WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition 4 CHAPTER The First Law of Thermodynamics: Control Volumes

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Chapter Summary A control volume differs from a closed system in that it involves mass transfer. Mass carries energy with it, and thus the mass and energy content of a system change when mass enters or leaves. 4-20

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Chapter Summary The mass and energy balances for any system undergoing any process can be expressed as 4-21

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition The mass and energy balances for any system undergoing any process can be expressed in the rate form as Chapter Summary 4-22

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Mass flow through a cross section per unit time is called the mass flow rate and is denoted m. It is expressed as where  = density, kg/m 3 (= 1/v) = average fluid velocity normal to A, m/s A= cross-sectional area, m 2 Chapter Summary. 4-23

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Chapter Summary The fluid volume flowing through a cross section per unit time is called the volume flow rate V. It is given by. 4-24

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Chapter Summary The mass and volume flow rates are related by 4-25

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Specific Properties and Enthalpy A specific property is an intensive quantity obtained by dividing an extensive property ( or its flow rate) by the total amount of the process material. For example the specific Volume ( m3/kg) and specific kinetic energy is ( J/kg). We will use the symbol ^ to denote the a specific property: will denote specific volume specific internal energy The enthalpy of the system H =U+PV The specific enthalpy

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Chapter Summary Thermodynamic processes involving control volumes can be considered in two groups: steady- flow processes and unsteady-flow processes. During a steady-flow process, the fluid flows through the control volume steadily, experiencing no change with time at a fixed position. The mass and energy content of the control volume remain constant during a steady-flow process. 4-26

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Taking heat transfer to the system and work done by the system to be positive quantities, the conservation of mass and energy equations for steady-flow processes are expressed as where the subscript i stands for inlet and e for exit. These are the most general forms of the equations for steady-flow processes. Chapter Summary for each exitfor each inlet 4-27

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition For single-stream (one-inlet--one-exit) systems such as nozzles, diffusers, turbines, compressors, and pumps, the steady flow equations simplify to In the above relations, subscripts 1 and 2 denote the inlet and exit states, respectively. Chapter Summary 4-28

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Chapter Summary During a uniform-flow process, the state of the control volume may change with time, but it may do so uniformly. Also, the fluid properties at the inlets and the exits are assumed to remain constant during the entire process. The conservation of energy equation for a uniform- flow process reduces to 4-29

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Chapter Summary When the kinetic and potential energy changes associated with the control volume and the fluid streams are negligible, the conservation of energy equation for a uniform-flow process simplifies to 4-30

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Steam Tables

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition (fig. 4-6) © The McGraw-Hill Companies, Inc.,1998 Velocity Profiles for Flow in a Pipe 4-1

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Volume Flow Rate 4-2 Volume flow rate is the volume of fluid flowing through a cross section per unit of time

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Mass Flow, Heat, and Work Affect Energy Content 4-3 The energy content of a control volume can be changed by mass flow as well as heat and work interactions

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Control Volume May Involve Boundary, Electrical, and Shaft Work (Fig. 4-9) 4-4

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Schematic for Flow Work (Fig. 4-10) 4-5

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition During Steady Flow Process, Volume Flow Rates are not Necessarily Conserved (Fig. 4-19) 4-6

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition A Water Heater Under Steady Operation 4-7 (Fig. 4-21).....

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Steady-Flow Devices Operate Steadily for Long Periods (Fig. 4-25) 4-8

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Nozzle and Diffuser Shapes Cause Large Changes in Fluid Velocities (Fig. 4-27) 4-9 Nozzles and Diffusers are shaped so that they cause large changes in fluid velocities and thus kinetic energies

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Schematic for Example

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Schematic for Example

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Throttling Valve Devices Cause Large Pressure Drops in Fluid (Fig.4-32) 4-12

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Ideal Gas Temperature Does Not Change During a Throttling 4-13 The temperature of an ideal gas does not change during a throttling(h =constant) process since h = h (T)

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition T-Elbow Serves as Mixing Chamber for Hot and Cold Water Steams (Fig. 4-35) 4-14 The T-ebow of an ordinary shower serves as the mixing chamber for hot- and cold-water streams.

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Heat Transfer Via Heat Exchanger Depends on System Selection 4-15 The heat transfer associated with a heat exchanger may be zero or nonzero depending on how the system is selected

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Schematic for Example

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Rigid Tank Charging From a Supply Line is an Unsteady-Flow Process (Fig. 4-47) 4-17 Charging of a rigid tank from a supply line is an unsteady-flow process since it involves changes within the control volume

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Temperature of Steam Rises Entering Tank, Flow Energy Converts to Internal Energy 4-18 (Fig. 4-54) The Temperature of Steam rises from 300 to 456°C as it enters a tank as a result of flow energy being converted to internal energy o o

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Enthalpy of a Saturated Vapor at a Given Pressure 4-19 In a pressure cooker, the enthalpy of the existing steam is H (enthalpy of the saturated vapor at the given pressure)