This document contains three thermodynamics assignment questions about ideal cycles: 1) A Brayton cycle with air has its pressure ratio doubled from 6 to 12, requiring calculations of the change in net work output and thermal efficiency. 2) An Otto cycle with air operating between 98 kPa and 27°C is analyzed, requiring calculations of heat input, net work output, thermal efficiency, and mean effective pressure. 3) A Rankine cycle steam plant with regeneration and reheating is modeled on a T-s diagram, requiring calculations of extraction fraction, thermal efficiency, and net power output.

1. ME 234 Thermodynamics II
Assignment No. 1 Due Date : December 08, 2014
Question No. 1. Consider a simple ideal Brayton cycle with air as the working fluid. The
pressure ratio of the cycle is 6, and the minimum and maximum temperatures are 300 and 1300
K, respectively. Now the pressure ratio is doubled without changing the minimum and maximum
temperatures in the cycle. Determine the change in (a) the net-work output per unit mass and (b)
the thermal efficiency of the cycle as a result of this modification. Assume constant specific heats
for air.
Question No. 2. An ideal Otto cycle has a compression ratio of 9.2 and uses air as the
working fluid. At the beginning of the compression process, air is at 98 kPa and 27°C. The
pressure is doubled during the constant-volume heat-addition process. Accounting for the
variation of specific heats with temperature, determine (a) the amount of heat transferred to the
air, (b) the net work output, (c) the thermal efficiency, and (d) the mean effective pressure for the
cycle.
Question No. 3. A steam power plant operates on an ideal reheat–regenerative Rankine
cycle with one reheater and two feedwater heaters, one open and one closed. Steam enters the
high-pressure turbine at 15 MPa and 600°C and the low pressure turbine at 1 MPa and 500°C.
The condenser pressure is 5 kPa. Steam is extracted from the turbine at 0.6 MPa for the closed
feedwater heater and at 0.2 MPa for the open feedwater heater. In the closed feedwater heater,
the feedwater is heated to the condensation temperature of the extracted steam. The extracted
steam leaves the closed feedwater heater as a saturated liquid, which is subsequently throttled
to the open feedwater heater. Show the cycle on a T-s diagram with respect to saturation lines.
Determine (a) the fraction of steam extracted from the turbine for the open feedwater heater, (b)
the thermal efficiency of the cycle, and (c) the net power output for a mass flow rate of 42 kg/s
through the boiler.