Water ingestion (jets)

[freewheeler]

Hello, and I'm wondering if anyone out there can help me understand how a jet engine can cope when flown into severe weather/precipitatation? I understand that a flame-out is possible in extremis, but what actually happens, and what are the design limits for certification; ie L/min or similar? water injections aids combustion in piston engines, what happens in the jet? does a small amount of vater vapour actually improve performance?

[barit1]

Water ingestion is a cooling effect in the compressor. Without water, as each stage of compression the air heats up perhaps 20-40 C and can exit the compressor as hot as 600-700 C.

If water enters with the air and vaporizes enroute through the compressor, it absorbs some of that heat. Also, since it was in liquid form in the inlet, the mass flow is increased, so more potential power is available. All good so far.

The problem arises with the stage-to-stage matching in the compressor. The specific heat of the gas flow is changing during the vaporization, and instead of each stage sharing the load equally, some stages may be near stalling. Even this may not be a problem, because a temperature sensor between the LP and HP stages can help to compensate.

The worst case is when flying in and out of pockets of very heavy rain. The rate at which the liquid content changes can be very quick, so the interstage temperature changes very quick. If the temp sensor and the engine control system are too slow to follow the temperature changes, you can encounter a compressor stall (surge).

[lomapaseo]

Water ingestion is a cooling effect in the compressor. Without water, as each stage of compression the air heats up perhaps 20-40 C and can exit the compressor as hot as 600-700 C.

If water enters with the air and vaporizes enroute through the compressor, it absorbs some of that heat. Also, since it was in liquid form in the inlet, the mass flow is increased, so more potential power is available. All good so far.

The problem arises with the stage-to-stage matching in the compressor. The specific heat of the gas flow is changing during the vaporization, and instead of each stage sharing the load equally, some stages may be near stalling. Even this may not be a problem, because a temperature sensor between the LP and HP stages can help to compensate.

The worst case is when flying in and out of pockets of very heavy rain. The rate at which the liquid content changes can be very quick, so the interstage temperature changes very quick. If the temp sensor and the engine control system are too slow to follow the temperature changes, you can encounter a compressor stall (surge).

agree but just to add that the even worse case is where the precipitation also contains freezing water such as hail. In this case a double state change in the compressor seriously degrades the work split in the engine cycle. SO242, Tacca, etc.

[Centaurus]

In the early days of the CM56 engines fitted to 737 Classics, double flame-outs occurred when the aircraft was descending at high airspeed at idle thrust and in very heavy rain. This was partially due to the airflow being compressed in the front part of the engine cowl and spilling over the edge while at the same time, rain being heavier formed the majority of air and water mixture entering the compressor. The fix was to increase the idle speed at closed throttle which in turn aided the airflow through the engine. Sorry if this is a pretty rough explanation.

[lomapaseo]

In the early days of the CM56 engines fitted to 737 Classics, double flame-outs occurred when the aircraft was descending at high airspeed at idle thrust and in very heavy rain. This was partially due to the airflow being compressed in the front part of the engine cowl and spilling over the edge while at the same time, rain being heavier formed the majority of air and water mixture entering the compressor. The fix was to increase the idle speed at closed throttle which in turn aided the airflow through the engine. Sorry if this is a pretty rough explanation.

I believe that the pointy spinner helped also in centifuging the heavier than air stuff outside the core-air splitter

[John Farley]

freewheeler

The Trent 500 series was certificated at over 1200l/min at low and high RPM If you go to the R-R website you can see a video of Trent 900 certification cases from bird strikes to blade off as well as water etc

JF

[freewheeler]

Cheers, gents, interesting and illuminating!:ok:

[Re-entry]

Water ingestion up to the saturation point reduces the compressor inlet temp. to near 'wet bulb' temp. The droplets then evaporate within the blade path, the adiabatic process causing the temp. to drop further. Since it takes less energy to compress cooler air, compressor work is saved. This translates to less load on the turbine, so the N1 increases.

This 'wet compression' process is used in some power stations to increase the efficiency of the combustion turbines.

Water ingestion beyond the saturation point (very heavy rain) results in water entering and evaporating in the N2 compressor and combustion sections. This results in reduction of the cycle temp. of the engine, reducing the work available to drive the N1 fan. Consequently, N1 decreases in this condition.

[Pugilistic Animus]

Very, interesting and solid explanation Re-entry, never quite heard it like that you must be into Physics! ---I'm going use that one, if you don't mind?

As an aside it would take extreme rain something on the order of 50g/cuCm or so, to even raise the possibility of a flameout your Wxr and guts wouldn't let you attempt such flight though!!

I've seen a video clip on Youtube of a Citation's engines running apparently under water after an overrun incident quite a testament to the durability of jet engines, indeed.

[FakePilot]

There's videos of water ingestion being tested on the Internet. After seeing that, I'm not worried unless the plane is flying through Niagra falls. Somehow I think that 25% of the ingested volume was water during the test?

[terrainpullup]

I've seen two heavy jets of the same type, departing one after each other in cold weather conditions, using different takeoff technique. One using static takeoff and the other rolling takeoff technique. What are the conditions (if any) that may require engines runup prior to takeoff roll and why?

[SNS3Guppy]

Water ingestion can lead to a flame-out. That's the biggest hazard. If it's cold water (eg, supercooled), inlet icing and even stator, fan, or compressor icing is a hazard. The recent spat of Beechjet dual engine flameouts is a good example of what can happen.

The turbofan is far less susceptible to the hazards of large amounts of water passing through the engine, as most of the water goes around the core.

If you're flying in an area of extreme precipitation, often the water is not the biggest hazard with which you must contend...and you probably ought not be there.

Ignitors on in heavy rainfall or precip is always a wise choice. While many aircraft have autoignition, many don't. If it gets heavy, going to manual ignition may be a wise choice.

Keeping engine speed up in heavy precip is also a plus for your engine. It's physically easier on the engine, reduces the AoA for each blade (and any attendant physical damage or icing) creates a larger and more rapid temp rise in the engine (compressor and turbine), and in general, makes it harder to put the fire out.

So far as static or rolling, neither one is particularly relevant so far as water ingestion, except that some engines at high power will "suck" water from the surface. Water ingestion as moved from the nosewheel is a big issue and could cause a flameout. In either case, static or rolling, the aircraft will have to transit the water at the same speed at some point in the takeoff, and will be exposed to the same levels of spray. If one area of standing water exists differently from the rest of the runway, then passing through this area slower is a plus, but not at the expense of runway length.

Standing vs. rolling is an issue of takeoff distance, in which case the aircraft manufacturer proceedures and company proceedures should be followed closely. A wet runway reduces one's ability to stop on an aborted takeoff, and may lead to hydroplaning issues on the go or the stop. Consequently, the amount of pavement ahead is really effectively diminished with respect to stopping the aircraft. If the field is critical, which is not only a function of runway condition in this case but weight, altitude, and temperature, holding brakes and running power to a predetermined value before release is the only way to gaurantee performance numbers.

You cited the case of two aircraft of the same type departing one after the other...but were they at the same weight? Probably couldn't tell, but that does make a difference.

[rubik101]

Go to Youtube.com and put in GE90 test and watch the video there. Water at the rate of 4.5 tons per MINUTE just produced a lot of steam out the back!

[lomapaseo]

Go to Youtube.com and put in GE90 test and watch the video there. Water at the rate of 4.5 tons per MINUTE just produced a lot of steam out the back!

Forget about being impressed by the apparant steam out the back. It ain't what goes through the fan and out the bypass that's the worry. As stated above it's the amount of water that has to be converted to a gas before it hits the combustor that's the worry. This part is unseen in the videos and can only be assessed via the performance chacteristics of the engine right up until it stall/surges. Some of the engine tests even go so far as to only spray the water into the core and leave out the eye candy effects out of the fan duct.

[barit1]

Right on. The fan rotor in front does a good job as a centrifuge to separate out liquid water (and anything else denser than air), effectively protecting the core inlet.

But even so, liguid water content in the core can be surprisingly high.