We were
discussing Otto cycle, an ideal cycle for internal
combustion spark ignition reciprocating engines or simply petrol engines and
also Diesel
cycle, the ideal cycle for the operation of internal combustion compression
ignition reciprocating engines
in our previous posts. We have also discussed the derivation of efficiency of Otto cycle and efficiency of Diesel cycle.
We will
see here the very important topic i.e. Open cycle gas turbine power plant or
open cycle gas turbine engine with the help of this post.
An open cycle gas turbine engine
Open cycle gas turbine engines are usually used in aircraft engines, marine engines and
also in automobile engines. Compressor, Turbine and combustion chamber are main
components of an open cycle gas turbine engine.
Let us see
the arrangements of various components of an open cycle gas turbine engine.
Fresh air
will enter in to the compressor at atmospheric pressure and temperature, where
pressure and temperature of air will be increased. Now air at high pressure and
high temperature will enter to the combustion chamber as shown in above figure.Â
Fuel will
be injected from outside in to the combustion chamber and therefore burning of
fuel will take place within the combustion chamber in presence of high pressure
and high temperature air. Hence combustion product i.e. gas will have high
pressure and high temperature at the discharge of the combustion chamber.
High pressure
and high temperature gas will enter in to the turbine, where high pressure and
high temperature gas will be expanded through the turbine. Pressure and
temperature of the gas, both will be dropped here.
There will
be drop in temperature of gas but still temperature of gas will be high, while
pressure of gas will be reduced to atmospheric pressure and hence we can say
that high temperature gas will be rejected to the atmosphere at atmospheric
pressure.
As we can
observe here that fresh air is entering in to the compressor at atmospheric
pressure and temperature and high temperature gas is rejected here to the
atmosphere at atmospheric pressure and exhaust gases leaving the turbine are not
re-circulated but also going to the atmosphere therefore this cycle will be
termed as open cycle gas turbine engine.
Work energy will be generated from the turbine during the expansion of high pressure
and high temperature gas and some part of this generated work will be used to
drive the compressor and hence compressor and turbine are assembled with common
shaft as shown in above figure.
Let us see the processes involved in an open cycle gas turbine engine
Process
1-2: Isentropic compression process, fresh air entering in to the compressor at
atmospheric pressure and temperature will be compressed here at high pressure
and high temperature.
Process
2-3: Constant pressure heat addition in to the combustion chamber
Process 3-4:
Isentropic expansion process, high pressure and high temperature gas will be expanded
through the turbine.
Let us see here the thermal efficiency of the open cycle gas turbine engine
We will
see here the various energy calculations for unit mass
Input work
energy to the compressor, WC = CP (T2-T1)
Output work
energy from the turbine, WT = CP (T3-T4)
Heat added
in the combustion chamber at constant pressure, QA= CP (T3-T2)
Net work
output, W= WT - WC = CP (T3-T4)
- CP (T2-T1)
η thermal
= Net work output / Heat added
η thermal = [CP (T3-T4) - CP (T2-T1)]/ CP (T3-T2)
Do you
have any suggestions? Please write in comment box.
We will
see another topic in our next post in the category of thermal engineering.
Reference:
Engineering
thermodynamics by P. K. Nag
Engineering
thermodynamics by Prof S. K. Som
Image
courtesy: Google
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