# Isentropic efficiency vs polytropic efficiency of a gas turbine

• jason.bourne
In summary, the conversation discussed the difference between isentropic and polytropic efficiencies, as well as how they work in practice. It was recommended to refer to textbooks for a better understanding and explanation of compressor design and determining efficiencies. It was also mentioned that in flight conditions, increasing the mass flow rate can be done by advancing the throttles, but there is a limit due to engines being optimized for efficiency at cruise altitude.
jason.bourne
definition wise i got the difference between these two efficiencies.

but in practice how does these two differ?

i was referring 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?

jason.bourne said:
definition wise i got the difference between these two efficiencies.

but in practice how does these two differ?

i was referring 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?

When you look at a turbine's http://en.wikipedia.org/wiki/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 http://www.wikipedia.org" ...

The http://en.wikipedia.org/wiki/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 http://en.wikipedia.org/wiki/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.

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i don't 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?

If you have $25 to spare, buy https://www.amazon.com/dp/1853108340/?tag=pfamazon01-20: 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, https://www.amazon.com/dp/1852606185/?tag=pfamazon01-20.

If you want a more detailed, and military, but still conceptual approach, https://www.amazon.com/dp/1423552660/?tag=pfamazon01-20.

* If you want a fairly realistic, 500+ page hands-on approach to designing a jet engine from RFP to delivery, https://www.amazon.com/dp/1563475383/?tag=pfamazon01-20.

** If you want an aerospace engineering text on the mechanics and thermodynamics of propulsion (both jets and rockets), https://www.amazon.com/dp/0201146592/?tag=pfamazon01-20.

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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?

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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.

okay. i got it.
thanks mugaliens

## What is the difference between isentropic efficiency and polytropic efficiency of a gas turbine?

Isentropic efficiency is a measure of how efficiently a gas turbine converts the energy of the incoming air into mechanical work, assuming that the process is reversible and adiabatic. Polytropic efficiency takes into account the effects of non-idealities, such as friction and heat transfer, on the efficiency of the gas turbine.

## Which efficiency is more commonly used in gas turbine performance calculations?

Isentropic efficiency is more commonly used in gas turbine performance calculations because it is a simpler and more idealized measure of efficiency. However, polytropic efficiency may be more accurate in real-world applications as it accounts for non-ideal conditions.

## How do isentropic efficiency and polytropic efficiency affect the performance of a gas turbine?

A higher isentropic efficiency means that the gas turbine is able to convert a larger portion of the incoming air's energy into mechanical work, resulting in higher power output. A higher polytropic efficiency indicates that the gas turbine is operating more efficiently, with less energy lost to non-idealities.

## What factors can influence the isentropic efficiency and polytropic efficiency of a gas turbine?

The design and operating conditions of the gas turbine, such as inlet temperature and pressure, can greatly affect both isentropic and polytropic efficiency. Additionally, the condition and maintenance of the compressor and turbine blades, as well as the quality of the combustion process, can also impact efficiency.

## Can the isentropic efficiency and polytropic efficiency of a gas turbine be improved?

Yes, the efficiency of a gas turbine can be improved through design modifications, such as increasing the number of compressor and turbine stages, or through improved maintenance and operational practices. However, there are physical limitations to how much efficiency can be improved.

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