非能动余热排出系统C型管换热器数值模拟

为保证在事故工况下非能动余热排除系统有效导出余热,对其主要设备PRHR热交换器进行换热特性研究,建立了非能动余热排出系统C型管换热器的内外耦合传热分析模型,采用一维均相流模型计算管内冷凝换热与CFD程序分析水池空间的自然对流。研究进口质量流量、进口流体含气率、管倾角和水箱温度对C型管换热器换热特性的影响。结果表明:C型管换热器入口倾斜段管内始终为饱和的两相流体,在竖直段与出口倾斜段,管内流体温度逐渐下降;管内压力、流体焓值和换热系数沿管长逐渐降低;大约在冷凝70 s后,管内流体参数趋于稳定;管壁温度在入口倾斜段迅速下降,在竖直段和出口段趋于平缓。增大进口质量流量与进口流体含气率,流体温度、流体焓以及管内外换热系数增加,并且沿流动方向受两者的影响逐渐减小;若管倾斜角度增大20°,出口倾斜段管内流体温度下降约3℃;当水箱温度升高10℃,汽泡生成与脱离速度加快,水箱内部换热增强,入口倾斜段外壁温升高2℃左右,出口倾斜段外壁温大约升高0.2℃。CFD模拟结果展示出水箱内汽泡大部分聚集在C型管上部并逐渐向上流动,致使热流体向上运动,冷流体向下流动,形成自然循环。     

高速燃气喷射管道表面自然对流散热分析

将热阻比拟为等效电阻,采用有限体积元法对超音速燃气喷射管道传热及表面自然对流散热进行数值计算,得到喷射管壁温度分布和表面不同自然对流散热及参数变化对壁温的影响.由于管两端自然对流散热作用,管中部温度高于两端;随着表面自然对流换热增强管壁温度逐渐降低;选用不同材质时,随材质导热系数增大管内外壁温差线性增大.     

封闭腔内Al_2O_3-EG纳米流体自然对流传热特性的数值研究

对梯形封闭腔内Al2O3-EG纳米流体自然对流传热进行了数值模拟,讨论了封闭腔尺寸比、瑞利数、纳米颗粒体积分数以及布朗运动对自然对流流动与传热特性的影响。数值模拟结果表明在考虑布朗运动时,腔体尺寸比与瑞利数对流动传热均有很大影响,且尺寸比为0.5时,对流换热平均Nusselt数达到最大值。随着纳米颗粒体积分数的增加,纳米流体换热效果逐渐增强;但当忽略布朗运动时,添加纳米颗粒削弱了换热效果。     

辐射侧加热腔内自然对流传热特性研究

为了明确辐射侧加热封闭方腔内半透明流体的自然对流传热现象及规律,采用有限体积法进行数值模拟研究,分析了瑞利数和光学厚度对流场、温度场以及传热特性的影响。结果表明：与传统侧壁加热腔内自然对流相比,辐射侧加热腔内等温线和流场分布规律不一致;随着瑞利数和光学厚度增加,涡心由中心位置沿直线向辐射入射侧斜上方偏移;随着瑞利数增加,等温线变得更均匀;随着光学厚度增加,等温线变密,努塞尔数Nu与瑞利数RaT的标度律指数减小,当光学厚度增加到一定时标度律不再变化,此时传热标度律与传统恒壁温侧加热腔内自然对流相当,满足Nu~Ra0.29T。     

理想流体对流传热问题的理论解

研究理想流体受迫对流传热和自然对流传热问题的理论解。采用流体无垂直于壁面法线方向运动(即无穿透)的条件取代黏性流体在壁面无滑移条件，解决了流体在边界上有滑移时计算对流传热系数的困难，给出了理想流体与平壁受迫对流传热、理想流体与竖直壁面自然对流传热和理想流体在管内受迫对流传热的理论解。结果表明：理想流体的对流传热与黏性流体同样存在着热边界层。在外部流动的情况下，无论受迫对流传热还是自然对流传热，对流传热系数都与流体的导热系数、密度和比热三乘积的二分之一次方成正比。在管内受迫对流的情况下，当无因次长度大于0．05时，局部Nu和界面无因次温度分布都不再变化，对于恒热流边界条件，Nu等于8，截面无因次平均温度等于2；对于恒壁温边界条件，Nu等于5．782，截面无因次平均温度等于2．316。     

相变储能堆积床传热特性的研究

基于多孔介质传热理论,建立了储能堆积床的传热模型.在此基础上分析了换热流体流量、温度、单元尺寸、相变材料导热系数和孔隙率等参数对堆积床传热特性的影响.研究表明,提高相变材料的导热系数,增大换热流体温差或减小单元尺寸对堆积床传热速率有明显提高,而提高换热流体流量,增大对流换热系数对传热速率的影响不明显.因而强化相变材料侧的传热过程是提高堆积床传热速率的有效途径.     

大空间管排自然对流的数值模拟

对具有2，4，6根管的管排在大空间内的自然对流进行了数值模拟。参数范围为10^2〈Ra〈10^4，S／D=1．25，2，3，5，9，11，Pr=0．7，流态为层流。数值计算结果表明：Ra和S／D对换热都有重要的影响，在小间距时对流作用处于劣势，管排中的管处于换热削弱状态，随着间距的增大对流作用逐渐占优势，管排中的各管从下至上逐渐受到加强，管排中出现了涡，这种涡起着增强局部换热系数的作用。     

A numerical investigation into the charge behaviour of a spherical phase change material particle for high temperature thermal energy storage in packed beds

基于高温相变材料,对填充床储热系统中储热单元球体的储热性能进行了模拟研究.研究了不同传热流体温度和球体直径对球体储热性能的影响规律,对导热为主的相变储热过程与导热和自然对流共同作用的相变储热过程进行了比较分析,同时还探讨了高温辐射换热的影响.结果表明,相变时间随球体直径的增大而增大,随传热流体温度的增大而减小.当考虑相变区域自然对流时,总的相变时间显著减少,和单纯导热相比,完全相变时间缩短了近16%.在导热和自然对流的基础上加上辐射传热后可以看出,辐射换热强化了球体内的传热过程,加快了相变材料的熔化速度,强化了自然对流的作用.     

含热源方腔的耦合自然对流换热的三维数值模拟

对具有内热源方腔的稳态层流耦合自然对流换热进行了三维的数值模拟，采用的模拟代码基于连续介质计算力学的开源库OpenFoam，解决了自然对流换热与固体传热的耦合问题。对外壁面为常温、方腔内充满含体积热源流体的自然对流计算结果表明，温度场、速度场与非耦合的工况有很大差异。     

不同磁场布置对空气自然对流的影响

为研究顺磁性气体介质在外磁场作用下自然对流换热的规律，进一步揭示热磁对流的实质，该文用数值方法模拟了两种钕铁硼永磁系统产生的磁场作用下二维封闭方腔内空气的自然对流换热，得到了工作空间内磁场强度和磁加速度的分布，获得了两种梯度磁场作用下空气自然对流的流场和温度场以及壁面局部对流换热系数。研究表明，不同的永磁系统布置可在工作空间形成不同的磁加速度分布，产生不同的磁场力，从而影响空气自然对流换热的强弱。     

Experimental investigation of heat transfer enhancement using impingement of multiple water jets

The problem of cooling electronic components has become a subject of special interest in recent years due to the increasing capacity and rapidly decreasing size of electronic components. Direct contact cooling using multiple jet impingement is considered the most effective method. The heat transfer problem is complex and a better understanding of the jet impingement method is essential for the proper application of this method for electronic cooling. Investigations were carried out using an electrically heated test plate. Heat flux in the range of 25 to $200 \ \hbox{W/cm}^{2}$ , which is a typical requirement for cooling high power electronic components was dissipated using 0.5‐mm diameter water jets arranged in a 7×7 array with a pitch of 3 mm. Temperature difference between the test plate and water was within $30 \ ^{\circ}\hbox{C}$ . Tests were performed in the flow rate range of 22 to 40 ml/min, resulting in a Reynolds number range of 1100 to 1750. Results show a significant increase in the heat transfer coefficient or Nusselt number with an increase in heat flux. The effect of the flow rate or Reynolds number on the heat transfer coefficient is found to be negligible. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20291     

Augmentation of heat transfer through passive techniques

The thermal performance of energy preservation systems is greatly improved by increasing miniaturization and boosting. These are imaginative (or Promethean) techniques to enhance heat transfer. Enhancement methods of heat transfer draw great attention in front of the industrial sector because of their ability to provide energy savings and raise the economic efficiency of thermal systems. Three techniques these methods are categorized; those are active, passive, and compound. Different types of components are used in passive methods because of the transfer/working fluid flow path to the enhancement of the heat transfer rate. In this article, the subject of the review was the passive heat transfer enhancement methods including inserts (conical strips, winglets, twisted tapes, baffles), porous materials, coil/helical/spiral tubes, rough surfaces (corrugated/ribbed surfaces), extended surfaces (fins) and nanofluids (mono and hybrid nanofluid). Recent passive heat transfer enhancement techniques are studied in this article as they are cost-effective and reliable, and also comparably passive methods do not need any extra power to promote the energy conversion systems' thermal efficiency than active methods. In the passive approaches, various components are applied to the heat transfer/working fluid flow path to improve the heat transfer rate. The passive heat transfer enhancement methods studied in this article include inserts (twisted tapes, conical strips, baffles, winglets), extended surfaces (fins), porous materials, coil/helical/spiral tubes, rough surfaces (corrugated/ribbed surfaces), and nanofluids (mono and hybrid nanofluid). From the pioneers' research work, it is clear that a lower twist ratio and lower pitch, lesser winglet angles can provide more heat transfer rate and a little bit more friction factor. In the case of nanofluids, a little bit of pumping power is enhanced. Finally, heat transfer enhancement is compared with the thermal performance factor, which is more than unity.     

中餐炒菜灶的系统热效率分析

传统的鼓风式中餐炒菜灶热效率仅为20%左右,通过增加辐射壁,可强化传统炒菜灶的燃烧与换热过程,提高系统热效率。采用均匀搅拌反应器(PSR)模型,研究了炉内的燃烧与换热过程,建立了相应的传热模型,并对系统热效率进行了分析。研究发现改造后的新型中餐炒菜灶热效率有大幅度提高,具有显著的节能效果和应用前景。     

套管换热器对流换热的数值模拟研究

本文建立了一种复杂的数学模型用于预测套管式换热器内流体的流动及传热特性。数学模型包括计算流体力学模型和计算传热学模型。其中,计算传热学模型中的湍流扩散系数是利用温度方差t2和温度方差耗散率εt来求解,而不是利用通常采用的Pr数假设值或实验测定值来求解。为验证新建立模型预测结果的准确性,本文将数值模拟结果与文献中的实验结果进行了比较,结果吻合较好。     

锥形内肋管传热与流阻性能的数值模拟

根据纵向涡强化传热技术提出了新型的强化换热管——锥形内肋管,运用数值模拟方法,研究了新型强化换热管结构参数锥底宽度a、导程P、肋深e和Re数对Nu、沿程阻力系数f及传热综合因子η的影响。结果表明：换热管内壁面边缘处产生了较多的微小涡流,有效破坏了流动边界层,强化了传热。在充分湍流的条件下,流体Re越小、e越小,其综合传热性能越强。当Re<15 000时,a对η的影响要大于P;在过渡点后, P对η影响较大。通过综合传热性能分析,给出了适合不同Re区间的锥形内肋优化参数。     

Inverse estimation of boundary conditions on radiant enclosures by temperature measurement on a solid object

In this paper, we present an inverse analysis to estimate the thermal boundary conditions over a two-dimensional radiant enclosure from the knowledge of the measured temperatures for some points on a solid object within the enclosure. The conduction heat transfer in the solid object and the radiative heat transfer between the surface elements of the enclosure are formulated by the finite volume method and the net radiative method, respectively. The resultant set of nonlinear equations is solved by the Newton's method. The inverse problem for estimation of boundary conditions over the radiant enclosure is solved by the conjugate gradient method.     

Application of Hermite wavelet method for heat transfer in a porous media

In this article, the flow and heat transfer for non-Newtonian viscoelastic fluid in an axisymmetric channel with a porous wall is investigated. Convective boundary conditions have been used in the problem formulation. We obtain coupled, highly nonlinear ordinary differential equations from the fundamental governing equations via appropriate similarity variables. The solution for velocity and temperature are computed by applying the Hermite wavelet method (HWM). The comparison between the results from the HWM, differential transform method, and numerical method are well in agreement which proves the capacity of HWM for solving such problems. The effects of Reynolds number and Prandtl number on the velocity and temperature are illustrated through graphs and tables for different values of an independent variable.     

对流传热计算中的烟气辐射放热系数α_1

本文提出锅炉对流传热计算中用到的烟气辐射放热系数α_(?)袭用炉内辐射传热的计算公式有误差,因为所对应的温压不同,应予改正。     

工业水管锅炉对流受热面受压元件壁温的选取

提出了确定锅炉对流受热面中金属壁温的新思路、新方法.同时,验证了GB/T9222中确定金属温度的经验式是安全的.该方法可用于强度计算中的金属壁温的确定.     

Enhancement in Evaporative Effectiveness of an Evaporative Tubular Heat Dissipator Using Experimental Design Approach

An experimental investigation of evaporative effectiveness and mass transfer coefficient on a bundle of tubes of an evaporative tubular heat dissipator is presented. Based on the experiments, correlations of evaporative effectiveness and mass transfer coefficient are derived using multiple regression analysis. A statistical model is developed to correlate the operating variables using design of experiment approach by selecting central composite design of a response surface methodology. Results shown in this article indicate that as the cooling film flow rate increases, evaporative effectiveness and mass transfer coefficient increases provided that the air flow rate is constant which is flowing from underneath the tubes of the evaporative tubular heat dissipator. Derived correlations are helpful in improvement of the design of heat transfer devices and many other engineering applications. Consideration of relative humidity of upstreaming air as one of the operating variables leads to the contribution to heat and mass transfer study of evaporative tubular heat dissipators in the present investigation.