Transport Phenomena In Microchannels Research Articles

Transport phenomena in microchannels in liquid–liquid extraction (LLE) systems operating in a slug flow regime—A review

AbstractIn the present work, the effects of various parameters (capillary size, shape, flow ratio, presence of additives, and presence of wall film) on both reactive and non‐reactive LLE systems are presented and analyzed. The literature data for a constant liquid flow ratio of 1 (Qc/QD = 1) has been correlated to give rise to two empirical correlations relating the Sherwood number (Sh), Reynolds number (Re), capillary number (Ca), and Schmidt number (Sc) have been developed for 0.1 < Re < 10 and 10 < Re < 200. The concept of j‐factor for micro‐channels has been introduced and named as Pandit–Kunzru–Ganguli analogy. Three new dimensionless numbers have been introduced, namely Pandit number (), Ganguli number , and Kunzru number (). The j‐factor is found to be a strong function of Capillary number for 10 < Re < 200 while it is a strong function of surface renewal for 0.1 < Re < 10. Empirical correlations to calculate j‐factor as a function of Re or Weber number (We) are also presented. Recommendations for future work have been presented based on the review in the present work.

Theoretical Aspects on Heat Transfer and Fluid Flow in Micro Channels

The past ten years have seen a lot of research on the issues of heat transfer and fluid flow in micro-channels. With an emphasis on upcoming research requirements, a critical analysis of the state of research as it stands is offered. Following a brief introduction, the study discusses six themes related to transport phenomena in micro-channels: condensation, cooling of electronics, single-phase gas flow, augmentation of single-phase liquid flow and flow boiling, and micro-scale heat exchangers. In this study, we investigate the three-dimensional heat transfer and water flow properties in a set of rectangular micro-channel heat sinks for advanced electronic systems. Over the past ten years, mini/micro channel type compact heat exchangers have received a lot of attention. The ability to make heat exchangers smaller, lighter, and cheaper than those in use now is the primary motivator. Additionally, emerging applications that call for the cooling of small things, such electronics and micro-electro-mechanical devices, need for heat exchangers with tiny channels. Theoretically, it is examined how a nanofluid moves and transfers heat through a horizontal micro channel while being affected by a magnetic field and an electric double layer (EDL). The flow problem for a micro channel with a large aspect ratio is handled as a two-dimensional nonlinear system. The magnetic field and EDL body force are taken into account while calculating momentum equation

Heat Transfer in Microchannels—2012 Status and Research Needs

Abstract Heat transfer and fluid flow in microchannels have been topics of intense research in the past decade. A critical review of the current state of research is presented with a focus on the future research needs. After providing a brief introduction, the paper addresses six topics related to transport phenomena in microchannels: single-phase gas flow, enhancement in single-phase liquid flow and flow boiling, flow boiling instability, condensation, electronics cooling, and microscale heat exchangers. After reviewing the current status, future research directions are suggested. Concerning gas phase convective heat transfer in microchannels, the antagonist role played by the slip velocity and the temperature jump that appear at the wall are now clearly understood and quantified. It has also been demonstrated that the shear work due to the slipping fluid increases the effect of viscous heating on heat transfer. On the other hand, very few experiments support the theoretical models and a significant effort should be made in this direction, especially for measurement of temperature fields within the gas in microchannels, implementing promising recent techniques such as molecular tagging thermometry (MTT). The single-phase liquid flow in microchannels has been established to behave similar to the macroscale flows. The current need is in the area of further enhancing the performance. Progress on implementation of flow boiling in microchannels is facing challenges due to its lower heat transfer coefficients and critical heat flux (CHF) limits. An immediate need for breakthrough research related to these two areas is identified. Discussion about passive and active methods to suppress flow boiling instabilities is presented. Future research focus on instability research is suggested on developing active closed loop feedback control methods, extending current models to better predict and enable superior control of flow instabilities. Innovative high-speed visualization and measurement techniques have led to microchannel condensation now being studied as a unique process with its own governing influences. Further work is required to develop widely applicable flow regime maps that can address many fluid types and geometries. With this, condensation heat transfer models can progress from primarily annular flow based models with some adjustments using dimensionless parameters to those that can directly account for transport in intermittent and other flows, and the varying influences of tube shape, surface tension and fluid property differences over much larger ranges than currently possible. Electronics cooling continues to be the main driver for improving thermal transport processes in microchannels, while efforts are warranted to develop high performance heat exchangers with microscale passages. Specific areas related to enhancement, novel configurations, nanostructures and practical implementation are expected to be the research focus in the coming years.

Transport, phase transitions, and wetting in micro/nanochannels: A phase field/DDFT approach

While the flow of a liquid in a macroscopic channel is usually described using hydrodynamics with no-slip boundary conditions at the walls of the channel, transport phenomena in microchannels involve physics at many different scales due to the interplay between the micrometric section of the channel and the micro- or nanometric roughness of the boundaries. Roughness can have many different effects such as increasing the friction between the liquid and the walls (leading to the macroscopic no-slip boundary condition) or on the contrary reduce it thanks to the Wenzel-Cassie-Baxter wetting transition induced by capillarity. Here we detail a phase-field/dynamic density functional theory model able to account for the wetting transitions, the resulting friction between the wall and the fluid, and compressible hydrodynamics at high viscosity contrast.

CFD simulation with detailed chemistry of steam reforming of methane for hydrogen production in an integrated micro-reactor

micro-reactor has drawn more and more attention in recent years due to the process intensification on basic transport phenomena in micro-channels, which would often lead to the improved reactor performance. Steam reforming of methane (SRM) in micro-reactor has great potential to realize a low-cost, compact process for hydrogen production via an evident shortening of reaction time from seconds to milliseconds. This work focuses on the detailed modeling and simulation of a micro-reactor design for SRM reaction with the integration of a micro-channel for Rh-catalyzed endothermic reaction, a micro-channel for Pt-catalyzed exothermic reaction and a wall in between with Rh or Pt-catalyst coated layer. The elementary reaction kinetics for SRM process is adopted in the CFD model, while the combustion channel is described by global reaction kinetics. The model predictions were quantitatively validated by the experimental data in the literature. For the extremely fast reactions in both channels, the simulations indicated the significance of the heat conduction ability of the reactor wall as well as the interplay between the exothermic and endothermic reactions (e.g., the flow rate ratio of fuel gas to reforming gas). The characteristic width of 0.5 mm is considered to be a suitable channel size to balance the trade-off between the heat transfer behavior in micro-channels and the easy fabrication of micro-channels.

Investigation of two-phase transport phenomena in microchannels using a microfabricated experimental structure

Microchannels (0.05–1 mm) improve gas routing in proton exchange membrane fuel cells, but add to the complexities of water management. This work microfabricates experimental structures with distributed water injection as well as with heating and temperature sensing capabilities to study water formation and transport. The samples feature optical access to allow visualization and distributed thermometry for investigation of two-phase flow transport phenomena in the microchannels. The temperature evolution along the channel is observed that the temperature downstream of the distributed water injection decreases as the pressure drop increases. As the water injection rate is lower than 200 μl/min, there exists a turning point where temperature increases as the pressure drop increases further. These micromachined structures with integrated temperature sensors and heaters are key to the experimental investigation as well as visualization of two-phase flow and water transport phenomena in microchannels for fuel cell applications.

Numerical simulation of microfluidic injection processes in crossing microchannels

The design and operation control of microfluidic devices have drawn a great deal of attention over the last decade due to the emerging lab-on-a-chip applications. Cross-shaped microchannels connecting liquid reservoirs are typical configurations of the microfluidic chips. Normally the microchannels have a large length-to-width aspect ratio (typically 1500:1), therefore, the transport phenomena in these microchannels are essentially multiscale and multidimensional problems. There are no analytical solutions existing for such kind of problems and it has been found that effectively and efficiently simulating the transport phenomena in such microchannels is very difficult. A numerical model developed here uses the designed boundaries to truncate the physical domain to a small computation domain in order to concentrate computing power in the areas exhibiting multidimensional phenomena (such as intersections) and apply analytical functions in the areas of one-dimensionality (such as fully developed flow region). This model is employed to simulate the flow and mass transport processes in a planar glass chip with a cross-shaped microchannel, and the model predictions are compared to the experimental results. Agreement between the model predictions and experimental results verified that this newly developed model is capable of accurately and efficiently simulating the transport phenomena in microfluidic devices.

NUCLEATION AND THERMODYNAMIC NONEQUILIBRIUM FOR BOILING IN MICROCHANNELS AND MICROSTRUCTURES

We review recent experimental and theoretical investigations that have contributed to developments and advances in boiling phase-change and transport phenomena in microchannels and microstructures, including bubble formation during boiling, phase transition, and the fluid flow and heat transfer characteristics. These investigations showed that bubble formation and phase transition in open and confined microstructures and the flow boiling heat transfer of pure water, pure methanol, and water-methanol binary mixtures through rectangular and triangular microchannels display some unusual phenomena and characteristics. The experimental results showed that the microchannel size, the geometric configuration including the aspect ratio (H/W) of the rectangular microchannels and the groove angle (6) of triangular microchannels, and the mole fraction of water-methanol binary mixtures have a significant impact on the flow boiling, especially for nucleate boiling, and on bubble formation and growth in microchannels. For boiling and nucleation in microchannels, two new concepts, evaporating space and fictitious boiling, were proposed to explain the unusual phase-change transport phenomena. We use classical thermodynamics to analyze the phase transition in microchannels and the thermodynamic aspects of boiling. For determining the phase transition in microchannels. we derive a dimensionless number and related criterion. The nonequilibrium effects during bubble formation and phase-change transitions are discussed theoretically, and the dynamics of microchannel flow boiling are analyzed to understand the phenomena. We propose a cluster model to describe the bubble formation and dynamic flow boiling process. The model shows that the phase-change transition and bubble formation are the consequence of cluster growth and interaction among clusters. The properties of nonequilibrium liquids with clusters are expected to be quite different from normal liquids, since they result in unconventional heat transport phenomena.