水平管内分层湿润角与周向传热分布

实验研究了不同当地蒸汽质量流速Gs和凝结液质量流速下,水平管内凝结液湿润角和管圆周方向传热系数的变化规律。研究发现,在一定蒸汽入口流量下,随着凝结速的增加,湿润角增加速度先快后慢,然后在低蒸汽流量下又变快;形成波状分层流时所需的Gs随着凝结液质量流速的增加而增加;波状分层流时,凝结液侧的传热系数平滑的变化到蒸汽侧传热系数。     

N7600凝汽器壳侧汽相流动与传热性能数值计算

采用自行编制的程序对N7600凝汽器壳侧蒸汽的流动与传热特性进行了数值计算,得到了壳侧蒸汽的速度与流线、压力、传热系数以及空气浓度等分布图。结果表明：N7600凝汽车器壳侧整体上呈现“汽流向两侧”的流动特点,与设计的初衰基本相符合,在凝汽器的底部流场中存在小的涡流区．说明管束布置还有不合理的地方;凝汽器汽阻为109．5Pa：平均传热系数为3237,15W／m2·K;随着蒸汽的不断凝结,从冷却管束外围到内部的空气浓度逐渐增大。     

水平肋片管外自然凝结换热特性研究

基于Nusselt凝结传热理论,沿肋片管圆周方向划分有限个微元角,建立了每个微元角内肋侧壁、肋间基管及肋顶三个区域的凝结传热模型,通过求解非淹没区和淹没区总传热量,推导管外平均传热系数计算式。计算不同肋片高度、肋密度时,R134a饱和蒸汽的管外平均凝结传热系数。结果表明:随肋密度的增加,平均传热系数先增大后减小,肋密度为25fpi时传热最佳;高肋片管的平均凝结传热系数大于低肋片管的,肋片高度达到一定值时,平均传热系数几乎不随肋高增加而增加。当R134a饱和蒸汽为20℃时,两种不同翅片密度的管外平均凝结传热系数随温差的增大而减小,并通过所建模型得到的计算值与Beatty-Kate模型进行了比较,平均误差分别为约16.1%和8.3%,故所建模型基本反映肋片管外蒸汽凝结传热机理。     

火电站直接空冷凝汽器积灰监测

火电站直接空冷凝汽器积灰是影响传热性能的重要因素,研究直接空冷凝汽器积灰对传热性能的影响规律并提出监测措施具有重要意义。通过分析汽轮机背压与汽轮机排汽量、冷却空气流量、凝汽器传热系数、凝汽器总传热面积以及环境温度之间的关系,得到了空冷凝汽器在维持汽轮机排汽量和冷却空气量不变时,汽轮机背压和传热系数之间的关系以及凝汽器积灰对汽轮机背压的影响。研究表明:凝汽器积灰会导致凝汽器传热系数降低,汽轮机背压升高,机组运行经济性下降。设计工况下,当蛇形翅片扁平管结构凝汽器积灰厚度达到1.2 mm时,汽轮机背压将增加50%左右。通过监测空冷机组运行过程中汽轮机背压的变化,可预报积灰的程度,为直接空冷凝汽器清洗提供一定的理论依据。     

不可凝气体对大扁管空冷凝汽器性能的影响分析

为了研究不可凝气体(non-condensable gases, NCG)对火电与光热发电机组上广泛使用的大扁管空冷凝汽器性能的影响,以工程机组凝汽器上普遍应用的通流面积220 mm×20 mm的大扁管为研究对象,针对汽轮机典型工况下的实际蒸汽流量,基于Lee相变方程、VOF方法以及组分扩散模型,对蒸汽与NCG混合气体管内两相流凝结换热进行数学建模与数值计算。结果表明：由于大扁管的狭窄通流几何结构与高蒸汽流量,NCG对管内蒸汽凝结的抑制效果要远低于预期;当入口空气质量分数按2%增加时,凝结管凝结换热系数仅下降2%左右,这与NCG导致低流量圆管凝结性能急剧下降的结论不同;空气正常泄漏不会导致空冷凝汽器性能下降而影响发电机组效率。     

改造前后凝汽器性能的数值模拟与分析

对某厂330 MW机组凝汽器改造前向心形管束布置和改造后TEPEE两山峰形管束布置的壳侧汽相流动与传热进行了数值模拟与比较.结果表明:改造前凝汽嚣结构设计不合理,上部管束得不到充分利用,导致凝汽器性能差;改造后TEPEE两山峰形管束布置使压降分布均匀,传热系数增大,更有利于凝汽器壳侧蒸汽的流动与传热.改造前后的性能试验结果表明:采用TEPEE两山峰形管束布置方式的凝汽器,其性能指标达到设计值,优于向心形管束布置方式;改造后凝汽器真空比改造前提高了1.743 kPa,端差下降4.63 K.     

含高浓度CO2的水蒸气在竖直平板上的凝结换热实验研究

对含有高浓度CO2的水蒸气在竖直平板上冷凝传热特性进行了实验研究,得到了混合蒸气流速为0．8 m／s、CO2质量分数为60．0％～94．0％工况下的凝结传热特性曲线。结果表明,CO2的加入会极大恶化蒸汽的凝结传热,随过冷度的增加凝结传热系数逐渐降低,且随着CO2质量分数的增大,凝结传热系数受过冷度的影响越来越小;随着CO2质量分数的增加,凝结传热系数继续降低,但降低幅度逐渐减缓。对比高CO2质量分数下膜状和珠状凝结传热特性发现,珠状凝结并没有明显强化传热优势。     

稠油地面蒸汽管线热力参数影响及保温厚度优化

提高蒸汽注汽品质和优化保温层厚度是改善地面蒸汽管线热力输运、实现稠油高效开采的关键。建立了油田地面蒸汽管线热力参数计算模型和保温层厚度经济性分析模型,基于分段微元方法求解沿程蒸汽干度、热损等特性参数,分析了锅炉出口蒸汽参数和注汽流量的影响规律,并结合经济厚度法的计算原理优化了保温层厚度。结果表明：提高锅炉出口蒸汽温度和压力,沿程蒸汽干度降低速度变快,沿程热损增加变快,与锅炉出口蒸汽温度313 oC,压力10.2 MPa相比,其热损最大增加11.15%;增加注汽流量,蒸汽干度提升且随管线沿程降幅缩小,等梯度注汽流量差下,高注汽流量时蒸汽干度降幅较小,其降幅为3.58%;经济厚度为0.33m时,其热损费用同比原有厚度可降低68.22%。     

气汽混合流体凝结液膜传热特性研究

液膜厚度对凝结传热具有较大影响,且传热管管型影响着凝结液膜形成及排除。为了通过改变管型降低液膜厚度达到强化传热的目的,对圆管、椭圆管及滴形管等三种管型凝结液膜建立了相应的物理及数学模型,并计算了液膜沿管壁的厚度分布及传热系数;分析了三种管型对液膜传热的影响。结果表明:在气汽混合流体凝结传热过程中,不同管型其凝结液膜厚度差别较大;壁面温度和混合流体速度对液膜传热均有影响;相同条件下滴形管管壁上所形成的液膜,其平均厚度较薄,传热系数较高,因此滴形管传热性能优于其他管型。     

直接空冷机组排汽管道流量分配特性数值模拟

对某电厂1000MW直接空冷机组的排汽管道系统进行研究,建立处于高位布置的排汽管道上升管段的几何模型。利用流体力学软件Fluent对汽轮机处于不同工况下,管道内蒸汽流动情况进行模拟,得出机组不同流量下管道压损和流量分配特征并采用变管径的方法降低了压损,均衡各分配管流量。模拟结果表明:蒸汽流量大小对汽机排汽管道蒸汽分配情况基本无影响,通过增大支管管径的方法可以显著改善排汽管道流量分配不均现象。     

Condensation flame of acetylene decomposition

Acetylene decomposition flame propagation was numerically analyzed and was found to be the result of the condensation reaction. Condensation processes provide reaction heat and act as a driving force for C₂H₂ flame propagation. The kinetic model reasonably predicts the level of burning velocity of the acetylene decomposition flame. The model does not demonstrate the relatively strong positive pressure dependence of burning velocity as was observed experimentally in the work of Cummings et al. [Proc. Combust. Inst. 8 (1962) 503–510]. Heat-release kinetics demonstrates a two-stage process. The first stage corresponds to heat release due to benzene formation, and the second stage of heat release corresponds to soot inception and carbonization processes. It was demonstrated that the burning velocity is sensitive to the surface growing rate constant. The use of a simplified form of presentation of the surface growing process [P.R. Lindstedt, in: Soot Formation in Combustion: Mechanisms and Models, Springer-Verlag, Berlin/New York, 1994, pp. 417–441] represents positive thermal feedback in the heat generation in a flame reaction zone.     

流速对非等温凝结表面Marangoni凝结换热特性的影响

在具有温度梯度的凝结表面上进行了水-酒精混合蒸气的Marangoni凝结的实验。研究了3种流速(V=2、4和6 m/s)对凝结表面上不同位置热流密度的影响。研究发现:流速对纯水蒸气和混合蒸气凝结时的表面温差和热流密度的影响是不同的。对纯水蒸气而言,流速增大后,表面温差和热流密度是增加的,并加剧了热流密度的分布不均(热流密度的相对差值在压力为31.2 kPa,流速2 m/s时为0.538,流速4 m/s时为0.6,流速6 m/s时为0.625)。对于混合蒸气,表面温差随流速的增加而减小,而热流密度增大很少(压力31.2 kPa,流速2 m/s时为0.186,4 m/s时为0.182,6m/s时为0.098)。     

Dropwise Condensation Studies on Multiple Scales

Recent advances in nanotechnology, chemical/physical texturing and thin film coating technology generate definite possibilities for sustaining a dropwise mode of condensation for much longer durations than was previously possible. The availability of superior experimental techniques also leads to deeper understanding of the process parameters controlling the relevant transport phenomena, the distinguishing feature of which is the involvement of a hierarchy of length/time scales, proceeding from nuclei formation, to clusters, all the way to macroscopic droplet ensemble, drop coalescence, and subsequent dynamics. This paper is an attempt to connect and present a holistic framework of modeling and studying dropwise condensation at these multiple scales. After a review of the literature, discussions on the following problems are presented: (i) atomistic modeling of nucleation; (ii) droplet–substrate interaction; (iii) surface preparation; (iv) simulation of fluid motion inside sliding drops; (v) experimental determination of the local/ average heat transfer coefficient; and (vi) a macroscopic model of the complete dropwise condensation process underneath horizontal and inclined surfaces. The study indicates that hierarchal modeling is indeed the way forward to capture the complete process dynamics. The microscopic phenomena at the three-phase contact line, leading to the apparent droplet contact angle, influence the shear stress and heat transfer. The nucleation theory captures the quasi-steady-state behavior quite satisfactorily, although the early atomistic nucleation was not seen to have a profound bearing on the steady-state behavior. The latter is strongly governed by the coalescence dynamics. Visual observation of dropwise condensation provides important information for building hierarchical models.     

Condensation in gas transmission pipelines.

Several pressure and temperature reductions occur along gas transmission lines. Since the pressure and temperature conditions of the natural gas in the pipeline are often close to the dew point curve, liquid dropout can occur. Injection of hydrogen into the natural gas will change the phase envelope and thus the liquid dropout. This condensation of the heavy hydrocarbons requires continuous operational attention and a positive effect of hydrogen may affect the decision to introduce hydrogen. In this paper we report on calculations of the amount of condensate in a natural gas and in this natural gas mixed with 16.7% hydrogen. These calculations have been performed at conditions prevailing in gas transport lines. The results will be used to discuss the difference in liquid dropout in a natural gas and in a mixture with hydrogen at pressure reduction stations, at crossings under waterways, at side-branching, and at separators in the pipelines.     

Condensation of bubbles in miscible liquids

We previously developed a theoretical envelope model for bubbles condensing in immiscible liquids. The envelope model defines two zones while condensing. In the first zone the bubble accelerates after detachment from the nozzle and the heat is transferred through a viscous boundary layer at the front of the bubble and through the wake at the rear. In the second zone the bubble decelerates, settles into the wake and the heat is transferred through the wake all around the bubble. At a third zone, the bubble reaches the terminal velocity while the condensation process is terminated. In this paper both models (viscous boundary layer model--VBLM; and envelope model--EM) are modified to suit also bubbles condensing in miscible liquids. According to our visualization study of bubbles condensing in miscible liquids, partly envelopment of bubbles takes place at the deceleration zone. Visualization studies also revealed that the condensate mixes immediately with the surroundings. The experimental results for freon-113 bubbles condensing in subcooled freon-113 and presented in this paper confirm these observations and therefore they are bounded by two theoretical models: the envelope model and the viscous boundary layer model.     

Condensation flow patterns in silicon microchannels

A simultaneous visualization and measurement experiment was carried out to investigate condensation flow patterns of steam flowing through an array of trapezoidal silicon microchannels, having a hydraulic diameter of 82.8 μm and a length of 30 mm. The degassed and deionized water steam flowing in the microchannels was cooled by flowing water of 8 °C from the bottom. The silicon microchannels were covered with a thin transparent pyrex glass from the top which enabled the visualization of flow patterns. Experiments were performed at different inlet pressures ranging from 4.15 × 10⁵ Pa to 1.25 × 10⁵ Pa (with corresponding mass fluxes decreasing from 47.5 g/cm² s to 19.3 g/cm² s) while the outlet pressure was maintained at a value of 10⁵ Pa. Different condensation flow patterns such as fully droplet flow, droplet/annular/injection/slug-bubbly flow, annular/injection/slug-bubbly flow, and fully slug-bubbly flow were observed in the microchannels. At a given inlet pressure and mass flux, the flow pattern depended on both the location and time. Of particular interest is that the vapor injection flow, consisting of a series of bubble growth and detachment activities, appeared and disappeared periodically. During the disappearance period of injection flow, the slug-bubbly flow at downstream changed to the single-phase liquid flow due to the reversed flow of outlet condensate, while the annular flow at upstream changed to the vapor flow due to the effect of incoming vapor. Therefore, two-phase flow and single-phase flow appeared alternatively in the microchannels, causing large fluctuations of wall temperatures as well as other measurements. It was also found that the occurrence of vapor injection flow moved from the outlet toward the inlet as the mass flux was decreased. The vapor injection flow and its induced condensation instabilities in microchannels are reported here for the first time.     

Condensation of steam in silicon microchannels

A visualization study was performed on condensation of steam in microchannels etched in a 〈100〉 silicon wafer that was bonded by a thin Pyrex glass plate from the top. The microchannels had a trapezoidal cross section with a hydraulic diameter of 75 μm. Saturated steam flowed through these parallel microchannels, whose walls were cooled by natural convection of air at room temperature. The absolute pressure of saturated steam at the inlet ranged from 127.5 kPa to 225.5 kPa, and the outlet was at atmospheric pressure at approximately 101.3 kPa with the outlet temperature of the condensate ranging from 42.8 °C to 90 °C. Stable droplet condensation was observed near the inlet of the microchannel. When the condensation process progressed along the microchannels, droplets accumulated on the wall. As the vapor core entrained and pushed the droplets, it became an intermittent flow of vapor and condensate at downstream of the microchannels. The traditional annual flow, wavy flow and dispersed flow observed during condensation in macrochannels were not observed in the microchannels. Based on a modified classical droplet condensation theory, it is predicted that the droplet condensation heat flux increases as the diameter of the microchannel is decreased. It is also predicted that the droplet condensation heat flux of saturated steam at 225.5 kPa can reach as high as 1200 W/cm² at ΔT=10 °C in a microchannel having a hydraulic diameter of 75 μm.     

Condensation of water by radiative cooling

Atmospheric humidity can be condensed as dew and used for example in small-scale irrigation. In arid locations, the most favourable conditions for dew collection persist in the late night and around sunrise. We study the possibility to use a dew collector for condensing atmospheric water vapour by exploiting the effect of radiative cooling. In particular, we study pigmented polymer foils with high solar reflectance and high thermal emittance. Suitable pigments are a mixture of TiO₂ and BaSO₄ particles or a novel SiO₂/TiO₂ composite. We calculate the condensation rate under different climatic conditions and report on initial field tests.     

Condensation Pressure Drop in Circular Microchannels

This paper presents procedures for computing stresses in floating-head and fixed-tubesheet heat exchangers. It does not cover U-tube exchangers. It was prepared in connection with the activities of the Special Working Group on Heat Transfer Equipment, Section VIII, of the ASME Boiler and Pressure Vessel Code Committee, for their use and consideration in the drafting of code rules for these items. The mathematical procedures are closely associated with the publications of K. A. Gardner and K. A. G. Miller [1–3].     

凝结水回收的节能效益

在介绍凝结水回收方式的基础上,对如何选择蒸汽凝结水回收系统和设备作了讨论,分析了蒸汽凝结水回收系统节能的经济效益。