加涡产生器的管片式换热器换热与阻力特性的数值研究

通过数值模拟的方法,研究了小翼式涡产生器对错排圆管管片式换热芯子换热与阻力特性的影响,比较了光板与加涡产生器强化板芯的速度场、横向平均Nu数以及平均对流换热系数、阻力系数的变化规律,为进一步提高其换热性能、改进翅片结构、设计新型换热器提供了理论依据。     

汽-汽再热器内流动与换热特性数值研究

为了揭示汽-汽再热器的换热特性,本文首次通过三维数值模拟的方法对汽-汽再热器内的流动、换热及熵产特性进行了研究。结果显示,当高温蒸汽流速较小时,高温蒸汽侧进口处的温度梯度较大,而低温蒸汽侧出口处的温度变化不明显;随着高温蒸汽流速增大,高温蒸汽沿流程的温度梯度逐渐减小,低温蒸汽出口平均温度逐渐增大,同时换热器内局部熵产值逐渐增大;随着管壳式换热器隔板数的增加,低温蒸汽出口平均温度逐渐增大,但增幅逐渐减小。     

T型圆管内蒸汽直接接触凝结数值模拟

选用流体体积分数两相流模型和大涡模拟湍流模型,并引入基于热平衡原理的凝结模型——双阻力模型的自定义函数(user defined function,UDF),在FLUENT平台上对T型圆管内低速蒸汽在缓慢流动冷水中的直接接触凝结进行了数值模拟。结果表明,该工况下的蒸汽直接接触凝结为典型的间歇(Chugging)凝结且凝结主要发生在界面附近很小的区域,同时蒸汽直接接触凝结导致压力瞬间急剧降低从而发生水锤现象。     

V形肋片型太阳能空气集热器换热特性的研究

本文运用数值模拟的方法,对带有多重V形肋片的平板型太阳能空气集热器的换热与流动特性进行了研究。在雷诺数5 000~20 000范围内,采用RNG k-ε湍流模型,研究了相对肋宽比(W/w)对集热器换热特性的影响规律,并对集热器内部流场进行了分析。研究结果表明,V形肋片增强了空气的对流换热。这是由于流体掠过肋片后,产生旋涡和回流,加强了流体扰动,从而使换热增强。相对其它肋宽比而言,W/w=6时,集热器具有较好的换热效果,努塞尔数为光滑壁面的2.54倍,但肋片的存在导致摩擦损失增加,换热性能因数最大为1.55。     

EHD_脉动流混合强化传热换热器的传热与阻力特性实验研究

以水为工质,实验研究了EHD(电场)与脉动流对单管换热器内的传热与阻力特性影响。实验中,电压值设定为0-40 kV,脉动流频率f=1、2、3 Hz,脉动流幅度A=1,脉动流条件下的管内时均流量q=0.1-0.5 m3/h。实验结果表明:管程脉动流单独作用时,同一流量条件下,f增加时传热系数α无明显变化,即脉动流单独作用对圆管传热强化作用不显著;EHD单独作用能显著强化传热,电压相对较小时,α增长缓慢,当电压U30 kV时α较快增长,但随着电压的继续增加渐趋平缓,α最大提高0.12倍;脉动流和EHD具有一定的复合作用,f对有电场强化条件下的α有较大影响,尤其是当U30 kV时,f越大,相同电场强度下管程α越大,对应的α最高可提高约0.25倍;EHD/脉动流混合作用时,随着Re的增大,管程阻力系数λ逐渐降低,并且在相同的Re情况下,脉动强化措施下的λ要明显高于无脉动情况;同一Re条件下,同频率工况时,电压的增大与否,对λ的改变并无影响,即阻力损耗主要来源于脉动流的作用,而电场对此的影响较小。     

套管式相变储热单元强化换热数值研究

建立套管式相变储热单元的二维模型,采用ANSYS FLUENT软件模拟分析了换热流体(HTF)侧Re和填充系数λ对储热单元传热性能的影响,采用有效储能比Est来表征相变储能单元的有效储能容量,在此基础上研究HTF侧增强传热对套管式相变潜热储能系统性能的影响.研究结果表明:随着Re从250增加至1 000,熔化时间从22...     

新型强化换热管管外流动与换热特性

针对新型强化换热管的几何特征,采用三维数值模拟方法,分别从速度—压力场协同性、综合评价因子ε、速度—温度场协同角三个方面对扭曲管、波节管、波纹管开展研究。对波纹管的实验测量结果与数值模拟结果最大相对偏差为2.35%。数值模拟结果表明,纹管管外Re从569~7 399变化,总压降从3.78增长到317.30Pa;扭曲管Re从508~7 622变化,总压降从0.84增大到62.50Pa;波节管Re为580~7 550变化,总压降从1.29增长到110.00Pa,波纹管阻力相比波节管最大提高186.7%,相比于扭曲管提高408.4%,Re<4 500时扭曲管Nu数比波纹管最大提高13.30%,比波节管最大提高6.97%,Re>4 500时波节管换热性能逐渐超越扭曲管以及波纹管,在计算范围内Nu比扭曲管最大提高5.17%,比波纹管最大提高5.40%;波节管的周期性波峰对于强化换热和减阻作用显著;波纹管的压降、强化换热表现均弱于波节管和扭曲管。根据计算结果拟合出Nu数与Re、Pr的实验关联式,为管壳式换热器设计提供一定的理论和依据。     

低压条件下紧凑式顺排管束满液型蒸发换热器的强化换热特性研究

提出了一种新型紧凑式顺排光滑管束组成的满液式蒸发换热器。在低压条件下对水平光滑顺排管束的小空间内沸腾强化换热特性进行了实验研究,确认了管距、管位置和运行压力对强化换热性能的影响。实验表明存在一个能得到最大强化换热效果的最佳管距,这一最佳管距接近沸腾气泡的脱离直径。压力对强化换热效果也有重要影响:随着压力降低,强化换热效果也逐步减弱。实验结果对高效节能型蒸发换热器设计提供了设计基础。     

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。     

三角扰流蛇形翅片管流动换热特性数值研究

蛇形翅片管作为空冷凝汽器基本换热面,其流动与换热性能对空冷电站经济性有着重要影响,基于此,对三角扰流蛇形翅片管的气侧换热及流动特性进行了数值研究,分析了流体温度、压力沿流程方向的变化规律以及迎面风速、扰流三角个数、扰流三角尺寸等因素对蛇形翅片管换热及流动性能的影响,结果表明三角扰流部位局部阻力是沿程阻力的25%～70%,扰流三角对换热有明显的促进作用,换热系数最大提高37%.本研究可为蛇形翅片管的设计及优化提供理论参考.     

Cold energy release characteristics of an ice/air direct contact heat exchanger

This paper deals with the cold energy release characteristics of an ice/air direct contact heat exchanger. Characteristics of the outlet temperature, humidity, and time history of heat release are examined when the initial height of the ice‐cube‐packed bed in the heat exchanger is changed. The following results were obtained in these experiments: (1) Inlet air of 30 °C is lowered to about 0 °C when passed through the heat exchanger, and the absolute humidity of the outlet air is reduced to about a quarter of that of the inlet air. (2) There is an optimum height of the ice‐cube‐packed bed to maximize the amount of cold energy release. (3) This heat exchange method can supply about twice as much cold energy as is released by an ordinary fin‐tube‐type heat exchanger even if the air velocity in the heat exchanger is reduced to about 0.38 times that of the fin‐tube‐type heat exchanger. (4) A nondimensional correlation equation for predicting the time taken for the ice‐cube‐packed bed to completely melt is derived. © 2001 Scripta Technica, Heat Trans Asian Res, 30(2): 95–113, 2001     

开缝布置方式对开缝翅片管换热器 传热与阻力特性的影响

为了获得开缝布置方式对开缝翅片管换热器传热与阻力特性的影响规律,对5种不同翅片管换热器进行了数值模拟研究,并进行了模化试验验证。结果表明：增加开缝会提高翅片管换热器的传热性能,但阻力也随之增加;与开缝位置相比,开缝数量对开缝翅片管换热器传热与阻力特性的影响更大;在Re=4800~7500日时,开缝翅片管换热器综合流动传热性能 随着Re数的增大而增大;在5种翅片中,开缝翅片的综合流动传热性能高于普通平直翅片;数值模拟与试验结果偏差较小,采用数值模拟方法能够比较准确地分析开缝翅片管换热器的传热与阻力特性。     

地源热泵地中换热器的非稳态传热数值研究

竖直U型管经常被用做地源热泵的地下换热器.文章建立了三维数值模型模拟U型管内的流动和地下耦合非稳态传热.使用商业软件GAMBIT和FLUENT构造数值模型并进行数值模拟.为了验证模型的有效性,将U型管出口水温的模拟结果与实测数据进行了比较:运行中出口水温模拟值稍大于实验值,随着运行时间的增加,两者相差越来越小.该结果显示,对于长时间运行的热泵系统,此模型的模拟数据更加真实可靠.     

矩形微槽群换热器换热特性的数值研究

设计了一种矩形微槽群换热器,分别对单个槽和整个换热器传热过程进行了数值模拟。对不同流量以及不同热流下的流场和温度场进行模拟,并与理论分析结果比较,两者相吻合。分析结果表明,微换热器的热阻随着流量的增大而变小,温度变低。当流量为200 mg/s时,微换热器最高温升为47 K,表明当达到一定流量的时候,微换热器温升能控制在有效地范围内,能很好地保证微器件的工作状态。     

A novel method for measuring spatial uniformity of irregular boiling bubbles in a direct contact heat exchanger

As the binary image fails to reflect the density distribution of the bubble, this paper proposes an improved new indicator for accurate measurement of spatial uniformity based on gray image analysis: moment. Compared with the inclination angle method in binary image, the test results show that the two methods have significant positive correlation. The experimental results also indicate that the evolution curve of bubble numbers is consistent with its moment evolution curve, and the moment evolution fitting curve can distinguish the high noise in the experiment. Compared with other methods, mixing performance index (t) obtained by moment evolution fitting curve is correlated with heat transfer performance (Pearson correlation coefficient a = −0.94). The variation in characteristic area of the bubble in the direction of bubble growth can characterize the heat transfer performance. The index of dispersion of the global moment and the local moment indicates that the local moment can be used to precisely quantify the spatial uniformity of boiling bubbles. This work could also be applied to study a variety of problems involving spatial uniformity through visualization techniques.     

Development of gas-solid direct contact heat exchanger by use of axial flow cyclone

A heat exchanger between particulate or granular materials and gas is developed. It makes use of a swirling gas flow similar to the usual cyclone separators but the difference from them is that the swirl making gas is issued into the cyclone chamber with downward axial velocity component. After it turns the flow direction near the bottom of the chamber, the low temperature gas receives heat from high temperature particles supplied from above at the chamber's center. Through this configuration, a direct contact and quasi counter-flow heat exchange pattern is realized so that the effective recovery of heat carried by particles is achieved. A model heat exchanger was manufactured via several numerical experiments and its performances of heat exchange as well as particle recovery were examined. Attaching a small particle diffuser below the particle-feeding nozzle brought about a drastic improvement of the heat exchange performance without deteriorating the particle recovery efficiency. The outlet gas temperature much higher than the particle outlet temperature was finally obtained, which is never realized in the parallel flow heat exchanger.     

矩形通道内冲孔矩形小翼涡流发生器的流动换热特性

雷诺数Re=214～10 703时,通过数值模拟方法对布置有冲孔和无孔的两种矩形小翼涡流发生器的矩形通道进行了传热和流阻特性的研究。计算结果表明:在低雷诺数下,冲孔矩形小翼涡流发生器的传热因子j值与无孔矩形小翼涡流发生器相差不大,而在高雷诺数下,冲孔涡流发生器的传热因子j值略低于无孔涡流发生器,大约低1.03%～3.05%。在相同的雷诺数下,无孔矩形小翼涡流发生器的阻力因子f大于冲孔涡流发生器,而且随着雷诺数的增大二者的差距也越来越大。通过对比综合性能指标可知,两种通道的综合性能指标均随着雷诺数的增加而减小,而且冲孔矩形小翼涡流发生器的综合性能要优于无孔矩形小翼涡流发生器。     

螺旋套管换热器传热特性研究

根据螺旋套管换热器的结构特点及传热特性,建立了水一水蒸气的流动与传热的三维几何模型.利用Fluem时不同工况下的螺旋套管进行了数值模拟,得出了湍流状态下螺旋套管内流体的温度场、速度场和压力场;利用搭建的螺旋套管换热器试验台,得出多种工况下的传热系数,为螺旋套管换热器的设计计算提供了依据.同时将试验结果和数值模拟结果进行...     

直接接触式高效低阻天然气锅炉排烟余热回收装置的研发

为了大幅度提高天然气锅炉效率,提出一种回收天然气锅炉排烟余热的直接接触式换热器。经实验研究,得出用于指导工程设计的经验关系式。并根据经验公式设计了配用2 t天然气锅炉的新型节能器,其使用效果良好。最后分析了影响该节能器换热效果的主要因素,研究了排烟温度随液气比、节能器阻力随烟气流速的变化趋势。     

A spatially advancing turbulent flow and heat transfer in a curved channel

Direct numerical simulation was performed for a spatially advancing turbulent flow and heat transfer in a two‐dimensional curved channel, where one wall was heated to a constant temperature and the other wall was cooled to a different constant temperature. In the simulation, fully developed flow and temperature from the straight‐channel driver was passed through the inlet of the curved‐channel domain. The frictional Reynolds number was assigned 150, and the Prandtl number was given 0.71. Since the flow field was examined in the previous paper, the thermal features are mainly targeted in this paper. The turbulent heat flux showed trends consistent with a growing process of large‐scale vortices. In the curved part, the wall‐normal component of the turbulent heat flux was twice as large as the counterpart in the straight part, suggesting active heat transport of large‐scale vortices. In the inner side of the same section, temperature fluctuation was abnormally large compared with the modest fluctuation of the wall‐normal velocity. This was caused by the combined effect of the large‐scale motion of the vortices and the wide variation of the mean temperature; in such a temperature distribution, large‐scale ejection of the hot fluid near the outer wall, which is transported into the near inner‐wall region, should have a large impact on the thermal boundary layer near the inner wall. Wave number decomposition was conducted for various statistics, which showed that the contribution of the large‐scale vortex to the total turbulent heat flux normal to the wall reached roughly 80% inside the channel 135^° downstream from the curved‐channel inlet. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20275