Turbofans Flashcards
(17 cards)
Why are turbofan engines more efficient than similar turbo jet engines?
If the net work produced by the engine core is only put into the kinetic energy of a jet, the jet velocity would be very high and consequently propulsive efficiency would be low.
The way around this is to use some or all of the available energy of the flow out of the core to drive a turbine which is used to move a much larger mass flow of air, either by a propeller or a fan in a duct.
A turbofan engine has a large fan at the front, which pressurizes a large volume of air, part of which bypasses around the engine core.
The objective is to increase thrust without increasing fuel consumption.
It achieves this by increasing the total air mass flow, while not having to burn fuel to heat the bypass air in the combustion chamber.
Describe how turbine blades are designed to withstand severe temperatures.
Turbine blades must meet demanding requirements: resistance to high temperatures, high fatigue strength, good creep resistance qualities, corrosion resistance, and manufacturing expense.
Internal Cooling:
- A continuous flow of cooling air is routed (after passing through compressor stages) to flow internally through the turbine blade and into the main gas stream.
Film Cooling:
- Cooling air or fluid flows through the blade and out through drilled holes along the leading edge, trailing edge, and/or the face of the blade. The expelled air forms a film or cooled boundary layer around the turbine blade which lowers the heat transfer rate to the blade and its surface temperature.
Transpiration Cooling:
- The blade is constructed with a porous wall, which allows cooling fluid or air to leak out all over the entire surface of the blade. This is like film cooling except there are no holes and cooling is over the entire surface.
Thermal Barrier Coatings:
- Blades are coated with a high temperature ceramic that insulates the metal, reducing the heat transfer rate and surface temperature.
What is the pressure recovery?
Defined as the ratio of the total pressures before and after the shock
Describe why solid rockets are more often used for air-launched missiles than liquid rockets.
Advantages of solid rockets are:
Ease of maintenance (since they have no moving parts like pumps or complex feed systems)
Only simple igniters are required
They can generally be stored for up to 20+ years
They are generally smaller than liquid rockets. For these reasons, solid rockets are more suitable for air-launched missiles than liquid rockets.
Describe why liquid rockets are more often used in large, ballistic missiles and space launchers than solid rockets.
Liquid rockets are more powerful than solid rockets in terms of both gross thrust and specific impulse, and can be throttled.
What flow does a condi nozzle create? What Mach number does this result in?
Condi-nozzle produces a supersonic exit velocity.
Consequently the flow must be sonic at the throat
M* = 1.0
Explain why most propeller-driven commercial aircraft or
transports use turboprop engines instead of piston aerodynamic engines.
Compared with a piston aerodynamic engine, a turboprop can produce a much higher shaft power so it can propel aircraft to much higher speeds.
Turboprops are also more fuel efficient and can operate at higher altitudes
Describe with the aid of diagrams the process of a convergent-divergent (condi) nozzle becoming choked as a result of progressively decreasing the downstream pressure.
Cases (1, 2)
As back pressure (Pb) is lowered, the static pressure through the duct decreases and the mass flow through the duct increases. Flow in both the convergent and
divergent portions of the duct is subsonic and Pe = Pb
.
Cases (3, 4)
This trend continues until the flow at the throat becomes sonic. When M=1.0 at the throat, there are two possible isentropic solutions for a given area ratio (A/A). The
flow can either decelerate to a subsonic exit Mach number, or accelerate to a supersonic exit Mach number. Since the flow through the convergent portion of the
duct cannot be accelerated further to supersonic velocities, the duct becomes choked at all pressure ratios below P*/P0
Case (5, 6)
If back pressure is further reduced (P
What are the assumptions of standard air cycles?
The fluid is represented by a fixed mass of air which can be assumed to be a perfect gas.
The combustion process is replaced by an equivalent heat energy input from an external source
The cycle is completed by a heat transfer to the surroundings which represents the intake and exhaust processes.
All processes are reversible hence second law applies
Why are real spark ignition engines not as efficient as the otto cycle?
Gas specific heat increases with temperature
Incomplete combustion occur
Combustion occurs at a finite rate
Heat transfers occur rapidly causing irreversibility
What are the ISFC and the BSFC?
The Indicated Specific Fuel Consumption
-It is a measure of engine efficiency
Brake Specific Fuel Consumption:
-Is the rate of fuel consumption per unit brake power
What is the cut-off ratio?
The ratio of the volume in a diesel cycle at which the heat supply is cut off (rc)
Why does the practical diesel cycle differ from the ideal one and how can it be better represented?
The corners become rounded off because there is an ignition delay period while fuel droplets evaporate.
Dual cycle contains both constant pressure and constant volume heat addition.
How does irreversibility arise?
Heat transfers across finite temperature differences
Fluid friction which dissipates energy
Which cycles are fully reversible and why?
Carnot
Ericsson
Stirling
Why is it impossible to construct a true Ericsson or Stirling Engine?
It is difficult to achieve isothermal compression or expansion at a reasonable speed
Pressure drops occur
For a regenerator of reasonable size significant temperature differences occur
Which cycle do you use for gas turbines? What are the limitations of these cycles?
Joule cycle
Variation in y around the cycle
Dissociation of combustion products and incomplete combustion
Fluid friction and irresponsibilities
Pressure loss during combustion
