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kurtverbose
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Countdown To Complete Chemical Burn

Tue Feb 24, 2015 7:24 pm

Maybe some engine buffs can chip in. I'm not sure of my facts.

My understanding is the maximum temperature of a complete chemical burn of air with aviation fuel is 2,230°C. At this temperature an engine can no longer rely on raising turbine inlet temperature to increase efficiency.

Given todays engines are running at approx 1600°C, and this seems to creep up with each generation, can we predict when complete chemical burn will be reached, and if so when will it be?

Second target is when can this be reached without cooling that lowers the efficiency of the engine - e.g. bleed air?

Also, is the target attainable at all, technically and given NOX emissions?

What kind of efficency gains can be achieved by reaching this temperature compared to today's engines?

Very grateful to those who can shed more light on this.
 
rwessel
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RE: Countdown To Complete Chemical Burn

Sun Mar 01, 2015 8:56 am

The adiabatic flame temperature of Jet-A is closer to 2100C, IIRC. And it's usually measured assuming a starting point of 20C. Remember that the adiabatic flame temp is basically the heat released from the combustion applied to the reacted mass, and is the resulting *rise* in temperature of that mass.

In the case of a jet engine, the air leaving the compressor is not 20C, but more like 300C, so the theoretical maximum turbine inlet temperature is more like 2400C
 
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DocLightning
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RE: Countdown To Complete Chemical Burn

Sun Mar 01, 2015 9:22 pm

Quoting rwessel (Reply 1):
In the case of a jet engine, the air leaving the compressor is not 20C, but more like 300C, so the theoretical maximum turbine inlet temperature is more like 2400C

Akiss would be the perfect one to answer this. I'll ping him and ask him to comment.
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akiss20
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RE: Countdown To Complete Chemical Burn

Mon Mar 02, 2015 4:22 am

Quoting kurtverbose (Thread starter):
My understanding is the maximum temperature of a complete chemical burn of air with aviation fuel is 2,230°C

Adiabatic flame temperature (temperature achieved when burning a fuel at the stoichometric ratios and without any heat removal) of Jet-A and air depends on both the combustor inlet temperature and also the combustion pressure, but you're in the right ball-park (see http://i.imgur.com/PNAKCo5.png, Peters & Hammond 1990) . Given modern OPRs, outlet temperatures are easily in the 700-800K range and combustor pressures are likely above 3MPa so around 2550K is about right.

Quoting kurtverbose (Thread starter):
Given todays engines are running at approx 1600°C, and this seems to creep up with each generation, can we predict when complete chemical burn will be reached, and if so when will it be?

I am not sure we can really predict with any real certainty, but if I had to wager a guess, we are looking in the 50-100 year time frame for that (in which hopefully we might be moving to an alternative propulsion system). This figure (http://i.imgur.com/sbXO8rK.png Cumpsty) shows the EIS dates and approximate T4 temperatures for Rolls-Royce engines. As you can see it took us about 70 years to go from 1000K to about the 1700-1800K we are now, with several huge technological shifts. To get further than this we really need to focus on CMCs and other advanced materials. Cooling technology is being stretched more and more and it would not be beneficial from an overall cycle perspective to draw more bleed air for the cooling side. There really isn't any low hanging fruit on the cooling tech anymore (although cooling and heat transfer isn't really my specialty).

Also to be noted, the actual thermal efficiency of the Brayton cycle (flow power/fuel power, not thrust power/fuel power) is actually quite insensitive to T4 and OPR at the values we are reaching (http://imgur.com/EzyLjwy , note in this figure T4 is labeled as T3, silly ramjet station notation). What is till a decent gain is specific work, or the amount of work produced per unit mass of air through the core. This is relevant as with more work per unit massflow, we can shrink the core relative to the fan and go for a low FPR fan.

Quoting kurtverbose (Thread starter):
Second target is when can this be reached without cooling that lowers the efficiency of the engine - e.g. bleed air?

I am doubtful it would ever happen. We would need a very large revolution in materials technology and I don't see such a huge upset on the horizon (but then I am not a metallurgist). The combination of high temperature and incredibly high stresses seen in the disks makes such a material almost unimaginable.

Quoting kurtverbose (Thread starter):
Also, is the target attainable at all, technically and given NOX emissions?

We have a long way to go in terms of technical progress, but it is potentially attainable. Like I said, if it's going to happen, we are looking at the 50-100 year time frame in my opinion. As to NOX emissions, remember that just because the T4 is approximately 1800K, does not mean there are not higher temperatures in the combustor. Combustion can only occur in a relatively narrow range of equivalence ratios (fuel-to-air ratio normalized by stoichometric fuel-to-air ratio), so stoichometric temperatures are essentially being achieved in the engine. It is in these hot zones that most NOx is produced. Only about 15-20% of the core massflow goes into the primary zone where these temperatures are reached. The remainder of the air is mixed in later in the downstream intermediate and dilution zones. This air is used to cool the combustor liners and bring the overall temperature down to a level that is acceptable to the combustor.

Quoting kurtverbose (Thread starter):
What kind of efficency gains can be achieved by reaching this temperature compared to today's engines?

Somewhat hard to say without doing a more complete cycle analysis, but stoich temperature would represent a T4/T2 of approximately 8.5. That specific work chart doesn't go that high, but I would estimate it to be about an 80% increase in specific work.

[Edited 2015-03-01 20:30:48]
Change will not come if we wait for some other person or some other time. We are the ones we've been waiting for. We are
 
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akiss20
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RE: Countdown To Complete Chemical Burn

Mon Mar 02, 2015 4:31 am

On a completely unrelated note, the forum posting software here is horrendous.
Change will not come if we wait for some other person or some other time. We are the ones we've been waiting for. We are
 
kurtverbose
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RE: Countdown To Complete Chemical Burn

Mon Mar 02, 2015 9:37 pm

Thanks Guys, that's very much appreciated. Will spend some time digesting that!

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