Thermodynamics

784 | Thermodynamics15–6C What is the air–fuel ratio? How is it related to thefuel–air ratio?15–7C Is the air–fuel ratio expressed on a mole basis identicalto the air–fuel ratio expressed on a mass basis?Theoretical and Actual Combustion Processes15–8C What are the causes of incomplete combustion?15–9C Which is more likely to be found in the products ofan incomplete combustion of a hydrocarbon fuel, CO or OH?Why?15–10C What does 100 percent theoretical air represent?15–11C Are complete combustion and theoretical combustionidentical? If not, how do they differ?15–12C Consider a fuel that is burned with (a) 130 percenttheoretical air and (b) 70 percent excess air. In which case isthe fuel burned with more air?15–13 Methane (CH 4 ) is burned with stoichiometricamount of air during a combustion process. Assuming completecombustion, determine the air–fuel and fuel–air ratios.15–14 Propane (C 3 H 8 ) is burned with 75 percent excess airduring a combustion process. Assuming complete combustion,determine the air–fuel ratio. Answer: 27.5 kg air/kg fuel15–15 Acetylene (C 2 H 2 ) is burned with stoichiometricamount of air during a combustion process. Assuming completecombustion, determine the air–fuel ratio on a mass andon a mole basis.15–16 One kmol of ethane (C 2 H 6 ) is burned with anunknown amount of air during a combustion process. Ananalysis of the combustion products reveals that the combustionis complete, and there are 3 kmol of free O 2 in the products.Determine (a) the air–fuel ratio and (b) the percentageof theoretical air used during this process.15–17E Ethylene (C 2 H 4 ) is burned with 200 percent theoreticalair during a combustion process. Assuming completecombustion and a total pressure of 14.5 psia, determine(a) the air–fuel ratio and (b) the dew-point temperature ofthe products. Answers: (a) 29.6 lbm air/lbm fuel, (b) 101°F15–18 Propylene (C 3 H 6 ) is burned with 50 percent excessair during a combustion process. Assuming complete combustionand a total pressure of 105 kPa, determine (a) theair–fuel ratio and (b) the temperature at which the watervapor in the products will start condensing.15–19 Propal alcohol (C 3 H 7 OH) is burned with 50 percentexcess air. Write the balanced reaction equation for completecombustion and determine the air-to-fuel ratio.Answer: 15.5 kg air/kg fuel15–20 Butane (C 4 H 10 ) is burned in 200 percent theoreticalair. For complete combustion, how many kmol of water mustbe sprayed into the combustion chamber per kmol of fuel ifthe products of combustion are to have a dew-point temperatureof 60°C when the product pressure is 100 kPa?15–21 A fuel mixture of 20 percent by mass methane (CH 4 )and 80 percent by mass ethanol (C 2 H 6 O), is burned completelywith theoretical air. If the total flow rate of the fuel is 31 kg/s,determine the required flow rate of air. Answer: 330 kg/s15–22 Octane (C 8 H 18 ) is burned with 250 percent theoreticalair, which enters the combustion chamber at 25°C.Assuming complete combustion and a total pressure of 1 atm,determine (a) the air–fuel ratio and (b) the dew-point temperatureof the products.C 8 H 18AIR25°CCombustionchamberP = 1 atmFIGURE P15–22Products15–23 Gasoline (assumed C 8 H 18 ) is burned steadily with airin a jet engine. If the air–fuel ratio is 18 kg air/kg fuel, determinethe percentage of theoretical air used during thisprocess.15–24 In a combustion chamber, ethane (C 2 H 6 ) is burned ata rate of 8 kg/h with air that enters the combustion chamberat a rate of 176 kg/h. Determine the percentage of excess airused during this process. Answer: 37 percent15–25 One kilogram of butane (C 4 H 10 ) is burned with25 kg of air that is at 30°C and 90 kPa. Assuming that thecombustion is complete and the pressure of the products is90 kPa, determine (a) the percentage of theoretical air usedand (b) the dew-point temperature of the products.15–26E One lbm of butane (C 4 H 10 ) is burned with 25 lbmof air that is at 90°F and 14.7 psia. Assuming that thecombustion is complete and the pressure of the products is14.7 psia, determine (a) the percentage of theoretical airused and (b) the dew-point temperature of the products.Answers: (a) 161 percent, (b) 113°F15–27 A certain natural gas has the following volumetricanalysis: 65 percent CH 4 , 8 percent H 2 , 18 percent N 2 , 3 percentO 2 , and 6 percent CO 2 . This gas is now burned completelywith the stoichiometric amount of dry air. What is theair–fuel ratio for this combustion process?15–28 Repeat Prob. 15–27 by replacing the dry air by moistair that enters the combustion chamber at 25°C, 1 atm, and85 percent relative humidity.15–29 A gaseous fuel with a volumetric analysis of 60 percentCH 4 , 30 percent H 2 , and 10 percent N 2 is burned to completionwith 130 percent theoretical air. Determine (a) the