Thermodynamics

58 | ThermodynamicsThe complete fission of 0.1 kg of uranium-235 releases16.73 10 10 kJ>kg210.1 kg2 6.73 10 9 kJof heat, which is sufficient to meet the energy needs of the car forEnergy content of fuelNo. of days Daily energy use 6.73 109 kJ 40,790 days165,000 kJ>daywhich is equivalent to about 112 years. Considering that no car will last morethan 100 years, this car will never need refueling. It appears that nuclear fuelof the size of a cherry is sufficient to power a car during its lifetime.Discussion Note that this problem is not quite realistic since the necessarycritical mass cannot be achieved with such a small amount of fuel. Further,all of the uranium cannot be converted in fission, again because of the criticalmass problems after partial conversion.Mechanical EnergyMany engineering systems are designed to transport a fluid from one locationto another at a specified flow rate, velocity, and elevation difference,and the system may generate mechanical work in a turbine or it may consumemechanical work in a pump or fan during this process. These systemsdo not involve the conversion of nuclear, chemical, or thermal energy tomechanical energy. Also, they do not involve any heat transfer in any significantamount, and they operate essentially at constant temperature. Suchsystems can be analyzed conveniently by considering the mechanical formsof energy only and the frictional effects that cause the mechanical energy tobe lost (i.e., to be converted to thermal energy that usually cannot be usedfor any useful purpose).The mechanical energy can be defined as the form of energy that can beconverted to mechanical work completely and directly by an ideal mechanicaldevice such as an ideal turbine. Kinetic and potential energies are thefamiliar forms of mechanical energy. Thermal energy is not mechanicalenergy, however, since it cannot be converted to work directly and completely(the second law of thermodynamics).A pump transfers mechanical energy to a fluid by raising its pressure, anda turbine extracts mechanical energy from a fluid by dropping its pressure.Therefore, the pressure of a flowing fluid is also associated with its mechanicalenergy. In fact, the pressure unit Pa is equivalent to Pa N/m 2 N ·m/m 3 J/m 3 , which is energy per unit volume, and the product Pv or itsequivalent P/r has the unit J/kg, which is energy per unit mass. Note thatpressure itself is not a form of energy. But a pressure force acting on a fluidthrough a distance produces work, called flow work, in the amount of P/rper unit mass. Flow work is expressed in terms of fluid properties, and it isconvenient to view it as part of the energy of a flowing fluid and call it flowenergy. Therefore, the mechanical energy of a flowing fluid can beexpressed on a unit mass basis ase mech P r V 22 gz(2–10)