed from the standard design: the gap between the back of the engine and the entry into the test bed exhaust silencer (muffler) happened to be just the right length to set up a standing wave in the gas flow, at the same frequency as a vibration mode of the last turbine stage in the engine. That mode would never have been excited in normal engine operation and therefore nobody had paid much attention to it - until 5 minutes into the first test run, when there was a very loud bang as the turbine disk and blades disintegrated!

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  • $\begingroup$ To add a bit onto number 2, it's second law of thermodynamics - since you aren't immediately exhausting to a heat sink, the "backpressure" you get from the turbine will cause a noticeable drop in efficiency of the original pulsejet. However, you usually will get more than enough energy out of the turbine that the the overall system efficiency will be larger. $\endgroup$ – Mark May 23 '18 at 18:19
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Yes, you can extract turbine energy from the exhaust of a pulse jet, but you need to evaluate the consequences before you think it worthwhile. I assume the pulse jet is on an aircraft, in which case, its exhaust normally jets into the atmosphere, and so the turbine needs to be attached to the aircraft and placed in the pulse jet exhaust stream, or a better idea may be to attach it directly by means of a short duct to the pulse jet exhaust. Regardless, one can see immediately that such a scheme greatly affects the operation of the pulse jet. If the turbine is mounted downstream in the free jet, the jet will cause a force that pushes on the turbine apparatus, in a direction opposite to the thrust of the pulse jet, thus reducing the net forward force on the aircraft. If the turbine is mounted to the exhaust of the pulse jet, the pressure drop of the turbine will increase the pulse jet exhaust pressure, thus reducing the performance of the pulse jet.

If the turbine is mounted downstream in the free jet, the resulting pressure disturbances in the exhaust stream can possibly travel upstream, resulting in an increase in the exit pressure of the pulse jet, which consequently reduces both the jet exit velocity and thrust of the jet. However, this upstream migration can happen only if the exhaust jet velocity is subsonic. Wherever this jet velocity is supersonic, the disturbance cannot travel upstream. The fact that the jet velocity is pulsed greatly complicates estimation of the magnitude of this reduced thrust, and also the magnitude of the backward-directed reaction force transmitted to the aircraft.

It seems to me that the simplest arrangement would be to mount the turbine in a short duct attached to the back of the pulse jet. But then, I wonder why not substitute this kind of contraption with a turbojet?

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