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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
In my time working on turbine aircraft both hobby and military (5k$-120k$ engines) between 5 to 82 lbs of thrust at 300 mph or less i get a fuzzy feeling in my gut. A feeling that magically shrinking a 111 inch diameter GEnx turbine (58k lbs thrust) down to 4 inch diameter (82 lbs thrust), doesn't allow the expected linear decrease in thrust per volume, per unit of mass or per specific fuel consumption.
So considering natural scaling laws make that kind of comparison useless, I ask if the Brayton cycle describes this difference.
Further more if we compare a tiny turbine using the brayton cycle (not otto) to a mid-sized piston engine, would we find a region on the graph where their power and efficiency start to overlap ?
Sorry if this is a messy question Im not good at thermodynamics.
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
your answer is on the first page
I think the other thing you need to consider is that viscous effects have a bigger impact as you reduce size. And you are probably aware that thrust is not directly related to energy ( M.dV vs M.(dV)^2)
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
Short answer : the efficiency of the Otto and Brayton cycles are the same, but Otto is best for a reciprocating engine and Brayton is better suited to continuous flow engines
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
Turns out speed of sound in a gas is only dependent on temperature (which I didn't know). In the turbine section, with temps of 1090C(2000F) speed of sound is 740m/s (2400ft/s.)
Registered Member #61406
Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268
Its also got to do with the chemical make up, H2,CO2,CH4 all have different numbers. The pressure is related to mocules by temperature. They get close to the speed of sound with winglet's and taper. If you can design a combustion chamber to handle large pressure differences and still work, worth aiming for.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Plasma wrote ...
If you can design a combustion chamber to handle large pressure differences and still work, worth aiming for.
I think the rotary engine holds promise for 500-750 lbs class drones. I was thinking of conventional propellers with the tips cut off running in a tube (i.e. f-16 ish inhaling)
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Turbine vs Piston Thermodynamics
