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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Typical modem fighter aircraft use two-spool, low by-pass ratio, turbojet engines to provide the thrust needed to carry out the combat manoeuvres required by present-day air warfare tactics. The dynamic characteristics of such aircraft engines are complex and non-linear. The need for fast, accurate control of the engine throughout the flight envelope is of paramount importance and this research was concerned with the study of such problems and subsequent design of an optimal linear control which would improve the engine's dynamic response and provide the required correspondence between the output from the engine and the values commanded by a pilot. A detailed mathematical model was derived which, in accuracy and complexity of representation, was a large improvement upon existing analytical models, which assume linear operation over a very small region of the state space, and which was simpler than the large non-analytic representations, which are based on matching operational data. The non-linear model used in this work was based upon information obtained from DYNGEN, a computer program which is used to calculate the steady-state and transient responses of turbojet and turbofan engines. It is a model of fifth order which, it is shown, correctly models the qualitative behaviour of a representative jet engine. A number of operating points were selected to define the boundaries used for the flight envelope. For each point a performance investigation was carried out and a related linear model was established. By posing the problem of engine control as a linear quadratic problem, in which the constraint was the state equation of the linear model, control laws appropriate for each operating point were obtained. A single control was effective with the linear model at every point. The same control laws were then applied to the non-linear mathematical model adjusted for each operating point, and the resulting responses were carefully studied to determine if one single control law could be used with all operating points. Such a law was established. This led, naturally, to the determination of an optimal linear tracking control law, and a further investigation to determine whether there existed an optimal non-linear control law for the non-linear model. In the work presented in this dissertation these points are fully discussed and the reasons for choosing to find an optimal linear control law for the non-linear model by solving the related two-point, boundary value problem using the method of quasilinearisation are presented. A comparison of the effectiveness of the respective optimal control laws, based upon digital simulation, is made before suggestions and recommendations for further work are presented.

A novel, fast switching, reliable, and economical fluidic gaseous fuel injector system designed for natural gas engines has been developed in this research. The system consists mainly of no-moving-part fluidic devices and piezo-electric controlling interfaces. The geometric parameters of a fluidic device seriously affect its performance. Traditionally, these parameters can only be optimised through "trial and error" exercise. In this research, a computer simulation model for the jet steady state attachment and dynamic switching has been developed. The good agreements between predicted results and experimental ones show that the model can not only explain the jet attaching and switching mechanism, but also optimise the design of geometric parameters of a fluidic device. The steady state and dynamic characteristics of the system were tested on a laboratory experimental rig. The results show that the system can handle the large gas volume flow rate required by natural gas engines and is capable of operating via pulse width modulation. A few typical commercial solenoid type gas injectors were also tested and the results were compared with those from the fluidic system. It was found that the fluidic gaseous fuel injector system has faster switching responses and smaller injection cycle-to-cycle variations.

Competitive pressure and stringent emissions legislation have placed an urgent demand on research to improve our understanding of the gas turbine combustor flow field. Flow through the air admission ports of a combustor plays an essential role in determining the internal flow patterns on which many features of combustor performance depend. This thesis explains how a combination of experimental and computational research has helped improve our understanding, and ability to predict, the flow characteristics of jets entering a combustor. The experiments focused on a simplified generic geometry of a combustor port system. Two concentric tubes, with ports introduced into the inner tube's wall, allowed a set of radially impinging jets to be formed within the inner tube. By investigating the flow with LDA instrumentation and flow visualisation methods a quantitative and qualitative picture of the mean and turbulent flow fields has been constructed. Data were collected from the annulus, port and core regions. These data provide suitable validation information for computational models, allow improved understanding of the detailed flow physics and provide the global performance parameters used traditionally by combustor designers. Computational work focused on improving the port representation within CFD models. This work looked at the effect of increasing the grid refinement, and improving the geometrical representation of the port. The desire to model realistic port features led to the development of a stand-alone port modelling module. Comparing calculations of plain-circular ports to those for more realistic chuted port geometry, for example, showed that isothermal modelling methods were able to predict the expected changes to the global parameters measured. Moreover, these effects are seen to have significant consequences on the predicted combustor core flow field.

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