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Fuel flexibility will drive the energy demand in the near future. The use of different syngas compositions from various sources will play a major role in the global fuel mix. CO2 in the blends will also be added as a mechanism to improve carbon capture and storage technologies. However, this can trigger instabilities such as thermoacoustics, flashback, autoignition and blowoff. In terms of blowoff, the phenomenon is still not entirely understood. This project presents a series of experiments to determine the behaviour and impact on the blowoff process at various swirl numbers, nozzle geometries and gas compositions. The Central Recirculation Zone was analyzed just before blowoff. The results show how the strength and size of the recirculation zones are highly influenced by these parameters. However, it seems that the CRZ dimensions/strength does not play an important role in the blowoff, whilst the composition of the mixture shows high correlation. Nevertheless, the CRZ intensity using these compositions can increase residence time, important for combustion improvement of other blends. [ABSTRACT FROM AUTHOR]

The measurement of power performance is an important procedure in the design verification and ongoing health monitoring of a tidal turbine. Standardised methods state that the performance should be measured relative to two independently located flow sensors, the arrangement of which is often non-trivial and necessitates additional cost. Recent interest in the usage of flow sensors mounted on the turbine has demonstrated their capabilities in profiling the rotor approach flow, but this instrument configuration is not recognised in the performance assessment standard. This study evaluates the merits of the turbine mounted configuration by measuring the performance of a tidal turbine relative to this reference and to a conventional seabed placed instrument. The turbine mounted sensor is found to provide a better reference of the free-stream conditions, evident from an improved agreement with theoretical predictions of device performance and a reduced amount of variation in the results. This new method could reduce both the costs and uncertainty associated with existing performance assessment best practices. • The power curve from a full-scale tidal turbine is measured. • Measurements from conventional seabed instrument inadequately capture turbine flows. • A new method is proposed using measurements obtained from a turbine mounted sensor. • Turbine sensor results show less variation and are in better agreement with theory. • New method reduces costs and uncertainties associated with performance assessment. [ABSTRACT FROM AUTHOR]

Extensive Research and Development have helped drive the tidal energy industry towards commercial viability, with full-scale prototypes starting to meet marine environmental challenges. This paper utilises velocity data collected from a site off the welsh coast with Computational Fluid Dynamics (CFD) to determine the effects of misaligned flows on the performance of a tidal stream turbine. The field observations indicate that the majority of the currents tend to fall within ±20° of the principal flow direction for economically viable velocities. The CFD modelling suggests a reduction in the non-dimensional performance parameters as the angle of misalignment increases between the axis of rotation and the free stream velocity. The resultant magnitude of the bending moments acting on the rotor end of the driveshaft for the misaligned turbines was found to be up to ten times greater than the aligned turbine. The paper also shows that an accurate definition of the tolerance to axial flow misalignment between the free stream velocity and the axis of rotation of a turbine is required to prevent any negative effects on performance and loading. [ABSTRACT FROM AUTHOR]

Detached eddy simulations (DES) have been performed for flow over a solitary hydrokinetic turbine for two different tip-speed ratios λ = 3.64 and 6.15 and three different tip-clearances (δ = 0.1 D , 0.2 D, and 0.35 D) to elucidate the effect of air-water interface on the turbine performance and wake recovery in shallow water conditions. The power predictions show good agreement with experimental data for λ = 3.64, but a large 12% error for λ = 6.15. The interface introduced unsteadiness in the power predictions, which increased with λ. The interface did not affect the mean power predictions for lower λ significantly, but reduced power for higher λ. The two-phase simulations for λ = 6.15 showed significant improvement in wake deficit and near-wake TKE prediction compared to the single-phase simulations. The improved predictions were identified because the interface causes early tip vortex entanglement and TKE burst. The DES predictions also show turbulence anisotropy maps consistent with the experimental data. Analysis reveals that turbulence is one-dimensional, dominated by streamwise fluctuations behind the blade tip region; two-dimensional, dominated by transverse and spanwise fluctuations in the TKE burst region; and approaches three-dimensional isotropy in the fully developed wake region. Detailed analysis of the vortical structures and wake characteristics reveals that the interface significantly affects the vortex advection, breakdown, and wake evolution. In particular, they enhance the wake deficit, which slows down the advection of the tip vortex filaments, causing an early vortex entanglement. The tip vortices show interaction with vortices generated underneath the interface, but these interactions seem localized. The tip vortex breakdown was identified due to instabilities generated during vortex entanglement and interaction with interface vortices for λ = 6.15 case, and because of tip and root vortex interaction for λ = 3.64. Although the two-phase simulations are encouraging compared to the single-phase case, TKE is still underpredicted in the near-wake region, possibly because of turbulence or rotating interface modeling issues. Two-phase simulations also predict significantly higher swirl, which needs further investigation. • High-fidelity simulation of three-bladed hydrokinetic turbine in shallow waters for two tip-speed ratios. • Validation of performance, and intermediate mean and turbulent wake predictions. • Effect of air-water interface on turbine performance and wake recovery. [ABSTRACT FROM AUTHOR]

The predictive capabilities of blade-resolved unsteady Reynolds averaged Navier-Stokes (URANS) and detached eddy simulation (DES), the most commonly used hybrid RANS/large eddy simulation (LES) model, are assessed for hydrokinetic turbine performance and mean and turbulent flows in the intermediate-wake region, and results for a range of tip-speed ratio encompassing design and off-design conditions are analyzed to understand the wake recovery mechanism. The performance predictions compared within 5% of the experimental data. Both URANS and DES models performed reasonably well for the near wake predictions, where the errors were <15%. DES outperformed URANS for both mean wake deficit and turbulence predictions in the intermediate-wake region and both quantities compared within 10% of the experiments. The improved prediction by DES is because of its ability to predict the tip vortex breakdown, which plays a critical role in the wake recovery, especially for higher tip speed ratios (λ). However, DES significantly underpredicted the turbulence predictions in the near-wake region, which could be partly due to the negligence of free-surface effects and partly due to modeling issues, namely modeled stress depletion. The study reveals that the tip vortex breakdown mechanism depends on λ. For lower values of λ, instabilities generated in the root vortex core are identified to be the cause of breakdown. For higher values, the breakdown occurred because of the instabilities generated during the vortex filament entanglement. Future work should focus on investigation of other hybrid RANS/LES models to address the limitations of the DES models, and extension of the study to include free-surface effects. [ABSTRACT FROM AUTHOR]

The paper presents an experimental campaign developed to contribute to the current research considering the operation of Horizontal Axis Tidal Turbines within stochastic flow conditions, namely turbulent and wake induced flows. The campaign was conducted at approximately a 1 / 20 t h -scale within a recirculating flume. Experiments were conducted over five differing setups, yielding a baseline low Turbulence Intensity case, two high turbulence cases and two upstream device generated wake cases. The experiments were conducted at a range of differing rotor velocities established, in a novel way, by utilising both fixed speed and fixed braking torque control. The paper presents analysis of flow measurements to statistically quantify the stochastic flow conditions impinging on the model-scale tidal turbine. The power, thrust, torque and blade root bending moment of single blade were recorded and analysed against the flow conditions generated under the five cases. The analysis showed that it may well be possible to exploit the accelerated region around an upstream turbine to capture marginally higher power (6 % increase) from downstream turbines. Lastly, it was found that the control scheme adopted has a significant impact on power and load fluctuations observed at differing rotor velocities. • Presents novel research into the combined effects of turbulence and devices.. • Two differing control schemes are used fixed torque and speed control. • The paper shows further evidence of the array 'packing' effect. • The paper presents the effects of differing levels of TI on device loading. [ABSTRACT FROM AUTHOR]

Before initiating a study on the interaction of multiple wakes, it is imperative that turbine wake hydrodynamics are studied in isolation. In this paper CFD computer simulations of downstream turbine wakes have been run using a scale-resolving hybrid turbulence model known as a detached eddy simulation. To allow validation of the CFD simulations the computer models were supported by flume measurements with a lab scale tidal stream turbine run at three tip-speed ratios and three turbulence conditions, varying both turbulence intensity and length-scale. From the study it was demonstrated that turbulence intensity has a significant impact on the wake development for both recovery and width. The turbulence length scales of between 0.25 and 1.0 rotor diameter did not have a significant impact on the wake. The turbine operating condition also had an impact on the resulting wakes. In the near wake, centreline velocity recovery was found to increase with increasing turbine thrust due to flow being diverted towards the turbine nacelle. For a volumetric averaged wake, greater power extraction was found to cause the greatest near-wake deficit. Wake width was found to increase with increasing tip-speed ratio (and therefore turbine thrust). • Wake widths and lengths for a tidal turbine using three different analysis metrics are presented. • Three levels of turbulent conditions are presented using upstream grids. • The impact of the turbine operating conditions, on the wake characteristics, are analysed. • The impact of turbulence intensity and Integral length scales on the wake characteristics are analysed. [ABSTRACT FROM AUTHOR]

The paper describes the development and characterisation of three 0.9 m diameter lab-scale Horizontal Axis Tidal Turbines. The blade development process has been outlined and was used to generate a design specification. Each turbine houses instrumentation to measure rotor thrust, torque and blade root bending moments on each blade, in both 'flapwise' and 'edgewise' directions. A permanent magnet synchronous machine and encoder are integrated to allow for servo-control of the turbine as well as to provide position and rotational velocity measurements, resulting in three turbines that can be individually controlled using speed or torque control. Analogue signals are captured via a real-time operating system and field programmable gate array hardware architecture facilitating sample rates of up to 2 kHz. Results from testing the pilot turbine at three differing facilities during the development process are presented. Here good agreement, less than 7 % variation, was found when comparing the testing undertaken at various flume and tow tank facilities. Lastly, the findings of a test campaign to characterise the performance of each of the three turbines are presented. Very good agreement in non-dimensional values for each of the three manufactured turbines was found. • Outlines the development of three 1/20th scale horizontal axis tidal turbines. • Presents the blade development undertaken to create an optimum turbine rotor. • Details of the drivetrain, instruments and control systems design are given. • Tests at differing facilities and the same facility for similar devices presented. • The paper discusses aspects of good practice for flume/tow-tank testing. [ABSTRACT FROM AUTHOR]

Abstract: Lean premixed swirl combustion is widely used in gas turbines and many other combustion Processes due to the benefits of good flame stability and blow off limits coupled with low NO x emissions. Although flashback is not generally a problem with natural gas combustion, there are some reports of flashback damage with existing gas turbines, whilst hydrogen enriched fuel blends, especially those derived from gasification of coal and/or biomass/industrial processes such as steel making, cause concerns in this area. Thus, this paper describes a practical experimental approach to study and reduce the effect of flashback in a compact design of generic swirl burner representative of many systems. A range of different fuel blends are investigated for flashback and blow off limits; these fuel mixes include methane, methane/hydrogen blends, pure hydrogen and coke oven gas. Swirl number effects are investigated by varying the number of inlets or the configuration of the inlets. The well known Lewis and von Elbe critical boundary velocity gradient expression is used to characterise flashback and enable comparison to be made with other available data. Two flashback phenomena are encountered here. The first one at lower swirl numbers involves flashback through the outer wall boundary layer where the crucial parameter is the critical boundary velocity gradient, Gf . Values of Gf are of similar magnitude to those reported by Lewis and von Elbe for laminar flow conditions, and it is recognised that under the turbulent flow conditions pertaining here actual gradients in the thin swirl flow boundary layer are much higher than occur under laminar flow conditions. At higher swirl numbers the central recirculation zone (CRZ) becomes enlarged and extends backwards over the fuel injector to the burner baseplate and causes flashback to occur earlier at higher velocities. This extension of the CRZ is complex, being governed by swirl number, equivalence ratio and Reynolds Number. Under these conditions flashback occurs when the cylindrical flame front surrounding the CRZ rapidly accelerates outwards to the tangential inlets and beyond, especially with hydrogen containing fuel mixes. Conversely at lower swirl numbers with a modified exhaust geometry, hence restricted CRZ, flashback occurs through the outer thin boundary layer at much lower flow rates when the hydrogen content of the fuel mix does not exceed 30%. The work demonstrates that it is possible to run premixed swirl burners with a wide range of hydrogen fuel blends so as to substantially minimise flashback behaviour, thus permitting wider used of the technology to reduce NOx emissions. [Copyright &y& Elsevier]

• Variable friction based on seabed morphology aids hydrodynamic model calibration. • High mesh refinement required to capture small but influential bathymetric features. • Implications of these model configurations for tidal energy developments are discussed Representation of the marine environment is key for reliable coastal hydrodynamic models. This study investigates the implications of common depth-averaged model configuration choices in sufficiently characterising seabed geometry and roughness. In particular, applications requiring a high level of accuracy and/or exhibiting complex flow conditions may call for greater detail in marine environment representation than typically adopted in coastal models. Ramsey Sound, a macrotidal strait in Pembrokeshire, Wales, UK is considered as a case study. The site contains various steeply inclined bathymetric features, including a submerged pinnacle named Horse Rock and a rocky reef called "The Bitches". The available energy in Ramsey Sound's tidal currents has attracted attention from tidal energy developers. There is interest in accurately modelling the energetic hydrodynamics surrounding its pronounced bathymetry. The coastal flow solver Thetis is applied to simulate the flow conditions in Ramsey Sound. It is shown that notable prominent bathymetric features in the strait influence localised and, most importantly, regional hydrodynamic characteristics. "The Bitches" consistently accelerate flow in the strait while Horse Rock induces a notable wake structure and flow reversals. The model is calibrated against bed- and vessel-mounted Acoustic Doppler Current Profiler (ADCP) observations, by altering seabed roughness parameterisations. A spatially variable and locally scaled Manning coefficient based on diverse seabed classification observations is found to improve model performance in comparison to uniformly applied constants, the latter a more common approach. The local impact of altering the Manning coefficient configuration is found to be greatest during spring flood periods of high velocity currents. Meanwhile, the effect of coarsening the computational mesh around bathymetric features towards values more typically applied in coastal models is investigated. Results indicate severe misrepresentation of seabed geometry and subsequent wake hydrodynamics unless refined to a mesh element size that adequately represents Horse Rock and "The Bitches". [ABSTRACT FROM AUTHOR]

A comparison of a tidal turbine's performance and structural loads is conducted using lab-scale numerical models and experimental testing under multiple current-only and wave-current conditions at the IFREMER wave-current flume. Experimental testing, used to validate CFD models, was accomplished using a 0.9 m diameter, 3-bladed tidal turbine and had a blockage ratio of 8% while the turbine was submerged. Initial investigations analysed the performance and loads on the turbine under uniform and profiled current-only conditions. The presence of a profiled velocity gradient was found to have a negligible effect on the average performance characteristics; however, transient thrust, torque and out of plane bending moment loads experienced much greater variations. These load fluctuations were further increased with increasing levels of shear in the velocity profile, while peaks in the turbine loads coincided with its rotational frequency. The addition of regular, Stokes 2nd Order Theory waves added to the complexity of the flow conditions experienced by the turbine. The effect on the average performance characteristics were negligible while the total turbine thrust and torque fluctuations increased by 35 times that of the current-only cases. Peaks in the loads aligned with the wave surface elevation, indicating the importance of transient analyses of dynamic loads. • The mode of the streamwise velocity accurately represents predominant flow conditions. • Mean turbine performance was unaffected by profiled velocity gradients and waves. • The turbine blade loading fluctuations increased with increasing velocity shear. • Blade loading fluctuations, due to waves, were the same as the highest shearing velocity. • Optimised free surface, multiphase, CFD models were successfully validated experimentally. [ABSTRACT FROM AUTHOR]

To understand the influence of complex hydrodynamic loads on tidal turbines, laboratory testing is necessary as a first approach. Previous laboratory work undertaken gave an indication that the use of speed control strategies may disguise the associated loading range that a turbine may be subjected to when this is operated with a variable speed control strategy. However, the preceding work was undertaken in a highly controlled environment without the influence of turbulent flows. The focus of this paper is directed towards the study of wave-induced loads on tidal turbines when these are controlled using two strategies and the impact that these parameters have on the turbine's performance when this is operated in a recirculating flume. Laboratory tests were undertaken with a 0.9 m diameter horizontal axis tidal turbine subjected to combined wave and current conditions with both regular and irregular waves. Constant speed and constant torque control strategies have been considered, for which rotor thrust, torque and blade root bending moment have been measured. Results show that similar to previous studies, average loads and power capture values remain unchanged between control strategies and the superposition of waves to the current. However, signal fluctuations are 2 to 3 times higher for torque control than for constant speed control strategy. A phase difference between the periodic signals of the turbine thrust and the incoming waves was also identified, in this case, the phase variation was lower when using torque than speed control. This work thus demonstrates the implication of studying strategies to control a marine converter from early stages of development. • Recirculating tank testing demonstrated the loading and performance effects when using two control methodologies. • Thrust, torque and root bending moment fluctuations are higher when using torque control than speed control. • Phase differences between the periodic signals of the turbine thrust and the incoming waves is analysed. • The effect of combined turbulent flows and regular and irregular wave interaction is presented. [ABSTRACT FROM AUTHOR]