Comparisons of Structured Surface Floors for Pool Boiling Enhancement at Low Heat Fluxes: Hands-On Learning Setup for Heat Transfer Classroom

Various enhanced surfaces have been proposed over the years to improve boiling heat transfer. This paper introduces an experimental setup designed for boiling demonstration in the graduate-level Heat Transfer course. The pool boiling performance of water under atmospheric pressure of 1.025 bar is investigated by using several structured surfaces at heat fluxes of 28 and 35 kW/m2. Surfaces with holes, rectangular grooves, and mushroom fins are manufactured by an NC-controlled vertical milling machine. The heat flux versus excess temperature graph is plotted by using thermocouple measurements of water and base temperatures of the boiling vessel. The separation, rise, and growth of individual vapor bubbles from the surface during boiling were recorded with a digital camera. The results for the plain surface are compared to the Rohsenow correlation. The enhancement of heat transfer coefficient (h) ranged between 15% - 44.5% for all structured surfaces. The highest heat transfer coefficient enhancement is observed between 41% - 56.5% for holed surface-3 (405 holes) compared to the plain surface. The excess temperature dropped around 29% - 34% for holed surface-3 (405 holes) compared to the plain surface. The heat transfer coefficient increases as the spacing between channels or holes decreases. While the bubbles on holed and mushroomed surfaces were spherical, the bubbles on the flat and grooved surfaces were observed as formless. The suggested economical test design could be appropriate to keep students focused and participating in the classroom.

Heat Transfer in Nucleate Pool Boiling of Binary and Ternary Refrigerant Mixtures

Heat transfer coefficients in nucleate pool boiling were measured on a horizontal copper surface for refrigerants, HFC-134a, HFC-32, and HFC-125, their binary and ternary mixtures under saturated conditions at 0.9MPa. Compared to pure components, both binary and ternary mixtures showed lower heat transfer coefficients.This deterioration was more pronounced as heat flux was increased. Experimental data were compared with some empirical and semi-empirical correlations available in literature. For binary mixture, the accuracy of the correlations varied considerably with mixtures and the heat flux. Experimental data for HFC-32/134a/125 were also compared with available correlated equation obtained by Thome. For ternary mixture, the boiling range of binary mixture composed by the pure fluids with the lowest and the medium boiling points, and their concentration difference had important effects on boiling heat transfer coefficients.

Heat Transfer Characteristic of Molten Pool for Twin-Roll Strip Casting

Body-fitted coordinate transformation equation was deduced and used to generate the body-fitted grids of molten pool for twin-roll strip casting.The orthogonality of the grids on the boundary was modified by adjusting source item.The energy equation and the boundary conditions were transformed from physical space to computational space.The velocity field model proposed by Hirohiko Takuda was used to calculate the temperature field of molten steel,and the influence of technical factors was also discussed.

Modeling transient fluid flow and heat transfer phenothena in stationary pulsed current TIG weld pool

A mathematical model is presented to describe transient behavior of heat transfer and fluid flow in stationary pulsed current tungsten inert gas (PC-TIG) weld pool, which considers three kinds of driving, forces for weld pool convection, i,e. buoyancyforce, electromagnetic force and surface tension force. furthermore. the effect of vaporization heat flux at the free surface of weld pool and the temperature coefficient of surface tenston which is a function of temperatuer and composition are considered in the model In order to accelerate the convergence of iteration the AST(additive source term)method which concerns with the thermal energv boundary conditions is extended successfully to deal with the momentum boundary conditions by which the transient momentum equation and energy equation are mutually coupled. At the same time. ADI (Alternating direction implicit) method and DBC (double blocks correction) technque are employed to solve the finite difference equations. The results of numerical simulation demonstrate the transient behavior of PC-TIG weld pool, as well as the periodic variation of fluid flow and heat transfer with the periodic variation of welding current in stationary PC-TIG weld pool. The theoretical predictions based on this model are, shown to be in good accordance with the experimental measurements.

Pool boiling heat transfer enhancement on porous surface tube

The passive residual heat removal exchanger (PRHR HX),which is a key equipment of the passive residual heat removal system,is installed in an elevated pool.Its heat transfer performance affects security and economics of the reactor,and boiling heat transfer in the liquid surrounding the exchanger occurs when the liquid saturation temperature exceeded.The smooth tubes,which are widely used as heat transfer tubes in PRHR HX,can be replaced by some enhanced tubes to improve the boiling heat transfer capability.In this paper,the pool boiling heat transfer characteristics of smooth tube and a machined porous surface tube are investigated by using high-pressure steam condensing inside tube as heating source.Compared with smooth tube,the porous surface tube considerably enhances the boiling heat transfer,and shortens the time significantly before reaching the liquid saturation temperature.Its boiling heat transfer coefficient increases from 68% to 75%,and the wall superheat decreases by 1.5oC.Combining effect of condensation inside tube with boiling outside tube,the axial wall temperatures of heat transfer tube are neither uniform nor linear distribution.Based on these investigations,enhance mechanism of the porous surface tube is analyzed.

NUMERICAL ANALYSIS OF THREE-DIMENSIONAL FLUID FLOW AND HEAT TRANSFER IN TIG WELD POOL WITH FULL PENETRATION

A model is established to analyze three-dimensional fluid flow and heat transfer in TICweld pools with full penetration.It considers the deformation of the molten pool surfaceat the condition of full penetrated workpieees,takes the are pressure as the drivingforce of the pool surface deformation,and determines the surface configuration of weldpool based on the dynamic balance of arc pressure,pool gravity and surface tension atdeformed weld pool surface. The SIMPLER algorithm is used to calculate the fluid flowfield and temperature distribution in TIG weld pools of stainless steel workpieces.TIGwelding experiments are made to verify the validity of the model.It shows the calculatedresults by the model are in good agreement with experimental measurements. professor,Dept of Welding Engineering,Harbin Institute of Technology,Harbin 150006,China

An Experimental Study of Porous Surface Tubes for Enhancement of Binary Liquid Mixture Pool Boiling Heat Transfer

Experiments have been conducted on the pool boiling of binary mixtures in JK tubeswith porous surface and ribbed channels,attempting to enhace the boiling heat transfer coefficientsinvolved.The binary mixtures used in this study are R113 and R11.The results show that the boilingfi1m heat transfer coefficients for the R113 and R11 binary mixtures in JK2 and JK1 tubes are 2—8tirnes and 2—5 times greater than those in smooth tubes respectively.Based on this experimentaldata and simple theoretical analysis,a correlation has been proposed to predict the boiling heattransfer coefficients for R113/R11 mixtures in a horizontal JK tube with a relative error of less than16%.

基于模糊聚类和改进Densenet网络的小样本轴承故障诊断

针对实际中轴承的故障数据少难以满足深度学习数据大量训练模型的要求,利用卷积神经网络的微小特征提取优势和模糊聚类不需要训练即可完成分类的特点,提出了一种基于模糊聚类和改进Densenet网络的小样本轴承故障诊断方法。首先将预训练微调的Densenet网络去掉分类只保留特征提取层,设计一个维度自适应全局均值池化层(GAP)代替全连接层(FC),其次利用模糊聚类代替Densenet网络的softmax分类层,不需要训练即可完成分类。实验结果表明:该算法利用小样本数据训练网络中的GAP参数,模型需要的训练样本大大减少,诊断时将轴承时域图像输入到网络中,在GAP层输出1 920个特征数据,不同故障状态的特征数据构建特征向量矩阵,利用模糊聚类方法求得模糊相似矩阵和模糊等价矩阵,当置信因子从大到小变化时,由对应布尔矩阵得到动态聚类图,从而实现轴承故障分类。

养老保险全国统筹对地方政府征管行为的影响研究

实现基本养老保险全国统筹是我国多层次、多支柱养老保险体系建设的重要环节。基于2012—2022年省级面板数据,将全国统筹初级阶段的中央调剂制度作为准自然实验,探讨中央调剂制度对地方政府征管行为的影响及作用机制。研究发现:中央调剂制度降低了地方政府对养老保险基金的征管强度,中央调剂制度实施后,基金缺口地区存在“压力缓解效应”,基金盈余地区存在“基金收益受损效应”。在经济发展水平高、财政压力小和老龄化程度低的地区,中央调剂制度对地方政府征管强度的负向影响更为明显。进一步分析表明,严征管和降费率改革在中央调剂制度与地方政府征管行为之间发挥调节效应。研究结论为加快实现养老保险全国统筹提供了经验依据,对增强地方政府征管治理能力具有重要启示。

微多孔铜表面参数对自驱射流强化池沸腾的影响

为强化池沸腾性能,本文基于一种适用于实际池沸腾强化的自驱射流冲击装置,使用R1336mzz(Z)作为工质,微多孔铜表面为加热表面,实验研究了自驱射流装置对沸腾性能的影响,并参数化研究了微多孔表面制备特征对池沸腾性能的影响,如烧结过程中的铜粉粒径尺寸(43µm、66µm、101µm)以及微多孔层厚度与粒径尺寸之比(4、6、10)。研究结果表明,自驱射流装置的置入确保了液体对表面的持续供给以及促进了气泡的脱离,从而有效增强了微多孔铜表面的临界热流密度(CHF)。然而,自驱射流的存在会对沸腾产生抑制作用,从而造成换热性能的轻微恶化,且抑制程度强弱与铜粉粒径尺寸大小呈正相关、与厚度与粒径之比大小呈负相关。参数化研究结果显示,沸腾换热性能随粒径增大而增大、随厚度与粒径之比增大先增大后降低。微多孔铜表面吸液能力均随铜粉粒径以及厚度与粒径之比的增加而提升,并且大部分情形下,搭配自驱射流装置与否,均不会影响其与CHF的线性正相关。但是,当粒径尺寸为101µm时,自驱射流装置会放大其过小的毛细力所带来的润湿劣势,进而造成CHF的偏低。与非微多孔铜表面的标准池沸腾性能相比,本研究的CHF和hNB@CHF提升幅度分别可达189.2%和337.5%,最佳为CHF=72.0W/cm^(2)和hNB@CHF=87.5kW/(m^(2)·K)。

铜表面-氧化石墨烯纳米涂层池沸腾换热特性实验

石墨烯材料因其优异的导热性能和化学惰性,在核反应堆先进换热设备领域有很大的应用前景。为探索石墨烯材料的强化换热性能,本文采用沸腾沉积法制备氧化石墨烯纳米涂层,开展了氧化石墨烯涂层-去离子水池沸腾换热特性实验。氧化石墨烯纳米流体工质浓度为0.001、0.003、0.005 mg/mL,加热时间控制为1 h和2 h,通过沸腾沉积在裸露铜表面制备6种氧化石墨烯涂层。以此为基础开展池沸腾实验,结果发现氧化石墨烯涂层对池沸腾换热系数的影响很小,但可显著增强池沸腾临界热流密度。从涂层表面润湿性、热导率及形貌结构变化等角度分析了氧化石墨烯涂层对池沸腾换热特性的影响机理。

CMT电弧增材制造铝合金传热传质与熔池行为数值模拟

为研究冷金属过渡(cold metal transition,CMT)电弧增材制造铝合金传热传质与熔池流动特性,基于Fluent软件建立了三维CMT电弧增材制造数值模型.模型中,采用动网格技术模拟焊丝竖直方向上的往复运动,利用流体体积法捕获气/液界面,焓-孔隙率法追踪固/液界面,并施加周期热量输入和阶段电弧力作用来等效电弧放电行为,研究分析了焊道成形传热传质过程与熔池动态行为.结果表明,焊道成形初期,熔池余高和坡度较大,形貌犹如半个球体,成形后期热量积累造成焊道余高后方较前方略小,而后端熔宽较前端略宽;单滴过渡周期内,焊丝机械回抽对熔池表面流动影响最为明显,液桥断裂产生较大反冲作用于熔池;熔池内部则是电磁力作为主导驱动力产生一股顺时针环流,环流随燃弧阶段周期切换而不断加强与减弱,并基本贯穿整个过渡周期,使得熔池内部热对流更加充分.模拟结果与试验结果显示吻合良好.

融合注意力机制和二次特征提取的ResNet小样本农作物病虫害识别

针对传统机器学习方法对于小样本和多类别的农作物叶片病虫害识别效果和时间不理想的问题,本文利用改进的ResNet模型来实现农作物病害识别.通过加入dropout层、激活函数、最大池化层和注意力机制来提高模型的鲁棒性、特征捕捉能力、实现了用较低的模型参数量来提高病虫害识别的准确率.首先对从公共数据集Plant Village获取的图像进行预处理和增强,将ReLU激活函数替换为PReLU,解决ReLU函数在小于0部分神经元坏死的问题;然后在全局平均池化层之前加入dropout层,设置合理的阈值,有效避免过拟合现象的发生,增强模型的鲁棒性;此外,在dropout与全局平均池化层之间加入最大池化层,不仅能扩大神经元的感受野,还能帮助模型获取局部病虫害的最显著特征,减小图片背景的噪声影响,实现二次特征提取;最后嵌入CBAM注意力机制,使模型自动学习输入特征图中最重要的通道信息,并对其进行通道与空间之间加权,从而更好地捕捉图像中的语义信息.实验结果表明,改进后的模型对测试集识别准确度达99.15%,模型参数量仅为9.13M,与Xception、InceptionV3、原ResNet等模型相比,准确率分别超过了1.01,0.68,0.59个百分点,降低了模型参数量,为农作物病虫害识别提供了一种先进的深度学习方法.

超临界CO_(2)池式传热流固耦合传热特性数值模拟

超临界类沸腾传热已有初步研究,对类气-类液界面的定义仍不统一.采用层流模型对超临界CO_(2)池式传热特性进行流固耦合数值模拟研究,d=70μm的铂丝为加热件,热流密度q_w范围为0-2000 kW/m^(2),压力P范围为8-10 MPa,加热丝附近采用多尺度网格进行建模,模拟值与实验数据吻合较好.通过与亚临界传热类比通过计算导热占比Q_(con)/Q_(t)沿r方向变化规律,将超临界划分为3个区,当TT_(M)为类气区(vapor-like,VL),与用超临界类沸腾干度x分区规律相同;根据通过计算导热占比确定的类气膜内平均导热λ_(ave)和导热热阻R_(G)随热流密度q_(w)变化曲线可很好地解释类蒸发区换热系数h随q_(w)的变化规律.本文提出了确定超临界池式传热类气-类液界面的新方法,较好地解释了类蒸发区传热机理,为后续超临界类沸腾池式传热的发展提供了理论基础.

多孔介质孔隙率对池沸腾传热性能影响机理的模拟研究

采用介观相变格子Boltzmann(lattice Boltzmann, LB)方法,在孔隙尺度下研究了多孔介质的孔隙率对池沸腾换热过程的影响,重点分析了不同孔隙率时气泡的运动过程,并对气泡在多孔介质中的典型状态进行了力平衡分析,进而探究了多孔介质孔隙率影响沸腾传热的机理.结果表明,与无多孔介质的平板表面相比,多孔材料能够有效地降低初始成核的壁面过热度,增强流体的扰动,并且能够显著提升临界热流密度(critical heat flux, CHF)值.在所研究的工况中,孔隙率ε=73.2%时,CHF值提升最大,约为平板的3.6倍,其余孔隙率的多孔介质最小也可将其CHF值提升至平板的2.3倍.研究发现,当孔隙率从97.7%开始逐渐减小时,CHF值逐渐增大,同时沸腾换热曲线向左上方移动,这是因为减小孔隙率能够增大有效换热面积,减小气泡成核的壁面过热度,从而强化沸腾换热.当孔隙率减小到ε=73.2%时,若继续减小孔隙率,热流密度将突然下降,沸腾传热性能显著降低.通过对沸腾过程中气泡的受力进行分析后发现,当孔隙率较小时,过小的孔隙直径显著增大了气泡的逸出阻力,降低了气泡的上升速度,延长了气泡脱离多孔介质的时间,且此时气泡会在蒸发动量力、接触压力以及摩擦力等的共同作用下聚集在加热器上表面,形成气膜,从而恶化沸腾传热.

池壳式研究堆停堆后非能动余热导出特性研究

本文以国内某池壳式研究堆为研究对象,采用RELAP5计算并分析了剩余释热功率以及堆水池初始温度对压力容器上部温度的影响。结果表明当剩余功率保持不变时转入间断冷却,随着堆水池初始温度的上升,压力容器上部温度达到50℃的时间会明显缩短。特别是,当堆水池的初始温度设定为10℃,且在剩余功率为0.5%FP的条件下转入间断冷却,压力容器上部的温度将始终保持在50℃以下。进一步证明了池壳式反应堆的安全性,并为停堆后的经济冷却方式选择和长期安全提供了依据。

电弧增材制造传热传质数值模拟技术综述

电弧增材制造过程涉及丝材的送入和熔化,熔融金属向熔池的过渡,熔池中液态金属的对流、凝固和成形。缺陷的形成与电弧增材制造过程中发生的复杂多物理场现象密切相关。因此,需要借助高保真数值模拟技术来深入理解这些物理现象,并将其作为优化工艺条件、制造高质量产品的理论依据。本文综述了电弧增材制造传热传质数值模拟涉及的关键技术,并对未来研究方向进行了展望:首先,介绍了几种典型的热源模型,鉴于电弧增材制造过程中熔池的形成与演变是多种驱动力共同作用的结果,分析了浮力、电磁力、表面张力、电弧压力、电弧剪应力模型对流体流动和熔池表面变形的影响;然后,总结了三种金属过渡模型,包括速度入口填充液态金属、指定位置添加球状质量源项以及直接建立固态金属焊丝;最后,介绍了常用的气液界面跟踪方法。

池式钠冷快堆复杂空间内流动与传热特性三维数值模拟研究综述

钠冷快堆是第四代先进核能系统的重要堆型之一。池式钠冷快堆安全裕量大,但结构较为复杂,其堆内一回路循环流动呈多尺度、复杂空间、多路径、三维流动等特点,给池式钠冷快堆计算与实验带来一定困难。近年来,计算流体力学(CFD)的快速发展为解决上述问题提供了重要技术路径,本文对池式钠冷快堆复杂空间内流动与传热特性三维数值模拟研究进行综述。对于全堆一体化整体计算,获取池式钠冷快堆典型对称/非对称工况下的三维温度场分布及关键热工参数瞬态变化,评价其余热排出能力;对于局部部件或区域精细化计算,获取局部三维流动与传热特征参数,也为全堆一体化计算提供关键输入。相关研究为池式钠冷快堆安全稳定运行及设计优化提供重要支撑。

基于刻蚀处理的池沸腾传热性能实验研究

为探究刻蚀处理铜板表面对池沸腾传热性能的改善效果,搭建了可视化实验平台,制备不同刻蚀浓度和刻蚀时间处理的表面,采用去离子水为工质,进行饱和池沸腾实验.结果表明,氨水刻蚀浓度越大,表面的换热性能越强,氨水刻蚀时间越长,表面换热性能越强.热流密度为43.2 W/cm^(2)时,0.05 mol/L氨水刻蚀4 h表面、0.05 mol/L氨水刻蚀5 h表面、0.05 mol/L氨水刻蚀8 h表面、0.03 mol/L氨水刻蚀4 h表面、0.05 mol/L氨水刻蚀4 h表面和0.10 mol/L氨水刻蚀4 h表面上的换热系数分别提高了46%、92%、105%、36%、45%、78%.此外,对其表面进行表征,结果说明了其优异的沸腾性能.刻蚀处理增强了表面粗糙度,有效面积,从而促进液体的补充,能够在较小的过热度下达到更高的临界热流密度.可视化研究结果表明,刻蚀表面具有更多的成核位点,更小的气泡离开直径和离开频率.这些结果显示出刻蚀表面强化池沸腾换热的潜力.

基于Rohsenow关联式的池沸腾HTC预测修正模型

针对去离子水池沸腾传热系数(HTC)预测,当前理论研究主要是基于Rohsenow关联式,采用固定数值的方式对关联式中的经验参数进行优化。本文基于Rohsenow关联式,通过拟合法建立关联式中两个经验参数与过热度的联系,进而搭建了池沸腾HTC预测修正模型。与3组去离子水铜表面池沸腾实验结果对比,修正模型对HTC的预测平均误差为17.79%、5.35%、18.14%,优于Rohsenow关联式模型和Li关联式模型。研究工作可以为HTC的预测提供一定的理论帮助。