Diffuser Shroud Research Articles

Low-flow ocean current turbine diffuser shroud concentrated and accelerated flow technical research

Ocean current energy is the kinetic energy of seawater flow and is a low-cost, low-carbon, clean, and renewable energy source. Ocean current energy is mainly captured by the Ocean Current Turbine (OCT). However, as the depth of the water increases, the flow rate of the water decreases significantly. The main purpose of this paper is to study the low efficiency of current turbine capture due to low current velocities, with the main objective of achieving the effect of gathering and increasing the speed of low current energy. Firstly, this paper investigates the mechanism and mathematical model of the concentrated and accelerated flow of the diffuser shroud. Secondly, the velocity and pressure flow fields of the basic diffuser shroud are simulated and analyzed under different low flow velocity conditions (flow velocity below 1.0 m/s), different aspect ratios, and different diffusion opening angles. This paper further investigates the effect of speed increase for four different airfoil sections of diffuser shroud. Finally, measurements and verification are carried out with Particle Image Velocimetry (PIV) circular water hole experiments. The experimental results demonstrate the concentrated and accelerated results of the diffuser shroud. The basic diffuser shroud has a 39% increase in flow field velocity. The airfoil section diffuser shroud has a better velocity increase on the flow field than the basic diffuser shroud. The flow field flow rate increased to 58%. The optimum location for the speed increase is near the inlet and less than 0.5 times the inlet diameter. Compared to the simulation results, the experimental results of the PIV circulating water hole show an average error of less than 4%. The results of this paper provide a research basis for the efficient capture of current energy in deep-sea low-flow waters and a theoretical basis for the design of low-flow current energy turbines.

Numerical and experimental study on temperature and heat transfer characteristics in centrifugal compressor diffuser

Lubrication oil aerosol deposit and coking in the compressor diffuser is a typical issue for the high power density internal combustion engine (ICE) using the closed crankcase ventilation (CCV) system. The aerosol coking significantly negatively influences the compressor's performance, leading to a penalty on ICE performance. The severity of the coking effect is highly dependent on temperature. In order to predict occurrence of coking and to clarify how to avoid aerosol coking in the compressor, the temperature distribution and heat transfer characteristics are investigated in this study. An experimental investigation on the diffuser temperature distribution at shroud side was carried out first. Numerical simulations that consider the heat transfer between the solid domain and the fluid domain are conducted, aiming at analyzing the temperature distribution of inner surface. The detailed simulation model of small high-pressure-ratio compressor for predicting coking risk is established. Furthermore, the simulation model is validated by the experimental results. The results indicate that the shroud side of the diffuser sees the highest temperature level in the diffuser. Under typical operation conditions, the temperature at the diffuser wall exceeds the threshold coking temperature of lubrication oil aerosol, this location most likely to have severe coking issues. Thus, the location of the shroud plate is the primary region that needs to be cooled down to mitigate the coking issue. Meanwhile, the intensity of heat transfer between the fluid and solid domain in the diffuser region degrades significantly along the flow direction and sees an 80% drop from diffuser inlet to outlet, indicating the requirements for further enhancement of heat transfer in diffuser shroud side along streamwise.

Impact of compact diffuser shroud on wind turbine aerodynamic performance: CFD and experimental investigations

In the present work, the influence of a compact diffuser on aerodynamic behavior of small-scale wind turbine is analyzed using both CFD and experimental approaches. The present work aims to obtain a compact diffuser shroud while ensuring high aerodynamic performance in order to reduce the fabricating costs, materials, and structural loads on the mast. In this regard, a unique optimization approach is employed to optimize the geometrical features of the shroud profile including length of both entrance and diffuser sections, radii of both diffuser and entrance area. The shroud profile is mathematically described using second-order polynomial functions. The optimization findings affirmed that the total length of optimized shroud profile is shorter by approximately 6 % than the baseline (Cii) configuration. However, the wind speed obtained from the optimized shroud profile shows an improvement of 1.58% compared to the baseline configuration at the throat area. A high-fidelity CFD simulation of the wind turbine equipped with an optimized shroud profile using commercial software SATR-CCM+ was performed to evaluate the overall aerodynamic performance of the wind lens system. Both conventional and shrouded wind turbines were investigated for two different cases: constant rotor speed, and constant wind speed. Furthermore, an experimental test was implemented for both conventional and shrouded turbines under various environmental conditions using an open-loop wind tunnel. A systematic comparison between CFD results with the corresponding experimental data for both turbines is performed. It is evident that placing an optimized shroud around the rotor improves the aerodynamic performance more than twofold compared to the bare turbine. Moreover, results revealed that Cp obtained from shrouded Wind Turbine increased by approximately 66.4 % compared to the conventional one based on CFD results, whereas, it has increased by 69.3 % according to the experimental data.

A Simple Theory and Performance Prediction for a Shrouded Wind Turbine with a Brimmed Diffuser

We developed a new wind turbine system that consists of a diffuser shroud with a broad-ring brim at the exit periphery and a wind turbine inside it. The shrouded wind turbine with a brimmed diffuser, which we called a “wind lens turbine” (WLT), has demonstrated power augmentation by a factor of about 2–5 compared with a bare wind turbine for a given turbine diameter and wind speed. The increase in power output depends on the diffuser shape and length and the brim height. However, a simple theory presented in this paper argues that only two performance coefficients are needed to predict the performance of WLT. The coefficients are the back pressure coefficient of the brim and the pressure recovery coefficient of the diffuser. We theoretically showed that the back pressure coefficient was particularly important for the performance of WLT. Finally, the simple theory was evaluated with experimental results. The results showed good agreement with each other.

Centrifugal Compressor Stall Control by the Application of Engineered Surface Roughness on Diffuser Shroud Using Numerical Simulations.

Downsizing in engine size is pushing the automotive industry to operate compressors at low mass flow rate. However, the operation of turbocharger centrifugal compressor at low mass flow rate leads to fluid flow instabilities such as stall. To reduce flow instability, surface roughness is employed as a passive flow control method. This paper evaluates the effect of surface roughness on a turbocharger centrifugal compressor performance. A realistic validation of SRV2-O compressor stage designed and developed by German Aerospace Center (DLR) is achieved from comparison with the experimental data. In the first part, numerical simulations have been performed from stall to choke to study the overall performance variation at design conditions: 2.55 kg/s mass flow rate and rotational speed of 50,000 rpm. In second part, surface roughness of magnitude range 0–200 μm has been applied on the diffuser shroud to control flow instability. It was found that completely rough regime showed effective quantitative results in controlling stall phenomena, which results in increases of operating range from 16% to 18% and stall margin from 5.62% to 7.98%. Surface roughness as a passive flow control method to reduce flow instability in the diffuser section is the novelty of this research. Keeping in view the effects of surface roughness, it will help the turbocharger manufacturers to reduce the flow instabilities in the compressor with ease and improve the overall performance.

Characterization of aerodynamic performance of wind-lens turbine using high-fidelity CFD simulations

Wind-lens turbines (WLTs) exhibit the prospect of a higher output power and more suitability for urban areas in comparison to bare wind turbines. The wind-lens typically comprises a diffuser shroud coupled with a flange appended to the exit periphery of the shroud. Wind-lenses can boost the velocity of the incoming wind through the turbine rotor owing to the creation of a low-pressure zone downstream the flanged diffuser. In this paper, the aerodynamic performance of the wind-lens is computationally assessed using high-fidelity transient CFD simulations for shrouds with different profiles, aiming to assess the effect of change of some design parameters such as length, area ratio and flange height of the diffuser shroud on the power augmentation. The power coefficient (Cp)is calculated by solving the URANS equations with the aid of the SST k–ω model. Furthermore, comparisons with experimental data for validation are accomplished to prove that the proposed methodology could be able to precisely predict the aerodynamic behavior of the wind-lens turbine. The results affirm that wind-lens with cycloidal profile yield an augmentation of about 58% increase in power coefficient compared to bare wind turbine of the same rotor swept-area. It is also emphasized that diffusers (cycloid type) of small length could achieve a twice increase in power coefficient while maintaining large flange heights.

Counter-Propagating Rotating Stall of Vaned Diffuser in a Centrifugal Compressor Near Design Condition

Abstract Unstable flow in the vaned diffuser of a centrifugal compressor stage near design condition is investigated by numerical simulation. The simulation is performed with the unsteady Navier–Stokes equations and three different turbulence models. Comparisons of the characteristic curves of the stage have been carried out between simulation and experiment, and good agreement is obtained. The fluctuation signals near the interface between the impeller and the diffuser are also quantitatively analyzed from macro and micro scales under a series of mass flow conditions. The simulation results show that an abnormal counter-propagating rotating stall in the vaned diffuser is detected near design condition. The rotating stall displays that four stall cells near the shroud side of the vaned diffuser propagate slowly at a rate of approximately 0.675% of the impeller rotating frequency along the opposite direction of the impeller rotation. The propagation speed increases as the mass flow decreases. The generation and propagation mechanisms of this phenomenon are elucidated, respectively. It is found that flow separation near the diffuser shroud side is produced due to the spanwise variation of flow angles near the impeller exit, which leads to the generation of stall cells in the diffuser. The circumferential propagation of the rotating stall cells is propelled by the accumulation and release of the fluid energy from impeller passages. Further studies show that this phenomenon can be restrained by modifying the installation angle of diffuser vanes.

Performance characteristics of shrouded horizontal axis hydrokinetic turbines in yawed conditions

During operations, river and tidal hydrokinetic turbines encounter changes in flow direction that decrease their performance. Evaluating hydrokinetic turbines in yaw conditions contributes to estimating turbine performance, power stability, and power delivered to the grid. Water tunnel tests using a 19.8 cm diameter horizontal axis model turbine in yaw operation show the performance reduction using three designs: no shroud, a convergent-divergent shroud, and a diffuser shroud. Experimental results obtained at three Reynolds numbers of 1.38 × 105, 1.77 × 105, and 2.17 × 105 show that the output power decreases in off-axis flows for all designs investigated. The reduction is initially negligible up to a 10° yaw angle, but then increases with increasing the yaw angle. The convergent–divergent shroud experiences significantly less performance loss compared to the other two designs. The maximum performance loss of the convergent-divergent shroud design is 6% on average for the Reynolds numbers investigated. Based on the experimental results and in analogy to the linear momentum theory cosine cubed rule, we propose new cosine relations for shrouded turbines in yaw operations. Depending on the shroud's geometry, the maximum power of a shrouded turbine decreases either with the cosine or the cosine squared of the yaw angle.

CFD Analysis of Shrouded Diffuser Wind Turbine

The power in the wind is well known to be proportional to the cubic power of the wind velocity approaching a wind turbine. This means that even small amount of its acceleration gives large increase in the power output. Brimmed diffuser shrouds for small wind turbines are being used to accelerate the wind velocity in small wind regimes. The objective of the Paper is to analyze the flow characteristics of brimmed diffuser shroud and to optimize the physical dimensions. CFD analyses are carried out by varying its physical dimensions with the aim of achieving augmented velocity. The effect of flow parameters with the presence of diffuser is analyzed by comparing it with bare wind turbine. The physical dimensions of brimmed diffusers are the parameters considered in this study. The study has been carried with proposed splitted diffuser design. The power yield of the turbine for shifting speeds is gotten and analyzed.The CFD tool CFX would be used to anlayse the flow field around the diffuser. Performance of wind turbine under various operating conditions is generally obtained through an experimental testing and could be cost prohibitive. In this case the computational fluid dynamics analysis provides better results. The capability of using computational fluid dynamics is a test to determine its viability for determining its performance parameters.

Computational fluid dynamic analysis of roof-mounted vertical-axis wind turbine with diffuser shroud, flange, and vanes

Advances in wind power and tidal power have matured considerably to offer clean and sustainable energy alternatives. Nevertheless, distributed small-scale energy production from wind in urban areas has been disappointing because of very low efficiencies of the turbines. A novel wind turbine design — a seven-bladed Savonius vertical-axis wind turbine (VAWT) that is horizontally oriented inside a diffuser shroud and mounted on top of a building — has been shown to overcome the drawback of low efficiency. The objective this study was to analyze the performance of this novel wind turbine design for different wind directions and for different guide vanes placed at the entrance of the diffuser shroud. The flow field over the turbine and guide vanes was analyzed using computational fluid dynamics (CFD) on a 3D grid for multiple tip-speed ratios (TSRs). Four wind directions and three guide-vane angles were analyzed. The wind-direction analysis indicates that the power coefficient decreases to about half when the wind is oriented at 45° to the main axis of the turbine. The analysis of the guide vanes indicates a maximum power coefficient of 0.33 at a vane angle of 55°.

Analysis and optimization of ducted wind turbines

Wind-lens turbines offer the potential for better energy efficiency and better suitability for urban and suburban environments compared to unshrouded or bare wind turbines. Wind-lenses, which are typically comprised of a diffuser shroud equipped with a flange, can enhance the wind velocity at the rotor plane due to the generation of a lower back pressure. In this article, the wind-lens efficiency is increased by designing the shroud and turbine shape as well as flange height through an optimization process that seeks to maximize the power while minimizing drag and thrust forces. The employed optimizer is a multi-objective genetic algorithm (MOGA). Bezier curves are used to define the chord and twist distribution of the turbine blades and a piece-wise quadratic polynomial is utilized to define the shroud shape. The power, thrust, and drag coefficients are calculated by solving the Reynolds-averaged-Navier-Stokes (RANS) equations with the k−ε turbulence model for the flow within and around the diffuser augmented wind turbine using the open source code software OpenFOAM. To reduce the computational cost, the turbine rotor itself is modeled by incorporating blade element momentum model body forces into the RANS equations. Realistic rotor data for the sectional lift and drag coefficients for all angles of attacks are utilized via look-up tables. Grid convergence studies for verification and comparisons with experiments for validation are carried out to demonstrate that the adopted methodology is able to accurately predict the performance of a wind-lens prior to performing shape optimizations. It will be demonstrated that the resulting optimal designs yield significant improvements in the output power coefficient.

Wind turbine designs for urban applications: A case study of shrouded diffuser casing for turbines

The increased demand for renewable energy and the development of energy independent building designs have motivated significant research into the improvement of wind power technologies that target urban environments. However, the implementation of wind turbines in urban environments is still very limited. There have been some studies analyzing different designs of urban wind turbines either using computational fluid dynamics (CFD), wind tunnel tests or field data for existing or new turbine designs. This paper reviews the state-of-the-art of urban wind energy by examining the various types of urban wind turbine designs, with a view to understand their performance and the synergy between the turbines and the urban environments. It also considers a flanged diffuser shroud mechanism - a fluid machine, mounted on rooftop of buildings used as casing for small wind turbines to improve turbine performance by using mainly CFD. The diffuser shroud mechanism can draw the airflow over buildings utilizing its special features such as, cycloidal curve geometry at the inlet and a vortex generating flange at the outlet, to guide and accelerate the airflow inside. The performance of the fluid machine is optimized parametrically. The mechanism is modeled on a building rooftop in a real test site in Montreal, Canada with real statistical wind data. The CFD result confirms the functionality of the fluid machine to take advantage of the airflow over buildings in complex built-environments for wind power generation.

Computational analysis of flow fields around flanged diffusers

Computation was done for flow fields around flanged diffusers to study small-type wind turbines. In these calculations, Spalart Allmaras &k–ε RNG turbulent models were used for solving corresponding Reynolds Average equations. Comparison of the computed results shows agreement with the corresponding experimental data. This indicates that the present models have the capability of providing reasonable predictions for the present complex turbulent flows. The wind turbine equipped with a flanged and inlet diffuser shroud which we study demonstrates by realizing remarkable increase in wind speed of 1.6–2.1 times that of the approaching wind speed. Our calculations under various conditions of diffuser opening angle and the flanged height, it is shown that the performance of a flanged diffuser strongly depends on the opening angle and flanged height because it greatly affects the nature of the separation appearing inside the diffuser. Finally, the effect of adding the diffuser inlet; explicitly reported.

Study of the Flow in a Vaneless Diffuser at Part Speed Operating Conditions

The aim of the current paper is to investigate the evolution of the flow in the vaneless diffuser of a scaled-up turbocharger compressor using the steady-state viscous calculations. First, the predicted flow patterns are compared with the detailed laser Doppler anemometry (LDA) and probe measurements acquired at different operating conditions. Then, the numerical results are analyzed further to understand the origins of the flow features observed in the measurements. It is shown that, despite the complexity of the flow, the main flow features in the vaneless diffuser can be predicted by computational fluid dynamics (CFD) at different operating conditions using a simple mixing length turbulence model. It is also demonstrated that, in the compressor studied, a vortical flow feature develops near the diffuser shroud at high flow coefficients. This flow feature is generated by strong pressure variations downstream of the impeller trailing edge and disappears at low flow coefficients, where the static pressure gradient becomes almost radial in the vaneless diffuser.

Performance Modeling of Ducted Vertical Axis Turbine Using Computational Fluid Dynamics

AbstractVertical axis turbines (VATs) excel over horizontal axis turbines in their independent flow direction. VATs that operate in an enclosure, e.g., a diffuser shroud, are reported to generate more power than unducted VATs. A diffuser-shrouded, high solidity of 36.67%, three-blade VAT with NACA 633-018 airfoil section is modeled in 2-D using the commercial software ANSYS-FLUENT®. Incompressible, unsteady, segregated, implicit, and second order in time and space solver is implemented in association with the Spalart-Allmaras turbulent model with a reasonable computational cost. The computational results are assessed against experimental data for unducted VAT at low tip speed ratios between 1 and 2 for further numerical analysis on diffuser models. Different diffuser designs are investigated using suitable nozzle size, area ratio, length-to-diameter ratio and angles between the diffuser inner surfaces. The numerical model shows that, for a specific diffuser design, the ducted VAT performance coefficient can be augmented by almost 90% over its unducted counterpart.

Behavior of the Blade Tip Vortices of a Wind Turbine Equipped with a Brimmed-Diffuser Shroud

To clarify the behavior of the blade tip vortices of a wind turbine equipped with a brimmed-diffuser shroud, called a “Wind-Lens turbine”, we conducted a three-dimensional numerical simulation using a large eddy simulation (LES). Since this unique wind turbine consists of not only rotating blades but also a diffuser shroud with a broad-ring brim at the exit periphery, the flow field around the turbine is highly complex and unsteady. Previously, our research group conducted numerical simulations using an actuator-disc approximation, in which the rotating blades were simply modeled as an external force on the fluid. Therefore, the detailed flow patterns around the rotating blades and the shroud, including the blade tip vortices, could not be simulated. Instead of an actuator-disc approximation, we used a moving boundary technique in the present CFD simulation to simulate the flow around a rotating blade in order to focus especially on blade tip vortices. The simulation results showed a pair of vortices consisting of a blade tip vortex and a counter-rotating vortex which was generated between the blade tip and the inner surface of the diffuser. Since these vortices interacted with each other, the blade tip vortex was weakened by the counter-rotating vortex. The results showed good agreement with past wind tunnel experiments.

An experimental investigation of the flow field between two radial plates

The flow field in a radial diffuser downstream of a radial impeller is highly complex, as the flow is turbulent, unsteady, viscous, and three-dimensional. Depending on the initial conditions at the diffuser inlet, the flow may separate from one or both walls of the diffuser. The objective of this experimental investigation of flow characteristics in a radial vaneless diffuser of a centrifugal blower is to enhance the performance of centrifugal compressors over a wide operating range. Numerical and experimental investigations of rotating stall effects on compressors and blowers have been conducted over the past four decades to achieve this objective. The flow characteristics in a radial diffuser were measured while changing the impeller speed and keeping other parameters such as diffuser radius and width ratio constant. The velocity and pressure distributions in the diffuser were measured along a radial path using a miniature X-wire probe and dynamic as well as static pressure transducers. The axisymmetrical radial and tangential mean velocity distributions and their statistics, as well as pressure distributions, are reported. Some of the results presented in this paper agree with those from earlier work on this subject. For example, the flow angle α (measured at half the diffuser width and from the diffuser inlet to its outlet) did not exceed 65°. This means that α < αc, although a recirculation region was present on the diffuser shroud wall. The critical flow angle αc required to initiate self-excited flow oscillations has been generally reported to be about 78°. Current data also indicate that the flow exiting the impeller is skewed, as revealed by the triple velocity product correlations. The current data are valuable for vaneless diffuser numerical model developers, since there are very limited similar data available in the open literature. The current data support the opinion that the flow separation is not enough to trigger or initiate stall at the diffuser.

A Shrouded Wind Turbine Generating High Output Power with Wind-lens Technology

We have developed a new wind turbine system that consists of a diffuser shroud with a broad-ring brim at the exit periphery and a wind turbine inside it. The shrouded wind turbine with a brimmed diffuser has demonstrated power augmentation by a factor of about 2–5 compared with a bare wind turbine, for a given turbine diameter and wind speed. This is because a low-pressure region, due to a strong vortex formation behind the broad brim, draws more mass flow to the wind turbine inside the diffuser shroud.

Development of a shrouded wind turbine with a flanged diffuser

We have developed a wind turbine system that consists of a diffuser shroud with a broad-ring flange at the exit periphery and a wind turbine inside it. The flanged-diffuser shroud plays a role of a device for collecting and accelerating the approaching wind. Emphasis is placed on positioning the flange at the exit of a diffuser shroud. Namely, the flange generates a low-pressure region in the exit neighborhood of the diffuser by vortex formation and draws more mass flow to the wind turbine inside the diffuser shroud. To obtain a higher power output of the shrouded wind turbine, we have examined the optimal form of the flanged diffuser, such as the diffuser open angle, flange height, hub ratio, centerbody length, inlet shroud shape and so on. As a result, a shrouded wind turbine equipped with a flanged diffuser has been developed, and demonstrated power augmentation for a given turbine diameter and wind speed by a factor of about 4–5 compared to a standard (bare) wind turbine. In a field experiment using a prototype wind turbine with a flanged diffuser shroud, the output performance was as expected and equalled that of the wind tunnel experiment.

Control of stall in a radial diffuser using rough surfaces

Fluid flow instabilities in a compression system set the lower limit of the stable operating range, and choking sets the upper limit of the flow rate. Experiments to investigate the feasibility of controlling the stall in the radial diffuser of a low-speed centrifugal blower were carried out using a very simple technique that involved using rough surfaces (i.e., sandpaper) attached to the diffuser shroud. This choice was based on measurements which showed that the reverse flow occurred on the shroud side for the tested impeller with radial blades. The experimental data showed that the flow instability in the diffuser (stall) was delayed to a lower flow coefficient, which was about 70% of its value with the smooth surface.