H. P. Hodson

Boundary Layer Development in Axial Compressors and Turbines: Part 3 of 4— LP Turbines

This is Part Three of a four-part paper. It begins with Section 11.0 and continues to describe the comprehensive experiments and computational analyses that have led to a detailed picture of boundary layer development on airfoil surfaces in multistage turbomachinery. In this part, we present the experimental evidence that we used to construct the composite picture for LP turbines that was given in the discussion in Section 5.0 of Part 1. We present and interpret the data from the surface hot-film gages and the boundary layer surveys for the baseline operating condition. We then show how this picture changes with variations in Reynolds number, airfoil loading, and nozzle-nozzle clocking.

Axial flow turbines

There are several distinctive types of axial turbine including both impulse and reaction, single and velocity compounded.

Boundary Layer Development in Axial Compressors and Turbines: Part 1 of 4—Composite Picture

Comprehensive experiments and computational analyses were conducted to understand boundary layer development on airfoil surfaces in multistage, axial-flow compressors and LP turbines. The tests were run over a broad range of Reynolds numbers and loading levels in large, low-speed research facilities which simulate the relevant aerodynamic features of modern engine components. Measurements of boundary layer characteristics were obtained by using arrays of densely packed, hot-film gauges mounted on airfoil surfaces and by making boundary layer surveys with hot wire probes. Computational predictions were made using both steady flow codes and an unsteady flow code. This is the first time that time-resolved boundary layer measurements and detailed comparisons of measured data with predictions of boundary layer codes have been reported for multistage compressor and turbine blading. Part 1 of this paper summarizes all of our experimental findings by using sketches to show how boundary layers develop on compressor and turbine blading. Parts 2 and 3 present the detailed experimental results for the compressor and turbine, respectively. Part 4 presents computational analyses and discusses comparisons with experimental data. Readers not interested in experimental detail can go directly from Part 1 to Part 4. For both compressor and turbine blading, the experimental results show large extents of laminar and transitional flow on the suction surface of embedded stages, with the boundary layer generally developing along two distinct but coupled paths. One path lies approximately under the wake trajectory while the other lies between wakes. Along both paths the boundary layer clearly goes from laminar to transitional to turbulent. The wake path and the non-wake path are coupled by a calmed region, which, being generated by turbulent spots produced in the wake path, is effective in suppressing flow separation and delaying transition in the non-wake path. The location and strength of the various regions within the paths, such as wake-induced transitional and turbulent strips, vary with Reynolds number, loading level, and turbulence intensity. On the pressure surface, transition takes place near the leading edge for the blading tested. For both surfaces, bypass transition and separated-flow transition were observed. Classical Tollmien-Schlichting transition did not play a significant role. Comparisons of embedded and first-stage results were also made to assess the relevance of applying single-stage and cascade studies to the multistage environment. Although doing well under certain conditions, the codes in general could not adequately predict the onset and extent of transition in regions affected by calming. However, assessments are made to guide designers in using current predictive schemes to compute boundary layer features and obtain reasonable loss predictions.

The Effect of Blade Tip Geometry on the Tip Leakage Flow in Axial Turbine Cascades

The phenomenon of tip leakage has been studied in two linear cascades of turbine blades. The investigation includes an examination of the performance of the cascades with a variety of tip geometries. The effects of using plain tips, suction side squealers, and pressure side squealers are reported. Traverses of the exit flow field were made in order to determine the overall performance. A method of calculating the tip discharge coefficients for squealer geometries is put forward. In linking the tip discharge coefficient and cascade losses, a procedure for predicting the relative performance of tip geometries is developed

Heat Transfer Committee Best Paper of 1995 Award: Distribution of Film-Cooling Effectiveness on a Turbine Endwall Measured Using the Ammonia and Diazo Technique

The distribution of adiabatic film-cooling effectiveness on the endwall of a large-scale low-speed linear turbine cascade has been measured using a new technique. This technique is based on an established surface-flow visualization technique, and makes use of the reaction between ammonia gas and a diazo surface coating. A new method of calibration has been developed to relate the result of the reaction to surface concentration of coolant. Using the analogy that exists between heat and mass transfer, the distribution of film-cooling effectiveness can then be determined. The complete representation of the film-cooling effectiveness distribution provided by the technique reveals the interaction between the coolant ejected from the endwall and the secondary flow in the turbine blade passage. Over- and undercooled regions on the endwall are identified, illustrating the need to take these interactions into account in the design process. Modifications to the cooling configuration examined in this paper are proposed as a result of the application of the ammonia and diazo technique.

Aerodynamic Aspects of Endwall Film-Cooling

This paper describes an investigation of the aerodynamic aspects of endwall film-cooling, in which the flow field downstream of a large-scale low-speed linear turbine cascade has been measured. The integrated losses and locations of secondary flow features with and without endwall film-cooling have been determined for variations of both the coolant supply pressure and injection location. Together with previous measurements of adiabatic film-cooling effectiveness and surface-flow visualization, these results reveal the nature of the interactions between the ejected coolant and the flow in the blade passage. Measured hole massflows and a constant static pressure mixing analysis, together with the measured losses, allow the decomposition of the losses into three distinct entropy generation mechanisms: loss generation within the hole, loss generation due to the mixing of the coolant with the mainstream, and change in secondary loss generation in the blade passage. Results show that the loss generation within the coolant holes is substantial and that ejection into regions of low static pressure increases the loss per unit coolant massflow. Ejection upstream of the three-dimensional separation lines on the endwall changes secondary flow and reduces its associated losses. The results show that it is necessary to take the three-dimensional nature of the endwall flow into account in the design of endwall film-cooling configurations.

High Lift and Aft Loaded Profiles for Low Pressure Turbines

This paper shows how it is possible to reduce the number of blades in LP turbines by approximately 15% relative to the first generation of high lift blading employed in the very latest engines. This is achieved through an understanding of the behaviour of the boundary layers on high lift and ultra high lift profiles subjected to incoming wakes. Initial development of the new profiles was carried out by attaching a flap to the trailing edge of one blade in a linear cascade. The test facility allows for the simulation of upstream wakes by using a moving bar system. Hot wire measurements were made to obtain boundary layer losses and surf ace mounted hot films were used to observe the changes in boundary layer state. Measurements were taken at a Reynolds number between 100,000 and 210,000. The effect of increased lift above the datum profile was investigated first with steady and then with unsteady inflow (i.e. with wakes present). For the same profile, the losses generated with wakes present were below those generated by the profile with no wakes present. The boundary layer behaviour on these very high lift pressure distributions suggested that aft loading the profiles would further reduce the profile loss. Finally, two very highly loaded and aft loaded LP turbine profile were designed and then tested in cascade. The new profiles produced losses only slightly higher than those for the datum profile with unsteady inflow, but generated 15% greater lift. NOMENCLATURE C p Pressure coefficient =

On the Interpretation of Measured Profile Losses in Unsteady Wake–Turbine Blade Interaction Studies

The interaction of wakes shed by a moving blade row with a downstream blade row causes unsteady flow. The meaning of the free-stream stagnation pressure and stagnation enthalpy in these circumstances has been examined using simple analyses, measurements, and CFD. The unsteady flow in question arises from the behavior of the wakes as so-called negative jets. The interactions of the negative jets with the downstream blades lead to fluctuations in static pressure, which in turn generate fluctuations in the stagnation pressure and stagnation enthalpy. It is shown that the fluctuations of the stagnation quantities created by unsteady effects within the blade row are far greater than those within the incoming wake. The time-mean exit profiles of the stagnation pressure and stagnation enthalpy are affected by these large fluctuations. This phenomenon of energy separation is much more significant than the distortion of the time-mean exit profiles that is caused directly by the cross-passage transport associated with the negative jet, as described by Kerrebrock and Mikolajczak. Finally, it is shown that if only time-averaged values of loss are required across a blade row, it is nevertheless sufficient to determine the time-mean exit stagnation pressure.

An investigation of boundary layer development in a multistage LP turbine

This paper describes an investigation of the behavior of suction surface boundary layers in a modern multistage Low-Pressure turbine. An array of 18 surface-mounted hot-film anemometers was mounted on a stator blade of the third stage of a four- stage machine. Data were obtained at Reynolds numbers between 0.9 × 10 5 and 1.8 × 10 5 . At the majority of the test conditions, wakes from upstream rotors periodically initiated transition at about 40 percent surface length. In between these events, laminar separation occurred at about 75 percent surface length

Aerothermal Investigations of Tip Leakage Flow in Axial Flow Turbines—Part I: Effect of Tip Geometry and Tip Clearance Gap

A numerical study has been performed to investigate the effect of tip geometry oil the tip leakage flow and heat transfer characteristics in unshrouded axial flow turbines. Base line flat tip geometry and squealer type geometries, namely, double squealer or cavity and suction-side squealer, were considered. The performances of the squealer geometries, in terms of the leakage mass flow and heat transfer to the tip, were compared with the flat tip at two different tip clearance gaps. The computations were performed using a single blade with periodic boundary conditions imposed along the boundaries in the pitchwise direction. Turbulence was modeled using three different models, namely, standard k-epsilon, low Re k-omega, and shear stress transport (SST) k-omega, in order to assess the capability, of the models in correctly predicting the blade heat transfer The heat transfer and static pressure distributions obtained using the SST k-omega model were-found to be in close agreement with the experimental data. It was observed that compared to the other two geometries considered, the cavity tip is advantageous both from the aerodynamic and from the heat transfer perspectives by providing a decrease in the amount of leakage, and hence losses, and average heat transfer to the tip. In general, for a given geometry, the leakage mass flow and the heat transfer to the tip increased with increase in tip clearance gap. Part II of this paper examines the effect of relative casing motion on the flow and heat transfer characteristics of tip leakage flow. In, Part III of this paper the effect of coolant injection on the flow and heat transfer characteristics of tip leakage flow is presented.