• Energy Research

AbstractThis paper discusses the findings from a measurement campaign on a rotating wind turbine blade operating in the free atmosphere under realistic conditions. A total of 40 pressure sensors together with an array of 23 usable hot‐film sensors (based on constant temperature anemometry) were used to study the behavior of the boundary layer within a specific zone on the suction side of a 30 m diameter wind turbine at different operational states. A set of several hundreds of data sequences were recorded. Some of them show that under certain circumstances, the flow may be regarded as not fully turbulent. Accompanying Computational Fluid Mechanics (CFD) simulations suggest the view that a classical transition scenario according to the growth of so‐called Tollmien–Schlichting did not apply. Copyright © 2016 John Wiley & Sons, Ltd.

Helical serrated finned-tubes are well established in many thermal systems. This paper presents the results of numerical calculations carried out for the performance improvement of these devices. The work is divided into three main investigations conducted for Reynolds numbers between Re = 600 and 2600. The first investigation shows the effect of the fin serration, where a comparison between performances of finned tubes with and without fin serration is presented. Another main investigation is conducted on the effect of fin twisting of the outermost part of the fin on the performance of the serrated finned-tubes. Here, twisting angles considered are between β = 0° and 25°. The third investigation deals with the effect of the number of fin segments per period.

Abstract In this work, different modeling approaches are used to study the hydrodynamics of gas–solid flows in a three-dimensional, lab-scale spouted fluidized bed. In particular, the simulation results obtained by the two-fluid model are compared with those of coupled CFD/DEM simulations. To explore the effects of the gas mass flow rate on the ability of the used simulation techniques to predict the flow behavior in the simulated test case, two different fluidization conditions are considered while maintaining the same computational set-up. The spouted fluidized bed has been evaluated by means of high-speed imaging for validating the simulation results. A comparative study of the two modeling approaches with experimental observations for bubble size in terms of the representative bubble diameter and the bed height has been carried out under both operating conditions. To identify significant simulation settings for the two-fluid model, a parameter study is carried out. The investigated input parameters include the gas–particle drag models, the granular temperature approach, the solid-phase wall boundary conditions in terms of specularity coefficient and the particle–particle restitution coefficient. At a gas mass flow rate of 0.005 kg/s, the predicted characteristics of the bubble formation and the bed expansion using both techniques are in very good agreement with experimental observations. Using the same validated settings, the two-fluid model shows some notable discrepancies when the gas mass flow rate is increased to 0.006 kg/s, while the DEM is more successful than the two-fluid model in reproducing realistic flow patterns. Although both techniques predict the right fluidization regimes and trends in bubble sizes and bed expansion, their results deviate significantly from the experimental data during the final stage of the bubble formation. The most important reasons for the differences in predicting the bed dynamics along with the advantages and limitations of the two approaches are discussed.

Abstract. Light detection and ranging (lidar) systems have gained a great importance in today's wake characteristic measurements. The aim of this measurement campaign is to track the wake meandering and in a further step to validate the wind speed deficit in the meandering frame of reference (MFR) and in the fixed frame of reference using nacelle-mounted lidar measurements. Additionally, a comparison of the measured and the modeled wake degradation in the MFR was conducted. The simulations were done with two different versions of the dynamic wake meandering (DWM) model. These versions differ only in the description of the quasi-steady wake deficit. Based on the findings from the lidar measurements, the impact of the ambient turbulence intensity on the eddy viscosity definition in the quasi-steady deficit has been investigated and, subsequently, an improved correlation function has been determined, resulting in very good conformity between the new model and the measurements.

Wall roughness is known to have a significant influence on particle-laden wall-bounded flows directly affecting the particulate and the continuous phase. For sufficiently high mass loading the fluid flow is also indirectly altered by the particles subjected to collisions with rough walls. The paper is concerned with the question how the effect of rough walls on the particulate phase can be modeled taking a minimum of measured or empirically determined physical quantities into account. Following Nikuradse’s idea, a sandgrain roughness model is proposed for the dispersed phase in which the wall is covered by a densely packed layer of sand grains idealized by mono-disperse spheres. Based on geometric considerations relying on generally used roughness parameters such as Rz or Rq the local inclination of the wall is determined in order to predict the inelastic collision of the particles with the wall including friction. The sandgrain model also takes the shadow effect into account leading to asymmetric probability density functions of the wall inclination angles, where the mean normal vector is turned towards the incoming particle trajectory. The wall model applicable in 3-D is evaluated in the context of four-way coupled large-eddy simulations for turbulent plane channel flow but is also applicable in direct numerical simulations or Reynolds-averaged Navier–Stokes predictions. A variety of test scenarios were considered including varying wall roughness values, several mass loadings and different particle sizes.

Abstract. The outlined analysis validates the dynamic wake meandering (DWM) model based on loads and power production measured at an onshore wind farm with small turbine distances. Special focus is given to the performance of a version of the DWM model that was previously recalibrated at the site. The recalibration is based on measurements from a turbine nacelle-mounted lidar system. The different versions of the DWM model are compared to the commonly used Frandsen wake-added turbulence model. The results of the recalibrated wake model agree very well with the measurements, whereas the Frandsen model overestimates the loads drastically for short turbine distances. Furthermore, lidar measurements of the wind speed deficit as well as the wake meandering are incorporated in the DWM model definition in order to decrease the uncertainties.

Abstract. Among a few field experiments on wind turbines for analyzing laminar–turbulent boundary layer transition, the results obtained from the DAN-AERO and aerodynamic glove projects provide significant findings. The effect of inflow turbulence on boundary layer transition and the possible transition mechanisms on wind turbine blades are discussed and compared to CFD (computational fluid dynamics) simulations of increasing fidelity (Reynolds-averaged Navier–Stokes, RANS; unsteady Reynolds-averaged Navier–Stokes, URANS; and large-eddy simulations, LESs). From the experiments, it is found that the transition scenario changes even over a single revolution with bypass transition taking place under the influence of enhanced upstream turbulence, for example, such as that from wakes, while natural transition is observed in other instances under relatively low inflow turbulence conditions. This change from bypass to natural transition takes place at azimuthal angles directly outside the influence of the wake indicating a quick boundary layer recovery. The importance of a suitable choice of the amplification factor to be used within the eN method of transition detection is evident from both the RANS and URANS simulations. The URANS simulations which simultaneously check for natural and bypass transition match very well with the experiment. The LES predictions with anisotropic inflow turbulence show the shear-sheltering effect and a good agreement between the power spectral density plots from the experiment and simulation is found in case of bypass transition. A condition to easily distinguish the region of transition to turbulence based on the Reynolds shear stress is also observed. Overall, useful insights into the flow phenomena are obtained and a remarkably consistent set of conclusions can be drawn.

Abstract Delta-winglet vortex generators (VGs) are known to enhance the heat transfer between the energy-carrying fluid and the heat transfer surfaces in plate-fin-and-tube banks. In this study optimal angles of attack of the delta-winglets are investigated based on the Pareto optimal strategy. The optimization process combines a CFD analysis, genetic algorithms and the response surface methodology. The angle of attack of a pair a delta-winglet-type VGs mounted behind each tube is varied between β = −90° and +90°. Three circular tube rows with inline and staggered tube arrangements are investigated for Reynolds numbers from 200 to 1200 (based on the inlet height and inlet velocity). The flow structure and heat transfer behavior is analyzed in detail for certain cases and the staggered and the inline tube arrangements are compared. Finally, for each of these arrangements the optimal sets of angles of attack for different Reynolds numbers are presented.