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THE HISTORICAL EVOLUTION OF TURBOMACHINERY 297 It is interesting to note that all the major aeroengine manufacturers started their jet engine work based on Whittle’s designs (Singh, 1996). The Rolls Royce Welland, Derwent, Nene, and Tay were based on the Whittle designs. Pratt and Whitney entered the gas turbine field after the war using the Rolls Royce Nene as a basis for their J-42 and J-48. General Electric started their jet engine work based on the Whittle designs and developed the I-A, J-31, and J-33. Technical Features of Whittle’s Engines Whittle had the genius to know that to achieve success, his designs had to be simple, robust, and have the best chance for rapid development. Whittle’s designs were masterpieces of simplicity in design and construction and low in weight. When Whittle pointed out the virtue of the simplicity of the engine to Lord Hives of Rolls Royce, Hives is reported to have dryly remarked, “Wait until we have worked on it for a while; we will soon design the simplicity out of it!” (Hooker, 1984). Looking at today’s complex aeroengines, this was a prophetic statement! Double Sided Centrifugal Impellers Whittle’s choice of a double-sided centrifugal compressor was made to obtain the maximum possible breathing capacity in proportion to size. The first experimental engine (WU) had a compressor tip diameter of 19 inches and had 30 blades. Whittle chose the largest number of blades possible based on manufacturing limitations in order to minimize the blade loading. Reverse Flow Combustors There were several reasons why Whittle elected to use reverse- flow combustors for his early developments. These included: • To permit the use of a short shaft that required only two bearings and eliminated the need for a flexible coupling. • To eliminate an expansion joint between the compressor and turbine. • To provide for even air flow to the primary combustion zone. • To screen the turbine blades from direct flame radiation. Vortex Design of Turbine Blades Whittle assumed that the BTH engineers were designing the turbines based on vortex theory. BTH engineers had not assumed vortex flow from the turbine nozzles and therefore had not designed the blades with adequate twist. Whittle’s insistence on this design approach soured relationships with some BTH engineers who resented this young engineer instructing them on how to design turbines. Design Problems and Failures It is easy for one to gain the impression that the course of engine development was simple and logical. This was hardly the case, and there were numerous problems that had to be surmounted by Whittle and his team with minimal resources and funds and always under intense time pressure. Several problems were the result of pushing the state-of-the-art. Several setbacks were the results of bad luck, lack of funds that forced cannibalization of parts, or environmental factors. Whittle believed, for example, that several of the early bearing failures that occurred at the dilapidated Ladywood Works were the result of a “rain” of foundry sand derived from the roof of the workshop, which was formerly a foundry! Some of the serious problems faced are presented below. Impeller, Turbine Blade, and Disk Failures There were several problems pertaining to impeller vibration and cracks, which have been covered by Voysey (1945). Problems started with the W.2/500 engine. Whittle found out that, at 14,000 rpm, the engine produced a “howling” sound. Tests showed that even a short run at the howling speed would result in resonance cracks over the length of its junction with the impeller disk, as shown in Figure 41. A front view of a wrecked Power Jets engine caused by an impeller failure is illustrated in Figure 42. Several methods of fundamental importance in analyzing blade and impeller vibration were developed. Problems also plagued turbine blading, especially with later versions of the Whittle engines, specifically in the W.2/800 where the blade lengths had increased. A Campbell diagram for the W.2/800 is illustrated in Figure 43. A wrecked impeller due to fatigue failure is shown in Figure 44. Figure 41. Position of Centrifugal Impeller Cracks. (Voysey 1945; Courtesy IMechE, UK) Figure 42. Whittle Engine Wreck Caused by Burst Impeller. (Development of the British Gas Turbine Unit, 1945; Courtesy IMechE, UK) The initial disks that utilized the De Laval type fixation method were subject to failure as shown in Figure 45. This failure occurred on the WU engine in February 1941, at a run time of 168 hours. The use of a fir-tree arrangement and better blade materials resolved this problem on future engines. Cracking in the root serrations on the W1.A engine is shown in Figure 46. In another case, the W.1 engine, which was putting in considerable time both on the E28 test aircraft and on the bench, suddenly encountered turbine blade failures. The mystifying factor in this case was that the location of the crack was not consistent (as would be expected by a fatigue type problem), occurring at times at the root, midspan, and at the blade tip. Whittle suspected that this was the effect of a thermocouple located three feet downstream from the turbine that was causing fluctuations in blade loading. Upon removal of the thermocouple, the blade failure problem disappeared (Whittle, 1979).PDF Image | THE HISTORICAL EVOLUTION OF TURBOMACHINERY
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