Vishwas Iyengar

Assessment of Rotor-Stator Interface Boundary Condition Techniques for Modeling Axial Flow Turbines

An existing 3-D Navier-Stokes analysis for modeling single stage compressor and turbine rotors has been modified to model multi-stage axial compressors and turbines. As part of this effort, several rotor-stator interface boundary conditions have been systematically evaluated. The first stage of the two stage fuel turbine on the space shuttle main engine (SSME) has been used to determine which of these boundary conditions conserve global properties such as mass, momentum, and energy across the interface, while yielding good performance predictions. All the methods gave satisfactory results and a characteristic based approach was found to work best.

Flash Atomization: A New Concept to Control Combustion Instability in Water-Injected Gas Turbines

The objective of this work is to explore methods to reduce combustor rumble in a water-injected gas turbine. Attempts to use water injection as a means to reduce NOX emissions in gas turbines have been largely unsuccessful because of increased combustion instability levels. This pulsation causes chronic fretting, wear, and fatigue that damages combustor components. Of greater concern is that liberated fragments could cause extensive damage to the turbine section. Combustion instability can be tied to the insufficient atomization of injected water; large water droplets evaporate non-uniformly that lead to energy absorption in chaotic pulses. Added pulsation is amplified by the combustion process and acoustic resonance. Effervescent atomization, where gas bubbles are injected, is beneficial by producing finely atomized droplets; the gas bubbles burst as they exit the nozzles creating additional energy to disperse the liquid. A new concept for effervescent atomization dubbed “flash atomization” is presented where water is heated to just below its boiling point in the supply line so that some of it will flash to steam as it leaves the nozzle. An advantage of flash atomization is that available heat energy can be used rather than mechanical energy to compress injection gas for conventional effervescent atomization.

Effects of Stator Flow Distortion on Rotating Blade Endurance: Part 2—Stress Analysis and Failure Criteria

Blade vibrations, with the possibility of failure, is one of the major factors controlling the reliability of compressors and turbines. The prospects of encountering high alternating stress environments in blades make efficient turbomachine operation a very challenging task. In many cases the compressor or turbine functions through a wide range of load, flow, temperature, and speed which affect blade vibration, thus the stress environment continuously changes as the operating conditions changes. Any flow disturbance upstream of the rotating blades and some disturbances downstream will produce repetitive wake pulses that excite the blades. Resonance occurs with any coincidence of repetitive pulses with structural natural frequencies of rotating blades or impellers resulting in substantial amplification of alternating stresses. Most OEM design practices control vibratory stresses by avoiding resonance with expected stator sources; those excitations that cannot be avoided are designed with sufficient endurance to prevent failure. Thus three aspects of rotor/ blade design affect reliability: 1) aerodynamic excitation level and frequency, 2) structural response and resonance margins, and 3) selection and control of materials, coatings and their fabrication process to withstand the service environment. The main objective of this study is to develop a mathematical model to simulate the stresses in the rotating blade row that evaluates all three aspects of design to assess long term endurance.This is a two part paper on high cycle fatigue (HCF) failure analysis procedure of rotating blades and impellers. Part 1 [1] discusses aerodynamic excitation caused by stator vane and its role in generation of blade vibration. Here comprehensive computational fluid dynamics (CFD) is used to get a better understanding of the stator-rotor flow interactions at different operating conditions. The results of the aerodynamic simulations are order related excitation spectrum that can be applied to the stress/pulsation relationship defined in this part of the paper.This paper, Part 2, discusses an empirical dynamic stress model developed by impulse testing, assessing material endurance strength, and evaluation of criteria for failure by HCF.Copyright

The Texas Cryogenic Oxy-Fuel Cycle (TCO): A Novel Approach to Power Generation With CO2 Options

A novel oxy-fuel based power cycle is presented that combines conventional oxy-fuel cycle technology with novel mixed gaseous compression and liquid pumping of CO2 to produce both useable power and provide transportable CO2 for transportation via pipeline for use in sequestration or enhanced oil recovery (EOR). CO2 emissions reduction is a central focus of climate change initiatives. Therefore, it is desired to have a power plant process cycle that reduces CO2 emissions associated with producing power. Once captured CO2 must be transferred to a sequestration site for long term storage or utilized in EOR operations. Recent research has demonstrated that CO2 is most efficiently transported as a liquid at high pressures via pipelines. A Cryogenic Oxy-Fuel cycle will be presented that captures all CO2 produced during combustion and inherently converts that CO2 to a sequestration-ready state that can be immediately placed into transportation pipelines and stored at the desired sequestration site. The proposed cycle deviates from conventional cycles in that during part of the process the CO2 is in cooled liquid form which allows; 1) Decreased power demand to increase the CO2 pressure because pumping has lower power requirement than compression, 2) The take-off of the CO2 is optimized for pipeline transport and no further compression or expansion is required, and 3) A high overall thermodynamic cycle efficiency can be reached with relatively low firing temperatures in the oxy-burner (around 1000°F). This significantly simplifies the combustor and expander designs required for the process. Additional benefits of the cycle include predicted efficiencies near state of the art IGCC’s, applicability to multiple fuel sources, and cost reduction associated with reduced component sizes utilized in the cycle. This presentation will focus on the overall operational and technological requirements of the novel cycle, a breakdown of the individual components utilized, and simulations demonstrating predicted performance. Technological challenges of implementing a working version of the cycle will be discussed and suggested development required for overcoming the challenges will be presented. This paper will include recent research and development of an oxy-fuel combustor utilized in the cycle as well as implementation of compression and pumping apparatus of recent development.Copyright

Effects of Stator Flow Distortion on Rotating Blade Endurance: Part 1—Aerodynamic Excitation Aspects

Blade vibrations, with the possibility of a failure, are one of the major factors controlling the reliability of all compressors and turbines. Flow disturbances upstream and downstream of rotor/ stator will produce wake pulses that excite the blades. This requires a structural dynamic model of the blade stress response for a given excitation and a method to estimate the pulsating forces acting on the rotating blades by the stationary components and, vice versa, for rotor pulsations acting on the stator. This paper discusses the efforts made to understand the aerodynamic instabilities caused by the vane and its role in generation of blade vibration. Here, comprehensive computational fluid dynamics (CFD) are used to get a better understanding of the stator-rotor flow interactions at different operating conditions and their effect on overall pulsation and vibration levels. This model is based on blade dynamic response measurements and on careful CFD simulations of basic flow altering scenarios. It is found that a surprisingly low misalignment angle (relative) could result in fatigue damage stress levels in most cases. This paper presents several example cases to demonstrate typical flow profiles for axial and radial compressors/ turbines with varying stator flow distortions. It is Part 1 of a two-part high cycle fatigue (HCF) failure analysis procedure, dealing with aerodynamic excitation aspects.Copyright

On Remaining Life Analysis of Turbine Disks Subjected to High Thermal Stresses

Disk failures can be caused by a number of mechanisms under the turbine operating conditions of high rotational speed at elevated temperatures. It is not uncommon for highly stressed turbine blades and disks to operate at temperatures in excess of 1,000°F, where increased exposure can affect their life. In the past, it has been adequate to analyze the life of these high temperature components using methods which calculate creep life and low cycle fatigue life independently in predicting service hours. More often than not, the parameters included in the creep life model are based on empirical data. Here, a practical methodology is presented to predict the remaining life of a turbine disk that utilizes a combination of Computational Fluid Dynamics (CFD), Finite Element Analysis (FEA) and a creep model. A full three-dimensional CFD analysis is performed on the turbine disks at design and off-design conditions, in order to accurately capture the thermal loads. A detailed FEA is performed on the turbine disk. The stress inputs for the creep life model are based on the stresses obtained from the FEA. A case study is presented that utilizes the proposed methodology. It is found that the methodology is beneficial for the remaining life analysis on highly loaded turbine disks. The accuracy of the methodology is somewhat dictated by the amount of historical operating data that is available.Copyright

Effect of Non-Uniform Blade Root Friction and Sticking on Disk Stresses

Stress levels predicted by conventional disk modeling assumptions are lower than expected to cause conventional creep or fatigue damage consistent with slot failures experienced in some compressor and turbine disks. It was suspected that disparate slot to slot friction at the blade root surface will result in sticking of some blade roots as the turbine is shut down while adjacent blades slip; the un-resisted stuck root would pry the steeples apart causing additional bending stress. Testing of a blade root/disk slot pair in a load frame found that the blade root will stick in place as imposed radial loads decrease. Simulation of blade root movement during shutdown indicates peak stress can increase by 20% or more depending on geometric factors. The slot stress only rises above its maximum speed condition on shutdown (at 80% Max Speed in the example case). This brief stress rise will not cause significant creep damage, but can shorten disk life based on low cycle fatigue or hold time fatigue damage.Copyright

Comprehensive Application of a First Principles Based Methodology for Design of Axial Compressor Configurations

Axial compressors are widely used in many aerodynamic applications. The design of an axial compressor configuration presents many challenges. It is necessary to retool the design methodologies to take advantage of the improved accuracy and physical fidelity of these advanced methods. Here, a first-principles based multi-objective technique for designing single stage compressors is described. The study accounts for stage aerodynamic characteristics and rotor-stator interactions. The proposed methodology provides a way to systematically screen through the plethora of design variables. This method has been applied to a rotor-stator stage similar to NASA Stage 35. By selecting the most influential design parameters and by optimizing the blade leading edge and trailing edge mean camber line angles, phenomena such as tip blockages, blade-to-blade shock structures and other loss mechanisms can be weakened or alleviated. It is found that these changes to the configuration can have a beneficial effect on total pressure ratio and stage adiabatic efficiency, thereby improving the performance of the axial compression system.Copyright

Flash Atomization: A New Concept to Control Combustion Instability in Water Injected Gas Turbines

The objective of this work is to develop and explore methods to reduce combustor rumble in an industrial gas turbine in co-generation service that is operated with water injection reducing NOX emissions. Attempts to use water injection as a means to reduce NOX emissions in gas turbines have been largely unsuccessful because of increased combustion instability levels experienced. The increase in pulsation causes chronic fretting, wear, and fatigue that damages combustor components resulting in higher operation costs due to repair or replacement of parts. This combustion instability can be tied to the insufficient atomization of injected water; relatively large water droplets evaporate non-uniformly that lead to energy absorption in non-uniform chaotic pulses. This added pulsation is amplified by the combustion process and acoustic resonance. Effervescent atomization, where a gas bubbles are injected with the liquid, is beneficial in producing finely atomized droplets, because the gas bubbles burst as they exit the nozzles creating additional energy to disperse the liquid. A new concept for effervescent atomization dubbed “Flash Atomization” is presented where water is heated to just below its boiling point in the supply line so that some of it will flash to steam as it leaves the nozzle. An advantage of Flash Atomization is that available heat energy can be used rather than mechanical energy to compress injection gas for conventional effervescent atomization.© 2010 ASME