Gas Turbine Systems Flashcards
What is the simple definition of a gas turbine?
An internal combustion engine consisting of an air compressor, a combustor chamber and a turbine.
What are the assumptions for the ideal simple gas turbine cycle?
- Steady processes
- Isentropic compression and expansion
- No pressure loss
- Constant specific heat and specific heat ratio
- Working fluid is ideal gas
- Added amount of fuel is neglected in energy balance relations
- Efficiencies for compression and expansion are unity
What does the first law of thermodynamics state?
The energy flowing into a system equals the energy flow out of the system
What forms can the conversed energy from first law of thermodynamics show up as?
Chemical energy
Kinetic energy
Potential energy
Mechanical energy
Heat
What is the key to high thermal efficiency and high specific work output?
Thermal: high pressure ratio
Work: high turbine inlet temperature
What are the types of industrial gas turbines?
- Single shaft engine
- Multiple shaft engine
What is a gas generator?
The compressor, the combustion chamber and the turbine makes the gas generator
What are numbering of stations used for?
Keeping track of the different components in a gas turbine engine when performing calculations
What are advantages to industrial gas turbines?
- No limits in size and weigh, unlike aero-engines
- Nearly zero exhaust speed
- About 100k operating hours between overhauls
- Compactness for a given power output
- Low vibration
- Reliable: low dynamic stresses
- Low cost per kg at larger sizes
- Low emissions, low installation costs, low-cost power generation
- Automatically changes from primary to secondary fuel while still operating
- Easy to transport and made mobile for certain applications
What are disadvantages to industrial gas turbines?
- Lower thermal efficiency compared to diesel engines
- Relatively poor dynamic performance
What is the principle of heat exchange cycle/recuperation?
High pressure air leaving the compresor is heated by transferring heat from the hot exhaust gases in a counterflow exchanger
What is the principle of the reheated cycle?
- Splitting expansion over a HP and LP turbine
- Reheating air between the HPT and the LPT to increase specific work
Where is the optimum reheating point in the flow?
When the pressure ratios of the HPT and the LPT are equal
How is the efficiency reduction in a reheated cycle overcome?
By adding a heat exchanger between the compressor and the combustor chamber.
- The hot exhaust gases are utilized in the heat exchanger
- The increase in work output is not offset by the increase in heat supply
- Efficiency is higher when the reheat is used together with a heat exchanger
What is the princple of the intercooled compression cycle?
- Splitting compression into a LP and HP compressor
- Intercooling the gas flow between the LPC and HPC
- Cycle efficiency is only increased if a heat exchanger is employed at the same time as the intercooler
When is the maximum effect of intercooling achieved?
When the pressure ratios of the LPC and the HPC are equal
What are disadvantages to intercooling?
Bulky and need large amount of cooling water
What are the assumptions of the real gas turbine cycle?
- Adiabatic flow (mechanical heat transfer magnitudes larger than heat transfer through wall)
- Ideal gas as working fluid
- Specific heat and specific heat ratio is dependent on and varies with temperature
- Added and extracted mass flows must be considered
- Efficiencies for compression and expansion are less than unity
- Polytropic compression and expansion
- Irreversible processes
- Most processes are unsteady
What is entropy and why is it caused?
A non-recoverable energy loss that occurs in all real cycles.
It is caused by molecular disorder in the fluid of the flow.
What are the different categories of gas turbines?
- Heavy-duty gas turbines
- Aircraft-derivatives: originally aircraft engines adapted for electricity generation
- Industrial-type gas turbines: extensively used in petro-chemical plants for compressor drive trains
- Small gasturbines: usually centrifugal compressors and radial inflow turbines
- Micro-turbines
What is a free power turbine?
The engine has two turbines, one that drives the compressor and the free turbine that drives the load and is completely disconnected from the compressor
