Objective 06: The Brayton Cycle

Question 1

What is another name for the Brayton Cycle?

Answer 1

Joule Cycle

Question 2

What does the Brayton Cycle apply to?

Answer 2

Gas Turbines

Question 3

How does the Brayton Cycle work?

Answer 3

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

Question 4

Why do the hot gases leaving the combustor mix with secondary compressed air in the Brayton Cycle?

Answer 4

To reduce it to a temperature that the turbine can safely handle

Question 5

What is the work produced in a Brayton cycle used for?

Answer 5

A portion is used to power the compressor while the remainder is available to drive an electrical generator or a process compressor

Question 6

Are most gas turbines open cycle design?

Answer 6

Yes

Question 7

Open Cycle Gas Turbines

Answer 7

Hot gases are exhausted to atmosphere from the power turbine

Question 8

Closed Cycle Gas Turbines

Answer 8

Exhaust Gases are recirculated back to the compressor intake

Question 9

What is an advantage of Closed Cycle Gas Turbines?

Answer 9

They allow for the use of a thermodynamically superior working fluid, e.g. helium

Question 10

Why are Open Cycle Gas Turbines more common than Closed Cycle?

Answer 10

They have progressed in efficiency sufficiently that Closed Cycle offers little advantage

Question 11

Are single, dual and triple shaft configurations for the Brayton Cycle quite common?

Answer 11

Yes

Question 12

What type of process is used by the Brayton Cycle?

Answer 12

Steady-Flow Process

Question 13

Why is the Brayton Cycle characterized as a Steady-Flow Process?

Answer 13

The working fluid flows steadily through the gas turbine (rather than being trapped in a cylinder)

Question 14

Describe Compression in the Brayton Cycle

Answer 14

Adiabatic and occurs in the compressor

Question 15

Describe Expansion in the Brayton Cycle

Answer 15

Adiabatic and occurs in the power turbine section

Question 16

Describe heat addition in the Brayton Cycle

Answer 16

Combustion occurs in the combustor and is continuously occurring at a constant pressure

Question 17

Describe heat rejection (open cycle) in the Brayton Cycle

Answer 17

Occurs in the external atmosphere when the hot exhaust gases cool to ambient temperature

Question 18

What kinds of modifications can be made to the gas turbine in the Brayton Cycle to make better use of the heat added?

Answer 18

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

Question 19

What compression ratios are achieved by gas turbines?

Answer 19

Usually between 10:1 and 15:1, although some engines exceed 20:1

Question 20

Why are high compression ratios difficult to achieve efficiently in the Brayton Cycle?

Answer 20

1. Increased compressor temperatures

2. Metallurgical limit to combustion and turbine components

Question 21

What is the efficiency of the majority of open cycle gas turbines?

Answer 21

30-35%