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LP/HP Intercooling Idea?  
User currently offlineGrunf From Sweden, joined Jan 2007, 54 posts, RR: 0
Posted (7 years 6 months 1 week 2 days 13 hours ago) and read 2570 times:

Hello there!

After lurking on Airliners forums for a long time, I joined the club. Please excuse me if my questions are trivial or allready being answered. I tried to search but I couldn't find anything.

Now to the question:

Jet engines are air-breathing machines. The more oxygen you can cram into combustion chamber, the more power you can get from it (as long as other things like EGT, RPM etc. are in check), right?

Air gets hotter when compressed. In ideal world it would follow ideal gas law, PV=nRT. In case of airplane engines, I guess it will get a tad hotter due to compressor inefficiencies. When air is compressed dozen times, it will be very hot when it reaches the burner cans, then get even hotter when fuel is burned.

Now what if you added a short heat exchanger between LP and HP compressor? You could use a short water circuit that would circulate the coolant trough say, leading edges (thus making a permanent anti-ice solution). This "Intercooler" (to use automotive terminology) would probably make little more resistance to the flow, but to make it more flow-friendly, you could incorporate it into stator vanes, thus making them both a cooling element and a stator?

A pessimist would probably say that such solution would be too complex and heavy, but hey...P&W are trying to make a geared fan for 20 years, and that's a tad more complex than circulating little coolant trough few bits and pieces  Smile

That way, you could achieve higher pressure ratios and make engine more effective??

Now shoot me down in flames...


Drink more milk, less kerosene!
13 replies: All unread, jump to last
 
User currently offlineBoeingFixer From Canada, joined Jul 2005, 529 posts, RR: 0
Reply 1, posted (7 years 6 months 1 week 2 days 11 hours ago) and read 2541 times:

The mass cooling required to cool the compressor discharge air to any significant degree would require immense surface area. More area than is available in the diffuser section. If is was feasible, LN2 would be a more likely candidate as a cooling medium than H2O but I feel that the surface area required would inhibit proper air-flow into the combustion chamber causing more problems than the benefits could provide. If this was worth the investment, engine manufacturers would have done this long ago. One thing you have to remember is weight is worth its price in gold. The current state of advanced alloys really negates the need for added weight in the form of a compressor cooling section.

As for geared fans, Garret has one on the TFE-731 so the technology is there. It's just a matter of the weight Vs benefits of the technology on a large scale.

Cheers,

John



Cheers, John YYC
User currently offlineGrunf From Sweden, joined Jan 2007, 54 posts, RR: 0
Reply 2, posted (7 years 6 months 1 week 2 days 9 hours ago) and read 2523 times:

Quoting BoeingFixer (Reply 1):

Thanx for answering. i hope you don't find my idea silly. I believe you slightly missunderstood reason for this. Reason for "intercooling" is not to save compressor blades from heat but to be able to pack more oxygen into runer cans, thus achieving higher pressure ratio while keeping EGT at bay. This is regular procedure in turbocharged cars etc.

[Edited 2007-01-16 22:13:28]


Drink more milk, less kerosene!
User currently offlineGrunf From Sweden, joined Jan 2007, 54 posts, RR: 0
Reply 3, posted (7 years 6 months 1 week 2 days 9 hours ago) and read 2511 times:

Hmm...it seems that GE already tought about it:

http://www.gepower.com/about/press/en/2003_press/120903h.htm

Quote:

In simple cycle, the LMS100 has an efficiency of 46 percent, which is 10 percent greater than GE’s highest efficiency gas turbine on the market today, the LM6000. In combined cycle, the efficiency is 54 percent. A key reason for the high efficiency is the use of off-engine intercooling technology within the compression section of the gas turbine. The LMS100 is the first modern production gas turbine in the power generation industry to employ this technology.

Probably too heavy for airborne use though...

Nice diagrams found here: http://www.energysolutionscenter.org/DistGen/Tutorial/CombTurbine.htm



[Edited 2007-01-16 22:38:25]


Drink more milk, less kerosene!
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 4, posted (7 years 6 months 1 week 2 days 7 hours ago) and read 2489 times:

You have to have a pretty big intercooler if the total pressure loss through the intercooler isn't to be significant enough to cause you bigger efficiency losses than what you gain. Volume and weight restrictions pretty much makes this a no-go as far as aviation engines go.

It has, however, been done in a way in aviation as well. Water can, and has been, injected into turbine engines giving the same effect (and added mass flow in addition).

Rgds,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineMarkC From United States of America, joined Apr 2006, 259 posts, RR: 0
Reply 5, posted (7 years 6 months 1 week 2 days 7 hours ago) and read 2485 times:

There is plenty of extra oxygen that is not burned in the combustion chamber. It would be done for cooling/durability. Its just impractical for the size. Its a huge volume of air.

For example, I own a twin turbo Camaro which generates 530 bhp. Its quite a lot for a car. This engine consumes 54 lbs of air per minute...slightly less than 1 lb per second. A PW4090 has almost 300 lbs of air per second through the combustion chamber. Your intercooler would need to have a very large frontal area, size and weight. A ground turbine on the other hand....


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 6, posted (7 years 6 months 1 week 11 hours ago) and read 2434 times:

The temperature going into the engine actually has a lot to do with how much power you can get out of it. This holds true for recips as well as turbines. The aforementioned Camaro will generate a lot less than 530 hp on a scorching hot summer day, while it may be able to put out more on a cold day (provided the mechanics can take it). The difference between the temperature going into the combustion chamber(s) and the temperature going out dictates how much energy (power) you can add in the form of fuel.

On the outgoing side, the temperature is limited by EGT/turbine inlet temperature, leaving you only the temperature of the air coming in to manipulate if you wish to increase the maximum possible power output without melting the exhaust ports/turbine.

The oxygen not taking part in the combustion process is a problem and a huge efficiency loss. In jet engines, the main reason for having more air through the engine than what is required to burn the fuel is cooling. Excess air is used to cool primarily the combustion chamber lining and the turbine blades. Turbine blades which can take higher temperatures enable the engine to have less "unburned" air going through it and increases the efficiency. That's why they're throwing the big money at turbine blade alloys.

If you could cool the air going into the engine, without a total pressure loss, you would primarily gain power, not durability. You could say this is achieved through cooling, but I'd consider that a rather indirect way of getting to the core (pun unintentional) of the matter.



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineThegeek From Australia, joined Nov 2007, 2638 posts, RR: 0
Reply 7, posted (6 years 8 months 7 hours ago) and read 2215 times:

The problem that the OP has not solved is what are you going to cool your intercooler with? I say use the fuel. Then you get the heat back when you come to burn the fuel. Ground based gas turbines' intercoolers don't get the heat back, they just exhaust the heat to the atmosphere. I can understand the an air-air intercooler, putting the heat into the bypass air, would weigh a lot and offer little benefit, but by using a liquid coolant should reduce the weight of the intercooler considerably.

User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 8, posted (6 years 8 months 7 hours ago) and read 2209 times:



Quoting Thegeek (Reply 7):
The problem that the OP has not solved is what are you going to cool your intercooler with? I say use the fuel. Then you get the heat back when you come to burn the fuel. Ground based gas turbines' intercoolers don't get the heat back, they just exhaust the heat to the atmosphere. I can understand the an air-air intercooler, putting the heat into the bypass air, would weigh a lot and offer little benefit, but by using a liquid coolant should reduce the weight of the intercooler considerably.

The problem with using fuel is that the engine fuel flow isn't nearly large enough to provide meaningful cooling, which means you're talking about rejecting heated fuel back to the tanks. Thanks to TWA800, heat rejection to the fuel tanks is a giant no-no.

You also still have the problem of contact area...there isn't enough surface area in the compressor to get meaningful intercooling, even with liquid coolant. To make it work, you need to add surface, which means pressure losses and weight.

Tom.


User currently offlineRwessel From United States of America, joined Jan 2007, 2310 posts, RR: 2
Reply 9, posted (6 years 7 months 4 weeks 1 day 9 hours ago) and read 2159 times:
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Actually the OP specified a heat exchanger between the HP and LP sections. Which is pointless, since you're just using a big heavy mechanism to move heat energy around in a circle (the newly heated LP air will promptly enter the HP section, and when compressed will be even hotter).

User currently offlineThegeek From Australia, joined Nov 2007, 2638 posts, RR: 0
Reply 10, posted (6 years 7 months 4 weeks 1 day 8 hours ago) and read 2155 times:

That's true. It would be worse than pointless, it would be a retrograde step. The amount of heating done in compressing a gas is directly proportion to its absolute temperature, so you might as well electrically power a heater in the compressor. Perhaps he meant a regenerator taking heat from after the turbine. But I think this only has a point if it heats up the air after the HP Compressor. And probably not even then, since your cooling the exhaust to heat the core flow.

What I was thinking is that the specific heat of a liquid is more than for a gas. Now that I looked it up, the difference is a lot less than what I remembered (it's actually about 2.2 times). So the effect of this intercooler would be very small. The only way it could work well is if either the fuel in the tanks were used as the heat sink, or if most of the heat could be discharged into the bypass. I'm not sure that TWA 800 should cause that much sensitivity - wasn't that thought to be due to arcing in the fuel tank causing an ignition?


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 11, posted (6 years 7 months 4 weeks 1 day 7 hours ago) and read 2148 times:



Quoting Thegeek (Reply 10):
I'm not sure that TWA 800 should cause that much sensitivity - wasn't that thought to be due to arcing in the fuel tank causing an ignition?

The direct cause is assumed to be electrical arcing in the fuel tank. However, TWA800 kicked off the idea that designing out ignition sources alone wasn't sufficient and that they should also reduce flammability. Heat rejection to the fuel has a very large impact on tank flammability.

Tom.


User currently offlineThegeek From Australia, joined Nov 2007, 2638 posts, RR: 0
Reply 12, posted (6 years 7 months 4 weeks 1 day 2 hours ago) and read 2139 times:

Ok, how about compensating for the added perceived risk with a nitrogen fire suppression system to keep oxygen out of the fuel tank?

User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 13, posted (6 years 7 months 4 weeks 13 hours ago) and read 2092 times:



Quoting Thegeek (Reply 12):
Ok, how about compensating for the added perceived risk with a nitrogen fire suppression system to keep oxygen out of the fuel tank?

That's exactly what the FAA suggested in a proposed rule last year. However, the way the rule is written you'd have to inert the main tanks as well if you were dumping significant heat into them. Right now, you just have to inert the center tank. Technically it's not a big deal but it would hike the cost/weight/installation complexity of the system.

Tom.


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