With thermal and mechanical nonequilbrium taken into consideration, the classical kinetic theory of boiling is modified to study unsteady-state homogeneous nucleation processes. Based on this newly developed model, the degree of superheat and the maximum nucleation rate corresponding to different rates of temperature rise in water are calculated and presented. For the first time, the initial nonequilibrium vapor pressure and the initial growth rate of bubble nuclei with different initial embryo sizes and different rates of temperature rise are accurately modeled. The resulting algorithm provides a method by which the details of bubble nucleation in a superheated liquid can be predicted, leading to a better understanding of the kinetics of boiling. Model validation, accuracy and application are also presented and discussed.

J. Li;G. P. Peterson;平 郑

International Journal of Heat and Mass Transfer

2005-7

An experiment has been performed for boiling heat transfer from a heated horizontal tube embedded in a porous medium under acoustic excitation. The effects of acoustical parameters, diameter and height of glass beads, and fluid subcooling have been investigated. The general trends of boiling heat transfer enhancement in porous medium by acoustic excitation are the same as those without porous medium, irrespective of fluid subcooling. The effects of glass beads diameter and glass beads height on heat transfer enhancement and boiling hysteresis are discussed. Mechanisms of heat transfer enhancement owing to these effects are analyzed.

D. W. Zhou;D. Y. Liu;平 郑

Journal of Enhanced Heat Transfer

2004-7

In this paper, the electrowetting dynamics of a droplet on a dielectric surface was investigated numerically by a mathematical model including dynamic contact angle and contact angle hysteresis. The fluid flow is described by laminar N-S equation, the free surface of the droplet is modeled by the Volume of Fluid (VOF) method, and the electrowetting force is incorporated by exerting an electrical force on the cells at the contact line. The Kilster's model that can deal with both receding and advancing contact angle is adopted. Numerical results indicate that there is overshooting and oscillation of contact radius in droplet spreading process before it ceases the movement when the excitation voltage is high; while the overshooting is not observed for low voltage. The explanation for the contact line overshooting and some special characteristics of variation of contact radius with time were also conducted.

芳军 洪;平 郑;Zhen Sun;慧英 吴

2010

Characteristics of air-water two-phase flow patterns in a miniature square channel having a gas permeable sidewall were investigated experimentally. Water was fed into the channel from its entrance, while air was injected uniformly into the channel along the permeable sidewall. This configured two-phase flow problem is encountered in direct feed methanol fuel cells. Flow patterns in both vertical upward and horizontal flows were identified using a high-speed motion analyzer. The visualization shows that the typical flow pattern encountered in the conventional co-current gas-liquid two-phase flow, such as bubbly flow, plug flow, slug flow and annular flow were also observed in the present work. However, unlike the conventional co-current gas-liquid two-phase flow in a channel with gas and liquid uniformly entering from one of its ends, for the flow configuration considered in this work, the stratified flow and wavy flow were not found in horizontal flow. And a so-called "single layer bubbly flow" was found in vertical upward flow, which is characterized by a mono small-gas-bubble layer existing adjacent to the surface of the permeable sidewall with the reminding space occupied by the liquid phase. Four transitional flow patterns such as bubbly-plug flow, bubbly-slug flow, plug-slug flow, and slug-annular flow, were found to exist between the distinct flow patterns. Finally, the flow regime maps for various liquid volumetric fluxes are presented in terms of mass quality versus the volumetric flux of gas phase.

H. Yang;天寿 赵;平 郑

International Journal of Heat and Mass Transfer

2004-12

Boundary-layer analysis is performed for mixed convection about a horizontal flat plate in a saturated porous medium with aiding external flows, where the governing parameter is found to be Ra (RePr) ^{3 2}. Similarity solutions are obtained for (i) horizontal flat plates at zero angle of attack with constant heat flux and (ii) stagnation point flows about horizontal flat plates with wall temperature varying as T_wα χ². Temperature and velocity profiles for these two cases at selected values of Ra (RePr) ^{3 2} are presented. The heat-transfer rate is shown to be asymptotically approaching the forced and free convection values as the value of Ra (RePr) ^{3 2} approaches the limits of 0 and ∞'. The criteria for pure and mixed convection about horizontal flat plates in porous media are established.

平 郑

International Journal of Heat and Mass Transfer

1977-9

A simultaneous visualization and measurement study has been carried out to investigate flow boiling of water in the 8 parallel silicon microchannels heated from below. It is found that there are two large-amplitude/long-period oscillating boiling modes exist in microchannels depending on the amounts of heat flux and mass flux. When the outlet water temperature is at saturation temperature and the wall temperatures are superheated, while the inlet water temperature is still subcooled, a Liquid/Two-phase Alternating Flow (LTAF) mode appears in the microchannels. This LTAF mode disappears when the inlet temperatures reaches the saturation temperature. As the heat flux is further increased such that the outlet water is superheated while the inlet water temperature is oscillating between subcooled and saturation temperature, a Liquid/Two-phase/Vapor Alternating Flow (LTVAF) mode begins. During these two unstable boiling modes, there are large-amplitude and long-period oscillations of water and wall temperatures with respect to time. Bubbly flow as well as some peculiar two-phase flow pattern are observed during the two-phase flow periods of the two unstable modes in the microchannels.

慧英 吴;平 郑

2003

Sample stacking in capillary electrophoresis is one of the effective techniques to concentrate sample species, thus improving their detection sensitivity. A 1-D mathematical model, including electrical potential distribution equation, buffer concentration equation, as well as sample electromigration and diffusion equation, is developed by proper simplifications and assumptions to study the sample-stacking process in capillary electrophoresis. These coupled governing equations are solved using finite-element method. The variations in the buffer concentration and the distribution of electrical field strength with time as well as the distribution of electrical potential in capillary during sample stacking were obtained. The sample stacking and the sample diffusion after stacking as well as the separation process of sample cations and anions are presented. It is found that the best stacking effect occurs near the entrance, where the species have not been separated well. With the increase in time, the stacking effect decreases whereas the distance between positively and negatively charged particles becomes larger, and the separation effect becomes better. The effect of buffer concentration ratio on sample stacking is also analyzed. It is found that the relationship between the sample-stacking effect and the buffer concentration ratio is not linear, and the maximum stacking effect is achieved within less time and less migration distance when the buffer concentration ratio is higher because of the stronger electrical field strength in sample plug region. It is anticipated that the numerical model developed in this study is helpful for the designing and optimization of sample stacking devices.

Jun CAO;芳军 洪;平 郑

Chinese Journal of Chromatography (Se Pu)

2007-3

The method of matched asymptotic expansions is applied to the problem of steady natural convection of a Darcian fluid about a semi-infinite inclined heated surf ace with a power law variation of wall temperature, i.e., T_w αX λ for x ͥ 0 where 0 ͤ X λ < 1. The leading edge of the inclined surface intercepts at an angle, Λ₀, with another impermeable unhealed surface extending upstream. The effects of the inclination angle α₀ (0 ͤ α₀ < < π/2) of the heated surface as well as the upstream geometry atx (as specified by Λ₀) on heat transfer and fluid flow characteristics near the heated surface are investigated. At a given inclination angle α₀, it is found that heat transfer from an upward-facing heated inclined surface is larger than that of a downward-facing heated surface, and that decreasing the intercepting angle (Λ₀) tends to lower the heat transfer rate. These effects become increasingly pronounced as the Rayleigh number is decreased.

C. T. Hsu;平 郑

Journal of Heat Transfer

1985-5

The problem of free convection along a vertical plane source or sink with uniform lateral mass flux in a porous medium is solved by the local non-similarity approximation. Solutions are carried out to the third level of truncation. Results for heat transfer characteristics as obtained from the third level of truncation are found in excellent agreement with a previous investigation using the finite difference method for a wide range of the mass flux parameter.

W. J. Minkowycz;平 郑

Letters in Heat and Mass Transfer

1982

The objective of this review is to provide a summary of modeling and experimental research efforts on transport phenomena in proton exchange membrane fuel cells (PEMFCs). Several representative PEMFC models and experimental studies in macro and micro PEMFCs are selected for discussion. No attempt is made to examine all the models or experimental studies, but rather the focus is to elucidate the macro-homogeneous modeling methodologies and representative experimental results. Since the transport phenomena are different in different regions of a fuel cell, fundamental phenomena in each region are first reviewed. This is followed by the presentation of various theoretical models on these transport processes in PEMFCs. Finally, experimental investigation on the cell performance of macro and micro PEMFC and DMFC is briefly presented.

洪潭 刘;Tianhong Zhou;平 郑

Journal of Heat Transfer

2005-12