Non-Intrusive Optical Validation of Two-Phase Flow Regimes in a Small Diameter Tube

A non-intrusive optical method for two-phase flow pattern identification was developed to validate flow regime maps for two-phase adiabatic flow in a small diameter tube. Empirical measurements of film thickness have been shown to provide objective identification of the dominant two-phase flow regimes, representing a significant improvement over the traditional use of exclusively visual and verbal descriptions. Use of this technique has shown the Taitel-Dukler, Ullmann-Brauner, and Wojtan et al. phenomenological flow regime mapping methodologies to be applicable, with varying accuracy, to small diameter two-phase flow.

Integration of a Pulsating Heat Pipe in a Flat Plate Heat Sink

The ongoing development of faster and smaller electronic components has led to a need for new technologies to effectively dissipate waste thermal energy. The pulsating heat pipe (PHP) shows potential to meet this need, due to its high heat flux capacity, simplicity, and low cost. A 20-turn flat plate PHP was integrated into an aluminum flat plate heat sink with a simulated electronic load. The PHP heat sink used water as the working fluid and had 20 parallel channels with dimensions 2 mm × 2 mm × 119 mm. Experiments were run under various operating conditions, and thermal resistance of the PHP was calculated. The performance enhancement provided by the PHP was assessed by comparing the thermal resistance of the heat sink with no working fluid to that of it charged with water. Uncharged, the PHP was found to have a resistance of 1.97 K/W. Charged to a fill ratio of approximately 75% and oriented vertically, the PHP achieved a resistance of .49 K/W and .53 K/W when the condenser temperature was set to 20°C and 30°C, respectively. When the PHP was tilted to 45° above horizontal the PHP had a resistance of .76 K/W and .59 K/W when the condenser was set 20°C and 30°C, respectively. The PHP greatly improves the heat transfer properties of the heat sink compared to the aluminum plate alone. Additional considerations regarding flat plate PHP design are also presented.

The Effect of Injection and Suction on the Interfacial Mass Transport Resistance in a Proton Exchange Membrane Fuel Cell Air Channel

Proton exchange membrane fuel cells are efficient and environmentally friendly electrochemical engines. The present work focuses on air channels that bring the oxidant air into the cell. Characterization of the oxygen concentration drop from the channel to the gas diffusion layer (GDL)-channel interface is a need in the modeling community. This concentration drop is expressed with the non-dimensional Sherwood number (Sh). At the aforementioned interface, the air can have a non-zero velocity normal to the interface: injection of air to the channel and suction of air from the channel. A water droplet in the channel can constrict the channel cross section and lead to a flow through the GDL. In this numerical study, a rectangular air channel, GDL, and a stationary droplet on the GDL-channel interface are simulated to investigate the Sh under droplet induced injection/suction conditions. The simulations are conducted with a commercially available software package, COMSOL Multiphysics.

Capillary Imbibition Dynamics in Porous Media

The phenomenon of capillary imbibition through porous media is important both due to its applications in several disciplines as well as the involved fundamental flow physics in micro-nanoscales. In the present study, where a simple paper strip plays the role of a porous medium, we observe an extremely interesting and non-intuitive wicking or imbibition dynamics, through which we can separate water and dye particles by allowing the paper strip to come in contact with a dye solution. This result is extremely significant in the context of understanding paper-based microfluidics, and the manner in which the fundamental understanding of the capillary imbibition phenomenon in a porous medium can be used to devise a paper-based microfluidic separator.

Finite Element Volume Analysis of Propane Preheated Air Flame Passing Through a Minichannel

A hybrid finite-element-volume FEV method is extended to simulate turbulent non-premixed propane air preheated flame in a minichannel. We use a detailed kinetics scheme, i.e. GRI mechanism 3.0, and the flamelet model to perform the combustion modeling. The turbulence-chemistry interaction is taken into account in this flamelet modeling using presumed shape probability density functions PDFs. Considering an upwind-biased physics for the current reacting flow, we implement the physical influence upwinding scheme PIS to estimate the cell-face mixture fraction variance in this study. To close the turbulence closure, we employ the two-equation standard κ-ε turbulence model incorporated with suitable wall functions. Supposing an optically thin limit, it needs to take into account radiation effects of the most important radiating species in the current modeling. Despite facing with so many flame instabilities in such small size configuration, the current method performs suitably with proper convergence, and the encountered instabilities are damped out automatically. Comparing with the experimental measurements, the current extended method accurately predicts the flame structure in the minichannel configuration.

Numerical Simulation of Bubble Growth During Nanofluid Flow Boiling in a Microchannel

In recent years, heat dissipation in micro-electronic systems has become a significant design limitation for many component manufactures. As electronic devices become smaller, the amount of heat generation per unit area increases significantly. Current heat dissipation systems have implemented forced convection with both air and fluid media. However, nanofluids may present an advantageous and ideal cooling solution. In the present study, a model has been developed to estimate the enhancement of the heat transfer when nanoparticles are added to a base fluid, in a single microchannel. The model assumes a homogeneous nanofluid mixture, with thermo-physical properties based on previous experimental and simulation based data. The effect of nanofluid concentration on the dynamics of the bubble has been simulated. The results show the change in bubble contact angles due to deposition of the nanoparticles has more effect on the wall heat transfer compared to the effect of thermo-physical properties change by using nanofluid.

The Effect of Temperature Dependent Electrical Conductivity on the MHD Natural Convection of Al2O3–Water Nanofluid in a Rectangular Enclosure

The present paper is concerned with the study of flow and heat transfer characteristics in the steady state free convective flow of Al2O3-waternanofluids in a square enclosure in the presence of magnetic field. Attention is given to the temperature variation of the electrical conductivity and its effect on the electromagnetic force induced by the motion of the nanofluid. A new experimental correlation recently presented in the literature was used for this aim. In all the earlier studies in this area the electrical conductivity variation of nanofluid with temperature was neglected. The fluid viscosity and thermal conductivity are assumed to vary as a function of temperature and this variation is modeled using the available experimental correlations. The governing differential equations are solved numerically using finite element method. The features of fluid flow and heat transfer characteristics are analyzed for various strengths of the magnetic field and different nanoparticle volume fractions. The results show that when the inclusion of the variation of the electrical conductivity with temperature in the numerical model noticeably affects the natural convection heat transfer in the studied rectangular cavity. The variations of Nusselt number for natural convection of Al2O3-water nanofluid with nanoparticle volume fractions are presented at various Rayleigh and Hartmann numbers.

A CFD Study on Film Thickness, Pressure Drop, and Heat Transfer of Slug Flows in Circular Microchannels

Slug flows in microchannels have been studied in the past two decades as a method to enhance heat transfer. Several correlations have been developed for predicting film thickness, pressure drop, and heat transfer, mostly based on experimental analysis. More recently, numerical simulation become an available tool providing deeper insight of these flows. In the present study, a number of numerical simulations of slug flows performed under different wall thermal conditions, the results have been compared to the available correlations, and gaps in existing models have been highlighted. The simulations have been carried out using the commercial package ANSYS Fluent. Film thickness, pressure drop, and heat transfer models under constant wall heat flux and constant temperature have been focused, and finally suggestion for future studies have been presented.

A Microchannel With Repeated Sharp Corners for Single Stream Particle Focusing

A new microfluidic device for fast and high throughput microparticle focusing is reported. The particle focusing is based on the combination of inertial lift force effect and centrifugal force effect generated in a microchannel with a series of repeated sharp corners on one side of the channel wall. The inertial lift force effect induces two focused particles streams in the microchannel, and the centrifugal force generated at the sharp corner structures tends to drive the particles laterally away from the corner. With the use of a series of the repeated, sharp corner structures, a single and highly focused particle stream was achieved near the straight channel wall at a wide range of flow rates. In comparison to other hydrodynamic particle focusing methods, this method is less sensitive to the flow rate and can work at a higher flow rate (high throughput). With its simple structure and operation, and high throughput, this method can be potentially used in microparticle focusing processes in a variety of lab-on-a chip applications.

Bubble Removal in Microfluidic Devices Using Nanofibrous Membranes

This paper presents an experimental study to determine bubble removal characteristics of nanofibrous membranes in microfluidic devices. It is well known that the presence of gas bubbles in fluidic channels can cause significant flow disturbances and adversely affect the overall performance and operation of microfluidic devices. In this study, a microfluidic device is designed and fabricated to generate and extract bubbles from a microfluidic channel. A T-junction is used to produce controllable bubbles at the entrance of fluidic channel. The generated bubbles are then transported to a bubble removal region and vented through a highly porous hydrophobic membrane. Four different hydrophobic PTFE membranes with different pore sizes ranging from 0.45 to 3 μm were used to permeate air bubbles. The fluidic channel width was 500 μm and channel height ranged from 100 to 300 μm. The effects of pore size, channel height, and liquid flow rate on the bubble removal rate are investigated. The results reveal that the rate of bubble removal increases with increasing the pore size and channel height but decreases with increasing the liquid flow rate.