Simulation on the Flow and Heat Transfer Characteristics of Confined Bubbles in Micro-Channels

3D simulations on confined bubbles in micro-channels with diameter of 1.24 mm were conducted. The working fluid is R134a with a mass flux range from 125kg/m2s to 375kg/m2s. The VOF model is chosen to capture the 2 phase interface while the geo-construction method was used to re-construct the 2-phase interface. A heated boundary wall with heat flux varying from 15kW/m2 to 102kW/m2 is supplied. The wall temperature was calculated. The effects of mass flux and heat flux are studied. The shape of the bubble was predicted by the simulation successfully and the results show that they are independent of the initial shape. Both thin film evaporation and micro convection enhance the heat transfer. However, the micro convection which is caused by bubble motion has greater contribution to the total heat transfer at the stage of bubble growth studied.

Two Phase Flow Patterns and Cooling Power of Mixed Refrigerant in Micro Cryogenic Coolers

Micro cryogenic coolers (MCCs) operating in the Joule-Thomson cycle with mixed refrigerants offer an attractive way to decrease the size, cost, and power draw required for cryogenic cooling. Recent studies of MCCs with mixed refrigerants have, when employing pre-cooling, shown pulsating flow-rates and oscillating temperatures, which have been linked to the refrigerant flow regime in the MCC. In this study we investigate those flow regimes. Using a high-speed camera and optical microscopy, it is found that the pulsations in flow correspond to an abrupt switch from single-phase vapor flow to single-phase liquid flow, followed by 2-phase flow in the form of bubbles, liquid slugs, and liquid slug-annular rings. After this period of 2-phase flow, the refrigerant transitions back to single-phase vapor flow for the cycle to repeat. Under different pre-cooling temperatures, the mole fraction of the vapor-phase refrigerant, as measured by molar flow-rate, agrees reasonably well with the quality of the refrigerant at that temperature as calculated by an equation of state. The frequency of pulsation increases with liquid fraction in the refrigerant, and the volume of liquid in each pulse only weakly increases with increasing liquid fraction. The cooling power of the liquid-flow is up to a factor of 7 greater than that of the 2-phase flows and single-phase vapor flow.

Effectiveness Analysis of Non-Mechanical Micro-Valvular Conduit in Single Phase Flow

The non-mechanical valvular conduit, which uses no moving parts but instead relies on a complex geometry to regulate flow, is studied through a combination of numerical, computational and experimental methods. This study is based on using water as the fluid at standard state properties. A numerical model is developed to evaluate the effectiveness of the non-mechanical valve’s intricate geometry. Then computational simulations of the oscillating/pumping sequence of the valvular conduit are conducted to examine the effectiveness of the valve when placed in use for a diaphragm pump. Results demonstrate that the non-mechanical valvular conduit can be an effective application for a diaphragm pump at the micro or macro-scale without requiring valvular mechanics. In computational simulations, when non-mechanical valves are positioned at both the inlet and exit of a diaphragm, the positive circulation of fluid is enhanced by 38% which is sufficient to meet the thermal dissipation requirements of an Intel Pentium D processor (i.e. 130 W). In addition, the experimental results in steady state condition demonstrated that the valvular design regulates the flow direction by producing diodicity (a measure of favorable flow direction) of 2.44.

Evaluation of Lattice Boltzmann Method for Reaction-Diffusion Process in a Porous SOFC Anode Microstructure

In the microscale structure of a porous electrode, the transport processes are among the least understood areas of SOFC. The purpose of this study is to evaluate the Lattice Boltzmann Method (LBM) for a porous microscopic media and investigate mass transfer processes with electrochemical reactions by LBM at a mesoscopic and microscopic level. Part of the anode structure of an SOFC for two components is evaluated qualitatively for two different geometry configurations of the porous media. The reaction-diffusion equation has been implemented in the particle distribution function used in LBM. The LBM code in this study is written in the programs MATLAB and Palabos. It has here been shown that LBM can be effectively used at a mesoscopic level ranging down to a microscopic level and proven to effectively take care of the interaction between the particles and the walls of the porous media. LBM can also handle the implementation of reaction rates where these can be locally specified or as a general source term. It is concluded that LBM can be valuable for evaluating the risk of local harming spots within the porous structure to reduce these interaction sites. In future studies, the information gained from the microscale modeling can be coupled to a macroscale CFD model and help in development of a smooth structure for interaction of the reforming reaction and the electrochemical reaction rates. This can in turn improve the cell performance.

Microfluidic Combinatorial Screening Platform

We propose a microfluidic droplet-based platform that accepts an unlimited number of sample plugs from a multi-well plate, performs splitting of these sample droplets into smaller daughter droplets and subsequent synchronization-free, reliable fusion of sample daughter droplets with multiple reagents simultaneously. This system consists of two components: 1) a custom autosampler which generates a linear array of sub-microliter plugs in a microcapillary from a multi-well plate and 2) A microfluidic chip with channels for sample plug introduction, reagent merging and droplet incubation. This novel system generates large arrays of heterogeneous droplets from hundreds to thousands of samples while concurrently screening these arrays against a large array of reagents. This high throughput system minimizes sample and reagent consumption and can be applied to a gamut of biological assays, ranging from SNP detection to forensic screening.

Experimental Investigation of Mixing in a T-Channel for Asymmetric Inlet Conditions

An experimental investigation of water flow in a rectangular T-channel with inner dimensions of 20 × 20 mm (inlet channel) and 20 × 40 mm (outlet mixing channel) has been conducted. The inlet Reynolds number Re, based on inlet hydraulic diameter, ranged from Re1 = 90 to 775. Inlet flow conditions were asymmetric, and inlet Re ratios of Re1/Re2 = 1.24, 1.65 and 2.45 were obtained by varying volumetric flow rate. Dynamical conditions and T-channel geometry are directly applicable to microscale mixing. Planar laser induced fluorescence (LIF) was used to characterize flow regimes and behaviors, including periodicity, in the inlet and outlet channels. Two distinct flow regimes were identified and characterized for asymmetric inlet Re. For low inlet Re, Re1 ≤ 150, and all Re1/Re2, flows were steady. For higher Re and all Re1/Re2, a jet flow regime, characterized by two counter rotating vortices and increasingly turbulent at higher Re dominated the flow in the junction. Qualitative mixing characteristics for all flow regimes, based on LIF visualizations in the outlet channel, are also presented.

Numerical Simulations of Hydrodynamics and Heat Transfer in Wavy Falling Liquid Films on Vertical and Inclined Walls

The flow of thin falling liquid films is unstable to long-wave disturbances. The flow instability leads to development of waves at the liquid-gas interface. The wave patterns depend on the properties of the liquid, the Reynolds number, the plate inclination angle, and the distance from the film inlet. The effect of the waves on heat and mass transfer in falling liquid films is a subject of ongoing scientific discussion. In this work numerical investigation of the wave dynamics has been performed using a modified Volume of Fluid (VOF) method for tracking the free surface. The surface tension is described using the Continuum Surface Force (CSF) model. At low disturbance frequency solitary waves of large amplitude are developed, which are preceded by low-amplitude capillary waves. At high disturbance frequency low amplitude sinusoidal waves are developed. The wave parameters (peak height, length, propagation speed) are computed from the simulation results and compared with available experimental correlations in a wide range of parameters. The effects of the disturbance frequency and the plane inclination angle on the wave dynamics have been studied. The interaction of waves initiated by simultaneous disturbances of two different frequencies has been investigated. The heat transfer in the wavy film has been simulated for constant wall temperature boundary condition. The effect of the Prandtl number and the disturbance frequency on the local and global heat transfer parameters has been investigated. It has been shown that the influence of waves on heat transfer is significant for large Prandtl numbers in a specific range of disturbance frequencies.

Multiphase Submerged Jet Impingement Cooling Utilizing Nanostructured Plates

An experimental setup is designed to simulate the heat dissipated by electronic devices and to test the effects of nanostructured plates in enhancing the heat removal performance of jet impingement systems in such cooling applications under boiling conditions. Prior experiments conducted in single phase have shown that such different surface morphologies are effective in enhancing the heat transfer performance of jet impingement cooling applications. In this paper, results of the most recent experiments conducted using multiphase jet impingement cooling system will be presented. Distilled water is propelled into four microtubes of diameter 500 μm that provide the impinging jets to the surface. Simulation of the heat generated by miniature electronic devices is simulated through four aluminum cartridge heaters of 6.25 mm in diameter and 31.75 mm in length placed inside an aluminum base. Nanostructured plates of size 35mm×30mm and different surface morphologies are placed on the surface of the base and two thermocouples are placed to the surface of the heating base and the base is submerged into deionized water. Water jets generated using microtubes as nozzles are targeted to the surface of the nanostructured plate from a nozzle to surface distance of 1.5 mm and heat removal characteristics of the system is studied for a range of flow rates and heat flux, varying between 107.5–181.5 ml/min and 1–400000 W/m2, respectively. The results obtained using nanostructured plates are compared to the ones obtained using a plain surface copper plate as control sample and reported in this paper.

Numerical Simulation of Thermal Transpiration in the Slip Flow Regime With Curved Walls

Nowadays, modeling gas flows in the slip flow regime through microchannels can be achieved using commercial Computational Fluid Dynamics codes. In this regime the Navier-Stokes equations with appropriate boundary conditions are still valid. A simulation procedure has been developed for the modeling of thermal creep flow using ANSYS Fluent®. The implementation of the boundary conditions is achieved by developing User Defined Functions (UDFs) by means of C++ routines. The complete first order velocity slip boundary condition, including the thermal creep effects due to an axial temperature gradient and the effect of the wall curvature, and the temperature jump boundary condition are applied. Motivation of the present work is the development of a simulation tool which will help in the pre-calculations and the preliminary design of a Knudsen micropump consisting of successively connected curved and straight channels and in a second step in the numerical optimization of the pump, in terms of geometrical parameters and operating conditions of the system.

Flow Pattern, Void Fraction and Pressure Drop During Adiabatic Two-Phase Gas-Liquid Flow in Vertical Micro-Channel

The experimental investigation is performed to study two-phase flow pattern, void fraction and pressure drop characteristics in a vertical micro-channel. The test section is a fused silica tube with a diameter of 0.53 mm and a length of 320 mm. Air and water are used as working fluid which is introduced to the test section in vertical upward direction. The test runs are done at superficial velocities of gas and liquid ranging respectively from 0.375 to 21.187 m/s and 0.004 to 2.436 m/s. Stereozoom microscope mounted together with camera are employed to conduct flow visualization from which slug flow, throat-annular flow, churn flow, annular flow and annular-rivulet flow are observed. Based on image analysis, void fraction data are obtained and found to be linear relationship with volumetric quality. The frictional pressure drop is relatively high when the formation of churn flow is established. Besides, the two-phase frictional multiplier is found to be strongly dependent on both mass flux and flow pattern.