Numerical Modelling of a Floating Wind Turbine Semi-Submersible Platform

A detailed study is undertaken of the computational modelling of a sub-platform for floating offshore wind using the software Star-CCM+ with the application of the RANS approach. First, a mathematical introduction to the governing equations is carried out. Then, the computational grid is defined, and the grid-independence of the solution is verified. A time-dependent study is performed with the selected time-step. Finally, two examples of 3D decay tests in heave of the sub-platform without and with moorings are presented, accompanied by a damping factor study, with the aim of providing a better understanding of the hydrodynamic damping of the platform. Throughout the process, three degrees of freedom (DoFs) are locked due to the limitations imposed by the use of a symmetry plane; this implementation allowed us to reduce the computational cost of each simulation by 50%. Therefore, three DoFs (heave, surge and pitch) are considered. The coupling study, adding a mooring system in the decay tests and the regular wave tests, shows good agreement between the experimental and computational results. The first half-period of the simulations presents a greater discrepancy due to the fact that the damping of the platform is lower in the computational simulation. However, this does not imply that the hydrodynamic damping is underestimated but may be directly related to the lock of various DoFs associated with the hydrodynamic damping.The current investigation was developed under the framework of the European Regional Development Fund through the “Interreg Atlantic Area Programme” under contract EAPA 344/2016, providing experimental inputs to complete this study

Study on the feasibility of the micromix combustion principle in low NOx H2 burners for domestic and industrial boilers: A numerical approach

The purpose of the present investigation is to explore the feasibility of applying the micromix combustion principle (MCP) to design low NOx burners using 100% H2 as fuel for domestic and industrial boilers. Previous investigations studying the MCP on stationary and aero gas turbine applications, showed low NOx emissions without flashback risk, which represent the two main issues when burning 100% H2. Since boiler burner operating conditions differ from gas turbine combustors, the present paper studies the MCP through CFD calculations under re-defined conditions for domestic and industrial burners, adapting the energy densities, air-fuel equivalence ratios and pre-heated air temperatures. A reference geometry was built to validate the selected numerical models with experimental results from literature. Afterwards, burners were re-dimensioned following an existing geometry-scaling methodology. The obtained results evidenced that MCP characteristics were still maintained for the defined cases, keeping low NOx values between 4 and 14 ppm. The numerical results were also validated through experimental NOx measurements in a laboratory scale micromix burner prototype. In order to assess the benefits of using hydrogen micromix burners in domestic and industrial boilers, the present work includes a final discussion with practical design considerations. The present study lays the groundwork for complementary experimental research work, which is being carried out in laboratory-scale prototypes.The authors are grateful to the Basque Government for the support given to this research through projects GIU19/029 and IT1314-19. The authors also want to thank Dr. Bobby Sethi from Cranfield University for the provided guidance in the FGM model for micromix combustion

Thermodynamic Analysis of a Regenerative Brayton Cycle Using H2, CH4 and H2/CH4 Blends as Fuel

Considering a simple regenerative Brayton cycle, the impact of using different fuel blends containing a variable volumetric percentage of hydrogen in methane was analysed. Due to the potential of hydrogen combustion in gas turbines to reduce the overall CO2 emissions and the dependency on natural gas, further research is needed to understand the impact on the overall thermodynamic cycle. For that purpose, a qualitative thermodynamic analysis was carried out to assess the exergetic and energetic efficiencies of the cycle as well as the irreversibilities associated to a subsystem. A single step reaction was considered in the hypothesis of complete combustion of a generic H2/CH4 mixture, where the volumetric H2 percentage was represented by fH2, which was varied from 0 to 1, defining the amount of hydrogen in the fuel mixture. Energy and entropy balances were solved through the Engineering Equation Solver (EES) code. Results showed that global exergetic and energetic efficiencies increased by 5% and 2%, respectively, varying fH2 from 0 to 1. Higher hydrogen percentages resulted in lower exergy destruction in the chamber despite the higher air-excess levels. It was also observed that higher values of fH2 led to lower fuel mass flow rates in the chamber, showing that hydrogen can still be competitive even though its cost per unit mass is twice that of natural gas

A Theoretical Study of the Hydrodynamic Performance of an Asymmetric Fixed-Detached OWC Device

The chamber configuration of an asymmetric, fixed-detached Oscillating Water Column (OWC) device was investigated theoretically to analyze its effects on hydrodynamic performance. Two-dimensional linear wave theory was used, and the solutions for the associated radiation and scattering boundary value problems (BVPs) were derived through the matched eigenfunction expansion method (EEM) and the boundary element method (BEM). The results for the hydrodynamic efficiency and other important hydrodynamic properties were computed and analyzed for various cases. Parameters, such as the length of the chamber and the thickness and submergence of the rear and front walls, were varied. The effects on device performance of adding a step under the OWC chamber and reflecting wall in the downstream region were also investigated. A good agreement between the analytical and numerical results was found. Thinner walls and low submergence of the chamber were seen to increase the efficiency bandwidth. The inclusion of a step slightly reduced the frequency at which resonance occurs, and when a downstream reflecting wall is included, the hydrodynamic efficiency is noticeably reduced at low frequencies due to the near trapped waves in the gap between the OWC device and the rigid vertical wall.The current investigation was developed under the framework of the Basque Government (IT1314-19 research group). The authors additionally thank the funding provided by CEMIE-Océano (Mexican Centre for Innovation in Ocean Energy). Project FSE-2014-06-249795 financed by CONACYT-SENER Sustentabilidad Energética

Thermal Performance Assessment of Walls Made of Three Types of Sustainable Concrete Blocks by Means of FEM and Validated through an Extensive Measurement Campaign

The thermal behavior of three different walls, made with and without by-products, is assessed by means of the Finite Element Method, aiming to evaluate its performance in terms of the sustainable construction of the blocks. Results were compared to those obtained from an experimental campaign, aiming at validation of the model. The by-products used for the blocks were “lime sludge” and “sawdust”, whose performance was compared against the traditional blocks made of concrete as a reference, aiming to demonstrate its sustainability, showing decreases of the thermal transmittance up to 10.5%. Additionally, following the same methodology, the thermal behavior of these above-mentioned blocks but now with added internal insulation made of “recycled cellulose” was assessed, showing higher decreases up to 25.5%, increasing sustainability by addressing an additional reduction in waste, so the right combination of using by-products and the insulating filler in their cavities has been revealed as a promising way of optimizing the walls, offering a relevant improvement in energy savings. Finally, when comparing the U-values of the blocks made of concrete without insulation versus those made of by-products, with insulation, improvements up to 33.3% were reached. The adaptation of the procedure through a moisture correction factor was also incorporated

Energia-teknologia: taulak eta abakoak

Helburuak: Ikasmaterial honen helburu nagusiak honako hauek dira: Beroaren transmisioa irakasteko behar diren taula, abako eta korrelazio batzuk biltzea, ikasleak ikasturtean zehar irakasgaia hobeto jarrai dezan. Horretaz gain energia-teknologiako beste arlo batzuen (errekuntza, aire hezea eta hozte teknologia) taulak eta abakoak biltzea ere. Gai hauekin erlazionaturiko ariketa multzoa ere ageri da testuan emaitzekin. Norentzat: Industria Ingeniaritzako 4. mailako ikasleentzat. Baita Ingeniaritza Kimikoan Beroaren Transmisioa ikasten dabiltzan 2. mailako ikasleentzat, edota Industria Antolakuntzan bigarren zikloko lehenengo mailako ikasleentzat. 2010-11 ikasturtetik aurrera indarrean egongo diren gradu ikasketetan bigarren mailan irakatsiko den Termoteknia izeneko irakasgaian lagungarria ere izango da.Liburu honetan atal hauek agertzen dira: Unitateak eta bihurketa-faktoreak, beroaren transmisioa (konduzkioa, konbekzioa eta erradiazioa), errekuntza, aire hezea, hozte-makinak eta ariketak.Liburu honek UPV/EHUko Euskara eta Eleaniztasuneko Errektoreordetzaren dirulaguntza jaso d

Experimental and numerical study of NOx formation in a domestic H2/air coaxial burner at low Reynolds number

Thermal NOx formation in H2 /air jet flames from a coaxial burner is studied experimentally and numerically. The aim is to study possible NOx reduction strategies for domestic gas boiler burners. Following a flame splitting method strategy, a single burner is studied at different inlet powers (from 0.2 to 1.0 kW). The effect of three different fuel-air ratios (or equivalence ratio φ) is considered by varying the coaxial air stream, with fuel-air ratios corresponding to values of φ < 1, relevant for domestic boiler applications (here φ = 0.77, φ = 0.83 and φ = 0.91). NOx concentrations increase with increasing inlet power between 0.2 and 0.6 kW and numerical results are in good correspondence with available experimental data. The opposite trend is observed above 0.6 kW and no numerical results are obtained, indicating a transition from laminar to turbulent flames. On the other hand, in contrast to the observations made in turbulent non-premixed flames, reducing the equivalence ratio implies higher NOx concentrations in the low Reynolds number flames considered. The numerical results in the laminar regime are used to highlight and quantify three competing main factors concerning NOx production in order to interpret the experimental observations: the volume of the region where NOx is produced, and within this region, the competition between residence time and NOx reaction rate. Based on this analysis, different design strategies for low NOx hydrogen diffusion burners are finally discussed.The authors are grateful to the Basque Government for funding this research through projects IT781-13 and IT1314-19. Part of this work is also supported at Ciemat by the project #PID2019-108592RB- C42/AEI/10.13039/50110 0 011033

The Influence of the Chamber Configuration on the Hydrodynamic Efficiency of Oscillating Water Column Devices

Based on the two-dimensional linear wave theory, the effects of the front wall thickness and the bottom profile of an Oscillating Water Column (OWC) device on its efficiency were analyzed. Using the potential flow approach, the solution of the associated boundary value problem was obtained via the boundary element method (BEM). Numerical results for several physical parameters and configurations were obtained. The effects of the front wall thickness on the efficiency are discussed in detail, then, various configurations of the chamber bottom are presented. A wider efficiency band was obtained with a thinner front wall. In a real scenario having a thinner front wall means that such a structure could have less capacity to withstand the impact of storm waves. Applying the model for the case of the Mutriku Wave Energy Plant (MWEP), findings showed that the proposed bottom profiles alter the efficiency curve slightly; higher periods of the incoming water waves were found. This could increase the efficiency of the device in the long-wave regime. Finally, the numerical results were compared with those available in the literature, and were found to be in good agreement.The present research has been developed under the framework of CEMIE-Océano (Mexican Centre for Innovation in Ocean Energy). Project FSE-2014-06-249795 financed by CONACYT-SENER- Sustentabilidad Energética. In addition, the authors would like also to express their gratitude for the funding provided by the UPV/EHU [PPGA20/26 research group] and the Basque Government (IT1314-19 research group)

CFD study of flameless combustion in a real industrial reheating furnace considering different H2 /NG blends as fuel

On the path towards decarbonisation of the steel industry, the use of H2/NG blends in furnaces, where high temperatures are needed, is one of the alternatives that needs to be carefully studied. The present paper shows the CFD study carried out for a full-scale reheating furnace burner case. The real operating conditions as well as experimental measurements provided by the furnace operator were used to validate the results and reduce simulation uncertainties. The burner under consideration (2.5 MW) works in flameless mode with natural gas and preheated air (813 K). Starting from this point, another three fuel blends with volumetric percentages of 23% (also known as G222), 50% and 75% of H2 in natural gas were considered. For this purpose, the open source CFD code OpenFOAM was used, where the novel NE-EDC turbulence-chemistry interaction model was implemented, which has already been successfully validated specifically for flameless combustion in a furnace. The implementation incorporated an enhanced approach for calculating the chemical time-scale, coupled with a specific post-processing solver to predict NOx emissions. The study analyses the relative impact of the considered fuel blends on NOx formation and flameless regime. The modelling results demonstrated the burner’s capability to operate efficiently with high concentrations of hydrogen, maintaining flameless regime in all cases. This condition ensured uniform temperature distributions and low levels of NOx emissions, reaching a maximum value of 86 mg/m. These results indicated the proper functionality of the existing natural gas-based burner with H2/NG blends, which was the primary requirement for the conversion process.Thanks to be given to the Research Group: IT1415-22 (ITSAS-REM) and to the ELKARTEK ERABILH2 programme (KK-2021/00086) and H2 PLAN 2023/00187 programme (A/20230061)