竖直矩形窄缝通道内过冷沸腾传热特性的实验研究

以去离子水为工质,在进口压力为0.1～0.3 MPa、质量流速为200～1400 kg·m^-2·s^-1、热流密度为20～320k W·m^-2的参数范围内,对截面参数为50mm×2mm的竖直矩形窄缝通道展开了传热实验研究。实验获得通道内部工质由单相状态到过冷沸腾状态的传热过程曲线,将过冷沸腾段实验值与8个经验公式提供的预测值进行了对比与分析,采用相似原理以及回归分析法,建立了适用于竖直矩形窄缝通道的过冷沸腾准则关系式。研究结果表明,在竖直矩形窄缝通道内,热流密度对过冷沸腾传热具有主导作用;对于本实验的窄缝通道,Bertsch传热公式对于过冷沸腾段的预测效果相较于其他公式更好,本研究所建立的准则关系式与实验数据符合良好。因此,本研究建立的公式能够用于竖直矩形窄缝通道过冷沸腾传热系数的预测。     

超临界压力下二氧化碳在套管换热器内传热特性数值模拟

在超临界二氧化碳布雷顿循环等热质循环输运过程中，存在超临界压力下冷热2股二氧化碳间的流动传热过程，其传热特性是影响相应系统性能的关键。本文以套管换热器为原型，对超临界压力下的冷热二氧化碳间的传热特性开展了数值模拟研究，分析了热流体入口温度、冷热流体入口流量对于传热特性的影响和周向的传热特性分布。结果表明:随着热流体入口温度的变化，热侧和冷侧的局部换热系数产生相应的变化和波动，同时冷侧局部换热系数在主流温度接近拟临界温度时，会出现明显的传热强化现象；另外，热侧二氧化碳质量流量的上升，会使得热侧换热系数提高，冷侧换热系数峰值减小且向冷流体入口处移动，而随着冷侧质量流量的上升，冷侧换热系数峰值增大且向冷流体出口处移动。这是由于套管换热器为水平布置，传热特性在周向上产生了明显的不均匀现象，其与流体密度变化在重力作用下的局部湍流效应增强和削弱有关。本研究对新型二氧化碳布雷顿循环等热质循环输运过程的开发和设计具有指导意义。     

Dual differential pressure used in multiphase flow double-parameter metering based on phase-isolation

The axial differential pressure and radial differential pressure can be generated by phase-isolation method. Once measuring the radial differential pressure between the pipe wall and pipe center of a cross section downstream of the swirler, the mass flowrate and phase holdup can be determined if either of the two parameters was known. If axial differential pressure also has a certain relationship with the mass flowrate and phase holdup, then multiphase flow double-parameter metering can be realized by dual differential pressure (the axial differential pressure and the radial differential pressure). Taking oil-water two-phase flow as an example, the mechanism of axial differential pressure used in multiphase flow measurement was studied and the experimental verification was carried out. The results show that the axial pressure difference between certain two sections downstream of the cyclone is a certain function of the total flow rate and volume oil content of the oil-water two-phase flow under the state of phase separation in the pipe. The relative error between the experimental value and the theoretical value of the two is within ±1.05% and ±9.84% respectively. Besides, the mass flowrate and oil cut can be determined by dual differential pressure with the relative errors are within ±1.13% and ±6.89% respectively, which verified the application feasibility of dual differential pressure used in multiphase flow double-parameter metering based on phase-isolation.     

Working prototype of an optoelectronic XOR/OR/YES reconfigurable logic device based on nanocrystalline semiconductors

Photoelectrodes made of nanocrystalline titanium dioxide modified with hexacyanoferrate anions exhibit unique photoelectrochemical properties: photocurrent direction can be switched from anodic to cathodic and vice versa upon changes in photoelectrode potential and incident light wavelength. This effect, called photoelectrochemical photocurrent switching (PEPS effect), can serve as a basis for construction of chemical logic gates with optical inputs and electric output. At certain potentials anodic photocurrent generated upon UV irradiation has the same intensity as the cathodic photocurrent generated upon visible irradiation. Under these conditions simultaneous irradiation with UV and visible light results in compensation of anodic and cathodic photocurrents and zero net photocurrent is observed. This process can be used for construction of unique light-driven chemical logic gates. Due to reversible electrochemical process leading to oxidation or reduction of the surface species the device can be programmed to perform XOR, OR or YES logic operations.     

Electrospinning and hydrothermal synthesis of recyclable MoS2/CNFs hybrid with enhanced visible-light photocatalytic performance

In this work, a two-step synthesis route combining an electrospinning method and a hydrothermal process was used to prepare MoS₂/CNFs hybrid. CNFs was applied as the matrix for the nucleation and growth of MoS₂ nanosheets. In this hybrid, the crisscrossed MoS₂ nanosheets were randomly aligned and densely packed over the surface of CNFs. We probed the photocatalytic activity of MoS₂/CNFs hybrid to degrade rhodamine B (Rh B) in an aqueous solution under visible light irradiation. The hybrid displayed higher photodegradation performance relative to MoS₂ and mechanical mixture of MoS₂ with CNFs, with 67% Rh B completely degraded over 5 h-period. We attributed such enhancement in photocatalytic activity to the enhanced absorption property and electrical conductivity due to the synergy between MoS₂ and CNFs. The hybrid can furthermore be easily separated from the solution and reused for the subsequent photodegradation cycles. We verified the negligible loss in the photodegradation activity of MoS₂/CNFs hybrid towards Rh B during the three subsequent cycles. The high photocatalytic activity and recyclability of the hybrid render its practical application to degrade organic pollutants (i.e., dye compounds) in industrial wastewater.     

纯水静态闪蒸中液膜高度演变的实验研究

设计搭建了静态闪蒸实验台，利用高速摄影对不同过热度、节流孔板直径和闪蒸速率下纯水静态闪蒸过程中液膜高度的演变规律进行了实验研究。实验中过热度为7.0~32.5 K，节流孔板直径为5、10、20 mm，闪蒸速率为0.004~0.073 s^-1。通过液膜膨胀率来衡量和比较液膜高度的变化，液膜膨胀率是指闪蒸过程中液膜实时液位与初始液位的比值。结果表明：最大膨胀率随过热度或节流孔板直径的增大而增大；节流孔板直径一定时，闪蒸速率可通过改变过热度来调节，此时最大膨胀率随闪蒸速率的增大而减小；当过热度一定时，闪蒸速率可通过改变节流孔板直径来调节，此时最大膨胀率随闪蒸速率的增大而增大。最后，根据实验结果拟合得到过热度、闪蒸速率与最大膨胀率的实验关联式，其计算值与实验值吻合良好。本文研究结果对工业闪蒸设备小型化、紧凑化和精细控制提供了重要的实验依据。     

Experimental PIV and LIF characterization of a bubble column flow

Experimental 2D Particle Image Velocimetry (PIV) measurements, with uniform background lighting and Laser Induced Fluorescence (LIF) of the tracking particles, were performed in order to characterize the air-water biphasic flow and the 2D bubble column rising velocity in static water. Some applications require knowledge of the simultaneity of two-phase flow characteristics. The two phase flow air/water are common application in industry as chemical, hydraulic and nuclear industry, water treatment by aeration, and measurements are implemented to characterize the behaviour of the air bubbles column flow. The bubble flow studied in this paper is related to the optimization of the aeration in hydraulic turbines with micro-bubbles. The first step of this study, presented in the paper, is a complete characterization of a bubble column issued from a metallic sparger with holes of 0.5 mm diameter. For its complete characterization is determined simultaneously, via image processing technics, the flow velocity field induced by the column of bubbles in water, and the bubbles features: the bubble ascension velocity, diameter variation, interfacial area and shape factor. The results are compared with bibliographical data.     

Concentration measurement with thermal probe for pulverized coal in the pneumatic pipeline of power plant

The concentration measurement of pulverized coal in a pneumatic pipeline is a challenging issue in power plant. A thermal probe manufactured with abrasion-proof steel was developed for coal concentration measurement in such a situation. The probe generates 15 W of heat. This method is based on the heat transfer between the thermal probe and the gas–solid two-phase flow. Experiments were conducted in a horizontal pneumatic pipeline to assess the accuracy of the thermal probe, where the gas was air and the solid was pulverized coal with mean diameter of 65 µm. The wall temperature of the thermal probe was found to be dependent on both coal powder concentration and air velocity. A new heat transfer correlation was proposed in terms of the modified Reynolds and Nusselt numbers for the gas–solid two-phase flow across the thermal probe. In the range of coal powder concentration from 0.1 to 0.65 kg/kg, the standard deviation is 0.01 kg/kg for the thermal probe. The thermal probe has potential application for concentration measurement of pulverized coal in the pneumatic pipelines of power plants.     

Experimental study on flash evaporation from superheated water jets: Influencing factors and formulation of correlation

This paper reports on the influencing factors and their effects on the intensity of flash evaporation that occurs in the superheated water jets. A variety of experiments were conducted and four variables were investigated namely velocity of flow, initial temperature, superheat degree and injection nozzle diameter. An exponentially decaying curve model that predicts temperature variation at the centerline of the upward flowing jet was proposed and employed in the demonstration of the nature of influence of each experimented variable. Evaporation end height was estimated and correlated by an empirical equation; this equation is thought to be useful in designing the flash evaporation chamber.