David Ransom

An Improved Catcher Bearing Model and an Explanation of the Forward Whirl/Whip Phenomenon Observed in Active Magnetic Bearing Transient Drop Experiments

Though many approaches have been proposed in the literature to model the reaction forces in a catcher bearing (CB), there are still phenomena observed in experimental tests that cannot be explained by existing models. The following paper presents a novel approach to model a CB system. Some of the elements in the model have been previously introduced in the literature; however, there are other elements in the proposed model that are new, providing an explanation for the forward whirling phenomena that has been observed repeatedly in the literature. The proposed CB model is implemented in a finite element rotordynamic package, and nonlinear time-transient simulations are performed to predict published experimental results of a high speed vertical sub-scale compressor; with no other forces present in the model, the agreement between simulations and experimental data is favorable.The results presented herein show that friction between the journal and axial face of the catcher bearing results in a forward cross-coupled force that pushes the rotor in the direction of rotation. This force is proportional to the coefficient of friction between the axial face of the rotor and catcher bearing and the axial thrust on the rotor. This force results in synchronous whirl when the running speed is below a combined natural frequency of the rotor-stator system, and constant frequency whip when the speed is above a whip frequency.Copyright

Engine Distress Detection in Gas Turbines With Electrostatic Sensors

Failures in gas turbines such as fretting at combustor assembly interfaces, blade rub, Thermal Barrier Coating (TBC) spalling, minor amounts of domestic or foreign object damage can be detected by mechanical vibration or gas turbine performance degradation, but it is usually too late for damage control by the time the failure is significant enough to be detected with these methods. Electrostatic charge sensors present a potential method for identifying the failure modes at an earlier stage before significant damage has occurred. In a gas turbine, there are potentially two sources of electrostatic charge in the exhaust gas flow stream: ionized plasma that is a natural byproduct of high temperature combustion, and any form of debris that has originated either in the compressor, combustor, or turbine sections of the gas turbine engine as a result of vibration or fatigue. For this reason, the electrostatic charge monitor becomes a very useful device for monitoring both combustion performance problems as well as potential damage related to debris in the exhaust stream. Electrostatic sensing technology has been proven to work in detecting ingested debris and engine debris on aerospace jet engines. However, the use of the sensors in industrial applications has shown that much research is still required especially in the areas of sensor placement and failure identification. This paper discusses results from testing conducted to identify the optimal placement location for the electrostatic charge sensors in a gas turbine exhaust stream. The results are presented for various sensor locations on small (112 kW) and medium (1.185 MW) frame gas turbines to evaluate distance, velocity, radial location, and gas turbine geometry effects. These tests are completed with the gas turbine ingesting varying amounts of TBC upstream of the compressor and administering power level changes to the gas turbine. The results of this experimental program demonstrate a clear sensitivity to sensor placement along the exhaust duct of a gas turbine as well as the radial location. There are variations in the particle flow pathlines in the exhaust duct at different gas turbine operating conditions. These variations influence the sensors response. Best results are obtained when the sensors are placed at the location with the fastest and hottest exhaust gas. Multiple sensors may be required to obtain comprehensive coverage for practical event detection.Copyright

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.

Enthalpy Determination Methods for Compressor Performance Calculations

In the field of compressor performance simulation and measurement, the most commonly used method to evaluate compressor performance is based on the analysis of inlet and discharge pressures and temperatures. Combined with gas mixture properties and known mass flow rate, it is a simple process to determine overall compressor power and efficiency. However, the critical step in this process is the conversion of pressure, temperature, and gas property information into both actual and ideal enthalpy differences. In addition to the abundance of equation of state (EOS) formulations, there are also multiple methods commonly applied for the calculation of the enthalpy differences. This paper reviews several of the methods used for this critical calculation and provides a comparison using multiple gas compositions.Copyright

Mechanical Performance of a Two Stage Centrifugal Compressor under Wet Gas Conditions

As subsea compression becomes a vital technology to the successful production of gas reserves in the North Sea, several technology issues will come to the forefront of the oil and gas industry. One of these important subjects is the capability to compress gas which includes a significant amount of liquids. Compressing wet gas requires knowledge in areas such as the prediction of turbomachinery performance with the mixed phase flow as well as the mechanical reliability of machinery in the same environment. This paper presents experimental results from a wet gas test campaign which, among other goals, is focused on characterizing the mechanical performance of a two stage compressor operating under wet gas conditions. Various mechanical parameters are monitored in the test program including rotor radial and axial vibration, rotor thrust, and shaft torque. A full array of wet gas conditions are tested with a suction pressure of 20 bar (300 psia) and liquid volume fractions in the range of 0.5 to 5%. The operating fluids are air and water, and the two stage compressor is operated at three speed lines ranging from high flow to low flow conditions. Significant variations are noted in the axial thrust, axial vibration and shaft torque. Thrust variations range from seemingly neutral thrust conditions at very low water injection rates to significant thrust increases (as compared to dry condition) for very high water injection rates. Rotor axial vibration is characterized by large amplitude and very low frequency, especially for the case in which the rotor thrust is balanced by the water injection. During higher levels of water injection, rotor axial vibration is generally characterized by relatively large amplitude and slightly higher frequency, although still very low as a percent of running speed. Variations in radial vibration are also noted, but to a much lesser extent.

Leveraging the Strengths of Commercial Finite Element Modeling Codes and Custom Engineered Software to Solve Atypical Rotordynamic Problems

Commercial finite element modeling codes have evolved over the past few decades into very user friendly environments, easily handling the many complications of finite element simulation. The significant steps of model generation (preprocessing), problem solution (analysis) and results viewing (post-processing) are easily handled for most of the typical finite element problems. However, there are still occasions when the necessary solution requirements fall outside the capabilities of any single finite element code. In this case, it is beneficial to the engineer to use some of the features of the commercial code, filing in the gaps with custom engineered software. This is especially true for the field of rotordynamics. In this paper, several of the complications involved in the finite element simulation of rotordynamics are discussed, and methods for leveraging the strengths of both commercial and custom engineered software are provided. The objective is to assist the practical engineer in the simulation of more complicated rotordynamic systems, including transient non-linear systems and three-dimensional coupled rotor-structure interaction systems.Copyright

Measured Performance of Two-Stage Centrifugal Compressor Under Wet Gas Conditions

The oil and gas industry is moving forward to access the most remote gas reserves and enhance the exploitation of the existing installation or postponing their tail-end. To achieve these accomplishments several technology challenges are being unveiled. In topside upstream application both offshore and onshore, one important technology issue is the capability to compress gas with a significant amount of liquids and it assumes a special interest in case of the facilities revamping. Nevertheless is in the subsea environment where this technology issue becomes really challenging. In order to properly design and size a compressor/motor system for subsea wet gas compression, one must be able to adequately predict the compressor performance with mixed phase flow.This paper presents the results from an experimental test program which investigated the performance of a centrifugal compressor at various wet gas conditions with elevated suction pressure. Performance tests are completed on a two stage centrifugal compressor with a mixture of air and water at suction pressures of 20 bar (300 psi). The compressor is subjected to flow with liquid volume fractions ranging from 0 to 5% along three speedlines.The performance measurements are made in accordance with ASME PTC-10 specifications with an additional torque measurement on the shaft between the compressor and gearbox. At each test condition, once the liquid is injected in the air flow, an increase in pressure ratio occurs. This testifies the compressor is still able to work in presence of water. However, increasing the amount of liquid injected a decreased polytropic head together with an increased absorbed actual power by the compressor cause a deterioration of its efficiency. Moreover when liquid is introduced into the flow, the discharge temperature of the compressor reduces significantly.The performance results and trends mentioned above are reviewed in the detail in this paper.Copyright

Gas Turbine Blade Stress and Temperature Sensitivity to Turbine Inlet Profile and Cooling Flow

Actual operating conditions in the hot section of a gas turbine vary from the design condition due to factors such as geographic location, component wear, and fuel composition. Turbine design practices typically use a conservative approach that requires checking the sensitivity of operating parameters such as turbine inlet profiles, cooling flows, and heat transfer correlations on component temperatures and stresses. In most cases, a sensitivity check is limited to analyzing the bounds of a range of values for only a few input parameters, whereby the inputs that produce the most conservative results are carried through the remainder of the analysis. For flow path components, however, multiple inputs must be evaluated over a range of values due to the interaction of the hot gas flow field and internal cooling systems. The study presented in this paper uses a probabilistic approach to develop surrogate models to evaluate the sensitivity of a set of operating parameters on the predicted blade temperatures and stresses. Commercially available software is utilized to predict blade temperatures and stresses for the first two stages of an industrial gas turbine. The operating parameters define the blade cooling flow and the shape and values of the turbine inlet profiles of total temperature and total pressure. The results of the study show the spatially resolved sensitivity of the operating parameters on blade temperature and stress distributions.Copyright

Mechanical Design Features of a Small Gas Turbine for Power Generation in Unmanned Aerial Vehicles

As part of the Intelligence Advanced Research Projects Activity (iARPA) Great Horned Owl (GHO) program, Southwest Research Institute® (SwRI®) developed and tested a small gas turbine for power generation in Unmanned Aerial Vehicles (UAV). This development program focused on advancing the state of the art in UAV power systems by meeting key metrics in weight, fuel efficiency, and noise generation.Design, assembly, and testing of the gas turbine were completed in-house at SwRI. Fundamental mechanical design features of the gas turbine include an integrated 7 kW motor-generator, minimal oil lubrication system, cantilevered compressor/turbine assembly, and can combustor with air-atomizing fuel nozzles. The compressor/turbine assembly is cantilevered directly off of the motor-generator shaft, which spins on hybrid ceramic bearings. Due to potential rotor natural frequencies in the design operating range, the rotor-dynamic design of this configuration was a special design challenge. The outboard rotor bearing is softly supported on O-rings to provide compliance and drive shaft natural frequencies below the operating range.The lube oil system is another interesting design feature of the GHO gas turbine. It is based on a minimal oil lubrication system previously used at SwRI. The minimal oil lubrication system relies on low oil flow rates and cooling air to pull droplets of oil through the bearing. The oil passes through the machine and is consumed during combustion. This system eliminates traditional oil recirculation hardware for simplicity and weight savings.The can combustor features a modular design and uses additive manufacturing techniques to facilitate easy and cost effective prototyping. All combustor components are manufactured from Inconel 718 using direct metal laser sintering (DMLS) with additional post-machining. These parts are particularly challenging for DMLS because of their thin walls and high aspect ratio. The custom air-atomizing fuel nozzles also highlight one of the exciting advantages of the DMLS process. Each nozzle would be difficult to machine using traditional techniques because of the tight internal flow passages; however, they are simple to construct using additive manufacturing.The GHO turbine developed by SwRI demonstrates interesting design features including a minimal oil lubrication system, a cantilever shaft with softly supported bearing, and combustor components built using additive manufacturing techniques. This design provides a platform for further development, testing, and demonstration of small gas turbine technology for UAV power generation.Copyright

Design of a Small Scale Gas Turbine for a Hybrid Propulsion System

As part of the Great Horned Owl (GHO) program Southwest Research Institute© (SwRI©) has developed a small, lightweight gas-turbine generator to provide power for an electric or hybrid electric Unmanned Aerial Vehicle (UAV). This original design for a fuel-to-electricity component of a hybrid propulsion system was designed, built and tested at the SwRI facility in San Antonio, TX. The design is based on a patented SwRI gas-turbine configuration and went through five major design iterations leading to the final configuration. The design iterations of the gas generator were driven by aggressive targets for weight, size and performance that were part of program requirements. The design of the GHO machine evolved from the initial concept based on lessons learned from previous testing at SwRI and considerations to improve manufacturability and operability. Improvements to the design were also incorporated to meet performance goals and increase life of hot section parts.This machine is low-cost and simple to operate and in addition to the original design intent of fuel-to-electricity use in a hybrid propulsion system can be used as a technology demonstration platform. SwRI plans to use the GHO machine in projects such as instrumentation development, as a test bed for new technologies such as ceramic or additive manufactured parts and for use as a component in a hardware-in-the-loop system.Copyright