Two-phase air-water flow in micro-channels: An investigation of the viscosity models for pressure drop prediction

Adiabatic two-phase air-water flow is experimentally studied in this work. Two channels, made of fused silica, with different diameters of 0.53 and 0.15 mm are used as test sections. The void fraction data for both tubes are obtained by image analysis. For the larger channel, the void fraction is found to be a linear relationship with the volumetric quality. In the case of the smaller one, however, the non-linear void fraction is obtained. The measured frictional pressure drop data are compared with the predictions regarding the homogeneous flow assumption. Several well-known two-phase viscosity models are subsequently evaluated for applicability to micro-channels.     

微通道中合成环氧脂肪酸甲酯

采用盐酸H2O2/HCOOH法,在微通道反应器内对不饱和脂肪酸甲酯进行环氧化反应。考察了双氧水用量、甲酸用量、反应温度及催化剂用量对反应的影响,得到最优的反应条件为:m(脂肪酸甲酯):m(甲酸):m(双氧水)=1:1.5:2,反应温度40℃,催化剂浓盐酸质量分数为3%(即浓盐酸质量占原料脂肪酸甲酯质量的百分数,下同),反应时间为110 s。在该条件下,产品环氧值为4.32%。     

Multi-objective optimization of a rectangular micro-channel heat sink using the augmented ε-constraint method

ABSTRACT

This article proposes the use of an improved version of the ?-constraint method for the design and optimization of a rectangular micro-channel heat sink. This study aims to optimize the geometric parameters describing the micro-channel, namely its width, its fin width and its depth. The thermal resistance and the pumping power, considered as indicators of the thermal performance, have been approximated by surrogated models based on response surface approximation. The optimization task is formulated as a nonlinear programming problem. This approach has been implemented in GAMS. Sufficient details on both the single-criterion and multi-criteria formulation of the problem have been provided. The implications of the mathematical modelling formulation and the interrelationship between criteria and estimated quantities have been clarified. The use of the augmented ε-constraint method for the multi-objective optimization of a rectangular micro-channel heat sink constitutes the main contribution of this work.     

Pressure-driven diffusive gas flows in micro-channels: from the Knudsen to the continuum regimes

Despite the enormous scientific and technological importance of micro-channel gas flows, the understanding of these flows, by classical fluid mechanics, remains incomplete including the prediction of flow rates. In this paper, we revisit the problem of micro-channel compressible gas flows and show that the axial diffusion of mass engendered by the density (pressure) gradient becomes increasingly significant with increased Knudsen number compared to the pressure driven convection. The present theoretical treatment is based on a recently proposed modification (Durst et al. in Proceeding of the international symposium on turbulence, heat and mass transfer, Dubrovnik, 3–18 September, pp 25–29, 2006) of the Navier–Stokes equations that include the diffusion of mass caused by the density and temperature gradients. The theoretical predictions using the modified Navier–Stokes equations are found to be in good agreement with the available experimental data spanning the continuum, transition and free-molecular (Knudsen) flow regimes, without invoking the concept of Maxwellian wall-slip boundary condition. The simple theory also results in excellent agreement with the results of linearized Boltzmann equations and Direct Simulation Monte Carlo (DSMC) method. Finally, the theory explains the Knudsen minimum and suggests the design of future micro-channel flow experiments and their employment to complete the present days understanding of micro-channel flows.     

微通道板质量污染对二代倒像微光管性能的影响

为解决二代微光倒像管研究中制管合格率低问题,对微通道板处理过程工艺质量和放气成分进行分析。结果表明,微通道板质量不合格、表面污染、性能一致性差是造成器件阴极灵敏度低、视场暗斑、分辨率低、背景大的主要因素。对其采用清洗,无油真空炉处理工艺,消除了工艺过程对微通道板的污染,研制成功了总体性能全面合格的二代倒像微光管。     

“Preliminary results of hydrogen production from water vapor decomposition using DBD plasma in a PMCR reactor”

The water decomposition is considered one of the most attractive chemical processes for the production of hydrogen. The present work describes the preliminary results obtained in the experimental study of the water vapor dissociation into hydrogen and oxygen species using Dielectric-Barrier Discharge (DBD) plasma in a plate micro-channel reactor (PMCR). The water vapor molecules are injected without using carrier gas into the PMCR reactor at pressure of 100 kPa and temperature of 573 K. The applied high voltage of the plasma was within range of 14–18 kV and different steam flow rates have been analyzed within range of 100–200 ml/h. The product gases have been separated in ice trap which it was connected directly to the PMCR reactor to prevent the recombination of hydrogen and oxygen species. The concentration of the outlet species has been measured in a gas phase chromatography (GC) instrument. The PMCR reactor heating temperature effect on the water vapor decomposition has been analyzed. It was found that the water vapor is dissociated into their constituent molecular elements of hydrogen and oxygen gas using plasma. The maximum obtained mole fraction, hydrogen flow rate and conversion rate were 2.3%, 9.42 g/h, 42.51% respectively, at steam temperature of 573 K, pressure 100 kPa, PMCR heating temperature 403 K, steam flow rate of 200 ml/h and the plasma discharge high voltage of 18 kV. It was observed that the amount of evolved hydrogen concentration increased with the increase of the PMCR reactor heating temperature. Also, the thermal efficiencies versus the heat supplied have been calculated and the maximum obtained efficiency was 49.32%. Consequently, the evolved hydrogen flow rate appears to depend mainly on the plasma voltage, PMCR reactor heating temperature and the separating temperature of outlet hydrogen and oxygen species. The steam dissociation experiment will be extended to separate hydrogen and oxygen species elements at high temperature conditions.     

Flame speed predictions in planar micro/mesoscale combustors with conjugate heat transfer

An analytical model for flame stabilization in meso-scale channels is developed by solving the two-dimensional partial differential equations associated with heat transport in the gas and structure and species transport in the gas. It improves on previous models by eliminating the need to assume values for the Nusselt numbers in the pre and post-flame regions. The effects of heat loss to the environment, wall thermal conductivity, and wall geometry on the burning velocity and extinction are explored. Extinction limits and fast and slow burning modes are identified but their dependence on structure thermal conductivity and heat losses differ from previous quasi one-dimensional analyses. Heat recirculation from the post-flame to the pre-flame is shown to be the primary mechanism for flame stabilization and burning rate enhancement in micro-channels. Combustor design parameters like the wall thickness ratio, thermal conductivity ratio, and heat loss to the environment each influence the flame speed through their influence on the total heat recirculation. These findings are used to propose a simple methodology for preliminary micro-combustor design.     

5英寸半球形微通道板光电倍增管的设计与研制

简述了5英寸半球形微通道板光电倍增管(简称:5”半球形MCP-PMT)的电子光学系统设计和优化试验;整管结构设计、工艺路径设计和光电参数设计.介绍了研制过程中,为确保MCP的增益特性和增益的稳定性,针对暗发射、真空性能等所采取的技术措施和相关工艺;分析产生暗电流(Id)和Id稳定性差(跳动)的各种因素,进行多次试验验证,解决了降低Id并使之趋于稳定的关键技术,优化了相关结构,在正常工作条件下,可达到Id≤30nA.为使器件更适用于进行单光子探测,本文还提出了研制量子效率更高、时间特性更好的全球形MCP-PMT的发展方向.     

神光原型诊断设备:门控针孔分幅相机的研制

为满足x射线针孔分幅相机在神光Ⅲ原型中的安装与正常测试,在原有分幅相机成像理论的基础上,研制了新型结构门控针孔分幅相机.设计了锥筒针孔成像系统,用于排除系统内杂散光干扰,从而在原型靶室内顺利实现小孔成像.设计了新型分幅像管结构,利用长微带结构充分改善了耦合匹配参数,利用新型荧光屏结构和装夹方式以及两块光纤面板耦合结构,...     

Constructal optimization of the geometry of an array of micro-channels

This paper documents the geometric optimization of an array of circular and non-circular ducts. The optimization was carried out numerically using finite volume method. As optimal dimensions were independent of the array configuration, the numerical simulation was performed on a unit cell. Numerical optimization for circular, square and isosceles right triangle cross-sections of channels was performed. Based on the results of this investigation, some correlations were proposed to predict the optimal hydraulic diameter and dimensionless heat transfer per unit volume. In addition to examining the effect of pressure drop on these parameters, it was showed that among the different geometries of this study, square cross-section has the most efficiency for a given volume. The numerical results of the present study were compared with approximate results reported in the literature which a good agreement was observed.