I’ve been researching (well, a lot of googling) about steam and heat engines lately and a potential engine design occurred to me that I couldn’t find an example of - that might well mean it’s just not a practical idea.
Anyway - in short, it’s a boiler-less steam-powered jet engine.
A conventional jet engine is powered by the expansion of burning fuel, but can that be replaced by the expansion of water flashing to steam?
I’m not talking about a turbine powered by high-pressure steam generated externally to the engine, in a boiler - instead, how about if the water is heated to near-boiling, then injected into a chamber where, upon meeting a heated element, it flashes to steam, expanding and driving the turbine.
I imagine this might not be so effective as an internally-fuel-burning jet engine, but is it workable at all?
This is what a hydrogen peroxide rocket is. The peroxide can be reacted to produce steam on demand which makes the most sense weight wise for a flight. But rocket cars have been made which use a heavy pressure tank to pre-react the peroxide and which is then released on demand. The problem with using fuel to produce the heat to produce steam is the fuel can be more efficiently combusted to produce thrust than to heat the water. Using steam for thrust requires taking it to very high temperature and pressure, and the few degrees of head start from water near the boiling point are insignificant.
There is a question about the semantics of jet versus rocket here. But that is mostly nit picking.
When we think of a jet engine we usually think of an engine that burns fuel and air - and generates thrust. Low speed jet engines need a turbine to keep the system working, but higher speed engines can work without a compressor - simply relying on the forward motion of the engine trough the air. A jet than blasts water out the back is simply one that is fuelled with hydrogen. There are of course scramjet designs that are so fuelled.
As Tripolar points out above - the issue is not so much the reaction mass (the steam going out the back) but the mechanism by which you power (in this case heat) the system. Burning some other fuel to heat the water is just adding complexity, and you might as well burn hydrogen right up front to make the already high temperature steam.
However, if we don’t burn stuff, what else might we do? Well of course we could heat the water with a nuclear reactor. That could make a lot of very hot steam. It isn’t a jet engine either, but is a rocket motor, and is quite practical. A rocket motor is most efficient the faster it ejects mass, and to be most efficient you want lightweight molecules. So nuclear rocket motors have been designed with hydrogen as the reaction mass. But if you are prepared to take a hit in efficiency, you can use water. And given the huge logistic advantages of water over liquid hydrogen, this might actually be not such a bad idea.
Not clear on whether you’re talking about a jet engine, or a gas turbine. You started off with the word “jet,” which implies that you’re talking about a device whose primary purpose is to produce thrust by expelling the exhaust gases at high velocity. Later on, you mentioned the steam driving a turbine, implying that you instead wish to extract mechanical power from the expanding steam and then use that mechanical power to drive some other device - maybe an alternator, maybe a ship’s propeller, maybe a cookie dough mixer.
The purpose of a conventional boiler is to add heat to the water; the water is then piped to and expanded through a turbine, where mechanical work is extracted. What you describe sounds sort of like a two-stage boiler:
Stage 1 - water is heated to near-boiling.
Stage 2 - near-boiling water hits a second heating element, where it flashes to steam.
I guess I’m not really seeing a significant technical difference between what you describe and a conventional boiler/turbine configuration.
Steam entering a turbine needs to be superheated to avoid condensing in the first stage of expansion. The system you describe sounds like it will deliver wet steam to the first row of blades. Which isn’t good from either a materials or energy conversion point of view.
A slight variation would be a system where high-pressure water at a temperature of 200[deg]C is flashed into steam by passing through a ring of nozzles, positioned so as to direct the high-velocity jets of steam onto the first stage of an impulse turbine.
I would say that you are still talking about a steam turbine here, not a jet engine. A jet engine or gas turbine is not primarily driven by the expansion of burning fuel, but by the large volume flow of air passing through the turbine. Just as in a car engine, the air is the working fluid; the fuel is the source of chemical energy to expand the air. You might not feel this is a nit-picky distinction, but it is effectively the difference between a steam turbine and a gas turbine, and is the reason a gas turbine has a compressor and a steam turbine does not.
In some sense this done already with jet engines that inject water into the combustion chamber. This can improve thrust characteristics, and is necessary for cooling in some conditions. However the effect of corrosion makes the use of this limited. IIRC the Harrier jet required up to 1000 lbs. of water for cooling when performing a vertical landing.
AFAIK, water is injected into compressor. Compression heats up the air, and hot gas takes more work to compress. Also, most jet engines are limited by the maximum temperature in the combustion chamber. Injecting water into compressor reduces the temperature of air as it enters the combustion chamber, so more fuel can be added.
I’ve heard of compression cooling, including research into the use of fuel for this purpose. Some of portion of this would end up in the combustion chamber. Are you saying that water is never injected directly into the combustion chamber?
I guess it depends what you mean by ‘driven’. The expansion of burning fuel is what makes jet engines go. The large volume of air passing through is what makes them go forward.
My question about a steam-powered jet turbine engine boils down to one thing, I think: can it develop more thrust than a simple steam reaction jet? I’m not sure it can, because the force of the expanding steam is not assisted by the intake compression (whereas the chemical reaction of burning fuel is).
In the latter case, the intake compression derives its energy from the burning fuel; the compressor section is powered by the turbine, which extracts energy from the expanding exhaust gases.
You would need to compare devices of equal input power.
Instead of fuel jets, water jets are used and water replaces kerosene as fuel for the jet engine.
Using the turbofan design, it’d be possible through flash pointing the water into steam to get the drive to one or two back turbines and produce plenty of thrust.
My friend that got me stuck on this water-as-fuel-jet-engine design had it in mind to use microwaves to flash the water into steam in the Jet Engine’s combustion chamber. I thought that was a great idea.
Also - I’ve seen a lot of people mentioning how in order to heat the water, you’d need to burn something else. POPPYCOCK.
So long as we have quality batteries, solar energy and human effort there’s no reason to use fuel.
Things that are necessary -
enough battery power to heat water to 210 degrees F in an insulated fuel tank within a reasonable amount of time. Remember kids, this is flying - nothing near as simple as “hopping in the car and going”. Because of that fact, a short warm up for your water tank shoudn’t be a problem.
a high enough watt microwave system in the heads of the nozzles of the “fuel” jets so that the hot water flashes into steam upon entry into the combustion chamber to mix with compressed air
Ideally, a recycling system at the exhaust port so that you could reuse most of the water that condenses
More research into what atmospheric pressure and temperature will do to the effectiveness of steam as an energy source - will the steam lose power when subjected to freezing temperatures at 30k feet?
In RE: to the above - could an agent be added to the water to prevent it from freezing without upsetting the flash point temperature?
I’m intrigued by this idea because I’ve recently been investigating the work that has been done on Human Powered Aircraft (basically bicycle pedal powered ultra lights that require inordinate amounts of effort to keep afloat.
With the integration of a pair of jet engines, the wingspan of the Human Powered Aircraft could be shortened - it’s currently ridiculous. I know it takes away the “human powered” element, but tell me that jet powered bicycle aircrafts fueled by water would not be a revolutionary transportation solution…
Anyway - more power to you and I hope that we start seeing designs for these simple, efficient engines.
Well yeah, if by quality you also mean *extremely *light and *extremely *high capacity, neither of which we have today.
Another big problem with H[sub]2[/sub]O[sub]2[/sub] is that it is highly reactive, very dangerous stuff. One malfunctioning torpedo powered with it is what destroyed the huge Russian submarine Kursk (well, the reacting H[sub]2[/sub]O[sub]2[/sub] caused the torpedo’s *warhead *to detonate, which sank it.)
Interesting point: water boils at 212F at STP, which is sea level: a jet flying at 30K’ would not require as much heat to create steam. OTOH, getting the many many pounds of water needed up to 30K’ would require quite a lot of energy.
There are a number of misunderstandings and misconceptions here.
Water can’t replace kerosene are a fuel. It doesn’t burn and thus can’t provide the energy source.
This isn’t using water as a fuel, this is simply using water as a way of moving energy about. What you describe is a steam turbine powering a ducted fan.
You need an energy source. The trick with kerosene is its energy density. This comes about because it is nothing more than carbon and hydrogen. The big win is that jet planes do not need to carry an oxidiser, they get that out of the air they travel through. Batteries are in a way burning their contents (both burning kerosene and batteries operating are redox reactions) but batteries have to carry both parts of the chemical reaction, which puts them at a distinct disadvantage relative to burning kerosene.
Getting the water to boiling point is only the beginning, and takes the least amount of the energy you need.
So what powers the microwave? There is nothing special about a microwave power source to stick energy into water, it is just a very complex way of doing it. In order to evaporate water you need a lot of energy. Water sitting at 212 degrees does not boil with the addition of a small amount of energy, in fact it takes 2260 kJ per kilogram to boil water that is already at 212 F, where as it only takes 418 kJ per kilogram to take water from just above freezing to the boiling point. This is the difference between latent heat of vaporisation and the specific heat.
So clearly this is only using water an energy transport mechanism. If you don’t let it pass out of the back of the turbine it can’t provide thrust, and you are only getting thrust from the fan.
Again, water isn’t the energy source, you are only using it to transfer energy from the batteries to the fan. The physics behind its efficiency at doing this is well understood, and was described by Nicolas Carnot in 1823. The key is that these are all heat engines, and the efficiency is determined by the difference between the hot part of the cycle and the cold part. In this case the hottest the steam ever gets, and the coldest. The bigger the difference the better the efficiency. Jet engines are governed by this absolute rule just as steam engines and internal combustion engines are. Modern jet engines get better efficiency by running combustion chamber temperatures that actually exceed the melting point of the turbine blades, and rely upon forced gas cooling through the blades to stop them melting. Steam turbines (such as you see in power generation stations rely upon massive cooling towers to keep the exhaust system cold, and do so with such effectiveness that the steam that exits the final low pressure stage of the turbine sees close to a vacuum in the exhaust system.
The problem is that I don’t see what you are trying to gain here. The water isn’t a fuel, it adds no energy of its own. It simply moves energy from one place to another. You can’t beat conservation of energy. A highly complex system of water heating, microwave generators, and water recovery is not more efficient than a simple chain.
Again, there is no efficiency gain. The water cycle does not make anything more efficiency, and in any realisable system will make it vastly worse. The most efficient water/steam power system we have are about 50% efficient. And they require very high temperatures, and very extensive cooling systems to manage this. A titanium chain can transmit all the power a human can generate with almost no loss at all and weighs a few hundred grams.
I had this very idea as a component of an ultra-efficient diesel engine: it all revolves around recycling heat energy otherwise lost out the tailpipe.
A heat exchanger pre-heats the diesel fuel, lowering the viscosity index and making the fuel spray easier to atomize, reducing soot production.
A second injector injects preheated clean-burning alcohol, which can combust cleanly in the remaining oxygen in the cylinder.
Finally, near-boiling water is injected through it’s own injector into the white-hot cylinder’s atmosphere, turning low-volume liquid into high-pressure gas, volume increasing by a factor of 1600.
water vapor can be distilled from the exhaust stream to be recycled indefinately.
Plenty of thrust can come only from a system that consumes plenty of energy. What you’ve described (battery-heated water, microwaves to flash it to steam) looks as if it would provide a very modest amount of energy, and thus little thrust.