An electrical generator is used to measure the torque available from the output shaft of a prime mover. The stator of the generator is carried on trunnion bearings and is restricted from rotating under the interaction of rotor and stator magnetic fields by means of a torque arm (see figure 2.1). (The torque arm must be horizontal during use so that the torque is the product of the net restraining force and the torque arm radius.) For a particular trial a torque arm of 0.5 m carries a fixed load of 120 N. This downward force is greater than can be supported by the torque transmitted from rotor to stator and the force difference required to restrain the stator is provided by a spring balance registering 15 N. Calculate the shaft power for a rotational speed of 3000 rev/min.
Throughout this volume all tabulated fluid properties are taken from ‘Thermodynamic and Transport Properties of Fluids’ by Y.R. Mayhew and G.F.C. Rogers published by Basil Blackwell, hereafter referred to as ‘tables’.
A mass of 0.5 kg of water at a temperature of 130 °C and a pressure of 1 000 000 Pa is heated isobarically until the final temperature of 200 °C is reached. Calculate the heat transfer, the change of volume and the work transfer. It is now cooled isochorically to a final pressure of 200 000 Pa. Determine the heat transfer.
The nozzles of a steam turbine receive steam at 3 bar and 150 °C with negligible velocity. The steam expands adiabatically through the nozzles and leaves at 1 bar with a dryness fraction of 0.96. Calculate
(a)
the exit velocity,
(b)
the total exit area for a mass flow rate of 10 kg/s.
A reciprocating compressor raises air from atmospheric conditions of 100 kN/m2 and 15 °C to 1000 kN/m2 at a rate of 2 kg per min. The air in the delivery pipe is at 155 °C and the internal diameter of the pipe is 50 mm. The inlet velocity is negligible. Calculate the heat transfer between the compressor and its surroundings when the power input to the compressor is 6 kW.
In this chapter a new fundamental concept appears — namely momentum — and in addition to the principles of conservation of mass and energy previously encountered we have to use the third principle — conservation of momentum which is embodied in Newton’s laws of motion.
The only equations used in this chapter are the characteristic gas equation and that expressing the principle embodied in Dalton’s law of partial pressures.
In stoichiometric calculations the student is continually being required to distinguish between mass balance in his equations, which must hold good, and molecular changes.
The chief aim in introducing the second law of thermodynamics is to derive the property entropy and the basic difficulty lies as much in the rather lengthy preamble as in any conceptual obscurity in the property itself.
In chapters 15 and 16 we attempt to apply the fundamental work of previous chapters to vapour and gas power cycles respectively. Three fundamental criteria need defining here.
In chapter 16 an attempt is made to compare the outputs and the efficiencies of various idealised gas power cycles by specifying as many common features as are consistent with the natures of the cycles considered.
Engineers have a prime concern with the production of mechanical work. Since all processes are irreversible in practice, the capacity to perform work is continually being degraded by virtue of the unwanted heat transfers which occur because of viscous friction and large temperature differences between the working fluid and its environment.
