30-08-2013, 02:49 PM
COMPUTATIONAL INVESTIGATION OF GAS TURBINE BLADE COOLING
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ABSTRACT
The nozzle guide vanes are subjected to the highest temperature of the gas
coming out of the combustor in a gas turbine engine. One method used to cool the
vanes are to use rows of film cooling holes to inject cooled air bled from the compressor
lower pressure stage. The main purpose of this project is to investigate computationally
the film cooling phenomena in gas turbine vane cascade. Computational fluid dynamics
(CFD) has been used for the simulation of the aerothermodynamics of film cooling. Two
types of holes have been investigated; simple cylindrical hole and fan-shaped hole.
Initially a three dimensional cascade with film cooling holes and plenum chamber was
developed, but due to the limitation of computational resources the model was aborted
and then a two dimension model was investigated. The modeling of the domain has
been done in Gambit 2.0.1. Unstructured grid (quad-pave scheme) was used for the
solver. The solver used for the CFD simulation was FluentTM 6.0.1. RNG Ÿ-Ɛ turbulence
model with standard wall function was selected for the simulation.
Background: -
India is a rapidly developing economy, with a need for dependable and reliable
supply of electricity and to be a power sufficient country is one of its prime concerns.
The present installed capacity of electricity in India is 132,110.21 MW which gives
the per capita consumption of power in 2005-06 as calculated by the Central Electricity
Authority about 631 Kilowatt Hours. While the per capita consumption of power in
developed countries like U.S is 13338 KWH. The National Electricity Policy envisages
that the per capita availability of electricity will be increased to over 1000 units by 2012.
So large number of new power projects are currently in progress. Thermal power plants
accounts for 64.7% of the installed capacity. Because of higher efficiency gas turbine
and combined cycle power plants are becoming more and more attractive with regard to
reduced fuel consumption and less emissions. Modern gas turbine engines tend to push
up the working gas temperature to further increase the cycle efficiency.
Gas Turbine: -
Of the various means of producing either thrust or power, the gas turbine engine
is one of the most satisfactory. Its main advantages are: exceptional reliability, high
thrust-to-weight ratio, and relative freedom of vibration. A gas-turbine engine consists of
the following main parts: an inlet, a compressor, a combustor, a turbine and an exhaust,
as shown in Fig. 2.1.
Nowadays there is a dire need to increase the performance of power plants and
jet engines because of dwindling non-renewable sources of energy and to cut the green
house gases (GHGs) emissions (the power companies has the largest share, about
60%, of the GHGs emissions). In general, speaking broadly, the efficiency of a gas
turbine depends on two main factors: (i) the basic cycle efficiency of the installation, and
(ii) the efficiency of the individual units. Still speaking broadly, it is said that first one is
inherent in design and is not subject to deterioration in any way, whereas the second
factor will vary in accordance with the condition of the equipment and the amount of
maintenance given to it.
Various Cooling Techniques: -
The turbine blades are exposed to a continuous flow of gas that may enter the
turbine at a temperature between 850oC to 1700oC as shown in fig. 2.5.
Modern turbine stage inlet temperatures exceed the melting point of turbine
blade materials. The HPT (High Pressure Turbine) first stage blade is one component
that is extremely vulnerable to high temperature. This temperature is far beyond the
melting point of current materials technology. The turbine blades are required to perform
and survive long operating periods at temperatures above their melting point. Various
internal and external cooling techniques are employed to bring down the temperature of
the blade material temperature below its melting point. Figure 2.6 shows the common
cooling technique with three major internal cooling zones in a turbine blade.
Film Cooling Parameters: -
Film-cooling has been one of the most extensively studied cooling methods
over the last two decades due to its wide variety of practical applications in high-
temperature systems such as turbine blades, end walls, combustors, and after-burners.
A lot of studies have been performed to enhance film-cooling effectiveness for various
injection systems in case of turbine vane. Designers are trying to achieve greater
cooling performance from less coolant air, particularly in next-generation high-efficiency
gas turbines. To make significant advances in cooling technology, it requires a
fundamental understanding of the physical mechanisms involved in film-cooling flow
fields. Definition of coordinates and dimensions of the cooling hole are given in fig.
2.12.The film cooling performance is influenced by variety of parameters which can be
listed as below:-
Summary of Past Studies: -
Goldstien et al [20, 21] provided an early review of these film cooling
techniques and measured experimentally the film cooling
effectiveness and heat
transfer coefficient distribution on inside hole surfaces, internal walls and on the
exposed surface for a blowing ratio of 0.2 to 2.2. Actually he used heat/mass transfer
analogy to convert mass transfer results into heat transfer results to avoid any error (it is
very difficult to measure the heat transfer rate inside the holes as such measurement
involves large conduction error due to variation in heat transfer rate resulting from sharp
temperature gradients at the hole entrance.) He showed that Sherwood no. is high on
the side of the hole due to the interaction of the coolant with the mainstream gas. The
mass transfer around the hole was dominated by the formation of horse shoe, side and
kidney vortices generated by jet stream and cross flow interaction as shown in fig. 2.15.
The film cooling effectiveness was high and uniform in the streamwise direction but not
in the lateral direction.