A 3D conjugate heat transfer simulation of a radially cooled gas turbine vane has been performed using STAR-CD™ code and the metal temperature distribution of the blade has been obtained. The study focused on the linear NASA-C3X cascade, for which experimental data are available; the blade is internally cooled by air through ten radially oriented circular cross section channels. According to the chosen approach, boundary conditions for the conjugate analysis were specified only at the inlet and outlet planes and on the openings of the internal cooling channels: neither temperature distribution nor heat flux profile were assigned along the walls. Static pressure, external temperature and heat transfer coefficient distributions along the vane were compared with experimental data. In addition, in order to asses the impact of transition on heat transfer profile, just the external flow (supposed fully turbulent in the conjugate approach) was separately simulated with TRAF code too and the behaviour of the transitional boundary layer has been analyzed and discussed. Loading distributions were found to be in good agreement with experiments for both conjugate and non conjugate approaches, but, since both pressure and suction side exhibit a typical transitional behavior, HTC profiles obtained without taking into account transition severely overestimate experimental data especially near the leading edge. Results confirm the significant role of transition in predicting heat transfer and, therefore, vane temperature field when a conjugate analysis is performed.
The preliminary design of new centrifugal stages often relies on one-dimensional codes implementing the concept of slip factor. This parameter plays a primary role in the stage design process since it directly affects the calculation of the impeller work coefficient and hence of the components situated downstream. Classical slip factor correlations may not always provide a satisfactory accuracy and generally they fail while attempting at covering a design space in a wide range of flow coefficients and peripheral Mach numbers. In that case the preliminary design has to be refined with more advanced tools, such as computational fluid dynamics (CFD). Often this process needs to be repeated several times before the design cycle ends. In order to predict more effectively the work coefficient as well as to reduce the number of iterations between 1D/CFD codes during the design activity, a new correlation has been developed, which is based on a large number of historical data from both CFD and experimental results. Accurate statistical analyses have shown that slip factor can be strongly linked to significant flow and geometry parameters by means of the outlet deviation angle. As the available calibration dataset gets more and more populated, the presence of specific constants in the structure of the correlation allows the designer to improve the accuracy of predictions.
The design of radial-inflow turbines usually relies on one-dimensional or mean-line methods. While these approaches have so far proven to be quite effective, they can not assist the designer in coping with some important issues, such as mechanical integrity and complex flow features. Turbo-expanders are in general characterized by fully three-dimensional flow fields, strongly influenced by viscous effects and passage curvature. In particular, for high pressure ratio applications, such as in organic Rankine cycles, supersonic flow conditions are likely to be reached, thus involving the formation of a shock pattern which governs the interaction between nozzle and wheel components. The nozzle shock waves are periodically chopped by the impeller leading edge, and the resulting unsteady interaction is of primary concern for both mechanical integrity and aerodynamic performance. This work is focused on the aerodynamic issues and addresses some key aspects of the CFD modelling in the numerical analysis of turbo-expanders. Calculations were carried out by adopting models with increasing level of complexity, from the classical steady-state approach to the full-stage, time-accurate one. Results are compared in details and the impact of the computational model on the aerodynamic performance estimation is discussed.
The growing concern for the role of man-made CO2 emissions with respect to global warming combined with the large increase in energy demand spurred by developing nations and a growing global population that is foreseen over the next 15 years have recently turned attention to potential CO2-neutral energy supply solutions. Waste heat recovery cycles applied to fossil fueled plants offer a local zero-emission solution to producing additional electric energy, thereby increasing the overall plant efficiency with a considerable reduction in the emission of CO2 per unit of energy produced. GE Oil & Gas with GE Global Research Europe has developed a new and attractive solution for recovering waste heat energy from a variety of thermal sources ranging from reciprocating combustion engines to gas turbines. This new recovery cycle is called ORegen™. The ORegen™ recovery cycle is a rankine cycle, with superheating, that recovers waste heat and converts it into electric energy by means of a double closed loop system. The ORegen™ system represents one of the very few viable solutions for recovering heat from sources (such as mechanical drive gas turbines) whose load may vary dramatically over time or where the equipment is located at a site where water is not readily available. For the temperature range of interest, a thorough comparison between many working fluids was performed, leading to the conclusion that the substance that delivers the highest efficiency is Cyclopentane. A high-efficiency Rankine cycle based on such a working fluid places a particularly high demand on the expansion ratio, which influences some of the basic architectural choices of the expander machine. This article introduces the ORegen™ recovery cycle and describes the process used in GE Oil & Gas to design the family of double supersonic stage turboexpanders, covering the power range of 2–17MW. Examples of the application of the ORegen™ cycle to gas turbine are also provided to demonstrate attractive opportunities to increase the overall plant efficiency.
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