Questions
Work at a rate of 25 kW is done by a paddle wheel on a slurry contained within a closed, rigid tank. Heat transfer from the tank occurs at a temperature of 250°C to the surroundings that, away from the immediate vicinity of the tank, are at 27°C. Determine the rate of entropy production, in kW/K, (a) for the tank and its contents as the system, and (b) for an enlarged system including the tank and enough of the nearby surroundings for the heat transfer to occur at 27°C. Assume steady state operation in both cases.
Air enters an insulated turbine operating at steady state at 6.5 bar, 687°C and exits at 1 bar, 327°C. Neglecting kinetic and potential energy changes and assuming the ideal gas model, determine (a) the work developed, in kJ per kg of air flowing through the turbine, (b) the entropy production per kg of air, in kJ/kg K, and (c) whether the expansion is internally reversible, irreversible, or impossible.
A power system operating at steady state consists of an air compressor having an isentropic compressor efficiency of 80%, a heat exchanger, and a turbine having an isentropic turbine efficiency of 90%. Air enters the compressor at 1 bar, 300K with a mass flow rate of 5.8 kg/s and exits at a pressure of 10 bar. Air enters the turbine at 10 bar, 1400 K and exits at a pressure of 1 bar. Air can be modeled as an ideal gas. Stray heat transfer and kinetic and potential energy effects are negligible. Determine, in kW, (a) the power required by the compressor, (b) the power developed by the turbine, and (c) the net power output of the overall power system.