10 Questions
What is the primary reason for operating gas turbines at four times the air needed for complete combustion?
To reduce excessive temperatures
What happens to the working fluid temperature when intercooling and reheating are utilized?
It decreases at the compressor and increases at the turbine
Why does the back work ratio of a gas-turbine cycle improve with intercooling and reheating?
Because the regeneration potential increases
What is the effect of intercooling and reheating on the thermal efficiency of a gas-turbine cycle?
It decreases the thermal efficiency unless accompanied by regeneration
What is the ideal limit of thermal efficiency that can be approached by a gas-turbine cycle with intercooling, reheating, and regeneration?
The Carnot efficiency
What is the primary goal of multistage compression with intercooling in a gas-turbine cycle?
To decrease the compression work by reducing the specific volume of the fluid
What happens to the compression process as the number of stages is increased?
The compression process becomes isothermal at the compressor inlet temperature
What is the effect of reheating on the expansion process in a gas-turbine cycle?
It increases the expansion work output without raising the maximum temperature
What is the principle behind the effect of multistage compression with intercooling and multistage expansion with reheating on the net work output?
The steady-flow work is proportional to the specific volume of the fluid
What is the ultimate goal of optimizing the compression and expansion processes in a gas-turbine cycle?
To increase the net work output of the cycle
Study Notes
Net Work of a Gas-Turbine Cycle
- Net work is the difference between turbine work output and compressor work input
- Net work can be increased by decreasing compressor work, increasing turbine work, or both
Multistage Compression with Intercooling
- Compression work can be decreased by carrying out compression in stages and cooling the gas in between
- As the number of stages increases, the compression process becomes nearly isothermal at the compressor inlet temperature
- Compression work decreases as the number of stages increases
Multistage Expansion with Reheating
- Turbine work output can be increased by expanding the gas in stages and reheating it in between
- Expansion process becomes nearly isothermal as the number of stages increases
- Maximum temperature in the cycle remains unchanged
Principle of Work
- Steady-flow compression or expansion work is proportional to the specific volume of the fluid
- Specific volume of the working fluid should be as low as possible during compression and as high as possible during expansion
- Intercooling and reheating accomplish this by reducing and increasing specific volume, respectively
Combustion in Gas Turbines
- Combustion typically occurs with four times the amount of air needed for complete combustion to avoid excessive temperatures
- Exhaust gases are rich in oxygen, allowing reheating by spraying additional fuel between expansion states
Effects of Intercooling and Reheating
- Working fluid leaves the compressor at a lower temperature and the turbine at a higher temperature
- Makes regeneration more attractive and increases potential for regeneration
- Back work ratio of a gas-turbine cycle improves
- Thermal efficiency decreases unless accompanied by regeneration
Ideal Gas-Turbine Cycle
- Ideal cycle with intercooling, reheating, and regeneration approaches the Ericsson cycle
- Thermal efficiency approaches the theoretical limit (Carnot efficiency) as the number of stages increases
- However, the contribution of each additional stage to thermal efficiency is less and less, and more than two or three stages are not economically justified
This quiz covers the principles of optimizing gas-turbine cycle network by decreasing compressor work or increasing turbine work, including multistage compression with intercooling. It's based on Chapter 7 and Fig. 9-42.
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