Effect of CeO_2 on heat transfer and crystallization behavior of rare earth alloy steel mold fluxes

To improve the heat transfer capability and the crystallization property of the traditional mold flux, CaF_2 was replaced with B_2O_3. Then, the influences of CeO_2 on the heat transfer and the crystallization of the CaF_2-bearing mold flux and the new mold flux with 10 wt% B_2O_3 were studied using a slag film heat flux simulator and X-ray diffraction(XRD). The results revealed that the addition of CeO2 reduced the heat transfer by increasing the solid slag thickness and the crystallization of two mold fluxes. However, CeO_2 had less effect on the B_2O_3-containing mold flux compared with the CaF_2-bearing mold flux. According to the analyses, the CeO_2 contents in the CaF_2-bearing mold flux and the B_2O_3-containing mold flux should not exceed 8 wt% and 12 wt%, respectively. Therefore, these experimental results are beneficial to improve and develop the mold flux for casting rare earth alloy steels.

Global air-sea surface carbon dioxide transfer velocity and flux estimated using 17 a altimeter data and a new algorithm

The global distributions of the air-sea CO2 transfer velocity and flux are retrieved from TOPEX/Poseidon and Jason altimeter data from October 1992 to December 2009 using a combined algorithm. The 17 a average global, area-weighted, Schmidt number-corrected mean gas transfer velocity is 21.26 cm/h, and the full exploration of the uncertainty of this estimate awaits further data. The average total CO2 flux （calculated by carbon） from atmosphere to ocean during the 17 a was 2.58 Pg/a. The highest transfer velocity is in the circumpolar current area, because of constant high wind speeds and currents there. This results in strong CO2 fluxes. CO2 fluxes are strong but opposite direction in the equatorial east Pacific Ocean, because the air-sea CO2 partial pressure difference is the largest in the global cceans. The results differ from the previous studies calculated using the wind speed. It is demonstrated that the air-sea transfer velocity is very important for estimating air-sea CO2 flux. It is critical to have an accurate estimation for improving calculation of CO2 flux within climate change studies.

Heat transfer in boundary layer stagnation-point flow towards a shrinking sheet with non-uniform heat flux

In this paper, the effect of non-uniform heat flux on heat transfer in boundary layer stagnation-point flow over a shrinking sheet is studied. The variable boundary heat fluxes are considered of two types： direct power-law variation with the distance along the sheet and inverse power-law variation with the distance. The governing partial differential equations （PDEs） are transformed into non linear self-similar ordinary differential equations （ODEs） by similarity transformations, and then those are solved using very efficient shooting method. The direct variation and inverse variation of heat flux along the sheet have completely different effects on the temperature distribution. Moreover, the heat transfer characteristics in the presence of non-uniform heat flux for several values of physical parameters are also found to be interesting.

Global air-sea surface carbon-dioxide transfer velocity and flux estimated using ERS-2 data and a new parametric formula

Using data from the European remote sensing scatterometer （ERS-2） from July 1997 to August 1998, glob- al distributions of the air-sea CO2 transfer velocity and flux are retrieved. A new model of the air-sea CO2 transfer velocity with surface wind speed and wave steepness is proposed. The wave steepness （6） is re- trieved using a neural network （NN） model from ERS-2 scatterometer data, while the wind speed is directly derived by the ERS-2 scatterometer. The new model agrees well with the formulations based on the wind speed and the variation in the wind speed dependent relationships presented in many previous studies can be explained by this proposed relation with variation in wave steepness effect. Seasonally global maps of gas transfer velocity and flux are shown on the basis of the new model and the seasonal variations of the transfer velocity and flux during the 1 a period. The global mean gas transfer velocity is 30 cm/h after area-weighting and Schmidt number correction and its accuracy remains calculation with in situ data. The highest transfer velocity occurs around 60°N and 60°S, while the lowest on the equator. The total air to sea CO2 flux （calcu- lated by carbon） in that year is 1.77 Pg. The strongest source of CO2 is in the equatorial east Pacific Ocean, while the strongest sink is in the 68°N. Full exploration of the uncertainty of this estimate awaits further data. An effectual method is provided to calculate the effect of waves on the determination of air-sea CO2 transfer velociW and fluxes with ERS-2 scatterometer data.

Arc Characteristic and Metal Transfer of Pulse Current Horizontal Flux-Cored Arc Welding

In this work, we utilized high-speed video photography to investigate the arc characteristics, metal transfer behavior, and welding spatter of the pulse-current flux-cored arc welding (P-FCAW) process in the horizontal position. The results indicate the presence of a stable “flux pole” during both the pulse-on and pulse-off periods when the mean current ranged from 140 to 170 A. The existence of this “flux pole” was beneficial for droplet transfer in the “axial droplet transfer” mode. With respect to welding spatter, with an increase in the welding current, we observed three kinds of spatter—explosive spatter, rebounded droplet spatter, and scattering spatter. With an increase in the pulse current from 310.6 to 345.6 A, the deflection of the arc reduced from 30.4° to 16.6°, which positively influenced the arc rigidity, particularly in the horizontal position. © 2017, Tianjin University and Springer-Verlag Berlin Heidelberg.

Droplet Transfer Behavior of Flux Cored Wire TIG Welding

This paper puts forward a new method to achieve flux cored wire TIG welding and uses high-speed photography to analyze the droplet transfer behavior and forces acting on the droplet. The droplet transfer forms include bridging transfer, slag column guided transfer, and non-contact transfer; each of these forms may be observed as the melting position of the welding wire changes. The important role of surface tension in the process of droplet transfer is proposed using static force balance theory and pinch instability theory. The phenomenon of droplet backward swing during welding process could be attributed to the vapor recoil force produced by vapors from the droplet. The welding experiments show that the proposed welding process is stable and that the weld quality produced is good.

Numerical Solution of MHD Convection and Mass Transfer Flow of Viscous Incompressible Fluid about an Inclined Plate with Hall Current and Constant Heat Flux

The present numerically study investigates the influence of the Hall current and constant heat flux on the Magneto hydrodynamic (MHD) natural convection boundary layer viscous incompressible fluid flow in the manifestation of transverse magnetic field near an inclined vertical permeable flat plate. It is assumed that the induced magnetic field is negligible compared with the imposed magnetic field. The governing boundary layer equations have been transferred into non-similar model by implementing similarity approaches. The physical dimensionless parameter has been set up into the model as Prandtl number, Eckert number, Magnetic parameter, Schmidt number, local Grashof number and local modified Grashof number. The numerical method of Nactsheim-Swigert shooting iteration technique together with Runge-Kutta six order iteration scheme has been used to solve the system of governing non-similar equations. The physical effects of the various parameters on dimensionless primary velocity profile, secondary velocity profile, and temperature and concentration profile are discussed graphically. Moreover, the local skin friction coefficient, the local Nusselt number and Sherwood number are shown in tabular form for various values of the parameters.

面向液态铅铋数值传热的湍流热通量模型构建综述

液态铅铋(LBE)数值传热受制于极低普朗特数特性,传统雷诺比拟方法不能准确封闭和描述平均能量方程中的湍流热通量(THF)及温度输运过程,因此需构建针对低普朗特数流体THF的封闭模型。模型分为4类:湍流普朗特数模型、代数热通量模型、二阶矩微分热通量模型及利用机器学习的数据驱动模型。本文通过梳理其研究进展、建模思想,对模型所展现的复杂程度、在工程应用中的适用性等特点展开评述,为后续THF封闭建模研究、优化及创新提供见解。在燃料棒束间LBE的强制对流换热模拟中,显式代数热通量模型展现出较高的应用前景,机器学习则为THF封闭提供了全新的视角。

基于平均热通量进行TMPI发动机传热的仿真与试验研究

本文对一款改进型TMPI发动机缸体缸盖温度场进行了仿真与试验研究。通过自然吸气MPI发动机台架性能试验,为一维热力学计算平均热通量提供校准基础,依据各模块平均热通量进行了有限元分析模型校核,并基于冷却水套CFD和缸内燃烧CFD计算的对流换热系数,为结构温度场进行预测。基于发动机交变工况,对发动机关键位置进行多通道瞬时温度采集,并与发动机温度场仿真值对比,确认其误差约8%,论证了基于平均热通量校核发动机温度场最高温度和温度分布的可行性,同时为改进型增压发动机温度场优化提供了理论依据。

STUDY ON METAL TRANSFER MODES OF SELF-SHIELDED FLUX-CORED WELDING WIRE

Metal transfer behavior of six kinds of self shielded flux cored wire(SSFCW) is studied using the apparatus of SSFCW high speed photography self made. Six kinds of metal transfer modes of SSFCW were obtained through observation for high speed photograph film and analysis. It is believed that the research is of magnificent for improving operative performance and mechanical properties of SSFCW and dynamics characteristic of welding power.

热风加热沥青路面冲击射流共轭传热特性

为提高热风加热沥青路面的就地再生加热效果,基于热风冲击射流对流换热和沥青路面内部导热的共轭传热过程,建立了热风加热沥青路面的冲击射流共轭传热理论模型,选取有限容积法得到了共轭传热模型的通用离散方程,采用压力-速度耦合半隐式算法(semi-implicit method for pressure linked equations,SIMPLE)获得了整个求解域内温度场分布,选取平均热流密度和平均换热系数反映沥青路面加热效果,通过正交试验研究了热风出口速度和热风出口温度对路面加热效果的影响程度。仿真和试验结果表明:理论计算与实验温度场分布趋势吻合度高,两者平均误差为8.4%;平均热流密度和平均换热系数在加热初期均从最大值急剧下降,而后下降幅度逐渐减小趋于平衡,两者的仿真计算与实验结果趋势相同,平均误差分别为6.4%和7.8%;热风出口速度和热风出口温度对平均热流密度均有显著影响,热风出口速度对平均换热系数有显著影响,热风出口温度对平均换热系数的影响相较于平均热流密度指标表现为不显著。研究结果为后续沥青路面就地热再生热风加热温度控制和加热器设计提供了理论依据。

基于平板热管技术的电子器件热管理研究进展

随着电子器件不断趋于小型化和高度集成化发展,电子器件功率过高引发的散热问题亟待解决。平板热管由于具有均温性好、散热效率高等优势已为许多大功率电子元器件的散热问题提供了有效的热管理解决方案。但面对如今越来越多元化的散热需求,开发适应新型电子设备散热需求的高效平板热管仍是当前研究的热点。基于此,在对平板热管工作原理及结构特点进行阐述的基础上,对平板热管的传热性能、影响传热性能的因素(毛细芯结构、腔体厚度、工质、充液率、倾角和热源)、强化平板热管传热性能的措施(不同润湿性表面和支撑柱结构等)及其在小型化电子设备、IGBT晶体管、LED和电池组等电子器件散热应用等方面进行了系统性综述,指出研究不同毛细芯结构与腔体厚度下平板热管内部流动传热特性、散热性能强化的机制和综合优化方法,以及如何在不同电子器件热管理需求下设计出空间适应性强而散热性能高效的平板热管等仍是当前平板热管有待进一步突破的关键技术,将为平板热管在电子器件散热领域的进一步深入研究与优化提供一定参考。

超高参数火箭煤油在小通道圆管内的流动换热特性

火箭煤油再生冷却过程具有高压、高温、高热流密度和高质量流速等特点。在超高参数条件下,采用低电压大电流电加热方法模拟火箭发动机推力室壁面热环境,在高温合金钢∅2 mm×0.5 mm圆管内研究了火箭煤油的流动换热特性。参数范围为压力25~65 MPa,质量流速8500~51000 kg/(m^(2)·s),流体温度常温为~500℃,热流密度最高为35 MW/m^(2)。研究表明:在所测试条件下,火箭煤油在小通道圆管内处于单相液态强制对流换热机制;换热性能主要受到流体温度和质量流速的影响;流体温度增加,换热系数增加;质量流速增加,换热系数增加;压力在25~65 MPa范围内对换热性能无显著影响;热流密度增加,内壁温显著增加,但热流密度变化对换热系数无显著影响;受入口强化换热效应的影响,换热系数增加,热流密度越高,入口效应越明显。超高参数尤其是超高压力条件下的火箭煤油换热特性,为火箭煤油再生冷却技术应用提供参考。

单向多孔材料在电弧中冷却性能的数值模拟

定向凝固制备的单向多孔材料在电弧加热器中可以替代水冷铜壁,提高电弧加热器的焓值和热效率,具有广阔的应用前景。针对单向多孔材料在电弧中的传热过程建立数学模型,利用数值模拟的方法研究了孔隙率、孔直径、通入角和注入率对单向多孔材料在电弧中冷却性能的影响规律。结果表明:孔隙率从7.9%增加到26.7%时,内壁面的热通量可降低约92%;在孔隙率和冷气流速不变的条件下,孔直径从0.3 mm增加到0.8 mm时,内壁面的平均温度从498 K增加到697 K;通入角的变化会同时影响单向多孔材料内壁面热通量和冷气注入率,进而影响材料的冷却性能;冷态气流注入率从14.2%增加到42.7%时,单向多孔材料内壁面热通量降低约17%。本研究结果可为单向多孔材料在电弧加热器中的应用提供参考。

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。

Hybrid-Vlasov simulation of soft X-ray emissions at the Earth’s dayside magnetospheric boundaries

Solar wind charge exchange produces emissions in the soft X-ray energy range which can enable the study of near-Earth space regions such as the magnetopause,the magnetosheath and the polar cusps by remote sensing techniques.The Solar wind Magnetosphere Ionosphere Link Explorer(SMILE)and Lunar Environment heliospheric X-ray Imager(LEXI)missions aim to obtain soft Xray images of near-Earth space thanks to their Soft X-ray Imager(SXI)instruments.While earlier modeling works have already simulated soft X-ray images as might be obtained by SMILE SXI during its mission,the numerical models used so far are all based on the magnetohydrodynamics description of the space plasma.To investigate the possible signatures of ion-kinetic-scale processes in soft Xray images,we use for the first time a global hybrid-Vlasov simulation of the geospace from the Vlasiator model.The simulation is driven by fast and tenuous solar wind conditions and purely southward interplanetary magnetic field.We first produce global X-ray images of the dayside near-Earth space by placing a virtual imaging satellite at two different locations,providing meridional and equatorial views.We then analyze regional features present in the images and show that they correspond to signatures in soft X-ray emissions of mirrormode wave structures in the magnetosheath and flux transfer events(FTEs)at the magnetopause.Our results suggest that,although the time scales associated with the motion of those transient phenomena will likely be significantly smaller than the integration time of the SMILE and LEXI imagers,mirror-mode structures and FTEs can cumulatively produce detectable signatures in the soft X-ray images.For instance,a local increase by 30%in the proton density at the dayside magnetopause resulting from the transit of multiple FTEs leads to a 12%enhancement in the line-of-sight-and time-integrated soft X-ray emissivity originating from this region.Likewise,a proton density increase by 14%in the magnetosheath associated with mirror-mode structures can result in an enhancement in the soft X-ray signal by 4%.These are likely conservative estimates,given that the solar wind conditions used in the Vlasiator run can be expected to generate weaker soft X-ray emissions than the more common denser solar wind.These results will contribute to the preparatory work for the SMILE and LEXI missions by providing the community with quantitative estimates of the effects of small-scale,transient phenomena occurring on the dayside.

柴油喷雾撞击高温壁面的瞬态传热特性

通过瞬态测温技术对定容燃烧室中柴油喷雾撞击高温壁面的瞬态传热特性进行了研究.结果表明:初始壁面温度从250℃升高到550℃,壁面上传热模式由膜态蒸发转化为核态沸腾,进而转化为过渡沸腾,这种转化使表面热流密度和换热系数先增大后减小,且分别在440℃和400℃达到最大值.增大喷射压力、减小撞壁距离和环境温度有助于提高表面热流密度和换热系数.初始表面温度的升高增加了喷射压力和撞壁距离对传热特性的影响,但降低了环境温度的作用.以毕渥数和傅里叶数表示的量纲为1表面热流密度曲线形态高度相似,且揭示了喷雾撞击高温壁面过程中的内部传热机制.此外,建立了一种准确性高、适用壁面温度范围广的预测最大表面热流密度的量纲为1模型.

垂直管内高质量流速超临界CO_(2)换热特性

利用已有的流动传热实验数据,对不同湍流模型预测超临界CO_(2)(S-CO_(2))传热能力进行了评价及选取,确定了SST k-ω湍流模型为最优模型。分析了入口温度、热流密度、质量流速、浮升力和流动加速效应对内径为10mm的垂直加热管内S-CO_(2)的对流传热特性的影响。结果表明:在一些工况条件下Bu<10^(-5)、Bu^(*)<5.6×10^(-7)和K_(v)<3×10^(-6)并不满足,表明浮升力和流动加速度效应并不能解释高质量流速下的数值模拟结果。基于超临界类沸腾理论,建立了垂直加热管内S-CO_(2)类沸腾传热模型,并阐述了S-CO_(2)传热恶化现象,径向方向上S-CO_(2)热物性和湍流的详细分布表明超临界传热受类气膜的厚度、类气膜的热性质和近壁区湍流动能的影响很大,成功解释了SCO_(2)在高质量流速下的传热机理。最后引入超临界沸腾数SBO,提出了适用高质量流速的传热关联式。

基于传热传质理论的路面除冰盐融冰过程及影响因素分析

为了解决不同环境下路面除冰盐的科学有效使用问题,降低由于除冰盐的滥用导致路面病害与环境污染,从路面除冰盐融冰传热传质机理出发,建立道路除冰盐融冰过程中的物质平衡方程与热通量平衡方程。采用COMSOL有限元软件建立了路面除冰盐融冰温度场模型,考虑多种因素如相变过程、风速、初始温度和太阳辐射的影响,分析融冰过程中温度场的变化规律。研究结果表明:在融冰过程中,盐溶液温度呈现先快速下降后缓慢回升的规律,在固态除冰盐完全溶解的时间段内(Ⅰ阶段),盐溶液温度迅速下降;除冰盐完全溶解后(Ⅱ阶段),盐溶液温度缓慢回升。外界环境温度的上升使得单位质量除冰盐的融冰体积增加,且外界环境温度相比风速和除冰盐用量对盐溶液最终温度的影响更大。风速越高,固体盐溶解溶解所需时间越短,Ⅰ阶段结束后盐溶液体系的温度也更低,盐溶液温度随之下降得更快。由于风速增大将加快盐溶液层与外部环境的换热,除冰盐融冰Ⅱ阶段升温明显,溶解后盐溶液的平均温度更高。另外,较高的除冰盐初始温度可以促进固态盐的溶解,且温度越高,这种促进作用更加明显。同时,随着反应初始温度的增加,融冰反应的终止温度也随之升高。除此之外,研究发现,太阳辐射对于除冰盐融冰过程的影响在Ⅰ阶段不显著,在Ⅱ阶段具有显著影响。研究结论可为道路冬季路面除冰盐的科学有效使用提供技术支撑与理论参考。

基于Workbench的高压圆盘气体轴承共轭传热研究

高压圆盘气体轴承流道间隙内高速气流的对流换热与轴承圆盘内部热传导紧密耦合在一起,是一个典型的共轭传热问题。基于ANSYS Workbench工作平台的Fluid Flow(Fluent)模块对高压圆盘气体轴承进行共轭传热数值模拟,获得轴承流道间隙内的速度和压力分布、流体域与固体域的温度分布以及共轭传热时流固耦合壁面的热流密度分布,并将其与非共轭传热恒温壁面条件下的计算结果进行对比,得到高压圆盘气体轴承共轭传热的一些基本特性。结果表明:2种情况下的计算结果存在较大差异,非共轭传热恒温壁面条件下,间隙内的气体只吸热,流体域耦合壁面上的热流密度均为正值;而共轭传热条件下流体域耦合壁面热流密度存在正负值,间隙内气体的吸热和放热同时存在,显示出轴承圆盘的热传导与间隙内气体的对流换热具有复杂的共轭作用机制;相比之下,采用共轭传热模型可以得到更为符合实际的结果。研究结果为该类轴承的设计和制造提供了有益的指导。