Effects of Free-Stream Turbulence Intensity on a Boundary Layer Recovering From Concave Curvature Effects

Experiments are conducted on a flat recovery wall downstream of sustained concave curvature in the presence of high free-stream turbulence (TI∼8%). This flow simulates some of the features of the flow on the latter parts of the pressure surface of a gas turbine airfoil. The combined effects of concave curvature and TI, both present in the flow over a turbine airfoil, have so far little been studied. Computation of such flows with standard turbulence closure models has not been particularly successful. This experiment attempts to characterize the turbulence characteristics of this flow. In the present study, a turbulent boundary layer grows from the leading edge of a concave wall then passes onto a downstream flat wall. Results show that turbulence intensities increase profoundly in the outer region of the boundary layer over the recovery wall. Near-wall turbulent eddies appear to lift off the recovery wall and a “stabilized” region forms near the wall. In contrast to a low-free-stream turbulence intensity flow, turbulent eddies penetrate the outer parts of the “stabilized” region where sharp velocity and temperature gradients exist. These eddies can more readily transfer momentum and heat. As a result, skin friction coefficients and Stanton numbers on the recovery wall are 20% and 10%, respectively, above their values in the low-free-stream turbulence intensity case. Stanton numbers do not undershoot flat-wall expectations at the same ReΔ2 values as seen in the low-TI case. Remarkably, the velocity distribution in the core of the flow over the recovery wall exhibits a negative gradient normal to the wall under high free-stream turbulence intensity conditions. This velocity distribution appears to be the result of two effects: 1) cross transport of kinetic energy by boundary work in the upstream curved flow and 2) readjustment of static pressure profiles in response to the removal of concave curvature.

Gas Turbine Topping Stage Based on Energy Exchangers: Process and Performance

Power generation in gas turbines is facing three main challenges today: • Low pollution prescribed by legal requirements. • High efficiency to obtain low operating cost and low CO2 emissions. • High specific power output to obtain low product and installation cost. Unfortunately, some of these requirements are contradictory: high efficiency and specific power force the development towards higher temperatures and pressures which increase NOx emissions and intensify the cooling and material strength problems. A breakthrough can be achieved by applying an energy exchanger as a topping stage. Inherent advantages are the self-cooled cell-rotor which can be exposed to much higher gas temperature than a steady-flow turbine and a very short residence time at peak temperature which keeps NOx emissions under control. The basic idea has been proposed long time ago. Fundamental research has now led to a new energy exchanger concept. Key issues include symmetric pressure-wave processes, partial suppression of flow separation and fluid mixing, as well as quick afterburning in premixed mode. The concept has been proven in a laboratory-scale engine with very promising results. The application of an energy exchanger as a topping stage onto existing gas turbines would increase the efficiency by 17% (relative) and the power by 25%. Since the temperature level in the turbine remains unchanged, the performance improvement can also be fully utilized in combined cycle applications. This process indicates great potentials for developing advanced gas turbine systems as well as for retrofitting existing ones.

Some Effects of Coolant Density on Film Cooling Effectiveness

Experiments have been conducted on a large wind tunnel model of the leading edge region of a turbine blade. The model had a semi-circular leading edge in which four rows of holes were symmetrically placed about the stagnation line, two at ±15° and two at ±44°. Air and alternatively CO2 were injected from the coolant holes after contamination with a known small percentage of propane. Using a flame ionization detector and the mass transfer analogy, the film cooling effectiveness was measured at various overall mass flow ratios and at various streamwise locations for each coolant type. The division of coolant flow rate from the two rows of holes was found to be more unequal for CO2 than for air, an effect which is predicted from a simple analysis of the coolant/free stream interaction and the hole discharge coefficient. This has practical implications for actual turbine operation since earlier cut-off of the coolant from the front row of holes, due to density differences, could have disastrous effects on the blade. This effect also further complicates any attempt to identify overall trends of coolant density on performance. It is not possible to conclude that air or CO2 coolant has a higher film cooling effectiveness, although, in general, air appears better close to the first row of holes, and CO2 better at some distance downstream of both rows. Based on the measurements, the effects of mass flow ratio, momentum flux ratio, relative hole placement in each row, and spanwise versus streamwise injection are discussed in the paper.

Investigation of Failures in Gas Turbines: Part 1 — Techniques and Principles of Failure Investigation

This paper is Part 1 of a two-part paper on the principles and methods of failure investigation in gas turbines. The qualities of a successful failure investigator are presented, and the most efficacious approaches to an investigation are discussed. An example of an aircraft accident that might have been avoided is used to support the necessity for thorough and conclusive investigations into failures. Two case histories involving heavy-duty industrial gas turbines are described to demonstrate different aspects of the logical approach to construction of hypotheses and the determination of the essential cause of a failure — the one event without which the failure would not have occurred.

An Optimum Balance Weight Search Algorithm

A mathematical description for an optimum balance weight search algorithm for single plane multipoint balance is presented. The algorithm uses influence coefficients, either measured or known beforehand, and measured complex vibration data to determine an optimum balance correction weight. The solution minimizes the maximum residual vibration. The algorithm allows user defined balance weights to be analyzed and evaluated. A test case is presented showing actual results and comparison with a least square solution algorithm. An efficient multiplane influence coefficient calculation scheme is also presented.

An Application of Octant Analysis to Turbulent and Transitional Flow Data

A technique called “octant analysis” was used to examine the eddy structure of turbulent and transitional heated boundary layers on flat and curved surfaces. The intent was to identify important physical processes that play a role in boundary layer transition on flat and concave surfaces. Octant processing involves the partitioning of flow signals into octants based on the instantaneous signs of the fluctuating temperature, t′; streamwise velocity, u′; and cross-stream velocity, v′. Each octant is associated with a particular eddy motion. For example, u′<0, v′>0, t′>0 is associated with an ejection or “burst” of warm fluid away from a heated wall. Within each octant, the contribution to various quantities of interest (such as the turbulent shear stress, −u′v′, or the turbulent heat flux, v′t′) can be computed. By comparing and contrasting the relative contributions from each octant, the importance of particular types of motion can be determined. If the data within each octant is further segregated based on the magnitudes of the fluctuating components so that minor events are eliminated, the relative importance of particular types of motion to the events that are important can also be discussed. In fully-developed, turbulent boundary layers along flat plates, trends previously reported in the literature were confirmed. A fundamental difference was observed in the octant distribution between the transitional and fully-turbulent boundary layers, however, showing incomplete mixing and a lesser importance of small scales in the transitional boundary layer. Such observations were true on both flat and concave walls. The differences are attributed to incomplete development of the turbulent kinetic energy cascade in transitional flows. The findings have potential application to modelling, suggesting the utility of incorporating multiple length scales in transition models.

Investigation of Failure in Gas Turbines: Part 2 — Engineering and Metallographic Aspects of Failure Investigation

This paper opens with a discussion of the various mechanisms of cracking and fracture encountered in gas turbine failures, and discusses the use of metallographic examination of crack and fracture surfaces. The various types of materials used in the major components of heavy-duty industrial and aeroderivative gas turbines are tabulated. A collection of macroscopic and microscopic fractographs of the various mechanisms of failure in gas turbine components is then presented for reference in failure investigation. A discussion of compressor damage due to surge, as well as some overall observations on component failures, follows. Finally, a listing of the most likely types of failure of the various major components is given.

Effect of Wind Tunnel Acoustic Modes on Linear Oscillating Cascade Aerodynamics

The aerodynamics of a biconvex airfoil cascade oscillating in torsion is investigated using the unsteady aerodynamic influence coefficient technique. For subsonic flow and reduced frequencies as large as 0.9, airfoil surface unsteady pressures resulting from oscillation of one of the airfoils are measured using flush-mounted high-frequency-response pressure transducers. The influence coefficient data are examined in detail and then used to predict the unsteady aerodynamics of a cascade oscillating at various interblade phase angles. These results are correlated with experimental data obtained in the traveling-wave mode of oscillation and linearized analysis predictions. It is found that the unsteady pressure disturbances created by an oscillating airfoil excite wind tunnel acoustic modes which have detrimental effects on the experimental results. Acoustic treatment is proposed to rectify this problem.

Nondestructive Eddy Current Evaluation of Anisotropic Conductivities of Silicon Carbide Reinforced Aluminum Metal-Matrix Composite Extrusions

Nondestructive eddy current methods were used to evaluate the electrical conductivity behavior of silicon-carbide particulate (SiCp) reinforced aluminum (Al) metal-matrix composite extrusions. The composites investigated included 2124, 6061 and 7091 Al base alloys reinforced by SiCp. The composite extrusions exhibited anisotropic conductivities with the maximum conductivity occurring along the extrusion plane. Microstructural characterization showed that the observed anisotropic conductivities could result from the preferred orientation distribution of SiCp. A theoretical model was formulated to quantify the influence of composite constituents (SiCp, intermetallics and Al base alloy) on the anisotropic conductivities of the composites. The theoretical predictions of conductivities were found to be in good agreement with the experimental results.

Fault Diagnosis in Gas Turbines Using a Model-Based Technique

Reliable methods for diagnosing faults and detecting degraded performance in gas turbine engines are continually being sought. In this paper, a model-based technique is applied to the problem of detecting degraded performance in a military turbofan engine from take-off acceleration type transients. In the past, difficulty has been experienced in isolating effects of some of the physical processes involved. One such effect is the influence of the bulk metal temperature on the measured engine parameters during large power excursions. It will be shown that the model-based technique provides a simple and convenient way of separating this effect from the faster dynamic components. The important conclusion from this work is that good fault coverage can be gleaned from the resultant pseudo steady-state gain estimates derived in this way.