Reinhard Willinger

Axial Turbine Tip-Leakage Losses at Off-Design Incidences

The tip-leakage losses in axial turbines with unshrouded rotor blades can account for as much as one third of the total losses. Various effects are influencing the tip-leakage flow and losses. This paper presents results of an experimental investigation concerning off-design incidences. Off-design incidences occur when the turbine operates at conditions different from the rated load condition. A low speed cascade wind tunnel has been used for the investigation. The geometry of the turbine cascade is an up-scale of the tip section of a low-pressure gas turbine rotor blade row (“Yaras–Sjolander cascade”) with a tip gap width of 2.5% of the chord length. The applied inlet flow angles consist of the design value as well as four off-design incidences in the range ±20°. Total pressures, static pressures and flow angles were obtained by traversing of a pneumatic five-hole probe in a plane about 0.3 axial chord lengths downstream of the turbine cascade. Based on the experimental results, a tip-leakage loss model is presented which can take into account off-design incidences. The model is applied to the present turbine cascade as well as to the turbine cascade of Yamamoto [1]. Due to its underlying concept, the model is able to predict, in addition to the losses, the flow underturning near the endwall caused by the tip-leakage vortex.Copyright

Development of a µ-Scale Turbine Expander for Energy Recovery

Waste heat is a primary source of energy loss in many applications. A number of developments around a micro rocket engine at the Austrian Research Centers (ARC) promise innovative energy recovery and micro power generation solutions. Here we focus on the investigation of micro technologies for application in HVAC (heating, ventilating, and air conditioning) systems. The use of μ-scale turbine expanders for work recovery in transcritical CO2 heat pump processes has been identified as most interesting and promising for the application in HVAC cases. One of the main drawbacks of transcritical CO2 heat pumps is the lower COP (coefficient of performance) compared to conventional heat pump systems which originates from the non isothermal heat rejection in the gas cooler. This drawback can be compensated by utilizing the pressure difference between the high pressure and low pressure part of the heat pump for work recovery. This is feasible as the pressure difference is considerably larger in case of CO2 heat pumps compared to conventional systems. Work recovery can be realized by substituting the expansion valve between the high and low pressure side by an expansion machine. Due to the low flow rate of the working fluid, the turbine type is based on the Pelton turbine with specific two phase flow turbine blades. In addition to the turbine part a magnetic coupling, miniature bearings and a small scale generator are important parts of the system. Thermodynamic simulations showed an absolute microturbine power yield between 60 W and 150 W for a 2 kW heating system.Copyright

THE ROLE OF ROTOR TIP CLEARANCE ON THE AERODYNAMIC INTERACTION OF A LAST GAS TURBINE STAGE AND AN EXHAUST DIFFUSER

An investigation of the aerodynamic interaction between a last gas turbine stage and an exhaust diffuser is presented. Special attention is given to the influence of the rotor tip gap on this interaction. Flow measurements downstream of a linear cascade of turbine blades with tip gap have been performed in a low speed cascade wind tunnel. The geometry of the cascade corresponds to the tip section of a gas turbine of fairly recent design. Five tip gaps lying in the typical range were investigated. The two essential results are the leakage loss and the underturning in the end wall region. The flow field of the turbine cascade supplied the inlet boundary condition for the subsequent numerical investigation of the flow field in an annular diffuser. The geometry of the annular diffuser is based on dimensions of exhaust diffusers of some heavy duty and aeroderivative gas turbines. The result of the investigation is the diffuser pressure recovery factor as a function of the gap of the upstream cascade. The results from the cascade measurements and the diffuser computations have then been coupled by means of an interaction model. For gaps of practical interest, specific work and efficiency of the last gas turbine stage followed by an exhaust diffuser are independent of the rotor gap.© 1998 ASME

Technical and Economic Assessment of Small to Medium Scale Bio-Ethanol Distilleries With Cogeneration Systems

This investigation presents an assessment of the potential use of sugar cane to support sustainable development in Colombia with a particular focus on devising ways of supplying electricity to regions without service (about 1.8 million people and 66% of the territory). The goal of this study is to evaluate the technical and economic feasibility of using small to medium scale modern technologies to exploit the co-production of two of the major forms of energy that can be derived from the sugar cane plant: ethanol (by fermentation and distillation of the cane juice) and electricity (by cogeneration of bagasse). The market potential for ethanol, electricity and carbon credits (for associated emissions reduction) is examined and a site selection analysis is performed to decide the most suitable location within the Non-Interconnected Zones (NZI) in Colombia. Development scenarios are constructed and analyzed in the context of the markets identified, combining different alternative technologies. Production costs of sugar cane, investment costs, operating costs, labour costs and maintenance costs are estimated for each development scenario, and economic and financial characteristics are quantified and compared, to determine which options are the most attractive. The study identifies that the key factors which affect the feasibility of distilleries with CHP systems in Colombia are: 1) specific location and conditions, 2) type of technologies used, efficiencies and maximum capacities of production, 3) total investment costs, 4) incomes for ethanol, electricity and emissions reduction and 5) national and global incentives.© 2007 ASME

On the Modeling of Tip Leakage Flow in Axial Turbine Blade Rows

The starting point of this paper is an established turbine tip leakage loss model based on energy considerations. The model requires a discharge coefficient as an empirical input. The discharge coefficient is the ratio of the actual to the theoretical tip gap mass flow rate, The nondimensional parameters influencing the discharge coefficient are determined by a dimensional analysis. These parameters are: gap width to length ratio, end wall speed to gap flow velocity ratio and gap Reynolds number. Ranges for these parameters, valid for typical turbine tip gap situations, are presented. The numerical investigation of the turbulent flow in a plane perpendicular to the blade chord line supplies the discharge coefficient versus the nondimensional gap width. Depending on the gap width to length ratio, various degrees of mixing of the flow downstream of the vena contracta can be detected. Based on these observations, a simple tip gap flow model is presented. The discharge coefficients computed by this model are compared with the numerical results as well as with experimental values from the literature. Finally, the model is used to calculate the discharge coefficients of improved tip gap geometries (squealers, winglets).Copyright

Design Modification of a Passive Tip-Leakage Control Method for Axial Turbines: Linear Cascade Wind Tunnel Results

Tip-leakage losses can contribute up to one third of the overall losses in unshrouded axial turbine blades. A passive tip-leakage flow control method is used to reduce the tip-leakage loss. Taking into account a modified discharge coefficient model, an inclination of the injection against the tip-leakage flow direction is said to have an even better effect on reducing the tip-leakage loss. To prove the effect, linear cascade measurements have been carried out at three different gap widths from 0.85% to 2.50% chord length. The used geometry is an up-scaled turbine blade tip cross section with weak turning. A single blade is fitted with an injection channel which is inclined by 45° against the tip-leakage flow direction. The flow field of the modified blade was measured 0.31 axial chord length downstream of the cascade using a pneumatic five-hole probe. The tip-leakage loss is reduced by passive tip-injection and further by inclined injection. The reduction can be significant at small gap widths. Detailed results are presented for a gap width of 1.40% chord length.Copyright

An Innovative Passive Tip-Leakage Control Method for Axial Turbines: Linear Cascade Wind Tunnel Results

Depending on the blade aspect ratio, tip-leakage losses can contribute up to one third of the total losses in an axial turbine blade row. In unshrouded turbine blade rows, the radial gaps allow working fluid to pass from the pressure to the suction sides. This tip-leakage flow does not contribute to the work output of the turbine stage. Therefore, any technique which tends to reduce tip-leakage losses has the objective to decrease the flow through the tip gaps. A frequently used method of reducing the tip-leakage flow is the modification of the blade tip geometry by so-called squealers or winglets. Since this method decreases the sensitivity of tip-leakage losses on tip gap width, it is called tip desensitization. This paper presents a new method for tip desensitization: the passive blade tip injection. A low speed cascade wind tunnel is used for experimental investigations. Geometry of the turbine cascade is the up-scale of the tip section of a gas turbine rotor row. Three different gap widths in the range from 0.85% to 2.50% chord length are used. Total pressure, static pressure and flow angles are obtained by traversing a pneumatic five-hole probe about 0.3 axial chord lengths downstream of the turbine cascade. For investigations of the tip injection effect, a single blade of the cascade is modified by an injection channel. Based on experimental results, it is shown that the passive tip injection method decreases tip-leakage losses. At small tip gaps, this reduction can be rather significant. Finally, the positive influence of blade tip injection on tip-leakage losses is described by an analytical model based on the discharge coefficient.Copyright

Combined Heat and Power Technologies: Application Studies of Options Including Micro Gas Turbines

Recently, several methods have been developed to select and optimize energy systems with the purpose of finding the most economical solution that supplies the required energy demand. This paper presents a theoretical study of cogeneration technolgies, which includes a qualitative survey, and technical and economical arguments to define which combinations of supply technologies can be expected to serve six different demand cases. The comparison from economical viewpoint, based on annualized capital and operatinal costs of the system, shows a trend to use backpressure steam turbine for small cases (power demand ≤ 5MW) and industrial gas turbine for big cases (power demand ≥ 5MW).© 2004 ASME

Rechnergestützte Verfahren zur aero-thermodynamischen Auslegung und Entwicklung

In einer Dampfturbine erfolgt die Umwandlung von potentieller Energie in mechanische Energie auf indirektem Weg uber die kinetische Energie des Arbeitsmittels. Daraus folgt, dass fur die rechnerische Beschreibung der Energieumsetzung Stromungsvorgange von entscheidender Bedeutung sind. Gerade im Bereich der Stromungen haben in den letzten Jahren rechnergestutzte Verfahren eine sturmische Entwicklung erfahren. Numerische Verfahren zur Berechnung von dreidimensionalen Stromungen (CFD = Computational Fluid Dynamics) werden heute im Auslegungs- und Entwicklungsprozess von Dampfturbinen zunehmend eingesetzt. Trotzdem bilden einfachere null-, ein- und zweidimensionale rechnergestutzte Berechnungsverfahren nach wie vor das Ruckgrat bei der Auslegung von Dampfturbinen. In diesem Kapitel werden die Methoden und ihre Grundlagen kurz beschrieben und es werden Beispiele fur deren Anwendung im Bereich der Auslegung und Entwicklung von Dampfturbinen gegeben.

Impact Of Different Shroud Configurations On Leakage Flow Of a LP Rotor

For this work, different shroud configurations of a LP turbine rotor and their influence on the tip leakage flow as well as on the main passage flow were experimentally and numerically investigated. The basic configuration of the LP rotor consists of a shroud with two straight sealing fins. Based on foregoing investigations in the basic setup, the geometry of the shroud was altered by drilling radially through the shroud in each rotor passage. The bore is located near the pressure side of the blade between the two sealing fins. The basic idea is to produce an aerodynamic blockage effect inside the shroud cavity and therefore reduce the cavity flow. The time-resolved measurement data from measurements with a Fast Response Aerodynamic Pressure Probe (FRAPP) downstream of the LP rotor is compared to results from CFD calculations. During the investigations it was found, that the radial bore not only influences the tip region downstream of the LP rotor, but also causes changes of the flow field close to the hub.