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The Research

Multi-objective efficiency optimization of a 30 kWe ORC turbine

Due to the demand of the district heating network and electric power grid ORC turbines can operate in the condensation and cogeneration modes. This approach requires the design of an expander which is characterized by high efficiency in each modes of operation. Multi-objective efficiency optimization of a one stage axial ORC turbine working on MM (Hexamethyldisiloxane) was carried out. An Implicit Filtering algorithm (IF) is used to find a flowpath with maximum efficiency. During the optimization the rotor profiles (hub and shroud) and the shape of endwall contours of the rotor domain were changed. Five optimization tasks were carried out with different weights of the efficiency of both modes of operation. Pareto frontiers were obtained and a decision-making method was used to select an optimum solution. The optimization of rotor row allows for significant efficiency improvements in two regimes of operation, with respect to a baseline geometric configuration designed with the help of classical methods. The turbine efficiency was increased by 3.1 pp. in the condensation mode and by 4.8 pp. in the cogeneration mode.

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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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Powering gas turbines with alternative fuels

The use of alternative fuels, such as bioalcohols, creates real opportunities for cleaner and more environmentally friendly operation of gas turbines in the aviation and energy sectors. At the same time, the development of gas turbine technologies with flexible fuel supply systems enables the use of alternative non-fossil fuels that can play a key role in global efforts to meet emission targets. Operation and emission characteristics of the GTM-140 jet engine fueled with mixtures of pentanol and aviation kerosene were developed and the results were compared with mixtures of kerosene with other biofuels - propanol (C3 alcohol) and butanol (C4 alcohol). All tested alternatives to liquid biofuels showed the potential to reduce regulated emissions such as NOX (on average by 40%) and CO (on average by 25%) for C5 alcohol. Increasing the proportion of pentanol in the mixtures results in lowering the temperature behind the combustion chamber. Engine fuel efficiency expressed as specific fuel consumption (TSFC) for all tested biofuels decreased by an average of 40% for C5 alcohol.

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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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