Aerospace Isentropic efficiency vs polytropic efficiency of a gas turbine

1. Dec 18, 2009

jason.bourne

definition wise i got the difference between these two efficiencies.

but in practice how does these two differ?

i was refering to this book:
Jet propulsion: a simple guide to the aerodynamics and thermodynamic design ... By N. A. Cumpsty

he has mentioned this:

"using polytropic efficiency makes algebra easier, but it also removes a bias in the isentropic efficiency when comparing machines of different pressure ratios"

can anyone please explain me in a much better way how these two efficiencies work in real practice and how they are different coz still m totally confused?

2. Dec 20, 2009

mugaliens

When you look at a turbine's compressor map, you'll notice it's depicted in terms of various throttle settings ranging from ground idle to climb, and with mass flow rate depicted along the x-axis while the pressure ratio is depicted along the y-axis.

In the upper-right of the graphic, you'll notice a subplot labeled "isentropic efficiency."

I take it you've heard of Wikipedia...

The isentropic process is one in which the entropy of the system remains constant. For axial-flow turbines, this means the engine system is neither gaining nor loosing energy, and whereby the sum total energies of all inputs equals that of the outputs. It also requires the energy to be at a constant rpm.

Since jet engines are designed primarily to operate at their greatest efficiencies at cruise altitude, it's appropriate to begin with isentropic maps and work from there.

The polytropic process is used for systems which obey the ideal gas law. While it applies to all ideal thermodynamic cycles, including the famous Carnot cycle, jet engine thermodynamic systems are not quite "ideal."

The compressor stages do not follow the adiabatic process, either, as successive compressor stages are warmer due to previously compressed air, and they've passed some of that heat to the vanes, which further heat incoming air. So it's not truly adiabatic, which requires that no heat is transferred to/from the working fluid. Clearly, the combustion stage is not adiabatic.

3. Dec 24, 2009

jason.bourne

i dont have a textbook in which i could find out how things work at the design stage.

can you briefly explain me how a compressor is designed?

and suppose if we are at a compressor test facility, with a compressor mounted on a test rig.

so if we want to determine these efficiencies (polytropic and isentropic), how do we do that?
like how the parameters (like pressure ratio, mass flow, temperature ratio etc) are varied and stuff?

4. Dec 26, 2009

mugaliens

If you have $25 to spare, buy Hunecke's Jet Engines: Fundamentals of Theory, Design and Operation. It's 236 pages will cover the design basics. More advanced texts begin around$100, and your local university aerospace professor should be able to guide you in the right direction.

If you want a basic non-technical approach, try this.

If you want a more detailed, and military, but still conceptual approach, try this.

* If you want a fairly realistic, 500+ page hands-on approach to designing a jet engine from RFP to delivery, try this.

** If you want an aerospace engineering text on the mechanics and thermodynamics of propulsion (both jets and rockets), try this.

5. Dec 28, 2009

jason.bourne

hey but i have this one thing to ask:

like in flight condition, at some altitude, suppose if we want to increase the mass flow rate of air, what do we do? like on what parameters does it depend on?

is it done only by increasing the engine RPM?

Last edited: Dec 28, 2009
6. Dec 29, 2009

mugaliens

Engines are optimized for efficiency at cruise altitude, with a little extra oomph built in for climb to that altitude. It's that extra oomph which pilots use to go faster than planned, but doing so uses more fuel, so the only time they're authorized to do so is if passengers missing connecting flights would prove more expensive to the airlines than the cost of the extra fuel used.

To increase the mass flow rate, simply advance the throttles. This introduces more fuel, which adds more energy to the system, resulting in greater rpm, and greater thrust.

As I said, however, there's a limit, as engines are bordering on stall for max efficiency, anyway, so there's not a lot of room for additional thrust.

7. Dec 29, 2009

jason.bourne

okay. i got it.
thanks mugaliens