The attached figure depicts an indirectly fired gas turbine engine. In the gas turbine engine, air is
compressed to a high pressure and then heat is added. The high pressure, high temperature air is then
expanded through a turbine with power being extracted. To a first approximation, the compressor and
turbine may be treated as adiabatic. In a typical gas turbine, the fuel is added to the air and combusted
and it is actually the combustion products that pass through the turbine. In the indirectly fired turbine,
as is used in this problem, the air is heated with a heat exchanger which allows heat sources that could
otherwise not be used (e.g. solar, coal, etc.). The net work out of the gas turbine engine is the work out
of the turbine less the work into the compressor. In a real engine these devices are normally all coupled
on a common shaft.
Since the work of compression decreases the net work out, anything you can do to decrease the
compression work will increase the net work out. One way to do that is to cool the air being
compressed. The attached diagram shows a proposed system where the compression is divided into
two stages and liquid water is sprayed into the air to cool it between the two stages. It is proposed that
this approach will increase power output and increase efficiency. Your task is to determine if this
approach is thermodynamically feasible. You may neglect the pressure drops between components and
within the heat exchanger and you may neglect kinetic energy. The following design parameters are
provided. For these design parameters, determine the effect of the water spray on efficiency (net work
out divided by heat input) and net power out per kg/s of dry air coming into the turbine. Do this for a
range of water spray inputs from 0 up to the amount required to achieve 70% relative humidity at point
3.
Point 1
T= _20C______
P= _0.85 bar______
Ø = __50%_____
Point 2
P= __3.5 bar_____
Point 3
P= ___10.0 bar_____
Point 4
T=__1220K_______
Point 5
P= P1
Compressor A: ?isen = __0.87___ Compressor B: ?isen = _0.83____ Compressor C: ?isen = __0.93