Peng Xiaofeng

Dynamic Bubble Behaviour during Microscale Subcooled Boiling

Bubble cycles, including initiation, growth and departure, are the physical basis of nucleate boiling. The present investigation, however, reveals unusual bubble motions during subcooled nucleate boiling on microwires 25 or 100 μm in diameter. Two types of bubble motions, bubble sweeping and bubble return, are observed in the experiments. Bubble sweeping describes a bubble moving back and forth along the wire, which is motion parallel to the wire. Bubble return is the bubble moving back to the wire after it has detached or leaping above the wire. Theoretical analyses and numerical simulations are conducted to investigate the driving mechanisms for both bubble sweeping and return. Marangoni flow from warm to cool regions along the bubble interface is found to produce the shear stresses needed to drive these unusual bubble movements.

Flow Structures Around Micro-bubbles During Subcooled Nucleate Boiling

The flow structures were investigated around micro bubbles on extremely thin wires during subcooled nucleate boiling. Jet flows emanating from the bubbles were observed visually with the fluid field measurement using high-speed photography and a PIV system. The jet flows induced a strong pumping effect around a bubble. The multi-jet structure was further observed experimentally, indicating the evolution of flow structure around micro bubbles. Numerical simulations explore that the jet flows were induced by a strong Marangoni effect due to high temperature gradients near the wire. The bubble interface with multi-jet structure has abnormal temperature distribution such that the coolest parts were observed at two sides of a bubble extending into the subcooled bulk liquid rather than at the top. Evaporation and condensation on the bubble interface play important roles not only in controlling the intensity of the jet flow, but also in bringing out the multi-jet structure.

Freezing?Thawing Characteristics of Botanical Tissues and Influence of Water Morphology

A series of visualization experiments were conducted to investigate the transport phenomena and interface behaviour during the freezing–thawing process of typical botanical tissues. Attention was paid to the growth of ice crystals and the advance of the phase-change interface. A comparison was made to identify the freezing/thawing behaviour for different tissues under various freezing conditions. Based on the experimental observation, analyses were conducted to explore the influence of water morphology on the freezing/thawing characteristics.

An advance on the theory of forced turbulent-flow film boiling heat transfer for subcooled liquid flowing along a horizontal flat plate

Abstract The semi-empirical theory for turbulent-flow film boiling of a subcooled liquid flowing with high velocity along a horizontal flat plate [ Int. J. Heat Mass Transfer 28 , 1049–1055 (1985)] is further advanced in this paper. It is found that the empirical constant m can be predicted analytically as 2 3 for different fluids. The another empirical coefficient k is revealed to be a function of thermophysical properties of the liquid and its vapor, the actual degree of surface superheat and degree of liquid subcooling.

Modeling of ion conductivity in Nafion membranes

A theoretical investigation was conducted to describe the ion transport behavior in a Nafion Membrane of proton exchange membrane fuel cells (PEMFC). By analyzing the surface energy configuration of the ionic clusters in a Nafion membrane, an equivalent field intensity, Ee, was introduced to facilitate the analysis of surface resistance against ion conduction in the central region of clusters. An expression was derived for ionic conductivity incorporating the influence of surface resistance. A face-centered cubic (FCC) lattice model for a spatial cluster distribution was used to modify the effect of water content on ionic conductivity in the polymeric matrix, i.e., the regions between clusters. Compared with the available empirical correlations, the new expression showed much better agreement with the available experimental results, which indicates the rationality to consider the structural influence on ion conduction in water-swollen Nafion membranes.

The effect of interfacial evaporation on heat and mass transfer of falling liquid film

Analysis of experimental data and estimation of the order of magnitude for interfacial mass diffusion have demonstrated that considerable excess evaporation exists on the free interface of falling liquid film, and that the capillary pressure caused by surface tension is the driving force of this excess interfacial evaporation, which we called the “capillarity-induced interfacial evaporation”. By correlating the experimental data, an empirical expression of the effective capillary radius,re, is obtained with which the evaporative rate formula we derived and reported previously has been modified to improve the prediction of the critical heat flux for film breakdown. Comparisons with the available predicting models show that our modified equation can predict the experimental results with much lower relative deviation.

Inner Structure of Boiling Nucleus and Interfacial Energy Between Nucleus and Bulk Liquid

A model of two-region structure of a nucleus is proposed to describe nucleus evolution. The interfacial tension between bulk liquid phase and nucleus is dependent on the density gradient in the transition region and varies with the structure change of the transition region. With the interfacial tension calculated using this model, the predicted nucleation rate is very close to the experimental measurement. Furthermore, this model and associated analysis provide solid theoretical evidence to clarify the definition of nucleation rate and understand the nucleation phenomenon with insight into the physical nature.