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12 Cards in this Set
- Front
- Back
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What are the air-standard assumptions?
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1. The working fluid is air as an ideal gas
2. Combustion is modeled heat addition from an outside source 3. Exhaust is modeled as heat rejection that restores the fluid to its initial state 4. All processes are internally reversible |
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For fixed maximum and minimum temperatures, what is the effect of the pressure ratio on (a) the thermal efficiency and (b) the net work output of a simple, open, ideal Brayton cycle? |
Higher pressure ratio translates to a higher thermal efficiency. The net work is increased by increased pressure ratio, reaches a maximum, then decreases. |
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What is the back work ratio? What are the typical back work ratio values for gas-turbines? |
Back Ratio is the ratio of compressor or pump work input to turbine work output. Typically between 0.4 and 0.6 |
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How do the inefficiencies of the turbine and compressor affect (a) the back work ratio and (b) the thermal efficiency of a gas-turbine. |
Back work ratio increases. Thermal efficiencies decrease. |
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How does regeneration affect the efficiency of an open, ideal Brayton cycle and how does it accomplish it? |
Regeneration increases the efficiency by sending the exhaust from the turbine through a heat exchanger to increase the temperature of the working fluid entering the combustion chamber. |
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Somebody claims that at very high pressure ratios, the use of regeneration actually decreases the thermal efficiency of a gas-turbine. Explain clearly! |
Ideal Brayton Cycles with pressure ratios greater than 11 experience decreased efficiency when regeneration is used. At high pressure ratios the temperature out of the turbine is less than the temperature exiting the compressor and efficiency decreases. |
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Is the effect of the turbine and the compressor irreversibilities of an open Brayton cycle to reduce (a) the net work, (b) the thrust, or (c) the fuel consumption rate? |
The net work |
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How does the ideal jet-propulsion cycle differ from the ideal Brayton cycle? |
The ideal Brayton cycle is maximizing work output.
The ideal jet propulsion cycle maximizes thrust or velocity out the turbine. |
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What is propulsive efficiency? How is it determined? |
Thrust*Velocity/Q_dot_in |
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Is the effect of the turbine and compressor irreversibilities of a turbojet engine to reduce (a) the net work, (b) the thrust, or (c) the fuel consumption rate? |
GROSS SAID NOT ON THE QUIZ: But if you’re curious, we don’t know without a full analysis. On one hand the increased heat could help, but decreased efficiency hurts. |
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What are the advantages or disadvantages of absorption refrigeration?
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Advantages: ideal for situations with cheap or free heat sources (steam, chemical), less work required (no compressor) less maintenance (no compressor) running costs, no foundation required to hold compressor.
Disadvantages: complex, expensive, low working pressure, high heat rejection. |
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In absorption refrigeration systems, why is the fluid in the absorber cooled, and the fluid in the generator heated? |
More ammonia can be dissolved with lower temperatures. The ammonia dissolving in solution is exothermic, so the absorber is kept cold with circulating water.
In the generator, heat transfer to the solution drives the ammonia out of the solution causing an endothermic process. |
