A large stationary Brayton-cycle gas turbine power plant delivers a power output of 100 MW to an electric generator. The minimum temperature

A large stationary Brayton-cycle gas turbine power plant delivers a power output of 100 MW to an electric generator. The minimum temperature in the cycle is 300 K, and the maximum temperature is 1600 K. The minimum pressure in the cycle is 100 kPa, and the compressor pressure ratio is 14. The isentropic efficiencies of a compressor and a turbine are 90% and 95%, respectively. Calculate the power output of the turbine. What fraction of the turbine output is required to drive the compressor? What is the thermal efficiency of the cycle? Make appropriate assumptions.

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  1. Answer:

    a) [tex]w_{NET}=511.7kJ/kg[/tex]

    b) [tex]q_H=965.7kJ/kg\\[/tex]

    c) [tex]\eta_{TH}=0.530[/tex]

    Explanation:

    Given That:

    Minimum temperature [tex]T_1[/tex] = 300 K

    Maximum temperature [tex]T_2[/tex] = 1600 K

    Compressor pressure ratio [tex]\frac{P_2}{P_1}= 14[/tex]

    k = 1.4

    For the compression in the compressor:

    [tex]T_2=T_1(\frac{P_2}{P_1})^{\frac{k-1}{k} } =300(14)^{\frac{1.4-1}{1.4} }=638.1K[/tex]

    [tex]w_c=h_2-h_1=C_{po}(T_2-T_1)=1.004(638.1-300)=339.5kJ/kg[/tex]

    For the expansion in the turbine:

    [tex]T_4=T_3(\frac{P_3}{P_4})^{\frac{k-1}{k} } =1600(\frac{1}{14} )^{\frac{1.4-1}{1.4} }=851.2K[/tex]

    [tex]w_t=h_3-h_4=C_{po}(T_3-T_4)=1.004(1600-752.2)=851.2kJ/kg\\w_{NET}=w_t-w_c=851.2-339.5=511.7kJ/kg[/tex]

    The overall net and cycle efficiency is given by:

    [tex]m=\frac{W_{NET}}{w_{NET}}=\frac{100000}{511.7}=195.4kg/s[/tex]

    [tex]W_t=mw_t=195.4*851.2=166.32MW\\w_c/w_t=339.5/851.2=0.399[/tex]

    The energy input to the combustor is:

    [tex]q_H=C_{po}(T_3-T_2)=1.004(1600-638.1)=965.75kJ/kg[/tex]

    [tex]\eta_{TH}=w_{TH}/q_H= 511.7/965.7=0.530[/tex]

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