THERMODYNAMICS QUESTIONS

1. FIG 1 below shows a combined Gas- steam cycle featuring two gas turbines T1 and T2 and a steam turbine T3. These turbines drive the compressor A and the electric generator G1 and G2. The compressor is of pressure ratio ɤpc adiabatic efficiencies of the compressor, the turbines and the pump are ηc, ηT1, ηT2 and ηp respectively. Numerical data are tabulated here under:
T1a T3a T5a T2b P2b P3b ɤPC ηC ηT1 ηT2 ηP
295 1200 440 500 30 10 6 82 85 82 78
---- (K) - - - - - - (C) (bar)- - - - - - - - - (%) - - - --
What would be (i) the mass flow rate ratio of air and steam (ṁa/ṁs), and (ii) the overall thermal efficiencies (ηth) of the combined cycle.

2. FIG 2 depicts possible application of the Brayton cycle for automotive propulsion.
Two adiabatic turbines C and D of efficiency ηT are employed, turbine C devoted to the
Compressor A (adiabaically) and turbine D, the power turbine for driving the drive
Wheels, interposed between the turbines is a throttle valve V. The compressor of
Efficiency ηC feeds the burner B (essentially a constant-pressure heat exchanger) which
Is supplied with fuel, the mass flow rate is negligible compared with the air (ideal gas of
Constant Cv and R). Two scenarios (I and II) are envisaged, the former being the full load
(wide-open throttle situation with P4= P5), and the latter the no- load (with partially
Open throttle situation and P5=P6). For each of the two scenarios,
(i) Draw the TS diagram and evaluate
(ii) P5, T5, P6, T6
(iii) The specific work output of the turbines
(iv) The energy conversion efficiency (scenario I only)
(v) Given Qcv as the calorific value of the fuel, what specific fuel consumption
(kg/KWh) occurs in scenario I?
(vi) The specific heat transfer in the burner
(vii) What air flow rate would be called for to support a particular power output Wdo.
Data are displayed:
ηT ηC Cv R Qcv Wd.o T1 T3 P1 P2 P5
(%) (KJ/Kg.K) (MJ/Kg) (KW) (K) - (bar)
80 75 0.719 0.287 44 250 300 1125 1 5 1