Water Injection

[Bruno Ogorelec]

Does anyone know -- is propane soluble in water, like CO2 ?

[tufty]

Does anyone know -- is propane soluble in water, like CO2 ?

Wikipedia Knows

Simon

[tufty]

Related question for those that have experimented:

Water has a very high SHC, which means you're taking a lot of heat out of the engine simply to heat it up to boiling point and then get it to change state. There's not much else out there that is affordable, has a lower SHC, and which changes state at the temperatures we're talking about, so if you want to do it, water is pretty much 'it', but it seems to me that you're losing a fair amount of energy in the state change.

Would using preheated water improve matters? If, instead of injecting water at ambient, you were injecting at boiling point, you'd surely be able to inject much more.

Just a thought, and one that's probably been tried already, but I haven't seen any results posted, soooooo...

[edit]Of course, the energy to heat the water has to come from somewhere. Maybe the above is a stupid question.

Simon

[larry cottrill]

[edit]Of course, the energy to heat the water has to come from somewhere. Maybe the above is a stupid question.

No, not stupid at all ... remember that heat is the one thing we have in bucketloads, and it is usually mostly wasted. You may recall that someone kindly calculated for me one time that the fuel heat value (using white gasoline) of my Dynajet's fuel consumption is a mere 87,000 watts! That would boil AND pre-heat a LOT of water, if you could capture it effectively.

L Cottrill

[Al Belli]

Hi Bruno,

Propane is slightly soluble in water, 65 cc/L.

Acetylene is a better choice at 1000 cc/L.

Source: CRC Handbook of Chemistry and Physics.

Al Belli

[larry cottrill]

[edit]Of course, the energy to heat the water has to come from somewhere. Maybe the above is a stupid question.

No, not stupid at all ... remember that heat is the one thing we have in bucketloads, and it is usually mostly wasted. You may recall that someone kindly calculated for me one time that the fuel heat value (using white gasoline) of my Dynajet's fuel consumption is a mere 87,000 watts! That would boil AND pre-heat a LOT of water, if you could capture it effectively.

... which leads me directly back to something I proposed a LONG time ago: Why not have a cooling jacket AROUND the pulsejet body where water is sprayed in and evaporated by the heat of the engine shell, like a primitive 'flash boiler', with the vapor blending with the tailpipe exhaust mass at (or before) the tail end? This could accomplish any or all of the following:

1. Extremely effective cooling of the engine mass as a whole
2. Exhaust mass enhancement at the tail end - useful if a following augmentor or 'SNECMA turbine pulsejet' rear section is used
3. Increased thrust force due to rearward acceleration of the vapor (if the shell is optimally designed - Bruno has pointed out that even heated air from such a shell could contribute slightly to thrust; surely water vapor would be much better!)
4. Possible noise reduction, since the emerging mass will be slower and heavier than the exhaust gas alone

L Cottrill

[kid jansen]

Propane produces 93.8MJ/m³, and 1m³ propane weighs a little less than 2kg. Eric said in one of my other topics, that a standard lockwood consumes one bottle (10.5kg propane) in approx. 4 minutes.
10.5/240=0.04375m³/second.

93.8MJ x 0.04375=4.1MW

And I can tell you that almost nothing of that gigantic amount of energy is used for propulsion, it all disappears as wasted heat.

Edit: I see that Larry posted only a few minutes before me, so I did not see his post until mine was posted. But I think it's a pretty good idea, that water can consume a lot of that wasted heat, and thus add a significant amount of thrust. The only thing is that you can only do it with high quality stainless pulsejets. I think mine (1mm mild steel) will be rusted away within minutes.

[tufty]

[edit]Of course, the energy to heat the water has to come from somewhere. Maybe the above is a stupid question.

No, not stupid at all ... remember that heat is the one thing we have in bucketloads, and it is usually mostly wasted. You may recall that someone kindly calculated for me one time that the fuel heat value (using white gasoline) of my Dynajet's fuel consumption is a mere 87,000 watts! That would boil AND pre-heat a LOT of water, if you could capture it effectively.

... which leads me directly back to something I proposed a LONG time ago: Why not have a cooling jacket AROUND the pulsejet body where water is sprayed in and evaporated by the heat of the engine shell, like a primitive 'flash boiler', with the vapor blending with the tailpipe exhaust mass at (or before) the tail end? This could accomplish any or all of the following:

1. Extremely effective cooling of the engine mass as a whole
2. Exhaust mass enhancement at the tail end - useful if a following augmentor or 'SNECMA turbine pulsejet' rear section is used
3. Increased thrust force due to rearward acceleration of the vapor (if the shell is optimally designed - Bruno has pointed out that even heated air from such a shell could contribute slightly to thrust; surely water vapor would be much better!)
4. Possible noise reduction, since the emerging mass will be slower and heavier than the exhaust gas alone

L Cottrill

Nice idea.

It wouldn't look as cool, though; you wouldn't have reddy-orangey glowing bits to remind people that you have made a very dangerous loud thing :)

I was thinking more along the lines of using an external heat exchanger to generate steam from the water reservoir and using that to preheat the water being injected, which would still be being taken from the reservoir in liquid form. Scavenging as much as possible from the lost heat and putting it back into the engine.

This wouldn't preclude steam-jacketing the engine either, I guess.

Simon

[Mike Everman]

By "effective sfc" I mean: water and fuel mass expended per pound of thrust per hour. Yes, water in this case is fuel. Cost specific fuel consumption is another matter of course.

It's all about how much expendable mass do you have to pack in to your airframe to go how far. Obviously you wouldn't inject water continuously if your 'specific mass expendature" is 5.

It's something I've been trying to calculate with my approach, and now I'm at the cut and try stage. I can say that my approach uses water for boost only, and conventional thrust for cruise.

[kid jansen]

You could also make a second vaporizing coil on your engine, but not for evaporating the fuel, but for the water.

[tufty]

It also occurs to me that there might be other issues with "steam-jacketing" a motor, viz:

Assuming 1 atmosphere of pressure, and that the steam jacket would be superheating the steam (not an unreasonable assumption, I think), the density of steam would run from .55 kg/m3 at 130c to .42 kg/m3 at 250c (figures from here)

The density of air at 1 atmosphere is 1.1 kg/m3 or thereabouts.

The upshot of this is that you would probably want to avoid the steam being sucked into the tailpipe, as you'd be reducing reaction mass, I think.

Am I way off the mark here?

Simon

[larry cottrill]

The upshot of this is that you would probably want to avoid the steam being sucked into the tailpipe, as you'd be reducing reaction mass, I think.

Am I way off the mark here?

Simon -

No, that occurred to me as a possible problem, too - I just had no idea how to quantify it.

We might need to work on a design that would allow cool outside air to bypass the steam jet in order to feed the tailpipe.

Yes, it's getting complicated, which is always bad when you're working with pulsejets.

L Cottrill

[Stuart]

I have been doing a lot of water injection over the past year or so, I have mentioned a few times the advantages of injecting water but only a few people even showed remote interest in doing it on their own, and other than my own comments I cant really remember much else on water injection.

Eric

Well, water injection makes a lot of sense on these systems because fuel consumption is so horrendous. One long term question would be corrosion, though this might be counteracted by decreased spalling.

Hum, I'm interested.

[larry cottrill]

Actually, the 'SNECMA Turbine Pulsejet' design gives us a way to aspirate the air without involving the water vapor. This is because it takes in its air at the (approx.) 2/3 point, rather than at the tail end. To build this engine, you need to support the tail end "tuned augmentor" part somehow, anyway - so, support it with two or three or four steam ducts. These would open into the rear skirt just beyond the throat - right where you want the added mass and fluid expansion to start working for you.

Some experimentation required ...

L Cottrill

[larry cottrill]

One thing worth mentioning re the jacketed 'flash boiler' idea is that if the water supply ever fails you, you're going to have a rapidly overheated engine, because its effective radiation will be highly diminished (the outer jacket will reflect back most of the radiated heat) and there won't be any cooling airflow around the chamber shell. Just thought I'd mention this as a potential difficulty.

L Cottrill