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The Research

Multidisciplinary optimization of turbine blading systems

The efficiency of turbine stages can be increased by optimisation of blading systems, including classical optimisation of stator and rotor blade numbers and stagger angles, as well as optimisation of blade sections (profiles) and 3D blade stacking lines. The possible large number of shape parameters of 3D turbine blading requires efficient optimisation techniques. The figure below shows the blade rim of efficient „compound lean” nozzles for an HP turbine stage of Alstom as well as a view on the stator blade „sweep back” and rotor blade twist for an exit LP turbine stage, also of Alstom.

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An HP stator with compound leaned blades of Alstom

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Adaptive control in lp turbines

Cogeneration of electric energy and heat in heat and power turbines requires application of adaptive control to adapt them to variable operating conditions. The main element of adaptive control is the so-called adaptive stage of flexible geometry located directly downstream of the extraction point. In a typical design of adaptive stage widely used by turbine manufacturers, the nozzles have movable leading edges blocking part of the blade-to-blade passage if necessary, thus reducing the mass flow rate. The mechanical construction of this stage with these so-called throttling nozzles is relatively simple, however its flow efficiency is low, both at full and low openings of the flow channel. At full opening, this is due to the low pitch-to-chord ratio, as the section of the leading edge should be able to block the entire blade-to-blade passage. At low levels of opening, a massive separation of flow can be expected downstream of the leading edge corner. The transport of vorticity from this separation through the nozzles, and especially through the rotating blade row downstream results in high unsteady components of forces acting at the blades of the adaptive stage.

Nozzles with movable leading edges (throttling nozzles - A) and nozzles with rotated trailing edges (flap nozzles - B): I – nozzles fully open, II – nozzles partly open/closed.

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Partial admission turbines

Partial admission is often used to control the power output in large power turbines. Steam flow is admitted through a number of nozzle boxes located at the annulus in the form of discrete arcs, and each nozzle box is equipped with a control valve. Each control valve has its own characteristics of opening. It has its own level of opening at a steady state, depending on the required turbine power. This type of power control requires the presence of a (usually one) control stage adapted to supply at partial-arc admission. Downstream, the control stage is followed by subsequent stages admitted at the full arc.

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a) Distribution of steam to the control stage.
b) Schematic of nozzle boxes in circumferential view.

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Investigations of steam turbine startup

During start-up from a cold state the temperature of the steam turbine metal elements increases by as much as 500K or more. This is accompanied by elongations, heat deformations and increase of stresses in the metal. Relative elongations of the inner and outer casing and the rotor appear. As the clearances are reduced, friction of metal against metal can occur. Frequent changes of heat load and large heating rates lead to thermal fatigue and metal cracking. Permanent thermal deformations such as the so-called „cat back” (longitudinal deformation) can occur.

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Tesla-type bladeless turbines

The first bladeless turbine, also known as a friction turbine, was designed and manufactured by a Serbian engineer and inventor Nicola Tesla in 1913. This unusual device makes use of viscous effects which occur in the boundary layer flow. Opposite to classical bladed turbines where viscous effects in flow are undesirable as a source of efficiency loss, these effects enable the rotational move of the rotor. The rotor consists of up to a few dozens of thin disks locked on a shaft perpendicular to its axis of revolution. The supply of a Tesla turbine is accomplished by one or several nozzles discretely located along the circumference. The nozzles are tilted under a certain angle to the disk tangent. Working fluid flows between the disks spirally from the outer to inner radius and transfers energy to the rotating disks. The medium flows out in the axial direction through a number of holes in the disks situated near the turbine shaft. The efficiency of the Tesla turbine depends on many parameters, namely on: pressure, temperature and velocity conditions between the disks, number, diameter, thickness and distance between the disks as well as on the state of the disk surface, rotational speed of the rotor, number and arrangement of the supply nozzles, etc.

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Rotor of a multidisc Tesla bladeless turbine
[patent documentation – Hicks Kenneth USA].

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Cogeneration in a small scale

Cogeneration is a simultaneous production of electric energy and heat, which leads to a more efficient use of primary energy. Sample quantitative gains from cogeneration are displayed in the figure below. As seen from the picture, in order to produce 21 units of electric energy and 33 units of heat in cogeneration (assuming the theoretical total cogeneration efficiency of 90%) there are 60 units of primary energy required, whereas 97 units of primary energy are needed to produce the same amount of final energies in separate generation.

Thus, cogeneration brings considerable increase of energy efficiency and contributes to decrease the level of harmful gases emissions into the environment. The opportunities for cogeneration are however usually determined by the demand on heat, which can vary seasonally and with the daytime.

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Production of electric energy and heat in a separate mode and in cogeneration.

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Secondary vortex structures in turbine stage rotors

The flow through a turbine stage is extremely complex due to the presence of numerous secondary flows and vortex structures in the stator and rotor cascades. Firstly, horseshoe vortices are formed at blade leading edges of the stator and rotor blades, near hub and tip endwalls. Inside the rotor passage these vortices are believed to mix, partially or entirely, with passage vortices forming as a result of passage cross flows. At the same time, the trailing edges of the stator and rotor blades are the sources of wakes shed downstream and being convected with the main flow into the next cascades. Also flow separations, occasionally observed at rotor passages, can frequently lead to the creation of additional large-scale vortices of various orientations. Permanent interactions between all the abovementioned main flow structures, not mentioning those revealing smaller or varying  intensity, such as corner or leakage vortices for instance, make studying the turbine flow extremely difficult.

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Vortex dynamics of stator-rotor interaction

One of the most widely recognized unsteady phenomena taking place in a turbine rotor is the rotor interaction with the stator wake (S/R interaction). For a continuous wake, as modelled in the majority of the S/R interaction studies, the wake characteristics at the rotor inlet are time-independent and successive rotor blades chop off identical wake segments, which are then expected to behave in the same way when passing the rotor passage. This, however, is not true any longer when the wake comprises a sequence of separate, active vortices. Active structures interact with each other and with the boundaries, and  their resultant trajectories differ from those estimated on the basis of pure main flow convection. Moreover, even if steady-state parameters of the vortex wake, such as vortex strength and distance between vortices, are kept constant, the characteristics of particular wake segments chopped off by rotor blades for different phase shifts differ by initial distribution of vortices with respect to the chopped-off section (see the phase shift definition below). Consequently, the deformation of the wake on its way through the rotor passage  can take different courses.

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