performance

Question 1

what is isentropic

Answer 1

constant entropy, -. pressure temperature ratio stays the same

Question 2

what is isobar?

Answer 2

constant pressure

Question 3

what are key differences in the real and ideal joule cycles

Answer 3

real: entropy always increases
- temp increases to higher at point 3
- same curve on 3->4

Question 4

what are assumptions for the real joule cycle? (3)

Answer 4

• inviscid flow
• ideal gas everywhere
• complete expansion in nozzle

Question 5

what is a total pressure ratio?

Answer 5

pi

total pressure,out/total pressure,in

Question 6

are pi and tau relations of static or total pressure and temp

Answer 6

total

Question 7

what values are dimensionless total temp are pressure referenced to

Answer 7

static temp and pressure outside free stream. (position zero)

Question 8

what does specific trust depend on for

Answer 8

• flight mach number
• temp rise (or pressure) rise in compressor -> cycle efficiency
• turbine entry temp -> energy addition in cycle

Question 9

what does specific trust depend on for turbijet

Answer 9

• flight mach number
• temp rise (or pressure) rise in compressor -> cycle efficiency
• turbine entry temp -> energy addition in cycle

Question 10

what is a trend that has resulted in turbojet engines from the results of the three specific thrust equation dependencies

Answer 10

higher material and cooling costs for the turbine allow higher turbine entry temps

Question 11

what is the conditions for maximum specific thrust of a turbojet

Answer 11

for a specific turbine entry temperature and flight mach number there is a given compression ratio to maximize static thrust

Question 12

what is the conditions for maximum specific thrust of a turbojet

Answer 12

for a specific turbine entry temperature and flight mach number there is a given compression ratio to maximize static thrust

Question 13

what is the condition for the maximum specific impulse of a turbojet engine?

Answer 13

no condition , no maximum

Question 14

what are (5) sources of losses in real cycles

Answer 14

• friction losses in flow
• incomplete expansion
• incomplete combustion
• gas constants change (eg heat capacity ratio)
• bleed air

Question 15

what is the inconsistant ratio of heat capacities lead to nonideal

Answer 15

cp varies a lot with heat. cv as well but not as much. leads to varying kappa

Question 16

what happens in the boundary layer that leads to losses from viscous flow?

Answer 16

no slip condition at wall -> in boundary layer there is flow losses and total pressure loss

Question 17

where can frictional losses come from?

Answer 17

• boundary layer

- shocks (supersonic)

Question 18

how do frictional losses affect S-T diagram

Answer 18

“isotropic” rxns actually increase entropy with temperature change.

Question 19

where do losses come from in the burner?

Answer 19

• incomplete combustion (normally 95-99%)

- viscous losses (strong because of mixing) (represented by the pressure ratio (pi b)

Question 20

where do losses in the nozzle come from

Answer 20

• viscous effect of flow moving over outlet

- expansion such that pressure does not equal outside pressure

Question 21

what direction does the air go in over, under and ideal expanded flow? Where are the shocks

Answer 21

under - out from nozle
over - in from nozzle
ideal - straight out

Question 22

where can bleed air go?

Answer 22

• turbine cooling
• anti-icing
• aircon systme

Question 23

what is the effect of bleed air on the cycle?

Answer 23

very detrimental

Question 24

what amount of core mass flow is bleed?

Answer 24

15-25%

Question 25

what type of nozzle is necessary at the different mach number levels of turbojets

Answer 25

ma < 1 convergent sufficient
ma > 1 increasing influence of losses to thrust
ma > 1.25 convergent-divergent nozzle necessary

Question 26

how does the importance of equal jet velocities and component losses vary with increasing bypass ratios in turbofan engines

Answer 26

increasing bypass ratio gives:
decreased importance of equal jet velocities
increased importance of component efficiencies