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The term transport phenomena is used to describe processes in which mass, momentum, energy and entropy move about in matter. Advances in Transport Phenomena provide state-of-the-art expositions of major advances by theoretical, numerical and experimental studies from a molecular, microscopic, mesoscopic, macroscopic or megascopic point of view across the spectrum of transport phenomena, from scientific enquiries to practical applications. The annual review series intends to fill the information gap between regularly published journals and university-level textbooks by providing in-depth review articles over a broader scope than in journals. The authoritative articles, contributed by internationally-leading scientists and practitioners, establish the state of the art, disseminate the latest research discoveries, serve as a central source of reference for fundamentals and applications of transport phenomena, and provide potential textbooks to senior undergraduate and graduate students.

This review book provides state-of-the-art expositions of major advances by theoretical, numerical and experimental studies from a molecular, microscopic, mesoscopic, macroscopic or megascopic point of view across the spectrum of transport phenomena, from scientific enquiries to practical applications. This new volume of the annual review "Advances in Transport Phenomena" series provides in-depth review articles covering the fields of mass transfer, fluid mechanics, heat transfer and thermodynamics.

This review book provides state-of-the-art expositions of major advances by theoretical, numerical and experimental studies from a molecular, microscopic, mesoscopic, macroscopic or megascopic point of view across the spectrum of transport phenomena, from scientific enquiries to practical applications. This new volume of the annual review "Advances in Transport Phenomena" series provides in-depth review articles covering the fields of mass transfer, fluid mechanics, heat transfer and thermodynamics.



Optimization Principles for Heat Convection

Human being faces two key problems: world-wide energy shortage and global climate worming. To reduce energy consumption and carbon emission, it needs to develop high efficiency heat transfer devices. In view of the fact that the existing enhanced technologies are mostly developed according to the experiences on the one hand, and the heat transfer enhancement is normally accompanied by large additional pumping power induced by flow resistances on the other hand, in this chapter, the field synergy principle for convective heat transfer optimization is presented based on the revisit of physical mechanism of convective heat transfer. This principle indicates that the improvement of the synergy of velocity and temperature gradient fields will raise the convective heat transfer rate under the same other conditions. To describe the degree of the synergy between velocity and temperature gradient fields a non-dimensional parameter, named as synergy number, is defined, which represents the thermal performance of convective heat transfer. In order to explore the physical essence of the field synergy principle a new quantity of entransy is introduced, which describes the heat transfer ability of a body and dissipates during hear transfer. Since the entransy dissipation is the measure of the irreversibility of heat transfer process for the purpose of object heating the extremum entransy dissipation (EED) principle for heat transfer optimization is proposed, which states: for the prescribed heat flux boundary conditions, the least entransy dissipation rate in the domain leads to the minimum boundary temperature difference, or the largest entransy dissipation rate leads to the maximum heat flux with a prescribed boundary temperature difference. For volume-to-point problem optimization, the results indicate that the optimal distribution of thermal conductivity according to the EED principle leads to the lowest average domain temperature, which is lower than that with the minimum entropy generation (MEG) as the optimization criterion. This indicates that the EED principle is more preferable than the MEG principle for heat conduction optimization with the purpose of the domain temperature reduction. For convective heat transfer optimization, the field synergy equations for both laminar and turbulent convective heat transfer are derived by variational analysis for a given viscous dissipation (pumping power). The optimal flow fields for several tube flows were obtained by solving the field synergy equation. Consequently, some enhanced tubes, such as, alternation elliptical axis tube, discrete double inclined ribs tube, are developed, which may generate a velocity field close to the optimal one. Experimental and numerical studies of heat transfer performances for such enhanced tubes show that they have high heat transfer rate with low increased flow resistance. Finally, both the field synergy principle and the EED principle are extended to be applied for the heat exchanger optimization and mass convection optimization.
Zhi-Xin Li, Zeng-Yuan Guo

Nonequilibrium Transport: The Lagging Behavior

Lagging behavior describes the non-instantaneous response between heat flux and temperature gradient in nonequilibrium heat transport. Extending over to mass transport, mass flux is in place of the heat flux while density/concentration gradient is in place of the temperature gradient. The lagging behavior may occur during the ultrafast transient; in times comparable to the intrinsic times characterizing the nonequilibrium transition of thermodynamic states. For heat transport, such intrinsic times include the mean free time of energy carriers and the thermalization time for the energy carriers to come to thermal equilibrium. For mass transport involving different species, on the other hand, the intrinsic times include the finite time required for the effective interdiffusion among the participating substances, the finite time required for the chemical reactions to take place in forming the interfacial substance, or in some cases the finite times required for releasing or absorbing specific species. This chapter is dedicated to the lagging behavior during the ultrafast response in nonequilibrium heat/mass transport. The process is termed “ultrafast” because the time scales involved are comparable to the intrinsic time scales governing the various nonequilibrium processes in heat/mass transport. The essence of thermal lagging will be first illustrated by well known examples in microscale heat transport, with emphasis on the admissibility within the framework of nonequilibrium thermodynamics. Equivalence of the lagging behaviors in heat and mass transport then follows to stretch the lagging response over to the growth of the ultrathin film/interfacial compound, bioheat transfer, and multistage interdiffusion for drug delivery in tumor cells. In the full spectrum from the electron/phonon interactions in femto- to picoseconds, the heat exchange between blood and tissues and drug delivery in minutes to hours, to the ultrathin film/interfacial compound growth in days, the sources for lagging are identified and the analytical expressions for the phase lags are derived. A regime map based on the response time is established to characterize the alterations between wave and high-order diffusion as the phase lags of the various orders activate/diminish at certain time scales. The concept of thermal lagging, which is a nonequilibrium behavior in time, is extended to cover the nonlocal response in space in making contacts with ultrafast heat transport through the phonon gas with a finite mass. The correlation length and intrinsic times are linked together in describing the ultrafast thermal transport in small scales.
D. Y. Tzou, Jinliang Xu

Microfluidics: Fabrication, Droplets, Bubbles and Nanofluids Synthesis

Present studies include a series of investigations on microfluidics from fabrication to application. In the application this study focuses on some fundamental problems regarding the manipulation of droplets and bubbles inside microfluidics, including the size control during generation, the critical condition for breaking droplets, chaotic mixing inside moving droplets and nanofluids synthesis.
A low-cost fabrication method for manufacturing glass-based microfluidic devices is developed in a routine laboratory without the requirement of a clean room. Direct bonding of glass material is realized without using any additives. The fabrication method is very reliable and the yield is larger than 90%. Channel surface modification from hydrophilic to hydrophobic is realized inside the microfluidic devices after fabrication. With the help of the surface modification, both of the two types of droplets, oil-in-water and water-in-oil, can be generated inside glass-based microfluidic devices.
Droplet formation under controlled flow rates and bubble formation under controlled pressures in confined T-shaped junctions are experimentally investigated by changing thermophysical properties of continuous phases and by changing the controlled dynamic parameters. A pressure-driven mechanism of droplet/bubble formation is experimentally discovered. The influence of the continuous phase thermophysical properties and the controlled dynamic parameters on droplet/bubble volume and formation time is systematically investigated. Empirical correlations are obtained for predicting the droplet volume and formation time.
Droplet breakup in either symmetrically or asymmetrically confined T-shaped junctions is experimentally studied. The critical condition with which microfluidic droplets will break equally is theoretically analyzed based on the pressure-driven mechanism. A semi-empirical correlation is obtained for predicting the equal breakup in symmetric T-shaped junctions. Besides the equal breakup, a new droplet breakup pattern, unequal breakup, is observed in the symmetric T-shaped junction. In asymmetric T-shaped junctions the droplet breakup is found to be very difficult.
Scaling analysis of the chaotic mixing inside moving droplets is conducted based on the idealized recirculating flow and the Baker’s transformation. Experimental investigations on the mixing efficiency inside the droplets moving in curved microchannels are performed with the help of the micro visualization system. It is found that the mixing efficiency is significantly enhanced by the chaotic advection inside the moving droplets and the full mixing time can be reasonably estimated by the scaling analysis. A significant mixing enhancement during the droplet formation process is observed, which is not so frequently reported by others. An effective microstructure is designed for mixing two or more individual droplets after their formation.
Synthesis of copper nanofluids is realized in microfluidic reactors. In contrast to the traditional method, the copper nanofluids synthesized in the microfluidic reactors have a narrower size distribution. The synthesis time is also reduced by one order of magnitude. It is also found that the particle size and size distribution are insensitive to the flow rate of reactants, the reactants concentration and the surfactant concentration.
Yuxiang Zhang, Liqiu Wang

Multi-scale Modelling of Liquid Suspensions of Micron Particles in the Presence of Nanoparticles

A combined continuous, discrete, and statistic mechanics (CCDS) method is proposed to model micron particle dynamics in the presence of nanoparticles – a highly asymmetric system. The CCDS method treats the liquid medium as a continuum and the micron particles as a discrete phase, whereas the statistics mechanics method is used to treat the nanoparticles. The treatment of the nanoparticles involves the use of the Ornstein-Zernike equation with Percus-Yevick approximation based on the hard-sphere interaction. Such an approach enables the effective coupling between different length scales. Sedimentation of micron particles in the presence of nanoparticles is used as a case study for the CCDS method. It is shown that, at a high salt concentration where electrostatic repulsive force is significantly screened, the structural force induced by both monodisperse and bidisperse nanoparticles could overcome the van der Waals attractive force between the micron particles and thus prevent particle flocculation. It is also shown that the introduction of disparity in the system complicates the effective interactions between the micron particles and consequently the particle dynamics.
Chane-Yuan Yang, Yulong Ding


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