Browsing by Author "Kalfas, A."
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Item Open Access Development of a Method for Enhanced Fan Representation in Gas Turbine Modeling(2011) Doulgeris, Georgios C.; Khaleghi, H.; Kalfas, A.; Pilidis, PericlesA challenge in civil aviation future propulsion systems is expected to be the integration with the airframe, coming as a result of increasing bypass ratio or above wing installations for noise mitigation. The resulting highly distorted inlet flows to the engine, make a clear demand for advanced gas turbine performance prediction models. Since the dawn of jet engine several models have been proposed and the present work comes to add a model that combines two well established compressor performance methods in order to create a quasi three dimensional representation of the fan of a modern turbofan. A streamline curvature model is coupled to a parallel compressor method, covering radial and circumferential directions respectively. Model testing has shown a close agreement to experimental data, making it a good candidate for assessing the loss of surge margin on a high bypass ratio turbofan, semi-embedded on the upper surface of a broad wing airframe.Item Open Access Off-design performance comparison between single and two-shaft engines: part 1 — fixed geometry(American Society of Mechanical Engineers, 2021-01-11) Nikolaidis, Theoklis; Pellegrini, Alvise; Saravanamuttoo, H. I. H.; Aslanidou, I.; Kalfas, A.; Pilidis, PericlesThis paper describes an investigation into the off-design performance comparison of single and two-shaft gas turbine engines. A question that has been asked for a long time which gas turbine delivers a better thermal efficiency at part load. The authors, notwithstanding their intensive searches, were unable to find a comprehensive answer to this question. A detailed investigation was carried out using a state of the art performance evaluation method and the answer was found to be: It depends! In this work, the performance of two engine configurations is assessed. In the first one, the single-shaft gas turbine operates at constant shaft rotational speed. Thus, the shape of the compressor map rotational speed line will have an important influence on the performance of the engine. To explore the implications of the shape of the speed line, two single-shaft cases are examined. The first case is when the speed line is curved and as the compressor pressure ratio falls, the non-dimensional mass flow increases. The second case is when the speed line is vertical and as the compressor pressure ratio falls, the non-dimensional mass flow remains constant. In the second configuration, the two-shaft engine, the two-shafts can be controlled to operate at different rotational speeds and also varying relationships between the rotational speeds. The part-load operation is characterized by a reduction in the gas generator rotational speed. The tool, which was used in this study, is a 0-D whole engine simulation tool, named Turbomatch. It was developed at Cranfield and it is based on mass and energy balance, carried out through an iterative method, which is based on component maps. These generic, experimentally derived maps are scaled to match the design point of a particular engine before an off-design calculation is performed. The code has been validated against experimental data elsewhere, it has been used extensively for academic purposes and the research activities that have taken place at Cranfield University. For an ideal cycle, the single-shaft engine was found to be a clear winner in terms of part-load thermal efficiency. However, this picture changed when realistic component maps were utilized. The basic cycle and the shape of component maps had a profound influence on the outcome. The authors explored the influence of speed line shapes, levels of component efficiencies and the variation of these component efficiencies within the operating range. This paper describes how each one of these factors, individually, influences the outcome.Item Open Access Transition to turbulence in the boundary layer of turbomachinery blading(Cranfield University, 1994-04) Kalfas, A.; Elder, R. L.This thesis is concerned with the investigation of boundary layer transition phenomena in relation to turbomachinery flows. The present study involved the experimental investigation of the flow field around flat plates with various leading edge configurations. This study has also included some theoretical predictions using computational fluid dynamics. The experimental investigation involved a variety of free stream environments which have attempted to simulate typical turbomachinery flows. Three flat plate configurations with a sharp, a semi-circular and a C4 leading edge shapes have been employed. The effect of free stream turbulence intensity over a range of free stream Reynolds numbers has been examined. Surface flow visualisation techniques have been applied to a total of 36 different experimental conditions in order to define the transitional characteristics of the boundary layer. This flow visualisation method was found to be appropriate for the large range of test cases involved and especially for the flat plate with the cylindrical leading edge. For the cylindrical leading edge configuration, a separation bubble has been detected in the vicinity of the leading edge, under all tested conditions. Hot-wire investigation of the boundary layer has been undertaken over the flat plate with the C4 leading edge, which has been regarded as the most relevant configuration for turbomachinery applications. This method provided high frequency velocity data which have been analysed in order to obtain information about the spatial distribution of integral parameters of the boundary layer, such as the mean velocity, the turbulence intensity and the skewness and flatness factors. Boundary layer spectra have also been acquired. Statistical analysis of this data has been employed in order to obtain the intermittency distribution. The present results have been found to be in good agreement with existing transition correlations. Calculations of the flow over flat plates with leading edge shapes similar to the experimental configuration have been undertaken using a two-dimensional elliptic Navier-Stokes solver. A solution of the flow field around a semi-circular and an elliptical leading edge has been obtained. Low Reynolds number k-Ɛ modelling has been applied in order to model the transitional characteristics of the boundary layer flow very near the wall. The use of the Nagano-Hishida version of the low-Reynolds number turbulence model led to predictions of an early start and end of transition.