• 한국태양에너지학회 논문집
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• 제34권3호
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• pp.98-106
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• 2014

The effect of blockage ratio on wind tunnel testing of small vertical-axis wind turbine has been investigated in this study. Height and rotor diameter of the three blades Darrieus vertical axis wind turbine used in present test were 0.4m and 0.35m respectively. We measured the wind speeds and power coefficient at three different wind tunnels where blockage ratio were 3.5%, 13.4% and 24.7% respectively. The test results show that the measured powers have been strongly influenced by blockage ratio, generally increased as the blockage ratio increases. The maximum power at higher blockage ratio has been obtained at relatively high tip speed ratio compared with that at low blockage ratio. The measured power coefficients under high blockage ratio can be improved from proper correction using the simple correction equation based on blockage factor. In present study, the correction error for power coefficient can be less than 5%, however correction effectiveness reveals relatively poor at high blockage ratio and low wind speed.

• Wind and Structures
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• 제1권4호
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• pp.351-364
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• 1998

In wind tunnel experiments, the blockage effect is a very important factor which affects the test results significantly. A number of investigations into this problem, especially on the blockage correction of drag coefficient, have been carried out in the past. However, only a limited number of works have been reported on the wind tunnel blockage effect on wind-induced vibration although it is considered to be fairly important. This paper discusses the aerodynamic characteristics of the square model and square model with corner cut based on a series of the wind tunnel tests with various blockage ratios and angles of attack. From the test results, the aerodynamic behavior of square models with up to 10% blockage ratio are almost the same and square models with up to 10% blockage ratio can be tested as a group which behaves similarly.

• Journal of Electrochemical Science and Technology
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• 제15권1호
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• pp.132-151
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• 2024

Proton exchange membrane fuel cell (PEMFC) has received extensive attention as it is the most common hydrogen energy utilization device. This research not only investigated the effect of obstacle number and shape on PEMFC performance, but also studied the effect of the blockage degree in the channel of PEMFC on its performance. It was found that compared with traditional scheme, longitudinally distributed obstacles scheme can significantly promote reactants transfer to catalyst layer, and the blockage degree in the channel effect PEMFC performance most. The scheme with 10 rectangular obstacles in single channel and 60% channel blockage had the best output performance and the most uniform distribution of reactants and products. Obstacle height distribution can significantly affect PEMFC performance, the blockage degree in the whole basin was large, particularly as the channel was blocked to higher degree in region 2 and region 3, higher net power density and better mass transfer effect can be obtained. Among them, the fuel cell with the blockage degree of 40%, 60% and 60% in region 1, region 2 and region 3 have the best PEMFC output performance and mass transfer, the net power density was 29.8% higher than that of traditional scheme.

• Wind and Structures
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• 제12권1호
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• pp.49-61
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• 2009

A method is presented to estimate the form drag and the base pressure on a triangular cylinder in the presence of blockage effect. The Strouhal number, which is found to increase with the flow constriction experimentally by Ramamurthy & Ng (1973), may be decoupled from the blockage effect when re-defined by using the velocity at flow separation and a theoretical wake width. By incorporating this wake width into the momentum equation by Maskell (1963) for the confined flow, a relationship between the form drag and the base pressure is derived. Independently, the experimental data of surface pressure from Ramamurthy & Lee (1973) are found to be independent of the blockage effect when expressed in terms of a modified pressure coefficient involving the pressure at separation. Using the potential flow model by Parkinson & Jandali (1970) and its subsequent development in Yeung & Parkinson (2000) for the unconfined flow, a linear relation between the pressure at separation and the form drag is formulated. By solving the two equations simultaneously with a specified blockage ratio and an apex angle of the triangular cylinder, the predictions of the drag and the base pressure are in reasonable agreement with experimental data. A new theoretical relationship for the Strouhal number, pressure drag coefficient and base pressure proposed in this study allows the confinement effect to be appropriately taken into consideration. The present approach may be extended to three-dimensional bluff bodies.

• 한국전산유체공학회지
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• 제13권4호
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• pp.33-38
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• 2008

For study on the unsteady blockage effect, flows around a rotationally oscillating circular cylinder with relatively low forcing frequency in closed test-section wind tunnels have been numerically investigated by solving compressible Navier-Stokes equations. The numerical scheme is based on a node-based finite-volume method with the Roe's flux-difference splitting and an implicit time-integration method coupled with dual time-step sub-iteration. The computed results of the oscillating cylinder in the test section showed that the fluctuations of lift and drag are augmented by the blockage effects. The drag further increases because of low base pressure. The pressure on the test section wall shows the harmonics having the oscillating and the shedding frequencies contained in the blockage effect.

• Wind and Structures
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• 제12권6호
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• pp.541-551
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• 2009

This paper explains how to correctly measure the drag coefficient of a circular cylinder in wind tunnels with large blockage ratios and for the sub-critical to the super-critical flow regimes. When dealing with large blockage ratios, the drag has to be corrected for wall constraints. Different formulations for correcting blockage effect are compared for each flow regime based on drag measurements of smooth circular cylinders performed in a wind tunnel for three different blockage ratios. None of the correction model known in the literature is valid for all the flow regimes. To optimize the correction and reduce the scatter of the results, different correction models should be combined depending on the flow regime. In the sub-critical regime, the best results are obtained using Allen and Vincenti's formula or Maskell's theory with ${\varepsilon}$=0.96. In the super-critical regime, one should prefer using Glauert's formula with G=0.6 or the model of Modi and El-Sherbiny. The change in the formulations appears at the flow transition with a variation of the wake pattern when passing from sub-critical to super-critical flow regimes. This parameter being not considered in the known blockage corrections, these theories are not valid for all the flow regimes.

• Wind and Structures
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• 제30권6호
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• pp.597-616
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• 2020

The blockage effect on the aerodynamic characteristics of tall buildings is a fundamental issue in wind tunnel test but has rarely been addressed. To evaluate the blockage effects on the aerodynamic forces on a square tall building and flow field peripherally, large eddy simulations (LES) were performed on a 3D square cylinder with an aspect ratio of 6:1 under the uniform smooth inflow and turbulent atmospheric boundary layer (ABL) inflow generated by the narrowband synthesis random flow generator (NSRFG). First, a basic case at a blockage ratio (BR) of 0.8% was conducted to validate the adopted numerical methodology. Subsequently, simulations were systematically performed at 6 different BRs. The simulation results were compared in detail to illustrate the differences induced by the blockage, and the mechanism of the blockage effects under turbulent inflow was emphatically analysed. The results reveal that the pressure coefficients, the aerodynamic forces, and the Strouhal number increase monotonically with BRs. Additionally, the increase of BR leads to more coherence of the turbulent structures and the higher intensity of the vortices in the vicinity of the building. Moreover, the blockage effects on the aerodynamic forces and flow field are more significant under smooth inflow than those under turbulent inflow.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.95-98
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• 2002

Three-dimensional flow analyses for two different ratios of radius to height of sedimentation bed are implemented to evaluate the effect of blockage ratio of center feed wall and angle of distributor on sedimentation efficiency, and to find the optimal value of those parameters. Sedimentation efficiencies for three different shapes are compared with and without rotation speed. And then, five different combinations of blockage ratio of center feed wall and angle of distributor are compared It reveals that the effect of blockage ratio of center fled wall and angle of distributor is considerable to sedimentation efficiency while rotation effect can be neglected and $0.55 and 33^{\circ}$for blockage ratio of center food wall and angle of distributor, respectively, ive the best sedimentation efficiency.

• Nuclear Engineering and Technology
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• 제54권8호
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• pp.3154-3165
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• 2022

During a loss-of-coolant accident (LOCA) in the pressurized water reactor (PWR), there is a possibility that high temperature and internal pressure of the fuel rods lead to ballooning of the cladding, which causes a partial blockage of flow area in a subchannel. Such flow blockage would influence the core coolant flow, thus affecting the core heat transfer during a reflooding phase and subsequent severe accident. However, most of the system analysis codes simulate the accident process based on the assumed channel blockage ratio, resulting in the fact that the simulation results are not consistent with the actual situation. This paper integrates the developed core Fuel Rod Thermal-Mechanical Behavior analysis (FRTMB) module into the self-developed severe accident analysis code ISAA. At the same time, the existing flow blockage model is improved to make it possible to simulate the change of flow distribution due to fuel rod deformation. Finally, the ISAA-FRTMB is used to simulate the QUENCH-LOCA-0 experiment to verify the correctness and effectiveness of the improved flow blockage model, and then the effect of clad ballooning on core heat transfer and subsequent parts of core degradation is analyzed.

• Journal of Hydrospace Technology
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• 제1권1호
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• pp.41-56
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• 1995

Flew control devices, such as flow liners, are frequently introduced in a cavitation tunnel in order to reduce the tunnel blockage effect, when a three-dimensional wake distribution is simulated using a complete ship model or a dummy model. In order to estimate the tunnel wall effect and to evaluate the effect of flow liners on the simulated wake distribution, a surface panel method is adopted for the calculation of the flow around a ship model and flow liners installed in a rectangular test section off cavitation tunnel. Calculation results on the Sydney Express ship model show that the tunnel wall effect on the hull surface pressure distribution is negligible for less than 5% blockage and can be appreciable for more than 20% blockage. The flow liners accelerate the flow near the afterbody of the ship model, so that the pressure gradient there becomes more favorable and accordingly the boundary layer thickness would be reduced. Since the resulting wake distribution is assumed to resemble the full scale wake, flow liners can also be used to simulate an estimated full scale wake without modifying the ship model. Boundary taper calculation should be incorporated in order to correlate the calculated wake distribution with the measured one.