Reports: DNI950554-DNI9: Gas Expanded Lubricants - Improving Energy Efficiency Using 'Smart' Fluids

Andres F. Clarens, PhD , University of Virginia

Lubricants serve an important function in the bearings and gears of power generation systems where they provide separation of solid surfaces, reduce wear, and dissipate heat. Power losses occur in these components due to the viscous shear of the lubricant, which can have a significant impact on the overall efficiency of a turbine. Since these viscous shear efficiency losses are a function of lubricant viscosity, the overall power losses are strongly correlated to temperature and loading, both parameters that can change rapidly. The goal of this research is to develop a novel lubrication technology that can adapt to changing conditions, minimize power losses, and provide for optimal performance while reducing environmental impacts. This technology, which we have called gas expanded lubricants or GELs, will consist of binary mixtures of synthetic lubricant and dissolved carbon dioxide. These mixtures can have their properties varied in real time to minimize power losses. Synthetic lubricants are used because they enhance compatibility with carbon dioxide, they have more predictable rheological properties when compared to petroleum-based lubricants, and many synthetic lubricants are biodegradable and can be made from renewable resources.

Building the foundation of the chemical understanding necessary to selecting the proper components and operating conditions is critical to delivering this technology to the power generation industry. In order to develop this understanding, the focus of this work is guided by two primary research objectives: (1) Characterize the relationship between GEL composition and bulk properties and (2) Establish the influence of GELs on gear behavior. In order to fulfill these objectives, two principal research tasks have been defined. During the first year of the project period, significant progress has been made in the development of both of these tasks, outlined below.

Task 1. Measure the phase stability and viscosity of a PAG-CO2 GEL formulation

The work in this task is focused on measuring three important chemical properties of binary CO2/lubricant mixtures: 1) the diffusion of CO2 into the lubricant 2) the oxidative stability of the mixtures and 3) the rheology of these mixtures under elevated temperatures and pressures. The diffusivity is being measured using custom built pressure vessels in which liquid and gas phase mixtures are combined and the pressure is measured for 20-hours. The experiments are performed in a temperature-controlled bath to minimize temperature fluctuation. The pressure history is then fit to an analytical expression using a non-linear regression tool to calculate constants for rate of change and equilibrium pressure. Results indicate greater diffusivity of CO2 into the lubricants with lower viscosities. A similar type of experimental setup is being used to measure oxidative stability. A pressure vessel is fitted with a two-way valve immediately above the CO2 intake to allow for pressurization, isolation, and subsequent disconnection from the syringe pump used to pressurize the vessels. For our purposes, oxidative degradation of lubricant has its most important impact on the viscosity of the lubricants. After allowing the mixtures to sit for 1-4 weeks at elevated temperatures, the lubricant is degassed and the viscosity of the lubricant is measured. Oxidation of synthetic lubricants generally results in decreased molecular weight and consequently viscosity.

The viscosity of GELs under a variety of conditions is being characterized under pressure using an Anton Paar MCR 301 rheometer equipped with a high-pressure measuring cell rated up to 15 MPa. Figure 1 demonstrates the relationship between the mass fraction of CO2 in the mixture and the viscosity. The data shows a good fit with Equation 1. Mass fraction values were obtained by extrapolating pressure-mass fraction data from previously published work. As predicted, the viscosity of the GEL decreases logarithmically with increasing mass fraction of CO2 according to equation 1:


The viscosity-composition data being generated here will form the cornerstone of the controllers that will be developed to regulate the composition of the GELs being delivered to the bearing.

Figure 1. Viscosity of PAG-CO2 mixtures as a function of mixture composition at 40°C and 100°C. The results were obtained experimentally while the predicted value is obtained using a theoretical equation. The modeled values were used to develop modeling estimates of power loss reductions in tilting pad journal bearings.

Task 2. Develop an experimental testbed to measure bearing performance using GELs

A major focus of the first year of the project has been to design the mechanical test rig to be used to measure the performance of GELs in tilting pad bearings. The test rig will utilize a 10 hp motor capable of reaching 10,000 rpm to drive a 1.5" rotor of approximately 42" in length. This rotor will be supported by two tilting-pad journal bearings supplied by Lufkin-RMT, Inc., who will also finalize all designs for the test rig and manufacture the individual components. This task was particularly challenging because custom bearing housings had to be designed to ensure that they would be capable of reaching pressures of up to 17 MPa. A finite element analysis was used to optimize key design parameters such as material selection and thickness, number and size of bolts, and accommodations for seals, instrumentation, and lubricant inlets and outlets. Figure 2 presents a rendering of the finite element model of the final housing design.

The design of the oil seals was especially challenging because of the pressures and speeds being used in this system. To meet these difficult conditions we maximized the face area exposed to the pressure and utilizing a Teflon-coated steel o-ring to allow the oil seal to form a tight seal without becoming overly rigid and acting as a bearing, which would introduce forces that could cause the test rig to become unstable. The final step in designing the GEL testbed was to develop a GEL delivery system. That system has been designed and the construction of the unit has been contracted out. All elements of the this test rig are now in production and we expect the rig to be built by end of 2011.

Description: Housing Assembly FEA Mesh.bmp

Figure 2. Finite element model used to analyze the effects of high pressure on a bearing housing design.

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