内插不同形状涡产生器管内层流流动与传热特性的对比分析

内插扰流元件是一种可操作性强的管内强化传热方式，其强化传热机理主要是在管内诱导产生了二次流。在均匀壁温热边界条件下，对内插不同形状涡产生器管内层流流动与传热特性进行了数值分析。研究发现：在扭带基础上裁去部分面积相同的条件下，管内插等腰梯形涡产生器的换热能力最强，直角梯形涡产生器次之，矩形涡产生器的换热能力最差，管壁上的局部Nusselt数的峰值所在圆周位置及其大小与涡产生器形状有关，而不同形状的涡产生器对管内流动的阻力系数影响较小。插入涡产生器后，管内二次流强度参数Se和平均Nusselt数Nu均随Reynolds数Re的增大而增大，二者随Reynolds数Re的变化规律具有一致性。平均Nusselt数Nu与二次流强度参数Se呈幂函数相关，内插涡产生器管内的二次流强度决定了其对流换热强度。     

润滑油在内插同轴交叉翼型涡产生器管内流动与传热特性分析

以润滑油为工质,采用数值方法对圆管内插同轴交叉等腰梯形涡产生器的管内流动与传热进行了数值模拟,分析了不同结构参数如扭率（T_r=3,4,5,6）、间距比（S_s/W=0.8,0.9,1.1,1.2）和基带宽度比（W_b/W=0.30,0.45,0.60,0.75）对圆管内插同轴交叉等腰梯形涡产生器的管内流动与传热特性的影响。结果表明：在相同Re下,平均Nusselt数Nu_m、二次流强度Se、强化传热因子JF均随扭率和间距比的减小而增大,而其与基带宽度比的变化没有明显规律,阻力系数f随着扭率的减小和基带宽度比的增大而增大,间距比对f的影响甚微。在相同结构参数下,JF和Se均随Re的增大而增大。在Re=50~1000范围内,相比于光滑圆管,内插不同结构参数的同轴交叉涡产生器的Nu_m增加了32.8%~208.6%,f增加了3.38~8.92倍,JF最大可达1.434。Nu_m与Se呈幂函数相关,内插同轴交叉翼型涡产生器管内的二次流强度决定了其对流换热强度。     

半周加热横纹管内熔盐强化传热特性

半周加热太阳能吸热器管给管内对流传热系数带来影响,基于此,建立半周加热横纹管内熔盐对流传热的实验台和数值计算模型,分析横纹管槽宽、槽深对管内熔盐传热性能的影响规律,结果表明：从横纹管绝热侧到加热侧,周向管内壁温度逐渐升高,周向管内局部Nusselt数先减小后缓慢增大至稳定。横纹管凹槽处轴向管内壁温度明显低于平滑段,凹槽处轴向管内局部Nu较大,凹槽后与平滑段交界处管内局部Nu最小。横纹管槽越宽,加热侧轴向管内局部Nu越小,管内平均Nu越小,传热综合性能评价因子PEC越大。横纹管槽越深,轴向管内局部Nu越大,管内平均Nu越大,传热强化倍数Nu/Nu _ST越大。槽宽对横纹管PEC的影响比槽深明显,槽深对横纹管内Nu/Nu _ST影响比槽宽大。通过线性拟合得到半周加热横纹管内熔盐传热Nu的关联式,其计算Nu与模拟值最大偏差在±7%以内。     

高低双螺旋片强化套管换热器壳侧换热

采用实验和数值模拟相结合的方法,研究了高低双螺旋片对套管换热器壳侧的强化换热效果.以仅带有高螺旋片的换热器结构为基础,研究了曲率ε分别为0.44、0.321和0.131时,Reynolds数在4000~20000范围内,低、高螺旋片高度之比l/W对壳侧换热平均Nusselt数Nu_m和流动阻力系数f的影响,考察了等泵功条件下换热器的综合传热性能.对ε=0.44的换热器研究结果表明:当l/W>1/2时双螺旋片强化传热效果显著,且研究工况中l/W=3/4时最优,此时与单一高螺旋片相比,Nu_m值平均提高了10.8%;研究范围内,综合强化传热因子PEC数在1.044~1.204.对不同曲率换热器的研究结果表明,同一l/W值下,PEC数随着曲率ε值的增大而增大,说明双螺旋片结构更适合强化曲率较大的套管换热器壳侧换热.     

射流卷吸边界层内相干结构的实验研究

在轴对称水射流实验台上,采用单帧长曝光图像法,测量了出口Reynolds数在1849~2509范围内的卷吸边界层内流场结构。发现在流向距离L=2D~3.5D,径向距离H=D~1.25D的区域内,流场中吞噬作用和侵蚀作用两种卷吸模式交替出现。分析得到,当Re>1915时,吞噬作用所占的比例随着Re的增大而增大,当Re>2311之后,Reynolds数对该结构发生概率的影响降低;通过快速傅里叶变换得到该相干结构发生的频率在10～19Hz之间;吞噬作用发生的同时在流场中观测到一些特殊涡结构。同时采用运动单帧长曝光图像法对射流流场进行拉格朗日法观测,发现射流流场中的涡结构一般存在于湍流与非湍流界面附近。     

基于停留时间分布的缠绕管内二次流研究

缠绕管内二次流能够显著强化管内传质、传热性能。通过测量缠绕管内液体的停留时间分布,利用无量纲方差σ²表征二次流强度,研究了缠绕直径、缠绕角度、缠绕管管径等结构参数对二次流的影响。结果表明：在径向流动未达到极限迁移距离的缠绕管中,随着液体Reynolds数Re的增大,σ²先减小后增大,对应缠绕管内依次出现的湍流作用区和二次流作用区;在径向流动达到极限迁移距离的缠绕管中,随着Re的增大,σ²先减小后增大最后趋于平稳,对应缠绕管内依次出现的湍流作用区、二次流作用区和二次流极限区。从湍流作用区转变为二次流作用区的临界Reynolds数Re_S随缠绕管缠绕直径和管径的减小而减小,缠绕角度对Re_S的影响较小。     

基于热舒适性纳米喷泉的流动与传热特性

目前关于纳米喷泉流动与传热特性及微区域热舒适度方面的研究较少,为了研究其流动与传热机理,本文基于气-液两相流模型和金刚石-乙二醇/水纳米流体高沸点、低冰点等优良特性,在有限柱体空间内建立了一种基于热舒适性的新型高效换热喷泉-“纳米喷泉”,对比研究了金刚石-乙二醇/水纳米流体、水射流工质分别对喷泉流动与传热的影响,同时讨论了纳米颗粒体积分数v和表征流体流动情况的雷诺数（Reynolds,Re）对喷泉流动与传热的影响,分析了局部温度分布、流线分布、平均温度和流体换热量在空间内的变化,并依据ASHRAE 55-1992标准和ISO7730标准对微区域的热舒适度进行了评价。结果表明：随着Re数和v增大,换热强度均得以不同程度地提升,热舒适程度也逐渐增加。Re=1.0×10⁵和Re=1.2×10⁵时,纳米颗粒体积分数从0增加至1%的换热强度提升较为明显,前后两种Re数可分别强化1.5%和2.8%;Re=1.4×10⁵时,体积分数从3%增至5%的换热强度可提升11.5%,强化效果最为明显。故较小Re数下,较低组分的纳米流体强化换热效果较好;而在较大Re数下,较高组分纳米流体的强化换热效果更好。     

自由摆动方柱强化微流体通道内传热传质

提出了一种基于自由摆动方柱提高微流体通道内传热传质效率的新方法。基于有限体积法并结合动网格技术，对微通道内液体流经自由摆动方柱（有旋转自由度、无平移自由度）时产生的扰流及强化传热传质现象进行了数值研究。研究显示，在低雷诺数（Re=10）下，自由摆动方柱几乎无运动，其对流动和传热传质的影响同固定方柱类似；随着雷诺数的增加，自由摆动方柱在流场作用下会自发产生周期性摆动，并在较低雷诺数（Re=50）下提前促使其后方产生交替性的涡脱落现象；随着雷诺数进一步增加（Re=100），方柱的自由摆动及其后方的涡街结构均显著增强。与同Re数下的固定方柱相比，自由摆动方柱能够更显著地扰动微通道内原有的泊肃叶流场，破坏通道内壁面处热边界层，提高其传热效率；同时通道内液体的横向流动可有效促进溶质混合，强化传质进程。当Re=100时，自由摆动方柱微通道内的平均换热努塞尔数Nu较固定方柱和无方柱时分别提高了17.5%和29.6%；同时，出口截面混合效率可较固定方柱和无方柱时分别提高了70.5%和65500%。     

Ross LPD型静态混合器内湍流传热与混合强化特性

在湍流状态Re=2640~17600下,采用恒热通量传热实验与数值模拟相结合的方法,系统研究Reynolds数Re和交错角对Ross LPD型静态混合器内湍流流动与传热性能影响,采用Nusselt数、Darcy摩擦系数、综合传热系数、速度场与温度梯度和压力梯度协同角等参数评价混合器内传热强化性能;基于CFD与LPT相耦合分析混合器内流体微元拉伸率。研究结果表明：SST k-ω模型预测Ross型静态混合器湍流阻力及传热结果与实验结果具有很好一致性;Ross混合器流场内形成与流场尺度较为接近的纵向涡,其涡心在圆形截面与半圆形截面中心间周期性迁移,横截面内湍流分散混合效率是Kenics的3.36~1.72倍;当Re>7040时,Ross LPD综合传热性能明显优于KSM;当叶片夹角为30°时,综合传热性能系数具有最大值;Ross LPD内插件具有高效低阻的技术优势和结构改进潜力。     

缠绕管式换热器壳程强化传热性能影响因素分析

由于内部流场信息缺乏，结构参数对流体流动的影响规律不明确，致使缠绕管式换热器壳程强化传热机理不明晰，阻碍其设计准则的进一步规范化和通用。针对上述问题，对缠绕管式换热器壳程流体流动进行几何建模及数值模拟，并通过文献中实验数据进行验证，进而基于该模型对壳程流体流场特性进行详细分析，分析关键结构参数对其壳程传热与阻力性能的影响，并探讨其强化传热机理。结果表明：Realizable k-ε湍流模型可较为准确地描述壳程流体流动；在双对数坐标系内，壳程Nusselt数随Reynolds数的增大而增大，阻力系数f则呈线性降低的趋势；壳程Nusselt数随缠绕管直径d与平均缠绕直径D的增大而增大，随螺距S的增大而减小，阻力系数f则相反；缠绕管直径d对壳程流体传热与阻力性能的影响最大，平均缠绕直径D的影响最小；增大缠绕管直径d与平均缠绕直径D有利于破坏流体速度边界层，增强流体扰动，加快温升速度，强化壳程传热，而增大螺距S则使速度边界层变厚，减小流动阻力的同时降低温升速度，不利于壳程强化传热。     

Influence of magnetic field on jet impingement heat transfer with molten salt

Experimental study of jet impingement heat transfer with molten salt under the influence of external constant magnetic field was generated by permanent magnets. Both stagnation correlation and radial distribution of Nusselt number under magnetic field were obtained. The results showed that the Nusselt number with magnetic field became higher than that without magnetic field at stagnation region and jet impingement heat transfer was comparatively enhanced, while in wall jet region, the enhancement of heat transfer was gradually weakened. In addition, when the Reynolds number was constant, the Nusselt number of molten salt increased with increasing of the intensity of magnetic field, and the most enhanced heat transfer existed at the stagnation point. Under the conditions of Reynolds number Re=6400 and the intensity of magnetic field B=2800 Gs, the stagnation Nusselt number of molten salt was about 6 % higher than that without magnetic field. It can be seen that the magnetic field may promote the jet impingement heat transfer of molten salt.     

不同工作因数下方波冲击射流的换热特性

应用数值模拟方法研究了不同工作因数下方波冲击射流的换热及流动特性,并分析了冲击靶面换热特性的变化规律。研究了冲击靶面换热特性随Reynolds数、脉冲频率、喷嘴距冲击靶面距离与喷嘴直径之比等参数的变化规律,重点分析了不同工作因数对冲击射流滞止区域换热与流动特性的影响,并将数值计算结果与连续冲击射流、脉动冲击射流实验结果进行对比验证。计算结果表明:当工作因数为0.5与0.7时,冲击靶面滞止区域Nusselt数非常接近;当工作因数为0.5时,壁面射流区域Nusselt数比工作因数为0.7时提高了10%;工作因数为0.9时,冲击靶面Nusselt数比连续冲击射流提高3%;工作因数为0.7时,相对于工作因数为0.5、0.9及连续冲击射流时,冲击靶面滞止区域存在强烈的涡结构变化。     

Experimental and Numerical Simulation Study of Heat Transfer Due to Confined Impinging Circular Jet

An experimental and numerical simulation study of heat transfer due to a confined impinging circular jet is presented. In this research, a stainless steel foil heated disk was used as the heat transfer surface of a simulated chip, and the thermocouples were mounted symmetrically along the diameter of the foil to measure the temperature distribution on the surface. Driven by a small pump, a circular air jet (1.5 mm and 1 mm in diameter) impinged on the heat‐transfer surface with middle and low Reynolds numbers. The parameters, such as Reynolds number and ratio of height‐to‐diameter, were changed to investigate the radial distribution of the Nusselt number and the characteristics of heat transfer in the stagnation region. Numerical computations were performed by using several different turbulence models. In wall bounded turbulent flows, near‐wall modeling is crucial. Therefore, the turbulence models enhanced wall treatment, such as the RNG κ‐? model, may be superior for modeling impingement flows. The numerical results showed reasonable agreement with the experimental data for local heat transfer coefficient distributions. The impinging jet may be an effective method to solve the cooling problem of high power density electronic packaging.     

圆形自由水射流冲击换热及喷嘴布置

引言 水射流冲击冷却由于具有较高的换热能力,广泛应用于机械和化工行业,以实现工件的快速冷却和控制工件的温度变化.在大型轴类工件(工件直径D=1000～3000 mm)喷水冷却装置中,多喷嘴圆孔自由水射流以特定阵列布置冲击至工件表面,相对短暂的沸腾换热结束后,阵列自由水射流即以强制对流方式实现工件冷却.因此,自由水射流冲击换热特性及多喷嘴布置形式对喷水冷却装置的结构设计至关重要.     

Numerical Study on the Effect of Cross-Flow on Turbulent Flow and Heat Transfer Characteristics Under Normal and Oblique Semi-confined Impinging Slot Jets

Impinging jets are commonly used in industrial dryers and electronics chip cooling. Since in industrial practice it is necessary to use multiple jets, the interaction between jets can have important effect on their heat transfer performance. Hence, the study of cross-flow caused by the spent flow of upstream jets is obviously significant. In this study, a computational fluid dynamics simulation was carried out of the flow and heat transfer characteristics for a single semi-confined turbulent slot jet of air impinging normally or obliquely into an imposed air cross-flow of the same or different temperature. The standard k-ε and the Reynolds stress models were used. Effects of the various flow parameter (e.g., jet-to-cross-flow mass ratio) and geometric parameters (e.g., nozzle-to-target spacing and jet angle) were evaluated at a fixed Reynolds number (11,000 and 12,000) for equal and unequal temperatures of the jet and cross-flow. Results indicate the significant degradation of the impingement heat transfer rates due to cross-flow and a relatively minor influence of the temperature difference between the jet and cross-flow over the ranges of parameters studied. Both the turbulence models produced comparable Nusselt number distributions along the impingement surface.     

Nanofluid impingement jet heat transfer

Experimental investigation to study the heat transfer between a vertical round alumina-water nanofluid jet and a horizontal circular round surface is carried out. Different jet flow rates, jet nozzle diameters, various circular disk diameters and three nanoparticles concentrations (0, 6.6 and 10%, respectively) are used. The experimental results indicate that using nanofluid as a heat transfer carrier can enhance the heat transfer process. For the same Reynolds number, the experimental data show an increase in the Nusselt numbers as the nanoparticle concentration increases. Size of heating disk diameters shows reverse effect on heat transfer. It is also found that presenting the data in terms of Reynolds number at impingement jet diameter can take into account on both effects of jet heights and nozzle diameter. Presenting the data in terms of Peclet numbers, at fixed impingement nozzle diameter, makes the data less sensitive to the percentage change of the nanoparticle concentrations. Finally, general heat transfer correlation is obtained verses Peclet numbers using nanoparticle concentrations and the nozzle diameter ratio as parameters.     

Highly selective fluorescent chemosensor for Zn2+ derived from inorganic-organic hybrid magnetic core/shell Fe3O4@SiO2 nanoparticles

Experimental investigation to study the heat transfer between a vertical round alumina-water nanofluid jet and a horizontal circular round surface is carried out. Different jet flow rates, jet nozzle diameters, various circular disk diameters and three nanoparticles concentrations (0, 6.6 and 10%, respectively) are used. The experimental results indicate that using nanofluid as a heat transfer carrier can enhance the heat transfer process. For the same Reynolds number, the experimental data show an increase in the Nusselt numbers as the nanoparticle concentration increases. Size of heating disk diameters shows reverse effect on heat transfer. It is also found that presenting the data in terms of Reynolds number at impingement jet diameter can take into account on both effects of jet heights and nozzle diameter. Presenting the data in terms of Peclet numbers, at fixed impingement nozzle diameter, makes the data less sensitive to the percentage change of the nanoparticle concentrations. Finally, general heat transfer correlation is obtained verses Peclet numbers using nanoparticle concentrations and the nozzle diameter ratio as parameters.     

Impingement heat transfer under a confined slot jet. Part I: Effect of surface throughflow

Local and average heat transfer coefficients were measured for a confined turbulent slot jet impinging on a permeable surface at which there may be throughflow. Local Nusselt number was obtained using a unique porous sensor designed for measurement of local heat transfer at a permeable surface which is subjected to rapidly and widely varying heat transfer. Measurements were performed for a wide range of jet Reynolds number and throughflow rates. Convective heat transfer coefficients was found to be enhanced by throughflow, and the enhancement factor in terms of Stanton number to be independent of jet Reynolds number and of extent of heat transfer area.     

低熔点熔盐圆管内强迫对流换热

熔融盐具有使用温度高、热稳定性和传热性能好等优点,被认为是太阳能热发电系统中最有前途的传热、蓄热介质之一。通过搭建槽式太阳能熔盐集热传热实验台,进行了低熔点熔盐管内受迫对流换热实验,获得了不同熔盐流速下套管式熔盐-水传热单元的总传热系数;通过最小二乘法获得了低熔点熔盐管内充分发展紊流段对流换热Nusselt数随Reynolds数的变化曲线和实验关联式,并与经典关联式进行了对比,结果表明,实验数据和Dittus-Boelter方程、Colburn方程、Seider-Tate方程以及Gnielinski方程最大偏差分别为+23%、+13%、-10%和-20%,实验数据和经典公式符合较好。