Chapter 6 FIRST-LAW ANALYSIS FOR A CONTROL VOLUME

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The Steady-State Process The control volume is stationary The state of the mass at each point in the control volume does not vary with time For mass flowing across the boundary, the mass flux and the state of mass at each area of flow on the control surface do not vary with time. The rates at which heat and work cross the control surface are constant.

Steady-State Devices: Heat Exchangers Heat transferred to/from fluids flowing through pipes Usually constant pressure No work gets done ΔKE and ΔPE usually small Little heat transfer with surroundings if C.V. includes both fluids

06-05 Steady-State Device: A Heat Exchanger

06-06 Steady-State Device: A Heat Exchanger

Steady-State Devices: Nozzles A device for creating high-velocity fluid streams Smooth transition to higher velocity produces lower pressures (Bernoulli’s equation, which is just another statement of the 1 st Law) No work done Little or no change in potential energy Little or no heat transfer Inlet KE usually negligible

06-07 Steady-State Device: A Nozzle

Steady-State Devices: Diffusers Anti-nozzle: A device for decelerating fluid flow to produce an increase in pressure As with nozzles, only inlet and exit enthalpies and inlet KE contribute to the 1 st Law

Steady-State Devices: Throttles Sudden restrictions in flow passage that produces a drop in pressure Not smooth like a nozzle; not much change in KE No change in PE No work done No heat transfer Net result: pressure drop at constant enthalpy Can involve a change in phase; e.g., an expansion valve in a refrigerator

06-08 Steady-State Device: A Throttle

Steady-State Devices: Turbines Rotary machines that produce shaft work at the expense of working fluid pressure Steam or gas Inlet pressure controlled by previous pumping or compression process Exit pressure determined by environment Two internal processes: – Nozzles to increase velocity and reduce pressure – High velocity fluid directed at rotating blades that turn the shaft and generate work; low-pressure, low-velocity fluid exits the turbine Negligible change in PE Negligible inlet KE Normally taken to be an adiabatic process Normally, work output is change in enthalpy from inlet to outlet

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06-09 Steady-State Device: A Turbine

Steady-State Devices: Compressors and Pumps Devices that use shaft work to increase pressure in the working fluid Two types: – Rotary; an anti-turbine – Piston/cylinder Usually taken to be adiabatic Negligible change in PE Negligible inlet KE Heat transfer negligible for rotary compressors; can be significant for piston/cylinder type

06-10 Steady-State Device: A Compressor

06-11 Steady-State Device: A Pump

06-12 Steady-State System: A Power Plant

06-13 Steady-State System: A Refrigerator

The Transient Process The control volume is stationary The state of the mass at each point in the control volume may change with time, but at any instant of time is uniform throughout the entire control volume The state of the mass crossing the control surface is constant with time, although flow rates may vary with time.

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