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V.The First Law of Thermodynamics 1.The First Law of Thermodynamics 2.Energy Balance for Closed Systems 3.Energy Balance for Steady-Flow Systems 4.Some Steady-Flow Engineering Devices 5.Energy Balance for Unsteady-Flow Processes

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1. The First Law of Thermodynamics Energy can be neither created nor destroyed. First law of thermodynamics, or the conservation of energy principle, is based on experimental observations. During an interaction between a system and its surroundings, the amount of energy gained by the system must be exactly equal to the amount of energy lost by the surroundings.

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Energy Balance

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2. Energy Balance for Closed Systems The first law of thermodynamics, or the conservation of energy principle for a closed system or a fixed mass, may be expressed as follows: or

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Net heat transfer across system boundaries Net work done in all form Net change in total energy of system

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For a stationary closed systems

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For a cyclic process

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Various forms of the first-law relation for closed systems.

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Examples Example 5-1: Cooling of a Hot Fluid in a Tank Example 5-2: Electric Heating of a Gas at Constant Pressure Example 5-3: Unrestrained Expansion of Water into an Evacuated Tank Example 5-4: Heating of a Gas in a Tank by Stirring Example 5-5: Heating of a Gas by a Resistance Heater Example 5-6: Heating of a Gas at Constant Pressure Example 5-7: Cooling of an Iron Block by Water

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3. Energy Balance for Steady-Flow Systems Mass balance for steady-flow systems:

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Energy balance for steady-flow systems:

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4. Some Steady-Flow Engineering Devices Nozzles and Diffusers Turbines and Compressors Throttling Valves Mixture Chambers Heat Exchangers Pipe and Duct Flow

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(Fig. 4-25)

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Nozzle and Diffuser

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Example 5-11 Deceleration of Air in a Diffuser Air at 10C and 80kPa enters the diffuser of a jet engine steadily with a velocity of 200m/s. The inlet area of the diffuser is 0.4 m2. The air leaves the diffuser with a velocity that is very small compared with the inlet velocity. Determine (a) the mass flow rate of the air and (b) the temperature of the air leaving the diffuser.

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Turbines and Compressors

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Example 5-13 Compressing Air by a Compressor Air at 100kPa and 280K is compressed steadily to 600kPa and 400K. The mass-flow rate of the air is 0.02 kg/s, and a heat loss of 16kJ/kg occurs during the process. Assuming the changes in kinetic and potential energies are negligible, determine the necessary power input to the compressor.

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Example 5-14 Power Generation by a Steam Turbine The power output of an adiabatic gas turbine is 5MW, and the inlet and the exit conditions of the hot gases are as indicated in Fig The gases can be treated as air. (a)Compare the magnitudes of h, ke, and pe. (b)Determine the work done per unit mass of hot gases. (c)Calculate the mass flow rate of the steam.

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Throttling Valves

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The temperature of an ideal gas does not change during a throttling(h =constant) process since h = h (T)

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Joule-Thomson Coefficient

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Example 5-15 Expansion of R-134a in a Refrigerator R-134a enters the capillary tube of a refrigerator as saturated liquid at 0.8MPa and is throttled to a pressure of 0.12MPa. Part of the refrigerant evaporates during this process and the refrigerant exists as a saturated liquid-vapor mixture at the final state. Determine the temperature drop of the refrigerant during this process.

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Mixing Chamber

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Heat Exchanger The heat transfer associated with a heat exchanger may be zero or nonzero depending on how the system is selected

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Pipe and Duct Flow.

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