This paper describes the role of tip leakage flow in creating the leading edge separation necessary for the onset of spike-type compressor rotating stall. A series of unsteady multipassage simulations, supported by experimental data, are used to define and illustrate the two competing mechanisms that cause the high incidence responsible for this separation: blockage from a casing-suction-surface corner separation and forward spillage of the tip leakage jet. The axial momentum flux in the tip leakage flow determines which mechanism dominates. At zero tip clearance, corner separation blockage dominates. As clearance is increased, the leakage flow reduces blockage, moving the stall flow coefficient to lower flow, i.e., giving a larger unstalled flow range. Increased clearance, however, means increased leakage jet momentum and contribution to leakage jet spillage. There is thus a clearance above which jet spillage dominates in creating incidence, so the stall flow coefficient increases and flow range decreases with clearance. As a consequence, there is a clearance for maximum flow range; for the two rotors in this study, the value was approximately 0.5% chord. The chordwise distribution of the leakage axial momentum is also important in determining stall onset. Shifting the distribution toward the trailing edge increases flow range for a leakage jet dominated geometry and reduces flow range for a corner separation dominated geometry. Guidelines are developed for flow range enhancement through control of tip leakage flow axial momentum magnitude and distribution. An example is given of how this might be achieved.

This paper describes the role of tip leakage flow in creating the leading edge separation necessary for the onset of spike-type compressor rotating stall. A series of unsteady multi-passage simulations, supported by experimental data, are used to define and illustrate the two competing mechanisms that cause the high incidence responsible for this separation: blockage from a casing-suction-surface corner separation and forward spillage of the tip leakage jet. The axial momentum flux in the tip leakage flow determines which mechanism dominates. At zero tip clearance, corner separation blockage dominates. As the clearance is increased, the leakage flow reduces the blockage, moving the stall flow coefficient to lower flow, i.e., giving a larger unstalled flow range. Increased clearance, however, means an increase in leakage jet momentum and the contribution to leakage jet spillage. There is thus a clearance above which jet spillage dominates in creating incidence, so the stall flow coefficient increases and the flow range decreases with clearance. As a consequence there is a clearance for maximum flow range; for the two rotors in this study, the value was approximately 0.5% chord. The chord-wise distribution of the leakage axial momentum is also important in determining stall onset. Shifting the distribution towards the trailing edge increases the flow range of a leakage jet dominated geometry and reduces the flow range of a corner separation dominated geometry. Guidelines are developed for flow range enhancement through control of tip leakage flow axial momentum magnitude and distribution. An example is given of how this might be achieved. INTRODUCTIONThere are two scenarios through which the flow in a compressor can make the transition from a nominally axisymmetric flow to the asymmetric regime known as rotating stall. The first involves the growth, over tens of revolutions, of a small amplitude propagating disturbance, with wave length on the order of the circumference, into rotating stall. The initial stage of this evolution is well described by linear stability analysis [1]. The second involves a disturbance with length scale on the order of several blade pitches and a time between initial detection (by casing static pressure measurements, for example) and fully developed rotating stall of less than ten rotor revolutions. The sharp up and down pressure signatures of the disturbance in this type of stall onset have led to them being referred to as spikes [2,3]. Computational and experimental studies [4][5][6][7] have determined that the structure of the spike is a vortex tube, with leading edge separation the source of the vorticity.

This paper describes an innovative, three-day, turbomachinery research project for Japanese and British high-school students. The project is structured using modern teaching theories that encourage student curiosity and creativity. The experience develops teamwork and communication and helps to break down the cultural and linguistic barriers between students from different countries and backgrounds. The approach provides a framework for other hands-on research projects that aim to inspire young students to undertake a career in engineering. The project is part of the Clifton Scientific Trust's annual UK–Japan Young Scientist Workshop Programme. This work focuses on compressor design for jet engines and gas turbines. It includes lectures introducing students to turbomachinery concepts, a computational design study of a compressor blade section, experimental tests with a low-speed cascade, and tutorials in data analysis and aerodynamic theory. The project also makes use of 3D printing technology, so that students go through the full engineering design process, from theory, through design, to practical experimental testing. Alongside the academic aims, students learn what it is like to study engineering at university, discover how to work effectively in a multinational team, and experience a real engineering problem. Despite a lack of background in fluid dynamics and the limited time available, the lab work and end-of-project presentation show how far young students can be stretched when they are motivated by an interesting problem.

This paper describes an innovative, three-day, turbomachinery research project for Japanese and British high-school students. The project is structured using modern teaching theories which encourage student curiosity and creativity. The experience develops team-work and communication, and helps to break-down cultural and linguistic barriers between students from different countries and backgrounds. The approach provides a framework for other hands-on research projects which aim to inspire young students to undertake a career in engineering. The project is part of the Clifton Scientific Trust’s annual UK-Japan Young Scientist Workshop Programme. The work focuses on compressor design for jet engines and gas turbines. It includes lectures introducing students to turbomachinery concepts, a computational design study of a compressor blade section, experimental tests with a low-speed cascade and tutorials in data analysis and aerodynamic theory. The project also makes use of 3D printing technology, so that students go through the full engineering design process, from theory, through design, to practical experimental testing. Alongside the academic aims, students learn what it is like to study engineering at university, discover how to work effectively in a multinational team, and experience a real engineering problem. Despite a lack of background in fluid dynamics and the limited time available, the lab work and end of project presentation show how far young students can be stretched when they are motivated by an interesting problem.