Laminar forced convective heat transfer for a gas flowing through a microchannel of arbitrary cross section with the axially-constant heat flux and circumferentially-varied wall temperature boundary condition in the slip-flow and temperature-jump regime is studied theoretically. The dimensionless temperature profile and the average Nusselt numbers are obtained by solving the fluid flow and energy equations for hydrodynamically and thermally fully developed incompressible slip flow by means of a computation-oriented method of the orthonormal function analysis. The work is then focused on studying the heat transfer characteristics of rectangular and triangular microchannels, which are more frequently encountered in real applications. The effects of Knudsen number, aspect ratio, and thermal boundary conditions on the heat transfer characteristics of these two types of microchannels are examined. The results show that the orthonormal function method is applicable to the fluid flow and heat transfer problems with the slip-velocity and temperature-jump boundary conditions for unsymmetrically heated microchannels with an arbitrary cross section. It is also shown that the increased thermal resistance caused by the temperature jump at the channel walls predominates in the overall heat transfer behavior in microchannels within a definite extension of Knudsen number, leading to a decrease in the heat transfer coefficient. Thus, the average Nusselt numbers in the slip flow are generally smaller than that in the no-slip-flow.

The experimental and theoretical heat generation behavior of a trickling biofilter treating toluene is discussed. The experimental results show that the temperature of the packed bed has a significant effect on the purification performance of the trickling biofilter and that an optimal operation temperature exists between 30 and 40℃. During the gas-liquid co-current flow, the temperature in the packed bed gradually rises along the direction of the gas and liquid flow due to the exothermic biodegradation of toluene. The temperature rise between the inlet and outlet of the trickling biofilter increases with an increase in the gas flow rate and inlet toluene concentration. In addition, a larger liquid flow rate leads to a smaller temperature rise. The heat generation process occurring in the trickling biofilter is modeled by representing the packed bed as an equivalent set of parallel capillary tubes covered by the biofilm. The temperature profile in the packed bed during the liquid-gas co-current flow is analyzed by simultaneously solving the problem of gas-liquid two-phase flow and heat and mass transfer through the liquid film and biofilm. It is shown that the model agrees well with the experimental data, predicting the variations of the temperature rise between the inlet and outlet of trickling biofilter with the increasing gas and liquid flow rates.

A numerical investigation on the mixed electro-osmotic and pressure-driven flows in triangle microchannels with constant wall temperature is reported in the present study. The Galerkin method is employed to solve the Poisson equation, energy equation and Navier-Stokes equations for the flow driven by electroosmotic and pressure gradient synchronously under the conditions of favorable pressure gradients and backpressure gradients. The physical properties of the electrolyte solution are considered to be varying with the temperature, and the dimensionless velocity profile, dimensionless temperature profile as well as dimensionless mass flux of the electrolyte solution are obtained. Furthermore, the parameters studies including pressure gradient, length ratio and Joule heating on mass flux of the electro-osmotic flows are performed, respectively. The numerical results show that a large Joule number leads to large dimensionless mass flux and dimensionless temperature of the electrolytic solution in the triangular microchannels for both conditions. For the electro-osmotic flows under the favorable pressure gradient, the increase in dimensionless mass flux resulted from Joule heating is enlarged with increasing pressure gradient and length ratio. However, for the electro-osmotic flows under the backpressure gradient, Joule heating results in a reverse flow in the channel, and the dimensionless mass flux of the reverse flow increases with increasing backward pressure gradient and decreasing length ratio. It is further found that the Joule heating induces a more significant increase in the dimensionless mass flux under favorable pressure gradient compared with that under backpressure gradient.

本文建立了固体表面上静止液滴的势能方程，根据能量最小化原理，当系统总势能取得最小值时，液滴将处于平衡状态。采用有限元方法，将初始自由液面离散化，通过曲面上节点的虚拟位移，改变自由液面的拓扑结构，使系统总势能取得最小值，从而得到静止液滴的形状。并应用该方法对均质表面和梯度表面能材料表面上的液滴界面进行了数值模拟，得到了均质材料表面和梯度表面能材料表面上静止液滴的界面形状及分布。

Experiments were carried out to study the heat transfer and friction characteristics for water, ethylene glycol, and ISO VG46 turbine oil flowing inside four tubes with three-dimensional internal extended surfaces and copper continuous or segmented twisted-tape inserts. During the experiments, Prandtl numbers ranged from 5.5 to 590 and Reynolds numbers from 80 to 50,000. The experimental results show that this compound enhanced heat transfer technique, a tube with three-dimensional internal extended surfaces and twisted-tape inserts, is of particular advantage to enhance the convective heat transfer for the laminar tubeside flow of highly viscous fluid. For the laminar flow of VG46 turbine oil, the average Stanton number could be enhanced up to 5.8-fold inside tubes with three-dimensional internal extended surfaces and continuous twisted-tape inserts compared with an empty smooth tube, and the friction factor was also increased by almost 6.5-fold. Inside the tubes with three-dimensional internal extended surfaces, replacement of the continuous twisted-tape inserts with the segmented twisted-tape inserts induced a greater decrease in the friction factor but a comparatively smaller decrease in the Stanton number.

Small triangle channels are frequently encountered in microelectromechanical systems (MEMS). In this paper, the mixed electroosmotic/pressure-driven flows in triangle microchannels are numerically investigated. Poisson and Navier–Stokes equations were numerically solved employing the Galerkin algorithm. The accuracy of the numerical algorithm was validated by refining the grids and using Richardson extrapolation. The numerical results show the mass flux of electrolytic solution increases with the increase of imposed positive pressure gradient. At small pressure gradients, the relative mass flux increment induced by the imposed positive pressure gradient is sensitive to the change in the pressure gradient. However, when the pressure gradient increases beyond a critical value, the relative mass flux increment increases smoothly with the increase of pressure gradient. It is also found that the mass flux increases with increasing the length ratio, jDh, at a given pressure gradient.

The dynamic behavior of liquid water emerging from the gas diffusion layer (GDL) into the gas flow channel of a polymer electrolyte membrane fuel cell (PEMFC) is modeled by considering a 1000 m long air flowmicrochannel with a 250μm×250μm square cross section and having a pore on the GDL surface through whichwater emerges with prescribed flowrates. The transient three-dimensionaltwo-phase flow is solved using Computational fluid dynamics in conjunction with a volume of fluid method. Simulations of the processes of water droplet emergence, growth, deformation and detachment are performed to explicitly track the evolution of the liquid–gas interface, and to characterize the dynamics of a water droplet subjected to air flowin the bulk of the gas channel in terms of departure diameter, flowresistance coefficient, water saturation, and water coverage ratio. Parametric simulations including the effects of air flow velocity, water injection velocity, and dimensions of the pore are performed with a particular focus on the effect of the hydrophobicity of the GDL surface while the static contact angles of the other channel walls are set to 45◦. The wettability of the microchannel surface is shown to have a major impact on the dynamics of the water droplet, with a droplet splitting more readily and convecting rapidly on a hydrophobic surface, while for a hydrophilic surface there is a tendency for spreading and film flow formation. The hydrophilic side walls of the microchannel appear to provide some benefit by lifting the attached water from the GDL surface, thus freeing the GDL-flow channel interface for improved mass transfer of the reactant. Higher air inlet velocities are shown to reduce water coverage of the GDL surface. Lower water injection velocities as well as smaller pore sizes result in earlier departure of water droplets and lower water volume fraction in the microchannel.

A numerical investigation of the dynamic behaviour of liquid water entering a polymer electrolyte membrane fuel cell (PEMFC) channel through a GDL pore is reported. Two-dimensional, transient simulations employing the volume of fluid (VOF) method are performed to explicitly track the liquid–gas interface, and to gain understanding into the dynamics of a water droplet subjected to air flow in the bulk of the gas channel. The modeled domain consists of a straight channel with air flowing from one side and water entering the domain from a pore at the bottom wall of the channel. The channel dimensions, flow conditions and surface properties are chosen to be representative of typical conditions in a PEMFC. A series of parametric studies, including the effects of channel size, pore size, and the coalescence of droplets are performed with a particular focus on the effect of geometrical structure. The simulation results and analysis of the time evolution of flow patterns show that the height of the channel as well as the width of the pore have significant impacts on the deformation and detachment of the water droplet. Simulations performed for droplets emerging from two pores with the same size into the channel show that coalescence of two water droplets can accelerate the deformation rate and motion of the droplets in the microchannel. Accounting for the initial connection of a droplet to a pore was found to yield critical air inlet velocities for droplet detachment that are significantly different from previous studies that considered an initially stagnant droplet sitting on the surface. The predicted critical air velocity is found to be sensitive to the geometry of the pore, with higher values obtained when the curvature associated with the GDL fibres is taken into account. The critical velocity is also found to decrease with increasing droplet size and decreasing GDL pore diameter.

Visualization experimental investigation on the phenomena and dynamic behavior of coalescence between two equal-size water drops on inclined surfaces is reported in this paper. The effects of inclination angle of the surface and drop size on the dynamics of liquid bridge, fore/back contact angle and triple-phase contact line of the coalescent drop are discussed. The results show that highly concave interface around the liquid bridge and highly convex interface at the retracted top-side parts of two drops built surface tension gradient to drive the interior liquid flow in early stage of the coalescence, while changeable surface tension gradient between highly convex interface at the forepart and near flattened interface at the back part of the coalescent drop and adverse gravity induced the interior flow and the oscillation process later. Coalescence promoted the motion of the coalescent drop on the inclined surface. The released surface energy and potential energy compensated the internal viscous dissipation and the friction, and the fraction for friction increased with increasing inclination angle and drop size. For coalescence of small drops on the inclined surface, the triple-phase contact line, liquid bridge radius and fore/back contact angle of the coalescent drop behaved as periodic damped oscillations. Larger inclination angle of the surface induced severe oscillations for the displacement of back-end and fore contact angle of the coalescent drop while subdued oscillations for the displacement of front-end and back contact angle as well as higher frequency and smaller amplitude for liquid bridge radius. For large drops coalescence, the displacements of front-end and back-end monotonously increased, the liquid bridge radius reduced rapidly without oscillation after passing through a peak value, and the fore/back contact angle increased/decreased quickly with slight oscillation to reach a stable value.

对自制可视化直接甲醇燃料电池单体阴极流场内液滴生长特性、氧气流量和氧气进气温度对流场水淹及电池性能的影响进行了实验研究。结果表明：平行流场中首个液滴大多在流场右上区域冒出；流场中新液滴的出现具有瞬间涌出特性，并优先在流场板和扩散层交界的夹角处及扩散层表面碳纤维束交叉处产生；液滴生长过程具有非连续性，与流道边壁相接触的液滴和液柱的生长速度均大于未接触流道边壁的液滴生长速度，而且液柱有逆气流方向反向生长现象。氧气流量及氧气进气温度的升高，均导致阴极流道内液态水和流场中大液滴数量及形成液柱的长度减少，促使电池性能提高。