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The mass flow value of the air was obtained by measuring the flow at the inlet of the gas turbine using an ion-gun velocimeter. Figure 3-18 shows the values obtained across the inlet. These values give an average flow rate of72O，868 lbslhr(327，667 kglhr). This flow rate is within experimental accu-racy. The temperature drop in the turbine is based on an energy balance and is given by the following equation:
-Wload +m的 a cPcavg (3-45)
eTtact Tgen(mj +m的 a)cPavg (m的 j +m的 a) cPtavg eTcact
where:
Wload -generator output in kilowatts
Tgen -generator efficiency
-turbine average specific heat
cPtavg
-compressor average specific heat
cPcavg
eTtact -temperature drop in turbine
64 in. (1630 mm)
49 68 42
43 68 39
37 68 49
48 68 52
Average Velocity = 55.3 ft/sec, 16.9 m/sec Assumed Blockage = 2.8
22
Inlet Area = 53.8 ft , 16.9 m
33
Average Density = 0.71 lb/ft , 1.14 kg/m 67 7161 Mass Flow Rate = 720,868 lb/hr, 327,667 kg/hr Percent Deviation = +0.1%
Figure 3-18. Typical inlet velocity profile for an industrial gas turbine.
The temperature drop calculated in this manner was compared to the drop calculated by subtracting the measured average exhaust temperature reading from the inlet temperature as obtained by the previous equation. The differ-ence between these two methods was about 2O. at the high-temperature exit.The second method gives a smaller drop， indicating that the temperaturerecorded is lower than the actual temperature. This result is expected， since the thermocouples are placed a distance downstream from the turbine blades and are not measuring the actual gas exhaust temperature. This comment isnot a criticism of the controlpackage， since that operates on a base exhaust temperature.
The turbine efficiency can now be calculated with the use of the following
relationship:
Tt - eTtact (3-46)
. .
. . 1 . .
Tt3 1 -. . ，-1. .
. Pt3 ，.
. Pt4 .
where the value of， was an average value in the turbine.
The gas turbine is coupled with a steam recovery boiler. The exhaust gas from the turbine is used to supplement fire the boiler. The thermal efficiency of the gas turbine alone was calculated by using the following relationship:
Wload ×K
Tad -(3-47)
(LHV)×Q
where:
K -3，412 BtU /kW-Hr(3，6OO KJ /kW-hr) LHV -heating value Btu/ft3(kJ/cu m) Qft -volume flow rate of fuel to turbine， ft 3/hr (cu m/hr)
The overall system efficiency is based on the following equation:
Wload×K Tsad -(LHV)×Q -m的 sb(hs -hfw)+(LHV)Qfb (3-48)
Figure 3-19. Combined cycle and simple cycle efficiency as a function of gas turbine load.
(%) load
Figure 3-20. Fuel consumption as a function of gas turbine load.
where:
m的 sb -mass flow of steam from recovery boiler
hs -enthalpy of the superheated steam
hfw -enthalpy of the feedwater
Qfb -volume flow rate of fuel to boiler
Figure 3-19 shows the thermal efficiency of the gas turbine and the Brayton-Rankin cycle (gas turbine exhaust being used in the boiler) based on the LHVof the gas. This figure shows that below 5O% of the ratedload，the combination cycle is not effective. At full load， it is obvious the benefits one can reap from a combination cycle. Figure 3-2O shows the fuel con-sumption as a function of theload， and Figure 3-21 shows the amount of steam generated by the recovery boiler.
Bibliography
Balje，O.I.， ""A Study of Reynolds .umber Effects inTurbomachinery，"" Journal oj Engineering jor Power， ASMETrans.， Vol.86， SeriesA，1964， p. 227.
中国航空网 www.aero.cn
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