Moran_9e_LectureSlides_ch04 PDF

Chapter 4 Lecture 17 Control Volume Analysis Using Energy  Learning Outcomes ►Describe key concepts related to control volume analysis, including distinguishing between steady-state and transient analysis, distinguishing between mass flow rate and volumetric flow rate,...

Chapter 4 Lecture 17 Control Volume Analysis Using Energy  Learning Outcomes ►Describe key concepts related to control volume analysis, including distinguishing between steady-state and transient analysis, distinguishing between mass flow rate and volumetric flow rate, and explaining the meanings of one- dimensional flow and flow work. ►Apply mass and energy balances to control volumes.  Learning Outcomes, cont. ►Develop appropriate engineering models for control volumes, with particular attention to analyzing components commonly encountered in engineering practice such as nozzles, diffusers, turbines, compressors, heat exchangers, throttling devices, and integrated systems that incorporate two or more components. ►Obtain and apply appropriate property data for control volume analyses.  Mass Rate Balance (1 of 4) time rate of change of time rate of flow of time rate of flow mass contained within the mass in across of mass out across control volume at time t inlet i at time t exit e at time t (Eq. 4.1) Mass Rate Balance (2 of 4)  Mass Rate Balance (3 of 4) In practice there may be several locations on the boundary through which mass enters or exits. Multiple inlets and exits are accounted for by introducing summations: (Eq. 4.2) Eq. 4.2 is the mass rate balance for control volumes with several inlets and exits.  Mass Flow Rate (One-Dimensional Flow) ►Flow is normal to the boundary at locations where mass enters or exits the control volume. ►All intensive properties are uniform with position over each inlet or exit area (A) through which matter flows. (Eq. 4.4b) where V is velocity v is specific volume  Mass Rate Balance (4 of 4) (Steady-State Form) ►Steady-state: all properties are unchanging in time. ►For steady-state control volume, dmcv/dt = 0. (Eq. 4.6) (mass rate in) (mass rate out)  Energy Rate Balance time rate of change net rate at which net rate at which net rate of energy of the energy energy is being energy is being transfer into the contained within transferred in transferred out control volume the control volume by heat transfer by work at accompanying at time t at time t time t mass flow (Eq. 4.9) Evaluating Work for a Control Volume The expression for work is (Eq. 4.12) where ► accounts for work associated with rotating shafts, displacement of the boundary, and electrical effects. ► is the flow work at exit e. ► is the flow work at inlet i.  Control Volume Energy Rate Balance (1 of 4) (One-Dimensional Flow Form) Using Eq. 4.12 in Eq. 4.9 (Eq. 4.13) For convenience substitute enthalpy, h = u + pv (Eq. 4.14)  Control Volume Energy Rate Balance (2 of 4) (One-Dimensional Flow Form) In practice there may be several locations on the boundary through which mass enters or exits. Multiple inlets and exits are accounted for by introducing summations: (Eq. 4.15) Eq. 4.15 is the accounting balance for the energy of the control volume. Control Volume Energy Rate Balance (3 of 4) (Steady-State Form) ►Steady-state: all properties are unchanging in time. ►For steady-state control volume, dEcv/dt = 0. (Eq. 4.18)  Control Volume Energy Rate Balance (4 of 4) (Steady-State Form, One-Inlet, One-Exit) ►Many important applications involve one-inlet, one-exit control volumes at steady state. ►The mass rate balance reduces to. Eq. 4.20a or dividing by mass flow rate Eq. 4.20b  Lecture 18 v First law applied to open systems – some applications at Steady State and flow v Nozzles and diffusers v Turbines v Compressors v Pumps v Heat Exchangers v Throttling devices v System integration – cycles, etc.  Nozzles and Diffusers ►Nozzle: a flow passage of varying cross- sectional area in which the velocity of a gas or liquid increases in the direction of flow. ►Diffuser: a flow passage of varying cross- sectional area in which the velocity of a gas or liquid decreases in the direction of flow.  Nozzle and Diffuser Modeling Eq. 4.20a ► ►If the change in potential energy from inlet to exit is negligible, g(z1 – z2) drops out. ►If the heat transfer with surroundings is negligible, drops out. (Eq. 4.21) Watch out for UNITS! So, we must convert kW to W or W to kW as the case might be to ensure units are consistent! If you don’t do this you will end up getting absurd results such as “negative” velocities, etc.  Turbines ►Turbine: a device in which power is developed as a result of a gas or liquid passing through a set of blades attached to a shaft free to rotate.  Turbine Modeling Eq. 4.20a ►If the change in kinetic energy of flowing matter is negligible, ½(V12 – V22) drops out. ►If the change in potential energy of flowing matter is negligible, g(z1 – z2) drops out. ►If the heat transfer with surroundings is negligible, drops out. Compressors and Pumps ►Compressors and Pumps: devices in which work is done on the substance flowing through them to change the state of the substance, typically to increase the pressure and/or elevation. ►Compressor : substance is gas ►Pump: substance is liquid Compressor and Pump Modeling Eq. 4.20a ►If the change in kinetic energy of flowing matter is negligible, ½(V12 – V22) drops out. ►If the change in potential energy of flowing matter is negligible, g(z1 – z2) drops out. ►If the heat transfer with surroundings is negligible, drops out.  Heat Exchangers ►Direct contact: A mixing chamber in which hot and cold streams are mixed directly. ►Tube-within-a-tube counterflow: A gas or liquid stream is separated from another gas or liquid by a wall through which energy is conducted. Heat transfer occurs from the hot stream to the cold stream as the streams flow in opposite directions.  Heat Exchanger Modeling (Eq. 4.18) ► ►If the kinetic energies of the flowing streams are negligible, (Vi2/2) and (Ve2/2) drop out. ►If the potential energies of the flowing streams are negligible, gzi and gze drop out. ►If the heat transfer with surroundings is negligible, drops out.  Throttling Devices ►Throttling Device: a device that achieves a significant reduction in pressure by introducing a restriction into a line through which a gas or liquid flows. Means to introduce the restriction include a partially opened valve or a porous plug.  Throttling Device Modeling Eq. 4.20a ► ►If the change in kinetic energy of flowing matter upstream and downstream of the restriction is negligible, ½(V12 – V22) drops out. ►If the change in potential energy of flowing matter is negligible, g(z1 – z2) drops out. ►If the heat transfer with surroundings is negligible, drops out. (Eq. 4.22)  System Integration ►Engineers creatively combine components to achieve some overall objective, subject to constraints such as minimum total cost. This engineering activity is called system integration. ►The simple vapor power plant of Fig 4.16 provides an illustration.  Lecture 19 v First law applied to open systems – some applications v Transient analysis of open systems v Mass balance v Energy balance  The Mass Balance (Transient Analysis) ►Transient: state changes with time. ►Integrate mass rate balance (Eq. 4.2) from time 0 to a final time t. (Eq. 4.2) This becomes (Eq. 4.23) where mi is amount of mass entering the control volume through inlet i, from time 0 to t. me is amount of mass exiting the control volume through exit e, from time 0 to t. Mass balance – transient analysis  The Energy Balance (Transient Analysis) ►Integrate energy rate balance (Eq. 4.15), ignoring the effects of kinetic and potential energy, from time 0 to a final time t. When the specific enthalpies at inlets and exits are constant with time, this becomes (Eq. 4.25) Energy balance – transient analysis