Thermodynamics

878 | Thermodynamics17–97C On a T-s diagram of Rayleigh flow, what do thepoints on the Rayleigh line represent?17–98C What is the effect of heat gain and heat loss on theentropy of the fluid during Rayleigh flow?17–99C Consider subsonic Rayleigh flow of air with aMach number of 0.92. Heat is now transferred to the fluidand the Mach number increases to 0.95. Will the temperatureT of the fluid increase, decrease, or remain constant duringthis process? How about the stagnation temperature T 0 ?17–100C What is the effect of heating the fluid on the flowvelocity in subsonic Rayleigh flow? Answer the same questionsfor supersonic Rayleigh flow.17–101C Consider subsonic Rayleigh flow that is acceleratedto sonic velocity (Ma 1) at the duct exit by heating. Ifthe fluid continues to be heated, will the flow at duct exit besupersonic, subsonic, or remain sonic?17–102 Consider a 12-cm-diameter tubular combustionchamber. Air enters the tube at 500 K, 400 kPa, and 70 m/s.Fuel with a heating value of 39,000 kJ/kg is burned by sprayingit into the air. If the exit Mach number is 0.8, determinethe rate at which the fuel is burned and the exit temperature.Assume complete combustion and disregard the increase inthe mass flow rate due to the fuel mass.P 1 400 kPaT 1 500 KV 1 70 m/sFuelCombustortubeFIGURE P17–102Ma 2 0.817–103 Air enters a rectangular duct at T 1 300 K, P 1 420 kPa, and Ma 1 2. Heat is transferred to the air in theamount of 55 kJ/kg as it flows through the duct. Disregardingfrictional losses, determine the temperature and Mach numberat the duct exit. Answers: 386 K, 1.64P 1 420 kPaT 1 300 KMa 1 255 kJ/kgFIGURE P17–10317–104 Repeat Prob. 17–103 assuming air is cooled in theamount of 55 kJ/kg.17–105 Air is heated as it flows subsonically through aduct. When the amount of heat transfer reaches 60 kJ/kg, theAirflow is observed to be choked, and the velocity and the staticpressure are measured to be 620 m/s and 270 kPa. Disregardingfrictional losses, determine the velocity, static temperature,and static pressure at the duct inlet.17–106E Air flows with negligible friction through a 4-indiameterduct at a rate of 5 lbm/s. The temperature and pressureat the inlet are T 1 800 R and P 1 30 psia, and theMach number at the exit is Ma 2 1. Determine the rate ofheat transfer and the pressure drop for this section of theduct.17–107 Air enters a frictionless duct with V 1 70m/s, T 1 600 K, and P 1 350 kPa. Lettingthe exit temperature T 2 vary from 600 to 5000 K, evaluate theentropy change at intervals of 200 K, and plot the Rayleighline on a T-s diagram.17–108E Air is heated as it flows through a 4 in 4 insquare duct with negligible friction. At the inlet, air is at T 1 700 R, P 1 80 psia, and V 1 260 ft/s. Determine the rate atwhich heat must be transferred to the air to choke the flow atthe duct exit, and the entropy change of air during thisprocess.17–109 Compressed air from the compressor of a gas turbineenters the combustion chamber at T 1 550 K, P 1 600kPa, and Ma 1 0.2 at a rate of 0.3 kg/s. Via combustion,heat is transferred to the air at a rate of 200 kJ/s as it flowsthrough the duct with negligible friction. Determine the Machnumber at the duct exit and the drop in stagnation pressureP 01 P 02 during this process. Answers: 0.319, 21.8 kPa17–110 Repeat Prob. 17–109 for a heat transfer rate of 300kJ/s.17–111 Argon gas enters a constant cross-sectional-areaduct at Ma 1 0.2, P 1 320 kPa, and T 1 400 K at a rateof 0.8 kg/s. Disregarding frictional losses, determine thehighest rate of heat transfer to the argon without reducing themass flow rate.17–112 Consider supersonic flow of air through a 6-cmdiameterduct with negligible friction. Air enters the duct atMa 1 1.8, P 01 210 kPa, and T 01 600 K, and it is deceleratedby heating. Determine the highest temperature that aircan be heated by heat addition while the mass flow rateremains constant.Steam Nozzles17–113C What is supersaturation? Under what conditionsdoes it occur?17–114 Steam enters a converging nozzle at 3.0 MPa and500°C with a negligible velocity, and it exits at 1.8 MPa. Fora nozzle exit area of 32 cm 2 , determine the exit velocity,mass flow rate, and exit Mach number if the nozzle (a) isisentropic and (b) has an efficiency of 90 percent. Answers:(a) 580 m/s, 10.7 kg/s, 0.918, (b) 551 m/s, 10.1 kg/s, 0.865