An Analytical Model for Over-Shroud Leakage Losses in a Shrouded Turbine Stage

This study presents an analytical model that uses directly measurable flow quantities to predict the effects of leakage on shrouded turbine stage performance. The model displays good predictive ability for the mass leakage fraction, for the tip leakage and for the mixing losses. The model resolves the negative incidence angle induced by mixing the leakage flow with the main stream and predicts the increment in the total mixing loss coefficient at increasing injection angles. The effects of the labyrinth seal geometry, such as the tip gap width and the number of seals, on the associated leakage losses as well as on the turbine stage performance are adequately represented. Overall, the present model exhibits a good qualitative and quantitative agreement with comparative benchmark data. It is concluded that increasing the labyrinth through-flow resistance by increasing the number of fins leads to a decrement in the leakage flow and its adverse effects but the effectiveness of this reduction decreases as the number of fins increases by more than three. The mass leakage fraction, tip leakage loss coefficient and total mixing loss coefficient increase linearly as the sealing gap ratio increases. A conventional injection angle of 90° increases the total mixing loss by about 28% compared to injecting parallel to the main passage flow.

The Interaction of Turbine Inter-Platform Leakage Flow With the Mainstream Flow

Individual nozzle guide vanes (NGV’s) in modern aeroengines are often cast as a single piece with integral hub and casing endwalls. When in operation, there is a leakage flow through the chord-wise interplatform gaps. An investigation into the effect of this leakage flow on turbine performance is presented. Efficiency measurements and NGV exit area traverse data from a low-speed research turbine are reported. Tests show that this leakage flow can have a significant impact on turbine performance, but that below a threshold leakage fraction this penalty does not rise with increasing leakage flow rate. The effect of various seal clearances are also investigated. Results from steady-state simulations using a three-dimensional multiblock Reynolds-averaged Navier-Stokes solver are presented with particular emphasis paid to the physics of the mainstream/leakage interaction and the loss generation.

The Interaction of Turbine Inter-Platform Leakage Flow With the Mainstream Flow

Individual nozzle guide vanes (NGVs) in modern aero engines are often cast as a single piece with integral hub and casing endwalls. When in operation there is a leakage flow through the chord-wise inter-platform gaps. An investigation into the effect of this leakage flow on turbine performance is presented. Efficiency measurements and NGV exit area traverse data from a low speed research turbine are reported. Tests show that this leakage flow can have a significant impact on turbine performance, but that below a threshold leakage fraction this penalty does not rise with increasing leakage flow rate. The effect of various seal clearances are also investigated. Results from steady-state simulations using a three-dimensional multiblock RANS solver are presented with particular emphasis paid to the physics of the mainstream/leakage interaction and the loss generation.

The Control of Shroud Leakage Flows to Reduce Aerodynamic Losses in a Low Aspect Ratio, Shrouded Axial Flow Turbine

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 Control of Shroud Leakage Flows to Reduce Aerodynamic Losses in a Low Aspect Ratio, Shrouded Axial Flow Turbine

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.