School of Water, Energy and Environment (SWEE)

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Browsing School of Water, Energy and Environment (SWEE) by Supervisor "Amaral Teixeira, Joao"

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    Heat removal in high pressure turbine seal segments
    (Cranfield University, 2017-02) Alenezi, Abdulrahman H.; Amaral Teixeira, Joao; Addali, Abdulmajid
    An important parameter for turbomachinery designers is “clearance control”, because the clearances between interfaces must be set to optimum values to maximize power output, operational life and efficiency. Leakage of hot gas result- ing from excessive clearance, can lead to flow instabilities, components overheat- ing, lower cycle efficiency and a dramatic increase in specific fuel consumption (SFC). Seal segments are used to reduce blade tip leakage, maintain coolant air flow and the stability of rotor-dynamic systems, helping to maximize blade perfor- mance. Seal segments in the High-Pressure Turbine (HPT) stages are one of the hottest components as they face the hot gases coming from the combustion chamber with temperatures which can reach 1700 0 C and which makes them sub- ject to oxidation, erosion, and creep. Thus, seal segments need to be protected. They are currently cooled using jet impingement techniques, passing cooling air (supplied by the high-pressure stage of the compressor) through channels to di- rectly impinge on the hot surfaces. The focus of this research was to improve the jet impingement cooling of the seal segments in HPTs by investigating methods that provide more effective heat removal. The role played by configurations of ribs (surface roughness using be- spoke turbulators), custom-made seal-segments, and surface features such as contouring, both in isolation and combination, were investigated using numerical methods. A set of 174 simulations were carried including the use of uniform and non-uniform roughness elements with different shapes and heights. Firstly, three different uniform roughness elements were tested, a square cross-sectional continuous rib, a hemi-spherical pin-fin and a cubical pin-fin for three jet impingement angles of α=90°, 60° and 45°. Each roughness element was also tested for six different heights (e) between 0.25 mm and 1.5 mm in increments of 0.25 mm. Results are presented in the form of average Nusselt number within and beyond the stagnation region. Secondly, the effect of using a roughness element with a square cross section in the shape of a circle, on the average Nu was investigated for four different radial locations (R), three jet angles (α) and six rib heights (e). Finally, the roughness element used was continuous, of square cross-sec- tion, in the shape of tear drops and reversed tear drops. This meant the rib did not act as a total barrier to flow in either the uphill or downhill direction.
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    Tidal turbine modelling from the perspective of design and operation
    (Cranfield University, 2016-06) Corsar, Michael; Amaral Teixeira, Joao
    The aim of this thesis is to study the effects of turbulent flow on a fixed pitch tidal current turbine from the perspective of turbine design and operation. A prototype turbine, Deltastream as it is known, is being developed by Tidal Energy Ltd for deployment in Ramsey Sound, Wales. It is well known that turbulence plays an important role in the fatigue life of marine turbines. Field measurements of tidal flow at the turbine site were analysed to establish the velocity spectra and turbulence intensity. This revealed a wide range of anisotropic turbulence which is dependent upon the tidal direction with intensities ranging from 5-20%. A numerical turbine model based on momentum theory was constructed in a time marching formulation that accounts for the effects of dynamic inflow and rotationally augmented airfoil stall delay properties. The turbine rotor design allows for load alleviation by regulation of the turbine tip speed ratio. At flow velocities above the rated velocity the tip speed ratio can be increased to reduce turbine loads. The model has been combined with a novel rotor speed control algorithm that estimates unsteady turbine inflow velocity from turbine loading without the requirement for external sensing of flow speed. When the turbine is subjected to three dimensional turbulent inflow the rotor speed controller has been shown to significantly reduce the fatigue effect of unsteady, turbulent flow. The turbine blade design has been developed using the model established. Experimental validation studies were carried out at 1/16th scale in turbulent conditions. Studies using the model have; identified the relationship between turbulence intensity and turbine fatigue load, established a controller schedule to significantly reduce fatigue loading and determined the blading fatigue life in realistic turbulent flows.