Abstract: Supercritical/near-critical fluid is very dense and highly expandable, and with its preferable flow and heat transfer properties, it has been valued in various kinds of energy conversion systems. The fluid critical transition and divergence are very important for both hydrodynamic study and heat transfer applications. The near-critical cabron dioxinde horizontal flow and its heat transfer performance in micro-scale channels were studied. Detailed numerical procedures were carried out with the Navier-Stokes equations as well as the energy and state equations, which were treated with special care for the sake of micro-scale investigations. In view of the thermal-mechanical effects of critical fluid, abnormal thermal convection structure and transient micro-scale vortex mixing evolution mode were found in micro-scale channels. Basic Kelvin-Helmholtz instability was identified for the near-critical fluid unstable convection. Due to the hot boundary vortex evolution, heat transfer of near-critical micro-scale channel flow was greatly enhanced, leading to fast thermal/heat transfer equilibrium process. The near-critical fluid convection problem is then characterized from a more general viewpoint in this study.
Abstract: Mechanisms of droplet evaporation on flat solid surface were investigated with the Lattice Boltzmann method. Effect of gravity on the droplet shape change during evaporation in the cases of different static contact angles was detailedly analyzed. The results show that, as the droplet size decreases, the gravity effect decreases; and when the size reaches a critical value, the gravity effect grows negligible. This critical value for a water droplet was calculated for specific parameters, which was 50% lower than the value given by the classical capillary theory. Moreover, the inside-droplet flow patterns are also considerably influenced by gravity and the droplet size.
Abstract: The dimple has bright prospect in the micro heat exchanger for smaller flow resistance and better heat transfer enhancement characteristics. Numerical and experimental study of heat transfer enhancement based on the structure of cooling rectangular channels with dimples was carried out. The flow structure and heat transfer characteristics in laminar air flow with different dimple depths and different Reynolds numbers were investigated, and the results were compared with those of the corresponding flat cases. The results show that: with the increasing Reynolds number, the heat transfer effect gradually increases; there exists the best dimple depth between 1 mm and 2 mm at the 3 Reynolds numbers (Re=500,1 000, 1 500); the flow separation occurs inside the dimple and the separation point is located in front of the dimple center, which results in the best heat transfer characteristics; at the same Reynolds number, the resistance characteristics decrease with the increasing dimple depth, and the thermal performance decreases with the increasing Reynolds number.
Abstract: To get white light emission, it’s common to use a blue LED (light emitting diode) chip to be coated with yellow emitting phosphor jel via a dispensing process. The phosphor jel dispensing process is of two-phase flow, which decides the morphology and properties of the phosphor gel, thus strongly influences both optical and thermal performances of the resulting LEDs. It is important to describe the dispensing process accurately and improve the coating quality. Based on the lattice Boltzmann method (LBM), a flow model of phosphor gel was established to simulate the dispensing process. The dispensing and shaping processes of phosphor gel on flat surface and square projection were analyzed respectively. Results show that LBM simulates the dispensing process of phosphor gel accurately and predicts the morphology well. The droplet contact line length changes as a power function of the droplet diameter on flat surface. The simulation results provide a theoretical basis for the optimization of the phosphor gel dispensing process.
Abstract: The flow characteristics in micro-channels with inner diameters of 0.4 mm and 0.5 mm at different pressure ratios were experimentally studied. The resistance coefficients in the micro-channels are 4 to 5 times of those of the conventional channels, and the phenomenon of early transition from laminar flow to turbulence in advance is well explained. The smaller the diameter of a micro-channel is, the greater the flowing pressure loss results.
Abstract: Micro-channel heat sinks have the advantages of small volume, low flow velocity and high heat transfer efficiency. With the rapid development of miniaturization industry, micro heat sinks are widely used. Previous studies have shown that the micro-channel’s heat transfer performance is mainly dependent on its geometric and flow conditons, and compared with triangle and trapezoid shapes, rectangle structures have better thermal transfer performance. Based on the finite element software ANSYS Workbench, micro-channel heat sinks with a length of 40 mm and different cross-sectional dimensions were analyzed numerically, to give the optimal micro-channel dimensions with small pressure drop but high heat transfer efficiency. Simulation of the optimized micro-channel shows, given an initial temperature, a mass velocity and a substrate heat flux, a heat flux can be dissipated with a pressure drop down to 230.2 Pa and a heat transfer capacity up to 5.254 W, promising good working performance.
Abstract: A novel structure of metal-foam filled microchannel heat sink was proposed for electronics cooling applications. Effects of the main parameters, such as porosity, pore density, metal foam materials (copper, nickel and aluminum) and coolants (water, ethylene glycol and nanofluid), were numerically studied to predict the pressure drop and heat transfer performance of laminar flow in the heat sink. The results show that the thermal performance of the microchannel heat sink is enhanced over twice after filling-in of metal foam, and it is also positive for the heat transfer efficiency to employ nanofluid as coolant. The results also show that the microchannel heat sink filled with metal foam is well qualified for cooling chips with heat flux of 200 W/cm2, which means that it has great potential for thermal management of electronics devices with high power density.
Abstract: In the process of through silicon via (TSV) for 3D system in package (3D SiP), thermal stress changes the mobility of carriers of silicon around TSV, and then influences the whole 3D SiP chip performance. Aimed at this problem, a new stress-releasing structure was proposed, named thermal-stress-releasing groove. Stress on the silicon substrate surface outside the groove can be isolated to a low level especially for large-sized TSV applications. Numerical simulation was used to obtain the relationship between the groove structure parameters and the resulting thermal stress distribution. Parameters including depth and width of the releasing groove and distance from the groove to the pad edge were also simulated to obtain the proportion of stress reduction. The numerical results show that, 40%~60% of the previous thermal stress could be reduced, so could keep-off-zone area, through the proposed stress-releasing groove design.
Abstract: The diffusion equation with the third-type boundary condition solved by the lattice Boltzmann method was theoretically and numerically studied. A new numerical algorithm based on the bounce-back method was constructed, to deal with the complex boundary problem. By asymptotic analysis, the compatibility of the numerical method was proved. The accuracy and stability of the algorithm were discussed via several numerical examples. Compared with the previous work, this numerical approach makes a significant improvement in the aspects of accuracy, stability and efficiency. Finally, through the numerical example of a reaction-diffusion problem with complex boundary, feasibility and effectiveness of the presented method are proved in the simulation of the multi-physical and chemical transport process in porous medium.
Abstract: Fully developed laminar flow properties and heat transfer characteristics in equilateral triangle micro-channels were studied. Based on the theory of Navier-Stokes equations, in the case that a steady heat flux was imported into the fluid at one side of the equilateral triangle, the approximate analytical results of velocity and temperature distributions of the fully developed laminar flow within the micro-channel, as well as the friction factor and Nusselt number of the fully developed convevtion, were given through solution of the momentum and energy differential equations. Then the laminar flow and heat transfer within the micro-channel were numerically simulated with the Fluent software to get the corresponding numerical values of velocity, temperature, friction factor and Nusselt number. In comparison, the analytical results fit the numerical ones well to verify the correctness of the presented method.
Abstract: A kind of superhydrophobic surface with controlled two-scale micro-nano textures was prepared. The lithography and ion etching technology were used to fabricate the micro structures on silicon wafers. Then the carbon nanotubes were controlled to grow on the prepared substrate by means of chemical vapor deposition (CVD) technology. The morphology and performance of the different surfaces have been examined by scanning electron microscope (SEM) and contact angle, rolling angle measurement. Particle image velocimetry (PIV) technology was used to capture the internal velocity distribution of water droplets rolling on the superhydrophobic surfaces with two-scale micro-nano textures. Compared with the one-scale micro-structured surface, the two-scale micro-nano-textured surfaces were found with lower rolling angle and higher rolling velocity for the droplets on them. A much higher slip velocity was found near the wall in the two-scale case, too, which may lead to significant drag reduction in the future research.
Abstract: The transport process and flow structure near an evaporating meniscus are highly complicated due to various coupling factors. A numerical model was developed to describe the physical process and motion of a micro-particle near an evaporating meniscus. The evaporation and its cooling effect on the interface, vapor diffusion in the gas domain, as well as thermocapillary flow were considered together with computational fluid dynamics (CFD). At the same time, the particle’s motion was tracked with discrete element method (DEM). The interaction between the micro flow field and the particle, including drag force and buoyancy force, was considered. The simulation results agree well with the experiments in previous published literatures.