摘要
换热设备在工业生产中占有重要地位,其传热性能的好坏对节能有着重要意义。目前,翅片管式换热器被广泛应用于能源动力、化工、冶金、空调、食品冷藏加工等诸多行业,特别是在制冷及空调领域,翅片管式换热器的应用更为广泛,而优化和提高翅片管式换热器空气侧的传热和流阻特性,也就相应的成为制冷空调领域的一个极其重要的研究课题。换热器空气侧性能受自身结构参数以及外界环境因素的影响,自身结构参数包括翅片间距、翅片厚度、管排数等,外界环境因素包括换热器进口空气干球温度、湿球温度和迎面空气流速、空气流量以及空气进风条件(自由进风、受限进风)等。
目前,结霜工况下工作的制冷换热器基本上采用平翅片,高效翅片的使用未见报道。干工况下所进行的理论研究及实验研究都表明,在平翅片上开小圆孔是一种简单有效的强化传热措施,其强化机理在于,小孔可以破坏边界层以及增强气流的扰动,从而使局部换热系数提高。对称圆孔翅片和三对称大直径圆孔翅片(又简称SK型翅片)就是依据此原理为改进依据发展起来的。
前期模拟及相关实验都证实,相对于矩形平翅片,SK型翅片具有最优的强化传热效果。方赵嵩利用FLUENT软件对平翅片、对称圆孔翅片和SK型翅片空气侧的流动及换热进行了数值模拟,对不同风速下的速度场、温度场和努谢尔特数分布的数值模拟结果进行了分析,结果表明:数值模拟的结果与早期实验结果吻合较好,误差控制在10%以内。同时,模拟结果还表明:同等条件下,SK型翅片的平均表面传热系数比平翅片平均高出25.7%。在模拟结果证实SK型翅片是一种传热效果优越的片型的基础上,方赵嵩利用风洞试验台在结霜工况下对平翅片、对称圆孔翅片和SK型翅片管式换热器进行了换热与制冷性能的对比性实验,实验结果表明:结霜工况下下,与平翅片及圆孔翅片相比,SK型翅片的强化传热效果最好。
为进一步说明SK型翅片优越的强化传热效果,并将科学研究成果转化为生产力,使SK型翅片可以量化生产。张杰等人制作了平翅片冷风机及SK型翅片冷风机各一台,并将已有的按冷库标准建造的人工气候室进行改造,对两台冷风机的节能性能进行了对比性工业样机试验。试验结果证实:本项目所开发出来的SK型制冷换热器是一种传热性能优异、能效比高的节能产品,
以上研究成果结果还不能全面反映SK型翅片的空气侧特性,目前,有关换热器空气侧性能的研究主要分为:实验研究和数值模拟研究。由于制冷工况下进行换热的两种流体在换热器内部的流态极其复杂,这就使得数值模拟难以得到可靠的研究结果。因此,目前对换热器空气侧特性的研究,主要还是以实验研究为主。
本文在前人实验研究和理论研究的基础上,利用已有风洞试验台,在结霜工况下,采用实验的方法研究了SK型翅片管式换热器的传热和流阻特性,分析了不同入口空气流速和相对湿度对SK性翅片管式换热器传热与流阻特性的影响情况,并回归出SK型翅片管的努谢尔特数Nu和摩擦系数f的准则关联式。实验结果表明:①入口空气温度恒定为28℃时,在实验时间段内,SK型翅片管表面霜层生长始终处于结晶生长期时,SK型翅片管式换热器的制冷量Φ、翅片表面传热系数h和空气侧压降ΔP值都随着入口空气流速和相对湿度的增大而增大。同时在相同的入口空气流速和相对湿度下,换热器的制冷量Φ、翅片表面传热系数h和空气侧压降ΔP值随着结霜时间的进行是不断增大的。②入口空气温度恒定18℃时, SK型翅片管表面结霜经历了三个阶段——结晶生长期、霜层生长期和霜层充分生长期。此时,SK型翅片管式换热器的制冷量Φ、翅片表面传热系数h、空气侧压降ΔP和COP值都随着入口空气流速和相对湿度的增大而增大。同时在相同的入口空气流速和相对湿度下,换热器的制冷量Φ、翅片表面传热系数h和COP值随着结霜时间的进行是先增大而后逐渐降低。
结霜工况下,对不同入口空气流速和相对湿度对SK型翅片管式换热器传热及阻力特性的影响进行深入分析,进一步阐明了结霜工况下SK型翅片管式换热器换热及内部流动的内在机理,加深了对SK型翅片管式换热器工作过程的理解,为SK型翅片管式换热器阻力和热工的优化设计提供基础依据,为其工业应用提供技术支撑。
The heat exchanger equipment holds important position in the industrial production, its heat-transfer performance's quality has the great significance to the energy conservation. At present, the finned tube type heat interchanger is widely applied in the energy and power, the chemical industry, the metallurgy, the air conditioning, and the food refrigeration processing and so on many industries, particularly in the field of refrigeration and air conditioning, the finned tube type heat interchanger's application is more extensive, so optimize and improve the air side heat transfer characteristic and flow resistance characteristic of the finned tube heat exchanger, also the corresponding field of refrigeration and air conditioning to become a very important research topic.The heat interchanger air side performance is influenced by its own structural parameters and external environmental factors, its own structural parameters includes the fin pitch, fin thickness, and number of tube rows and so on,and the external environmental factors including the heat exchanger temperature of inlet air dry bulb temperature,the wet bulb face temperature and the air velocity,the air flow as well as the air inlet conditions (free air intake, restricted into the wind) and so on.
At present, working in the frost conditions is basically plain fin heat exchanger, but the highly effective wing piece's using has not been reported. Under the dry operating mode carries on the fundamental research and the experimental study indicated that opens a small round hole on the even wing piece is one kind of simple effective measure of strengthened heat transfer, its strengthening mechanism is enhanced boundary layer air holes can destroy the disturbance, thus causes the partial heat transfer coefficient increased. The symmetrical round hole wing piece and the three symmetrical large diameter round hole wing piece (also refered to as SK fin) to rest on this principle to develop for the improvement basis.
The earlier period simulation and the related experiment confirmed that, raletive to the rectangular flate fin, the SK wing piece has the most superior strengthened heat transfer effect. Fang Zhaosong using the FLUENT software to the even wing piece, the symmetrical round hole wing piece and the SK wing piece air side's flowing and the heat transfer has carried on the numerical simulation, to under the different wind speed's velocity field, the temperature field and the Nuxieerte number distributed numerical simulation result has carried on the analysis,the simulation results indicated: Numerical simulation's result agree well with the early time experimental result tallies well, error control within 10%. At the same time, the simulation results also indicated: Under the same level condition, the SK wing piece's average heat transfer coefficient is higher than 25.7% compared to the even wing piece average. On the basis of the simulation results demonstrate that SK-type heat transfer fin is a superior film-type, Fang Zhaosong used the wind tunnel test platform under the rime operating mode to the even wing piece, the symmetrical round hole wing piece and the SK finned tube type heat interchanger has carried on the heat transfer and the refrigeration performance relative property experiment, the experimental result indicated: under frosting conditions, compared with plain fin and fin hole, the SK fin heat transfer effect is best.
To further explain that the SK-type fin superior strengthening heat transfer effect, and transforms the scientific research achievement into the productive forces, enables the SK-type fin to be possible the quantification production. Zhang Jie et al. have manufactured a plain fin air conditioner forced draft fan and SK-type fin air conditioner forced draft fan each, and the construction of the existing standards by cold artificial climate chamber to transform, has carried on the relative property industry prototype experiment to the two air conditioner forced draft fan's energy conservation performance. The experimental result confirmed that: This project develops the SK-type refrigeration heat interchanger is one kind of heat-transfer property is a energy conservation product with outstanding heat transfer performance and high energy efficiency.
The results of these studies can not fully reflect the SK-type fin’s air-side characteristics, at present, the related air side heat exchanger performance study is divided into: experimental research and numerical simulation. As a result of the two heat transfer fluid in the flow inside the heat exchanger is extremely complex, making it difficult to obtain reliable simulation results. Therefore, the current characteristics of the air side heat exchanger research, mainly experimental study primarily.
Based on the previous experimental research and theoretical study, using the existing wind tunnel test platform, in the frost conditions, using the experimental method to study the SK-type finned tube heat exchanger heat transfer characteristic and flow resistance characteristic, and return to the SK-type finned tube Nu and f correlation criteria. The experimental results show that:①When inlet air temperature constantly for 28℃, in the experimental period, SK fin surface frost layer growth is always in crystal growth stage, the cooling capacityΦ, the fin surface heat transfer coefficient h and the pressure dropΔP values of the SK-type finned tube heat exchanger along with inlet air velocity and relative humidity increases increases. While at the same inlet air velocity and relative humidity, the cooling capacity of heat exchangerΦ, fin surface heat transfer coefficient h and the pressure dropΔP values are constantly increasing with the frosting time increases.②When inlet air temperature constantly for 28℃, SK-type fin surface frost has gone through three stages:crystal growth period, frost layer full growth period and the frost layer growth period. At this point, the cooling capacityΦ, the fin surface heat transfer coefficient h, the pressure dropΔP values and the COP of the SK-type finned tube heat exchanger along with inlet air velocity and relative humidity increases increases. While at the same inlet air velocity and relative humidity, the cooling capacity of heat exchangerΦ, fin surface heat transfer coefficient h and the COP are carried out with the frosting time increases first and then gradually decreased.
Under frosting conditions, carries on the thorough analysis to the different inlet air velocity and the relative humidity to the SK finned tube type heat interchanger heat transfer and the resistance characteristic influence, and further clarified SK fin tube heat exchanger internal flow of heat exchange and the internal mechanism of deepening of the SK-type finned tube heat exchanger working understanding of the process under frosting conditions, the SK-type finned tube heat exchanger thermal resistance and the basis for the optimal design based on its provide technical support for industrial applications.
引文
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