Interfacial heat-transfer between A356-aluminium alloy and metal mould

The interfacial heat-transfer coefficient at casting/mould interface is a key factor that impacts the simulation accuracy of solidification progress.At present,the simulation result of using available data is comparatively different from the practice.In the current study,the methods of radial heating and electricity measurement under steady-state condition were employed to study the nature of interfacial heat-transfer between A356 Aluminum alloy and metal mould.The experimental results show that the interfacial heat-transfer between A356 Aluminum alloy and the outer mould drops linearly with time while that of A356 aluminum alloy and the inner mould increases with time during cooling.The interfacial heat-transfer coefficient between A356 aluminum alloy and mould is inversely proportional to the electrical resistance.

Fluid Mechanics and Heat-Transfer Operations Combination Involved in Urea Unit of Fertilizer Complex

Fertilizer plants are most complex plants in the world (Rashid et al., 2013, Process Safety Progress) and its good opportunity to learn science from operations involved in these plants. Fluid mechanics and heat transfer operations combination involved in fertilizer complexes are explored in this article.

Temperature Distributions in Single Cell of Polymer Electrolyte Fuel Cell Simulated by an 1D Multi-plate Heat-Transfer Model and a 3D Numerical Simulation Model

The purpose of this study is to verify an 1D multi-plate heat-transfer model estimating the temperature distribution on the interface between polymer electrolyte membrane and catalyst layer at cathode in single cell of polymer electrolyte fuel cell, which is named as reaction surface in this study, with a 3D numerical simulation model solving many governing equations on the coupling phenomena in the cell. The results from both models/simulations agreed well. The effects of initial operation temperature, flow rate, and relative humidity of supply gas on temperature distribution on the reaction surface were also investigated. It was found in both 1D and 3D simulations that, the temperature rise （i.e., Treact-Tini） of the reaction surface from initial operation temperature at 70℃ was higher than that at 80℃ irrespective of flow rate of supply gas. The effect of relative humidity of supply gas on Treact- Tini near the inlet of the cell was small. Compared to the previous studies conducted under the similar operation conditions, the Treact - Tini calculated by 1D multi-plate heat-transfer model in this study as well as numerical simulation using 3D model was reasonable.

Heat-transfer model on the improvement of continuous casting slab temperature

A heat transfer model on the solidification process has been established onthe basis of the technical conditions of the slab caster in No.3 steel works of Wuhan Iron & SteelCorporation, and the temperature field in the solidifying slab was calculated which was verified bythe measured slab surface temperature. The influences of the main operating factors includingcasting speed, spray cooling patterns, superheat of melt and slab size on the solidification processwere analyzed and the means of enhancing the slab temperature was brought forward. Raising thecasting speed to 1.3 m/min, controlling the flowrate of secondary cooling water and improving thecooling pattern at the lower segments of secondary cooling zone could improve the slab temperatureeffectively. And the increasing the superheat is adverse to the production of high temperature slab.

Numerical simulation of the drag and heat-transfer characteristics around and through a porous particle based on the lattice Boltzmann method

Porous particle flow is universal in nature and industry.However,in previous numerical simulations,porous particles have usually been assumed to be solid.It is necessary to study the flow and heat-transfer characteristics around porous particles because they are greatly different from those of impermeable particles.In this study,two-dimensional steady flow and heat transfer around and through a porous particle with a constant temperature placed in a cold fluid were numerically investigated.The effects of the Reynolds number(Re)and Darcy number(Da)on the flow and heat-transfer characteristics were investigated in detail.The investigated ranges of the parameters were 10≤Re≤40 and 10^(−6)≤Da≤10^(−2).It is sophisticated to simulate porous particles with traditional simulation methods because of their complicated structure.Therefore,the lattice Boltzmann method was used to solve the generalized macroscopic governing equations because of its simplicity.The drag coefficient decreased with increasing Re or Da,but the decrease was not prominent in the range 10^(−6)≤Da≤10^(−4).The heat-transfer efficiency of the front surface was much stronger than that of the rear surface.The heat-transfer efficiency between the particle and the fluid increased with increasing Re or Da.However,for 10^(−6)≤Da≤10^(−4),the increase was not prominent and the heat-transfer enhancement ratio was slightly larger than one.Furthermore,the effect of Da became more prominent at larger Re.In addition,new correlations for the drag coefficient and surface-averaged Nusselt number were obtained based on the simulated results.

Time-accurate CFD conjugate analysis of transient measurements of the heat-transfer coefficient in a channel with pin fins

Heat-transfer coefficients(HTC)on surfaces exposed to convection environments are often measured by transient techniques such as thermochromic liquid crystal(TLC)or infrared thermography.In these techniques,the surface temperature is measured as a function of time,and that measurement is used with the exact solution for unsteady,zero-dimensional(0-D)or one-dimensional(1-D)heat conduction into a solid to calculate the local HTC.When using the 0-D or 1-D exact solutions,the transient techniques assume the HTC and the free-stream or bulk temperature characterizing the convection environment to be constants in addition to assuming the conduction into the solid to be 0-D or 1-D.In this study,computational fluid dynamics(CFD)conjugate analyses were performed to examine the errors that might be invoked by these assumptions for a problem,where the free-stream/bulk temperature and the heat-transfer coefficient vary appreciably along the surface and where conduction into the solid may not be 0-D or 1-D.The problem selected to assess these errors is flow and heat transfer in a channel lined with a staggered array of pin fins.This conjugate study uses three-dimensional(3-D)unsteady Reynolds-averaged Navier-Stokes(RANS)closed by the shear-stress transport(SST)turbulence model for the gas phase(wall functions not used)and the Fourier law for the solid phase.The errors in the transient techniques are assessed by comparing the HTC predicted by the time-accurate conjugate CFD with those predicted by the 0-D and 1-D exact solutions,where the surface temperatures needed by the exact solutions are taken from the time-accurate conjugate CFD solution.Results obtained show that the use of the 1-D exact solution for the semi-infinite wall to give reasonably accurate“transient”HTC(less than 5%〇relative error).Transient techniques that use the 0-D exact solution for the pin fins were found to produce large errors(up to 160%relative error)because the HTC varies appreciably about each pin fin.This study also showed that HTC measured by transient techniques could differ considerably from the HTC obtained under steady-state conditions with isothermal walls.

η_m FOR THE HEAT-TRANSFER LAW q∞△1/T

Curzon and Ahlborn considered firstly the irreversibility of heat-transfer in the Carnot cycle, and derived the efficiency of a Carnot cycle at maximum power

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.

甲烷/乙烷混合工质纳米液膜蒸发传质特性的研究

采用分子动力学模拟方法研究了五种不同配比甲烷/乙烷混合工质纳米液膜的汽化过程,从蒸发现象、能量变化、涨落耗散规律及相对分子汽化量四个方面分析了混合工质的蒸发传质特性.结果表明:混合工质纳米液膜中甲烷配比越大,汽化起始时间越早,汽化过程越剧烈,汽化的持续时间越短,固定吸附层越薄,模拟体系动能和势能的增量越小,乙烷的存在会抑制甲烷的汽化;由涨落耗散分析可得,甲烷配比越大,对乙烷汽化的促进作用越强,汽化弛豫时间越短,整体汽化速率越快,传质效果越好.

基于能量流向的变压器饼式绕组散热分析及试验研究

变压器热点温度作为运行经济性、安全性的关键指标,是设备在线监测及状态评估中的重点。该文对变压器内部散热进行研究,着眼于饼式绕组及其油道结构,基于能量流向建立绕组温度变化的物理模型。基于这一模型,在一台内置分布式传感光纤的110kV三相ONAN变压器上开展试验研究,使用分布式光纤测温(distributed temperature sensing,DTS)技术对运行状况下的绕组整体温度分布进行实时监测,分析绕组在ONAN冷却方式下的散热状况。在变压器启动初期,绕组各处散热量较低,温升速率较快。约2 h后,各饼散热量基本与损耗相一致,散热率可达98%以上,因此将这一阶段称为准稳态。准稳态阶段,绕组整体散热率基本一致。负载变化前期不同位置散热量的差异是温度梯度形成的主要原因。基于DTS手段及散热器进出口处油温,提出绕组每饼平均对流换热系数的计算方法,基于无量纲数建立绕组内外表面局部对流换热系数的计算方法,对不同位置、负载率下两种对流换热系数的变化规律进行分析获得了绕组运行过程中对流换热系数分布规律及变化趋势。

基于自产热和外传热的锂离子电池热学模型参数辨识方法

锂离子电池热学模型参数(热容和热阻)的准确辨识对电池热电耦合建模及状态参数估计至关重要。然而传统测量方法成本高且测试周期长,如何利用充放电工况结合产热和传热机理研究快速热参数辨识方法具有重要意义。以8A×h软包锂离子电池为研究对象,建立分布式热路模型;设计双向脉冲工况实验,采用自适应粒子群算法(APSO)进行辨识;同时采用其他工况进行验证,实验和仿真温度误差小于0.1℃。另外,将热容和热阻转换为比热容和导热系数,并与其他文献中同类电池的参数进行比对,量级接近。研究结果表明,该方法可以有效解决层叠式软包锂离子电池热学模型参数辨识难的问题,且简便易行、成本低。

石墨烯低温远红外辐射干燥稻谷干燥特性与品质研究

为研究稻谷的石墨烯低温远红外干燥特性及其对稻谷干燥品质的影响,以辐射温度、排粮流量和除湿风量为影响因素,以整精米率和应力裂纹指数增值为评价指标,用自制的循环式石墨烯低温远红外干燥机进行稻谷干燥试验,通过BBD(Box-Behnken设计)响应面法,分析了低温远红外干燥对稻谷干燥品质的影响以及工艺参数优化。结果表明:影响稻谷干燥特性和品质的最主要因素是辐射温度,其次是排粮流量和除湿风量。随着辐射温度的升高,稻谷干燥速率和应力裂纹指数增值逐步增大,整精米率则逐步降低。与同温度的热风干燥相比,石墨烯低温远红外干燥平均干燥速率和干燥品质均有显著提高。经优化后,稻谷最佳石墨烯低温远红外干燥工艺条件为:辐射温度43℃、排粮流量4 kg/min、除湿风量193 m^(3)/h,此时应力裂纹指数增值为9,整精米率为79.75%,稻谷干燥品质最佳。这说明利用石墨烯低温远红外干燥稻谷,可以明显提高干燥速率并改善稻谷干燥品质。

含端壁涡轮导叶红外测温的误差影响机理研究

由于涡轮叶片所处的工作环境复杂,受到壁面及燃气辐射干扰,导致测温十分困难。为进一步分析涡轮叶片表面在端壁影响下的温度分布情况,采用数值模拟结合自定义编程的方式进行了含端壁涡轮导叶的温度误差验证计算。利用自定义编程对经典同心球模型进行了角系数,有效辐射等可靠性验证计算,验证方法可靠。基于该方法进行了端壁与涡轮导叶各网格单元的角系数,叶片表面间的辐射换热计算,输出涡轮导叶的表面辐射特性分布,运用玻耳兹曼定律反推导出该工况涡轮导叶所受到的辐射能流分布情况;计算分析了不同误差影响机理下,含端壁的涡轮导叶温度误差分布情况,探明了热辐射环境下对叶片表面辐射特性的影响。结果表明:当进口黑体辐射温度在1 400~1 800 K之间,进口辐射强度是对叶片换热影响最大的因素;利用有效辐射和所计算的误差分布可知,进口辐射温度影响区域主要是叶片前缘区域,最大计算误差不超过2.82%;通过有效辐射及误差分布可见,出口黑体辐射温度的变化对于涡轮导叶的影响较小,受温度影响区域也主要为叶片前缘区域,最大计算误差不超过2.35%;叶片表面发射率与温度成正相关,当叶片表面发射率增大时,叶片表面温度随之均匀升高,在真实工况下,叶片材料物性的改变较小,在发动机设计中可以近似忽略由于叶片温度变化导致物性参数改变带来的影响。

电厂锅炉实验虚拟仿真系统建设与应用

为开展能动专业核心课程“锅炉原理”的实验教学,校企合作开发了电厂锅炉实验虚拟仿真系统。基于两相流动与辐射对流传热原理构建的仿真系统,由风烟和汽水两大模型构成,严格遵守质量、能量、动量守恒方程。变负荷虚拟实验显示,随锅炉负荷增加,热效率降低、燃料耗量增加。根据辐射传热原理,结合火焰平均温度计算发现:只有炉膛出口烟温增加,才能保证炉膛传热负荷增加,继而抬升排烟温度,导致排烟热损失变大、锅炉热效率降低。锅炉原理实验虚拟仿真,可训练学生运用锅炉本体热力计算原理分析问题的思维,是落实创新能力培养的重要举措。

黄土地基中能量桩群桩承载变形性状模型试验

基于室内缩尺试验研究了含能量桩群桩在4次冷热循环过程中的桩身热力学特性及桩基承载变形特性变化规律,分析冷热循环对桩土温度、承台沉降、桩顶荷载及桩身摩阻力的影响。试验结果表明:冷热循环过程中能量桩较浅位置处桩身及桩侧土温度始终高于较深处,热循环过程中能量桩的传热效率高于冷循环过程。冷热循环会使承台顶面的工作荷载及群桩中各基桩的桩顶荷载发生往复变化,相应地引起承台发生往复倾斜,热循环引起的承台倾斜稍大于冷循环。热循环引起的能量桩桩身上部摩阻力为负,下部为正,冷循环时则相反。

管内充分发展流动与传热数值模拟的教学方法探讨

由于缺乏数值计算的基础理论知识,工科本科生在学习流体流动与传热过程的数值模拟方法并进行程序设计时,往往觉得难度较大。本文围绕圆管内不可压缩流体充分发展流动与传热,采用边界层积分法,推导出无量纲控制方程组;针对层流工况和湍流工况,开发出相应的数值方法。基于该方法的程序代码易于理解,且计算结果表明该方法具有预测精度高、收敛速度快的优点。本文的工作可以为计算流体力学、数值传热学及热工计算等系列课程的本科教学提供参考。

20钢/316L复合管固液复合制坯工艺研究

双金属复合管在石油运输行业中应用前景广阔,结合性能是考察复合管是否合格的重要指标之一。为了能在轧制成形过程中得到高品质的复合管材,在复合管制坯过程中要求实现界面处的冶金复合。以20钢和316L为原材料,基于结合温度及元素扩散情况,采用固液复合方式,并且结合界面的温度模型及通过Pro CAST软件模拟,获得了外模最佳预热温度为1050—1100℃。通过现场试制界面处实现了冶金复合,在最佳温度范围既保证了固态20钢和液态316L之间能达到冶金复合,又能使元素有充分的扩散时间而增强了界面结合强度,同时又可以防止因元素过渡扩散而导致316L腐蚀性能下降。该方法为后续复合管连轧提供了优良的管坯,可实现高品质复合管的现场生产。

4种用于温室除湿的亲水翅片管蒸发器热湿传递性能分析

温室冬季高效除湿需求已经成为制约中国温室产业高质量发展的技术难题之一。为了提高温室用冷冻除湿系统中亲水翅片管蒸发器的热湿传递性能,该研究建立了低温高湿工况下平翅片、带涡产生器平翅片、波纹翅片、开缝翅片4种亲水翅片管蒸发器空气侧的数值传热模型,采用蒸发器的换热量Q、努塞尔特数Nu、摩擦因子f、单位翅片面积析湿量和强化传热因子JF等评价参数,对比分析了低温高湿工况下4种亲水翅片管蒸发器空气侧热湿传递性能。结果表明,与平翅片相比,带涡产生器平翅片、波纹翅片、开缝翅片空气侧的Nu、Q和f均高于平翅片,且开缝翅片的Q和f在相同条件下均最大;4种翅片的f随入口风速的增大而大幅度减小,而相对湿度对f的影响较小;单位翅片面积析湿量均随入口风速和相对湿度的增大而增大,波纹翅片的除湿能力最优;在入口风速1~4 m/s和相对湿度80%~95%条件下,波纹翅片管蒸发器的JF因子平均值最大,其热性能最优;在冬季寒冷地区低温高湿的温室中,推荐选用波纹亲水翅片管蒸发器对温室内空气进行除湿。该研究可为温室低温高湿环境下除湿系统用亲水翅片管蒸发器的设计与应用提供参考。

燃煤锅炉改燃兰炭燃烧工艺及效益分析

兰炭因热值高、硫含量低的特点被广泛应用于散煤替代,燃煤锅炉改燃兰炭是实现资源合理利用、降低污染物排放和改善企业经济效益的有效途径。以河北某循环流化床锅炉改燃兰炭工程为例,燃烧系统采用掺烧灰渣的方法,解决改燃后机械未完全燃烧导致损失大和物料循环不稳定问题,确定了兰炭和灰渣的最佳掺混比为9∶1;配风系统采用控制总量并降低一次风率的方法,解决兰炭着火困难的问题,确定了最佳一次风比例为55%;换热系统采用增加锅炉尾部受热面的方法,解决因燃料热值提升较大产生的蒸汽超温和锅炉效率下降问题。改燃扩容后锅炉烟尘、SO2和NOx排放分别比原锅炉降低了73%、94%和55%,以原锅炉35 t/h的额定功率运行,每小时节约燃料成本5.51%,产吨蒸汽所需燃料成本降低6.47元。

基于响应面法的矿用翅片管空冷器参数优化

矿用空冷器是矿井降温系统常用的末端设备,优化矿用空冷器换热性能对提升矿井降温系统能效具有重要意义。采用Fluent建立了平直翅片管矿用空冷器模型,研究了翅片间距、翅片厚度、横向管间距和纵向管间距对传热因子、阻力因子和综合性能评价指标的影响规律;采用Box-Behnken响应面法,分析了双参数协同作用下传热因子、阻力因子和综合性能评价指标变化规律。结果表明:随着翅片间距、横向管间距的减小,以及翅片厚度增加,传热因子、阻力因子和综合性能评价指标逐渐增加;综合性能评价指标则随纵向管间距的增加而先增加后降低;横向管间距、翅片厚度是影响空冷器传热及流动性能最显著的两个参数,而翅片间距、纵向管间距的影响最小;得到了平直翅片管矿用空冷器的最优参数,传热因子提升16.3%、阻力因子增加26.3%、综合性能评价指标提升8.3%。研究结果可为矿用翅片空冷器设计参数优化提供指导。