Axial Turbines and Combustion Flashcards
Describe the basic design features of an axial turbine
- turns flow away from engine axis
- flow area decreases
- average velocity increase
- average pressure decreases
- extracts energy from hot gases and drives the compressor
Give differences between inpulse and reaction turbine
Impulse:
all pressure drops is in the nozzle effect on blades is momentum only
Reaction:
Pressure drop in both rotor and stator
How does Cw vary between turbine and compressor
Cw is greater in Turbine
What is stage loading
defines how much work can be obtained from a stage
work increases usually means efficiency decrease
What does a high value flow coefficient mean
Flow coefficient = Cx / U
high flow coefficient - low pitch-chord ratio
low flow coefficient - high pitch-chord ratio
What does a Smith chart show
temperature drop coefficient vs. flow coefficient
- used as a guide for a new turbine design
List requirements for combustors
- high combustor efficiency (fuel burnt and heat liberated)
- reliable smooth ignition
- wide stability limits
- free from pressure pulsations
- low pressure loss
- good outlet temp traverse (blade and nozzle life)
- min. cost and ease of maintencance
- durability
- size constraints
What is the purpose of a baffle
used to create a flow reversal and recirculation region in which a flame can be sheltered and sustained
How is pressure loss reduced in comrpessor
air flow is decelerated using a diffuser
reduced by a factor of 5
- optimal diffuser 7-12°
- frictional losses
- short diffusers are required
What is an approximate AFR
AFR - Air-Fuel Ratio
around 35:1
- outside the flammability limits so air is introduced in stages
Why is air introduced in stages in a combustor
Air-Fuel ratio is outside the flammability limits so introducing in stages allows for ignition
What is the cold pressure loss
this is the combined pressure losses due to the diffuser and air-fuel holes
List advantages and disadvantages of tubular combustion system
A:
- mechanically robust
- fuel and airflow patterns easily matched
- rig testing easy small fraction of engine flow
D:
- Bulky and heavy
List advantages and disadvantages of annular combustor
A:
- min. length and weight
- min. engine frontal area
- min. pressure loss
- easy light around
D:
- outer liner buckling loads
- rig test require high airflows
- difficult to match fuel and air patterns
- inlet velocity profiles can effect outlet traverse
List advantages and disadvantages of tubo-annular combustion system
A:
- mechanically robust
- fuel and air patterns easily matched
- rig testing easy small fraction of engine flow
- low pressure loss
- shorted and lighter than tubular
D:
- less compact than annular
- requires interconnectors
- cooling requirements
List the stages of initial to a developed spray
- dribble
- distorted pencil
- onion stage
- tulip stage
- fully developed spray
How does blade stress loading vary to accommodate the varying temperatures seen
tip - temp low
med - temp high
root - temp low
There blade stress loading decreases from hub to tip so can allow temperatures up the span
List alternative fuels
- Biofuel
- Hydrogen
- Methane
- Ethanol
List reasons why Biofuel not used
- argued that this would reduce global warming
- risk of food production reduced
- emission would still be there
List reasons for hydrogen fuel
- emissions are just water vapour
- density too small however
Why is kerosene used
- easy to handle
- stable within a wide range of temperatures
- highly calorific
What is the reason for cooling turbine blades
- so blades can cope with increase in combustor outlet
- this is so efficiency increases by having a higher specific work out
How much can cooling decrease the temp of blade by
200-300K
How are blades made
cast - single uni-directional crystal
or
forged
