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This thesis investigates the concept of thrust rating as a means towards reducing the life cycle costs of engine ownership. Towards this end, this thesis has discussed the concept of thrust rating, developed computer programs for mechanical load type failures, which include creep, LCF, and combinations thereof, and conducted simulations of improving life usage and reducing life cycle costs. A study was performed on a military engine, under an original design mission mix, that showed significant gains in creep-LCF life of the HPT blade could be achieved, especially With the recently proposed and presumably more accurate criterion- ductility exhaustion, by thrust rating. The savings were expressed in terms of an approximate reduced life accumulation rates and life cycle costs. The net result was a 50% increase in creep-LCF life with a savings of $50.4 million. These calculations were based on a Feet of 300 engines having the designed lifetime of 8,000 operating hours per engine. Throughout the thesis, mention is also made of employing the thrust rating concept on other engines. To this end, the thesis will also give a blueprint for conducting a feasibility study to employ thrust rating as a maintenance tool. In addition to the technical aspects, the role of maintenance and aircraft operations policy will also be studied to determine the interrelationships that exist between thrust rating technology and its practical application.

The objective of the work described in this thesis is twofold; to elucidate the nature of stall and surge in an axial flow aeroengine compressor, and to improve on current computational stall modelling techniques. Particular attention is paid to the initial stages of the stall/surge transient, and to the possibility of using active control techniques to prevent or delay the onset of stall/surge. A detailed analysis is presented of measurements of the stalling behaviour of a Rolls- Royce VIPER jet engine, showing a wide variety of stall inception and post-stall behaviour. Stall transients are traced from disturbances through to stable rotating stall or axisymmetic surge. The stall inception pattern at nearly all speeds is shown to conform to the short circumferential length scale pattern described by Day [1993a]. A multiple compressors in parallel stall model is developed using conventional stall modelling techniques, but extended to include the effects of the jet engine environment The model is shown to give a good representation of the overall stalling behaviour of the engine, although the details of the stall inception period are not accurately predicted. A system identification technique is applied to the results of the model in order to develop a method of active control of stall/surge. A new stall model is introduced and developed, based on a time-accurate three dimensional (but pitchwise averaged) solution of the viscous flow equations, with bladerow performance represented by body forces. The flow in the annulus boundary layers is calculated directly, and hence this new method is sufficiently complex to model the initial localised disturbances that lead to stall/surge. At the same time the computational power required is compatible with application to long multistage compressors.

A boost controlled continuously variable geometry turbocharger prototype has been designed, manufactured and tested. The prototype has been first rig tested and later fitted to a Perkins T6.354 diesel engine. The engine tests have included both steady state and transient runs. Torque back up has been improved considerably increasing from 34.3% to 55.8%, the former occurring at 1400rpm while the latter at 1200rpm. In the experimental programme, compressor surge has been the limiting parameter while in the theoretical investigations a wide mass flow compressor has been assumed and the limiting parameter was maximum cylinder pressure. In the theoretical investigations lower compression ratio and retarded injection timing have been considered to further improve the scope for higher torque back up and improved transient response. In addition the performance of the variable geometry turbocharged engine using a simple boost controlled turbine restriction schedule has been simulated. It is concluded that a simple boost controlled system will present sfc penalties in the part load regime and thus more sophisticated multi-variable schemes will have to be studied if sfc optimization is to be achieved. The experimental programme has been conducted using the 'zip fastener' design. This design has been found to be strongly non-linear with respect to turn down ratio in response to turbine restriction but will offer the required effects at the expense of a slight drop in turbine efficiency. However, in future investigations initial calibration studies have to be undertaken to ensure comparable swallowing capacities with the standard turbine which the variable geometry turbine replaces.

The vibration of a turbocharger blade and dynamic characteristics of bladed packets connected by a lacing wire have been studied. The study was carried out using three analytical and experimental methods. They are: Modal Testing, Electronic Speckle Pattern Interferometry (ESPD and Finite Element Analysis (FEA)). Vibration modes of a turbocharger blade with aerodynamic profile, with and without a lacing wire, were identified using model blades with simplified geometry. The separation of coupled modes was achieved using ESPI tests. The modes of vibrations of bladed packets were identified. The effect of inter-blade coupling through a lacing wire is that a cluster of sub-modes are generated in bladed packets corresponding to each fundamental mode of the freestanding blade, the number of the sub-modes being equal to the number of blades in the packet. Apart from the fundamental sub-mode, the vibration of all other submodes are out of phase with different phase relations. The stiffness of the lacing wire and its location with respect to the blade make great contributions towards certain mode clusters in terms of mode shapes and natural frequencies. The nonlinearity of the stiffness of the deformed lacing wire caused by centrifugal force was established. The coupling of this non linearity with different vibration amplitudes, due to different phase relation, results in the dynamic mistuning in lacing wire stiffness. This mistuning is considered to be a major attribute in reducing the responses at resonance.

This thesis initially describes the experimental work which was carried out in order to examine the effects of parameters such as the blade profile, free stream turbulence intensity etc. on the onset and, extent of transition in the boundary layer on a turbine blade. A series of four different blade profiles with identical pressure surfaces but different suction surfaces, were tested. The suction surfaces differed in the severity and extent of their initial favourable pressure gradients. Tests were carried out for a range of throat Mach numbers and free stream turbulence intensities. From the results obtained it is suggested that the most significant factor is 'the severity of the initial. favourable pressure gradient. This appears to have an inhibiting effect on the influence of the free stream turbulence and on the growth of the turbulent stresses in the transition region. Using these hypotheses, modifications were made to an existing boundary layer prediction program in order to provide a better transition-model and to include the effect of free stream turbulence.

This research theme is originated from a development project that is going on in South Africa, for the design and construction of a closed cycle gas turbine plant using gas-cooled reactor as the heat source to generate 115 MW of electricity. South African Power utility company, Eskom, promotes this developmental work through its subsidiary called PBMR (Pebble Bed Modular Reactor). Some of the attractive features of this plant are the inherent and passive safety features, modular geometry, small evacuation area, small infrastructure requirements for the installation and running of the plant, small construction time, quick starting and stopping and also low operational cost. This exercise is looking at the operational aspects of a closed cycle gas turbine, the finding of which will have a direct input towards the successful development and commissioning of the plant. A thorough understanding of the fluid dynamics in this three-shaft system and its transient performance analysis were the two main objectives of this research work. A computer programme called GTSI, developed by a previous Cranfield University research student, has been used in this as a base programme for the performance analysis. Some modifications were done on this programme to improve its control abilities. The areas covered in the performance analysis are Start-up, Shutdown and Load ramping. A detailed literature survey has been conducted to learn from the helium Turbo machinery experiences, though it is very limited. A critical analysis on the design philosophy of the PBMR is also carried out as part of this research work. The performance analysis has shown the advantage, disadvantage and impact of various power modulation methods suggested for the PBMR. It has tracked the effect of the operations of the various valves included in the PBMR design. The start-up using a hot gas injection has been analysed in detail and a successful start region has been mapped. A start-up procedure is also written based on this. The analysis on the normal and emergency load rejection using various power modulation devices has been done and it stress the importance of more control facilities during full load rejection due to generator faults. A computational fluid dynamics (CFD) analysis, using commercial software, has been carried out on some geometry of the PBMR design to find out whether its flow characteristic will have any serious impact on the performance on the cycle during the load control of the plant. The analysis has demonstrated that there will not be much impact on the performance, during load control using pressure level changes, from this geometry. However, some locations in the geometry have been identified as areas where the flow is experiencing comparatively high pressure losses. Recommendations, which include modification in the physical design, were made to improve this. The CFD analysis has extended to a cascade to compare the flow behaviour of Air and Helium with an objective of using air, being inexpensive, to test the helium flow characteristic in a test rig to simulate the behavioural pattern of helium in the PBMR pressure vessel. The specification of a hypothetical test rig and the necessary scaling parameters has been derived from this exercise. This will be useful for designing test rigs during the developmental and operational stage of the PBMR project.

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