The losses generated by fluid leaking across the shrouds of turbine blade rows are known to form a significant proportion of the overall loss generated in low aspect ratio turbines. The use of shrouds to encase the tips of turbine blades has encouraged the development of many innovative sealing arrangements, all of which are intended to reduce the quantity of fluid (the leakage fraction) leaking across the shroud. Modern sealing arrangements have reduced leakage fractions considerably, meaning that further improvements can only be obtained by controlling the leakage flow in such a way so as to minimise the aerodynamic losses incurred by the extraction and re-injection of the leakage flow into the mainstream. There are few published experimental investigations on the interaction between mainstream and leakage flows to provide guidance on the best means of managing the leakage flows to do this. This paper describes the development and testing of a strategy to turn the fluid leaking over shrouded turbine rotor blade rows with the aim of reducing the aerodynamic losses associated with its re-injection into the mainstream flow. The intent was to extract work from the leakage flow in the process. A four stage research turbine was used to test in detail the sealing design resulting from this strategy. A reduction in brake efficiency of 3.5% was measured. Further investigation suggested that much of the increase in loss could be attributed to the presence of axial gaps upstream and downstream of the shroud cavity which facilitated the periodic ingress and egress of mainstream fluid into the shroud cavity under the influence of the rotor potential field. This process was exacerbated by reductions in the leakage fraction.

The aerodynamics of stator shroud leakage and mainstream flow interactions have been investigated using a linear cascade in which leakage mass flow and tangential velocity could be varied independently. Experimental measurements confirm the detrimental effect of increasing leakage mass flow rate, but also show clearly that increasing cavity tangential velocity improves the performance of the blade row. Shroud leakage is also found to modify significantly the spanwise distribution of blockage, flow turning and loss at exit of the stator passage. The mechanisms behind these large-scale effects are identified and a description of the interaction process based on momentum thickness arguments is proposed. Experimental and numerical results show that the amount of leakage is a linear function of the difference between cavity pressure and average pressure on the hub upstream of the stator. This result is shown to be an axisymmetric result, and thus largely independent of the pitchwise variation in blade loading. Departures from linearity at low leakage fractions are associated to the blade loading inducing pitchwise non-uniformities in the structure of the flow at the interface between the shroud cavity and the mainstream. These non-uniformities involve vortical structures, which participate in the exchange of fluid and momentum between the cavity and the mainstream, even at zero net leakage. Experiments also show that these non-uniformities in flow structure contribute to the generation of loss.

As CFD has matured to the point that it is capable of reliable and accurate flow simulation, attention is now firmly fixed on how best to deploy that CFD as part of a process to improve actual products. This “process” consists of capturing and controlling the geometry of a suitable portion of an aeroengine (e.g., a blade row, or an internal cooling system or a fan-plus-nacelle), building a mesh system, solving the flow and responding to an appropriately visualized flow field by changing or accepting the geometry. This paper looks at that process from the point of view of identifying any bottlenecks and argues that current research should be directed at the CAD-to-mesh-to-solution cycle time rather than, as has been traditional, just looking at the solver itself and in isolation. Work aimed at eliminating some of these bottlenecks is described, with a number of practical examples.

The following paper examines the key aspects of technology that deliver an advanced hybrid meshing capability that is operational today, and is ideally suited to complex, realworld geometries. The paper shows how particular benefits are derived from detailed algorithmic features at octree level, from the overall meshing procedure and from the software's client-server architecture and its distributed-memory parallel implementation. Two two complex geometry examples are used to demonstrate the benefits in terms of complex geometry capability, of ability to modify geometry and of and ease-of-use. Nomenclature ϕ = distance field derived around the surfaces defining the geometry 1

The losses generated by fluid leaking across the shrouds of turbine blade rows are known to form a significant proportion of the overall loss generated in low aspect ratio turbines. The use of shrouds to encase the tips of turbine blades has encouraged the development of many innovative sealing arrangements, all of which are intended to reduce the quantity of fluid (the leakage fraction) leaking across the shroud. Modern sealing arrangements have reduced leakage fractions considerably, meaning that further improvements can only be obtained by controlling the leakage flow in such a way so as to minimize the aerodynamic losses incurred by the extraction and re-injection of the leakage flow into the mainstream. There are few published experimental investigations on the interaction between mainstream and leakage flows to provide guidance on the best means of managing the leakage flows to do this. This paper describes the development and testing of a strategy to turn the fluid leaking over shrouded turbine rotor blade rows with the aim of reducing the aerodynamic losses associated with its re-injection into the mainstream flow. The intent was to extract work from the leakage flow in the process. A four stage research turbine was used to test in detail the sealing design resulting from this strategy. A reduction in brake efficiency of 3.5 percent was measured. Further investigation suggested that much of the increase in loss could be attributed to the presence of axial gaps upstream and downstream of the shroud cavity which facilitated the periodic ingress and egress of mainstream fluid into the shroud cavity under the influence of the rotor potential field. This process was exacerbated by reductions in the leakage fraction.