Thermodynamics

9–137 Repeat Problem 9–136 using constant specific heatsat room temperature.9–138 A Carnot cycle is executed in a closed system anduses 0.0025 kg of air as the working fluid. The cycle efficiencyis 60 percent, and the lowest temperature in the cycleis 300 K. The pressure at the beginning of the isentropicexpansion is 700 kPa, and at the end of the isentropic compressionit is 1 MPa. Determine the net work output per cycle.9–139 A four-cylinder spark-ignition engine has acompression ratio of 8, and each cylinder has amaximum volume of 0.6 L. At the beginning of the compressionprocess, the air is at 98 kPa and 17°C, and the maximumtemperature in the cycle is 1800 K. Assuming the engine tooperate on the ideal Otto cycle, determine (a) the amount ofheat supplied per cylinder, (b) the thermal efficiency, and(c) the number of revolutions per minute required for a netpower output of 60 kW. Assume variable specific heats for air.9–140 Reconsider Problem 9–139. Using EES (orother) software, study the effect of varying thecompression ratio from 5 to 11 on the net work done andthe efficiency of the cycle. Plot the P-v and T-s diagrams forthe cycle, and discuss the results.9–141 An ideal Otto cycle has a compression ratio of 9.2and uses air as the working fluid. At the beginning of thecompression process, air is at 98 kPa and 27°C. The pressureis doubled during the constant-volume heat-addition process.Accounting for the variation of specific heats with temperature,determine (a) the amount of heat transferred to the air,(b) the net work output, (c) the thermal efficiency, and (d) themean effective pressure for the cycle.9–142 Repeat Problem 9–141 using constant specific heatsat room temperature.9–143 Consider an engine operating on the ideal Dieselcycle with air as the working fluid. The volume of the cylinderis 1200 cm 3 at the beginning of the compression process,75 cm 3 at the end, and 150 cm 3 after the heat-additionprocess. Air is at 17°C and 100 kPa at the beginning of thecompression process. Determine (a) the pressure at the beginningof the heat-rejection process, (b) the net work per cycle,in kJ, and (c) the mean effective pressure.9–144 Repeat Problem 9–143 using argon as the workingfluid.9–145E An ideal dual cycle has a compression ratio of 12and uses air as the working fluid. At the beginning of thecompression process, air is at 14.7 psia and 90°F, and occupiesa volume of 75 in 3 . During the heat-addition process,0.3 Btu of heat is transferred to air at constant volume and1.1 Btu at constant pressure. Using constant specific heatsevaluated at room temperature, determine the thermal efficiencyof the cycle.9–146 Consider an ideal Stirling cycle using air as the workingfluid. Air is at 350 K and 200 kPa at the beginning of theChapter 9 | 547isothermal compression process, and heat is supplied to airfrom a source at 1800 K in the amount of 900 kJ/kg. Determine(a) the maximum pressure in the cycle and (b) the network output per unit mass of air. Answers: (a) 5873 kPa,(b) 725 kJ/kg9–147 Consider a simple ideal Brayton cycle with air as theworking fluid. The pressure ratio of the cycle is 6, and theminimum and maximum temperatures are 300 and 1300 K,respectively. Now the pressure ratio is doubled without changingthe minimum and maximum temperatures in the cycle.Determine the change in (a) the net work output per unitmass and (b) the thermal efficiency of the cycle as a result ofthis modification. Assume variable specific heats for air.Answers: (a) 41.5 kJ/kg, (b) 10.6 percent9–148 Repeat Problem 9–147 using constant specific heatsat room temperature.9–149 Helium is used as the working fluid in a Braytoncycle with regeneration. The pressure ratio of the cycle is 8,the compressor inlet temperature is 300 K, and the turbineinlet temperature is 1800 K. The effectiveness of the regeneratoris 75 percent. Determine the thermal efficiency and therequired mass flow rate of helium for a net power output of60 MW, assuming both the compressor and the turbine havean isentropic efficiency of (a) 100 percent and (b) 80 percent.9–150 A gas-turbine engine with regeneration operates withtwo stages of compression and two stages of expansion. Thepressure ratio across each stage of the compressor and turbineis 3.5. The air enters each stage of the compressor at300 K and each stage of the turbine at 1200 K. The compressorand turbine efficiencies are 78 and 86 percent, respectively,and the effectiveness of the regenerator is 72 percent.Determine the back work ratio and the thermal efficiency ofthe cycle, assuming constant specific heats for air at roomtemperature. Answers: 53.2 percent, 39.2 percent9–151 Reconsider Problem 9–150. Using EES (orother) software, study the effects of varying theisentropic efficiencies for the compressor and turbine andregenerator effectiveness on net work done and the heat suppliedto the cycle for the variable specific heat case. Let theisentropic efficiencies and the effectiveness vary from 70 percentto 90 percent. Plot the T-s diagram for the cycle.9–152 Repeat Problem 9–150 using helium as the workingfluid.9–153 Consider the ideal regenerative Brayton cycle. Determinethe pressure ratio that maximizes the thermal efficiencyof the cycle and compare this value with the pressure ratiothat maximizes the cycle net work. For the same maximumto-minimumtemperature ratios, explain why the pressureratio for maximum efficiency is less than the pressure ratiofor maximum work.9–154 Consider an ideal gas-turbine cycle with one stage ofcompression and two stages of expansion and regeneration.