A COMPARISON BETWEEN EXPERIMENTAL AND NUMERICAL ANALYSIS OF A WELLS TURBINE

Wave energy is one of the renewable energy sources with the highest potential. Several pilot plants have been built based on the principle of the Oscillating Water Column (OWC). Among the different solutions that have been suggested, the Wells turbine has gained particular attention due to its simplicity and reliability. The majority of available studies concentrate on the steady operation of the Wells turbine, while only few analyze its performance under an unsteady and bi-directional air flow, as determined by the presence of the OWC system. In this work, experimental and numerical performance of a high-solidity Wells turbine with NACA0015 profiles under the bi-directional flow generated by a hydraulic piston is compared. The numerical simulations have been conducted using commercial CFD software and focus on unsteady predictions, with particular attention to the behavior of the flow upstream and down-stream of the rotor, flow hysteresis between acceleration and deceleration phases and differences between intake and exhaust strokes due to the non-symmetrical configuration of the machine

An investigation of higher-order multi-objective optimisation for 3D aerodynamic shape design

We investigate the performance of different variants of a suitably tailored Tabu Search optimisation algorithm on a higher-order design problem. We consider four objective func- tions to describe the performance of a compressor stator row, subject to a number of equality and inequality constraints. The same design problem has been previously in- vestigated through single-, bi- and three-objective optimisation studies. However, in this study we explore the capabilities of enhanced variants of our Multi-objective Tabu Search (MOTS) optimisation algorithm in the context of detailed 3D aerodynamic shape design. It is shown that with these enhancements to the local search of the MOTS algorithm we can achieve a rapid exploration of complicated design spaces, but there is a trade-off be- tween speed and the quality of the trade-off surface found. Rapidly explored design spaces reveal the extremes of the objective functions, but the compromise optimum areas are not very well explored. However, there are ways to adapt the behaviour of the optimiser and maintain both a very efficient rate of progress towards the global optimum Pareto front and a healthy number of design configurations lying on the trade-off surface and exploring the compromise optimum regions. These compromise solutions almost always represent the best qualitative balance between the objectives under consideration. Such enhancements to the effectiveness of design space exploration make engineering design optimisation with multiple objectives and robustness criteria ever more practicable and attractive for modern advanced engineering design. Finally, new research questions are addressed that highlight the trade-offs between intelligence in optimisation algorithms and acquisition of qualita- tive information through computational engineering design processes that reveal patterns and relations between design parameters and objective functions, but also speed versus optimum quality

Robust design optimisation of gas turbine compression systems

Engineering design commonly assumes nominal values for uncertain parameters to simplify the design process: the design of a gas turbine, or one of its modules, is generally approached with some specific operating conditions in mind (its design point). Unfortunately, engine components never exactly meet their specifications and do not operate at just one condition, but over a range of power settings. This simplification can then lead to a product that exhibits performance significantly worse than nominal in real-world conditions. This problem is exacerbated in the presence of heavily optimised designs, which tend to lie in extreme regions of the design space.15 In gas turbine design, safe and satisfactory off-design operation must be guaranteed and is generally evaluated before moving to the next phase of the design process. This approach, while guaranteeing that some minimum requirements are met, introduces a further loop in the design process and does not ensure the final design will be optimal with respect to this new requirement. The introduction of some robustness considerations into the design process can reduce the level of fragmentation and iteration typical of gas turbine engine design and produce further (and more robust) improvements relative to the traditional method. In this study, two approaches for dealing with off-design performance analysis are presented, integrated into an automatic optimisation system and applied to the preliminary design of a core compression system from a three-spool modern turbofan engine. Designs that are more robust than those found if only design-point performance is considered are successfully identified

Balancing Configuration and Refinement in the Design of Two-Spool Multistage Compression Systems

With limited resources, time spent refining a design is time not spent in selecting its optimal configuration. A multi-fidelity optimization scheme is applied to the configuration and refinement of a generic core engine compression system. The best designs result from expending between half and three quarters of the total design effort on configuration selection. The performance of the refinement phase is a weak function of the preceding configuration phase when the latter is well into diminishing returns. By exploiting this behavior, the time taken to obtain equally good designs with the same analysis tools and computational resource may be halved

Numerical Evaluation of Entropy Generation in Isolated Airfoils and Wells Turbines

In recent years, a number of authors have studied entropy generation in Wells turbines. This is potentially a very interesting topic, as it can provide important insights into the irreversibilities of the system, as well as a methodology for identifying, and possibly minimizing, the main sources of loss. Unfortunately, the approach used in these studies contains some crude simplifications that lead to a severe underestimation of entropy generation and, more importantly, to misleading conclusions. This paper contains a re-examination of the mechanisms for entropy generation in fluid flow, with a particular emphasis on RANS equations. An appropriate methodology for estimating entropy generation in isolated airfoils and Wells turbines is presented. Results are verified for different flow conditions, and a comparison with theoretical values is presented.Regione Autonoma Sardegna (funding for University of Cagliari co-authors

Correction to: Numerical evaluation of entropy generation in isolated airfoils and Wells turbines

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