Afterburner Water Injection

[voltech444]

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 im 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

[racket]

Hi Jordon

You MUST NOT create more pressure in the afterburner , that will send the gas producers temperatures too high and engine could surge , you would need to size the afterburner nozzle a little bigger to accommodate the steam volume produce , the pressure must stay the same .

Because there can't be any more fuel burn in the afterburner as theres no more oxygen available , any water addition will lower the gas temperatures and lower their energy potential for producing velocity in the nozzle , you would need to find the "sweet spot" of just enough water but not too much so as to produce maximum thrust .

Cheers
John

[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.

[racket]

Hi Jordon

You could use a larger freepower wheel , but your fuel burn rate will be horrendous , the problem with using water is the huge amount of energy required to turn it from water into steam at the same temperature ( latent heat of evaporation) , that phase change uses up a lot of temperature which then means less temperature drop available for making power .

If you burnt a water methanol mix of the correct ratio off water to methanol the heating value from the methanol balances out the water phase change energy requirement , you simply end up with more mass flow at the same temperature .

Cheers
John