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Topic: New Heat Exchanger Lodged Inside Jet Engines?
Username: faro
Posted 2012-11-28 12:55:46 and read 7286 times.

From this Reuters article:

http://www.reuters.com/article/2012/...e-spaceplane-idUSBRE8AR0V220121128

the following fair use excerpts:

"A small British company with a dream of building a re-usable space plane has won an important endorsement from the European Space Agency (ESA) after completing key tests on its novel engine technology."
***
"What the company has right now is a remarkable heat exchanger that is able to cool air sucked into the engine at high speed from 1,000 degrees Celsius to minus 150 degrees in one hundredth of a second."
***
"The challenge for the engineers was to find a way to cool the air quickly without frost forming on the heat exchanger, which would clog it up and stop it working....Using a nest of fine pipes that resemble a large wire coil, the engineers have managed to get round this fatal problem that would normally follow from such rapid cooling of the moisture in atmospheric air."
***
"Chief executive Tim Hayter believes the company could have an operational engine ready for sale within 10 years if it can raise the development funding....The company reckons the engine technology could win a healthy chunk of four key markets together worth $112 billion a year, including space access, hypersonic air travel, and modified jet engines that use the heat exchanger to save fuel."


That last sentence is fascinating: could it be that such hyper-efficient heat exchanger can be used to cool compressor air to achieve higher compression ratios or would it work otherwise?


Faro

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: zeke
Posted 2012-11-28 16:37:19 and read 7167 times.

Quoting faro (Thread starter):
That last sentence is fascinating: could it be that such hyper-efficient heat exchanger can be used to cool compressor air to achieve higher compression ratios or would it work otherwise?

Looks like they are using it to pre-cool the air before it gets into the compressor so it is almost at the liquid phase. It is not really for aircraft as we know it today, it is for Mach 5+ cruise.

http://www.bbc.co.uk/news/science-environment-17874276
http://www.reactionengines.co.uk/lapcat.html
http://www.reactionengines.co.uk/tec...docs/EUCASS07_scimitar_5_08_03.pdf

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: LU9092
Posted 2012-11-28 17:44:48 and read 7135 times.

Quoting zeke (Reply 1):
Looks like they are using it to pre-cool the air before it gets into the compressor so it is almost at the liquid phase. It is not really for aircraft as we know it today, it is for Mach 5+ cruise.

I wonder if it would be possible to use this between low and high compressors, or even between high compressor stages. Depending on the weight and how much energy the process eats up, I could see this tech making a pretty significant difference in performance and efficiency in a number of applications including subsonic turbine engines.

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: zeke
Posted 2012-11-28 21:50:09 and read 7030 times.

Quoting LU9092 (Reply 2):

I am sure it would be possible, however very complicated. They would also need to find a space to store the coolant.

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: WingedMigrator
Posted 2012-11-29 17:02:10 and read 6725 times.

I think this is a bogus concept, at least for orbital launch.

There isn't a lot to be gained from harvesting the oxidizer from the atmosphere in flight. Even at a lowly Mach 5 (one fifth of orbital velocity) it takes a lot of energy to accelerate the oxidizer to the same speed as the vehicle-- and then some, because I assume you'd be liquifying an even greater amount of nitrogen as well. The latter is useless for propulsion. The only savings are in the potential energy of not having to lift all that oxidizer up to altitude, and those pale in comparison to the kinetic energy that must be imparted to the oxidizer, all the energy wasted to accelerate nitrogen, and the drag losses from flying low & fast enough to harvest anything.

When kinetic energy >> potential energy, lifting oxidizer from the ground is not such a horrible idea.

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: tdscanuck
Posted 2012-11-29 21:45:09 and read 6652 times.

Quoting faro (Thread starter):
That last sentence is fascinating: could it be that such hyper-efficient heat exchanger can be used to cool compressor air to achieve higher compression ratios or would it work otherwise?

Yes. It's called an intercooler. Along with a reheater (basically the same thing in on the turbine side) it's relatively common in stationary power generation (both recip and turbine engines) and cogen systems because it allows you to drive the system efficiency way up. The theory has been around for a really long time and the application, in general, has been proven for a really long time.

However, intercoolers and reheaters are big, bulky, and heavy. Hence you don't see them on jet engines today. If these guys can fix the weight/bulk issue (basically the same problem, for heat exchangers) then we already know the concept works...this is one of those things where it's not the idea, it's the implementation, that screws you.

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: BMI727
Posted 2012-11-29 22:10:24 and read 6641 times.

Quoting tdscanuck (Reply 5):
Yes. It's called an intercooler.

That's what I was thinking. The basic idea is sound but there are very good reasons why they haven't been used on aircraft yet.

Maybe they really do have an amazingly good heat exchanger, but I think I have to be skeptical on this one.

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: flipdewaf
Posted 2012-11-30 04:19:49 and read 6573 times.

Quoting WingedMigrator (Reply 4):
The only savings are in the potential energy of not having to lift all that oxidizer up to altitude,

Some 70% by mass of the saturn 5 was in the first stage and that was gotten rid of at ~220 000ft, sounds like quite the saving to me.

It's all about not having to tank as much oxygen with you, if you can make it while you go through the atmosphere then you are likely on to a winner. As far as I know it was never about making an intercooler and will not produce benefits in the same way as an intercooler.

Fred

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: WingedMigrator
Posted 2012-11-30 09:47:10 and read 6493 times.

Quoting flipdewaf (Reply 7):
Some 70% by mass of the saturn 5 was in the first stage and that was gotten rid of at ~220 000ft, sounds like quite the saving to me.

I don't think the present proposal envisions staging, does it?

Either way, the part you don't mention is that the Saturn V first stage accelerated the rocket to Mach 8+. That's nearly 4 MJ/kg of kinetic energy compared to 0.6 MJ/kg of potential energy... so the point is that lifting stuff is relatively cheap, but accelerating it is not. That brings us back to this in-flight oxidizer harvesting scheme, which ends up accelerating all the oxidizer just the same as a conventional rocket. Considering all the other drawbacks (such as accelerating vast quantities of nitrogen and flying through a much higher dynamic pressure profile) this concept is fatally flawed.

I'm amazed they can spend hundreds of millions on an idea that doesn't even pencil out on a napkin.

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: LU9092
Posted 2012-11-30 11:07:29 and read 6467 times.

Quoting WingedMigrator (Reply 8):

I may not be reading you correctly, and forgive me if that's the case, but I don't believe the concept involves harvesting oxidizer for flight beyond Mach 5. My understanding is that the engine would be air-breathing up to the point the plane is at an altitude where sufficient oxygen is no longer available to sustain combustion, and then it would transition to rocket mode, burning its fuel with LOX carried internally. The heat exchanger brings down the temp of intake air only to -150 C, which isn't cold enough to liquefy the gases. That said, I certainly don't claim to know enough to assess the viability of the concept.

With regard to the heat exchanger/intercooler itself, Reaction Engines does claim to have reduced to the mass enough to make the technology practical for aerospace use:

http://www.reactionengines.co.uk/heatex_rel.html

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: faro
Posted 2012-11-30 11:55:55 and read 6454 times.

Quoting LU9092 (Reply 9):
With regard to the heat exchanger/intercooler itself, Reaction Engines does claim to have reduced to the mass enough to make the technology practical for aerospace use:

http://www.reactionengines.co.uk/hea....html

Indeed, this is very exciting:

"REL's heat exchangers are 100 times lighter than current technology allowing them to be used in weight-critical aerospace applications. This is achieved through the use of extremely thin walls to separate the hot and cold fluids within the heat exchangers, coupled with advanced manufacturing techniques needed to bond these fine structures whilst maintaining their strength, durability and low weight.

For example, REL has made the tube walls for its Pre-cooler as thin as possible - in our most recent demonstration the tube walls were only 27 microns thick but are bonded together to resist pressures greater than 150 bar - that's 150 times greater than atmospheric pressure at temperatures ranging from over 1,000°C to less than minus 150°C ."


I would be looking actively at this technology if I were a conventional engine manufacturer...


Faro

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: faro
Posted 2012-11-30 12:13:51 and read 6442 times.

Quoting tdscanuck (Reply 5):
However, intercoolers and reheaters are big, bulky, and heavy. Hence you don't see them on jet engines today.

From the link referenced in the previous post:

"REL's heat exchangers are extremely powerful. The Pre-cooler designed for the SABRE engine displaces 400 Mega-Watts of heat energy (equivalent to the power output of a typical gas-powered power station) yet weighs less than 1¼ tonnes."

If 400 MW is a typical gas-fired power station, how much heat energy does a CFM56 HP compressor produce per second for comparison purposes? Just scale down the 1.25 tonnes as a function of the ratio of these two powers...


Faro

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: tdscanuck
Posted 2012-11-30 15:43:52 and read 6385 times.

Quoting faro (Reply 10):
in our most recent demonstration the tube walls were only 27 microns thick but are bonded together to resist pressures greater than 150 bar

How does a 27 micron tube handle impact...of anything? It seems like a snowflake going Mach 5 would trash that.

Quoting faro (Reply 11):
If 400 MW is a typical gas-fired power station, how much heat energy does a CFM56 HP compressor produce per second for comparison purposes?

I'm not sure about a CFM56, but a JT-9 HP compressor is moving about 40 MW...if it scales linearly that would be a ~250lb heat exchanger.

Tom.

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: faro
Posted 2012-12-01 03:30:54 and read 6252 times.

Quoting tdscanuck (Reply 12):
a JT-9 HP compressor is moving about 40 MW

Does that power reflect the total energy imparted to the air in the HPC (energy of compression + heat energy + losses) or the heat energy only? Seems a mighty big figure...

250 lbs is rather weighty, yes...but if the heat exchanger were thus sized to handle 40 MW of heat input, it would mean that it would give you compressed air coming out of the HPC at essentially its initial intake temperature. IIRC, air coming out of the HPC can reach up to 450° C, so if the air were coming in at say 150° C that's a huge increase in efficiency.

Just might make it worthwhile if the technology is valid/implementable.


Faro

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: starlionblue
Posted 2012-12-01 05:07:15 and read 6242 times.

Quoting WingedMigrator (Reply 8):
I'm amazed they can spend hundreds of millions on an idea that doesn't even pencil out on a napkin.

I seriously doubt they spent hundreds of millions of dollars on the concept if it didn't pencil out on a napkin.

Many many advances have been made in the face of skepticism. We hardly have all the information. As usual we'll have to wait and see if it actually pans out.

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: tdscanuck
Posted 2012-12-01 09:02:12 and read 6178 times.

Quoting faro (Reply 13):
Does that power reflect the total energy imparted to the air in the HPC (energy of compression + heat energy + losses) or the heat energy only? Seems a mighty big figure...

It's total energy imparted to the air in the HPC (technically, the enthalpy change from inlet to outlet of the HP compressor). Since there is no heat energy input in the compressor (ignoring thermal conduction losses into the blades), it's basically all energy of compression + losses. And yes, it's a huge figure. There is more energy moving back and forth between than the turbine and compressor than total power output of the engine:
http://upload.wikimedia.org/wikipedia/commons/thumb/3/3c/Brayton_cycle.svg/745px-Brayton_cycle.svg.png

The area inside the P-V curve is the power generated by the cycle. But the energy coming out of the turbine is the *entire* area under the 3-4 portion of the curve (the compressor is under 1-2). Most of the energy moving around inside the engine nets to zero between the turbine and compressor but there are *huge* amounts moving between the two.

Put another way, the engine puts an absolutely huge amount of energy into the compressor, then extracts a slightly more huge amount from the turbine. The difference that's left is what we get for useful power. Since, in a jet engine, useful power is a big number, the amount of power moving between the two engine components is larger by, roughly, the reciprocal of the efficiency.

Quoting faro (Reply 13):
250 lbs is rather weighty, yes...but if the heat exchanger were thus sized to handle 40 MW of heat input, it would mean that it would give you compressed air coming out of the HPC at essentially its initial intake temperature. IIRC, air coming out of the HPC can reach up to 450° C, so if the air were coming in at say 150° C that's a huge increase in efficiency.

Yes, the point would be to lower the compressor output temperature. If you really get fancy (as stationary power generation does), you re-inject the heat removed from the compressor into the turbine and it gets even better.

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: jetmech
Posted 2012-12-01 19:39:27 and read 6093 times.

Quoting zeke (Reply 3):
They would also need to find a space to store the coolant.
Quoting LU9092 (Reply 2):
Quoting BMI727 (Reply 6):
Maybe they really do have an amazingly good heat exchanger, but I think I have to be skeptical on this one.
Quoting faro (Reply 10):
I would be looking actively at this technology if I were a conventional engine manufacturer...

As mentioned prior by Zeke, one of the reasons the Reaction Engines heat exchanger is able to produce such a fantastic rate of cooling is the availability of a huge amount of cryogenic hydrogen.

http://www.reactionengines.co.uk/images/sabre/sabre_cycle_1024.jpg

http://www.reactionengines.co.uk/images/sabre/sabre_cycle_1024.jpg

If an intercooling scheme were implemented on a gas turbine what would be used for coolant? I suppose we could use the outside air, which is very cold at altitude, however, this would most probably entail some sort of ram drag loss as part of the process. The fuel also becomes very cold at altitude; however, I don’t think there would be enough mass flow rate to make an appreciable amount of cooling to the air flowing through the core of the engine.

Regards, JetMech

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: tdscanuck
Posted 2012-12-01 20:36:13 and read 6077 times.

Quoting jetmech (Reply 16):
The fuel also becomes very cold at altitude; however, I don’t think there would be enough mass flow rate to make an appreciable amount of cooling to the air flowing through the core of the engine.

You're absolutely right. The fuel/air ratio (in mass terms) in a modern jet is so low that you usually assume it's zero for initial calculations. The engine is passing orders of magnitude more air mass than fuel mass (this is *not* generally true of rockets).

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: jetmech
Posted 2012-12-01 23:46:31 and read 6023 times.

Quoting faro (Reply 11):
Quoting tdscanuck (Reply 12):

My thermodynamics is extremely rusty, but I think a good estimate could be calculated from the following specifications for the CFM56-7B26;

Airflow (lbs./sec) = 779
Bypass ratio = 5.1 to 1
Pressure ratio = 32.8

This would give a core mass flow rate of 152.745 lbs./sec (69.284 kg/sec). Assuming 100% isentropically efficient compression, the corresponding temperature ratio for a 32.8 to 1 pressure ratio would be 2.710818 to 1. Assuming air at 23 degrees C (296.15K), we have a specific heat capacity of 1.012 kJ / (kg.K) .

Thus, for the given conditions, I am calculating a minimum compressor exit temperature of 802.808 K (529.658 degrees C), which is a 506.568K temp rise. This in turn gives a minimum rate of heat addition to the air of 35,525 kJ/s, or 35.525 Mw.

http://www.cfmaeroengines.com/engines/cfm56-7b
http://en.wikipedia.org/wiki/Heat_capacity

Regards, JetMech

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: faro
Posted 2012-12-02 08:24:00 and read 5977 times.

Quoting jetmech (Reply 16):
however, I don’t think there would be enough mass flow rate to make an appreciable amount of cooling to the air flowing through the core of the engine.

This indeed seems like a limiting condition; regardless of the efficiency of the heat exchanger it is difficult to see how/where 35-40 MW of heat energy can be disposed of in subsonic flight, even if it is at high altitude...you need a much bigger external heat sink...


Faro

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: faro
Posted 2012-12-04 02:57:16 and read 5783 times.

Quoting tdscanuck (Reply 15):
It's total energy imparted to the air in the HPC (technically, the enthalpy change from inlet to outlet of the HP compressor). Since there is no heat energy input in the compressor (ignoring thermal conduction losses into the blades), it's basically all energy of compression + losses. And yes, it's a huge figure. There is more energy moving back and forth between than the turbine and compressor than total power output of the engine:

Thanks for the details and the diagram, that's quite impressive and I guess explains why land-based derivatives of jet engines are used in electric power generation. Which leads me to the question: how big a power output could a land-based GE90-115B derivative generate? Must be a surprising figure...

Quoting faro (Reply 19):
This indeed seems like a limiting condition; regardless of the efficiency of the heat exchanger it is difficult to see how/where 35-40 MW of heat energy can be disposed of in subsonic flight, even if it is at high altitude...you need a much bigger external heat sink...

But now that I think about it, you don't *need* to dispose of 100% of the heat input, you only want to get rid of as much as you can to make the contraption economic. It may be workable after all...time will tell.


Faro

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: tdscanuck
Posted 2012-12-04 08:39:36 and read 5744 times.

Quoting faro (Reply 20):
Which leads me to the question: how big a power output could a land-based GE90-115B derivative generate?

On the order of 85MW or more. Very rough rule of thumb is that you get about 1 hp of shaft output per pound of rated takeoff thrust.

Quoting faro (Reply 20):
But now that I think about it, you don't *need* to dispose of 100% of the heat input, you only want to get rid of as much as you can to make the contraption economic.

You need to send it somewhere useful...otherwise you're dumping energy overboard and your efficiency will absolutely go to heck. In the Reaction Engines design all that heat is going into the fuel and hence being recovered in the engine. If you just dump it to atmosphere it's wasted. In a jet engine application, I think you'd have to combine it with a reheater in the turbine.

Tom.

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: faro
Posted 2012-12-05 02:36:32 and read 5639 times.

Quoting tdscanuck (Reply 21):
In a jet engine application, I think you'd have to combine it with a reheater in the turbine.

Yes, this is among the technology implementation points discussed in this Flightglobal article on the Reaction Engines cooling technology published yesterday:

http://www.flightglobal.com/news/art...revolutionise-aero-engines-379772/

Article also discusses using cooled air to feed turbine blade cooling. I wonder though, how steep a temperature gradient you can use in turbine blade cooling before the material starts to experience inacceptable heat 'fatigue'. There must be a limit somewhere there you cannot cross with present materials.

Seems the (classic) jet engine cooling potential of the technology is getting quite some attention; the article mentions 5%-10% efficiency gain potential. Money-wise, it's probably the most promising application for the technology although it will take some time and a lot of R&D spending. Spaceplanes are still a pipe dream, at best several decades away...


Faro

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: JamBrain
Posted 2012-12-09 03:53:36 and read 5331 times.

Quoting tdscanuck (Reply 21):
You need to send it somewhere useful...otherwise you're dumping energy overboard and your efficiency will absolutely go to heck.

There is a good paper here from MTU that explains some advanced cycles with 2 heat exchangers these would work well with the reaction engine technology:-
http://www.mtu.de/en/technologies/en...ued_Intercooled_recuperated_en.pdf
Quote:
recover heat from the hot exhausts to the combustion chamber, and to decrease the burner temperature rise; the fuel consumption is therefore reduced. ...The intercooler itself features a single passage, counter-flow arrangement providing the compressor air with a temperature drop of approximately 100K. The cooling flow is accelerated and discharged by a third additional nozzle; the higher exit temperature of the cooling flow results in a higher exhaust velocity, .

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: jetmech
Posted 2012-12-10 05:30:51 and read 5180 times.

Quoting tdscanuck (Reply 21):
You need to send it somewhere useful...otherwise you're dumping energy overboard and your efficiency will absolutely go to heck.

Very interesting! For many years I always assumed that intercooling or reheating alone were techniques for increasing thermal efficiency, but as you state, this is not so. Either technique alone would certainly increase the work output of the cycle, but probably would not increase thermal efficiency. An increase in thermal efficiency from intercooling and reheat would require a combined heat exchange system such as that posted by JamBrain.

I was under a similar mistaken belief about turbocharging, in that I always thought it improved thermal efficiency. A few years ago, to my surprise, I learned that although turbocharging certainly increases the work output of the cycle, it again does not improve thermal efficiency.

The key subtlety that I never realised, was that although a turbocharger made use of heat that was otherwise wasted, the resulting compressed air by itself did very little or nothing. To take advantage of this compressed air required the expenditure of additional fuel!

A power recovery turbine also uses heat that would be otherwise wasted heat to spin a shaft, however in this case, the shaft rotation is directly fed back to the crank. Hence, unlike the turbocharger, additional power is provided without having to expend extra fuel, hence an increase in thermal efficiency.

http://www.avweb.com/newspics/pp31_prt_sch.jpg

Regards, JetMech

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: WingsFan
Posted 2012-12-10 14:08:02 and read 5116 times.

Quoting LU9092 (Reply 9):
REL has made the tube walls for its Pre-cooler as thin as possible - in our most recent demonstration the tube walls were only 27 microns thick but are bonded together to resist pressures greater than 150 bar

This is quiet small.I wonder how the pressure drop across this coil bundle will be for the coolent. Higher pressure drop means the pump size and pump energy requirements will be high.

This is almost shell and tube exchanger.


Aditya

Topic: RE: New Heat Exchanger Lodged Inside Jet Engines?
Username: Aesma
Posted 2012-12-15 13:01:24 and read 4821 times.

ESA is involved so it has to have some merit, they know a thing or two about rockets.

Quoting faro (Reply 22):
Spaceplanes are still a pipe dream, at best several decades away...

I'm guessing you mean space airliners ? Because space planes are already flying. They just don't go far.


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