Deck 9: Gas Power Cycles

Question 1

An Otto cycle with air as the working fluid has a compression ratio of 8.2. Under cold air standard conditions, the thermal efficiency of this cycle is

A)12%
B)43%
C)52%
D)57%
E)75%

Answer 1

57%

Question 2

For specified limits for the maximum and minimum temperatures, the ideal cycle with the lowest thermal efficiency is

A)Carnot
B)Stirling
C)Ericsson
D)Diesel
E)All are the same

Answer 2

Diesel

Question 3

A Carnot cycle operates between the temperatures limits of 400 K and 1600 K, and produces 3600 kW of net power. The rate of entropy change of the working fluid during the heat addition process is

A)0
B)1.8 kW/K
C)3.0 kW/K
D)2.3 kW/K
E)9.0 kW/K

Answer 3

3.0 kW/K

Question 4

Air in an ideal Diesel cycle is compressed from 4 L to 0.25 L, and then it expands during the constant pressure heat addition process to 0.50 L. Under cold air standard conditions, the thermal efficiency of this cycle is

A)49.0%
B)38.6%
C)61.4%
D)67.0%
E)79.5%

Question 5

Helium in an ideal Diesel cycle is compressed from 4 L to 0.25 L, and then it expands during the constant pressure heat addition process to 0.50 L. Under air standard conditions, the thermal efficiency of this cycle is

A)20.5%
B)84.3%
C)79.5%
D)61.4%
E)67.4%

Question 6

Helium gas in an ideal Otto cycle is compressed from 12<°C and 2 L to 0.25 L, and its temperature increases by an additional 800°C during the heat addition process. The temperature of helium before the expansion process is

A)1668°C
B)1182°C
C)1941°C
D)812°C
E)575°C

Question 7

In an ideal Otto cycle, air is compressed from 1.15 kg/m³ and 2.2 L to 0.26 L, and the net work output of the cycle is 620 kJ/kg. The mean effective pressure (MEP) for this cycle is

A)713 kPa
B)807 kPa
C)638 kPa
D)96 kPa
E)367 kPa

Question 8

In an ideal Brayton cycle, air is compressed from 84 kPa and 25°C to 840 kPa. Under cold air standard conditions, the thermal efficiency of this cycle is

A)48%
B)52%
C)10%
D)60%
E)74%

Question 9

Consider an ideal Brayton cycle executed between the pressure limits of 1100 kPa and 100 kPa and temperature limits of 20°C and 1200°C with argon as the working fluid. The net work output of the cycle is

A)136 kJ/kg
B)96 kJ/kg
C)227 kJ/kg
D)439 kJ/kg
E)456 kJ/kg

Question 10

An ideal Brayton cycle has a net work output of 220 kJ/kg and a backwork ratio of 0.4. If both the turbine and the compressor had an isentropic efficiency of 75%, the net work output of the cycle would be

A)165 kJ/kg
B)79 kJ/kg
C)293 kJ/kg
D)379 kJ/kg
E)182 kJ/kg

Question 11

In an ideal Brayton cycle, air is compressed from 100 kPa and 25°C to 800 kPa, and then heated to 1200°C before entering the turbine. Under cold air standard conditions, the air temperature at the turbine exit is

A)540°C
B)184°C
C)150°C
D)813°C
E)825°C

Question 12

In an ideal Brayton cycle with regeneration, argon gas is compressed from 100 kPa and 25°C to 600 kPa, and then heated to 1200°C before entering the turbine. The highest temperature that argon can be heated in the regenerator is

A)337°C
B)246°C
C)586°C
D)719°C
E)446°C

Question 13

In an ideal Brayton cycle with regeneration, air is compressed from 80 kPa and 10°C to 400 kPa and 180°C, is heated to 450°C in the regenerator, and then further heated to 1200°C before entering the turbine. Under cold air standard conditions, the effectiveness of the regenerator is

A)26%
B)42%
C)65%
D)57%
E)89%

Question 14

Consider a gas turbine that has a pressure ratio of 8 and operates on the Brayton cycle with regeneration between the temperature limits of 20°C and 900°C. If the specific heat ratio of the working fluid is 1.33, the highest thermal efficiency this gas turbine can have is

A)42%
B)49%
C)58%
D)63%
E)96%

Question 15

An ideal gas turbine cycle with many stages of compression and expansion and a regenerator of 100 percent effectiveness has an overall pressure ratio of 8. Air enters every stage of compressor at 290 K, and every stage of turbine at 1500 K. The thermal efficiency of this gas-turbine cycle is

A)19%
B)45%
C)65%
D)100%
E)81%

Question 16

Air enters a turbojet engine at 180 m/s at a rate of 20 kg/s, and exits at 850 m/s relative to the aircraft. The thrust developed by the engine is

A)17 kN
B)13 kN
C)0.8 kN
D)22 kN
E)34 kN