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THE HISTORICAL EVOLUTION OF TURBOMACHINERY 309 Work of Osborne Reynolds in the Theoretical Development of Lubrication Theory While Tower’s elegant experiments had tremendous practical value, there was still no explanation of the behavior noted. This explanation would come from the work of Osborne Reynolds. Osborne Reynolds was born in Belfast in 1842 and had served as a mechanical apprentice prior to entering Cambridge where he studied mathematics. In 1883 he published his classic paper on the flow of fluids in pipes and channels from which the term Reynolds number was derived. In a meeting in Montreal in 1884, Reynolds first mentioned the classic hydrodynamic lubrication equation. Later, in a detailed 77 page paper, Reynolds (1886) provided his observations of Beauchamp Tower’s work and laid the theoretical foundation of bearing hydrodynamic theory. In this paper, he showed that friction changes could be linked to changes in oil viscosity and changes in journal speed and load. He also considered the effect of temperature on the differential growth between the bearing and the brass liner. The German physicist Arnold Sommerfeld, who was well- known for his work on atomic theory, provided tribologists with the famous Sommerfeld equation, which was a solution to Reynolds’ equation. His solution showed that the displacement of the journal (i.e., its eccentricity ratio) could be characterized by a dimensionless combination of parameters including the load, the journal surface velocity, and the bearing clearance. In 1952, Fred William Ocvirk, under the sponsorship of a NACA program, solved the Reynolds equation for a short bearing. Ocvirk accurately derived the load-bearing capacity and provided vital information as to how the journal center moved in a bearing and the lubricant flow required to supply it. He formulated the concept of a “capacity number” (also referred to as the Ocvirk number), which was the product of the Sommerfeld number and the square of the bearing length to diameter ratio. Thrust Bearing Developments The concept of tilting-pad bearings was independently developed by Kingsbury in the US and Mitchell in Europe—their designs being used to this day. Albert Kingsbury entered Cornell University in 1887, but had to leave because of a lack of funds. Because of his experience as a machinist apprentice in Ohio, he was assigned the task of testing bearings provided by the Pennsylvania Railroad Company. It was during these tests that Kingsbury noticed the benefits of film lubrication. Several years later, while experimenting with a test device involving a six inch cylinder containing a piston, he noticed that spinning the piston caused the piston to float within the cylinder. He then conducted tests on this air lubricated bearing. This was an important step in his development of the tilting-pad thrust bearing that bears his name. He studied Reynolds’ report dealing with the pressure that would be generated by a slight tilt of a flat surface and conceived of the idea of a tilt-pad thrust bearing. His concept involved a series of flat blocks arranged in a circle, each having a pivot on the back and facing a thrust collar attached to the rotating shaft. This allowed a dramatic increase in thrust load capability. Kingsbury filed for a patent for his bearing in 1907, even though he was testing tilt-pad bearings as early as 1898. His US Patents No. 247 and 242 were obtained in 1910. Kingsbury, who was working at Westinghouse at that time, had considerable difficulty in getting his design accepted. In one case, he had to pay for the manufacture of the bearing that Westinghouse was willing to try. Finally when Kingsbury wanted to sell his patent rights to Westinghouse just for the cost of obtaining the patent, Westinghouse refused, at which point, Kingsbury started his own bearing company in 1912. In a few years, this type of bearing was very common, especially on vertical machinery. CLOSURE This paper has covered several centuries of development in the turbomachinery field and has traced the evolution of technology that has resulted in the high efficiency turbomachines of today. Since the 1940s, turbomachinery development has been led by gas turbine and aeroengine development, and the growth in power within the past 60 years has been dramatic. The growth in thrust, turbine inlet temperature, and materials capability is shown in Figure 76. Figure 76. Turbomachinery Development for Aircraft Engines, 1940 to Present, Including SFC, TIT, and Material Capability. Current fourth-generation aeroengines operate with very high bypass ratios and include the GE 90, the Rolls Royce Trent, and the Pratt and Whitney 4084. These engines have applied a host of state-of-the-art technologies including: • Wide chord fans • Full authority digital engine control systems (FADEC) • Single crystal blading • Active clearance control • 3-D aerodynamic design • Low emissions combustors A current day fighter aeroengine has a thrust to weight ratio of about 10:1. Work is underway to increase this to 20:1 early in this new millennium. In the industrial turbine arena, the high demand for power has caused a proliferation of combined cycle power plants including gas turbines operating at rotor inlet temperatures of 2600 to 2700°F. A 50 Hz GE Frame 9H gas turbine with an airflow rate of 1519 lb/sec is shown in Figure 77. This machine and its 60 Hz counterpart, the 7H, are offered in combined cycle configurations only and operate at pressure ratios of 23:1 developed in an 18-stage axial flow compressor. Rotor inlet temperature of these machines is 2600°F, with the nozzles and blades being steam cooled. Other OEMs have advanced technology machines including the GT 24/26, which is an intercooled reheat gas turbine operating at a 30:1 pressure ratio. In a recent ASME talk, Povinelli stated that currently efforts are underway to study “breakthrough propulsion physics” (Povinelli, 1999). The concepts being investigated include the study of questions such as, “Are there forces that exist in the universe that we can utilize to push against?” “Are there clues in expanding dark matter that can lead to thrust or buoyance or lift forces?” and “Can the concept of cosmic energy or energy differential in space be utilized for providing large scale propulsive forces?” These concepts are attracting the attention of some leading physicists.PDF Image | THE HISTORICAL EVOLUTION OF TURBOMACHINERY
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