This article examines the role of slip conditions within surface-embedded microchannels for reducing entropy production of external flows with convective heat transfer. Viscous dissipation of mechanical energy into internal energy within the boundary layer leads to pressure losses and other irreversible losses of energy availability. These exergy losses entail additional input power needed to deliver a fixed mass flow across the surface, subject to a specified rate of heat transfer to/from the wall. By selectively altering geometrical and surface parameters which minimize the net entropy production, the benefits of drag reduction due to the slip-flow conditions can outweigh the higher irreversibility arising from added microchannel area. Predicted results illustrate the changes of optimal Reynolds number and entropy generation number with varying surface parameters for embedded parallel and diverging microchannels. Based on these results, it is viewed that surface micro-profiling offers a useful new technique of taking advantage of slip-flow microfluidic conditions for reducing drag and simultaneously increasing heat transfer effectiveness in external flows.

In recent years, micro-technologies have become very important to cutting edge industries such as aerospace and biotechnology. In the fluidic research area, two-phase flow study in microchannels has been an emerging topic in the past few years. Characteristics of two-phase flow in microchannels such as flow regimes, pressure drop, void fraction, and heat transfer are now being extensively studied by numerous groups. The ultimate goal of this research ranges from compact heat exchangers to small-sized refrigeration systems. One of the major drawbacks in this area to date is the lack of a universal flow regime map enabling the prediction of flow regimes in microchannels. In the present study, a new test rig was designed and constructed to extend the range of the existing data on flow regimes. Several flow regime maps, for a hydraulic diameter of less than 1.0 mm obtained from a comprehensive literature review, are tested and compared. A total of approximately 1475 experimental data points from present and previous studies performed in channels with Dh ≤ 1.0 mm were used for comparison. Two universal flow regime maps were created, one each for horizontal and vertical channels with a hydraulic diameter ranging between 0.1 mm and 1.0 mm. The resulting universal maps presented here are based on flow regimes observed in all the studies on two-phase flow in microchannels. We suggest that the use of these flow regimes could help diminish the amount of discord present among research groups pertaining to the definition of flow regimes. Based on the comparisons between the universal maps and different experimental flow regime maps, appropriate conclusions on the effects of channel orientation and geometry are suggested.

By solving Maxwell's equations, we observed photon localization and strong electromagnetic field enhancement in laser-irradiated, one-dimensional random multilayer and two-dimensional random media, with particle size around the laser wavelength. Higher refractive index contrast and degree of randomness result in stronger localization. The probability density and the location of enhanced fields are investigated as functions of refractive index and particle size distribution. For weakly dissipative media, local absorption enhancement is observed, and the expectation intensity distribution is obtained by averaging among many realizations. For comparison, the equation of radiative transfer (ERT) is also solved for random porous media, using the two-flux model. Since no interference effects are allowed, the classical diffusion, rather than localization, is predicted. Consequently, ERT can represent a good statistical average of the intensities only as absorption is dominant over localization. As expected, the particle treatment of ERT does not allow for predicting photon localization and field enhancement. 236pt

We investigate the thermal response of a silicon crystal irradiated by a pico-femto heat pulse by using molecular dynamics technique and a linear response theory–based statistical analysis. The thermal susceptibility is first defined in terms of computed quantities and then convoluted with time Gaussian temperature pulses. The qualitative difference between the responses to various pulse durations is explained and the results are compared to those of classical Fourier model. Non-Fourier behaviors are emphasized and a mean phonon relaxation time is identified.

Recent advances in cold cathode field emitters have produced improvements in emission efficiency and thus enhanced the applicability of such devices. This article considers heating due to electron field emission from a polycrystalline diamond cathode. Temperature measurements from infrared thermography reveal highly localized heating due to nonuniform emission from the cathode. Power dissipation of 0.36 W produces a peak temperature increase of 12.5 K on the back surface of the water-cooled anode. Parameter estimation techniques are employed to predict the effective size of the heat source and the fraction of total power that is dissipated near hot spots. The estimates indicate effective heated radii of order 1 mm and power fractions in the range of 25% to 50%.

Intense centrifugal fields can be exploited to increase the electrochemical reaction rates in energy conversion devices, such as batteries and fuel cells, to improve their overall performance characteristics. This paper describes how “spin-enhanced natural circulation,” created under high centrifugal acceleration, can be used as a means for promoting an increase in mass transfer and reaction rates in three-dimensional porous electrodes. Some relevant scaling laws and new theoretical insights will be discussed to identify the main governing parameters in the design of centrifugal electrochemical energy conversion systems. Recent findings obtained during the development and testing of a small experimental prototype centrifugal zinc-air battery are presented to indicate possible performance trends and future prospects for this novel approach.

In this study, direct simulation Monte Carlo (DSMC) method, the Navier-Stokes equations, and the Burnett equations are applied to micro-Couette flow to investigate flow and heat transfer characteristics in the slip and transition flow regimes. The unique solutions of the Burnett equations are obtained through determination of additional boundary condition for pressure using the results of DSMC. The effect of rarefaction on velocity, temperature, and pressure distributions is analyzed. The nondimensional parameter PrE characterizing convective heat transfer in Coutte flow increases as flow becomes more rarefied. The prediction of shear stress and heat flux at wall shows that the solution of the Burnett equations is superior than that of the Navier-Stokes equations at relatively high Kn number in the slip flow regime, but the Burnett equations with the slip boundary condition, which is proportional to Kn number, cannot be extended to the transition flow regime.

We analyze one-dimensional plane Couette flows in the entire Knudsen regime with the objective of modeling shear-driven rarefied gas flows encountered in various microelectromechanical system (MEMS) components. Using the linearized Boltzmann solutions available in the literature and hard sphere direct simulation Monte Carlo (DSMC) results, we develop a unified empirical model that includes analytical expressions for the velocity distribution and shear stress for steady plane Couette flows. We also present extension of this model to time-periodic oscillatory Couette flows. Comparisons between the extended model and ensemble averaged unsteady DSMC computations show good agreements in the quasi-steady flow limit, where the Stokes number (β) based on the plate separation distance and oscillation frequency is ≤ 0.25. Overall, the new model accurately predicts the velocity distribution and shear stress for steady and quasi-steady (β ≤ 0.25) flows in a wide Knudsen number range (Kn ≤ 12), and it is strictly valid for low subsonic flows with Mach number ≤ 0.3.

Heat exchangers with corrugated walls are well known as macro heat exchangers. In previous systematic analyses heat exchangers with crosswise corrugated walls at large Reynolds numbers were analyzed. The wall corrugation was generated by sinusoidally shaped walls as well as by crosswise corrugated cylinders of different cross sections on plane walls, such as those being used as spacers in membrane technology. The low-Reynolds-number range of these structures is also interesting for application as micro heat exchangers. Therefore first results will be presented, where such structures are experimentally analyzed in the low-Reynolds-number range up to Re S 10, especially interesting for applications as micro heat exchangers. For the use of corrugated walls in the free-flow arrangement, first new results at low Reynolds numbers for this heat exchanger geometry are presented, where the transition from turbulent to laminar flow range is dominated by the Görtler-Dean instability.

Momentum transfer in a sheared liquid is analyzed with a focus on the intermolecular momentum transfer (IMT). The IMT, which determines shear viscosity of liquids, is examined based on the result of molecular dynamics (MD) simulations for a simple liquid modeled by the Lennard-Jones (12-6) potential. Distortion of liquid structure, which is characterized by spherical symmetry breaking in the pair distribution of molecules, is reproduced by the MD simulation and its effect on the characteristics of the IMT, which includes a negative momentum transfer in a certain range of intermolecular distance, is discussed. The appearance of negative IMT depends on characteristics of the intermolecular potential, and it is predicted that the IMT will show a different response for repulsive soft-sphere liquids.