Theory Flashcards
Name the two ways a specified thrust force can be produced?
- Small mass of gas and a large increase in velocity
- Large mass of gas and a small increase in velocity
What two assumptions can you make to simplify the thrust equation?
- Mass fuel rate of fuel is approximately 0
- Air and Exit pressure are equal
What are advantages of thrust reversers?
- Reduces landing distance
- Improves ground handling in wet/icy conditions
- Reduce brake and tyre wear (especially in wet/icy conditions)
- Improves rejected take-off margins
- Military: Operate from shorter runways (increases operational flexibility)
What are disadvantages of thrust reversers?
- Hot gases cannot impinge on wind
- Must be impossible to deploy in flight
- Hot gases cannot be re-ingested into the engine
- Need to minimise lift to improve braking performance
What’s the difference between hot and cold stream thrust reversers?
Hot Thrust Reversers: Core/all flow reversed (turbojet/small turbofan)
Cold thrust Reversers: Bypass flow reversed (large turbofan engines)
What’s the definition of compressibility?
The fractional change in volume of a fluid element per unit change in pressure
From what Mach Number does Compressibility become important for high speed flows?
Greater than 0.3
What’s the difference between heat, work and internal energy?
Heat is the form of energy that is transferred due
to a difference in temperature
Work is basically any form of energy transferred that isn’t due to a temperature difference
Internal energy is the energy stored in the form of
excited atoms (function of temperature only for an ideal gas)
What’s the difference between enthalpy and entropy?
Enthalpy is the capacity to do work (function of temperature only for an ideal gas)
Entropy is a measure of the reversibility of a process (J/K)
When is a flow reversible in terms of entropy?
When ∆s=0 when the temperature between two places are equal (Thot=Tcold)
What’s the definition for a steady flow?
No properties within the control volume, at the boundaries of a control volume or heat and work interactions DON’T change with time
What’s the definition for flow work?
Work that is required to push mass into or out of a control volume
What are two reasonable assumptions for aero engines?
- Adiabatic combustion chamber (Q=0)
- No shaft power output (Wshaft =0)
What’s the definition for Propulsive Efficiency?
Propulsive efficiency is a measure of how well an engine produces thrust power relative to the amount of kinetic energy it adds to the airflow.
What’s the definition for Thermal Efficiency (cycle efficiency)?
Thermal efficiency is a measure of how well an engine adds kinetic energy to the airflow relative to the total energy input from the fuel.
What’s the two main ways of improving thermal efficiency?
- Increase overall pressure ratio
- Increase turbine entry temperature
Whats the the definition for overall efficiency?
Combination of propulsive and thermal efficiency
What are the equations for thrust specific fuel consumption and specific thrust?
tsfc = Mf/F
specific thrust = F/Ma
Whats true about disturbances in supersonic flows?
In supersonic flow disturbances cannot propagate upstream
Explain what happens to Area, Pressure and Velocity in a nozzle at subsonic mach number.
dA < 0 area is decreasing
dV > 0 velocity is increasing
dp < 0 pressure is decreasing
Explain what happens to Area, Pressure and Velocity in a Diffuser at subsonic mach number.
dA > 0 area is increasing
dV < 0 velocity is decreasing
dp > 0 pressure is increasing
Explain what happens to Area, Pressure and Velocity in a nozzle at supersonic mach number.
dA > 0 area is increasing
dV > 0 velocity is increasing
dp < 0 pressure is decreasing
Explain what happens to Area, Pressure and Velocity in a Diffuser at supersonic mach number.
dA < 0 area is decreasing
dV < 0 velocity is decreasing
dp > 0 pressure is increasing
When can the mach number be 1 in a nozzle?
When dA=0 so when the duct walls are parallel.
The flow is with the minimum of supersonic, maximum of subsonic or at sonic velocity.
How do oblique shocks come about?
Nozzle exit pressure is lower than for normal shock at exit.
A series of oblique shock waves occur outside the nozzle.
What happens to under expanded flow through a con-di nozzle?
A series of oblique expansion waves outside the nozzle continue the expansion until the external pressure is
reached.
Whats the definition for the degree of reaction?
Measure of the extent to which the rotor contributes to
the overall static enthalpy change in the stage
What type of pressure gradient occurs in a compressor?
Adverse pressure gradient (p2>p1)
Explain what happens if the degree of reaction is 0 in a compressor
- Pressure rise is entirely over stator (pressure drop if turbine)
- Pressure gradient through stator is high (sealing difficult)
- Boundary layer prone to separation
Explain what happens if the degree of reaction is 0.5 in a compressor
- Pressure rise is split evenly between stator and rotor
- Pressure gradient is modest (sealing easy)
- Boundary layer likely to remain attached
What happens across a stator in terms of work done and enthalpy?
No work done across a stator so ∆ho=0 (ho1=ho2)
