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A closed loop ideal Brayton cycle is being designed to operate on water vapor.
A closed loop ideal Brayton cycle is being designed to operate on water vapor. It is hypothesized that since water vapor has a high heat capacity and density the cycle efficiency will improve over air. The maximum temperature for the cycle is limited to 1000◦C while a pressure ratio of 10 has been selected. The inlet of the compressor (state 1) will need to be 50◦C to reject heat to the surroundings at 30◦C. The pressure at state 1 is 125kPa to prevent surrounding air from entering the system in the event of a leak. Unfortunately, water would like to be a liquid at these conditions, and Brayton cycles do not like liquids. To deal with this it is proposed that the water vapor can be mixed with another gas to form a gas-vapor mixture where the water will not reach the dew point in the cycle.
- What gas can carry the most water vapor at the state 1 conditions (i.e. has the highest ratio of water vapor mass to carrier gas mass)?
- Regardless of your answer to the previous question, helium has been selected as the carrier gas for water vapor. Can this mixture be treated as an ideal gas everywhere in the cycle? (calculate the compressibility factor at each state to answer)
- Ignoring the compressibility factors, treat the mixture as an ideal gas. What is the thermal efficiency of the cycle?
- What would the efficiency be using pure helium?
