Multi-level adaptive particle refinement method with large refinement scale ratio and new free-surface detection algorithm for complex fluid-structure interaction problems

Fluid-Structure Interaction (FSI) is a crucial problem in ocean engineering. The smoothed particle hydrodynamics (SPH) method has been employed recently for FSI problems in light of its Lagrangian nature and its advantage in handling multi-physics problems. The efficiency of SPH can be greatly improved with the Adaptive Particle Refinement (APR) method, which refines particles in the regions of interest while deploying coarse particles in the left areas. In this study, the APR method is further improved by developing several new algorithms. Firstly, a new particle refinement strategy with the refinement scale ratio of 4 is employed for multi-level resolutions, and this dramatically decreases the computational costs compared to the standard APR method. Secondly, the regularized transition sub-zone is deployed to render an isotropic particle distribution, which makes the solutions between the refinement zone and the non-refinement zone smoother and consequently results in a more accurate prediction. Thirdly, for complex FSI problems with free surface, a new free-surface detection method based on the Voronoi diagram is proposed, and the performance is validated in comparison to the conventional method. The improved APR method is then applied to a set of challenging FSI cases. Numerical simulations demonstrate that the results from the refinement with scale ratio 4 are consistent with other studies and experimental data, and also agree well with those employing the refinement scale ratio 2. A significant reduction in the computational time is observed for all the considered cases. Overall, the improved APR method with a large refinement scale ratio and the new free-surface detection strategy shows great potential in simulating complex FSI problems efficiently and accurately.Comment: 47 pages, 26 figures, accepted to be published by Journal of Computational Physic

Multiscale Micro/Nanostructured Heat Spreaders for Thermal Management of Power Electronics

In this chapter, we describe surface modification techniques for enhancing heat/mass transfer and evaporation on heated surfaces. The effect of asymmetrical structure in designing a vapor chamber, patterned with multiscale micro/nanostructured surfaces will be introduced. The wettability patterned surface and its mechanism for improving the evaporation rate of a droplet and the thermal performance of nucleate boiling are discussed. An ultrathin vapor chamber based on a wettability patterned evaporator is introduced as a case for the application of the wettability pattern. Besides, modifying the surface with nanostructure to form a multiscale micro/nanostructured surface or superhydrophobic surface also enhances the phase change. Several types of heat spreaders are proposed to investigate the effects of multiscale micro/nanostructured surface and nanostructured superhydrophobic condenser on the thermal performance of the heat spreaders, respectively. The effects of multiscale micro/nanostructured evaporator surfaces with wettability patterns will be analyzed and experimental data will be presented

Do surfaces with mixed hydrophilic and hydrophobic areas enhance pool boiling?

We demonstrate that smooth and flat surfaces combining hydrophilic and hydrophobic patterns improve pool boiling performance. Compared to a hydrophilic surface with 7 degree wetting angle, the measured critical heat flux and heat transfer coefficients of the enhanced surfaces are up to respectively 65 and 100% higher. Different networks combining hydrophilic and hydrophobic regions are characterized. While all tested networks enhance the heat transfer coefficient, large enhancements of critical heat flux are typically found for hydrophilic networks featuring hydrophobic islands. Hydrophilic networks indeed are shown to prevent the formation of an insulating vapor layer.Comment: accepted August 10 in Applied Physics letter

Two-dimensional fringe probing of transient liquid temperatures in a mini space

A 2D fringe probing transient temperature measurement technique based on photothermal deflection theory was developed. It utilizes material's refractive index dependence on temperature gradient to obtain temperature information from laser deflection. Instead of single beam, this method applies multiple laser beams to obtain 2D temperature information. The laser fringe was generated with a Mach-Zehnder interferometer. A transient heating experiment was conducted using an electric wire to demonstrate this technique. Temperature field around a heating wire and variation with time was obtained utilizing the scattering fringe patterns. This technique provides non-invasive 2D temperature measurements with spatial and temporal resolutions of 3.5 mu m and 4 ms, respectively. It is possible to achieve temporal resolution to 500 mu s utilizing the existing high speed camera. (C) 2011 American Institute of Physics. [doi:10.1063/1.3584872

Interfacial Dynamics of Immiscible Liquid-Liquid Displacement in Capillary Tubes

The interfacial liquid film dynamics of two immiscible fluids in a micropipe is of interest in microfluidics and two-phase flows. Liquid film displacement is essential for fluid dynamics, heat and mass transfer. Understanding interfacial dynamics and liquid film thickness in two immiscible fluids is important in two-phase flow research because the interfacial area of fluid film and its thickness have a significant impact on the heat and mass transfer processes. However, the processes of liquid film formation, development and breakup are complex which involve the interfacial tension between fluid-fluid and fluid-solid, the beating process of fluid dynamics, the thermal effect and the capillary induced instability of thin liquid film breakup. As a consequence of the small length scale, the capillary forces play a fundamental role in the physics of the phenomena. For the high surface area and volume ratio will affects the evaporation of the film fluid. In this paper, annual type thin water film displacement, velocity field and breakup in a two immiscible fluid system in a capillary tube are presented. A novel method for measuring the interfacial liquid film thickness between immiscible liquids of annual oil liquid and an oil slug/droplet (oil-water-oil) in a capillary tube is proposed. A water plug/droplet was introduced into kerosene oil and pushed forwarded under a constant pressure. Water droplet adhesion to the glass capillary wall was observed and the dynamic changes of interfacial liquid film thickness were visualized and measured against different capillary numbers by using light absorption techniques utilizing high speed CCD camera and Beer-Lambert law. The mean film propagation velocity was measured by micro resolution particle image velocimetry (μPIV). The experimental results show the film thickness against the capillary number at certain cross-section obeys a power law. The measured velocity can also be used to demonstrate the liquid film break up processes. Furthermore, a ripple pattern film thickness was presented when a long annual water film was formed