Effect of nanoparticles on CHF enhancement in pool boiling of nano-fluids

To investigate the characteristics of CHF (Critical Heat Flux) enhancement using nano-fluids, pool boiling CHF experiments of two water-based nano-fluids with titania and alumina nanoparticles were performed using electrically heated metal wires under atmospheric pressure. The results showed that the water-based nano-fluids significantly enhanced CHF compared to that of pure water. SEM (Scanning Electron Microscopy) observation subsequent to the pool boiling experiments revealed that a lot of nanoparticles were deposited on heating surface during pool boiling of nano-fluids. In order to investigate the role of the nanoparticle surface coating on CHF enhancement of nano-fluids, pool boiling CHF of pure water was measured using a nanoparticle-coated heater prepared by pool boiling of nano-fluids on a bare heater. It was found that pool boiling of pure water on the naonoparticle-coated heater sufficiently achieved the CHF enhancement of nano-fluids. It is supposed that CHF enhancement in pool boiling of nano-fluids is mainly caused by the nanoparticle coating of the heating surface.  

Theoretical Aspects of Nucleate Pool Boiling with Dielectric Liquids

Direct cooling with inert,dielectric liquids may well become the technique of choice for the thermal manage-ment of future electronic systems.Due to the efficiency of phase-change processes and the simplicity of naturalcirculation,nucleate pool boiling is of great interest for this application.This paper examines the characteristicsof vapor bubbles and nucleate pool boiling of the dielectric liquids.The results provide a theoretical foundationfor understanding and interpreting the often complex empirical results reported in the literature.  

水平窄空间沸腾传热的实验研究

通过对五种尺寸的窄空间试验元件分别以水和乙醇做工质进行实验。研究了窄空间间距、窄空间尺寸、不同工质及不同热流密度对窄空间沸腾性能的影响。结果表明：当窄空间尺寸与热流通等因素组合恰当时。其换热系数可比大空间池沸腾提高3～6倍；临界热流密度有所降低。  

强润湿性液体池沸腾传热的实验研究和机理分析

对强润湿性液体的池沸腾传热实验而言，本文提出了行之有效的实验程序，并严格按照实验程序进行了R113池沸腾传热的实验研究，具体研究了表现老化和液体过冷度对池沸腾传热曲线及起沸点的影响，实验中观察到了三个反常现象，最后，从强润湿性液体的沸腾传热机理的角度对其给出了相应的解释。  

沸腾表面凹坑的尺度分布特征

在沸腾表面上储气(或汽)凹坑是形成活化核心的关键因素。文中从类似Sierpinski地毯的经典分形表面出发，成功构造了凹坑以不同尺度分布的表面。研究结果表明凹坑的尺度分布雄数与表面形貌的分形维数是两个含义不同的概念，机械表面凹坑(或凸点)的尺度分布特征符合分形理论。分析活化核心的测量结果，表明活化核心的尺度分布同表面上凹坑的尺度分布一样具有分形特征。  

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures ：Part I—Analytical Model for Boiling Heat Transfer of Pure Liquids on Smooth Tubes

A mechanism is proposed for nucleate pool boiling heat transfer along with a general model for both pure liquids and binary mixtures. A combined physical model of bubble growth is also proposed along with a corresponding bubble growth model for pure liquids on smooth tubes. Using the general model and the bubble growth model for pure liquids, an analytical model for nucleate pool boiling heat transfer of pure liquids on smooth tubes is developed.  

Wettability of heated surfaces under pool boiling using surfactant solutions and nano-fluids

The wettability of the heated surface under pool boiling of surfactant solutions and nano-fluids has been investigated. Tri-sodium phosphate (TSP, Na₃PO₄) solutions (0.01, 0.05, 0.1, 0.3, 0.5, 0.8 wt.%) and Aluminum oxide (Al₂O₃) nano-fluids (NF) (0.5, 1, 2, 4 vol.%) were prepared for experiments. Stainless steel (SUS 304) strips (30 × 30 × 3 mm) were heated by an alcohol lamp and quenched in the prepared solutions. Before complete quenching, when the surface temperature was 150 ± 10 °C (nucleate boiling region), the strip was taken out and excessive liquid on the surface was removed. Contact angles of pure water and the solutions on the quenched surface and fresh surface were measured. Contact angles of pure water on the quenched surfaces (5°–25°) were much smaller than those on the fresh surface (65°–70°). The solutions (TSP, NF) on the quenched surface shows the smallest contact angle (5°–15°). Surfaces deposited TSP and nano-particle could affect surface energy of the strips and enhance hydrophilicity of the surfaces. Several implications of the experimental results on the pool boiling CHF model and CHF enhancement using TSP and NF were discussed.  

Experimental study of void fraction profiles in pool boiling on a vertical surface

An experimental study of saturated nucleate pool boiling of n-pentane on a vertical surface was conducted. The boiling surface is 120 mm high and 60 mm width. Using hot wire anemometer, void fraction profiles were measured along the height, width, and the normal of the heated surface for different heat fluxes. The output signals of the hot wire anemometer were treated in order to characterize the boiling flow. The contribution of long, average, and short bubbles along the width and the height of the heated surface are identified for various heat flux. Three different boiling regimes were defined and the transition heat flux density was determined and compared to the results of Gaertner [5]. The wavelength of vapor waves formed on the heater was measured and is higher than the critical wavelength characteristic of the Taylor instability. The experimental results showed that the wavelength is independent of the heat flux and increased along the heater height. However, the number of the wetting zones decreased because of the waves merging and stretching. The physical mechanisms that triggering critical heat flux were identified.  

Features of Boiling on an Artificial Surface—Bubble Formation, Wall Temperature Fluctuation and Nucleation Site Interaction

The artificial surfaces are applied to study the pool boiling features, including the bubble behaviors, the surface temperature fluctuation, the heat transfer characteristics and nucleate site interaction. Three sets of experiments are carried out to investigate the influences of cavity shape, cavity size, cavity spacing on the boiling phenomena. Experimental results reveal that bubbling from the cylindrical as well as reentrant cavity is generally stable. The influence of cavity diameter on the bubble behaviors and the temperature fluctuation seems very weak while the effect of cavity depth cannot be neglected. As for the two cavity conditions, the bubble behaviors show the different features depending on the dimensionless cavity spacing. Three significant factors (thermal interaction, hydraulic interaction, bubble coalescence) control the nucleation site interaction, and the competition and dominance of the factors yield four interaction regimes.  

Heat transfer and size distribution of active nucleation sites in boiling propane outside a tube

Size distributions of active nucleation sites in pool boiling of propane on the outside of single horizontal copper and steel tubes with different surface finishes (emery ground and fine sandblasted) have been calculated from heat transfer measurements over wide ranges of heat flux and pressure. The model assumptions of Schömann for the heat transfer near growing and departing bubbles, which were applied in the calculations, have been modified and the calculated results have been compared to experimental investigations by high-speed video techniques. New measurements of the departure diameter d_A and frequency f of the bubbles at the same conditions as the heat transfer measurements show that d_A and f are influenced by the azimuthal position of the active nucleation sites on the tube and that the calculated results react sensitively to the input data for d_A and f . A modification of the model is proposed.