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• W2551736593 abstract "Dense Gases are single-phase vapors of molecularly complex fluids operating close to saturation conditions. In such a region, the perfect gas law is invalid and has to be replaced by more complex equations of state. In recent years, great attention has been paid to certain fluids, known as the Bethe— Zel'dovich—Thompson (BZT) fluids, which are theoretically predicted to exhibit in the vapor phase, for a whole range of temperatures and pressures above the upper saturation curve, negative values of the Fundamental Derivative of Gasdynamics Γ [1], i.e. reversed isentrope concavity in the p-v plane. In the transonic and supersonic regime, this can lead to nonclassical gasdynamic behaviors, such as expansion shocks and mixed waves. A particularly interesting application is represented by efficiency enhancement in Organic Rankine Cycles (ORCs). ORCs’ working fluids are in fact heavy organic compounds with large heat capacities. Interestingly, some of the organic fluids used in ORCs also possess BZT properties. One major source of losses in ORC turbines is due to wave drag, as they usually operate in the transonic/supersonic regime: the use of a BZT fluid could avoid shock formation and, ideally, allow isentropic turbine expansion. Unfortunately, simply utilizing a BZT working fluid is not sufficient to maximize the reduction in losses. Operating the turbine cascade at a pressure and temperature near the thermodynamic region where BZT effects appear is also necessary. On the other hand, the thermodynamic region where BZT effects appear, i.e. the inversion zone, is of quite limited extent. As a consequence, a reduction in the cascade pressure ratio is also required in order to operate the cascade entirely within the inversion zone. Now, it is known from thermodynamic theory that a too small pressure ratio leads to poor global thermal cycle efficiency. Thus, the development of BZT Organic Rankine Cycles needs to find a reasonable tradeoff between two opposite requirements: on the one hand, turbine expansion must happen as close as possible to the inversion zone, in order to get maximal benefit from BZT effects, on the other one, the turbine pressure ratio must be sufficiently high for achieving high global cycle efficiency and power output. Previous studies about inviscid and viscous dense gas flows through turbine cascades [2] have shown that the use of a BZT working fluid allows, for a given cascade pressure ratio, an efficiency improvement of about 3% over air, and even greater benefits with respect to steam. The benefit is obtained for a range of thermodynamic conditions whose extent depends on the cascade pressure ratio. Namely, the higher the pressure ratio, the harder is to exploit dense gas effects to obtain improved cascade efficiency over a large range of conditions. Efficiency improvements observed in previous studies were simply due to the special nature of the working fluids, as the blade shapes considered were typical gas turbine blade sections, not specifically adapted for dense gas flows. The objective of the present study is to find optimal blade shapes providing high turbine efficiency over a large range of operating conditions for turbulent flows trough highly loaded turbine cascades, characterized by high values of the pressure ratio. To this end, a multi-point optimization of the blade shape is undertaken, using a multi-objective genetic algorithm. Viscous dense gas flows are modeled by the compressible Reynolds-Averaged Navier-Stokes equations for single-phase, non-reacting flows, completed by the realistic equation of state of Martin-Hou [3], and by the simple Baldwin-Lomax turbulence model. A structured grid solver (SGS), using the thirdorder accurate centred scheme of Ref. [4] is used. The solution is advanced in time using a four-stage Runge-Kutta scheme. Local time-stepping, implicit residual smoothing and multigrid are used to efficiently drive the solution to the steady state. The flow solver is coupled with a multi-objective genetic algorithm (MOGA). Genetic algorithms have proved their interest with respect to gradient-based methods because of their high flexibility and also because of their ability to find global optima of multi-modal problem. The MOGA applied in this study is the Non-Dominated Sorting Algorithm (NGSA) proposed by Srinivas and Deb [5]. For multi-objective problems, a Pareto-based genetic algorithm is applied. The MOGA has been" @default.
• W2551736593 created "2016-11-30" @default.
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• W2551736593 date "2008-01-01" @default.
• W2551736593 modified "2023-09-24" @default.
• W2551736593 title "Optimal Blade Shapes For Viscous Dense Gas Flows Through Turbine Cascades" @default.
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