Concept explainers
n-Octane [C8H18(l)] is burned in the constant-pressure combustor of an aircraft engine with 70 percent excess air. Air enters this combustor at 600 kPa and 327°C, liquid fuel is injected at 25°C, and the products of combustion leave at 600 kPa and 1227°C. Determine the entropy generation and exergy destruction per unit mass of fuel during this combustion process. Take T0 = 25°C.
(a)
The entropy generation from the combustion chamber per unit mass of fuel.
Answer to Problem 92P
The entropy generation from the combustion chamber per unit mass of fuel is
Explanation of Solution
Write the energy balance equation using steady-flow equation.
Here, the total energy entering the system is
Substitute
Here, the enthalpy of formation for product is
Calculate the molar mass of the
Here, the number of carbon atoms is
Write the expression for entropy generation during this process.
Write the combustion equation of Equation (IV)
Here, the entropy of the product is
Determine the entropy at the partial pressure of the components.
Here, the partial pressure is
Write the expression for exergy destruction during this process.
Here, the thermodynamic temperature of the surrounding is
Determine the entropy generation per unit mass of the fuel.
Conclusion:
Perform unit conversion of temperature at state 1 from degree Celsius to Kelvin.
For air temperature enter in the combustion chamber,
For then liquid injected temperature in the combustion chamber,
For air temperature exit in the combustion chamber,
Write the combustion equation of 1 kmol for
Here, liquid octane is
Express the stoichiometric coefficient of air by
Substitute
Refer Appendix Table A-18, A-19, A-20, and A-23, obtain the enthalpy of formation, at 298 K, 600 K, and 1500 K for
Substance | ||||
-249,950 | --- | --- | --- | |
0 | 8682 | 17,929 | 49,292 | |
0 | 8669 | 17,563 | 47,073 | |
-241,820 | 9904 | --- | 57,999 | |
-393,520 | 9364 | --- | 71,078 |
Refer Equation (X), and write the number of moles of reactants.
Here, number of moles of reactant octane, oxygen and nitrogen is
Refer Equation (X), and write the number of moles of products.
Here, number of moles of product carbon dioxide, water, oxygen and nitrogen is
Substitute the value of substance in Equation (II).
Therefore the heat transfer for
Substitute 8 for
Refer Equation (VI) for reactant and product to calculation the entropy in tabular form as:
For reactant entropy,
Substance |
(T, 1 atm) | ||||
1 | --- | 466.73 | 14.79 | 451.94 | |
21.25 | 0.21 | 226.35 | 1.81 | 4771.48 | |
79.9 | 0.79 | 212.07 | 12.83 | 15,919.28 | |
For product entropy,
Substance |
(T, 1 atm) | ||||
8 | 0.0757 | 292.11 | -6.673 | 2390.26 | |
9 | 0.0852 | 250.45 | -5.690 | 2305.26 | |
8.75 | 0.0828 | 25.97 | -5.928 | 2309.11 | |
79.9 | 0.7563 | 241.77 | 12.46 | 18,321.87 | |
Substitute
Substitute
Substitute
Thus, the entropy generation from the combustion chamber per unit mass of fuel is
(b)
The exergy destruction from the combustion chamber per unit mass of fuel.
Answer to Problem 92P
The exergy destruction from the combustion chamber per unit mass of fuel is
Explanation of Solution
Determine the exergy destruction from the combustion chamber per unit mass of the fuel.
Conclusion:
Substitute
Thus, the exergy destruction from the combustion chamber per unit mass of fuel is
Want to see more full solutions like this?
Chapter 15 Solutions
Thermodynamics: An Engineering Approach
- The gas mixture formed as a result of the combustion of the stoichiometric mixture consisting of methane (CH4) and humid air reaches a temperature of 1500 K and a pressure of 80 bar. In this case, calculate the specific entropy value of the products on a molar and mass basis. Take the specific humidity of the humid air as 0.01 kg sb/kg Hk.arrow_forwardIdentify if the property of the following steam: Pressure Specific Enthalpy Specific Entropy MPa kJ/kg kJ/kg-K 13 2483 ?arrow_forwardLiquid octane is burned steadily with air in a jet engine. If the air–fuel ratio is 18 kg air/kg fuel, determine the percentage of excess air used during this process.arrow_forward
- In a combustion chamber, ethane (C2H6) is burned at a rate of 8 kg/h with air that enters the combustion chamber at a rate of 176 kg/h. Determine the percentage of excess air used during this process.arrow_forwardA gaseous fuel with 80% butane, 15% nitrogen and 5% oxygen (on a mole basis) is burned to completion with 120 percent theoretical air that enters the combustion chamber at 30°C and 100 kPa. Determine the volume flow rate of air required to burn fuel at a rate of 2 kg/min.arrow_forwardSteam (H2O) enters a turbine at 35 m/s and specific enthalpy of 3500 kJ/kg. Thesteam leaves the turbine as a mixture of vapor and liquid having a velocity of 60m/s and a specific enthalpy of 2400 kJ/kg. If the flow through the turbine is steady,adiabatic, and changes in elevation are negligible, calculate the work output per unitmass (kJ/kg) for the turbine.arrow_forward
- 16-Water vapor enters an isentropic turbine at a pressure of 3 MPa and a temperature of 400°C and exits the turbine at a pressure of 100 kPa. What is the degree of dryness at the turbine outlet? A) % 92,8 B) % 81,5 C) % 76,4 D) % 95,5 E) % 89,9arrow_forwardHow did the ideal gas laws apply in internal combustion engines?arrow_forwardWater vapor enters an adiabatic turbine at a pressure of 20 MPa and a temperature of 650 oC, with a flow rate of 2 kg / s, and expands to a pressure of 0.2 MPa. The isentropic efficiency of the turbine is 0.75. a) Calculate the temperature of the steam at the turbine outlet. b) Which phases are in Case 1 and 2 and what tables were used? c) Calculate the power of the turbine.arrow_forward
- In a closed feedwater (FW) heater, a mixture enters the heater with a mass flow rate m1 and enthalpy h1 and leaves with an enthalpy h2. A second stream into the heater has a mass flow rate m2 and enthalpy h3 and it leaves with an enthalpy h4. What is a correct first law equation for this FW heater? m1·(h3 - h4) = m2·(h1 - h2) m1·(h1 - h2) = m2·(h4 - h3) m1·(h1 - h2) = m2·(h3 - h4) m1·m2 = (h1 -h2)·(h3 - h4) m1·(h1 - h3) = m2·(h2 - h4)arrow_forwardH1. A stack gas containing 71.9% N2, 10% CO2 and 18.1% CO is passed through a turbine. This gas stream flows at 500 m3 STP/min and cools from 1000°C to 500°C. a. How much shaft work is done if the rate of heat removal from the turbine is 2000 kW? b. What was the flow rate of air (m3/s) to the combustor if the air was fed at 50°C and 6 atm? c. What is the theoretical amount (mol/s) of air required for the combustor? d. Generally excess air is used. Why?arrow_forwardIn the boiler design, it is desirable to have the flue gas exit temperature above the dew point. Estimate the partial pressure of water vapor of the flue gas in kPa produced by combustion having the gravimetric analysis of: N2=71.84%, CO2=20.35% , O2=3.61%, H2O=4.20%arrow_forward
- Elements Of ElectromagneticsMechanical EngineeringISBN:9780190698614Author:Sadiku, Matthew N. O.Publisher:Oxford University PressMechanics of Materials (10th Edition)Mechanical EngineeringISBN:9780134319650Author:Russell C. HibbelerPublisher:PEARSONThermodynamics: An Engineering ApproachMechanical EngineeringISBN:9781259822674Author:Yunus A. Cengel Dr., Michael A. BolesPublisher:McGraw-Hill Education
- Control Systems EngineeringMechanical EngineeringISBN:9781118170519Author:Norman S. NisePublisher:WILEYMechanics of Materials (MindTap Course List)Mechanical EngineeringISBN:9781337093347Author:Barry J. Goodno, James M. GerePublisher:Cengage LearningEngineering Mechanics: StaticsMechanical EngineeringISBN:9781118807330Author:James L. Meriam, L. G. Kraige, J. N. BoltonPublisher:WILEY