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This work focuses on two main subjects: first, the development and validation of a robust generic performance code for industrial gas turbines (GTSI) and, second, the study of an innovative carbon dioxide/argon semi-closed cycle burning low calorific gas coming from coal gasification. GTSI will be able to simulate open, closed and semi-closed cycles at design and off-design conditions. A comprehensive thermodynamic study of the properties of the most common working fluids has been carried out, introducing the results in GTSI for a wide range of temperatures and pressures, being easy to add other gases. To make the code very general, in addition to gas turbines conventional components, such as inlet, compressor, intercooler, regenerator, combustor, turbine and exhaust system, GTSI can model evaporative intercooler, steam injection and reheat. The possibility of variable geometry was introduced in the compressor, steam injector and turbine modules. Given the high temperatures in modem and future turbines, a detailed cooling system modeling has been developed, being able to predict cooling flow requirements according to different technology levels. The control of the complete power plant has also been considered in detail, allowing the user to select among different options. The code incorporates a simple steam turbine bottoming cycle for a preliminary analysis of the combined cycle arrangement. In addition to the conventional off-design simulations it is possible to carry out studies involving engine deterioration and modification, or substitution, of components. A validation process was carried out using different gas turbine arrangements. The result has been satisfactory, although additional configurations should be examined when more data is available. As a direct application of GTSI, the conceptual design of the carbon dioxide/argon semi-closed cycle was conducted. Several key performance factors were considered in this study, such as the working fluid composition, the gas turbine arrangement and the cooling technology. Other main parameters were selected according to the state-of-the-art technology. Advanced concepts such as cryogenic precooling and turbine stator internal cooling, together with improved component efficiencies and higher temperatures were contemplated for a mid-long term future design. The results obtained for the conventional cycles have not been very promising, with slightly better values for the advanced cycles. Five of the most interesting cycles were selected for off-design studies, evaluating the part-power behaviour, the variable geometry requirement, etc. To complete the investigation, the starting sequence of one of them was performed. Also, considering the possibility of using existing turbomachinery, designed for air, in a semi-closed cycle pilot plant, the operation of several gas turbine configurations was analysed.

World developments have led the armed forces of many countries to become more aware of how their increasingly stringent financial budgets are spent. Major expenditure for military authorities is upon aero-engines. Some in-service deterioration in any mechanical device, such as an aircraft's gas-turbine engine, is inevitable. However, its extent and rate depend upon the qualities of design and manufacture, as well as on the maintenance/repair practices followed by the users. Each deterioration has an adverse effect on the performance and shortens the reliable operational life of the engine thereby resulting in higher life cycle costs. The adverse effect on the life-cycle cost can be reduced by determining the realistic fuel and life-usage and by having a better knowledge of the effects of each such deterioration on operational performance. Subsequently improvements can be made in the design and manufacture of adversely-affected components as well as with respect to maintenance/repair and operating practices. For a military aircraft's mission-profiles (consisting of several flight-segments), using computer simulations, the consequences of engine deterioration upon the aircraft's operational-effectiveness as well as fuel and life usage are predicted. These will help in making wiser management decisions (such as whether to remove the aero-engines from the aircraft for maintenance or to continue using them with some changes in the aircraft's mission profile), with the various types and extents of engine deterioration. Hence improved engine utilization, lower overall life-cycle costs and the optimal mission operational effectiveness for a squadron of aircraft can be achieved.

Three steady state combustion models, two turbulence models and a model for tK'6 prediction of NOx were implemented and investigated on a simple backward facing step experiment as well as an experimental lean prevaporised premixed (LPP) combustor. The three combustion models included the simple Eddy Break-up model as well as a presumed probability density function (pdf) model and a form of the BML crossing frequency flamelet model. These models were adapted to consider a variable mixture fraction to account for a non-homogeneous fuel air mixture. The two turbulence models used were the k-ε and second moment models. Despite being unable to capture the flame front spreading in the case of the backward facing step, these predictions provided insight into the performance and implementation of the models. All three of the combustion models, after appropriate tuning, worked well for the LPP test combustor. This illustrates that such time averaged models are useful for flows which do not contain large transient coherent structures, such as that of the LPP test combustor and most practical engine combustors designed today. The second moment closure turbulence model was found to have the greatest impact on the flame front through the flow field predictions rather than through counter gradient diffusion. The Eddy Break-up and BML crossing frequency models both performed very well, qualitatively predicting the correct trends. The additional consideration of flame front straining in the BML crossing frequency model did not appear to significantly influence the flame front. This is because the type of model adopted to predict this effect had a relatively uniform influence everywhere in the flow. The presumed pdf model also performed well and was additionally found to self ignite without the existence of hot products when the inlet temperature was high enough. The NOx model faired well for a simple experimental geometry. However, it considerably over predicted the NOx formed within the LPP test combustor, which was most probably due to poor boundary conditions. Despite this overprediction, the results give insight into how to improve the NOx emissions for the experimental combustor.

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