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Subject: search.filters.subject.Electrohydrodynamics

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Now showing 1 - 6 of 6
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    A coupled immersed interface and grid based particle method for three-dimensional electrohydrodynamic simulations
    (2019-12-01)
    Hsu, Shih-Hsuan
    ;
    Hu, Wei-Fan
    ;
    Lai, Ming-Chih
    ;
    應用數學系
    ;
    Department of Applied Mathematics
    In the present work, we propose a coupled immersed interface and grid based particle method to solve two-phase electrohydrodynamic problems in three dimensions. The problem considers a leaky dielectric (weakly conducting) droplet immersed in another leaky dielectric fluid under electric field where the non-homogeneous droplet surface charge effect is taken into account. Due to the mismatch of electrical properties between two fluids, the electric potential satisfying Laplace equation with jump conditions across the droplet surface is coupled with the conservation equation for the surface charge density. Consequently, we first develop a three-dimensional augmented immersed interface method (IIM) which incorporates some known jump conditions naturally along the normal direction and check the desired accuracy. Here, the grid based particle method (GBPM) is used to track the interface by the projection of the neighboring Eulerian grid points so no requirement for stitching of parameterizations nor body fitted moving meshes. Within the leaky dielectric framework, the electric stress can be treated as an interfacial force so that both the surface tension and electric force can be formulated in a unified continuum force in the Navier-Stokes equations. A series of numerical tests have been carefully conducted to illustrate the accuracy and applicability of the present method to simulate droplet electrohydrodynamics. In particular, we investigate the droplet equilibrium dynamics under weak and strong electric fields in detail. It is interesting to find out a chaotic tumbling motion with irregular rotating modes which we believe that is the first numerical verification to the recent experiments. (C) 2019 Elsevier Inc. All rights reserved.
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    Influence of electrode configuration on the heat transfer performance of a LED heat source
    (2014-10-01)
    Chen, Ing Youn
    ;
    Chen, Chien-Jen
    ;
    Wang, Chi-Chuan
    ;
    機械工程學系
    ;
    Department of Mechanical Engineering
    In this study, the effects of electrode configuration on the cooling of LED are presented. A total of six needle type electrodes, including sharp, 0.08, 0.12, 0.37, 0.49 R and flat tip, are used to generate ionic wind for cooling of the LED. The effects of electrode configuration, vertical separation height, and tilt angle on the cooling performance of a LED are reported in this study. It is found that the thermal resistance is reduced with the supplied voltage and a maximum 50% reduction is achieved before the spark-over voltage. For the same supplied voltage, the thermal resistance with a larger vertical separation is also higher. However, the operational range is also longer. The effect of tilt angle on the cooling performance of LED depends on the supplied voltage. With a supplied voltage being less than 6.5 kV, it is found that the thermal resistance is increased when the tilt angle is reduced with a tip radius of 0.12 R. However, the trend is reversed when the supplied voltage is higher than 6.5 kV where the thermal resistance of a smaller tilt angle is lower than that of a larger tilt angle. For the same mesh ground, the effect of electrode configurations cast little influence on the final thermal resistance. All the electrodes show an approximately 50% reduction of thermal resistance in association with its original thermal resistance. However, the corresponding threshold voltage and operational voltage differs significantly. The threshold voltage is lowest for the sharp needle and is increased with the rise of tip radius. On the other hand, the corresponding operational range becomes narrower when the tip radius is increased. Yet the spark-over voltage is also insensitive to change of electrode configuration. (C) 2014 Elsevier Ltd. All rights reserved.
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    Item type:Item,
    A hybrid immersed boundary and immersed interface method for electrohydrodynamic simulations
    (2015-02-01)
    Hu, Wei-Fan
    ;
    Lai, Ming-Chih
    ;
    Young, Yuan-Nan
    ;
    應用數學系
    ;
    數學建模與科學計算所(含中心)
    In this paper, we develop a hybrid immersed boundary (IB) and immersed interface method (IIM) to simulate the dynamics of a drop under an electric field in Navier-Stokes flows. Within the leaky dielectric framework with piecewise constant electric properties in each fluid, the electric stress can be treated as an interfacial force on the drop interface. Thus, both the electric and capillary forces can be formulated in a unified immersed boundary framework. The electric potential satisfies a Laplace equation which is solved numerically by an augmented immersed interface method which incorporates the jump conditions naturally along the normal direction. The incompressible Navier-Stokes equations for the fluids are solved using a projection method on a staggered MAC grid and the potential is solved at the cell center. The interface is tracked in a Lagrangian manner with mesh control by adding an artificial tangential velocity to transport the Lagrangian markers to ensure that the spacing between markers is uniform throughout the computations. A series of numerical tests for the present scheme have been conducted to illustrate the accuracy and applicability of the method. We first compute the potential and its gradient (electric field) to perform the accuracy check for the present augmented IIM. We then check the convergence of the interfacial electric force and the fluid variables. We further run a series of simulations with different permittivity and conductivity ratios and compare with the results obtained by the small deformation theory and other numerical results in literature. In addition, we also study the electric effect for a drop under shear flow. (C) 2014 Elsevier Inc. All rights reserved.
  • Thumbnail Image
    Item type:Item,
    Influence of electrode configuration on the heat transfer performance of a LED heat source
    (2014-10-01)
    Chen, Ing Youn
    ;
    Chen, Chien-Jen
    ;
    Wang, Chi-Chuan
    ;
    機械工程學系
    ;
    Department of Mechanical Engineering
    In this study, the effects of electrode configuration on the cooling of LED are presented. A total of six needle type electrodes, including sharp, 0.08, 0.12, 0.37, 0.49 R and flat tip, are used to generate ionic wind for cooling of the LED. The effects of electrode configuration, vertical separation height, and tilt angle on the cooling performance of a LED are reported in this study. It is found that the thermal resistance is reduced with the supplied voltage and a maximum 50% reduction is achieved before the spark-over voltage. For the same supplied voltage, the thermal resistance with a larger vertical separation is also higher. However, the operational range is also longer. The effect of tilt angle on the cooling performance of LED depends on the supplied voltage. With a supplied voltage being less than 6.5 kV, it is found that the thermal resistance is increased when the tilt angle is reduced with a tip radius of 0.12 R. However, the trend is reversed when the supplied voltage is higher than 6.5 kV where the thermal resistance of a smaller tilt angle is lower than that of a larger tilt angle. For the same mesh ground, the effect of electrode configurations cast little influence on the final thermal resistance. All the electrodes show an approximately 50% reduction of thermal resistance in association with its original thermal resistance. However, the corresponding threshold voltage and operational voltage differs significantly. The threshold voltage is lowest for the sharp needle and is increased with the rise of tip radius. On the other hand, the corresponding operational range becomes narrower when the tip radius is increased. Yet the spark-over voltage is also insensitive to change of electrode configuration. (C) 2014 Elsevier Ltd. All rights reserved.
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    Item type:Item,
    Effect of oscillatory EHD on the heat transfer performance of a flat plate
    (2013-06-01)
    Sheu, Wen-Junn
    ;
    Hsiao, Jen-Jei
    ;
    Wang, Chi-Chuan
    ;
    機械工程學系
    ;
    Department of Mechanical Engineering
    The present study investigates the effect of oscillatory EHD on the heat transfer performance of a flat plate. A needle type electrode with positive polarity is used to generate ionic wind with applied voltage ranging from 4 to 9.5 kV. The wave forms of the input voltage are either steady or stepped and the frequency ranges from 0.5 to 2 Hz with the separation distance between the electrode and the test plate being 5 and 15 mm, respectively. It is found that at the same applied voltage, the heat transfer performance subject to the oscillatory EHD is always inferior to that of the steady EHD for all frequencies. A parabolic dependence of the ionic current with the supplied voltage is seen under steady EHD operation. However, the ionic current vs. supplied voltage shows a rather linear dependence pertaining to the oscillatory EHD. For a smaller separation distance of 5 mm, the heat transfer performance for f = 0.5 Hz is slightly higher than that of f = 2 Hz. However, the trend is reversed when the separation distance is increased to 15 mm. (C) 2013 Elsevier Ltd. All rights reserved.
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    Item type:Item,
    Enhanced cooling for LED lighting using ionic wind
    (2013-01-15)
    Chen, Ing Youn
    ;
    Guo, Mei-Zuo
    ;
    Yang, Kai-Shing
    ;
    Wang, Chi-Chuan
    ;
    機械工程學系
    This study employs ionic wind to augment heat transfer of a LED mounted on a substrate. The size of the LED chip is 0.9 mm x 0.9 mm with a nominal power of 1 W. A needle type electrode is used to generate ionic wind with the applied voltage ranging from 4 to 11 kV. The effects of aligned angle, electrode polarity, separation distance, and ground configuration on the thermal resistance of the LED substrate are examined in this study. For the same applied voltage, it appears that the thermal resistance for the negative polarity is lower than that for the positive one and the negative electrode also has a wilder operation range. The thermal resistance can be reduced as much as 50% in the test range. The thermal resistance is slightly reduced when the aligned angle is increased from 0 degrees to 20 degrees, but a further increase of aligned angle casts no further reduction on the thermal resistance. It is found that the influence of vertical separation distance between the needle and ground electrode is moderately higher than that of horizontal separation distance. Test results also indicate that the mesh ground electrode shows moderately lower thermal resistance than those of point or line electrode. (C) 2012 Elsevier Ltd. All rights reserved.