tailieunhanh - Lecture Thermodynamics: An engineering approach (8/e) - Chapter 5 (Part 2) - Yunus A. Çengel, Michael A. Boles

Chapter 5 (Part 2) - Mass and energy analysis of control volumes. The objectives of this chapter are to: Solve energy balance problems for common steady-flow devices such as nozzles, compressors, turbines, throttling valves, mixers, heaters, and heat exchangers; apply the energy balance to general unsteady-flow processes with particular emphasis on the uniform-flow process as the model for commonly encountered charging and discharging processes. | Chapter 5 Part 2 Mass and Energy Analysis of Control Volumes Study Guide in PowerPoint to accompany Thermodynamics: An Engineering Approach, 8th edition by Yunus A. Çengel and Michael A. Boles 7 Turbines If we neglect the changes in kinetic and potential energies as fluid flows through an adiabatic turbine having one entrance and one exit, the conservation of mass and the steady-state, steady-flow first law becomes 28 Example 5-5 High pressure air at 1300 K flows into an aircraft gas turbine and undergoes a steady-state, steady-flow, adiabatic process to the turbine exit at 660 K. Calculate the work done per unit mass of air flowing through the turbine when (a) Temperature-dependent data are used. (b) Cp,ave at the average temperature is used. (c) Cp at 300 K is used. Control Volume: The turbine. Property Relation: Assume air is an ideal gas and use ideal gas relations. Process: Steady-state, steady-flow, adiabatic process 29 Conservation Principles: . | Chapter 5 Part 2 Mass and Energy Analysis of Control Volumes Study Guide in PowerPoint to accompany Thermodynamics: An Engineering Approach, 8th edition by Yunus A. Çengel and Michael A. Boles 7 Turbines If we neglect the changes in kinetic and potential energies as fluid flows through an adiabatic turbine having one entrance and one exit, the conservation of mass and the steady-state, steady-flow first law becomes 28 Example 5-5 High pressure air at 1300 K flows into an aircraft gas turbine and undergoes a steady-state, steady-flow, adiabatic process to the turbine exit at 660 K. Calculate the work done per unit mass of air flowing through the turbine when (a) Temperature-dependent data are used. (b) Cp,ave at the average temperature is used. (c) Cp at 300 K is used. Control Volume: The turbine. Property Relation: Assume air is an ideal gas and use ideal gas relations. Process: Steady-state, steady-flow, adiabatic process 29 Conservation Principles: Conservation of mass: Conservation of energy: According to the sketched control volume, mass and work cross the control surface. Neglecting kinetic and potential energies and noting the process is adiabatic, we have 30 The work done by the air per unit mass flow is Notice that the work done by a fluid flowing through a turbine is equal to the enthalpy decrease of the fluid. (a) Using the air tables, Table A-17 at T1 = 1300 K, h1 = kJ/kg at T2 = 660 K, h2 = kJ/kg 31 (b) Using Table A-2(c) at Tave = 980 K, Cp, ave = kJ/kg K (c) Using Table A-2(a) at T = 300 K, Cp = kJ/kg K 32 Compressors and fans Compressors and fans are essentially the same devices. However, compressors operate over larger pressure ratios (P2/P1) than fans. Axial flow compressors are made of several “fan blade” like stages as shown on the pervious slide. If we neglect the changes in kinetic and potential energies as fluid flows through an adiabatic compressor having one entrance and one