I am afraid that your system will never be fast enough to create any shockwave at all. The shock wave that you want to add, should that be 'superposed' on the steady flow, or on the rig containing the steady flow? What pressures are you looking for?

Thanks for your answer. The shockwave would be superposed on to a low Mach steady flow inside a test rig. I agree that the speed of opening the valve is critical, clearly a slow opening would just create a flow and a jet into the rig. There are solenoid valves which open quite quickly, I've seen opening times of 5 to 15 mS. These are very inexpensive and come from the process industries, will take up to 10Bar. The fastest valve I've seen can run at 1kHz How could I judge if this is fast enough?

cheers Martin

Even if the valves open quickly enough to create a (small) shock wave, they will normally not create a shock wave that is powerful to be sustained into your test rig due to the very low flux that is coming through the valves. In that case you will end up with a small shock wave in the neighborhood of your valve opening but that dissipates very rapidly to become a simple acoustic wave... I have no idea what your test rig is about, but could you combine your test rig with a shock tube? Or have a small-scale detonation at the inlet side?

Since you have developed a test rig for this, I assume that you also have an application in mind where you have this superposition of steady flow and a shock wave. What is the origin of the shock wave in that case?

Dear Frederik

My interest is in unsteady combustion. I supervised a PhD student to study the interaction of a premixed laminar flame with a shockwave in a shock tunnel. The gave the expected result Richtmaier Meshkov instability and huge increase in the rate of combustion. This gave the huge increases of pressure we were looking for. Now I want to repeat the process with shockwaves at a frequency (100's Hz). My ultimate aim is pressure rise combustion for a gas turbine, fuel consumption could be cut by circa 10%

Hi Martin,

Sorry, but this is too much outside my regular field (blast and ballistic impact). No idea how you could get repeated shock waves superposed on a laminar flaw. I have a couple of colleagues that I will talk to about your problem and that might have an idea. I'll come back to you if I get any useful answers. In the meanwhile: good luck with your research!

I've been thinking about this technique to make shockwaves, I still think it should work. Previous authors have used a shock tube to make a shockwave and then connected up small turbes to the shock tube to carry the shock to (for example) a linear cascade. So the only difference is that I want to make the shock in the small tube. As a believer in continuum mechanics the small tube is just a small shock tube. The difference is the lower Reynolds number, the different geometry of the valve cf the diaphragm and the speed of opening. Also the small shock pipe has an advantage of a plenum upstream that is much larger than the pipe diameter. The Reynolds number problem can be addressed by maximising the diameter of the pipe and by maximising the pressure of the gas in the plenum (and downstream rig). I guess we could also adjust the temperature to shift the viscosity. The detail of the valve seat will cause additional losses of pressure but this will raise the static temperature of the gas and cause pressure waves to speed up and encourage the formation of the shock in a few diameters of the pipe. I have found on the web high speed 'jet' solenoid valves with opening times of order 0.5 ms and 5mm diameter pipe. I think I shall try this out. cheers

Hi Martin,

It has been a long time since I tackled shock wave problems for the last time, but let me try once again. Perhaps, depending on the nature of the laminar flow you are creating in your rig, a fast-spinning wheel could be added to act as a mock - injector. I mean a solid wheel having conical, skewed channels drilled and located along a circle. The wheel would have to spin real fast ... and be supplied with gas at an elevated pressure coming from a separate source. Surely, it would all be very susceptible to leaks, but if the diameter of the rig tube is small you may get scot - free. We have build something like this (a slapdash) when working on a two-phase fuel injection concept, but had to give it up exaclty because of the side, parasite flow around the rotor and flow instabilities downstream the aft side of the rotor. I am pretty sure we had shock waves in the "aft" region, because the pressure difference was sufiiciently high. For gas-liquid systems there is another, failry simple way to get such shocks, but traveling inside the mist, so they would dissipate fairly quickly. Could it be, that the combustion system you are after requires some liquid fuel and that we are speaking about the same idea? Please, see how such a dual-phase flow disperses in the appended picture (pressure difference is below 10 bar, most of the time).

Greetings from rainy Warsaw

Jerzy 

Dear Jerzy

Thanks for your reply. I have considered introducing a rotating shock generator into the combustion chamber. The problem is temperature if we are near to the primary zone the temperature will approach stoichiometric and since the wheel is spinning it will see even higher relative temperature. So say 2600K for primary zone +100 (so 2700K) for blade speed. In a turbine things are much more benign there we have had dilution air and NGV cooling air say, "only" 1900 K and the relative speed of the blades further reduces the relative temperature by around 75K (so Trel is  only 1825K).

 I guess one might have a non-burning gap between the wheel and the flames somehow? If it is possible to introduce shockwaves through small pipes into a combustor:  that is what I would like to do. It is intended to use swirling fuel spray burners which will not (as they stand) introduce any compressibility events.

Greatings from a sunny Brighton (it has briefly stopped raining after a couple of months continuous!)

cheers Martin

Hi, Martin

I use my smartphone right now, so let me only utter one word for you to contemplate and write some more longhand as soon as I reach my den. The word is Gatling Gun liquid fu

Dear Martin,

Sorry, I had an incoming call, another fellow Pole angy about the political situation in this country. Consequently, I lost the internet connection.....

There were some attempts in the '60 to combine turbine inter-fin voids with combustion chambers and all failed miserably, exactly due to the thermal problems you have mentioned. However, there is a way out of this predicament, I believe.

Enclosed, you will find a copy of an old patent of mine. This is a water mist extinguisher I have concocted when working for TELESTO. You may buy exactly the same extinguisher in the UK from Saffire or Firelincs. If you were to load this extinguisher with extraction naphta and pressurise it using nitrogen you could extinguish a sizeable pool fire of same naphta, not as efficiently as with a water load, but YES you would. Point is that  even a very combustible mist plume does not burn at any wind conditions, there is something like a flameout. In short, there is a simple way to push heat produced by the flame away from the rotor.

The "rotative" device inside the Telesto extinguisher cylinder is called (okay, I have named it) "the pulsator".  It may be printed on a 3D printer using metal powder, except that the rotor should not be a shutter-deflector combination as in the picture, but a solid wheel I have aready described. We have used a passive rotor driven by gas and liquid flows, at some stage a quick response solenoid valves were used instead, but all this is not as important. Passive rotor was pulsing the mixed-phase flow satisfactorily. Converted pulsator works like a Gatling gun, except that series of explosions occur aft the rotor. I come from an aeronatics family and every relation who has seen this described it as a V1 bis, a trick von Braun's has missed.

My intent was to come with a quick solution to diesel engine caused pollution using a dual-fluid injection, but for my purposes the rotor was too much of a good thing (sluggish)  and another solution had to be found. However, could be that the specifics of the problem you are interested in and the rig geometry has some elements allowing for some improvements and it all may we worth a try.  I believe that shocks and mist have to be somehow intermingled, for a better effect.

Hope it helps.

Cheers,

Jerzy

PS. SO why do they call in Bright-on, if it rains there all the time? JZK

Dear Martin,

Reviewing some old files for my work on current patents I came across some snapshots of prototype pulsers (pulsators) used to create mist-shockwave combination I have mentioned to you the other day. The ones mimmicking the V1 gated jet engine.

I no longer keep the original prints made of metal powder (we had a little explosion, so they had to be scrapped), however the "preprints" in PLA working on air-water streams survived. Here they are. Size of an AA battery.

Cheers,

Jerzy

Dear Jerzy

Let me see if I understand. You are using high pressure nitrogen gas from a bottle to drive a rotor with holes in it? I guess the holes are used to modulate a water flow and you get water spray with gas shockwaves traveling through it. The shockwaves originate from the fast opening of the gas flow pipes due to the rotation of the rotor. I guess the shockwave will smash the water into a finer spray than would be achieved through a simple injection nozzle. Am I about right?

For my case I am looking at the combustion chamber of a turbofan. The pressure is high 40bar or so but the velocities are low (10 to 100 m/s). This is the condition currently used in aero engines. It is called isobaric. Such combustion is approximately steady and as the gases get hot they expand massively. This process does work on all the surrounding gases and the static temperature of the burning gases drops below the value it would see if the combustion chamber was constant volume (isochoric). The end result is that the entropy rises much more in the isobaric case than it would in the isochoric case. If we were to switch to isochoric combustion "pressure rise combustion" in the engine the cycle efficiency would be much better between 5% for a high pressure ratio engine (40+) and upto 20% for a small cheap low pressure ratio gas turbine. MY proposal to get this benefit (in part) is to inject shockwaves into what is otherwise a conventional combustor. I have begun to look at this experimentally in Stuttgart where we used a shock tunnel and studied shock:laminar flame (premixed) interaction. We saw huge increases in pressure steps of 2 bars as shocks and reflections passed through the flame. Now I want to get the same combustion physics using sprayed kerosene and a series of shockwaves (maybe 100's of Hz).

I guess your devise could be used we would inject both fuel and a special additional high pressure air supply (maybe 100 bar) though the rotor and the net effect would be fine fuel spray and also shockwaves. provided we could provide an intense heat source we would get combustion and the shockwaves would grow in intensity when the heat release and the pressure rise are in phase (Reyleigh).

I have a proposal into Brussels EU for a 6 university collaborative project on pressure rise combustion. I have my fingers crossed!

cheers Martin

Dear Martin,

Yes, you "are about right", just one little correction near the end of my epistle. To be precise, I do not advocate replacing the turbine you work on with my contraption, but to use it only as a pulser. In my student years, at the Warsaw Tech there were gun tunnels all around me. They put me in a corner of a lab and told me to "bifurcate". Spent almost two years calculating shockwave profiles and comparing results with the experimental data. I remember well how distorted these profiles were; due to the asymetry in membrane rupture mechanics, reflections and dissiptation .... my programming errors notwithstanding :-)

Surely, the phenomenon you are working on is  completely new to me, moreover, my knowledge of combustion chemistry is limited only to the fire-fighting issues. On the other hand, I see you would like to project a squence of shockwaves into a combustion chamber, hoping the shock profiles are steep enough, do not disspate too fast....etc. etc. In an ideal situation these shocks should be produced by a device mimmicking a common rail system fitted withe a fast-response solenonoid valve.  However, faster such a shutter works, more prone it is to mechanical damage, higher the pressure jump, more distorted the initial profile would be.  So I was thinking, perhaps, an escape is to make it cheap and self-lubricating. The pulsator we have "fried" last year jammed because we used different metals, the carcass was machined out in aluminum, we did not take thermal expansion differences into acount. But, returning to my point, any rotor is  a very good shutter and if every second supply channel supplies a liquid, than this liquid works both, as a plug (a plunger) and a lubricant. Only the upstream side of the contraption is thight, the other side is open (there were two pulsator versions in the picture I sent, an in-line and a free-standing types). At the exit, only three streamlined prongs hold the axle.  

Question is could it produce micro detonation waves in case the liquid is some fuel,  or even water? Surely, not in a rendition I have produced, but I believe it just could it.

Spinning wheel at pressure difference values you have mentioned could easily produce 100 Hz pulse rate, be very reliable, cheap to make and test. On the other hand, Frederik worried about how to produce shock waves fast enough?  What could better satisfy a military man than a fast, liquid-fuel fueled Gatling gun printed 3D

Come to think of it, fuel in jet engines is often supplied by effervescent nozzle systems, should they not shock-pulse the fuel mist just to see if it changes anything?

Kind regards,

Jerzy

Good Evening, Martin

Luckily RG let me append this mammoth file. Hope it reaches you intact. In this video you may see a "squirt" from the KARAS atomiser, in high resolution and slow speed. KARAS is a version of  pulsator, a new type, absolutely different from the one I have designed for the Telesto several years ago. In KARAS we used a spinning disc (of an advanced design). This is the pulsator that is too sluggish for piston engine applications, I have mentioned to you before. KARAS is fine for some extinguisher and spray/decontamination applications, where we hold a patent.

In this video you may notice some strobbing of the mist plume caused by the shutter action of the rotor. This is what I had in mind, except of adding a barrel at the exit of the pulser to get small detonation waves out, rather than having them smuged by flame fronts. There is an option of breeching channels in the disc to get internal explosions. This could be developed rather quickly, should you feel that combining fuel injection and shock/detonation waves is fine and dandy, why not give it  "shot"? Don't believe there is anybody working on this or any legal claims exist forsuch a layout.

Cheers,

Jerzy

Status of this project. 14 months later we have built and tested the shockwave generator device: a simple plenum with solenoid valve. We installed three Druck pressure transducers and logged them at 105 kHz (48 kHz anti-alias). We see nice clear shock waves. At 250 Hz we can have Mach 1.5 with a plenum pressure of 6 bar. We can also generate weak shockwaves (1.07) at 500 Hz. So the next step is to see what happens when we put these shocks into a combustion chamber. A new research project to do this is due to start in May this year.

Martin, chapeau bas. This was an ordeal, I am sure, but success always has a sweet taste. One question, for the application you have in mind it would probably be better to produce arrays of shockwaves in a crisscross pattern, say,as at an exit of a jet  engine. Are you sure no shock wave bounces around in this combustion chamber, or better yet, it could bounce somehow?