Gas Turbine Afterburner Water Injection

[voltech444]

Hey I've been thinking of this idea for a while,

If I had a small boat with a home made gas turbine engine on it to power it solely with thrust. If I put an afterburner on it and nozzle; and had a small intake for water that filtered out all of the crud. Would it be possible to generate more thrust by injecting the water into the afterburner? Not like other types of water injection that inject water before the compressor or in the combustion chamber, but after the power turbine wheel in an afterburner like device. Not adding any extra fuel in the afterburner just spraying a mist of water.

My thinking is that the super hot gases coming out of the power turbine would super heat this water mist into steam. This would create a lot of extra pressure and mass and add another gas coming out to produce more thrust. The water droplets would flash vaporize into steam and cause a lot more pressure and mass.

I know a similar technique is used in other aircraft for short periods of time to get extra power but they are injecting the water before the compressor and in the combustion chamber. In the setup I am talking about it wouldn't change the operation of the turbine it would just take advantage of the hot gases coming out as thrust.

I understand why this technique is not used on commercial aircraft because of the heavy weight of the water required, but on a boat where there's unlimited water something like this could be viable

What do you think?
Thank you,
Jordan

 

[jim meyer]

By raising the pressure in the exhaust stream are you not increasing the back pressure on the main turbine? Very bad idea because that saps a lot of power from the main drive turbine.

 

[voltech444]

Wouldn't the same be true if u injected fuel into an afterburner? Doesn't that create back pressure too?

 

[AtomRamjet]

Wouldn't the same be true if u injected fuel into an afterburner? Doesn't that create back pressure too?

Yes it would if the nozzle wasn't variable in geometry. The purpose of varying the nozzle geometry is to increase the throat area so that the nozzle can accept the amount of airflow that you're feeding it after correcting for the massive increase in stagnation temperature. You may need a variable geometry nozzle as well that can adjust the throat area to account for the variation in mass flow rate and stagnation temperature and pressure. What you would need to be concerned about is the fact that you aren't adding energy and enthalpy to the flow but rather diluting the specific internal energy/enthalpy as well as lowering stagnation temperature. This runs contrary to what a conventional afterburner does, which is heat up the air to increase exhaust velocity and raise thrust.

What your thrust augmentation system would do is greatly increase the mass flow rate, lower the stagnation temperature and most likely lower the stagnation pressure as well. My reasoning behind the stagnation pressure lowering is the fact that heat transfer dissipates stagnation pressure according to simple Rayleigh Flow heuristics. The reason why certain aircraft inject water into their engines is to raise mass flow rate and cool the turbine to allow for greater pressure ratios and fuel-to-air ratios for greater Brayton cycle and thrust performance. All that said, I'm sure you can derive a fair amount of increase in thrust from injecting water downstream of the turbine but I'm not sure exhaust velocity would improve that much as a result of the losses in stagnation properties. This means you may need to design a fairly large intake to draw in a lot of water to maximize the flow rate while taking care to avoid the issues of excessive specific enthalpy dilution and stagnation temperature reduction that result from the heat transfer from hot gas to water.

This all may end up proving to be more trouble than it's worth.

 

[voltech444]

I'm wondering if you could utilize this extra volume to turn a larger free turbine which could turn an alternator. My thinking is that the water injection could utilize the waste heat in the exhaust stream and maybe make a more efficient turboshaft. I know it's different than what I originally posted, I was just trying to get an idea of what would happen. It's kinda like combining a combined cycle into one cycle; maybe it wouldn't be as efficient as a true combined cycle but it would be a lot simpler. The water would be lost; a radiator could be installed to capture some of the water.