Abstract :
GAS TURBINE
Of the
various means of producing mechanical power, the turbine is in many respects
the most satisfactory. The absence of reciprocating and rubbing members means
that balancing problems are few, that the lubricating oil consumption is
exceptionally low, and that reliability can be high. The inherent advantages of the turbine were
first realized using water as the working fluid, and then it changed to steam
for electricity generation. Since the
cost of setting up an installation for producing steam is very high hot gases
themselves started to be used for running the turbine. Then the turbines were called Gas Turbines.
In a Gas
Turbine the working fluid, which is compressed to a very high pressure is
allowed to expand. The power developed
by the turbine can be increased by the addition of energy or increasing the
temperature of the working fluid prior to expansion. The expansion of the hot working fluid then
produces a greater power output. The
following fig represents the gas turbine system in its simplest form.
The
compressor takes atmospheric air and compresses to a very high pressure. Then in the combustion chamber it is imparted
high energy. Then it is allowed to expand through the turbine. We will get the power O/P at the shaft.
Depending
on how the working fluid is guided to expand and the shape of the turbine
blades, they are classified into Radial flow and Tangential flow turbines. Even if the increase in inlet temperature of
the working fluid increases the work O/P and hence the efficiency of the
turbine there are certain limitations for that, so we must cool the turbine
using appropriate cooling methods.
GAS TURBINE
BLADE
Gas turbine
blade is that part of the turbine which will guide the gas through the required
angle to expand with minimum loss. It is
usually small in size and the largest one will be only of the size of the palm
of the hand.
The gas
enters the row of nozzle blades called stator blades or nozzle guide vanes with
a static pressure and temperature P1&T1 and a velocity C1. It is expanded to P2&T2 and leaves with
an increased velocity C2 at an angle a2. The rotor
blade inlet angle will be chosen to suit the direction b2 of the
gas velocity V2 relative to the blade at the inlet.b2 and V2
are found by vectorial subtraction of the blade speed from the absolute
velocity C2. After being deflected the
gas get expanded in the rotor blade passages and leaves at P3&T3 with
relative velocity V3 at angle b3. Vectorial
addition of U yields the magnitude and direction of the gas velocity at exit
from the stage C3 and a3. a3 is known as swirl angle.
In
multistage turbine C1and a1 will be probably equal to C3 and a3
LIQUID
COOLING
In liquid
cooling some liquid is used as coolant.
1. Spray
cooling
In spray
cooling the coolant is sprayed to the blade surface or to the stator, which
ever is to be cooled according to the amount and rate of cooling required.
2. Forced
convection
In forced
connection small passages are made inside the parts to be cooled through which
coolant is passed and the heat is taken away. Depending upon the amount of
coolant flow and the coolant used the blade temperature can be decreased to
various extend. It is shown in the
graph.
LOSSES DUE
TO COOLING
1. There is a direct loss of turbine work due to
reduction in turbine mass flow.
2. The expansion is no longer adiabatic, and
furthermore, there will be negative reheat effect in multi stage turbines.
3. Due to the mixing of spend cooling air with main
stream at the blade tips, there is a pressure loss.
4. Some pumping work is done by the blade on the
cooling air when is passed radically
outwards through the cooling passages.
5. Due to all of these reasons there is a loss of 1 to
3 percent of turbine efficiency compared to uncooled one.
ADVANTAGES
OF COOLING
1. Use of a higher blade loading co-efficient y- The blade
loading co-efficient expenses the work capacity of a stage. So by increasing the blade loading
co-efficient we are reduce the no. of stages to a min.
2. Higher pitch/chord ratio- this will help number of
blades in a row.
3. Higher flow co-efficient q - which
implies a blade of smeller comber and hence smaller surface area.
4. The time for 0.2% increase as the blade temperature
decrease.
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