Thermodynamics

550 | Thermodynamics9–179 An ideal Brayton cycle has a net work output of 150kJ/kg and a back work ratio of 0.4. If both the turbine and thecompressor had an isentropic efficiency of 85 percent, the network output of the cycle would be(a) 74 kJ/kg (b) 95 kJ/kg (c) 109 kJ/kg(d) 128 kJ/kg (e) 177 kJ/kg9–180 In an ideal Brayton cycle, air is compressed from100 kPa and 25°C to 1 MPa, and then heated to 1200°Cbefore entering the turbine. Under cold-air-standard conditions,the air temperature at the turbine exit is(a) 490°C (b) 515°C (c) 622°C(d) 763°C (e) 895°C9–181 In an ideal Brayton cycle with regeneration, argongas is compressed from 100 kPa and 25°C to 400 kPa, andthen heated to 1200°C before entering the turbine. The highesttemperature that argon can be heated in the regenerator is(a) 246°C (b) 846°C (c) 689°C(d) 368°C (e) 573°C9–182 In an ideal Brayton cycle with regeneration, air iscompressed from 80 kPa and 10°C to 400 kPa and 175°C, isheated to 450°C in the regenerator, and then further heated to1000°C before entering the turbine. Under cold-air-standardconditions, the effectiveness of the regenerator is(a) 33 percent (b) 44 percent (c) 62 percent(d) 77 percent (e) 89 percent9–183 Consider a gas turbine that has a pressure ratio of 6and operates on the Brayton cycle with regeneration betweenthe temperature limits of 20 and 900°C. If the specific heatratio of the working fluid is 1.3, the highest thermal efficiencythis gas turbine can have is(a) 38 percent (b) 46 percent (c) 62 percent(d) 58 percent (e) 97 percent9–184 An ideal gas turbine cycle with many stages of compressionand expansion and a regenerator of 100 percenteffectiveness has an overall pressure ratio of 10. Air entersevery stage of compressor at 290 K, and every stage of turbineat 1200 K. The thermal efficiency of this gas-turbine cycle is(a) 36 percent (b) 40 percent (c) 52 percent(d) 64 percent (e) 76 percent9–185 Air enters a turbojet engine at 260 m/s at a rate of 30kg/s, and exits at 800 m/s relative to the aircraft. The thrustdeveloped by the engine is(a) 8 kN (b) 16 kN (c) 24 kN(d) 20 kN (e) 32 kNDesign and Essay Problems9–186 Design a closed-system air-standard gas power cyclecomposed of three processes and having a minimum thermalefficiency of 20 percent. The processes may be isothermal,isobaric, isochoric, isentropic, polytropic, or pressure as a linearfunction of volume. Prepare an engineering report describ-ing your design, showing the system, P-v and T-s diagrams,and sample calculations.9–187 Design a closed-system air-standard gas power cyclecomposed of three processes and having a minimum thermalefficiency of 20 percent. The processes may be isothermal,isobaric, isochoric, isentropic, polytropic, or pressure as a linearfunction of volume; however, the Otto, Diesel, Ericsson,and Stirling cycles may not be used. Prepare an engineeringreport describing your design, showing the system, P-v andT-s diagrams, and sample calculations.9–188 Write an essay on the most recent developments onthe two-stroke engines, and find out when we might be seeingcars powered by two-stroke engines in the market. Whydo the major car manufacturers have a renewed interest intwo-stroke engines?9–189 In response to concerns about the environment, somemajor car manufacturers are currently marketing electric cars.Write an essay on the advantages and disadvantages of electriccars, and discuss when it is advisable to purchase an electriccar instead of a traditional internal combustion car.9–190 Intense research is underway to develop adiabaticengines that require no cooling of the engine block. Suchengines are based on ceramic materials because of the abilityof such materials to withstand high temperatures. Write anessay on the current status of adiabatic engine development.Also determine the highest possible efficiencies with theseengines, and compare them to the highest possible efficienciesof current engines.9–191 Since its introduction in 1903 by Aegidius Elling ofNorway, steam injection between the combustion chamberand the turbine is used even in some modern gas turbinescurrently in operation to cool the combustion gases to ametallurgical-safe temperature while increasing the massflow rate through the turbine. Currently there are several gasturbinepower plants that use steam injection to augmentpower and improve thermal efficiency.Consider a gas-turbine power plant whose pressure ratio is8. The isentropic efficiencies of the compressor and the turbineare 80 percent, and there is a regenerator with an effectivenessof 70 percent. When the mass flow rate of air through the compressoris 40 kg/s, the turbine inlet temperature becomes 1700K. But the turbine inlet temperature is limited to 1500 K, andthus steam injection into the combustion gases is being considered.However, to avoid the complexities associated with steaminjection, it is proposed to use excess air (that is, to take inmuch more air than needed for complete combustion) to lowerthe combustion and thus turbine inlet temperature whileincreasing the mass flow rate and thus power output of the turbine.Evaluate this proposal, and compare the thermodynamicperformance of “high air flow” to that of a “steam-injection”gas-turbine power plant under the following design conditions:the ambient air is at 100 kPa and 25°C, adequate water supplyis available at 20°C, and the amount of fuel supplied to thecombustion chamber remains constant.