Pool Boiling on Modified Surfaces Using R-123

Saturated pool boiling of R-123 was investigated for five horizontal copper surfaces modified by different treatments, namely, an emery-polished surface, a fine sandblasted surface, a rough sandblasted surface, an electron beam-enhanced surface, and a sintered surface. Each 40-mm-diameter heating surface formed the upper face of an oxygen-free copper block, electrically heated by embedded cartridge heaters. The experiments were performed from the natural convection regime through nucleate boiling up to the critical heat flux, with both increasing and decreasing heat flux, at 1.01 bar, and additionally at 2 bar and 4 bar for the emery-polished surface. Significant enhancement of heat transfer with increasing surface modification was demonstrated, particularly for the electron beam-enhanced and sintered surfaces. The emery-polished and sandblasted surface results are compared with nucleate boiling correlations and other published data.     

Computational Analysis of the Effects of Process Parameters on Molten Pool Transport in Cu-Ni Dissimilar Laser Weld Pool

A three-dimensional, transient numerical model is used for analyzing the effects of process parameters such as laser power and the laser scanning speed on turbulent momentum, heat and mass transport in a typical dissimilar metal weld pool of a copper-nickel binary couple. The conservation equations are solved in a coupled manner using a semi-implicit pressure linked algorithm in the framework of a finite-volume approach. Turbulence effects are taken care of by employing a suitably modified k–? model, which accounts for solid–liquid phase change in a turbulent environment. The solid–liquid phase change aspects are addressed using a modified enthalpy-porosity technique. Subsequently, the developed turbulent transport model is used to simulate continuous welding of a copper-nickel binary couple in a butt joint configuration for different values of the laser power and laser scanning speed, in order to assess their influences on the pool geometry, cooling rate, and heat, momentum and species transport inside the molten pool. In order to investigate the effects of turbulence, the results of the turbulent transport simulations are compared with the results obtained from the simulations without turbulent transport for the same values of process parameters. Significant differences are observed on comparing the results obtained based on simulations with and without turbulent transport, which provide valuable insights for controlling the process parameters based on manufacturing needs. It has been observed, in general, that the enhanced diffusive mixing in turbulent transport leads to lower maximum values of mean velocity, temperature, and cooling rate than those obtained from the corresponding simulations without turbulent transport.     

高拱坝水垫塘体型设计优化方案试验

通过整体水工模型试验,以流态和动水压力为指标,对某高拱坝水垫塘体型的合理性进行了论证。结果表明,射流水舌在水垫塘内落点集中,加之水垫厚度不足,底板局部冲击压力和脉动压力过大;水垫塘长度不足,水流在塘内无法形成完整水跃,直接冲击翻越二道坝。为此,提出水垫塘体型优化方案,优化后水垫塘内流态明显改善,底板动水压力显著降低且分布均匀,可见优化后的水垫塘体型趋于合理,可直接指导工程设计。     

Bubble Dynamics and Heat Transfer During Pool Boiling on Wettability Patterned Surfaces

Surfaces with spatial wettability patterns have been proven to enhance heat transfer coefficient and critical heat flux in pool boiling. To understand the physical mechanism behind this phenomenon and obtain the correlation among some critical parameters (bubble departure frequency, bubble size, nucleation site density, surface tension), pool boiling experiments were conducted. A Pyrex glass with a layer of indium-tin-oxide was used as the substrate. Hydrophobic patterns will serve as nucleation sites. Experiments were conducted in deionized water under atmospheric pressure at a relatively low heat flux. The processes of nucleation, growth, and departure of individual bubbles were visualized by using a high speed camera through the bottom of the heater surface. It has been found that the patterned surface performed the best in heat transfer for subcooled pool boiling when compared with hydrophilic and hydrophobic surfaces. The nucleation site density of the biphilic surface was much higher, when compared with that of the homogeneous surface. The individual bubbles always nucleate on the edge of the hydrophobic and hydrophilic area, and then move onto the hydrophobic pattern. Most of the individual bubbles detach from the wettability patterned surface in the diameter range from 300 µm to 450 µm (around 77.3%). The bubble departure periods scatter in the range from 80 ms to 1500 ms.     

西藏低压环境对油池火火焰形状的影响

通过拉萨和合肥两地开展相同尺寸的甲醇和汽油油池火燃烧实验,研究了两地油池火燃烧特性参数的不同,分析低氧低压条件对于燃烧特性参数的影响机理.研究发现,在燃料类型和尺寸相同的情况下,燃料在拉萨的失重速率比合肥慢,产生的CO浓度更高;采用火焰图像分析对两地火焰图像进行处理,发现相同小尺寸的油盘燃料燃烧,拉萨火焰面积比合肥要大...     

Review of the Manufacturing Techniques for Porous Surfaces Used in Enhanced Pool Boiling

Continuous development of high-performance microelectronic chips requires efficient cooling systems to dissipate large amount of heat produced over a small footprint. Pool boiling is capable of dissipating large heat fluxes while maintaining low wall superheat and is receiving renewed interest. Porous surfaces have been investigated extensively for pool boiling enhancement. This paper presents a review of different manufacturing techniques employed to manufacture porous surfaces in pool boiling application. Different types of surfaces developed using these techniques are reviewed and their pool boiling performance is discussed.     

Initiation of CASO4 Scale Formation on Heat Transfer Surfaces under Pool Boiling Conditions

The success of innovative fouling mitigation techniques such as ion implantation depends upon the early stage of scale formation on the heat transfer surface. This is because the first crystalline nuclei that appear on the surface during the initial period dictate how fouling would develop in latter stages. In this study, the initial period of deposition of calcium sulfate on heat transfer surfaces has been investigated under pool boiling conditions. The independent variables were heat flux and calcium sulfate concentration. The experimental results show that the time until the heat transfer coefficient reaches its intermediate maximum decreases with an increasing concentration and heat flux, and is also significantly affected by the surface finish. Neural network architectures were utilized to correlate the experimental results during the initial deposition period. A satisfactory agreement between predicted and measured heat transfer coefficients has been achieved with an average error of 8.7%.     

手工氩弧焊封底焊工艺过程分析

分析了采用手工钨极氩弧焊方法进行封底焊接时易出现的焊接缺陷,并提出了解决这些缺陷的方法。同时结合工作实际列举了吊管焊接时采用的手工钨极氩弧焊方法封底及相应的焊接要求,焊接方法和焊接质量。     

Investigation of Nucleate Pool Boiling of Saturated Pure Liquids and Ethanol-Water Mixtures on Smooth and Laser-Textured Surfaces

ABSTRACT

Nucleate pool boiling experiments were performed on plain and five laser-textured stainless-steel foils using saturated pure water, 100% ethanol, 0.4% and 4.2% mole fraction ethanol – water mixtures. All laser-textured samples contained untreated, smooth 0.5 mm wide regions and intermediate textured surfaces, that differ in the width of the laser patterned regions (from 0.5 mm to 2.5 mm). For smooth surfaces, we measured significant decreases in average heat transfer coefficients (HTC) and increases in bubble activation temperatures in comparison with the laser-textured surfaces for all the tested working fluids. Significant enhancement in HTC (280%) on a textured heating surface with 2.5-mm-wide laser pattern was recorded using pure water. For pure ethanol, the highest enhancement of 268% was achieved on a heating surface with 1.5-mm-wide laser pattern. The highest enhancement of HTC for the tested binary mixtures was obtained using 2.0-mm wide-laser-textured regions (HTC improved by 235% and 279% for the 0.4% and 4.2% mixtures, respectively). Our results indicate that laser texturing can significantly improve boiling performance when the intervals of the laser-textured patterns are close to the capillary lengths of the tested fluids.     

Nucleate Pool Boiling Heat Transfer of Hydro-Fluorocarbon Refrigerant R134a on TiO2 Nanoparticle Coated Copper Heating Surfaces

ABSTRACT

Pool boiling heat transfer performance of hydro-fluorocarbon refrigerant R-134a on titanium dioxide (TiO₂) nanoparticle coated surface is experimentally studied in the article. The test surfaces, viz, 100 nm, 200 nm and 300 nm thick TiO₂ nanoparticle coated surfaces over 100 nm thin film surface are used in this experimentation. The surfaces are synthesized and fabricated by simple and cost-effective electron beam evaporation method. The test surfaces were characterized by scanning electron microscope and atomic force microscope to uncover the formation of crystalline structure on coated surfaces. These surfaces are utilized in pool boiling test rig using refrigerant R134a at 10°C saturation temperatures. The result indicated that a maximum of 87.5% augmentation in the boiling heat transfer has been achieved by higher thickness of TiO₂ coated surface than the bare copper surface. In addition, the incipience wall superheat is reduced for higher thickness coated surface. The augmentation of heat transfer coefficient might be the reason for increase in micro/nano-porosity, active nucleation site density and surface area of the heating surface. It is observed that with the increase of sub-cooling temperature of liquid, the bubble departure diameter was reduced while the heat transfer coefficient has been increased.     

参数辨识法在锅炉受热面故障诊断中的应用

通过采用低集总参数模型近似高阶分布参数模型 ,将基于换热器机理动态数学模型的参数辨识法用于锅炉受热面的故障诊断 ,不仅大大减少了计算工作量 ,而且具有很好的实时诊断效果     

利用压力相似预测高原低压环境小尺寸池火燃烧速率的变化特性

运用火灾动力学中的压力相似分析理论,推导出高海拔低压环境下的池火燃烧速率与常压环境下的相似关系,并结合前人实验数据,得到非受限燃烧条件下,等效直径在10～50 cm池火燃烧速率随气压降低呈线性变化的规律.通过开展拉萨低压地区及合肥常压地区的池火燃烧对比实验,并结合前人的实验数据对理论关系式进行验证,研究结果表明,压力相似分析虽仅从动力学角度出发,没有包含对燃烧化学机理的探讨,但仍然能够较好地反映出燃烧速率随环境压力的变化规律,对于指导实验研究,预测实验结果具有重要意义.     

应用遗传算法的非线性转子-轴承系统参数辨识

为了识别非线性油膜力作用下的转子．轴承系统的不平衡量，给出了一种适合参数识别的非线性油膜力表达式，并引入遗传算法来识别非线性油膜力的动力系数和不平衡的大小和位置。首先运用达朗伯原理结合里茨法建立了非线性油膜力作用下的多自由度转子-轴承系统的运动方程．进而利用遗传算法对非线性油膜力参数和不平衡量进行识别。结果表明：该方法能够有效识别转子的不平衡量，并能达到较高的识别精度，可以用于无试重动平衡。图5表1参6     

Thermal Contact Analysis Using Identification Method

The system identification method is one of the most important topics in engineering. An interesting application of this method is to investigate the heat transfer from the exhaust valve, especially the valve burning at high temperatures. This study consists of experimental and analytical work. During experimentation, two co-axial rods were used to transfer heat at their contact surfaces. Using the measured temperatures at different locations on the rods and the analytical method, the temperature distribution of the rods and the heat transfer coefficient of the contact surface were calculated. Using the above calculated temperatures at both sides of the contact surface and applying the system identification method, the temperature transfer function was estimated. Using the transfer function, a computational model was created. The results were compared to previous research work. An experimental apparatus, including an analog to digital board, was designed and set up for the experiment.     

利用同步辐射验证正庚烷反应过程中的庚酮类物质

利用用步辐射单光子电离和分子束取样技术并结合飞行时间质谱仪,在低压、预混、燃油当量比为1.0的情况下研究含有正庚烷燃料层流火焰的燃烧特性.共进行了7组不同燃料的对比实验:正庚烷;正庚烷/MTBE(体积比1:1);正庚烷/乙醇(体积比1:1);异辛烷/正庚烷(体积比9:1);异辛烷/正庚烷/乙醇(体积比8.1:0.9:1.0);异辛烷/正庚烷/MTBE(体积比8.1:0.9:1.0);异辛烷(作为对比参照样品).发现含正庚烷燃料燃烧中间产物均含有电离能为(9.14±0.1)eV、分子量为114的物质,无正庚烷燃料燃烧中间产物则未发现,根据光电离效率谱(PIE)和电离能查表初步断定为庚酮类物质.利用实验误差分析方法分析确定为2-庚酮、3-庚酮、4-庚酮,出现的相对概率分别为19.49%、55.37%和25.13%.检测与分析结果对发展正庚烷氧化机理提供了参考,同时在利用同步辐射探测预混层流火焰成分的实验中,对鉴别物质的方法进行了改进.     

细水雾扑灭油池火的临界条件

通过模拟灭火实验发现细水雾灭火过程中由于雾特性参数的变化而形成两种相异的灭火过程,进而通过理论分析与计算得到了细水雾通过火焰冷却扑灭油池火所需的临界条件,并通过实验室尺度实验与全尺度实验对计算结果进行了验证．在实验中还发现对于下喷式系统而言,细水雾能否穿透烟气羽流层对于细水雾系统能否灭火具有至关重要的影响,为此引入了烟气羽流与细水雾的推力比这一概念,并通过实验确定了采用下喷方式时,细水雾灭火的临界推力比大致为10．     

氧气射流对电炉熔池搅拌的研究

根据新冶钢四炼钢8号电弧炉实际冶炼工况,建立了用于模拟计算的模型。采用数值计算方法对炼钢电弧炉内氧气和钢液的流动状况进行了数值模拟。分析模拟结果可得出如下结论:氧气经过拉瓦尔管后,最大速度671 m/s,吨钢比搅拌功率为6 507 W/t;氧气射流在流动过程中速度不断衰减,熔池表面处氧气的最大速度约121 m/s,比搅拌功率为1 799 W/t;炉内钢液受氧气射流搅拌后具有的比搅拌功率50 W/t;若取拉瓦尔管入口处氧气比搅拌的功率为100,则拉瓦尔管出口处氧气、熔池表面处氧气和炉内钢液具有的比搅拌功率分别71.70、19.82和0.55。     

轻钢彩板坡形屋面在小区住宅方面的尝试

轻钢彩板坡型屋面与传统屋面比较具有自己独特的优越性,本文通过其在哈尔滨通站小区的应用实例说明其在住宅小区建设中应用的尝试。     

平面叶栅气动设计的变域变分原理及有限元计算

采用变域变分原理,对以流函数描述的无粘流体流动的亚音速平面叶栅反设计问题进行了理论分析和有限元计算。由于变域变分能将未知形状的几何变量及控制方程结合在一个变分泛函中,使得数学描述简洁、紧凑,且几何变量及控制方程的求解能耦合地进行。采用变分有限元方法计算结果表明,该计算原理具有较高的精度,能获得较为满意的叶栅反设计的结果。     

Experimental Investigations of Pool Boiling Heat Transfer Characteristics on a Vertical Surface Using CuO Nanoparticles in Distilled Water

Experiments were conducted on pool boiling heat transfer using dilute dispersions of CuO nanoparticles in distilled water at and above atmospheric pressure. Pool boiling characteristics of CuO nanofluid were studied at different pressures and concentrations. Characterization of the heating surface was done both qualitatively and quantitatively by taking the scanning electron microscopy (SEM) images and by subsequent measurement of surface roughness of the heater. SEM images of the heater surface showed nanoparticle deposition on the heater surface, suggesting surface modification. Thorough visualization showed microcavities on the heater surface, which provide an excellent location for nucleation sites enhancing heat transfer. However, these microcavities, once filled up with the suspended nanoparticles, reduced active nucleation sites, deteriorating the boiling heat transfer coefficient. Based on the experimental investigations it was concluded that there is an optimum thickness of nanoparticles coating at which heat flux is maximum and beyond this coating boiling heat transfer coefficient decreases. At higher pressures, boiling heat transfer coefficient and specific excess temperature remained nearly the same. This showed that pressure has negligible or no role to play in boiling heat transfer using nanofluids.