平直翅片管与波浪翅片管流动换热性能比较

扁管外焊蛇形平直翅片是直接空冷凝汽器翅片管的一种常见形式,为进一步提高空冷凝汽器的冷凝效率,建立了波浪翅片扁管的三维物理数学模型。通过数值模拟,获得了不同空气入口流速下,波浪翅片扁管和平直翅片扁管外冷却空气的流场和温度场,通过对流换热系数和流动损失的对比分析表明,波浪翅片扁管在空气侧换热系数和流动损失方面比平直翅片扁管有一定的优势,在低风速的工况下,优势较为明显。     

火电厂直接空冷凝汽器传热性能实验研究

直接空冷凝汽器的传热性能是火电厂空冷机组优化运行和设计的重要依据.针对实际运行的某典型600 MW直接空冷机组的冷端系统,利用红外热像仪测量了不同空冷凝汽器翅片管束表面的温度分布,并与热电偶测量的温度进行比较分析.发现,受管内蒸汽流向和凝汽器冷却三角内空气流场的综合影响,顺流和逆流凝汽器的壁面温度都表现出从上至下逐渐上升的趋势,为进一步预测凝汽器空气侧流场特性和进行空气侧的传热强化提供了参考;通过实验研究,获得了机组空冷凝汽器单元空气侧传热系数关联式.本文的结果对变工况条件下空冷机组的优化运行以及空冷凝汽器设计的改进具有指导意义.     

空冷岛扁平管外翅片空间的大涡模拟研究

本文基于计算流体力学软件FLUENT采用大涡模拟方法(LES)及Smagorinsky-lilly亚格子模型,对空冷岛扁平管外翅片空间的流动与换热特性进行了三维瞬态湍流数值模拟。计算结果表明大涡模拟方法能较好地模拟空冷岛扁平管外翅片空间的瞬态流动特性,能够捕获更多的流场细节信息。可得出流场中温度、压力、涡量等各参数的瞬态变化特性以及翅片空间尾流区瞬态三维涡的发展演化过程,更好地预测多尺度湍流。为进一步揭示扁平管温度场和流场的协同关系以及大型空冷系统多尺度输运规律和非线性尺度效应机理打下基础。     

内部导流对空冷单元流动传热特性的影响

通过改变空冷单元内冷却空气流场温度场,可提高空冷凝汽器的传热性能.针对600 MW直接空冷机组空冷凝汽器的典型结构,建立了内部安装空气导流装置的空冷单元冷却空气流动传热过程的物理数学模型.通过CFD模拟获得了冷却空气流场和温度场,分析了不同环境因素影响下加装空气导流装置后空冷单元内空气流动传热特性的变化规律.结果表明:在空冷单元内安装导流装置能提高冷却空气流场的均匀性,降低冷却空气温度,改善空冷凝汽器的运行特性.     

扁平管交错蛇形短翅片强化传热的研究

针对火电空冷凝汽器采用的扁平管蛇形翅片长度较大,空气在翅片间流动对强化传热的效果受到边界层发展抑制的缺陷,根据锯齿翅片通过破坏边界层发展强化传热的思想,提出一种扁平管交错蛇形短翅片结构。实验结果表明,扁平管交错蛇形短翅片的传热性能优于原有结构,在不同雷诺数Re范围,努塞尔数Nu增加了1.4%～16%;同时空气侧流动阻力也明显增加,摩擦系数f增加了18%～45%。由综合评价指标PEC也可以得到,扁平管交错蛇形翅片有效地强化了空气侧的换热。     

一种空冷单元空气导流装置流动传热特性

通过对空冷单元冷却空气进行合理导流,可改善空冷凝汽器的传热性能。针对600MW机组空冷凝汽器典型结构,设计了一种空冷单元空气导流装置,建立了空冷单元内冷却空气流动传热过程的物理数学模型。通过CFD模拟,获得了不同环境因素影响下加装空气导流装置后空冷单元内流动传热特性的变化规律。结果表明:在空冷单元内安装导流装置能提高空气流场的均匀性,降低管束表面局部高温区的温度,为空冷单元的优化设计提供一定依据。     

带纵向涡发生器的椭圆管翅片换热器数值分析

对带纵向涡发生器的椭圆管翅片换热器和圆管翅片换热器的空气侧表面的换热和流动特性进行了三维数值模拟.结果表明,在模拟的流速范围内,与圆管翅片换热器相比,带纵向涡发生器的椭圆管翅片换热器的换热效果Num平均强化了32.4%,其综合换热性能(Nu/f)提高了28.93%,明显好于圆管翅片换热器.纵向涡强化换热的内在机理可以用场协同原理进行解释,它改善了速度场和温度场的协同,使全场的速度和温度梯度之间的夹角减小.     

开孔矩形翅片椭圆管流动与换热特性的数值研究

对电站直接空冷系统的基本换热元件矩形翅片椭圆管建立三维物理数学模型,对空气侧流动和传热性能进行数值研究．分析了不同迎面风速下翅片上无扰流孔和开有扰流孔两种情况下矩形翅片表面的局部表面传热系数分布规律,发现椭圆基管后存在的尾流区使得翅片的强化换热作用减弱。比较了扰流孔的尺寸、数目和位置对管外空气侧流动与换热的影响,结果表明:扰流孔尺寸对流动与换热存在明显影响,而扰流孔数目和位置的影响相对比较小．     

椭圆管矩形翅片间空气流动的扰流特征

针对椭圆管矩形翅片间的空气流动特性,将应用于直接空冷电站椭圆管矩形翅片的几何中心相对于椭圆管的几何中心偏置.针对不同偏心距离x,以及不同入口风速u,得到空气侧对流换热系数h和压差P随u的变化规律.模拟结果表明翅片的偏置会对着空气侧的流动与换热产生不同影响:翅片向前偏置,低速尾流区对翅片表面换热的影响减小,空气侧总的表面...     

空温式翅片管气化器自然对流换热的数值模拟

建立了深冷空温式星形翅片管气化器的空气侧自然对流换热模型.利用fluent软件,采用SST k-ω湍流模型和SIMPLEC算法,对多组不同结构参数、不同内壁面温度的气化器空气侧自然对流换热进行了三维数值模拟.由数值模拟结果分析了气化器各结构尺寸大小与壁温对翅片管自然对流换热的影响,并拟合了气化器空气侧自然对流换热的Nu...     

扁平管管内负压冷凝流动阻力特性研究

本文采用试验的方法对扁平管内的负压蒸汽冷凝过程进行了研究。研究结果表明,这类扁管的特殊结构使得管内的蒸汽充满度高,冷凝液占据整个流动截面的份额小,流动顺利,阻力小,是很好的有利于冷凝液流动的管型。研究结果给出了这类扁平管的管内冷凝流动摩擦阻力的计算方法,为这类管型在空冷凝汽器的应用提供了设计依据。     

Flow patterns and heat-transfer characteristics of a top heat mode closed-loop oscillating heat pipe with check valves (THMCLOHP/CV)

The aim of this research was to investigate the flow patterns and heat transfer of a top heat mode closed-loop oscillating heat pipe with check valves (THMCLOHP/CV). In this study, the heat pipe was made of a high-quality glass capillary tube with an inner diameter of 2.4 mm bent into 10 meandering turns. The number of check valves was 2 and the tube was filled with R141b at a filling ratio of 50% of internal volume of the tube. The combined lengths of the evaporator, adiabatic and condenser sections were equal to 50 mm. The pipe was operated at the top heat mode, and the angles of inclination were 20°, 40°, 60°, 80°, and 90°. The heat applied at the evaporator section was controlled at 85°Cto 105°C, and 125°C. The results show that in the evaporator section, bubbles are produced and grow as a result of the continuous nucleate boiling. They coalesced and their volume expanded. Similarly, in the condenser section the vapor plug condensate caused the bubbles to collapse and accumulate as a liquid mass at the lower section of the U-bend tube. A new slug then developed and the bubbles coalesced in an upward flow. Heat flux increased when the evaporator temperature and inclination angle increased causing the average length of the vapor plug to decrease and the average velocity of vapor plug to increase. The maximum heat flux occurred at an evaporator temperature of 125°C and an inclination angle of minus 90°.     

非稳态横掠管束周期性充分发展对流换热的数值模拟

本文采用非稳态数学模型对横掠管束的周期性充分发展流动与换热进行了数值模拟。结果显示,在所考虑的参数条件下,Re≥100以后流场和温度场随时间发生振荡,并且振幅随Re增加而增大;进一步增加Re到一定值后,在管束间会有交替出现的涡。计算得到的平均Nusselt数与现有文献中的实验关联式基本是吻合的。平均Nusselt数的计算值随流动方向管间距的增加而增加。     

Visualization of shock waves around hypersonic spiked blunt cones using electric discharge

The hypersonic flow fields around a 120⁰ apex angle blunt cone fitted with spikes have been visualized using electric discharge technique at Mach 5.75 in a hypersonic shock tunnel. The blunt cone has been fitted with a flat tipped spike and a flat aerodisc tipped spike assembly. The length of the aerodisc tipped spike assembly has been kept equal to the model base diameter (L/D=1) and the length of the flat tipped spike has been kept at 12mm. The flow field pictures reveal all the salient features of the flow fields around the spiked bodies. Numerical simulations carried out to complement the experiments agree well with the experimental pictures.     

INTERNAL FLOW PATTERNS OF AN INCLINED CLOSED TWO-PHASE THERMOSYPHON AT CRITICAL STATE: CASE STUDY II,EFFECT OF BOND NUMBER

The effects of Bond number (Bo) on internal flow patterns of an inclined, closed, two-phase thermosyphon at critical state are considered in this article. The thermosyphons used employed a glass tube for the evaporator and the adiabatic section, and a copper tube for the condenser section. The internal diameters of the thermosyphon were 10, 12, and 28.5 mm. The aspect ratios were 5 and 30, with inclination angles of 90 degrees, 60 degrees, and 5 degrees (against the horizontal axis). R123 was used as working fluid, with a filling ratio of 80% and 150% of the evaporator section. A solution of water and ethylene glycol was used to carry heat from the condenser, while water was employed to supply heat to the thermosyphon. A video camera was used to record all the flow patterns, while a still camera was used to record the internal flow patterns at specific times. Heat transfer rate was measured by means of a calorimeter in the condenser section. It was found from the experiments that, as under normal operating conditions, an Le/d of 10 is an important value at which changes of internal flow patterns can be observed. At an Le/d< 10 and with low Bo, the internal flow pattern at critical state changed from bubble flow at vertical to slug flow at an inclined position. Although the flow patterns were the same as with high Bo, the bubbles generated were smaller and appeared only on the upper side. In cases of Le/d>10, the pattern changed from annular flow at vertical to stratified flow at inclined positions for all Bo. Generated bubbles were comparatively small at low Bo. A flow pattern map at each angle was also created by using modified Kutateladze numbers and Reynolds numbers of the vapor.     

海勒式间接空冷系统变工况特性理论研究

本文提出了海勒式间接空冷系统凝汽器压力变工况计算的数学模型,分析了海勒式间接空冷系统运行性能的影响因素,通过实例计算得出了各因素对凝汽压力的影响规律,为提高海勒式间接空冷系统的性能提供理论依据。     

平面波照射下无限大导体板上周期孔阵屏蔽效能的解析研究

本文针对平面电磁波对无限大导体平板上周期孔阵的透射问题,首先用Bethe小孔理论将孔阵表示成偶极子阵列,然后用平均化处理得到均匀的等效磁化/极化强度,进而引入等效面源导出透射电磁波表达式,最终给出了孔阵对平面电磁波屏蔽效能的解析公式.该公式分别针对横电和横磁两种极化方式,将屏蔽效能表示成孔阵周期面积、孔的极化系数、波长和入射角的简单函数,其计算结果与全波仿真结果一致性好.结果表明,透射场强的幅值与孔极化系数和波频率成正比,与周期面积成反比;在横电极化方式,波频率和周期面积不变的情况下,透射场强的幅值与入射角的余弦值成正比,入射角越大屏蔽效能越高;在横磁极化方式下,透射场强幅值与入射角的关系相对复杂,但在入射角较小时与入射角的余弦值近似成反比,总体上入射角越大屏蔽效果越低.     

太阳模拟器中椭球面聚光镜参数的确定

椭球面聚光镜是太阳模拟器设备的重要组成部分,其能量收集的效率决定着太阳模拟系统的能量传递效率,而太阳模拟器光学系统中的椭球面聚光镜参数一直没有理论上的设计依据,结合氙灯的发光特性并通过对MATLAB 中建立的椭球面聚光镜聚光过程数值分析模型给出了椭球面聚光镜包括第一焦距、最大成像放大倍率、包容角范围及前后开口直径的确定依据,并通过在Lighttools 中建立的4种仿真模型验证了理论分析的正确性。第一焦距由光源光中心高确定,最大成像放大倍率由光学积分器相对孔径及椭球镜包容角范围共同确定,椭球镜包容角范围不小于30～120,前开直径口由椭球镜的最大孔径角确定,后开口直径由最小孔径角和光源的径向调节量共同确定。该结论给椭球面聚光镜的设计提供了理论支撑,有利于设计完成高能量收集效率的椭球面聚光镜。