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Re: Turbine bypass?

Unread postPosted: 01 May 2017, 04:28
by collimatrix
johnwill wrote:
collimatrix wrote:Using the mass flow rate and pressure ratio figures for a J79 from wikipedia, I get that assuming perfectly isentropic compression the compressor in a J79 is eating up 21.2 kW. An LM1500, which is basically an industrial turbine J79 and will even swap parts with an aero J79, produces 7.4 kW. Obviously, gas turbine compressors are not perfectly isentropic, but back-of-the-envelope calculation says that the compressor of a gas turbine engine is consuming about three times the net power that the same engine can produce as a turboshaft.

So I really don't think there's all that much wiggle room to be robbing the turbine of power.


Wiki says the LM1500 produces 7.42 MEGA watts.



Ooops.

Well, what's three orders of magnitude between friends?

Re: Turbine bypass?

Unread postPosted: 01 May 2017, 13:42
by zhangmdev
I think dumping hot gas directly into the nozzle will decrease thrust. The reasoning is like this: to increase thrust (useful work) is to move as much air (mass) as possible and as fast as possible. Air flow rate depends on rotation rate of the compressor, which is driven by the turbine. Less gas goes through turbine will decrease power to drive compressor, doesn't help increasing air mass flow. Turbine acts like a cylinder, very hot very high pressure but very slow moving gas expand inside, getting cooler and less dense, giving up energy to the turbine, and moving a lot faster afterwards. Do not go through turbine will further decrease velocity of air flow.

Part of the gas expansion is done inside the nozzle, but, unless combustion chamber pressure is impossibly high, like those inside rocket engines, or air flow velocity is very high, like those inside ramjet engines, hot gas won't expand enough to generate useful thrust by using nozzle only.

Thermodynamic cycle of gas turbine

https://www.grc.nasa.gov/WWW/K-12/airplane/brayton.html

Re: Turbine bypass?

Unread postPosted: 01 May 2017, 23:38
by count_to_10
collimatrix wrote:
johnwill wrote:
Wiki says the LM1500 produces 7.42 MEGA watts.



Ooops.

Well, what's three orders of magnitude between friends?

:doh: I didn't even think to check.
That said, now I think your estimate of how much power the compressor takes to operate must be really low.

I was reading the wiki page for combustion chambers, and it claims that most modern combustors are annular. I was thinking that complicates things, but maybe not. One of the possibilities I had in mind is that the compressed bypass air from a single engine could be routed to two widely separated nozzles with thrust vectoring. You could get pitch, role, and yaw control with just vertical vectoring (yaw by varying the thrust of each nozzle), all without the risk of catastrophicly asymmetric thrust that comes with two widely spaced engines.

Re: Turbine bypass?

Unread postPosted: 13 May 2017, 13:03
by count_to_10
One more thought before I let this thread die an ignominious death.
The engine in this idea would always, outside of idling, be at peak RPM, with the power being determined entirely by how much fuel was being injected into the compressed bypass air (in combination with its nozzle setting). As such, thrust should respond much faster to the throttle.