performance Flashcards
what is isentropic
constant entropy, -. pressure temperature ratio stays the same
what is isobar?
constant pressure
what are key differences in the real and ideal joule cycles
real: entropy always increases
- temp increases to higher at point 3
- same curve on 3->4
what are assumptions for the real joule cycle? (3)
- inviscid flow
- ideal gas everywhere
- complete expansion in nozzle
what is a total pressure ratio?
pi
total pressure,out/total pressure,in
are pi and tau relations of static or total pressure and temp
total
what values are dimensionless total temp are pressure referenced to
static temp and pressure outside free stream. (position zero)
what does specific trust depend on for
- flight mach number
- temp rise (or pressure) rise in compressor -> cycle efficiency
- turbine entry temp -> energy addition in cycle
what does specific trust depend on for turbijet
- flight mach number
- temp rise (or pressure) rise in compressor -> cycle efficiency
- turbine entry temp -> energy addition in cycle
what is a trend that has resulted in turbojet engines from the results of the three specific thrust equation dependencies
higher material and cooling costs for the turbine allow higher turbine entry temps
what is the conditions for maximum specific thrust of a turbojet
for a specific turbine entry temperature and flight mach number there is a given compression ratio to maximize static thrust
what is the conditions for maximum specific thrust of a turbojet
for a specific turbine entry temperature and flight mach number there is a given compression ratio to maximize static thrust
what is the condition for the maximum specific impulse of a turbojet engine?
no condition , no maximum
what are (5) sources of losses in real cycles
- friction losses in flow
- incomplete expansion
- incomplete combustion
- gas constants change (eg heat capacity ratio)
- bleed air
what is the inconsistant ratio of heat capacities lead to nonideal
cp varies a lot with heat. cv as well but not as much. leads to varying kappa
what happens in the boundary layer that leads to losses from viscous flow?
no slip condition at wall -> in boundary layer there is flow losses and total pressure loss
where can frictional losses come from?
- boundary layer
- shocks (supersonic)
how do frictional losses affect S-T diagram
“isotropic” rxns actually increase entropy with temperature change.
where do losses come from in the burner?
- incomplete combustion (normally 95-99%)
- viscous losses (strong because of mixing) (represented by the pressure ratio (pi b)
where do losses in the nozzle come from
- viscous effect of flow moving over outlet
- expansion such that pressure does not equal outside pressure
what direction does the air go in over, under and ideal expanded flow? Where are the shocks
under - out from nozle
over - in from nozzle
ideal - straight out
where can bleed air go?
- turbine cooling
- anti-icing
- aircon systme
what is the effect of bleed air on the cycle?
very detrimental
what amount of core mass flow is bleed?
15-25%
what type of nozzle is necessary at the different mach number levels of turbojets
ma < 1 convergent sufficient
ma > 1 increasing influence of losses to thrust
ma > 1.25 convergent-divergent nozzle necessary
how does the importance of equal jet velocities and component losses vary with increasing bypass ratios in turbofan engines
increasing bypass ratio gives:
decreased importance of equal jet velocities
increased importance of component efficiencies
