Flashcards in Objective 06: The Brayton Cycle Deck (21)
What is another name for the Brayton Cycle?
What does the Brayton Cycle apply to?
How does the Brayton Cycle work?
1. Air is compressed in a compressor
2. Fuel is added and burned in a combustor
3. The hot gases leaving the combustor mixes with secondary compressed air
4. The hot gas/air mixture then expands through a turbine, producing work
Why do the hot gases leaving the combustor mix with secondary compressed air in the Brayton Cycle?
To reduce it to a temperature that the turbine can safely handle
What is the work produced in a Brayton cycle used for?
A portion is used to power the compressor while the remainder is available to drive an electrical generator or a process compressor
Are most gas turbines open cycle design?
Open Cycle Gas Turbines
Hot gases are exhausted to atmosphere from the power turbine
Closed Cycle Gas Turbines
Exhaust Gases are recirculated back to the compressor intake
What is an advantage of Closed Cycle Gas Turbines?
They allow for the use of a thermodynamically superior working fluid, e.g. helium
Why are Open Cycle Gas Turbines more common than Closed Cycle?
They have progressed in efficiency sufficiently that Closed Cycle offers little advantage
Are single, dual and triple shaft configurations for the Brayton Cycle quite common?
What type of process is used by the Brayton Cycle?
Why is the Brayton Cycle characterized as a Steady-Flow Process?
The working fluid flows steadily through the gas turbine (rather than being trapped in a cylinder)
Describe Compression in the Brayton Cycle
Adiabatic and occurs in the compressor
Describe Expansion in the Brayton Cycle
Adiabatic and occurs in the power turbine section
Describe heat addition in the Brayton Cycle
Combustion occurs in the combustor and is continuously occurring at a constant pressure
Describe heat rejection (open cycle) in the Brayton Cycle
Occurs in the external atmosphere when the hot exhaust gases cool to ambient temperature
What kinds of modifications can be made to the gas turbine in the Brayton Cycle to make better use of the heat added?
1. A regenerator used to be used to transfer heat from the power turbine exhaust gases to preheat the compressed air, prior to combustion
2. Intercooling may be used in the compressor to increase efficiency and power output
3. Combined Cycle, where a heat exchanger is added at the turbine exhaust to produce steam to expand through a steam turbine and power an electrical generator or for industrial or commercial heating purposes
What compression ratios are achieved by gas turbines?
Usually between 10:1 and 15:1, although some engines exceed 20:1
Why are high compression ratios difficult to achieve efficiently in the Brayton Cycle?
1. Increased compressor temperatures
2. Metallurgical limit to combustion and turbine components