ME354 Thermodynamics 2

Problem Set #7: Brayton Cycle/Jet Propulsion

Problem 7: 9-73

An ideal Brayton-cycle gas turbine with regeneration but no reheat or intercooling operates at full load with an inlet temperature to the compressor of 60 oF and an inlet temperature to the turbine of 1540 oF. The cycle is also to be operated at part load at a pressure ratio of 3.0 with a turbine inlet temperature of 1100 oF. Compare the thermal efficiency and net work output per lbm flow for the two operating points if the working fluid (air) is treated as a perfect gas and the maximum possible regeneration is obtained.

Answer:

full load:	nt = 56.2%, Wnet = 109 Btu/lbm
part load:	nt = 54.3%, Wnet = 55 Btu/lbm

Solution

(73.2 KBytes)

Problem 7: 9-75

Redo Problem 5: 9-73 using compressor and turbine isentropic efficiencies of 0.8 and 0.85, respectively, and a regenerator effectiveness (Qact/Qmax) of 0.85.

Answer:

full load:	nt = 33.3%, Wnet = 58.9 Btu/lbm
part load:	nt = 27.7%, Wnet = 28.2 Btu/lbm

Solution

(72.5 KBytes)

Problem 7: 3

A simple gas-turbine engine operates with air as the working fluid, a pressure ratio of 18:1, and a maximum temperature of 700 oC. The air enters the compressor at 100 kPa, 20 oC. Determine the thermal effieiency, specific heat addition, and state of air exhausted by the turbine when it is modeled by the Brayton cycle.

Answer:

n = 0.562

Q2-3/m = 305.1 kJ/kg

T4/m = 426 K

Solution

(70.1 KBytes)

Problem 7: 4

A simple gas-turbine engine operates with air as the working fluid, a pressure ratio of 18:1, and a maximum temperature of 700 oC. The air enters the compressor at 100 kPa, 20 oC. The compression and expansion isentropic efficiencies are 0.87 and 0.94, respectively. Calculate the specific entropy generation and chaange in availability for each process in the cycle. The temperature of the energy source reservoir is the same as the maximum cycle temperature, and the temperature of the energy sink reservoir is the same as the minimum cycle temperature.

Solution

(94.5 KBytes)

Problem 7: 5

An aircraft with a turbofan propulsion system is flying at a speed of 200 m/s at an altitude where pressure is 60 kPa and temperature is 0 oC. The compressor has a pressure ratio of 8:1, the maximum Brayton cycle temperature is 400 oC, and the area of the opening servicing the fan engine is 3 m2, and the mass flow rate ratio, defined as rm = (mt + me)/me, is 4:1. Where mt is the thrust generating mass flow rate and me is the mass flow rate needed to supply the Brayton cycle engine. Determine the thrust force generated by this propulsion unit.

Answer:

Ft = 36.52 kN

Solution

(73.1 KBytes)

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