Download In your analysis, let the bolt have a

FE-study of turbine disc behavior
Project description for the course
TMMI37, autumn 2013
in collaboration with
SIEMENS Industrial Turbomachinery, Finspång
Introduction
In this project, which constitutes a mandatory examination task in the
course TMMI37 (spring 2013), you will have the opportunity to apply the
knowledge and skills gained in the course on a concrete engineering
problem; this year set up in collaboration with SIEMENS Industrial
Turbomachinery, Finspång, which is a world leading producer of steamand gas turbines. More specifically you are to
I. get some insight into the loading experienced by a typical gas turbine
disc and how it can be analyzed by using FEM
II. study a new concept of gas turbine rotor design, where only one single
central bolt is used
General aim
The general aim of the project work (complemented by the guest lecture,
see below) is that you will see how the Finite Element Method can be used
in an industrial context, and to become more experienced as an FE-user.
Part I
As described above, you are here to get some insight into the loading
experienced by a typical turbine disc and how it can be analyzed by using
FEM.
In detail you are to calculate the displacements and stresses in a turbine
disc of relevant geometry (see the drawing), with focus on the radial
displacements at the outer rim of the disc, the tangential stresses at its
central hole, and the equivalent stress (according to von Mises) at its
central hole and bolt hole, resp.
Furthermore, also the contraction (in the thickness direction) of the disc at
the bolt holes is to be studied, since that will affect the pre-tensioning of the
bolts. However, you do not need to model the bolts. Please observe that the
contraction is to be expressed as the total reduction of the thickness at the
bolt holes (be careful, such that you really find the total reduction in
thickness)! This contraction analysis is also to be performed without any
thermal loading, since that will not affect the pre-tensioning.
The disc is supposed to rotate with a speed of 9 000 rpm, and have a central
hole of radius 4 cm. In addition to the inertia of the disc itself, the thermal
loads (at stationary conditions) and the effect of the turbine blades are to be
considered. The former (ambient gas temperatures) with associated heat
covection coefficients can be found in the appended geometry drawing for
Part I, while the effect of the blades is to be simulated by adding a radial
stress of 150 MPa on the outer surface of the disc.
The disc is supposed to contain 8 bolt holes of equal tangential spacing (see
the separate drawing) having the diameter 18 mm. By this it follows that a
3D-model of a disc section needs to be analyzed (making use of the
symmetry). Furthermore, in order to take into account the thermal as well
as the mechanical loads, a coupled thermo-elastic analysis needs to be
carried out, where the output data from the stationary heat transfer analysis
acts as input data to the mechanical analysis. Finally, when calculating
displacements and stresses, also the influence of the mesh density is to be
studied by refining the mesh at interesting areas. Please note that we in this
work do not consider any inelastic behavior (plasticity and/or creep).
Due to the high temperatures and loading conditions, turbine discs are
typically made of wrought nickel-based superalloys, such as for example
Inconel 718. Typical temperature independent material data (a
simplification of course), to be used here, are
 E=200 GPa , υ=0.29 , ρ=8.2*103 kg/m3
 k=12 W/Km , α=14E-6 1/K
Part II
As described above, you are here to study a new concept of disc bolting,
where only one single central bolt is used. The traditional type of fastening
is illustrated in Fig. 1 below, where the two discs are connected by a
number of tangentially distributed bolts (see also Part I).
Since we here may assume axial symmetry, we can analyze the behavior of
the assembly by an axisymmetric analysis, which will greatly reduce the
number of elements needed (and thus also the computation time).
Fig.1 Traditional bolting
An alternative type of disc bolting (not in use by SIEMENS in Finspång)
would be to use only one single central bolt as shown in Fig. 2 below
Fig. 2 Conceptual bolting
Your task will here be to investigate to which extent the pre-tensioning of
the bolt (discussed in Part I) will be reduced in service (when the machine
is running at stationary conditions). The bolt is assumed to be made of the
same material as the rest of the assembly, and the initial pre-tensioning is to
be of the order 420 Mpa. The same mechanical loading (speed and
influence of blading) as in Part I is to be adopted, while the thermal input
data can be found in Fig. 3 below.
The geometry data will be provided in the form of an input file to the FEsoftware ABAQUS. You may then try to import it to Workbench and run
the analysis there. However, since this may not be that easy, you can
instead start from the length of the bolt (48,125 cm; the length of the
bottom line in the figure above) and then find the rest of the needed
dimensions by a simple scaling of lengths measured in the figure.
Furthermore, in order to carry out the analysis above, you will need to
incorporate contact conditions and to apply a pre-tensioning to the bolt
(these issues will be discussed in the lab hall).
Fig. 3 Thermal data
In your analysis, let the bolt have a homogeneous temp. of 365 °C.
Guest lecture
Strongly coupled to the project work is the guest lecture by MSc. Daniel
Nilsson, SIEMENS Industrial Turbomachinery, Finspång, where you will
get some more insight into how gas turbines work and how FE-simulations
are used in the design of them. It is also to be noted that Tekn.Lic. Per
Almroth has been contributing in the planning of this project work.
Reporting
Your work and results are to be described and discussed in a written report
which is to be handed in according to the guidelines found in the course
information (and in both printed and electronic form). Note specifically
that you are to
 carefully specify all thermal and mechanical boundary conditions
and material data used in the FE analyses; it should be possible to
fully understand how the boundary conditions have been applied. For
boundary conditions already specified, you can add an appendix with
the given information.
 present and discuss your results; temperature distributions,
contraction of the bolt hole in Part I (reduction of thickness, in mm),
stresses, radial displacements (in Part I), relaxation of stresses due to
pre-tensioning of the bolt (in Part II), etc etc- the report needs to be
in color (for both the electronic and printed version)
 specify all the FE-meshes used
 show convergence studies (for Part I) and discuss the associated
results
 make sure that the mesh is not too coarse at the bolt hole in Part I
Hint
For Part I you first make a stationary heat transfer analysis, after which
geom. and results are exported to an elasto-static analysis (by creating
“pipelines” from the former to the latter). The disc is to be free to expand,
but its axial rigid body motion is to be locked. Furthermore, symmetry is to
be taken into account in a relevant way.
For Part II you are first to use the Finite Element Modeler, in which you
import the FE-mesh, which is then to be “pipelined” to a stationary heat
transfer analysis. In doing this, you are to change the dimensions to mm at
two places. Note also that a separate analysis (without thermal or rotational
loading is to be done, in order to find appropriate boundary conditions for
the pre-straining of the bolt). Further details will be discussed in the lab
hall.
Evaluation criteria and grading of the reports
Your grading (uk/Fx, 3/C, 4/B, 5/A) will be primarily based on the report,
but also your activity and performance on the mandatory project
presentation seminar may to some extent be incorporated in the final
grading. The evaluation criteria used for assessing the report will be
 the quantity of the work, i.e. to what extent the different tasks have
been addressed and executed
 the quality of the work; i.e. how well the FE-modeling has been
carried out, how boundary conditions have been chosen and
motivated, how well results have been presented and discussed etc
 the quality of the written text; organization of the text, language etc
Please note that we did not state “the quantity of the written text”. Of
course we would like to see good, comprehensive reports with a lot of
interesting results and discussions, but at the same time you should avoid
putting in unnecessary things, since neither we in the academy nor our
colleagues in the industry have unlimited time to go through reports.
Please, also put effort on language, since a poorly written report will not be
well received, independently of how good results that have been achieved.
Please also note that the reports can be written in both English and
Swedish.
Looking forward to take part of your work!