31-08-2012, 02:46 PM
Design and Analysis of Steam Turbine Blades using FEM
1Design and Analysis.pdf (Size: 321.94 KB / Downloads: 376)
Abstract
The analysis of Stress values that are produced while the turbine is running
are the key factors of study while designing the steam turbines. Hot section
blades typically fail because of creep, oxidation, low cycle fatigue and High
cycle fatigue.
Therefore this report has primarily focused on heat transfer characteristics,
centrifugal and thermal stresses in the blade. The maximum stresses obtained
from different analysis by using innovative high heat resistant material N155
are found to be within the yield strength of the material. Results are obtained
interms of thermal stresses, Vonmisses stress are compared with high heat
resistant material INCONEL 718.Inspite of all operating conditions the blade
with material INCONEL 718 is expected to perform well .The object is to
improve the life and efficiency of future generation of engines.
Introduction
The purpose of turbine technology are to extract the maximum quantity of energy
from the working fluid to convert it into useful work with maximum efficiency by
means of a plant having maximum reliability, minimum cost, minimum supervision
and minimum starting time.
A steam turbine is a mechanical device that extracts thermal energy from
pressurized steam, and converts it into rotary motion. An impulse turbine has fixed
nozzles that orient the steam flow into high speed jets. These jets contain significant
kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation
68 A. K. Matta et al
as the steam jet changes direction. A pressure drop occurs across only the stationary
blades, with a net increase in steam velocity across the stage. As the steam flows
through the nozzle its pressure falls from inlet pressure to the exit pressure. Due to
this higher ratio of expansion of steam in the nozzle the steam leaves the nozzle with a
very high velocity. The steam leaving the moving blades has a large portion of the
maximum velocity of the steam when leaving the nozzle.
Material
The turbine blade is simultaneously subjected to high temperature gradients and
centrifugal forces that require a unique blend of material properties such as high
strength. The materials chosen for the application are N155 and INCONEL 718
.INCONEL 718 is niobium, chromium and Nickel alloy .N155 is Nickel based Super
alloy .INCONEL 718 have high strength at elevated temperatures, corrosion
resistance and the relative ease of manufacturing. And N155 has high temperature
properties and do not depend upon Age hardening it has good ductility, strength,
corrosion resistance and excellent oxidation.
Mechanical Properties
The physical properties of the materials such as Elastic modulus, Poisson ratio,
Coefficient of thermal expansion vary with the temperature. Accurate stress
calculations would therefore require accurate data of the elastic properties of the
material.
Finite Element Modeling
The finite Element model [2] used in this analysis. The time steps are varied and the
results are stored in load step files. ANSYS allows the thermal analysis to be
conducted first and then the results are automatically transferred to the structure
model.
For the finite element representation of a problem the nodal values of the field
variable become the unknowns. Once these unknowns are found, the interpolation
functions define the field variable throughout the assemblage of elements. Clearly, the
nature of the solution and the degree of approximation depend not only on the size
and number of the elements used but also on the interpolation functions selected. As
one would expect, we cannot choose functions arbitrarily, because certain
compatibility conditions should be satisfied. Often functions are chosen so that the
field variable or its derivatives are continuous across adjoining element boundaries.
These are applied to the formulation of different kinds of elements. Thus far we have
briefly discussed the concept of modeling an arbitrarily shaped solution region with
an assemblage of discrete elements, and we have pointed out that interpolation
functions must be defined for each element. We have not yet mentioned, however, an
important feature of the finite element method [3] that sets it apart from other
numerical methods. This feature is the ability to formulate solutions for individual
elements before putting them together to represent the entire problem. This means, for
example, that if we are treating a problem in stress analysis, we find the force–
displacement or stiffness characteristics of each individual element and then assemble
the elements to find the stiffness of the whole structure. In essence, a complex
problem reduces to considering a series of greatly simplified problems.
Conclusions
The results and conclusions are presented for a study concerning the durability
problems experienced with gas turbine engines. The investigation encompassed the
design and failure history of hot gas path components in the engines. The maximum
operational Vonmises Stresses are within the yield strength of the material but the
deformation is comparatively better for material INCONEL 718.