07-10-2016, 04:30 PM
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Abstract
In this Diploma thesis the results of the investigation of a Numerical Simulation of the Flow
Field in a Friction-Type Turbine is presented. The Tesla turbine, an unconventional
turbomachinery that uses smooth disks instead of blades, is described principally by the
loading coefficient curve, the efficiency and degree of reaction vs. the flow rate parameter. In
order to describe its behaviour, the rotational speed is maintained constant and the flow rate is
changed with the purpose of simulate a virtual brake, and obtain the performance curves of
the Tesla turbine. Since the flow is characterized in a transitional regime, both laminar and
turbulent cases are simulated. The work presented represents an initial significant step
towards the analysis of this type of flow using CFD tool. Starting from a simple axisymmetric
model of the flow between two co-rotating disks in two dimensions, the model is
improved including the outlet of the turbine with the casing, and at the end a 3D simulation of
a single disk is performed including the effects of the nozzles. A complete model of a Tesla
turbine is restricted by computer resources.
Introduction
The flow inside a Tesla Turbine as well as the flow of a fluid between two parallel corotating
disks is of general interest in the technical field. The turbomachinery applications have
several variants, each idea comes to help to construct the world of power. One of this ideas
was put into real world by Nikola Tesla, in his application of a friction turbine, with new
concepts of energy transfer, using the properties of the fluids as viscosity, adhesion and
cohesion instead of the conventional energy transfer mechanism in bladed turbines. The
application of this special turbine is not on the normal spectrum range of turbomachinery for
as powerplants or aeroderivative turbines, but its use is intended for small applications.
Different concepts and theories have been used to explain the behaviour of this machine in the
analytical field, besides the physical testing has been also used by researches, and with the
upcoming of the computer age and the availability of numerical methods, the numerical
solution has been conducted over an extensive formulation of the NS equations using several
methods and assumptions.
In the present work CFD tools are used to understand the overall flow inside the turbine. It
presents the numerical simulation of the flow field in a friction-type turbine. The commercial
Computational Fluid Dynamics code FLUENT as well as the grid generation software
GAMBIT have been used for the investigation. The aim of the work is to provide an objective
point of view of the flow in 2D and 3D flow space.
With a selected turbine configuration, which is supported by experimental flow results, is
showed a 2D simulation over an isolated rotor (flow between two disks). It is important to
state that the analysed geometry is not optimised, as the researcher of this turbine states in
reference [32], who takes the initial geometry from the patent of Tesla [37]. In addition, the
CFD method is used to compute the flow field in a Tesla turbine consisting of rotor and stator
(nozzles), using the 3D capabilities of modern computational fluid dynamics codes for
complicated 3D-geometries. The computed results are analysed and compared with theoretical
and experimental facts gained from the literature. The modelling of the 3D flow in the whole
turbine is of great practical relevance but the limitation and restrictions of computer resources
dictate a different approach of modelling this turbine instead of simulating a complete 3D
model. The obtained results are diskussed. The work concludes with a summary of
conclusions and some guidelines concerning future flow research on this field.
2. Literature Review
This chapter presents a short history of this turbomachine and a summary of some of the
works conducted by different researches of Tesla turbine; they described the analytical
approach as well as experimental results.
2.1. Introduction and History of the Tesla Turbine
One of the inventions that the engineer and inventor Nikola Tesla conceived was the Tesla
disk turbine. With this device he proposed to make an useful and efficient handle of the
energy especially on electric generation, fluid power and engines field. Therefore, the Tesla
turbine also called and denoted in literature as Tesla turbomachines, multiple-disk, shear,
shear force or boundary layer turbomachinery, is a rotatory fluids machine that works with
compressible and incompressible fluid. The direction of the fluid flows in the radial and
tangential direction, forming spirals streamwise and operates principally on the laminar
regime. He referred to it as a thermodynamic converter in his original patented.
More over of conceiving the Tesla turbine, Nikola Tesla provided an useful design for other
machines operating the principle of the Tesla disk. Examples of these machines are an air
compressor, an air motor engine, a vacuum exhauster or vacuum pump. These machines use
the Tesla method of “fluid propulsion” that is based on two basic principals of physics of the
fluids: “adhesion and viscosity”. These types of turbomachinery can be applied as liquid
pumps, liquid or vapor or gas turbines, and gas compressors [37].
On October 21st, 1909 Nikola Tesla filled a patent for a pump, which uses smooth rotating
disks inside a volute casing. In the patent (which he received May 6th, 1913, U.S. Patent
No.1,061,142) Tesla began by pointing out the benefits of a smooth transition of energy:
"In the practical application of mechanical power based on the use of fluid as the vehicle of
energy, it has been demonstrated that, in order to attain the highest economy, the changes in
velocity and direction of movement of the fluid should be as gradual as possible."
The Tesla turbine invention was discussed in the semitechnical press at the time of the
invention [13-40].
What Tesla claims in his patents was a high efficiency due to the form of energy transfer,
based on the assumption that a highest economy will be attained when the changes in velocity
and direction of the movement of the fluid is as gradual as possible. This can be accomplished
causing the propelling fluid moving in natural paths or streamlines of least resistance, free
from constraints and disturbances caused by vanes or intricate devices in common
turbomachinery, and changing the fluid velocity and direction of movement by imperceptible degrees.
LITERATURE REVIEW
Laminar approach [24,6,5,22,18,42,15,43].
Turbulent approach [11,47,48,72].
Solution with incompressible fluid [most of literature].
Solution with compressible fluid [18,43,30].
Heat transfer in frictions turbines [15,14]
Multiphase fluids [43,42]
Most of the literature has considered the fluid to be laminar and incompressible. In general, it
has been found that the efficiency of the rotor can be very high, at least equal to that achieved
by conventional bladed rotors. Nevertheless the Tesla turbine accept all kind of fluids, most of
the literature assumed a newtonian fluid for simplifications except experimental test or those
studies with two phases fluid.
The initial assumptions of Rice [32], use an overall friction factor as a bulk parameter to
undertake the viscosity effects, in order to avoid the extensive and complex equations of NS,
incompatible with the aim of a first approach. In addition Rice developed some turbine
performance parameters, for calculating the pressure drop through the entire turbine, (rotor
and nozzle) and overall parameter such as torque, power and efficiency. Moreover, Rice
constructed six disk turbines and report some aspects of them, with the purpose of
determining the feasibility of this kind of turbomachinery. Starting from the description of the
Tesla´s patent, the turbine was operated with compressed air exhausting to the atmosphere,
Some changes as the angle of the nozzle and the use of a supersonic nozzle were made; in no
case these changes improve the performance of the turbine. Later, some improvements were
made as reducing the gap and increasing the number of disks. The comparison between
analytical data and results from these experiment revealed that the geometry, flow rate, and
speed combination used in the turbine where not near to those indicated by the analyses for
optimum turbine efficiency. Therefore, the experienced gained from this prototypes were use
to constructed the others four air turbines, Rice report the last version, the most successful of
this turbines.