Mass Transfer In Microchannels Research Articles

Using Direct Simulation Monte Carlo With Improved Boundary Conditions for Heat and Mass Transfer in Microchannels

Micro-electromechanical systems and nano-electromechanical systems have attracted a great deal of attention in recent years. The flow and heat transfer behaviors of micromachines for separation applications are usually different from that of macro counterparts. In this paper, heat and mass transfer characteristics of rarefied nitrogen gas flows in microchannels are investigated using direct simulation Monte Carlo with improved pressure boundary conditions. The influence of aspect ratio and wall temperature on mass flowrate and wall heat flux in microchannels are studied parametrically. In order to examine the aspect ratio effect on heat and mass transfer behaviors, the wall temperature is set constant at 350 K and the aspect ratio of the microchannel varies from 5 to 20. The results show that as the aspect ratio increases, the velocity of the flow decreases, so does the mass flowrate. In a small aspect ratio channel, the heat transfer occurs throughout the microchannel; as the aspect ratio of the microchannel increases, the region of thermal equilibrium extends. To investigate the effects of wall temperature (Tw) on the mass flowrate and wall heat flux in a microchannel, the temperature of the incoming gas flow (Tin) is set constant at 300 K and the wall temperature varies from 200 K to 800 K while the aspect ratio is remained unchanged. Results show that majority of the wall heat flux stays within the channel entrance region and drops to nearly zero at the halfway in the channel. When Tw<Tin, under the restriction of pressure-driven condition and continuity of pressure, the molecular number density of the flow decreases along the flow direction after a short increase at the entrance region. When Tw>Tin, the molecular number density of the flow drops rapidly near the inlet and the temperature of the gas flow increases along the channel. As Tw increases, the flow becomes more rarefied, the mass flowrate decreases, and the resistance at the entrance region increases. Furthermore, when Tw>Tin, a sudden jump of heat transfer flux and temperature are observed at the exit region of the channel.

Modeling Micro Mass and Heat Transfer for Gases Using Extended Continuum Equations

This paper presents recent contributions to the development of macroscopic continuum transport equations for micro gas flows and heat transfers. Within the kinetic theory of gases, a combination of the Chapman–Enskog expansion and the Grad moment method yields the regularized 13-moment equations (R13 equations), which are of high approximation order. In addition, a complete set of boundary conditions can be derived from the boundary conditions of the Boltzmann equation. The R13 equations are linearly stable, and their results for moderate Knudsen numbers stand in excellent agreement with direct simulation Monte Carlo (DSMC) method simulations. We give analytical expressions for heat and mass transfer in microchannels. These expressions help to understand the complex interaction of fluid variables in microscale systems. Additionally, we compare interesting analogies such as a mass flux and energy Knudsen paradox. In particular, the R13 model is capable of predicting and explaining the detailed features of Poiseuille microflows.

Direct numerical simulations of mass transfer in square microchannels for liquid–liquid slug flow

Microreactors for the development of liquid–liquid processes are promising technologies since they are supposed to offer an enhancement of mass transfer compared to conventional devices due to the increase in the surface/volume ratio. But impact of the laminar flow should be negative and the effect is still to be evaluated. The present work focuses on the study of mass transfer in microchannels by means of 2D direct numerical simulations. We investigated liquid–liquid slug flow systems in square channel of 50 – 960 μ m depth. The droplet velocity ranges from 0.0015–0.25 m/s and the ratio between the channel depth and the droplet length varies between 0.4 and 11.2. Droplet side volumetric mass transfer coefficients were identified from concentration field computations and the evolution of these coefficients as a function of the flow parameters and the channel size is discussed. This study reveals that mass transfer is strongly influenced by the flow structure inside the droplet. Moreover, it shows that the confinement of the droplets due to the channel size leads to an enhancement of mass transfer compared to cases where the droplets are not constrained by the walls.

A detection system for microfluidic chips based on epifluorescence videomicroscopy and its analytical potentials

An epifluorescence videomicroscopy system with a light-emitting diode as an excitation source and a charge-coupled device matrix as a radiation detector was developed and studied. The system was used for studying mass transfer in microchannels of single- and two-phase systems. It was shown that the developed detection system is suitable for studying mass transfer on a microchip. A one-dimensional physical and mathematical model was developed for mixing processes on a chip. A comparative analysis of experimental results and model concepts demonstrated the diffusion nature of the fluid flow mixing processes. Mass transfer in the two-phase water-octanol system was studied. Dye concentration profiles in a two-phase flow were recorded depending on the time of phase contact. The diffusion nature of mass transfer in the two-phase system was found.