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Unformatted text preview: 2-1 Chapter 2 ENERGY, ENERGY TRANSFER, AND GENERAL ENERGY ANALYSIS Forms of Energy 2-1CIn electric heaters, electrical energy is converted to sensible internal energy. 2-2CThe 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. 2-3CThe macroscopicforms of energy are those a system possesses as a whole with respect to some outside reference frame. The microscopicforms 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. 2-4CThe 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. 2-5CThe 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. 2-6CThermal energy is the sensible and latent forms of internal energy, and it is referred to as heat in daily life. 2-7CThe mechanical energyis 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. 2-2 2-8A 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. Assumptions1 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. PropertiesWe take the density of water to be = 1000 kg/m3. AnalysisNoting 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/kg887./sm1000kJ/kg12)m/s3(m)90)(m/s(9.81222222mech=+=+=+=Vghkepee90 m River 3 m/sThe power generation potential of the river water is obtained by multiplying the total mechanical energy by the mass flow rate, kg/s500,000/s)m00)(5kg/m1000(33===V&&mMW444=====kW000,444kJ/kg)7kg/s)(0.88000,500(mechmechmaxemEW&&&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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This note was uploaded on 03/19/2008 for the course MAE 301 taught by Professor Hassan during the Spring '08 term at N.C. State.
- Spring '08