Lecture 3 Flashcards
(16 cards)
What does isentropic efficiency change with?
Pressure ratio
What is polytropic efficiency?
Isentropic efficiency for an elemental stage in a process such that it is constant through the whole process.
Why is isentropic efficiency not that useful sometimes?
It has a different value for every pressure ratio
What happens to the pressure isobars in the T/s diagram?
They diverge
Relationship between change in jet velocity and propulsive efficiency?
As the velocity change through an engine increases, propulsive efficiency decreases.
This isn’t a major issue in military aircraft (eurofighter for eg. has huge deltaC) because priorities are different, compared to the sustainability focus of civil a/c.
Average bypass ratio of turboprop?
25-30
What is thermal efficiency?
Measure of how much Ek is gained by the engine jet relative to the energy contained in the fuel.
What is overall efficiency?
Propulsive efficiency * Thermal efficiency
What is specific fuel consumption?
Mass flow rate of fuel (kg/h) / thrust of engine
What is specific thrust?
Thrust per unit mass of air flow
Want this to be high
What governs overall efficiency of engine?
Pressure ratio
What is specific work output? What’s it a function of?
Excess available enthalpy for thrust.
Function of turbine entry temperature.
What is transmission efficiency?
Effciency that takes into account mechanical losses, like those from bearings and windage.
What properties vary with temperature?
Cp and gamma
When do you want to highest enthalpy in a jet engine?
After the turbine, so that the nozzle can convert this into high Ek.
