T. N. Wong

A Global Mass Correction Scheme for the Level-Set Method

The level-set method is used to study the evolution of a bubble carried by a primary phase in (1) a straight channel, (2) a double-bend channel, and (3) a constricted channel. Special attention is given to the conservation of mass for the phases. A global mass correction scheme is proposed to ensure mass conservation. Surface tension effect is modeled using the continuum surface force approach. A finite-volume method is used to solve the governing equations. The CLAM schemes are used to model the convection of the level-set equations. The results compare well with the solutions of the volume-of-fluid (VOF) method.

Frequency-dependent velocity and vorticity fields of electro-osmotic flow in a closed-end cylindrical microchannel

The frequency-dependent electro-osmotic flow in closed-end cylindrical microchannels is analyzed in this study. A dynamic ac electro-osmotic flow field is obtained analytically by solving the Navier–Stokes equation using the Green function formulation in combination with a complex variable approach. Onsagers principle of reciprocity is demonstrated to be valid for transient and ac electro-osmotic flow. The effect of a frequency-dependent ac electric field on the oscillating electro-osmotic flow is studied. The induced pressure gradient is analyzed under the effects of the channel dimension and the frequency of electric field. Based on the Stokes second problem, the solution of the slip velocity approximation is presented for comparison with the results obtained from the analytical solution developed in this study. In addition, the expression for the electro-osmotic vorticity field is derived, and the characteristic of the vorticity field in ac electro-osmotic flow is discussed.

Particle Transport in Microchannels

Particle transport in microchannel is presented. This article focuses on situations in which the sizes of the particles are comparable to the sizes of the channels. These solid bodies are sufficiently large that momentum is exchanged between the bodies and the flowing fluid. As a result, the solid bodies affect the fluid flow significantly, and vice versa, resulting in a transient process in which the motions of the solid bodies and the flow field are strongly coupled. The flow field and the particulate flow must then be solved simultaneously. The solid bodies are modeled as a fluid constraint to move with rigid body motion. The solid–fluid interface is described using a distance function. For demonstration purposes, the finite-volume method is used to solve the resulting set of governing equations. The present approach is validated against (1) flow around stationary, (2) flow around forced rotating, (3) flow around freely rotating cylinders, and (4) sedimentation of a circular cylinder under gravity. Finally, the motion of particles carried by an incompressible fluid in a microchannel system is studied.

Development and Characterization of Thermal Enhancement Structures for Single-Phase Liquid Cooling in Microelectronics Systems

In this paper, the development and characterization of thermal enhancement structures for single-phase liquid cooling in microelectronics systems are presented. Miniature heat sinks with three different types of thermal enhancement structures were examined. The first type was a metallic microchannel heat sink (MMCHS) made of aluminum with the channel dimensions of 15 mm (length) × 0.2 mm (width) × 2 mm (height). The second type was the silicon microchannel heat sinks (SMCHS) made through the deep reactive ion etching technique on a 6 inch wafer, with identical channel height of 0.45 mm and average channel widths of 66.6 μ m and 46.6 μ m, respectively. The last type was the metallic foam heat sinks (MFHSs), which were formed by brazing porous foam materials of high pore density onto copper base plates. All three types of heat sinks were fabricated and experimentally characterized by incorporating into electronic packages in standard flip chip ball grid array format. Characterization results indicate that, given a thermal window of 50°C, both the MMCHS and MFHSs can achieve an equivalent package heat dissipation above 100 W/cm2 at moderate pressure drops and the SMCHS above 200 W/cm2 at larger pressure drops. A comparison of thermal enhancement and manufacturability of the three types of heat sinks is also presented.

Fast Dynamic Visualizations in Microfluidics Enabled by Fluorescent Carbon Nanodots

Microfluidic systems have become a superior platform for explorations of fascinating fluidic physics at microscale as well as applications in biomedical devices, chemical reactions, drug delivery, etc. Exploitations of this platform are built upon the fundamental techniques of flow visualizations. However, the currently employed fluorescent materials for microfluidic visualization are far from satisfaction, which severely hinders their widespread applications. Here fluorescent carbon nanodots are documented as a game-changer, applicable in versatile fluidic environment for the visualization in microfluidics with unprecedented advantages. One of the fastest fluorescent imaging speeds up to 2500 frames per second under a normal contionous wave (CW) laser line is achieved by adopting carbon nanodots in microfluidics. Besides better visualizations of the fluid or interface, fluorescent carbon nanodots-based microparticles enable quantitative studies of high speed dynamics in fluids at microscale with a more than 90% lower cost, which is inaccessible by traditionally adopted fluorescent dye based seeding particles. The findings hold profound influences to microfluidic investigations and may even lead to revolutionary changes to the relevant industries.

Submerged liquid jet impingement cooling

In this paper, submerged liquid jet array impingement is proposed as a solution for the thermal management of the high performance electronics. Experiments were done using a submerged jet array of 285 nozzles of 0.5 mm diameter spaced 5.5 mm apart, machined on a 5 mm thick nozzle plate, with an impingement height of 3 mm. The temperature difference between the average heat source temperature and the inlet temperature decreases with the flow rate and increases with the heating power. The experimental Nusselt numbers agree well with the reported data in previous studies. Numerical studies were also conducted on a selected 16-nozzle module using the renormalization group (RNG) k-ε turbulence model. The temperature and velocity fields were obtained, and the average heat transfer coefficients from the numerical studies show good agreement with the experimental results.

Microfluidic on-chip fluorescence-activated interface control system

A microfluidic dynamic fluorescence-activated interface control system was developed for lab-on-a-chip applications. The system consists of a straight rectangular microchannel, a fluorescence excitation source, a detection sensor, a signal conversion circuit, and a high-voltage feedback system. Aqueous NaCl as conducting fluid and aqueous glycerol as nonconducting fluid were introduced to flow side by side into the straight rectangular microchannel. Fluorescent dye was added to the aqueous NaCl to work as a signal representing the interface position. Automatic control of the liquid interface was achieved by controlling the electroosmotic effect that exists only in the conducting fluid using a high-voltage feedback system. A LABVIEW program was developed to control the output of high-voltage power supply according the actual interface position, and then the interface position is modified as the output of high-voltage power supply. At last, the interface can be moved to the desired position automatically using this feedback system. The results show that the system presented in this paper can control an arbitrary interface location in real time. The effects of viscosity ratio, flow rates, and polarity of electric field were discussed. This technique can be extended to switch the sample flow and droplets automatically.

Theoretical investigation of two-fluid electroosmotic flow in microchannels

This paper presents theoretical investigations of the pressure-driven two-liquid flow in microchannels with electroosmosis effect. For a fully developed, steady state, laminar flow of two liquids combined the pressure gradient and the electroosmotic effects, we have derived analytical solutions that relate the velocity profiles and flow rates to the liquid holdup, the aspect ratio of the microchannel, the viscosity ratio of the two liquids and the externally applied electric field.

Numerical modeling of annular flow in microchannel

Recent experimental studies on flow boiling in micro-channels reveal that annular flow is the dominant two-phase flow pattern. This study is an initial effort towards the simulation of annular flow in micro-channels. In this article, two-phase axisymmetric flows with phase change are studied. The level-set method is used to track the interface between the phases. To overcome the mass conservation problem of the level-set method, a local mass correction (LMC) scheme is proposed. With the proposed LMC, mass is shown to conserve well even phase change occurs.

Submerged liquid jet array impingement cooling with high gravity effects

Submerged liquid jet array impingement has been considered as one of the most effective cooling technologies. By combining the impingement and forced convection effects, it provides high heat transfer coefficients and compact cooling designs. The present studies are to extend the liquid jet array impingement for the thermal management of ai rborne electronic devices, which are subjected to conditions with high gravities. The high gravity studies were conducted on an Submerged liquid jet array cooling system, which had a nozzle plate with 14×21 circular nozzles arranged in a uniform square pattern with aj et-to-jet distance of 7.0 mm. The diameter oft he nozzles was 0.3 mm and the jet-to-target spacing was 3.0 mm. The high gravity test results showed that an heat transfer coefficient of about 7000 W/(m2.K) could be achieved at a flow rate of 4.5 L/min for a heated plate up 12g in all directions. The computational fluid dynamics (CFD) numerical simulations of the jet array impingement were conducted. The heat transfer coefficient results agree well with the experimental data and som e information of the temperature and velocity fields was obtained.