直接空冷单元流动换热特性数值研究

对A型直接空冷单元流动换热特性进行了数值研究,分析了环境风速、风向、风温、平台高度等对空冷单元流动换热特性的影响。计算结果表明:环境侧风通过两方面影响空冷单元的性能,水平分速度严重影响通风量在散热器上的分配均匀性,垂直分速度严重影响风机的通风量。随着环境风速的增大,散热器的通风量减小,散热器温度升高,热回风率增加,风机压头有效利用系数降低;在同样环境风速下,Y向风是更不利的风向,应尽量避免;环境温度增加一方面提高了空冷单元的进风温度,另一方面减少了风机的通风量,使流动和换热恶化;在所研究的平台高度内,平台高度对空冷单元的流动和换热影响不大。研究结果可为空冷凝汽器的运行及优化设计提供参考。     

环境风影响下空冷系统换热特性数值模拟

环境风是影响直接空冷系统运行特性的主要因素之一。以某2×300 MW直接空冷机组空冷系统为研究对象,通过数值模拟,获得了不同炉后风速下空冷岛的空气流量和热风回流率,分析了空冷岛冷却空气流动换热特性。计算结果表明:不同炉后风速对空冷岛换热特性影响差别较大;随着炉后风速的增加,直接空冷岛空气流量不断降低;热风回流率先随炉后风速增大而增加,而后随炉后风速增大而降低;空冷岛各单元热风回流现象表现出明显的区域特性,处于风场上游的空冷单元具有较大的热风回流率。     

表面式间接空冷散热器传热特性分析北大核心

以表面式间接空冷散热器为例,建立了间接空冷散热器冷却单元的一维传热数学模型。通过该模型,计算并分析了冷却水流量、温度及空气进口温度(环境温度)、流速等对空冷散热器传热性能的影响。计算结果显示,随着冷却水流量、温度和空气流速、温度的增大,散热器总传热系数逐渐增大;散热器总传热系数更接近于空气侧表面传热系数,其中空气流速对总传热系数的影响尤为明显;积灰对散热器传热性能的负面影响较大,积灰越厚,散热器总传热系数越小。另外,利用本文模型,得到了一定条件下间接空冷系统冷却水最佳流量,为间接空冷机组的运行了提供一定的理论依据。     

散热器热负荷分配对间接空冷系统运行的影响

以某SCAL型间接空冷塔为研究对象,根据散热管束的实际尺寸和冷却三角与塔体的布置方式,建立了详细的空冷塔几何模型,并基于RNGk-ε湍流模型和多孔介质模型对空冷塔内外流场进行三维数值模拟,分析了环境风速和散热器热负荷对其运行性能的影响,并针对散热器非满负荷运行,提出了改变冷却扇区热负荷分配的优化方案.结果表明:空冷塔通风量和塔出口速度随散热器热负荷的降低而减小,且随环境风速的增大减小趋势变慢;改变热负荷分配后,塔内流场明显改善,且通风量和塔出口速度增大.     

环境风对直接空冷系统塔下热回流影响的试验研究

对发电厂空冷系统热回流产生的原因及其危害进行了分析,提出了定量分析热回流的评价标准.在直接空冷系统模型内导入恒温热水构成循环系统,在低速风洞中对直接空冷模型进行热效应试验研究,并在不同来流条件下,通过测量风机入口和翅片管束出口空气的温度,计算各测点回流率的大小,探求环境来流风速和风向的变化对空冷塔下回流率的影响,结果发现在实验风速范围内,空冷塔下平均回流率随着来流风速的增加而增加,不同来流风速下空冷塔下平均回流率随风向角变化趋势基本相同;实验风向角β=0时,空冷塔下平均回流率最大.     

1000MW直接空冷机组凝汽器污垢热阻对热经济性影响特性研究

直接空冷机组运行一段时间后,其散热器管内、外污垢热阻会对机组的热经济性产生一定的影响.以某1 000MW直接空冷机组为例,取污垢热阻的变化范围为0～0.001m2·K/W,用η-NTU建立了机组排汽压力的管内、外污垢热阻数学模型.分析了热负荷一定的情况下,不同环境温度、迎面风速下,散热器管内、外污垢热阻对排汽压力的影响特性.结合当地经济因素,比较了不同管内、外污垢热阻阻值对机组年运行费用的影响程度,为机组的经济运行提供了参考.     

环境风对空冷岛内部涡流分布的影响

随着环境风速的变化,空冷岛内部涡流分布对空冷单元换热器性能的影响愈加重要。以2×600MW的直接空冷机组为研究对象,采用Fluent软件模拟了环境因素对空冷岛内部流场的影响。针对不同的环境风速,分析了空冷机组周围流场温度和压力的分布情况,研究了涡流产生的机理和变化规律,以及环境风速对空冷机组风机流量的影响。结果表明:随着环境风速的增大,热空气羽流倾角增大,空冷单元与周围环境的换热量相对减少,换热效率降低。空冷单元呈现更多的负压区域,流场内出现涡旋并逐渐在增大,此时出现回流情况。另外,各空冷单元风机进口流量整体呈下降趋势,其中迎风侧的第一个风机f1的进口流量降幅最大,整体下降298.54kg/s;其功率的降幅也最大,下降了14.338kW。对此提出了优化运行改进措施。     

600MW直接空冷机组冷端换热面积计算与分析

分析了空冷凝汽器总传热系数的计算方法,通过迭代计算和设定的误差确定管内凝结换热系数,并将管外换热系数表达成迎面风速的函数,最终得出了总换热系数与迎面风速的关系式.建立了换热面积的目标函数,并进行了变工况计算,得出换热面积随迎面风速、排汽压力的增大而减小,随环境温度的增加而增加.通过分析初始温差ITD、迎面风速与设计气温对换热面积的影响,提出在空冷系统优化设计中,应以初始温差和迎面风速作为变量参数.     

1000MW直接空冷机组散热单元换热特性的数值模拟

以某1 000MW直接空冷机组为例,对机组散热器外部流场进行了数值模拟,分析了不同迎面风速、环境温度、翅片间距以及翅片厚度对散热器外部换热和流动特性的影响.结果表明:随着迎面风速的增大,散热器外部的传热系数和流动阻力均显著增大,环境温度对散热器外部换热和流动特性的影响并不明显,但对总散热量影响较大;较大的翅片间距能增大散热器外部的传热系数、减小流动阻力,但会使单位管长的换热面积减小,总散热量减小;对应于一定的迎面风速,存在较为合理的翅片间距和翅片厚度,迎面风速越大,合理的翅片间距越大,翅片厚度越小.     

SCAL型间接空冷塔内外空气流动和传热性能数值研究

利用计算流体力学软件CFD,对自然通风状态下,某电厂2×300 MW机组两机一塔的SCAL型间接空冷散热器空冷塔内外空气的流动和传热性能进行数值模拟、分析和研究。确定考核工况基准下,不同环境风速对空冷塔通风量和间接空冷散热器散热量以及机组背压的影响。模拟结果显示,随着环境风速的增加,空冷塔内外流场发生复杂变化,空冷塔通风量和散热器散热量降低,机组背压升高,将引起很大的经济损失。这为间接空冷系统空冷散热器的优化设计提供了参考。     

侧风条件下带消能导流装置的直接空冷岛冬季冻结风险模拟研究

哈密某电厂在其空冷岛中使用了一种新型消能导流装置来抵抗侧风影响、稳定机组背压。然而，该电厂冬季空冷单元散热管束冻结的情况依然存在。为了探究该装置对空冷岛冬季防冻的影响，利用Fluent软件对该电厂冬季大风时空冷岛的流动传热特性及各空冷单元的冻结风险进行了模拟研究。研究表明：空冷岛“消能导流装置”整体上对空冷单元的防冻起负面作用；在冬季大风条件下该装置迎风侧空冷单元的散热量平均超出警戒值27%以上，最高达到50%;消能导流装置主要通过提升轴流风机空气流速来增加对应空冷单元的换热量，该装置迎风侧空冷单元轴流风机的轴向空气流速甚至能达到与环境侧风相同的水平，这导致对应空冷单元换热量激增，更容易出现冻结事故；大风条件下该装置在空冷岛下方形成的高压区域分布并不均匀，临近主厂房与相邻空冷岛一侧的高压区域压力更高、面积更大，这些区域空冷单元的冻结风险更高。     

基于互连思想的直接空冷机组运行优化技术研究

将不同机组间的空冷凝汽器单元或者不同机组空冷凝汽器进行互连,充分利用停运或低负荷机组的空冷散热面积,提高运行机组的散热面积和散热能力。通过对直接空冷单元互连和直接空冷凝汽器互连两种设计方案的分析表明,运行机组的运行背压获得较大幅度的下降,同时显著提高了运行机组在夏季高温条件下的带负荷能力和节能运行水平。随着环境温度或机组运行负荷的降低,通过互连改造后运行机组背压降低的幅度逐渐减小。     

Space characteristics of the thermal performance for air-cooled condensers at ambient winds

Ambient winds may lead to poor fan performance, exhaust air recirculation and mal-distribution of the air across the tube bundles of the air-cooled condensers in a power plant. Investigations of the impacts of the ambient winds on the air-cooled condensers are key area of focus. Based on a representative 2 × 600 MW direct dry cooling power plant, the physical and mathematical models of the air-side fluid and heat flow in the air-cooled condensers at various ambient wind speeds and directions are set up by introducing the radiator model to the fin-tube bundles. The volumetric flow rate, inlet air temperature and heat rejection for different air-cooled condensers as a whole, condenser cells and fin-tube bundles are obtained by using CFD simulation. The results show that the thermo-flow performances for the air-cooled condenser as a whole, condenser cells and heat exchanger bundles vary widely in space. The thermal performances of the air-cooled condensers, condenser cells and fin-tube bundles at the downstream are generally superior to those at the upwind. It is of use for the upwind fan regulations and the A-frame condenser cell geometric optimization to investigate the space characteristics of the thermal performance for the air-cooled condensers in a power plant.     

Performance prediction of an improved air-cooled steam condenser with deflector under strong wind

For an air-cooled steam condenser (ACSC), environmental wind can cause a large flow rate reduction in the axial fans mainly near the windward side of the air-cooled platform due to cross-flow effects, resulting in a heat transfer reduction. This leads to an increase of turbine back pressure, and occasional turbine trips occur under extremely gusty conditions. A new method is proposed in this paper to remove the strong wind effect by adding deflecting plates under the air-cooled platform, which contributes to forming a uniform air mass flow rate in the axial fans by leading enough cooling air to the fans in the upwind region. Numerical simulation is made of the thermal-flow characteristics and heat transfer performance of the improved ACSC with deflectors. A heat exchanger model is used for simulating the flow and heat transfer in the ACSC, in which the heat exchanger is simplified to a porous medium and all flow losses are taken into account by a viscous and an inertial loss coefficient. A fan model is used for reaching the flow condition at the heat exchanger inlet with the actual performance curves of the fan. It is found that the improved ACSC with deflector shows a significant enhancement in both the cooling air mass flow rate and the heat rejection rate compared with the conventional ACSC. The higher the wind speed is, the larger the heat transfer enhancement of the improved ACSC is. The effect of the plate inclination is also investigated, and the inclination angle of 45° is found to be the optimum value for the arrangement of the deflector.     

Operation strategies of axial flow fans in a direct dry cooling system under various meteorological conditions

For a direct dry cooling system, the turbine back pressure fluctuates with the meteorological conditions. Moreover, the operation of axial flow fans plays an important role in the cooling performance of air-cooled condensers (ACC). It is of significant use to study the operation strategies of axial flow fans under various ambient conditions. Based on typical 2 × 660 MW direct dry cooling power generating units, the ACC model coupled with the turbine thermodynamic characteristics is developed, by which the thermo-flow performances of the ACC are predicted in the dominant wind direction, and then the standard coal consumption is calculated. The results show that the increased ambient temperature and wind speed, or the reduced fan rotational speed leads to the high turbine back pressure. At the low ambient temperature and wind speed, the standard coal consumption rate of the unit can be reduced by reducing the speed of axial flow fans appropriately, with the maximum drop in coal consumption rate reached 0.734 g/(kWh) when the ambient temperature is 10°C without wind. If the wind speed exceeds 12 m/s or the ambient temperature reaches 25°C, 110% of the rated fan rotational speed is recommended.     

表面式间接空冷散热器换热特性的数值研究

利用计算传热学软件Fluent,对自然通风状态下某间接空冷塔内布置的表面式空冷散热器的流动和换热性能进行了数值模拟和分析,并将间接空冷散热器考核工况下的模拟结果与设计点进行了比较.结果表明:在3 m/s环境风速下,空冷塔散热器的换热量为设计工况下的95.6%;环境风对空冷塔的出流产生压制作用.数值模拟结果还需进一步进行试验验证和工程检验;考虑到空冷塔间的相互影响,在获得厂区布置图后,还需对其进行更加详细的数值研究.采用数值模拟的方法可为间接空冷系统的优化设计提供帮助.     

Operation optimization of cold-end system of direct air-cooled units considering the effect of environmental wind

An air-cooled island can significantly alter the heat transfer performance of an air-cooled condenser due to the reflow of hot air caused by environmental wind. This can result in a considerable deviation between the backpressure calculated by traditional air-cooled condenser models and the actual value. To address the issue, a research study was conducted on a 600-MW direct air-cooled unit. Numerical simulation methods were used to obtain the corresponding air flow rates and fan inlet air temperatures for each air-cooled heat exchanger, which were then combined to establish a backpressure calculation model. From the above model, the backpressure prediction model and unit net output of full conditions were established using a backpropagation neural network. Therefore, taking the net output as the optimization objective, a genetic algorithm was used to compute the optimal backpressure and optimal fan speed in off-design situations. Compared with traditional calculation approaches, the model produces backpressure predictions that were closer to the actual situation under the effect of ambient wind. The results indicate that both the optimal backpressure and fan speed were positively correlated with the exhaust flow and ambient temperature. It has been observed that when a unit was affected by different wind directions, the effect of the forwarding wind on the backpressure was smaller than that of other wind directions, especially under high-load conditions. Moreover, the fan group operates close to full capacity under high-temperature and high-load conditions. Therefore, considering the influence of ambient wind, the obtained optimal backpressure and fan speed under variable working conditions were more realistic.     

锅炉侧面来风对空冷机组换热效率的影响

环境风特别是横向环境风会引起热风回流,造成空气凝汽器换热恶化,影响直接空冷机组的安全、经济性运行。以国内某电厂300MW直接空冷系统为例,利用Fluent软件,针对锅炉侧面风给直接空冷机组带来的不利影响,对空冷岛外部流场进行了数值模拟研究。主要分析了锅炉侧面来风的不同风速下空冷平台流场情况,研究了锅炉侧面风在对空冷风机冷空气吸入量和空冷凝汽器换热性能的影响。     

直接空冷凝汽器加装防风网的数值模拟

利用Fluent软件,以国内某600 MW直接空冷机组为例,对迎风侧2个相邻空冷凝汽器进行了数值模拟,得到了不同环境横向风速下空冷凝汽器的传热效率随着风速的增大而降低,而且被环境风冲刷越严重的凝汽器受影响程度越大的结论.针对横向风对空冷凝汽器传热效率的影响,在凝汽器挡风墙下延方向加装了防风网.对加装防风网后的空冷凝汽器进行了数值模拟,分析了防风网的不同结构参数(开孔率、高度)对空冷凝汽器传热效率的影响,结果表明:在加装防风网后凝汽器传热效率显著提高,防风网的结构参数对其防风效果有明显影响,为直接空冷机组的防风网改造提供了理论依据.     

空冷凝汽器椭圆翅片椭圆管束外空气的流动与传热特性

研究空冷凝汽器椭圆翅片椭圆管管束外空气的流动与传热特性,对火电站空冷岛的设计与运行具有重要意义.通过CFD模拟,获得了椭圆翅片椭圆管管束外冷却空气的流场和温度场,计算得到了空冷凝汽器冷却空气对流换热平均Nu和摩擦系数f随Re的变化规律,并采用最小二乘法拟合得到了相应的关联式.结果表明:随冷却空气流动Re的增大,Nu增大,f减小.