Simon Hogg

Interaction of Rim Seal and Annulus Flows in an Axial Flow Turbine

A combined computational fluid dynamics (CFD) and experimental study of interaction of main gas path and rim sealing flow is reported. The experiments were conducted on a two stage axial turbine and included pressure measurements for the cavity formed between the stage 2 rotor disc and the upstream diaphragm for two values of the diaphragm-to-rotor axial clearance. The pressure measurements indicate that ingestion of the highly swirling annulus flow leads to increased vortex strength within the cavity. This effect is particularly strong for the larger axial clearance. Results from a number of steady and unsteady CFD models have been compared to the measured results. Good agreement between measurement and calculation for time-averaged pressures was obtained using unsteady CFD models, which predicted previously unknown unsteady flow features. This led to fest response pressure transducer measurements being made on the rig, and these confirmed the CED prediction.

Measurement and Computation of Energy Separation in the Vortical Wake Flow of a Turbine Nozzle Cascade

This paper describes the observation, measurement, and computation of vortes shedding behind a cascade of turbine nozzle guide vanes that have a blunt trailing edge. At subsonic discharge speeds, periodic wake vortex shedding was observed at all times at a shedding frequency in the range 7-11 kHz. At high subsonic speeds the wake was susceptible to strong energy redistribution. The effect was greatest around an exit Mach number of 0.95 and results are presented for that condition. An unusually cold flow on the wake centerline and hot spots at the edges of the wake were measured. These were found to be a manifestation of Eckert-Weise effect energy separation in the shed vortex street. Experimental identification of these phenomena was achieved using a new stagnation temper ature probe of bandwidth approaching 100 kHz. Using phase-averaging techniques, it was possible to plot contours of time-resolved entropy increase at the downstream traverse plane. Computational work has been undertaken that gives qualitative confirmation of the experimental results and provides a more detailed explanation of the fine scale structure of the vortex wake. The topology of the wake vortical structures bchind blunt trailing-edged turbine blades is becoming clearer. These measurements are the first instantaneous observations of the energy separation process occurring in turbine blade wake flows. This was also the first demonstration of the use of the probe in the frequency. Mach number, and temperature ranges typical of operation behind the rotors of high-performance turbomachines such as transonic fans.

Installation of a Turbine Stage in the Pyestock Isentropic Light Piston Facility

The Isentropic Light Piston Facility (ILPF) at Pyestock has been upgraded to include a single stage, high pressure turbine. All major non-dimensional parameters are accurately scaled during the 0.4s run time, enabling heat transfer and aerodynamic measurements to be made at engine representative conditions. The ILPF was previously an annular cascade facility. This paper describes the design and integration of the rotor module and the results of the commissioning tests. An important feature is a novel, patented, turbobrake which is shown to maintain the turbine at a constant speed during the run.Copyright

Porosity modeling of brush seals

A new model for predicting leakage flows through the bristle pack of brush seals is developed. In the model, the bristle pack is treated as a porous medium. Good agreement is demonstrated between predictions from a one-dimensional form of the model and a wide range of experimental data available from the literature, for seals with a bristle pack to rotor interference fit. The results demonstrate that both viscous and inviscid effects contribute significantly to the drag on the bristles within the pack. The model uses a linear superposition of viscous and inertial losses, with a resistance coefficient assigned to each contribution. Formulas that have been deduced for flow in packed beds, are adapted for use in assigning values to the resistance coefficients in the one-dimensional model. Finally, extension of the method to multiple dimensions is discussed, with a view to incorporating the model into a CFD code to form a general predictive capability for brush seal flows.

An Experimental and Theoretical Study of Brush Seal and Shaft Thermal Interaction

This paper presents a detailed experimental study and theoretical analysis of the thermal interactions of brush seal, shaft and airflow. The tests were performed in the Oxford University Brush Seal Test Facility. The facility, which was originally designed for the brush seal aerodynamic study, has been enhanced for thermal measurements. A variety of brush seals, typically applicable to aero and power generation gas and steam turbines, have been tested. The thermal interaction caused by changes in rotor and housing alignment was also studied. In the tests, in addition to the conventional aerodynamic parameters, such as the pressure, mass flow and torque, the temperatures of the rotor were measured and heat generated in the bristle/rotor contact were deduced using the finite element analysis. A theoretical analysis of the system was developed which predicts the heat transfer to the airflow from the bristle pack. This analysis is used to give a correlation to predict heat input to the rotor. The study provides valuable design information for the use of brush seals.Copyright

Wind energy: UK experiences and offshore operational challenges

This paper presents a discussion of the development of wind energy generation in the United Kingdom and the challenges faced by the wind industry including reliability, performance and condition monitoring, particularly in the offshore environment. The worldwide installed capacity of offshore wind has now risen to over 7 GW, with an ever increasing deployment rate of new assets. About 90% of the global currently installed capacity is in Northern Europe, with the United Kingdom having the worlds largest share at 4 GW. Capacity factor data from UK offshore wind farms is presented, providing an insight into the current performance of large Round 2 offshore wind farms compared to the earlier Round 1 farms and to onshore farms. The data reveal that the United Kingdoms Round 2 offshore farms are achieving an average monthly capacity factor of 38.3% with a peak value of 75.8%. The older Round 1 farms have a lower average capacity factor of 33.6% while large onshore farms with capacities above 100 MW have achieved 25.6%. Offshore wind turbine performance has improved over time, and the industry is applying the learning from early experiences to achieve better performances at the more recently installed farms. Despite these improvements in turbine availability, the cost of energy from wind, particularly offshore, remains too high for it to be a commercially viable form of generation without subsidies. Reducing the cost of energy from wind to economically sustainable levels is the most important challenge facing the industry today. Operation and maintenance costs constitute up to 30 % of the total cost of energy from wind in large farms. The industry must overcome the challenges associated with improving component reliability and the development and adoption by operators of appropriate condition monitoring systems and maintenance strategies, in order to reduce costs to sustainable levels. Research and development work carried out with these goals in mind is also reviewed in the paper.

A Literature Review of Low Pressure Steam Turbine Exhaust Hood and Diffuser Studies

This paper summarizes the findings from research studies carried out over the last 30 years, to better understand the flows in steam turbine low pressure exhaust hoods and diffusers. The work aims to highlight the areas where further study is still required. A detailed description of the flow structure is outlined and the influence of the last turbine stage and the hood geometry on loss coefficient is explored. At present, the key challenge faced is numerically modeling the three-dimensional, unsteady, transonic, wet steam exhaust hood flow given the impractically high computational power requirement. Multiple calculation simplifications to reduce the computational demand have been successfully verified with experimental data, but at present there is no ‘best-practice’ approach to reduce the computational time for routine design exercises. This paper highlights the importance of coupling the exhaust hood to the last stage steam turbine blades to capture the interaction; ensuring the total pressure and swirl angle profiles, along with the tip leakage jet are accurately applied to the diffuser inlet. The nonaxial symmetry of the exhaust hood means it is also important to model the full blade annulus. More studies have emerged modeling the wet steam and unsteady flow effects, but more work is required in this area to fully understand the impact on the flow structure.

Development of Brush Seal Technology for Steam Turbine Retrofit Applications

Increased cylinder efficiency is one of the main drivers in the steam turbine retrofit market, particularly for HP and IP modules. To-date most retrofit turbine suppliers have concentrated their efforts on improving the aerodynamic efficiency of blades and other steampath components, optimizing stage numbers and reducing leakage losses. Developments in all of these areas rely heavily on improved design and analysis tools to refine existing designs and evaluate new concepts. The opportunity exists to further reduce leakage losses by replacing conventional labyrinth seal designs by more advanced turbomachinery sealing technology. Brush seals, which have now been used successfully in some gas turbine (mainly aero-engines) applications for several years, are a natural candidate for steam turbine retrofits. Careful thought is needed when applying brush seals as the mechanical integrity of the cylinder needs to be maintained at all times. Attempts to increase performance should never be at the expense of availability and reliability. This paper describes the development work undertaken by the authors’ company and covers research in the areas of brush seal design, performance improvement, operational issues, and life assessment.Copyright

Multiple Brush Seals in Series

Rolls-Royce has identified phenomena associated with aeroelastic instability of brush seal bristles, when multiple seals are operated in series at high pressure loading. This paper gives a general overview of test experience to date. A brush seal test facility commissioned by Rolls-Royce at Oxford University is described, along with results from double and triple brush seal tests carried out using the facility. The effect of bristle pack instability on sealing performance is clearly identified in the paper. A design modification aimed at suppressing the instability is described. Results from the Oxford University tests show that the device significantly increases the operating range of multiple seal arrangements.Copyright

Performance Guarantee and Testing of Steam Turbine Retrofits

The steam turbine retrofit business has grown in recent years, largely due to advances in the aerodynamic design of steam path components, to the point that retrofitting non-OEM equipment is now common place within the industry. This paper discusses different steam turbine retrofit scenarios and how improvements in component performance translate into tangible power and heat rate benefits for plant operators. Methods for guaranteeing the performance of steam turbine retrofit cylinders, that allow the guarantees to be verified by testing approaches that minimize expense and uncertainty are described. Depending on the retrofit solution, guarantees may be simple turbine cylinder efficiency guarantees, performance improvement guarantees or absolute performance guarantees. Different types of testing are appropriate to verify these different forms of guarantee, and they are discussed in the paper.Copyright