Thermodynamics

256 | Thermodynamicsunsteady-flow processes. During a steady-flow process, thefluid flows through the control volume steadily, experiencingno change with time at a fixed position. The mass and energycontent of the control volume remain constant during asteady-flow process. Taking heat transfer to the system andwork done by the system to be positive quantities, the conservationof mass and energy equations for steady-flowprocesses are expressed asa m # aoutm #inQ # W # aoutm # a h V 2for each exit2 gz b ainm # a h V 2for each inletThese are the most general forms of the equations for steadyflowprocesses. For single-stream (one-inlet–one-exit) systemssuch as nozzles, diffusers, turbines, compressors, andpumps, they simplify tom # 1 m # 2 S 1 v 1V 1 A 1 1 v 2V 2 A 22 gz b⎫⎪⎪⎪⎬⎪⎪⎪⎭⎫⎪⎪⎪⎬⎪⎪⎪⎭Q # W # m # c h 2 h 1 V 2 2 2 V 1 g 1z22 z 1 2dIn these relations, subscripts 1 and 2 denote the inlet and exitstates, respectively.Most unsteady-flow processes can be modeled as a uniformflowprocess, which requires that the fluid flow at any inlet orexit is uniform and steady, and thus the fluid properties do notchange with time or position over the cross section of an inletor exit. If they do, they are averaged and treated as constantsfor the entire process. When kinetic and potential energychanges associated with the control volume and the fluidstreams are negligible, the mass and energy balance relationsfor a uniform-flow system are expressed asm in m out ¢m systemQ W aoutmh ainmh 1m 2 u 2 m 1 u 1 2 systemwhere Q Q net,in Q in Q out is the net heat input andW W net,out W out W in is the net work output.When solving thermodynamic problems, it is recommendedthat the general form of the energy balanceE in E out E system be used for all problems, and simplifyit for the particular problem instead of using the specific relationsgiven here for different processes.REFERENCES AND SUGGESTED READINGS1. ASHRAE Handbook of Fundamentals. SI version.Atlanta, GA: American Society of Heating, Refrigerating,and Air-Conditioning Engineers, Inc., 1993.2. ASHRAE Handbook of Refrigeration. SI version. Atlanta,GA: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc., 1994.3. Y. A. Çengel and J. M. Cimbala, Fluid Mechanics:Fundamentals and Applications. New York: McGraw-Hill, 2006.PROBLEMS*Conservation of Mass5–1C Name four physical quantities that are conserved andtwo quantities that are not conserved during a process.*Problems designated by a “C” are concept questions, and studentsare encouraged to answer them all. Problems designated by an “E”are in English units, and the SI users can ignore them. Problemswith a CD-EES icon are solved using EES, and complete solutionstogether with parametric studies are included on the enclosed DVD.Problems with a computer-EES icon are comprehensive in nature,and are intended to be solved with a computer, preferably using theEES software that accompanies this text.5–2C Define mass and volume flow rates. How are theyrelated to each other?5–3C Does the amount of mass entering a control volumehave to be equal to the amount of mass leaving during anunsteady-flow process?5–4C When is the flow through a control volume steady?5–5C Consider a device with one inlet and one outlet. If thevolume flow rates at the inlet and at the outlet are the same,is the flow through this device necessarily steady? Why?5–6E A garden hose attached with a nozzle is used to fill a20-gal bucket. The inner diameter of the hose is 1 in and it