Sintering, porosity & combustion synthesis

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cudabean
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Sintering, porosity & combustion synthesis

Post by cudabean » Wed Nov 19, 2003 8:57 pm

This discussion about sintering and porosity of various materials reminds me of some other material and net-shape manufacturing techniques that I've studied.

My friend worked at a scientific glass-blowing shop and used to be involved in making sintered glass filters. Temperature control allowed them to create filters of various grades or porosities. So sintering might be a solution, but that requires a kiln and the ability to carefully control temperatures.

Another interesting technique of materials manufacture is combustion synthesis. This process is used in making cermets (ceramic metalic materials). I might have described this in this forum before, but here it is again: take some Aluminum Powder, Titanium Dioxide and Boron Oxide and thoroughly mix them together in a Stoichiometric ratio and set them ablaze with a super-hot igniter (a sparkler left over from celebrations, 4th of July, etc.). The powders reassemble themselves into Titanium Diboride--TiB(2) and Alumina--Al(2)O(3). If you compress the initial reactants into a cookie and ignite them, you will end up with a solid cookie where the porosity varies depending on the initial pressure applied.

Titanium Diboride has an incredibly high melting point and is used in modern tank armor and for electrodes in Aluminum arc furnaces. You might be able to make one of these vaporizers suitable for pulsejet use without having to buy or bring to bear a lot of equipment:
http://www.vapore.com/tech_howto.htm

On the above howto page it shows two wafers having different porosities joined together. You could make this in a two-step process by igniting a wafer-shaped pocket of reactant powders at high pressure. Then taking the net shape and packing a second wafer-shaped pocket of reactant powder right below and compressing to medium pressure. Finally afix a slightly raised orfice section to the top. Hint: you can use non-reactive sand as a medium to hold the reactive powders in place while you compress it with a vice or something.

cudabean

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Post by Mike Everman » Thu Nov 20, 2003 6:55 pm

Trying to coalesce my thinking in this area:
Isn't the holy grail here that we want to have liquid fuel delivery that is vaporized instantly where we want it to be?
Is my understanding correct that the problem with vaporizing liquid fuel by means of a coiled tube around the exhaust limits amount of fuel we can deliver through this line because it is now gaseous, and also hurts our ability to throttle because the reservoir of pressurized gas in the tube is too great?
Spraying or atomizing liquid fuel has the disadvantage that there is very little time for the droplets to vaporize in the fresh air charge, and so most of it happens after combustion has begun?

So, that being said, (and please correct my ignorance on the above), can it be as simple as an insulated fuel feed line to a porous metal "sponge" (Bruno coined that one) in the combustion chamber, that by it's location is red-hot, and by it's nature is extremely high surface area and heat transfer, vaporizing the fuel right where we want it.
Seems as if this is happening to a lesser degree in the burner cans on conventional turbo-jets, and perhaps in their case, they want the slower burn of slower droplet vaporization, but I think PJ guys want as quick a deflagration as possible...
Pardon the thinking out loud, but I'm still being educated by you guys... >;-)
This is fascinating
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Post by Bruno Ogorelec » Thu Nov 20, 2003 9:07 pm

Mike Everman wrote:I'm still being educated by you guys...
Mike, if you can be educated by us, boy, you're in trouble...

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Post by Mike Everman » Sat Nov 22, 2003 11:12 am

bruno wrote:Mike, if you can be educated by us, boy, you're in trouble...
Cute, but am I on the right track with these fueling opinions?
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Post by Bruno Ogorelec » Sat Nov 22, 2003 1:56 pm

Mike Everman wrote:Is my understanding correct that the problem with vaporizing liquid fuel by means of a coiled tube around the exhaust limits amount of fuel we can deliver through this line because it is now gaseous, and also hurts our ability to throttle because the reservoir of pressurized gas in the tube is too great?
Mike, what do you need a quick throttle response for, if you're flying with jet power? Turbojets have about a two-day throttle lag and still work well.
Mike Everman wrote:can it be as simple as an insulated fuel feed line to a porous metal "sponge" (Bruno coined that one) in the combustion chamber, that by it's location is red-hot, and by it's nature is extremely high surface area and heat transfer, vaporizing the fuel right where we want it.
I don't think this is where we want it. Mixing with air will be less than perfect. I am still holding out for the intake -- it's hot enough for vaporization but gives the vapor some travel time until the chamber to socialize with air and get to know it better before they are thrown together in a fiery union.

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Post by Mike Everman » Sat Nov 22, 2003 6:42 pm

If you have continous flow at the intakes, doesn't fuel get blasted out the intake every cycle, or does it suck it all back in?
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Post by Bruno Ogorelec » Sat Nov 22, 2003 6:57 pm

A good question. Depends on the pressures. I admit that inside the chamber, internal pressure would stop ejection of fuel into chamber while combustion was going on. In teh intake, the passing stream may actuall draw additional fuel out... Hmmm... back to the drawing board.

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Post by Mike Everman » Sat Nov 22, 2003 7:16 pm

Makes a good case for many fuel orifices at reduced pressure.
Hey, Bruno, check out two things for me, OK? The jamjar turbine in valveless thread and the fluidic oscillator circuit in tools and construction. I'm firing on all cylinders right now, with no resposibilities but to rest my back. I want to come out of this vacation with something to build!
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Post by Viv » Wed Nov 26, 2003 4:24 pm

brunoogorelec wrote:A good question. Depends on the pressures. I admit that inside the chamber, internal pressure would stop ejection of fuel into chamber while combustion was going on. In teh intake, the passing stream may actuall draw additional fuel out... Hmmm... back to the drawing board.
If you remember Bruno we had a big problem with this on the early BCVP tests.

Changing to high pressure propane hose (orange industriel stuff) from soft neoprien (calor) hose had a huge effect on the engine fueling.

the soft hose was basically springing with the pulses in the combustion chamber, when we changed to the high pressure hose the pulses were pushing the gas back further up the pipe so he engine seemed to run leaner.

Check valves are a good idea to help isolate the fuel system.

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Post by Bruno Ogorelec » Wed Nov 26, 2003 4:41 pm

Viv wrote:If you remember Bruno we had a big problem with this on the early BCVP tests. Changing to high pressure propane hose (orange industriel stuff) from soft neoprien (calor) hose had a huge effect on the engine fueling.
Frankly, I remember the flying gloves much better :o) So much more spectacular than bulging hoses.

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Post by Mike Everman » Wed Nov 26, 2003 6:14 pm

viv wrote:the soft hose was basically springing with the pulses in the combustion chamber, when we changed to the high pressure hose the pulses were pushing the gas back further up the pipe so he engine seemed to run leaner.

Check valves are a good idea to help isolate the fuel system.
Stands to reason. Can we hang our hat on the assumption that fuel inflow during combustionis wasted fuel? Maybe not, but if the supply pressure is less than the CC pressure spike, the stiffness of your supply line or stiffness of the check valve spring and the distance from the CC to the check valve all conspire to set how much fuel will come out before being shut off by the combustion pressure spike. A simple experiment would be to put a check valve in your supply line and vary the length of a soft supply line to the engine, which would vary this delayed shut-off timing nicely. Seems a critical thing to do to any engine for fuel efficiency; as important as the delivery itself. Whether you’re a timed injection believer or not, it seems you can easily have pulsed fuel delivery without a complicated system. (and if you use Mark’s “pressure capacitorâ€
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Post by Viv » Wed Nov 26, 2003 6:27 pm

Mike Everman wrote:
viv wrote:the soft hose was basically springing with the pulses in the combustion chamber, when we changed to the high pressure hose the pulses were pushing the gas back further up the pipe so he engine seemed to run leaner.

Check valves are a good idea to help isolate the fuel system.
Stands to reason. Can we hang our hat on the assumption that fuel inflow during combustionis wasted fuel? Maybe not, but if the supply pressure is less than the CC pressure spike, the stiffness of your supply line or stiffness of the check valve spring and the distance from the CC to the check valve all conspire to set how much fuel will come out before being shut off by the combustion pressure spike. A simple experiment would be to put a check valve in your supply line and vary the length of a soft supply line to the engine, which would vary this delayed shut-off timing nicely. Seems a critical thing to do to any engine for fuel efficiency; as important as the delivery itself. Whether you’re a timed injection believer or not, it seems you can easily have pulsed fuel delivery without a complicated system. (and if you use Mark’s “pressure capacitorâ€
"Sometimes the lies you tell are less frightening than the loneliness you might feel if you stopped telling them" Brock Clarke

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Post by hinote » Wed Nov 26, 2003 6:37 pm

[quote="Mike Everman"]

Stands to reason. Can we hang our hat on the assumption that fuel inflow during combustionis wasted fuel?
Maybe not, but if the supply pressure is less than the CC pressure spike, the stiffness of your supply line or stiffness of the check valve spring and the distance from the CC to the check valve all conspire to set how much fuel will come out before being shut off by the combustion pressure spike. A simple experiment would be to put a check valve in your supply line and vary the length of a soft supply line to the engine, which would vary this delayed shut-off timing nicely. Seems a critical thing to do to any engine for fuel efficiency; as important as the delivery itself. Whether you’re a timed injection believer or not, it seems you can easily have pulsed fuel delivery without a complicated system. (and if you use Mark’s “pressure capacitorâ€

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Post by Bruno Ogorelec » Wed Nov 26, 2003 8:39 pm

I wish all old hands in the forum to take note of the calm way I have taken Bill Hinote's notion that timed injection will be necessary in a pulsejet to achieve good running, not to mention notable SFC improvements. I am NOT throwing a tantrum. I am not foaming at the mouth. I am not even shaking with repressed anger. If this forum conveyed sounds, you'd hear me whistle a ditty. Amazing, how people can change, even in my age.

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Sponges, Timers, and Un-Necessary Fluidic Emmisions

Post by Mike Kirney » Wed Nov 26, 2003 9:03 pm

I like this sintered hi-temp fuel sponge idea a lot. As for the issue of timed injection, I think a lot could be accomplished with a Seebeck-effect probe, a transistor, and a solenoid-actuated valve. The temp peak of combustion would send a voltage signal to the transistor which would allow current to open the valve to admit a little bit of fuel to be soaked up and evapourated by the sponge. This timed ignition debate has been going on since the forum's inception in the late 20th century. Bruno has maintained that it is immoral while almost every other voice seems to say its just what the pulsejet needs. If you search the archives, you will find discussions of all sorts of schemes for using the combustion pressure pulses to drive the fuel system and eliminate the wastage inherent in continuous fuel-feed systems. By the way Bruno, I did notice that you maintained decorum throughout this entire thread.

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