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
How to increase heat transfer in blade cooling
make airflow turbulent
What are NGV
Nozzle guide vanes:
- just after the combustor
- do not rotate
- this means less stresses so higher temp possible
- oxidation often the biggest problem
How do high rotational speeds effect stresses and stage loading
Higher rotational speed:
- increase stresses
- reduce stage loading
- also cooler temperatures seen as V2 seen
List 3 cooling methods
- impingement
- passage
- film cooling
What is impingement cooling
- internally air is drawn in at high compression and blasted onto the side walls of the turbine blade
What is passage cooling
- high turbulent air is passed through tubes within the blade
What is film cooling
- cooler air then exhaust is bled over the blade to create a barrier
Give equation for cooling efficiency
efficiency = Tcool_out - Tcool_in / Tblade - Tcool_in
Give equation for cooling effectiveness
effectiveness = Tgas - Tblade / Tgas - Tcool_in
What is the compromise when trying to reduce pressure loss
- reduce pressure loss by decrease speed
- decrease speed by increase area
- increase area means longer blades
- longer blades mean more cooling and more friction and more likely to break-down aerodynamics
What are the penalties for choosing to cool blades
- Cost of manufacturing blades
- more high pressure air required from compressor (6.5%)
- film cooling disrupts the boundary layers, potentially increasing the profile losses
Describe how varying where airflow is ejected from blades changes the situation
ejecting from tip:
- requires additional work which reduces total power output
- also a waste but mechanical integrity is not always possible to eject elsewhere
ejecting from trailing edge:
- coolant flow will have been turned in the rotor cooling passages and will thus do some useful work
- reduced entropy generation due to lower mixing losses
- will mean trailing edge design has to be thicker however meaning a poorer wake
- ejecting coolant here fills in that wake and ideally coolant velocity = mean velocity
