Gas Turbine Off-Design Flashcards
What is the ideal turbojet cycle
Intake nozzle - isentropic compression
Compressor - isentropic compression
Combustor - constant pressure heat addition
Turbine - isentropic expansion
Propelling nozzle - isentropic expansion
What are the key differences between a aircraft gas turbines and a shaft power gas turbine
The useful power output of an aircraft is produced wholly or mostly by expansion in a propelling nozzle - shaft power gas turbine systems have two turbines, one to drive compressor and one to generate power
Also have to include the effect of forward speed and altitude on the engine performance
What considerations are employed for intakes
To is constant
Po is constant if isentropic
Forward air speed
Considerate pressure increase
Requirements to minimise pressure loss while ensuring uniform pressure and velocity at compressor inlet - to avoid surge and vibrations
Why do mechanical losses occur between turbine and compressor
Bearing friction
Windage
What is the relationship between number of stages and overall isentropic efficiency for compressors and turbines
Compressors - Efficiency decreases as stages increase
Turbines - Efficiency increases as stages increase
What is the role of the combustion chamber
Provide smooth stream of uniformly heated gas to turbine
Requires minimum losses in pressure and maximum heat release
What difficulties occur in combustion chambers
Burning large quantities of fuel
Coping with extensive volume of air
Coping with high speed air
Working at very high temperature
What does off design mean
Often case that one component works reasonably at certain operating conditions while others encounter undesirable conditions - turbine and compressor have to be run at same rotational speed - off design matches values to find the best design point
How can you avoid surge during start-up
Bleed off air from compressor
Increase nozzle area
How does a constant speed propeller work
Has a variable pitch that automatically adjusts to maintain chosen rotational speed meaning gas turbine can function at its optimum operation conditions both thermodynamically and aerodynamically
Why might a gas turbine be combined with a diesel engine
To get optimum operation at a range of operating conditions for a significant time
Why does altitude play a part in gas turbine design
Altitude determines the ambient pressure and temperature
These have strong impact on gas turbine performance
Why is off design important
Variation in SFC with reduced power is important especially where large times are spent at this condition
Give an example of off design
Water sprays injected into gas turbine inlet to cool down intake air on hot day - increases power output
Surplus electricity used to produce ice and applied to cool inlet air during day time - better than having a larger unit whole year round working away from design point
List three gas turbine configurations
Single shaft gas turbine delivering shaft power
Free turbine engine where gas generator turbine drives compressor and power turbine drives load
Simple jet engine with propelling nozzle
What is dimensional analysis
Means of simplifying physical problem by appealing to dimensional homogeneity to reduce number of relevant variables
What are the two non-dimensional parameters
Non dimensional flow
Non dimensional rotational speed
What is equilibrium point matching
Finding corresponding points on characteristics of each component when engine is running at a steady speed or in equilibrium
What is the impact of connected compressor and turbine shafts
Turbine pressure ratio and non-dimensional speed can be used to get turbine efficiency
What is flow compatibility/matching
Checking the compatibility of non-dimensional flow between compressor and turbine
What is work compatibility
Equating the thermal work of the compressor with turbine (including mechanical efficiency)
Summarise the calculations for a gas generator
Select point on compressor characteristic and use values to determine temperature ratio
Guess turbine pressure ratio to obtain non-dimensional mass flow for turbine from turbine characteristics and temperature ratio from flow compatibility
Calculate non-dimensional speed
Find turbine efficiency from turbine characteristic
Non-dimensional temperature drop can be calculated and used to calculate another value of temperature ratio
Compare values of temperature ratio - if they don’t agree guess another value for turbine pressure ratio
Summarise the calculations for a choked propelling nozzle
Calculate pressure ratio and see if nozzle is choked
Choked - pressure and temperature found from critical relations
Perform static matching of gas generator turbine and nozzle - same mass flow between the components and fixed by compressor characteristics and turbine pressure ratio
Compare mass flow from nozzle characteristic with gas generator value - if not the same then compressor characteristic point needs to be changed
Summarise the calculations for an unchoke propelling nozzle
Calculate pressure ratio and see if the nozzle is unchoked
Pressure is the same as atmospheric pressure
Why do we look at forward speed as a function of Mach number
Forward speed increase ram pressure
Increased pressure before nozzle so higher pressure ratio
Nozzle non-dimensional flow maximum when nozzle is choked then independent of pressure ratio (and forward speed)
Turbine unchanged due to flow compatibility
What are the components of thrust
Momentum thrust (change in velocity)
Pressure thrust (change in pressure)
Why is mechanical speed important to engine life
Turbine stresses
Turbine entry temperature
Maximum don-dimensional speed decreases as ambient temp increases so reduced mass flow and pressure ratio (and so red. thrust)
How can fuel consumption dependence on ambient conditions be ignored
Use non-dimensional values based on ambient conditions
What does SFC identify
The efficiency of the engine
Obtained from thrust and fuel consumption curves
Improves with altitude as ambient temperature decreases
What is the specific thrust equation for gas turbine engine
Choked F/m = (u5 - ua) + (1/(rho_c * u5))*(Pc - Pa)
Unchoked F/m = (u5 - ua) — No pressure thrust
What is the work compatibility equation
Cpg * (To3 - To4) = nm * Cpa * (To2 - To1)
What is the general procedure for gas turbine calculations
Start from atmospheric condition and move along each component sequentially i.e. intake -> compressor -> combustor -> turbine -> nozzle
Give example steps for simple gas turbine calculation
Given initial conditions
Find To1 from standard relation
Find Po1 using Po/P relation in LBOTF
Find Po2 from compressor ratio
Find the compressor temperature rise from LBOTF
Find the turbine temperature drop (work matching)
Find turbine inlet pressure (either stated or same as compressor outlet)
Find To4’ from definition of expansion isentropic efficiency
Find Po4 using isentropic flow relation of pressure and temperature ratios
Find nozzle to atmospheric pressure ratio
Find critical pressure ratio and check if choked or unchoked
Find nozzle static temperature
Find nozzle static pressure
Find nozzle density
Find nozzle velocity
Find area/mass flow - A5/m = 1/(rho_c * u5)
Find the specific thrust (pressure in pascals)
Find the theoretical far from combustor temperature rise from LBOTF figure
Find actual far = theoretical/combustor efficiency
Find TSFC = f/Fsp
Give steps for choked nozzle
T5 is critical temperature so found from relation
P5 is critical pressure so found from relation
U5 is the sonic speed
F/m = (u5 - ua) + (1/(rho_c * u5))*(Pc - Pa)
Give steps for unchoked nozzle
To4 - T5 can be found from standard expansion equation
To4 = To5 = Static + Dynamic temperatures
To4 - T5 = (u5^2)/2Cp so can find u5
(u5 - ua) — No pressure thrust
What is the mass flow parameter
mass flow rate * sqrt(T)/P
What is the rotational speed parameter
N/sqrt(RT)
What does compatibility of rotational speeds mean
Compressor and turbine shafts are connected so rotational speeds are the same
N/sqrt(To3) = N/sqrt(To1) * sqrt(To1/To3)
What does flow compatibility mean
Mass flow of air through components is the same
mass flow rate * sqrt(To3)/Po3 = (mass flow rate * sqrt(To1)/Po1) * Po1/Po2 * Po2/Po3 * sqrt(T/To1)
Same can be done to find the turbine exit non-dimensional flow and compare with nozzle flow
What is the nozzle non-dimensional flow
C5 * A5 * rho_5 * sqrt(To4)/Po4
