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Unformatted text preview: 2-1Chapter 2 ENERGY, ENERGY TRANSFER, AND GENERAL ENERGY ANALYSIS Forms of Energy2-1CIn 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 isconverted 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 someoutside reference frame. The microscopicforms of energy, on the other hand, are those related to themolecular structure of a system and the degree of the molecular activity, and are independent of outsidereference 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 dailylife.2-7CThe mechanical energyis the form of energy that can be converted to mechanical work completelyand directly by a mechanical device such as a propeller.It differs from thermal energy in that thermalenergy cannot be converted to work directly and completely. The forms of mechanical energy of a fluidstream are kinetic, potential, and flow energies. 2-22-8A 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. 2The velocity given isthe average velocity. 3The mechanical energy of water at the turbine exit is negligible.PropertiesWe take the density of water to beU= 1000 kg/m3.AnalysisNoting that the sum of the flow energy and the potential energy is constant for a given fluidbody, we can take the elevation of the entire riverwater to be the elevation of the free surface, andignore the flow energy. Then the total mechanicalenergy of the river water per unit mass becomeskJ/kg887./sm1000kJ/kg12)m/s3(m)90)(m/s(9.81222222mech...Vghkepee90 mRiver3 m/sThe power generation potential of the river water is obtained by multiplying the total mechanical energy bythe mass flow rate,kg/s500,000/s)m00)(5kg/m1000(33V&&UmMW444kW000,444kJ/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 09/21/2009 for the course ME 311 taught by Professor Ferrenberg during the Fall '09 term at Nevada.
- Fall '09