Browsing by Author "Tsui, Yeng-Yung"

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A FINITE-VOLUME-BASED APPROACH FOR DYNAMIC FLUID-STRUCTURE INTERACTION
(2013)
Tsui, Yeng-Yung
;
Huang, Yi-Cheng
;
Huang, Chun-Lung
;
Lin, Shi-Wen
;
機械工程學系
It is common to adopt finite-element methods to solve solid mechanics problems and finite-volume methods for fluid dynamics computations. The use of different methods causes complication of the solution procedure for problems involving both fluids and solids. In this study, a partitioned approach based on the finite-volume method for dynamic fluid-structure interaction is presented. The method is formulated in a way suitable for an unstructured mesh with arbitrary grid geometry. The variables for the fluid are stored at the centroids of grid cells, whereas those for the solid at the grid nodes. The latter arrangement makes it more suitable for large structure deformation. After spatial discretization for the solid using the finite-volume approach, the resulting system of ordinary differential equations is solved implicitly using the dual-time-stepping scheme. As for the fluid calculation, a pressure-based algorithm is employed and the time step is advanced in a prediction-correction manner. The finite-volume method for the solid is assessed by calculating the deformation and dynamics of a cantilever under various loads. Good agreement with analytical solutions is obtained. Then, the solution procedure is applied to two cases with coupled fluid flow and structure dynamics. One is the flow over a vertical plate with one end fixed on the floor in a channel. The other is the flow over a cylinder with a plate attached to it on the lee side.
A Novel Means Combining Corona Discharge and Electrostatic Force-Induced Vibration for Convective Heat Transfer
(2020-08-01)
Tsui, Yeng-Yung
;
Wei, Ting-Kai
;
Wang, Chi-Chuan
;
機械工程學系
;
Department of Mechanical Engineering
A new design, not reported in the existing literature, combining features of ionic wind and mechanical vibration to induce appreciable airflow is developed. Its feasibility is demonstrated in a cooling system to enhance heat transfer. Ionic wind is generated using a thin, flexible plate as the emitting electrode and a heated, vertical plate as the collecting electrode. By placing a metal inductor close to the discharge electrode, an electrostatic field is formed. The electrode is attracted and thus moves toward the inductor owing to the electrostatic force created. To sustain periodic oscillation and produce large vibrational amplitudes, the inductor is grounded using current-limiting resistors. Vibrational characteristics are highly dependent on the corona voltage, resistance of the resistor, and position of the induction plate, which are examined in the experiments. It was found that the heat transfer enhancement is not improved at high corona voltages because the ionic wind overwhelms the mechanical effect of vibration. The vibrational effect becomes more prominent at low corona voltages with which the electrical field created by the corona discharge is not so intense. The maximum increase of heat transfer coefficient over that without vibration can be as large as 13.4% at the lowest corona voltage considered in the tests.
A novel peristaltic micropump with low compression ratios
(2012-07-20)
Tsui, Yeng-Yung
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Chang, Tso-Chang
;
機械工程學系
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Department of Mechanical Engineering
It is common for peristaltic micropumps to have large compression ratios. In the limit, the chamber of the pump is completely blocked by the membrane to prevent back flow. Different from this kind of pump, a micropump with small compression ratios is proposed in this study. With small oscillation amplitudes the membrane of the pump can reciprocate at high frequencies to improve its pumping flow. Both the multidimensional method and the lumped-element method are employed for analysis. For this kind of peristaltic micropump the working fluid is allowed to flow freely in the forward and backward directions. Therefore, the operating sequences for the high-compression ratio type of pumps are not appropriate. It is shown that the theoretical net flow rate is zero for the four-phase and six-phase modes of sequence and becomes negative for the three-phase mode unless regulators, such as the nozzle/diffusers, are incorporated to rectify the flow. However, this pump becomes very attractive by reversing the operating sequence of the three-phase mode because positive net flow is yielded. It is seen that with the reversed three-phase mode and the nozzle/diffuser as connecting channels, the pumping effectiveness is greatly enhanced. The pumps with both two chambers and three chambers are under consideration in the study. Copyright (c) 2011 John Wiley & Sons, Ltd.
A study of heat transfer enhancement via corona discharge by using a plate corona electrode
(2017-06-01)
Tsui, Yeng-Yung
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Huang, Yu-Xiang
;
Lan, Chao-Cheng
;
Wang, Chi-Chuan
;
機械工程學系
This paper is concerned with the corona discharge used to enhance heat transfer. Both theoretical and experimental procedures are employed. To examine the heat transfer resulting from corona discharge, a thin plate is employed as the corona electrode and a heated plate is grounded to form the collecting electrode. Experiments show that the heat transfer coefficient at the center of the heated plate is increased by a factor in the range 2.6-4.8 times comparing with natural convection. Optical images reveal that corona discharge occurs at the two side corners of the plate electrode when the corona voltage is low and a bluish sheet of discharge issuing from the front edge of the electrode is visible at sufficiently high voltages. Comparison between predicted temperatures and measurements indicates that significant differences exist at low corona voltages and good agreement is obtained when the voltage is high enough. This result is mainly attributed to the 2-D assumption employed in the simulation and the heat loss appearing in the experiment. The corona discharge is stronger for small electrode gaps. However, the corona voltage allowed before formation of the electrical spark is higher for large electrode gaps, leading to better heat transfer. (C) 2017 Elsevier B.V. All rights reserved.
A VOF-Based Conservative Interpolation Scheme for Interface Tracking (CISIT) of Two-Fluid Flows
(2013-04-01)
Tsui, Yeng-Yung
;
Lin, Shi-Wen
;
機械工程學系
;
Department of Mechanical Engineering
A simple interface-reconstruction scheme (CISIT), based on the volume-of-fluid (VOF) formulation and applicable to unstructured grids with arbitrary topology, is developed to track the interface in free-surface flows. The interface is represented by the contour surface of VOF value f=0.5, which is constructed simply by interpolation. The advancing of the interface is made by imposing mass conservation in the cells containing the interface. However, problems arise when the interface moves across the cell face and into another cell. The cell originally occupied by the interface may be overfilled (f>1) or underfilled (f<1) as the interface advances (moving in the positive direction of the interface) to leave the cell. In contrast, the cell may be overdepleted (f<0) or underdepleted (f>0) as the interface retreats (moving in the negative direction of the interface) to move out of the cell. These situations are remedied via adjusting the transported volume of fluid across the cell face by following the conservation law so that the VOF becomes 1 or 0 in these cells, depending on advancing or retreating of the interface. The resulting VOF distribution is uniform, either in 1 or 0, in the region outside the interface cells, and the interface occupies only one cell in its width. To smooth the fluid properties across the interface region in the velocity calculation, an average smoothing technique is adopted. Application to a number of test cases, including model cases with known velocities and a number of real flows, reveal that this method is accurate and robust.
Airflow Management on the Efficiency Index of a Container Data Center Having Overhead Air Supply
(2017-12-01)
Wang, Cheng-Hao
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Tsui, Yeng-Yung
;
Wang, Chi-Chuan
;
機械工程學系
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Department of Mechanical Engineering
Effect of airflow managements on the efficiency index of a small container data center having overhead air supply is reported in this study. Seventeen arrangements and configurations regarding the airflow and blockage arrangements are experimentally examined and compared. Test results indicate an appreciable hot air recirculation occurring for rack arrangement without any blockage, and the hot spot occurs at the second rack alongside the cold aisle. The hot spot had moved to the first rack when the blockage plate is installed on the rack top. Rack locations relative to air handler casts a negligible effect on the efficiency index, and it is comparatively more effective by sealing the trailing of the cold aisle. A smaller cold-aisle spacing helps to lower the temperature distribution, and an additional opening of the supplied vent will not help in removal of hot spot. Shutting off the grille in the center of cold aisle is also unable to fix the hot air recirculation and may even incur hot air reversal. The hot air reversal can be removed by adding additional blockage plate at the flow reversal section. Higher supplied air flow rate also improves the efficiency index considerably.
Analysis of the flow agitated by disc impellers with pitched blades
(2008)
Tsui, Yeng-Yung
;
Lin, Shih-Chao
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Shen, Shih-Jhen
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Hu, Yu-Chang
;
機械工程學系
In this study we are concerned with the flow in a tank stirred by disc impellers with pitched blades. A mathematical method, based on multiframe of reference and unstructured grid technology, was employed in the flow simulation. It was learned from the study by Tsui et al. [12] that for a pitched-blade impeller, which does not include the disc, the flow pattern in the agitated tank is transformed from the axial type to the radial type at a particular angle when the blade angle is increased. A similar phenomenon was observed in the present study. However, in contrast to the results of Tsui et al., the change of flow pattern brings about a sharp decrease in power requirement and a jump in pumping capacity. The cause of the difference is related to the disc which obstructs the fluid when it flows in the axial direction at low blade angles and makes it flow smoothly in the radial direction along the disc at high blade angles. It was shown in the results that the transition angle is increased when the impeller size, the disc size as well as the off-bottom clearance are reduced.
Analysis of the flow in grooved pumps with specified pressure boundary conditions
(2006-11-06)
Tsui, Yeng-Yung
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Jung, Shiaugh-Ping
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機械工程學系
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Department of Mechanical Engineering
In this study the numerical method is employed to examine the flow in pumps with spiral grooves. A methodology suitable for the use of unstructured grids is developed. In order to have the greatest feasibility, the governing equations are formulated in such a manner that the grids are allowed to rotate with the rotor. Different from common practices, pressures are specified on the inlet and the outlet boundaries. Therefore, special treatments are required to calculate the mass flow rate and the inlet and outlet velocities. The methodology developed here is assessed via comparison with existing measurements and good agreement is obtained. Examination of pump parameters reveals that in order to obtain maximum flow rate, both the spiral angle and the groove height need to be carefully determined. The force balance over the entire flow channel shows that for a fixed pressure rise across the pump, the pressure difference between side walls is proportional to tangent of the spiral angle and the groove width, but inversely proportional to the groove height and the rotor length. These relationships can roughly be seen in the calculated results. (c) 2005 Published by Elsevier Ltd.
Blade angle effects on the flow in a tank agitated by the pitched-blade turbine
(2006-07-01)
Tsui, Yeng-Yung
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Chou, Jian-Ren
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Hu, Yu-Chang
;
機械工程學系
;
Department of Mechanical Engineering
This paper presents a study of the influence of the blade angle on the flow in a tank stirred by the pitched-blade turbine. The flow induced by the pitched-blade turbine is usually described as an axial type with a principal ring vortex dominating the flow structure. However it is known that as the blade becomes vertical, i.e., as the pitch angle of the blade becomes 90 deg, the flow is of the radial type, with two main ring vortices occupying the tank. Thus, a transition of flow type must take place when the blade angle is varied. This motivates the current study. A computational method was developed, which incorporates the unstructured grid technique to deal with the complex geometry in the tank. Multiframe of reference was employed to handle the rotation of the impeller. The results show that the transition from the axial type to the redial type is not progressive, but occurs all of a sudden at a particular angle, depending on the configuration. This critical angle decreases as the off-bottom clearance and the impeller size are increased. Its influences on the flow angle of the discharge stream, the power requirement, the induced flow rate through the impeller, and the pumping efficiency are discussed. The mechanism to cause the sudden change of flow type is addressed through observing the flow on the surface of the turbine blade.
Coupled level-set and volume-of-fluid method for two-phase flow calculations
(2017)
Tsui, Yeng-Yung
;
Liu, Cheng-Yen
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Lin, Shi-Wen
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機械工程學系
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Department of Mechanical Engineering
In simulating two-phase flows, the volume-of-fluid (VOF) method has the advantage of mass conservation while with the level-set (LS) method, the surface tension force can be calculated more accurately. In this study, we present a coupling method which combines the advantages of both methods. The volume-of-fluid (VOF) method adopted in the calculation is the conservative interpolation scheme for interface tracking method proposed recently by the authors. Based on the location of the interface calculated from the VOF, the LS function is obtained by solving the equation used in the LS method for re-initialization without needing to solve its advection equation. A high-resolution-bounded scheme within the frame of finite-volume methods is used to solve the re-initialization equation. This scheme is verified by considering a variety of interface geometries. A circular bubble at equilibrium is used to assess the coupled LS and VOF method by examining the spurious currents generated in the bubble. Three-dimensional calculations are conducted to study the rising of a bubble in the quiescent water.
Evaluation of the mixing performance of the micromixers with grooved or obstructed channels
(2008-07-01)
Tsui, Yeng-Yung
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Yang, Ching-Shiang
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Hsieh, Chung-Ming
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機械工程學系
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Department of Mechanical Engineering
The mixing flows in microchannels were examined using numerical methods. To speed up fluid mixing, it is essential to generate lateral transport of mass. In this study, the mixing flow is disrupted by either placing grooves or block obstacles on the walls of the channels. Since the grooves or the blocks appear in a periodic configuration, the velocity is solved only in a section of the channel. With the repeating cycle of flow velocity field, the fluid concentration can be calculated throughout the entire length of the channel. Good agreement with experiments in the mixing performance justifies the present methodology. Two different channel configurations are under consideration: grooved channels and obstructed channels. The results reveal that with straight grooves, a well organized vortex flow is formed in the vertical plane along the groove, which leads to a helical flow in the channel. The mixing performance can be enhanced by having grooves on both the top and the bottom walls arranged in a staggered manner, by which the transversal velocity is largely increased. It is seen that the strength of the secondary flow and, thus, the mixing can be improved by suitably choosing geometric parameters of the groove, such as the depth, the width, and the oblique angle. It is also shown that the efficient mixing for the staggered herringbone type groove is due to the fluid stratification caused by the exchange of position of the resulted counter-rotating vortices. As for the obstructed channels, the flows are in essence two dimensional. Very strong transversal velocity can be produced by narrowing down the flow passage in the channel. However, the efficient mixing is obtained at the cost of large pressure head loss.
Evaluation of the performance of a valveless micropump by CFD and lumped-system analyses
(2008-11-04)
Tsui, Yeng-Yung
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Lu, Shiue-Lin
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機械工程學系
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Department of Mechanical Engineering
The flow in a valveless micropump is analyzed using both the CFD method and the lumped-system method. In the multidimensional simulation of the CFD model, the Navier-Stokes equations are solved using a finite volume method suitable for the use of unstructured grids. The moving membrane is modeled by imposing a reciprocating velocity boundary condition. It is seen that a good agreement with measurements can be obtained for various back pressures by adopting an appropriate membrane shape blending the parabolic and the trapezoidal profiles. The multidimensional predictions serve as benchmark solutions to the lumped-system analysis. In the latter analysis two correlations for the loss coefficients of the nozzle and the diffuser are employed. The results show that with a more accurate one of the two correlations, a better agreement with the multidimensional calculations is yielded. The performance of the pump can be evaluated by considering the pumping efficiency. The pumping efficiency can be approximately formulated in two different ways, depending on the average ratios of the outlet flow rate to the inlet flow rate in the pumping and supply stages. In the averaging process to determine the mean ratios, the transient region between the pumping stage and the supply stage is excluded. This leads to even closer agreement to the multidimensional calculations in the pumping efficiency. (C) 2008 Elsevier B.V. All rights reserved.
Experimental investigation on thermal management for small container data center
(2019-01-01)
Chu, Wen-Xiao
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Hsu, Chang-Sheng
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Tsui, Yeng-Yung
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Wang, Chi-Chuan
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機械工程學系
The energy consumptions of data centers are increasing dramatically and cooling systems using for controlling the operating temperature usually consume more than half of these energy. Thus, effective strategies on thermal management in data centers may have benefit on energy savings. In order to improve the power usage effectiveness (PUE) of data centers, the effects of air supply flowrate by computer room air handler (CRAH), intake flowrate of rack cooling fans, air supply grilles and heat load distribution on the cooling performance of a typical small container data center having overhead air supply system are investigated in this paper. The rack cooling index (RCI) and supply heat index (SHI) are incorporated to evaluate the local and average cooling performance of data racks. It is found that the intake flowrate of rack cooling fans plays dominant role, which is suggested to be greater than the air supply flowrate in normal operation. Meanwhile, the volumetric flowrate distribution of grilles is studied. The additional front grille before entering the cold aisle may improve the RCI and SHI of front rack (rack A), in which severe hot-air recirculation may occur in typical operation. Conversely, additional rear grille at the exit of cold aisle has negative effect on cooling performance of the last rack (rack E). Based on the measurement of local temperature distribution, the non-uniform heat load distribution in racks is proposed. Result shows that the RCI can be improved to 64% if more power is placed at lower half part, which is eligible for energy saving. However, the non-uniform heat load distribution shows limited improvement on the rear rack (rack E).
FLOW CHARACTERISTICS IN MIXERS AGITATED BY HELICAL RIBBON BLADE IMPELLER
(2011-09-01)
Tsui, Yeng-Yung
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Hu, Yu-Chang
;
機械工程學系
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Department of Mechanical Engineering
The main concern of this study is to investigate the flow mixing generated by helical ribbon blade impellers and to show that with the help of CFD the performance of the mixing system can be significantly improved by optimizing the geometric configuration of the impeller. To fulfill this objective, a numerical model is developed to solve the Navier-Stokes equations for the flow field. However, difficulties arise due to the rotation of the impeller in the vessel. In order to ease the problem, the velocity field is assumed to be in a quasi-steady state and the multiframe of reference is adopted to tackle the rotation of the impeller. For discretization the fully conservative finite volume method, together with unstructured grid technology, is incorporated. It is shown that the flow in the mixer can be regarded as a flow in an open channel with a wall moving at an angle with respect to the channel. The influences of the blade pitch, the blade width, and the clearance gap between the blade and the surrounding wall are examined. The mechanism to cause these effects is delineated in detail. It is demonstrated that after optimization of the blade geometry, the circulating flow rate induced by the impeller is largely increased, leading to significant reduction in mixing time. In addition, the power demand is reduced. It is also evidenced that by enlarging the clearance, it is difficult for the fluid in this region to be mixed.
Flux-blending schemes for interface capture in two-fluid flows
(2009-11-01)
Tsui, Yeng-Yung
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Lin, Shi-Wen
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Cheng, Tong-Ting
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Wu, Tian-Cherng
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機械工程學系
This paper is aimed at developing a robust algorithm (FBICS) to capture the interface between two immiscible fluids without the need of interface reconstruction. The advection equation of the volume fraction is solved using the fully conservative finite volume method. Determination of the convective flux through each cell face is based on blending of high resolution schemes and compressive schemes to preserve the sharpness and boundedness of the interface. The flux-blending practice is fulfilled with the use of flux limiters. Test on simple advection flow problems indicates that the well-known CICSAM and HRIC schemes lose accuracy as the Courant number increases. In contrast, the present method maintains high-accuracy performance for Courant numbers up to one. The capability of the method to cope with the complicated dynamics of free surface flows is demonstrated via calculation of the collapsing flow of a water column with an obstacle. (c) 2009 Elsevier Ltd. All rights reserved.
Impact of Overhead Air Supply Layout on the Thermal Performance of a Container Data Center
(2020-03-01)
Chu, Wen-Xiao
;
Wu, Jui-Lin
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Tsui, Yeng-Yung
;
Wang, Chi-Chuan
;
機械工程學系
This study focused on the improved designs of airflow management in container data centers having overhead air supply. The computational fluid dynamics (CFD) model is first validated with experimental results. Then, the impact of grille diameter, deflector angle, and air supply layout on the data center thermal performance is investigated. The results show that the larger grille diameter may reduce the volumetric flowrate through the upstream grille, causing insufficient air supply and strong hot-air recirculation at the first rack A1. By decreasing the grille diameter from 335 mm to 235 mm, the average rack cooling index (RCI) and supply heat index (SHI) can be improved from 25.4% and 0.292 to 65% and 0.258, respectively. However, implementing small diameter grilles is not an economic way for data center performance improvement as far as the energy consumption is concerned due to the high pumping power. Meanwhile, raising the deflector angle below 30 deg in grille S1 can provide moderate improvement on temperature of the A1 rack. A further rise in the deflector to 40 deg may impose severe deterioration with a pronounced hot-spot area. The data center performance can be improved by changing from center-cold-aisle arrangement to center-hot-aisle layout. The layout provides much higher return air temperature and the RCI and SHI can be improved by 32.7% and 34.5%, respectively.
Improvements of Airflow Distribution in a Container Data Center
(2015-01-01)
Wang, I-Nuo
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Tsui, Yeng-Yung
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Wang, Chi-Chuan
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機械工程學系
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Department of Mechanical Engineering
The present study aims at improvements of the airflow and temperature distribution in a container data center. Simulation is carried out based on a raised floor design. It is found that the height of raised floor casts significant impact on the flow distribution. Generally, the flow rate of the perforated tile near the entrance region of CRAC unit is lower than that of the rear part of the perforated tile. The mal-distribution becomes even more severe when the height of the raised floor is reduced. The mal-distribution can be significantly improved using a smaller opening perforated tile but it also gives rise to higher pumping power and impairs the effective flow rate from CRAC. Hence, a variable opening design with a larger opening near the CRAC outlet, followed subsequently with smaller opening perforated tiles, the mal-distribution can be eliminated without additional pumping power. For improvement of the airflow/temperature distribution into the rack, we propose a drawer-type rack which can increase the effective hot aisle space and reducing the cold aisle space. Through this design, hot air recirculation and cold air bypass will be significantly reduced. As a result, the rack maximum inlet temperature of drawer-type rack can be reduced as much as 13.3 degrees C when compared with the traditional one. (C) 2015 The Authors. Published by Elsevier Ltd.
Mixing flow characteristics in a vessel agitated by the screw impeller with a draught tube
(2008-04-01)
Tsui, Yeng-Yung
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Hu, Yu-Chang
;
機械工程學系
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Department of Mechanical Engineering
The circulating flow in a vessel induced by rotating impellers has drawn a lot of interests in industries for mixing different fluids. It used to rely on experiments to correlate the performance with system parameters because of the theoretical difficulty to analyze such a complex flow. The recent development of computational methods makes it possible to obtain the entire flow field via solving the Navier-Stokes equations. In this study, a computational procedure, based on multiple frames of reference and unstructured grid methodology, was used to investigate the flow in a vessel stirred by a screw impeller rotating in a draught tube. The performance of the mixer was characterized by circulation number, power number, and nondimensionalized mixing energy. The effects on these dimensionless parameters were examined by varying the settings of tank diameter, shaft diameter, screw pitch, and the clearance between the impeller and the draught tube. Also investigated was the flow system without the draught tube. The flow mechanisms to cause these effects were delineated in detail.
MODELING OF HEAT TRANSFER ACROSS THE INTERFACE IN TWO-FLUID FLOWS
(2014)
Tsui, Yeng-Yung
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Lin, Shi-Wen
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Ding, Kuen-Je
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機械工程學系
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Department of Mechanical Engineering
A model is presented in this article to deal with heat transfer across the interface separating two immiscible fluids. It is suitable to be incorporated into interface-tracking methods, such as volume-of-fluid (VOF) methods, because a sharp interface is available in these approaches. The temperature at the interface and the heat flux through it are calculated in such a way that the continuity of the two properties at the contact surface is satisfied explicitly. With use of these values, the temperature either at the centroid or on a face of the interface cell can be estimated, which serves as Dirichlet boundary condition for the energy equation. The temperature field is then calculated by solving the energy equations for the two fluids simultaneously in an implicit way. This method is first assessed via testing on two heat conduction problems in which two solids are in contact. Good agreement between numerical solutions and theory is obtained. To demonstrate its capability, it is applied to two kinds of heat transfer problems, one being the collapse of a heated water column in a cavity, and the other the falling of a molten tin droplet in an oil tank. The effect of fluid flow on the heat transfer is clearly illustrated.
Modeling of Valveless Micropumps
(2009)
Tsui, Yeng-Yung
;
Wu, Shin-En
;
機械工程學系
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Department of Mechanical Engineering
There are two difficulties encountered in modeling valveless micropumps using lumped-element methods. The pressure loss coefficient for fluidic diodes used in valveless pumps to rectify flow depends on the flow direction. A problem arises in choosing the proper loss correlation because the flow direction is not known a priori. Another problem is the quadratic form of the equation for the flow through the fluidic diodes, which brings about multiple solutions. The above problems become even more serious in multi-chamber cases. They are overcome in this study by suitably formulating the flow resistance. In addition, the flow inertia is accounted for in the unsteady model. The steady and unsteady models are evaluated by comparing with CFD simulations, which also serve to illustrate the flow field in more detail. It is shown that, compared with the steady model, the variation of the flow rate and pressure predicted by the unsteady model behaves in a close manner to those obtained by multidimensional calculations.