• Journal of Korean Society of Coastal and Ocean Engineers
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• v.24 no.2
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• pp.77-83
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• 2012

Numerical analyses through Computational Fluid Dynamics have been performed to investigate the seawater flow field characteristics for various shrouds used in horizontal axis tidal current turbine systems. Seawater flow characteristics are largely influenced under constant tidal current velocity by the shroud geometry and there is considerable difference in fluid velocity distributions around the shrouds. Especially the location and magnitude of maximum seawater flow velocity directly affect turbine performance for power generation. For the cylinder-diffuser type shroud system whose cylinder and diffuser parts have the same length accelerated flow region is formed in the overall cylinder part while maximum velocity in the nozzle-diffuser type whose nozzle and diffuser parts have the same length with symmetry, locally appears near the minimum sectional area. In case of cylinder-diffuser type shroud fluid velocity increases rather high compared with current velocity. And fluid velocity at the centerline gradually increases from the entrance, and then decreases rapidly after reaching a peak close to the middle of the cylinder part unlike the nozzle-diffuser while there is not much variation near the rear of the shroud. These results of the seawater flow characteristics with various shroud geometries can be applied to optimal design for the development of efficient tidal current power generation systems.

• Proceedings of KOSOMES biannual meeting
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• 2017.04a
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• pp.94-94
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• 2017

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.11
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• pp.3639-3648
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• 1996

The effect of 15% pinched diffuser in a centrifugal air compressor with a cascade airfoil diffuser on the aerodynamic performance is investigated using a numerical approach. The commercial CFD code for three-dimensional, turbulent, compressible flow fields is executed for various mass flow rates at a design speed which can be obtained as long as the calculation succeeds. The pinched diffuser is found to help improve the instability of flow within vaneless diffuser space, especially the reverse flow near shroud, and to change both stall/surge line and choking line to increase the surge margin. It is also found to generate more favorable increase of static pressure in diffuser region, and to increase the resulting pressure ratio and efficiency.

• The KSFM Journal of Fluid Machinery
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• v.15 no.2
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• pp.45-50
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• 2012

The flow characteristics in a vaneless diffuser with a backswept radial impeller have been experimentally investigated according to the variation of discharge flow rate. Particle image velocimetry(PIV) system was applied to measure velocity fields with several operating conditions and on some diffuser horizontal planes. Pressure transducers were installed on hub wall of the diffuser in order to analyze the pressure fluctuations and their corresponding velocity fields. The results show that the location of the main flow center moves from the hub to the shroud side as the flow rate decreases, and the reverse flow is locally generated on the hub side.

• The KSFM Journal of Fluid Machinery
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• v.2 no.1 s.2
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• pp.64-72
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• 1999

CSCM upwind flux difference splitting compressible Navier-Stokes method has been used to predict the transonic flows in a centrifugal compressor diffuser. The modified cyclic. TDMA and the mass flux boundary conditions were used as boundary conditions of the diffuser analysis. Broad flow separation on the suction surface near the hub and shroud was observed from the results of the mass flow rates 5.8, 6.0 and 6.2kg/s at 27000 rpm. The three-dimensional flow analysis predicted successfully that the static pressure increased and the total pressure decreased through the flow passage of the channel diffuser when compared to two-dimensional analysis due to the strong effect of the three-dimensional flow. The mass averaged loss coefficients and pressure coefficients were also studied.

• 유체기계공업학회:학술대회논문집
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• 1998.12a
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• pp.217-222
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• 1998

CSCM upwind flux difference splitting compressible Navier-Stokes method has been used to predict the transonic flows in centrifugal compressor diffuser. The modified cyclic TDMA and the mass flux boundary conditions were used as boundary conditions of the diffuser analysis. With the mass flux boundary condition and the $130{\times}80{\times}40$ grid, the compressible upwind Navier-Stokes method predicted the transonic diffuser flowfield successfully. Plow changes in the impeller exit region due to the strong interaction between impeller exit and vaned diffuser, broad flow separation on the suction surface near hub and shroud was observed from the results of the mass flow rates 6.0 and 6.2kg/s at 27000 rpm. The static pressure increased and the total pressure decreased through the flow passage of the channel diffuser, which were predicted better from the three-dimensional analysis than from the two-dimensional analysis due to the strong effect of the three-dimensional flow. The mass averaged loss coefficients and pressure coefficients were also studied.

• Wind and Structures
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• v.19 no.2
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• pp.199-217
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• 2014

Reported experimental and computational fluid dynamic (CFD) studies have demonstrated significant power augmentation of diffuser shrouded horizontal axis micro wind turbine compared to bare turbine. These studies also found the degree of augmentation is strongly dependent on the shape and geometry of the diffuser such as length and expansion angle. However study flow field over the rotor blades in shrouded turbine has not received much attention. In this paper, CFD simulations of an experimental diffuser shrouded micro wind turbine have been carried out with the aim to understand the mechanisms underpinning the power augmentation phenomenon. The simulations provide insight of the flow field over the blades of bare wind turbine and of shrouded one elucidating the augmentation mechanisms. From the analysis, sub-atmospheric back pressure leading to velocity augmentation at the inlet of diffuser and lowering the static pressure on blade suction sides have been identified as th dominant mechanisms driving the power augmentation. And effective augmentation was achieved for ${\lambda}$ above certain value. For the case turbine it is ${\lambda}$ greater than ${\approx}2$.

• Nuclear Engineering and Technology
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• v.50 no.3
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• pp.368-378
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• 2018

Understanding of turbulent flow in the reactor coolant pump (RCP) is a premise of the optimal design of the RCP. Flow structures in the RCP, in view of the specially devised spherical casing, are more complicated than those associated with conventional pumps. Hitherto, knowledge of the flow characteristics of the RCP has been far from sufficient. Research into the nonintrusive measurement of the internal flow of the RCP has rarely been reported. In the present study, flow measurement using particle image velocimetry is implemented to reveal flow features of the RCP model. Velocity and vorticity distributions in the diffuser and spherical casing are obtained. The results illuminate the complexity of the flows in the RCP. Near the lower end of the discharge nozzle, three-dimensional swirling flows and flow separation are evident. In the diffuser, the imparity of the velocity profile with respect to different axial cross sections is verified, and the velocity increases gradually from the shroud to the hub. In the casing, velocity distribution is nonuniform over the circumferential direction. Vortices shed consistently from the diffuser blade trailing edge. The experimental results lend sound support for the optimal design of the RCP and provide validation of relevant numerical algorithms.

• International Journal of Fluid Machinery and Systems
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• v.6 no.2
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• pp.87-93
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• 2013

In order to study the effect of the fluid-structure interaction (FSI) on the simulation results, the external characteristics and internal flow features of a diffuser pump were analyzed with a two-way flow solid coupling method. And the static and dynamic structure analysis of the blade was also caculated with the FEA method. The steady flow field is based on Reynolds Averaged N-S equations with standard $k-{\varepsilon}$ turbulent model, the unsteady flow field is based on the large eddy simulation, and the structure response is based on elastic transient structural dynamic equation. The results showed that the effect of FSI on the head prediction based on CFD really exists. At the same radius, the van mises stress on the nodes closed shroud and hub was larger than other nodes. A large deformation region existed near inlet side at the middle of blades. The strength of impeller satisfied the strength requirement with static stress analysis based on the fourth strength theory. The dynamic stress varied periodically with the impeller rotating. It was also found that the fundamental frequency of the dynamic stress is the rotating frequency and its harmonic frequency. The frequency of maximum stress amplitude at node 1626 was 7 times of the rotating frequency. The frequency of maximum stress amplitude at node 2328 was 14 times of the rotating frequency. No matter strength failure or fatigue failure, the root of blades near shroud is the key region to analyse.

• International Journal of Fluid Machinery and Systems
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• v.5 no.2
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• pp.65-71
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• 2012

The objective of the present investigation is to explore the possibility of improving the performance of a centrifugal fan at low Reynolds numbers using a simple passive means, namely Gurney flap (GF). GFs of 1/$8^{th}$ inch brass angle (3.175 mm) corresponding to 15.9% of blade exit height or 5.1% of blade spacing at the impeller tip are attached to the impeller blade tip on the pressure surface. Performance tests are carried out on the centrifugal fan with vaneless diffuser at five Reynolds numbers (viz., 0.30, 0.41, 0.55, 0.69, $0.82{\times}10^5$, i.e., at five speeds respectively at 1,100, 1,500, 2,000, 2,500 and 3,000 rpm) without and with GF. Static pressures on the vaneless diffuser hub and shroud are also measured for each speed at four flow coefficients [${\phi}$=0.23 (below design flow coefficient), ${\phi}$=0.34 (design flow coefficient), ${\phi}$=0.45 (above design flow coefficient) and ${\phi}$=0.60 (above design flow coefficient)] with and without GF. From the performance curves it is found that the performance of the fan improves considerably with GFs at lower Reynolds numbers and improves marginally at higher Reynolds number. Similar improvements are observed for the static pressures on the diffuser hub and shroud. The effect of Reynolds number on the performance and static pressures is considerable. However the effect is reduced with GFs.