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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This thesis investigates a novel secondary flow reduction method. The inlet boundary layer to a linear turbine cascade is skewed by injection of air through an upstream slot to oppose regular generated negative stream wise vorticity. Other methods from the pertinent literature are reviewed on a broad basis. Detailed measurements of the flowfield in the Durham Linear Cascade facility have shown that substantial reductions in secondary flows and losses are possible. If the kinetic energy required for the blowing is taken into account by means of an availability analysis, no net gain in loss is achieved. Tests are performed at two different angles, of which the higher is typical for film cooling applications, and at a wide range of injection ratios. Calculation of the mixed-out losses show the tangential rather than spanwise momentum of the injected air is more effective in countering the generation of secondary flows. Computations using a state-of-the-art Navier-Stokes solver indicated shortcomings in modelling a flow governed by complex vortex dynamics. Improvements in the turbulence model and injection geometry could remedy this. The evaluation of turbulent and laminar production rates obtained without injection helps to explain total pressure loss generation mechanisms. The comparison of calculated and experimental eddy viscosities reveals the inadequacy of the Boussinesq assumption for high turning flows. The results obtained in this work are relevant to endwall film cooling applications. The tangential injection of air in front of the leading edge provides coolant in an optimum manner whilst possibly reducing secondary losses to a large extent. Disc cooling air, present in a real engine to prevent the ingestion of hot air from the mainstream, could be used to supply the injection.

The three-dimensional calculation of unsteady flows is increasingly gaining importance in the prediction of turbomachinery flow problems. A three-dimensional Euler/Navier-Stokes solver incorporating the time-linearized method and the nonlinear harmonic method in the frequency domain has been developed for predicting unsteady turbomachinery flows. In the time-linearized method, the flow is decomposed into a steady part and a harmonic perturbation part. Linearization results in a steady flow equation and a time-linearized perturbation equation. A pseudo-time time-marching technique is introduced to time-march them. A cell centred finite volume scheme is employed for spatial discretization and the time integration involves a four stage Runge Kutta scheme. Nonreflecting boundary conditions are applied for far field boundaries and a slip wall boundary condition is used for Navier-Stokes calculations. In the nonlinear harmonic method, the flow is assumed to be composed of a time-averaged part and an unsteady perturbation part. Due to the nonlinearity of the unsteady equations, time-averaging produces extra unsteady stress terms in the time-averaged equation which are evaluated from unsteady perturbations. While the unsteady perturbations are obtained from solving the harmonic perturbation equation, the coefficients of perturbation equations come from the solution of time-averaged equation and this interaction is achieved through a strong coupling procedure. In order to handle flows with strong nonlinearity, a cross coupling of higher order harmonics through a harmonic balancing technique is also employed. The numerical solution method is similar to that used in the time-linearized method. The numerical validation includes several test cases involving linear and nonlinear unsteady flows with specific attention to flows around oscillating blades. The results have been compared with other well developed linear methods, nonlinear time-marching method and experimental data. The nonlinear harmonic method is able to predict strong nonlinearities associated with shock oscillations well but some limitations have also been observed. A three-dimensional prediction of unsteady viscous flows through a linear compressor cascade with 3D blade oscillation, probably the first of its kind, has shown that unsteady flow calculation in the frequency domain is able to predict three-dimensional blade oscillations reasonably well.

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