Water-Enhanced Turbofan (WET) concept by MTU

[Basil]

Interesting emission- and contrail reduction concept - a long description and a shorter one:

The Water-Enhanced Turbofan as Enabler for Climate-Neutral Aviation

A significant part of the current aviation climate impact is caused by non-carbon-dioxide emissions, mainly nitrogen oxides (NOx) and contrails. It is, therefore, important to have a holistic view on climate metrics. Today’s conventional, but already well-developed, aero-engines are based on the...

www.mdpi.com

A brief guide: How the WET concept works

The water-enhanced turbofan is a gas-turbine-based concept that approaches climate neutrality.

aeroreport.de

 

[Nik]

Is this related to the water injection used for eg Pegasus engine in VTOL mode ??

 

[Basil]

Is this related to the water injection used for eg Pegasus engine in VTOL mode ??

No. Just read the short version.

 

[Scott Kenny]

So, they're trying to trap all the water vapor out of the exhaust, and then spray that recovered water into the combustion chamber to increase the turbine inlet temperatures?

Uh, higher TIT equals higher NOx.

 

[F119Doctor]

The water or steam injection lowers the combustion temperature, so you can burn more fuel (more heat release, more power) at lower TIT.

The one place where they lost me was that you can pressurize the recovered water with a low energy liquid pump for injection into the high pressure combustor. Or you could expand the heated pressurized water in a steam turbine to add to the turbine power. But I don’t think you can do both - once the steam is expanded in steam turbine it is low pressure that can’t make it into the high pressure combustor. I can see one or the other.

One interesting point was that high overall pressure ratios are not needed or desirable for this concept. In a standard Brayton cycle, the higher the OPR and TIT, the higher the thermodynamic efficiency. This water/steam recycling appears similar to the recuperator heat exchange cycle in that high OPR makes the compressor discharge temperature too high compared to the exhaust temp, which makes recycling not work well. Low to moderate OPR is best for overall efficiency in either the steam or recuperator cycles, which does make the compressor smaller, lighter, and probably higher polytropic compression efficiency.

 

[TomcatViP]

I am not convinced that steam will help in Nox reduction. Nox are not temperatur related (@Scott Kenny ) but driven by the time transfer in the hot spot of the combustion procress.
Injecting atomized water (steam) will simply decrease the combustion uniformity and then create more Nox. They are probably witnessing a decrease in emissivity as they are focused on thrust. As said above, steam (water) injection will be favorable for thrust at a lesser fuel gradiant...


Then there is the mass of the system... Storing enough water that the steam impart significant changes in the exhausts and drive the blades of a compressor continuously... Humm.

 

[Scott Kenny]

The water or steam injection lowers the combustion temperature, so you can burn more fuel (more heat release, more power) at lower TIT.

Right, that's the usual use of water injection. Burn more fuel, add more mass flow.

But what's this "recovering exhaust heat" comment, then?

The carbon footprint of WET is unique: it reduces CO2 and NOx emissions primarily by recovering exhaust heat and injecting steam into the combustor. It also curbs the formation of contrails by filtering out any particulate matter from the exhaust gas stream. Recovering the exhaust heat, and thus improving thermal efficiency, reduces energy consumption.

The engine’s efficiency is increased by recovering the exhaust energy that would otherwise be lost—this also lowers fuel consumption and CO2 emissions. First, a vaporizer produces hot steam. The exhaust gas cools down in the process. As it cools further in the condenser, the water it contains begins to condense. The condensation heat is fed to the bypass flow and the liquid water is separated from the exhaust gas in a water separator. Condensation nuclei are also washed out of the exhaust gas in the process, which reduces the formation of contrails. The water is then brought to a high pressure level by means of a pump and delivered to the steam generator. The steam expands in a steam turbine before being channeled into the combustor. The power is fed into the low-pressure shaft. Injecting the hot steam into the combustor not only increases the engine’s efficiency, but also reduces nitrogen oxide (NOx) emissions.

Trying to use water as a recuperator stage? I don't see how that's going to work without some very high pressure pumps...

The one place where they lost me was that you can pressurize the recovered water with a low energy liquid pump for injection into the high pressure combustor. Or you could expand the heated pressurized water in a steam turbine to add to the turbine power. But I don’t think you can do both - once the steam is expanded in steam turbine it is low pressure that can’t make it into the high pressure combustor. I can see one or the other.

One interesting point was that high overall pressure ratios are not needed or desirable for this concept. In a standard Brayton cycle, the higher the OPR and TIT, the higher the thermodynamic efficiency. This water/steam recycling appears similar to the recuperator heat exchange cycle in that high OPR makes the compressor discharge temperature too high compared to the exhaust temp, which makes recycling not work well. Low to moderate OPR is best for overall efficiency in either the steam or recuperator cycles, which does make the compressor smaller, lighter, and probably higher polytropic compression efficiency.

I am not convinced that steam will help in Nox reduction. Nox are not temperatur related (@Scott Kenny ) but driven by the time transfer in the hot spot of the combustion procress.
Injecting atomized water (steam) will simply decrease the combustion uniformity and then create more Nox. They are probably witnessing a decrease in emissivity as they are focused on thrust. As said above, steam (water) injection will be favorable for thrust at a lesser fuel gradiant...

I'm assuming that this is for a fairly high bypass turbofan, so that you don't care about core thrust but instead have something absurd like 90% of the thrust coming off the fan anyways.

Turbo-diesel engines have been pushing for higher peak cylinder pressures and therefore temperatures for years (boost times natural compression ratio) to get better fuel economy, but that came with a side order of increased NOx. Same problem with gas engines. Higher combustion temperatures, higher NOx.

Then there is the mass of the system... Storing enough water that the steam impart significant changes in the exhausts and drive the blades of a compressor continuously... Humm.

Yeah, ignoring the steam generator and turbines, dragging a multi-megawatt steam turbine water source is going to be heavy... I wouldn't be surprised if that required 10klbs of water per engine.

 

[Nicknick]

@F119Doctor : take a look:

Injector - Wikipedia

en.wikipedia.org

@TomcatViP : Nox are very much temperature related, If the combustion temperature stays below about 1700 °C nearly no NOx is formed (in fast combustion processe like in engines and very likely also true for turbines). In a Diesel engine, as well as in a burner of a turbine, the combustio is only partially homogenous, that means even with much lower mean temperatures, around the burning fuel droplets NOx will be formed.

The process is thermodynamically sound, but surly not an entirely new idea. I guess, there are a couple of problems waiting to be solved, like soot production, weight, erosion, flame outs etc.. Many things could be much easier to solve with hydrogen than with carbon fuel.

One question which I find rather intresting, is, can the condenser produce extra thrust by heating the air after the fan, or will the additional drag be higher than the "afterburner" effect of the added heat.

 

[Scott Kenny]

@F119Doctor : take a look:

Injector - Wikipedia

en.wikipedia.org

@TomcatViP : Nox are very much temperature related, If the combustion temperature stays below about 1700 °C nearly no NOx is formed (in fast combustion processe like in engines and very likely also true for turbines). In a Diesel engine, as well as in a burner of a turbine, the combustio is only partially homogenous, that means even with much lower mean temperatures, around the burning fuel droplets NOx will be formed.

The process is thermodynamically sound, but surly not an entirely new idea. I guess, there are a couple of problems waiting to be solved, like soot production, weight, erosion, flame outs etc.. Many things could be much easier to solve with hydrogen than with carbon fuel.

One question which I find rather intresting, is, can the condenser produce extra thrust by heating the air after the fan, or will the additional drag be higher than the "afterburner" effect of the added heat.

Many emissions things would be easier to solve with hydrogen. Basic packaging, storage and handling, and safety are greatly complicated by hydrogen.

 

[Nicknick]

True, but if you see it the other way aound and your job is to design a hydrogen plane (for whatever reasons) you can make use of new options. Maybe you will end up with wet cycle turboprop powered flying wing (for more volume) design.

 

[TomcatViP]

@F119Doctor : take a look:

Injector - Wikipedia

en.wikipedia.org

@TomcatViP : Nox are very much temperature related, If the combustion temperature stays below about 1700 °C nearly no NOx is formed (in fast combustion processe like in engines and very likely also true for turbines). In a Diesel engine, as well as in a burner of a turbine, the combustio is only partially homogenous, that means even with much lower mean temperatures, around the burning fuel droplets NOx will be formed.

The process is thermodynamically sound, but surly not an entirely new idea. I guess, there are a couple of problems waiting to be solved, like soot production, weight, erosion, flame outs etc.. Many things could be much easier to solve with hydrogen than with carbon fuel.

One question which I find rather intresting, is, can the condenser produce extra thrust by heating the air after the fan, or will the additional drag be higher than the "afterburner" effect of the added heat.

Yes, you said it all, fast combustion
In open combustion (aicrfat engines, Rocket), the temperature is dictated by the size of the flame core, relatively to the size of the volume where the combustion happen. Which means the time that the particles can spend in this zone.
The synergy happens when the temperature is raised: the circulation is increased, the trajectory includes reverse flow and o2 is attracted to the flame core. With high temperature, the momentum effect increases the speed of exhaust, raising thrust.

It's a very intuitive process.

 

[Nicknick]

OK,might be the case, that the combustion takes much longer in a turbine than in a Diesel engine, which makes sense, since the pressure and temperatures are higher in a Diesel engine. This would also explain, why even modern gas turbines produce more NOx per kwh than even a vintage Diesel without any aftertreatment.

I was writing about additional thrust from the condenser, not from the exhaust temperature, which btw. will be very low.