Experimental and Numerical Investigation of a Small-Scale Steam Turbine

This paper focuses on a small-scale radial steam turbine which is part of a fan-turbine unit (FTU) realizing the anode off-gas recirculation of a Solid Oxide Fuel Cell (SOFC). Due to the small size of the turbine (15mm diameter), it is not possible to determine its power by directly measuring the torque. Therefore, the performance parameters are evaluated using thermodynamics. However, heat transfers within the FTU have a non-negligible impact on the evaluation of the turbine power and total-to-total isentropic efficiency. New temperature and pressure measurements were thus conducted to refine the experimental results and the numerical model. The analysis of the experimental and numerical results highlights the significant influence of heat transfers, and provides a range for the turbine power and total-to-total isentropic efficiency, which are on the order of 34W and 39%, respectively. Furthermore, a heat investigation was carried out on the FTU in order to localise and determine the heat losses to the environment by using a simplified model. Through investigation, the primary source of heat loss was located at the turbine outlet.

Application of the Transient Heater Foil Technique for Heat Transfer and Film Cooling Effectiveness Measurements on a Turbine Vane Endwall

Dominique Charbonnier, Magnus Jonsson, Peter Ott

The paper presents an application of the transient heater foil measurement technique using thermochromic liquid crystals (TLC) to endwall heat transfer and film cooling investigations in a transonic turbine vane cascade. The film cooling configuration consists of an upstream slot, representing the leakage flow area between the interface of the combustor and the turbine, and several rows of cylindrical and fan-shaped holes within the passage. With the transient method chosen, the heat transfer and adiabatic film cooling effectiveness distributions can be obtained simultaneously together with the local heat release within the heater foil. Therefore, the heat release in the foil is not required to be uniform, and the foil can contain discrete holes in the film cooling configuration. Some new developments are presented, which are directed towards improved application of the transient heater foil method for such a complex configuration. This includes tailoring the foil heat-release distribution towards the expected heat transfer patterns, supported by numerical Finite-Element computations and the use of a double-TLC mixture for improved time-wise TLC indications. Additionally, CFDsimulations were used to evaluate the recovery temperature distribution through the vane cascade without film cooling. The experiments were performed in the linear cascade facility at the EPFL-Lausanne. A compressor provides a continuous air flow at near-ambient temperature regulated with heat exchangers. Carbon dioxide is used as coolant in order to achieve engine-representative density ratio between coolant and main flow. Multiple experiments with the same main and coolant flow settings but varying heat flux levels and coolant injection temperatures have been performed and simultaneously analysed using nonlinear regression analysis. The time required between successive experiments to return to homogenous initial conditions, as required by the transient method, has been analysed using an analytical solution for heat-on-heat-off conditions. This permits the model assumption of onedimensional conduction within a semi-infinite wall with a heat releasing layer on the top. Example results for cases without cooling, with film cooling from rows of discrete holes and the addition of slot film cooling are used to illustrate the benefit of the new approaches for the investigated vane cascade.

American Soc Mechanical Engineers, Three Park Avenue, New York, Ny 10016-5990 Usa2008

Detailed heat transfer distributions of narrow impingement channels for cast-in turbine airfoils

Alexandros Terzis

Gas turbine operation at elevated temperatures ensures increased thermal efficiency and useful power output. However, the industrial tendency to push further firing temperatures is limited by the capabilities of the current turbine blade cooling technologies which approach their limits. Nowadays, the capabilities of the foundry industry to produce high integrity and dimensionally accurate castings allow the production of integrally cast turbine airfoils with the so-called double-wall cooling technology. Cast-in turbine airfoils include a more distributed form of cooling, where the coolant can be injected in the form of impingement jets within the wall rather than the hollow of the airfoil increasing dramatically the heat exchanged capabilities. Narrow impingement cooling passages can be therefore generated close to the external hot gas flow. In this study detailed heat transfer distributions for all internal surfaces of narrow impingement channels are evaluated. The test models consist of a single row of five impingement jets investigated over a range of engine representative Reynolds numbers. Effects of jet-to-jet spacing, channel width and height and impingement jet pattern are independently examined composing a test matrix of 22 large-scale geometries. For the evaluation of the heat transfer coefficients, the transient liquid crystal technique is used considering also thermocouple thermal inertia effects. Given that the use of different impingement geometries could play an important role on the distribution of convection coefficients, and hence, on the homogeneity of blade metal temperatures, the experimental results of this study can be used along with the external heat load for the development of thermal design models for integrally cast turbine vanes and blades.

2014

Etude du comportement dynamique des turbines Francis

A common phenomenon in Francis turbines is draft tube surge. In these fixed-bladed machines, a strong runner outlet swirl can develop under off-design conditions. The action of the diffuser, and in particular the draft tube elbow, on the rotating flow field induces synchronous pressure and discharge fluctuations. These low frequency disturbances are of special interest, because they can easily propagate throughout the whole hydraulic system. The dynamic response of the system to the natural excitations can inhibit the normal usage of the powerplant. If the natural excitation occurs at frequencies close to an eigenfrequency of the hydraulic system, an important amplification of the hydraulic fluctuations can be expected. Passive measures, such as air admission or minor design modifications, are commonly taken as a last resort to try to reduce these phenomena. This work explores an active control approach to alleviate the problem. It is shown that the hydroacoustic fluctuations in a hydraulic installation can strongly be reduced by "injecting" the inverse signal of the turbine's natural excitation. While the main theme is the application of the active control approach to improve the operation stability of Francis turbines, the scope of this study is larger and addresses the following scientific topics: Modeling of an hydroelectric installation The dynamic behavior of an installation is studied based on an one-dimensional model. A compilation of the basic modeling tools was done. Using these tools, the dynamic model of a hydraulic installation and an external hydroacoustical source is presented. It is used to explain how the overall hydroacoustic fluctuations can be reduced using a hydraulic exciter mounted in the wall of a draft tube's cone. Improving the operation stability by active control The hydraulic fluctuations associated with off-design operating conditions of Francis turbines are often very periodic and characterized by the presence of a dominant frequency component. The aim is to cancel out this component for which the amplitudes are often excessive. An active control system, composed of a hydraulic exciter and its controller, has been developed for a Francis turbine. The exciter employs a pulsed flow injection in the draft tube to generate an antiexcitation at a given frequency, which is then synchronized with the turbine's natural excitation. A self-tuning extremum control algorithm optimizes the operating parameters of the exciter to minimize the overall fluctuations in the hydraulic installation. Laboratory tests show the efficiency of the active control system. The amplitude of the pressure and discharge fluctuations at the dominant frequency component are reduced to background noise levels. Multiple configurations of the exciter have been tested. A spectral analysis of the pressure fluctuations is presented, the system's energy balance has been performed and the control algorithm of the system is analysed. The energy balance is very encouraging: the exciter system needed only about 1 percent of the hydraulic power of the Francis turbine. Prediction of the operation stability of turbines based on model tests Laboratory tests on a scale model, homologous to the turbine, are an indispensable step in the development of a hydroelectric powerplant. Unlike the static characteristics, the dynamic behavior of a turbine installation is difficult to predict. An original identification method of the dynamic characteristics of a model turbine is proposed. The method is verified on a theoretical case study and experimentally tested. It constitutes the basis of the prediction of the operation stability of the powerplant based on model tests.

EPFL2000

Application of the Transient Heater Foil Technique for Heat Transfer and Film Cooling Effectiveness Measurements on a Turbine Vane Endwall

Dominique Charbonnier, Magnus Jonsson, Peter Ott

American Soc Mechanical Engineers, Three Park Avenue, New York, Ny 10016-5990 Usa2008

Etude du comportement dynamique des turbines Francis

EPFL2000