Papers with Whittle Lab authors at ASME Turbo Expo, June 6-110 2011, Vancouver

Stall Warning by Blade Pressure Signature Analysis

Anna Young, Ivor Day, Graham Pullan

Paper GT2011-45850; Session TC-34-1; Tuesday 2:30pm-5:00pm

At low mass flow rates axial compressors suffer from flow instabilities leading to stall and surge. The inception process of these instabilities has been widely researched in the past .ANP primarily with the aim of predicting or averting stall onset. In recent times, attention has shifted to conditions well before stall and has focussed on the level of irregularity in the blade passing signature in the rotor tip region. In general, this irregularity increases in intensity as the flow rate through the compressor is reduced. Attempts have been made to develop stall warning/avoidance procedures based on the level of the flow irregularity, but little effort has been made to characterise the irregularity, or to understand its underlying causes.

Work on this project has revealed for the first time that the increase in irregularity in the blade passing signature is highly dependent on both tip-clearance and eccentricity. In a compressor with small, uniform, tip-clearance, the increase in blade passing irregularity which accompanies a reduction in flow rate will be modest. If the tip-clearance is enlarged, however, there will be a sharp rise in irregularity at all circumferential locations. In a compressor with eccentric tip-clearance, the increase in irregularity will only occur in the part of the annulus where the tip-clearance is largest, regardless of the average clearance level.

In this paper, some attention is also given to the question of whether this irregularity observed in the pre-stall flow field is due to random turbulence, or to some form of coherent flow structure. Detailed flow measurements reveal that the latter is the case. From these findings, it is clear that a stall warning system based on blade passing signature irregularity will not be viable in an aero-engine where tip-clearance size and eccentricity change during each flight cycle and over the life of the compressor.

Blade loading and its application in the mean-line design of the low pressure turbine

John D. Coull and Howard P. Hodson

Paper GT2011-45238; Session ThA-30-6; Thursday 8:00am

Increasing the lift of LP turbine blades is desirable because it reduces engine weight and cost, but there is likely to be an efficiency penalty at cruise conditions. In order to minimise the number of design iterations, it is therefore important to capture this trade-off at the preliminary design stage.

This paper examines a design study for a repeating-stage LP turbine. The analysis demonstrates that the Zweifel coefficient is not a suitable measure of blade loading as it inherently depends on the flow angles. An alternative lift coefficient based on circulation instead proposed.

Several of the most common correlations for predicting profile and secondary losses are examined for their suitability to mean-line design. Importantly, the correlations derived from Ainley and Mathieson (1957) predict the wrong efficiency trends with respect to flow coefficient and stage loading. The profile loss model of Coull and Hodson (2010) and the secondary loss models of Craig and Cox (1971) and Traupel (1977) gave reasonable predictions, but further improvements should be pursued.

The analysis quantifies the likely trade-off between blade loading and performance. The figure shows the retreat of efficiency contours across the Smith chart as blade loading (Co) is increased. Reasonable predictions of Reynolds number lapse rates may also be made.

The Aero-thermal Performance of a Cooled Winglet Tip in a High Pressure Turbine Cascade

Chao Zhou, Howard Hodson, Ian Tibbott and Mark Stokes

Paper GT2011-46369; Session WC-10-14; Wednesday 2:30-3:00pm

The aero-thermal performance of a winglet tip with cooling holes on the tip and on the blade surface near the tip is reported in this paper. The investigation was based on a high pressure turbine cascade. Experimental and numerical methods were used. The effects of the coolant mass flow rate are also studied.

Because the coolant injection partially blocks the tip leakage flow, more passage flow is turned by the blade. As a result, the coolant injection on the winglet tip reduces the deviation of the flow downstream of the cascade due to the tip leakage flow. However, the tip leakage loss increases slightly with the coolant mass flow ratio.

Both the CFD tools and experiments using the Amonia-Diazo technique were used to determine the cooling effectiveness. On the blade pressure side surface, low cooling effectiveness appears around the holes due to the lack of the coolant from the cooling hole or the lift-off of the coolant from the blade surface when the coolant mass flow is high. The cooling effectiveness on the winglet tip is a combined effect of the coolant ejected from all the holes.

On the top of the winglet tip, the average cooling effectiveness increases and the heat load decreases with increasing coolant mass flow. Due to its large area, the cooled winglet tip has a higher heat load than an uncooled flat tip at engine representative coolant mass flow ratio. Nevertheless, the heat flux rate per unit area of the winglet is much lower than that of an uncooled flat tip.

Effects of Endwall Motion on the Aero-thermal Performance of a Winglet Tip in an HP Turbine

Chao Zhou, Howard Hodson, Ian Tibbott and Mark Stokes

Paper GT2011-46373; Session WC-10-14; Wednesday 3:30-4:00pm

Contra-rotating Open Rotor Operation for Improved Aerodynamics and Noise at Takeoff

A. Zachariadis, C. A. Hall, A. B. Parry

Paper GT2010-45205; Session ThC-35-2; Thursday 2:30pm-5:00pm

The contra-rotating open rotor is, once again, being considered as an alternative to the advanced turbofan to address the growing pressure to cut aviation fuel consumption and carbon dioxide emissions. One of the key challenges is meeting community noise targets at takeoff. Previous open rotor designs are subject to poor efficiency at takeoff due to the presence of large regions of separated flow on the blades as a result of the high incidence needed to achieve the required thrust. This is a consequence of the fixed rotor rotational speed constraint typical of variable pitch propellers.

Within the study described in this paper, an improved operation is proposed to improve performance and reduce rotor-rotor interaction noise at takeoff. Three-dimensional computational fluid dynamics (CFD) calculations have been performed on an open rotor rig at a range of takeoff operating conditions. These have been complemented by analytical tone noise predictions to quantify the noise benefits of the approach.

The results presented show that for a given thrust, a combination of reduced rotor pitch and increased rotor rotational speed can be used to reduce the incidence onto the front rotor blades. This is shown to eliminate regions of flow separation, reduce the front rotor tip loss and reduce the downstream stream tube contraction. The wakes from the front rotor are also made wider with lower velocity defect, which is found to lead to reduced interaction tone noise. Unfortunately, the necessary increase in blade speed leads to higher relative Mach numbers, which increases rotor alone noise.

In summary, the combined CFD and aero-acoustic analysis in this paper shows how careful operation of an open rotor at takeoff, with moderate levels of re-pitch and speed increase, can lead to improved front rotor efficiency as well as appreciably lower noise across all directivities on a dBA basis.

Influence of Film Cooling Hole Angles and Geometries on Aerodynamic Loss and Net Heat Flux Reduction

Chia Hui Lim, Graham Pullan, Peter Ireland

Paper GT2011-45721; Session ThC-11-12; Thursday 2:30pm-5:00pm

Turbine design engineers have to ensure that film cooling can provide sufficient protection to turbine blades from the hot mainstream gas, while keeping the losses low. Film cooling hole design parameters include inclination angle (alpha), compound angle (beta), hole inlet geometry and hole exit geometry. The influence of these parameters on aerodynamic loss and net heat flux reduction is investigated, with loss being the primary focus. Low-speed flat plate experiments have been conducted at momentum flux ratios of IR = 0.16, 0.64 and 1.44.

The film cooling aerodynamic mixing loss, generated by the mixing of mainstream and coolant, can be quantified using a three-dimensional analytical model that has been previously reported by the authors. The model suggests that for the same flow conditions, the aerodynamic mixing loss is the same for holes with different alpha and beta but with the same angle between the mainstream and coolant flow directions (angle kappa). This relationship is assessed through experiments by testing two sets of cylindrical holes with different alpha and beta: one set with kappa=35deg, another set with kapp=60deg. The data confirm the stated relationship between alpha, beta, kappa and the aerodynamic mixing loss. The results show that the designer should minimise kappa to obtain the lowest loss, but maximise beta to achieve the best heat transfer performance. A suggestion on improving the loss model is also given.

Five different hole geometries were also tested: cylindrical hole, trenched hole, fan-shaped hole, D-Fan and SD-Fan. The D-Fan and the SD-Fan have similar hole exits to the fan-shaped hole but their hole inlets are laterally expanded. The external mixing loss and the loss generated inside the hole are compared. It was found that the D-Fan and the SD-Fan have the lowest loss. This is attributed to their laterally expanded hole inlets, which lead to significant reduction in the loss generated inside the holes. As a result, the loss of these geometries is 50% of the loss of the fan-shaped hole at IR = 0.64 and 1.44.

Self-Regulating Casing Treatment for Axial Compressor Stability Enhancement

Stephanie Weichert, Ivor Day, Chris Freeman

Paper GT2011-46042; Session FA-29-15 ; Friday 8:00am-10:00am

The operating range of an axial compressor is often restricted by a safety imposed stall margin. One possible way of regaining operating range is with the application of casing treatment. Of particular interest here is the type of casing treatment which extracts air from a high pressure location in the compressor and re-injects it through discrete loops into the rotor tip region. Existing re-circulation systems have the disadvantage of reducing compressor efficiency at design conditions because worked flow is unnecessarily re-circulated at these operating conditions. Re-circulation is really only needed near stall. This paper proposes a self-regulating casing treatment in which the re-circulated flow is minimized at compressor design conditions and maximized near stall. The self-regulating capability is achieved by taking advantage of changes which occur in the tip clearance velocity and pressure fields as the compressor is throttled toward stall.

In the proof-of-concept work reported here, flow is extracted from the high pressure region over the rotor tips and re-injected just upstream of the same blade row. Parametric studies are reported in which the flow extraction and re-injection ports are optimized for location, shape and orientation. The optimized design is shown to compare favorably with a circumferential groove tested in the same compressor. The relationship between stall inception type and casing treatment effectiveness is also investigated.

The self-regulating aspect of the new design works well: stall margin improvements from 2.2 to 6.0% are achieved for just 0.25% total air re-circulated near stall and half that near design conditions. The self-regulating capability is achieved by the selective location and orientation of the extraction hole; a simple model is discussed which predicts the optimum axial location.

Large Eddy Simulation of a Hot High Speed Coflowing Jet

Simon Eastwood, Paul Tucker and Hao Xia

Computations are made of a short cowl coflowing jet nozzle with a bypass ratio 8 : 1. The core flow is heated, making the inlet conditions reminiscent of those for a real engine. A large eddy resolving approach is used with a 12 x 106 cell mesh. Since the code being used tends towards being dissipative the sub-grid scale (SGS) model is abandoned giving what can be termed Numerical Large Eddy Simulation (NLES). To overcome near wall modelling problems a hybrid NLES-RANS (Reynolds Averaged Navier-Stokes) related method is used. For y+ less than 60 a k$B!](Bl model is used. Blending between the two regions makes use of the differential Hamilton-Jabobi (HJ) equation, an extension of the eikonal equation. Results show encouraging agreement with existing measurements of other workers. The eikonal equation is also used for acoustic ray tracing to explore the effect of the mean flow on acoustic ray trajectories, thus yielding a coherent solution strategy.