We wish to do an ideal thermodynamic analysis of the General Electric T700 gas turbine engine, which is used to power the Army Black Hawk helicopter. Consider the schematic diagram of the engine shown in the figure below:
Notice that there are two turbines operating on independent output shafts. The High Pressure (first) turbine, named the Gas Generator Turbine, is directly connected by a shaft to the compressor. Its sole purpose is to drive the the axial/centrifugal compressor, thus the energy output of this turbine must equal the energy consumed by the compressor. The Low Pressure (second) turbine, named the Power Turbine, is connected via gearing to the helicopter rotor.
Problem
6.9 - Assume
that the compressor and both turbines are isentropic, and that
the combustion process occurs at constant pressure (isobaric).
Using the information shown on the schematic diagram above, do
the following:
(Data obtained through private communication with Dr. Tom Scott)
Note: Because of the large temperature variation throughout this problem we will need to consider the temperature dependence of the Specific Heat Capacities of Air. All thermo texts that we know of present a method of doing this using the tabulated function s0, relative pressure Pr, and relative specific volume vr. We prefer the simpler approach of using a constant specific heat capacity CP and ratio of specific heats k, where the values are chosen at the average system temperature. This has always provided an answer within around 1% accuracy. In the above schematic diagram we see that the temperature extremes of the system are 16°C - 1000°C (289 K - 1273 K), giving an average temperature of 781 K. From the table of Specific Heat Capacities of Air we see that at 800 K, CP = 1.099 [kJ/kg.K] and the ratio of specific heat capacities k = 1.354, thus we use those values throughout this problem.
Solution Approach:
Notice that the various temperatures and pressures shown on this diagram are evaluated and plotted as we progress with the solution. Notice
Note that the actual power putput of the T700 engine is around 1800 hp, which is significantly less than the above value. This is because we have assumed that the compressor and both turbines are isentropic, which will never occur in practice. As an exercise try to do the following extension to this problem:
C = 88%, and that each turbine has an efficiency T = 86%. Show the effects of these efficiencies on
the h-s diagram, and discuss how the power output of both
turbines will be affected. Determine the actual power output
of the power turbine (MW).During Fall Quarter 2008 Dr. Tom Scott presented an inspiring and animated demonstration of the cutaway T700 gas turbine engine as shown below:
During the talk he mentioned that one could unclamp and remove the power turbine at the rear and replace it with a nozzle to form a turbojet engine. This captured our imagination, hence we would like to do an initial ideal thermodynamic analysis of this conversion. Consider the schematic diagram of this conversion shown in the figure below:
Notice that the h-s diagram has retained the same shape and characteristics, with the only difference being that the change in enthalpy on the nozzle gives rise to a kinetic energy increase, rather than work output.
Links: General Electric T700
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Engineering Thermodynamics by Israel Urieli is licensed under a
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