Convective heat transfer in a parallel plate microchannel gas flow is investigated analytically and numerically, considering the effects of viscous dissipation, pressure work, shear work, axial conduction and rarefaction. Analysis is performed with constant wall temperature and constant wall heat flux boundary conditions for both gas cooling and heating. The results presented demonstrate the significance of the combined effect of pressure work and viscous dissipation, shear work, rarefaction degree and axial conduction on microchannel convective heat transfer, in both the thermally developing and fully developed flow regions. Viscous dissipation and pressure work in a pressure-driven microchannel gas flow are of comparable magnitudes and may not be neglected from the energy equation. The shear work at the wall, which is effectively the combined effect of viscous dissipation and pressure work, needs to be included in the Nusselt number for better predictions of heat transfer.

Heat handling has been a significant problem of the high power fiber laser systems as the output power increases rapidly. Cladding power stripper (CPS) which is used to deal with the unwanted optical power and light is required for higher cooling ability. So the methods of stripping the unwanted light attracted much attention recently, and the thermal effect is given. However, few investigations focus on the dissipation of the heat converted from the unwanted light. In this paper, an approach of active cooling for CPS is demonstrated. This is achieved by using microchannel cooling technology in heat sinking in CPS to improve the efficiency of heat exchange. In order to explain the mechanism of CPS the function of it and consistence of categories of the unwanted light are detailed firstly. Then microchannel heat sinking is proposed and verified by the heat exchange theory. At last, the design of the CPS with microchannel heat sinking is shown and following experiment is conducted. The final temperature of the device with 1000 W cladding power was demonstrated at last to verify the ability of heat distribution of the CPS component. This suggests that these CPSs can be used to stripe a thousand of watts of light in high power double cladding fiber lasers.

This study investigated the effect of rarefaction on microchannel gas flow by measuring pressure profiles in microchannels with various aspect ratios. Pressure-sensitive paint (PSP) was applied in rectangular microchannels to obtain the global flow field by using detailed pressure data. The effect of rarefaction on the microchannel gas flow was clearly observed in the microchannels through the pressure data obtained using PSP measurements. A nonlinear pressure distribution was observed inside the microchannels, and this distribution decreased as the Knudsen number (Kn) increased because of the rarefaction effect. The dimensionless pressure deviation from the linear assumption dropped from 0.25 to 0 when the outlet Kn number increased to 0.066 in the 100-μm-wide microchannel, and the dimensionless location of the maximum deviation moved upstream because of the gaseous slip at the wall. The nonlinear pressure distribution also decreased in the 50-μm-wide microchannel as the outlet Kn number increased; however, the peak of the maximum deviation could no longer be identified because of the characteristic of the narrow channel.

The stability of pressure-driven liquid-liquid viscosity-stratified microchannel flow is investigated by considering the combined effects of electrical double layer (EDL) and boundary slip. The electrical streaming currents determined by the Streaming Electrical Current Balance (ECB) and the boundary slip are considered by Navier slip assumption. The stability of flow is studied by the small disturbance theory. Numerical results indicate that the effect of boundary slip on the flow stability is strongly depended on the EDL. The boundary slip stables the flow when EDL effect is weak and destabilizes the flow when EDL effect is strong. The effects of boundary slip and EDL on flow stability is also determined by viscosity ratio, the height of channel, interface position and conductivity ratio.

Newtonian and non-Newtonian dispersed phase droplet formation in non-Newtonian continuous phase in T-junction and cross junction microchannels are investigated by the immiscible lattice BGK model. The effects of the non-Newtonian fluid power-law exponent, viscosity and interfacial tension on the generation of the droplet are studied. The final droplet size, droplet generation frequency, and detachment point of the droplet change with the power-law exponent. The results reveal that it is necessary to take into account the non-Newtonian rheology instead of simple Newtonian fluid assumption in numerical simulations. The present analysis also demonstrates that the lattice Boltzmann method is of potential to investigate the non-Newtonian droplet generation in multiphase flow.

Fluid-structure interaction (FSI) problems in microchannels play prominent roles in many engineering applications. The present study is an effort towards the simulation of flow in microchannel considering FSI. Top boundary of the microchannel is assumed to be rigid and the bottom boundary, which is modeled as a Bernoulli-Euler beam, is simulated by size-dependent beam elements for finite element method (FEM) based on a modified couple stress theory. The lattice Boltzmann method (LBM) using D2Q13 LB model is coupled to the FEM in order to solve fluid part of FSI problem. In the present study, the governing equations are non-dimensionalized and the set of dimensionless groups is exhibited to show their effects on micro-beam displacement. The numerical results show that the displacements of the micro-beam predicted by the size-dependent beam element are smaller than those by the classical beam element.

In this paper the pressure distribution of the gaseous flow in a microchannel is studied via a lattice Boltzmann equation (LBE) method. With effective relaxation times and a generalized second order slip boundary condition, the LBE can be used to simulate rarefied gas flows from slip to transition regimes. The Knudsen minimum phenomena of mass flow rate in the pressure driven flow is also investigated. The effects of Knudsen number (rarefaction effect), pressure ratio and aspect ratio (compression effect) on the pressure distribution are analyzed. It is found the rarefaction effect tends to the curvature of the nonlinear pressure distribution, while the compression effect tends to enhance its nonlinearity. The combined effects lead to a local minimum of the pressure deviation. Furthermore, it is also found that the relationship between the pressure deviation and the aspect ratio follows a pow-law.

The effects of boundary slip on the stability of finite miscible/immiscible liquid-liquid stratified microchannel flow were investigated. In this approach, the boundary slip was considered by Navier slip assumption and the finite-miscible liquid-liquid interface was modeled by double film model. The stability of the flow was studied by the small disturbance theory. The results indicated that the effects of boundary slip on the instability of finite miscible stratified microchannel flow with different viscosity ratio, interface location and the property of interface (i.e. thickness and viscosity distribution of mixed layer) are distinct and complex. The effect intensity of upper and lower boundary slip on flow stability is determined by viscosity ratio, interface structure (different Ns) and film thickness. When the interface changes from the channel center to the wall, the critical Re number is enhanced by boundary slip and especially markedly near the critical line and after across the critical line it suddenly decreases to a small value (even to 424). The flow stability always increased by boundary slip.

Gases in microfluidic structures or devices are often in a non-equilibrium state. The conventional thermodynamic models for fluids and heat transfer break down and the Navier-Stokes-Fourier equations are no longer accurate or valid. In this paper, the extended thermodynamic approach is employed to study the rarefied gas flow in microstructures, including the heat transfer between a parallel channel andpressure-driven Poiseuille flows through a parallel microchannel andcircular microtube. The gas flow characteristics are studied and it is shown that the heat transfer in the non-equilibrium state no longer obeys the Fourier gradient transport law. In addition, the bimodal distribution of streamwise and spanwise velocity and temperature through a long circular microtube is captured for the first time.

We present the results of a numerical simulation of a mixing process of two parallel streams of nitrogen (N2) and carbon monoxide (CO), in a microchannel with a splitter plate mounted at the inlet. As an attempt to enhance mixing efficiency, bumps are added on the channel walls, downstream the splitter. Typically, in such a passive micro device, the gas flow is laminar, rarefied, and characterized by a Knudsen number in the range of about 0.01 to 0.5, that is in the transition flow regime. The Direct Simulation Monte Carlo (DSMC) method is employed to explore the influence of the bumps and the splitter plate, and to analyze the sensitivity of the gas mixing length to parameters such as the inlet velocity as well as the height and location of the bumps. For a selected range of parameters, quantitative analytical and numerical results are in very good agreement.

An analytical study on entropy generation considering viscous dissipation effect in the circular microchannel is reported. The fluid flow is steady, laminar, hydrodynamically fully developed and thermally developing. In the first law analysis, appropriate dimensionless variables are applied to solve the energy equation in the thermal entrance region of microchannel. Subsequently the obtained temperature field is used to derive an expression for entropy generation rate. The effect of Knudsen number and Brinkman number on the entropy generation rate and Bejan number in different axial location is presented.

This paper investigates the flow boiling heat transfer in microchannels with the aim ofdeveloping compact cooling systems which can be adapted to miniaturized power components.Nano- and micro-surface treatments were used as innovative techniques to improve the heattransfer performance as well as to delay the intermittent dryout. It was observed that themicro-structured surfaces show significant enhancements (up to 85%) in heat transfercompared to the smooth surfaces. Especially, using the highly-wetted structured surface,the intermittent dryout is improved.

In this work we present an experimental investigation of the employment usage application of the vertical pixel binning (PB) option for PIV (particle image velocimetry) measurements. The PB option increases the speed of a CCD, at the cost of loosing spatial resolution. Consequently, it is expected that PB will positively impact the dynamic velocity range of the PIV measurements. In order to show the benefit of the CCD PB option in PIV measurements, we have carried out series of microPIV experiments on laminar flows, seeded with 1 μm fluorescent polystyrene microparticles and passing through a 200 μm × 200 μm × 50000μm microchannel. The flow images were recorded at normal, 2 × 1, and 3 × 1 vertical PB modes of a monochrome CCD camera. The experimentally obtained velocity profiles were calculated using the ensemble-averaged cross-correlation method and Gaussian sup-pixel interpolation and then compared with theoretically calculated velocity profiles. We found that the error introduced by the PB option did not exceed the inherent uncertainty of the PIV system used. For a particular PIV system CCD camera, using the PB option allowed an increase in the dynamic velocity range of a PIV system by more than a factor of two, without extra investments.

This paper models and analyzes flows in linear and curved microchannels on a rotating Laboratory Compact Disk (LabCD). The effects of centrifugal force are introduced into the governing equations of the microchannel flow to promote the fluidic velocity in the microchannel. The microchannel types on the LabCD must be designed following a process of mathematical identification. A flow model which takes into account the combined effects of viscosity, capillary forces, pressure difference and rotation is developed. A reduction-order technique is applied to obtain linear and nonlinear governing equations for flows in straight and curviform microchannels, respectively. The analytical solutions for the flow in the tubular microchannel are obtained using the Laplace transform method, while the numerical solutions for the curviform microchannel or microchannel with a varying cross-section are obtained using a piecewise linear method. The results show that the analyzed models are easily presented by a mathematical expression for the case of a tubular microchannel and simulated using a numerical program for the case of special microchannels. The modeling presented in this paper enables the performance of LabCD devices to be significantly enhanced by providing insights into the fluid flow behavior in microchannels of varying configurations under different rotational velocities.