lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

Ideal Jet Engine Question?

Wed Apr 12, 2006 10:56 am

Ideally, the fuel burned in a combustion chamber can be considered as a heat interaction into the system, raising the temperatures with a certain amount of energy per unit mass.

Will another source of heat work, without burning anything, like an electric coil for example?
The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
727EMflyer
Posts: 538
Joined: Wed Mar 09, 2005 3:22 am

RE: Ideal Jet Engine Question?

Wed Apr 12, 2006 11:13 am

I may be out to lunch here, but doesn't the combustion also increase the volume (mass may be a better word) of the gas at the same time as the heat increases the pressure, thereby adding even more energy potential?

The amount of power that is gained from combustion engines versus weight of the machine and the fuel, and the cost of each, is about as close to ideal as you can get in the real world. Sure, designs can be tweaked here and there for better perfomance and efficiency, but as a group when compared to other means of power combustion is just king!

Just out of curiosity, if you were to use electric coils to heat the gas, where would you get the power from?
 
darkblue
Posts: 227
Joined: Thu Sep 11, 2003 10:27 pm

RE: Ideal Jet Engine Question?

Wed Apr 12, 2006 12:36 pm

Quoting 727EMflyer (Reply 1):
I may be out to lunch here, but doesn't the combustion also increase the volume (mass may be a better word) of the gas at the same time as the heat increases the pressure, thereby adding even more energy potential?

No. Several things to address here.

1.) The only increase in mass in combustion is due to addition of fuel. The amount of mass added compared to the total airflow of a high bypass engine is less than 0.1%.

2.) Gas turbine engines operate using the Brayton cycle. Combustion occurs ideally at constant pressure. In a real combustor, pressure will drop slightly due to losses.

3.) In a gas turbine engine all you need is heat addition. Whether the heat comes from a combustor, nuclear reactor or electric coil, it doesn't really matter where the heat comes from. In the case of a combustor, the entire point is to convert the chemical energy in the fuel into thermal energy. That's it. There is no added benefit of mass flow or pressure increase.

Now, back to the original question, could an electric coil work in a gas turbine engine? Yes it could be done, however the problem with this method is the heat transfer effectiveness of the coil. Not very good at all. It would be very difficult to get this to work efficiently in large gas turbines. The one place where a electric coil would work, and in fact I believe it has been done, is in a micro turbine engine. I don't have time right now, but if no one gets to it before I do, I'm sure there are some sites on the web describing these engines.

Darkblue
 
bri2k1
Posts: 952
Joined: Sun Dec 19, 2004 4:13 am

RE: Ideal Jet Engine Question?

Wed Apr 12, 2006 1:31 pm

Quoting DarkBlue (Reply 2):
Now, back to the original question, could an electric coil work in a gas turbine engine? Yes it could be done, however the problem with this method is the heat transfer effectiveness of the coil.

Not only efficiency of heat transfer, but efficiency as measured by heat addition capacity per weight added. Aircraft fuel efficiency is inversely proportional to weight, and all of the other methods developed to add heat in the appropriate cycle of a turbine engine, inlcuding nuclear heating of a fluid, have been far too heavy to be sustainable. A reactor can produce huge amounts of heat with virtually no fuel addition for a long time, but the weight required to transport all of the required equipment more than offsets the benefit compared to today's gas turbine engines.
Position and hold
 
tepidhalibut
Posts: 182
Joined: Fri Dec 10, 2004 8:19 pm

RE: Ideal Jet Engine Question?

Wed Apr 12, 2006 4:56 pm

Quoting Lehpron (Thread starter):
Will another source of heat work, without burning anything,

Yes, you can, tho' I'm not sure many have been flown. The most eye-popping example would be nuclear power, for example the X-36.

http://en.wikipedia.org/wiki/Nuclear_aircraft
 
DH106
Posts: 592
Joined: Wed Jun 15, 2005 5:32 pm

RE: Ideal Jet Engine Question?

Wed Apr 12, 2006 5:07 pm

Quoting TepidHalibut (Reply 4):
The most eye-popping example would be nuclear power, for example the X-36.

Not sure if you're refering to the converted B-36 (NB36), or to the propsed X-6 here. It's important to note that the in the case of the NB36 the nuclear reactor was in no way involved in the propulsion of the aircraft - the conversion from the B-36 was just an experiment with the aim of taking a 'working' nuclear reactor aloft without 'connecting' it to the engines. This presumably was the first step in experiments towards a nuclear powered aircraft, and the X-6 proposal followed.
...I watched c-beams glitter in the dark near the Tanhauser Gate....
 
kaddyuk
Posts: 3697
Joined: Thu Nov 08, 2001 1:04 am

RE: Ideal Jet Engine Question?

Wed Apr 12, 2006 9:03 pm

Quoting 727EMflyer (Reply 1):
Just out of curiosity, if you were to use electric coils to heat the gas, where would you get the power from?

Plug it in and drag an extension lead around with you... Just DONT TOUCH THAT PLUG!
Whoever said "laughter is the best medicine" never had Gonorrhea
 
User avatar
Starlionblue
Posts: 17055
Joined: Fri Feb 27, 2004 9:54 pm

RE: Ideal Jet Engine Question?

Wed Apr 12, 2006 9:09 pm

Quoting 727EMflyer (Reply 1):
I may be out to lunch here, but doesn't the combustion also increase the volume (mass may be a better word) of the gas at the same time as the heat increases the pressure, thereby adding even more energy potential?

Just for the record, you can't add mass that way. This would be tantamount to creating matter. Tricky at best  Wink


As has been pointed out, you can heat the air with other methods, but chemical energy is still the most efficient application from an energy storage (weight and volume) perspective..
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
User avatar
lightsaber
Crew
Posts: 11740
Joined: Wed Jan 19, 2005 10:55 pm

RE: Ideal Jet Engine Question?

Thu Apr 13, 2006 12:42 pm

Quoting Lehpron (Thread starter):
Will another source of heat work, without burning anything, like an electric coil for example?

Yes. It all comes down to creating a volumetric expansion that is mostly done by heating the gas in the core. The fan is just there to improve propulsion efficiency... but that can be a long discussion on its own.  Wink

There are proposals to generate electricity in the cargo hold (via fuel cells burning... something) and power electrically powered ducted fans on the wings. That is the way to go if you don't want to use chemical energy.

Quoting DarkBlue (Reply 2):

1.) The only increase in mass in combustion is due to addition of fuel. The amount of mass added compared to the total airflow of a high bypass engine is less than 0.1%.

Its a wee bit more if you talk core air. You're about right for a high bypass. F/A stoich is 0.068 (6.8%) The core overall runs at a F/A out the combustor of about 0.035 (takeoff). While this 3.5% fuel is then diluted by cooling air to about 2%. The fan drops this to 0.2% At cruise, with lower f/a ratios in the combustor, we're talking 0.12%. Sorry... used to be my job.  Wink

Quoting TepidHalibut (Reply 4):
he most eye-popping example would be nuclear power, for example the X-36.

No reason not to! Ok, one runway over-run could be the end of that airport...  duck 

Quoting Starlionblue (Reply 7):

As has been pointed out, you can heat the air with other methods, but chemical energy is still the most efficient application from an energy storage (weight and volume) perspective..

To expand: chemical liquid energy. The cost in pumping energy and weight for a gas fuel... would be impractical. I can't help but notice every hydrogen powered aircraft makes tank assumptions that put them at a wait that's... amazingly light. Lighter than tanks I've worked on for nitrogen or air in an aircraft... But academic studies never design for the 9G crash load either...  Wink

Lightsaber
"They did not know it was impossible, so they did it!" - Mark Twain
 
pavlin
Posts: 391
Joined: Sun Oct 23, 2005 5:34 am

RE: Ideal Jet Engine Question?

Thu Apr 13, 2006 9:47 pm

If you have electricity ( a lot of it like small powerplant) on an aircraft you would need only electric motors with fans or propellers to drive aircraft forward. It couldn't be simplier. But remember you can't fit a whole powerplant even A380.

Who is online

Users browsing this forum: No registered users and 15 guests