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ThermoSolutions-CHAPTER02

# ThermoSolutions-CHAPTER02 - 2-1 Chapter 2 ENERGY ENERGY...

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2-1 Chapter 2 ENERGY, ENERGY TRANSFER, AND GENERAL ENERGY ANALYSIS Forms of Energy 2-1C In electric heaters, electrical energy is converted to sensible internal energy. 2-2C The forms of energy involved are electrical energy and sensible internal energy. Electrical energy is converted to sensible internal energy, which is transferred to the water as heat. 2-3C The macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame. The microscopic forms of energy, on the other hand, are those related to the molecular structure of a system and the degree of the molecular activity, and are independent of outside reference frames. 2-4C The sum of all forms of the energy a system possesses is called total energy . In the absence of magnetic, electrical and surface tension effects, the total energy of a system consists of the kinetic, potential, and internal energies. 2-5C The internal energy of a system is made up of sensible, latent, chemical and nuclear energies. The sensible internal energy is due to translational, rotational, and vibrational effects. 2-6C Thermal energy is the sensible and latent forms of internal energy, and it is referred to as heat in daily life. 2-7C The mechanical energy is the form of energy that can be converted to mechanical work completely and directly by a mechanical device such as a propeller . It differs from thermal energy in that thermal energy cannot be converted to work directly and completely. The forms of mechanical energy of a fluid stream are kinetic, potential, and flow energies.

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2-2 2-8 A river is flowing at a specified velocity, flow rate, and elevation. The total mechanical energy of the river water per unit mass, and the power generation potential of the entire river are to be determined. Assumptions 1 The elevation given is the elevation of the free surface of the river. 2 The velocity given is the average velocity. 3 The mechanical energy of water at the turbine exit is negligible. Properties We take the density of water to be U = 1000 kg/m 3 . Analysis Noting that the sum of the flow energy and the potential energy is constant for a given fluid body, we can take the elevation of the entire river water to be the elevation of the free surface, and ignore the flow energy. Then the total mechanical energy of the river water per unit mass becomes kJ/kg 887 . 0 /s m 1000 kJ/kg 1 2 ) m/s 3 ( m) 90 )( m/s (9.81 2 2 2 2 2 2 mech ¸ ¹ · ¨ © § ¸ ¸ ¹ · ¨ ¨ © § . . . V gh ke pe e 90 m River 3 m/s The power generation potential of the river water is obtained by multiplying the total mechanical energy by the mass flow rate, kg/s 500,000 /s) m 00 )(5 kg/m 1000 ( 3 3 V & & U m MW 444 kW 000 , 444 kJ/kg) 7 kg/s)(0.88 000 , 500 ( mech mech max e m E W & & & Therefore, 444 MW of power can be generated from this river as it discharges into the lake if its power potential can be recovered completely.
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