Roger W. Ainsworth

Unsteady pressure measurement

Pressure measurement and turbomachinery have been intimately linked since flow field diagnosis was employed in verifying the operation of the first gas turbines. In the early years, time-mean pressures were required and extensive use of pneumatic connections between the measurement points and pressure transducers was made. Over the last two decades or so there has been a requirement to measure time-varying pressures in turbomachinery applications to bandwidths of order 100 kHz and the silicon piezoresistive pressure sensor has been the device which has been at the heart of many of the measurements. Although we mention other new developments in technology that are under way, this paper concentrates on the silicon piezoresistive sensor. The operation of the device in the context of gas turbine applications is outlined and some of the special issues which arise and must be addressed for accurate measurements are discussed. Following this, two example fields of the application of piezoresistive sensors are discussed in some detail, namely, rotating blade static pressure measurements and fast response aerodynamic probes. In both these cases instrumentation design considerations are discussed, technological implementation details given and sample data displayed and briefly discussed. Contemporary work elsewhere is included in the discussion. Finally, conclusions are drawn and the future context for the piezoresistive device is outlined.

Time-Resolved Vane-Rotor Interaction in a High-Pressure Turbine Stage

This paper describes the time-varying aerodynamic interaction mechanisms that have been observed within a transonic high-pressure turbine stage; these are inferred from the time-resolved behavior of the rotor exit flow field. It contains results both from an experimental program in a turbine test facility and from numerical predictions. Experimental data was acquired using a fast-response aerodynamic probe capable of measuring Mach number, whirl angle, pitch angle, total pressure, and static pressure. A 3-D time-unsteady viscous Navier-Stokes solver was used for the numerical predictions. The unsteady rotor exit flowfield is formed from a combination of four flow phenomena: the rotor wake, the rotor trailing edge recompression shock, the tip-leakage flow, and the hub secondary flow. This paper describes the time-resolved behavior of each phenomenon and discusses the interaction mechanisms from which each originates. Two significant vane periodic changes (equivalent to a time-varying flow in the frame of reference of the rotor) in the rotor exit flowfield are identified. The first is a severe vane periodic fluctuation in flow conditions close to the hub wall and the second is a smaller vane periodic fluctuation occurring at equal strength over the entire blade span. These two regions of periodically varying flow are shown to be caused by two groups of interaction mechanisms. The first is thought to be caused by the interaction between the wake and secondary flow of the vane with the downstream rotor; and the second is thought to be caused by a combination of the interaction of the vane trailing edge recompression shock with the rotor, and the interaction between the vane and rotor potential fields.

A new approach to flow-field measurement—A view of Doppler global velocimetry techniques

Abstract Nonintrusive techniques for determining velocities in fluid flowfields, based on the use of laser technology, have been the subject of much research over the last three decades. Many of these techniques, such as laser-Doppler-anemometry (LDA) and laser-two-focus-anemometry (L2F), have been based on point-wise measurements until the advent, in the last decade, of particle image velocimetry (PIV). There is much interest in pursuing the goal of nonintrusive, whole-field velocity measurements, although such techniques are still some way from maturity. A new technique has recently emerged to challenge the position of PIV: the technique of Doppler global velocimetry (DGV). This paper outlines the origins of the technique, the physical principles involved, and the potential advantages of DGV over PIV. A review is made of the technology used by the small number of groups researching this new method, drawing comparisons between the major aspects of their differing techniques. Finally, examples are given of “test case” measurements of two well-established velocity fields: those of a rotating disc, and those in a subsonic open jet.

Wake, Shock, and Potential Field Interactions in a 1.5 Stage Turbine—Part I: Vane-Rotor and Rotor-Vane Interaction

The composition of the time-resolved surface pressure field around a high-pressure rotor blade caused by the presence of neighboring blade rows is investigated, with the individual effects of wake, shock and potential field interaction being determined. Two test geometries are considered: first, a high-pressure turbine stage coupled with a swan-necked diffuser exit duct; secondly, the same high-pressure stage but with a vane located in the downstream duct. Both tests were conducted at engine-representative Mach and Reynolds numbers, and experimental data was acquired using fast-response pressure transducers mounted on the mid-height streamline of the HP rotor blades. The results are compared to time-resolved computational predictions of the flowfield in order to aid interpretation of experimental results and to determine the accuracy with which the computation predicts blade interaction. The paper is split into two parts: the first investigating the effect of the upstream vane on the unsteady pressure field around the rotor (vane-rotor interaction), and the second investigating the effect of the downstream vane on the unsteady pressure field around the rotor (rotor-vane interaction). The paper shows that at typical design operating conditions shock interaction from the upstream blade row is an order of magnitude greater than wake interaction and that with the design vane-rotor inter-blade gap the presence of the rotor causes a periodic increase in the strength of the vane trailing edge shock. The presence of the potential field of the downstream vane is found to affect significantly the rotor pressure field downstream of the Mach one surface within each rotor passage.

The development of turbine exit flow in a swan-necked inter-stage diffuser

This paper describes both the migration and dissipation of flow phenomena downstream of a transonic high-pressure turbine stage. The geometry of the HP stage exit duct considered is a swan-necked diffuser similar to those likely to be used in future engine designs. The paper contains results both from an experimental programme in a turbine test facility and from numerical predictions. Experimental data was acquired using three fast-response aerodynamic probes capable of measuring Mach number, whirl angle, pitch angle, total pressure and static pressure. The probes were used to make time-resolved area traverses at two axial locations downstream of the rotor trailing edge. A 3D time-unsteady viscous Navier-Stokes solver was used for the numerical predictions. The unsteady exit flow from a turbine stage is formed from rotor-dependent phenomena (such as the rotor wake, the rotor trailing edge recompression shock, the tip-leakage flow and the hub secondary flow) and vane-rotor interaction dependant phenomena. This paper describes the time-resolved behaviour and three-dimensional migration paths of both of these phenomena as they convect downstream. It is shown that the inlet flow to a downstream vane is dominated by two corotating vortices, the first caused by the rotor tip-leakage flow and the second by the rotor hub secondary flow. At the inlet plane of the downstream vane the wake is extremely weak and the radial pressure gradient is shown to have caused the majority of the high loss wake fluid to be located between the mid-height of the passage and the casing wall. The structure of the flow indicates that between a high pressure stage and a downstream vane simple two-dimensional blade row interaction does not occur. The results presented in this paper indicate that the presence of an upstream stage is likely to significantly alter the structure of the secondary flow within a downstream vane. The paper also shows that vane-rotor interaction within the upstream stage causes a 10° circumferential variation in the inlet flow angle of the 2nd stage vane.© 2003 ASME

Unsteady loss in a high pressure turbine stage

Abstract An investigation into the unsteady losses in a high pressure turbine stage has been performed experimentally at engine-representative conditions. This has been done by making time-resolved measurements of entropy at stage exit at all vane and rotor-relative positions over a wide range of radial height. Due to the considerable difficulty of obtaining accurate measurements at engine-representative conditions, these measurements provide a unique set of experimental data. Five distinct flow features have been identified and their effects on the stage efficiency estimated, showing good overall agreement with an independent efficiency measurement. Comparisons with a particular numerical prediction of the flow field, Unstrest, have shown very good agreement in both the flow structure and the entropy generation. Only the rotor dependence of the loss mechanisms is examined here: the vane dependence will be presented in a subsequent paper.

The Development of Fast Response Aerodynamic Probes for Flow Measurements in Turbomachinery

The advent of a new generation of transient rotating turbine simulation facilities, where engine values of Reynolds and Mach number are matched simultaneously together with the relevant rotational parameters for dimensional similitude (Dunn et al., 1988 ; Epstein and Guenette, 1984 ; Ainsworth et al., 1988), has provided the stimulus for developing improved instrumentation for investigating the aerodynamic flows in these stages. Much useful work has been conducted in the past using hot-wire and laser anemometers. However, hot-wire anemometers are prone to breakage in the high-pressure flows requiredfor correct Reynolds numbers. Furthermore, some laser techniques require a longer run-time than these transient facilities permit, and generally yield velocity information only, giving no data on loss production. Advances in semiconductor aerodynamic probes are beginning to fulfill this perceived need. This paper describes advances made in the design, construction, and testing of two and three-dimensional fast response aerodynamic probes, where semiconductor pressure sensors are mounted directly on the surface of the probes, using techniques that have previously been successfully used on the surface of rotor blades (Ainsworth et al., 1991). These are to be used to measure Mach number and flow direction in compressible unsteady flow regimes. In the first section, a brief review is made of the sensor and associated technology that has been developed to permit a flexible design of fast response aerodynamic probe. Following this, an extensive program of testing large-scale aerodynamic models of candidate geometries for suitable semiconductor scale probes is described, and the results of these discussed. The conclusions of these experiments, conducted for turbine representative mean and unsteady flows, yielded new information for optimizing the design of the small-scale semiconductor probes, in terms ofprobe geometry, sensor placement, and aerodynamic performance. Details are given ofa range of wedge and pyramid semiconductor probes constructed, and the procedures used in calibrating and making measurements with them. Differences in performance are discussed, allowing the experimenter to choose an appropriate probe for the particular measurement required. Finally, the application of prototype semiconductor probes in a transient rotor experiment at HP turbine representative conditions is described, and the data so obtained are compared with CFD solutions of the unsteady viscous flow-field.

The effect of an upstream turbine on a low-aspect ratio vane

The interaction between a high-pressure rotor and a downstream vane is dominated by vortex-blade interaction. Each rotor blade passing period two co-rotating vortex pairs, the tip-leakage and upper passage vortex and the lower passage and trailing shed vortex, impinge on, and are cut by, the vane leading edge. In addition to the streamwise vortex the tipleakage flow also contains a large velocity deficit. This causes the interaction of the tip-leakage flow with a downstream vane to differ from typical vortex blade interaction. This paper investigates the effect these interaction mechanisms have on a downstream vane. The test geometry considered was a low aspect ratio second stage vane located within a S-shaped diffuser with large radius change mounted downstream of a shroudless high-pressure turbine stage. Experimental measurements were conducted at engine-representative Mach and Reynolds numbers, and data was acquired using a fastresponse aerodynamic probe upstream and downstream of the vane. Time-resolved numerical simulations were undertaken with and without a rotor tip gap in order to investigate the relative magnitude of the interaction mechanisms. The presence of the upstream stage is shown to significantly change the structure of the secondary flow in the vane and to cause a small drop in its performance.© 2004 ASME

Wake, shock and potential field interactions in a 1.5 stage turbine: Part II: Vane-vane interaction and discussion of results

The composition of the time-resolved surface pressure field around a high-pressure rotor blade caused by the presence of neighboring blade rows is investigated with the individual effects of wake, shock and potential field interaction being determined. Two test geometries are considered: first, a high-pressure turbine stage coupled with a swan-necked diffuser exit duct: secondly, the same high pressure stage but with a vane located in the downstream duct. Both tests were conducted at engine-representative Mach and Reynolds numbers and experimental data was acquired using fast-response pressure transducers mounted on the mid-height streamline of the HP rotor blades. The results are compared to time-resolved computational predictions of the flow field in order to aid interpretation of experimental results and to determine the accuracy with which the computation predicts blade interaction. In the first half of this paper it is shown that, in addition to the two main interaction mechanisms (upstream vane-rotor and rotor-downstream vane interactions, presented in Part I of this paper) a third interaction occurs. This new interaction mechanism is shown to be caused by the interaction between the downstream vanes potential field and the upstream vanes trailing edge potential field and shock. The unsteady rotor surface static pressure fluctuations caused by this interaction are shown to occur on the late rotor suction surface at a frequency corresponding to the difference in the numbers of upstream and downstream vanes. The second part to the paper discusses the mechanisms that cause vane-rotor-vane interaction. The rotors operating point is periodically altered as it passes the downstream vane. It is shown that for a large downstream vane, the flow conditions in the rotor passage, at any instant in time, are close to being steady state.

Improved fast-response heat transfer instrumentation for short-duration wind tunnels

This paper describes the recent innovations associated with the fabrication and application of fast-response heat flux gauges in short-duration wind-tunnel facilities in Oxford. This work has been driven by the need to measure the heat transfer in an environment with large spatial and temporal variations in surface heat flux, caused by a flow-field that has significant high-frequency periodic fluctuations in both recovery temperature and heat transfer coefficient (namely, an axial flow turbine). A new approach to the fabrication of miniature surface heat flux thin-film gauges is described; this involves the use of a pulse laser to ablate (etch) the gauge and connecting track patterns directly onto electrically insulated and platinized components. This automated technique is shown to produce gauges of high geometrical tolerance at precisely known locations, and to create arrays of gauges on the experimental components rapidly. Typically, the platinum resistance elements are 1 mm by 0.08 mm in plan form and have closely repeatable characteristics (for example, resistance and temperature coefficient). A novel technique for measuring the adiabatic wall temperature and heat transfer coefficient in short-duration facilities is also presented; this includes the capability to measure the deterministic component of these unsteady parameters in periodic (pulsating) flows. The combination of the new high spatial density gauge arrays and the capability to measure unsteady adiabatic wall temperature allows unprecedented insight into the flow physics that influences the unsteady heat flux. In particular, it has been possible to establish whether heat transfer rate fluctuations in a turbine experiment are caused by changes in flow recovery temperature or by changes in heat transfer coefficient. Some example measurements taken in a high-speed turbine test facility are described. Finally, some of the recently manufactured heat transfer gauge systems are presented.