【作者】 李俊超；

【导师】 吴金星；

【作者基本信息】 郑州大学 ， 热能工程， 2013， 硕士

【摘要】 本课题是郑州大学节能技术研究中心与河南力威热能设备制造有限公司的合作项目,是在企业产品实际应用中发现问题的基础上,结合用户反馈信息,针对性地开展了一系列研究工作。首先介绍了课题的研究背景和意义,提出了燃气真空锅炉中换热管束存在的换热效率低的问题,然后分析了换热管束换热效率低的根源,分别对直径大小不同的换热管的强化换热方法进行了研究,最后通过改善蒸汽在管束空间的流通方式,进一步提高了换热管束部分的换热效率。本课题研究内容,可概况为以下四个方面：(1)结合锅炉应用实践,找出了现有燃气真空锅炉中常用平直换热管束存在的两种主要缺陷：首先是平直换热管内部热阻大,流体流经换热管内表面时,在管壁附近形成边界层,流体的速度梯度变化较大,管内会形成很大的热阻层,严重影响传热性能；其次,由于平直换热管束的密集布置,不能使换热空间充分展开,造成蒸汽通道过窄,而且蒸汽冷凝过程产生的冷凝水不能及时排除,管外冷凝热阻增大。(2)开发了一种新型等弦波形强化换热管,在场协同理论的基础上,分析了等弦波形管的强化传热机理；同时分析了等弦波形管由于其自身特殊的结构,强迫管内主流区流体向边界层及边界层向主流区的流动,改善了温度场与速度场的协同性,减小了协同角,起到了强化传热的作用。通过正交试验法对实验方法的有效设计,大大的减少了试验次数,运用CFD中的Fluent软件,对16种模型进行了逐一模拟,用极差分析法得到了等弦波形管的最佳结构组合。通过计算得出了等弦波形管内流体温度场与速度场的协同角。通过综合热工性能评价,证实等弦波形换热管有较好的热工性能,对流传热系数比直管在相同条件下提高47.6%。最后设计了等弦波形换热管的加工方法,该方法具有操作简单、加工成本低的优点,容易实现企业的规模化生产。(3)对于管径较大的换热管,改变换热管本身结构来强化管内换热性能难度较大,因而开发了一种流体阻力较小的新型格栅纽带,并对格栅纽带在大管径中的强化换热进行了全面的理论分析,重点分析了格栅纽带强化换热的三种方式,即插入格栅纽带当量直径减小引发的强化换热特性、管内流体螺旋流动速度提升的强化换热特性,以及格栅纽带特点和螺旋流动引发二次流的强化传热特性等。接着,构建了结构参数相同的普通螺旋纽带模型并进行了数值模拟对比,在同等工况下,格栅纽带具有较好的综合换热性能,即虽然传热系数有所降低,但在考虑压降的情况下,比普通纽带有更好的综合价值。最后,建立了五种格栅纽带模型,分别设定了五种试验工况,一共做了二十五组数值模拟试验,得出了不同模型不同工况下格栅纽带的换热和压降特点,总结了格栅纽带的换热系数和压降随雷诺数和扭比的变化规律,为格栅纽带在不同条件下的应用指明了方向。(4)结合锅炉生产厂家及用户的反馈信息,针对燃气真空锅炉现有的换热管束结构存在的不足,为强化管束外相变换热,在锅炉内部设置了新型导流板和折流板等结构,改变了蒸汽冲刷换热管束的方向,实现了真空锅炉中换热管束外蒸汽与冷凝液膜的同向流动,大大促进了液膜在壁面的快速剥离,强化了蒸汽与换热管束壁面的冷凝换热。应用场协同等强化传热理论,对锅炉中换热管束外相变冷凝的强化换热机理进行了详尽的分析。上述研究成果对强化燃气真空锅炉中换热管束的对流换热及其工程应用奠定了基础。更多还原

【Abstract】 This topic is the project of Zhengzhou University Research Center of Energy Saving Technology and Hennan Liwei heat energy equipment manufacturing Co Ltd. Based on the practical application of products and the feedback information of users, a series of research about vacuum boiler is carried out. At first, the background and significance of the research is introducted, and the problem of low heat efficiency in vacuum boiler is put forward and analysed. Next, the article researches the diffirent methods of heat transfer enhancement about two type tubes. At last. it improve steam circulation in order to optimize heat transfer.The main works and productions in this paper are as follows:(1) According to the direction of study, the topic points out the disadvantages about vacuum boiler. It summarizes two disadvantages. One is that the common heat transfer tube has the larger thermal resistance. Because the fluid near the tube wall velocity is almost zero, forming a greatly thermal resistance. Another is that the condensation water between tubes can not be timely eliminated, weakening the condensation heat transfer.(2) The research sets up a model of equal chord waveform tube. It analysis the tube’s heat transfer mechanism based on the principle of field synergy. The tube can make the fluid flow, which mainstream fluid come to boundary or boundary fluid come to mainstream. This flow improves the collaborative feature of temperature field and velocity field and reduce the synergy angle, achieving the effect of heat transfer enhancement. Then, by means of orthogonal test and Fluent, we simulation16models one by one. And we get the best structure by the methods of range analysis. Next, we get the synergy angle of temperature field and velocity field by calculation. Based on the calculation of overall thermal performance, the convective heat transfer coefficient of equal chord waveform tube increased by47.6%than commen tube. At last, we put forward the processing method of qual chord waveform tube.(3) Be directed against large diameter heat transfer tube, we also develop the reearch. Becanse it is difficulty to change the structure of large diameter tube, the article put forward the method of inserting the spiral ligament to enhance heat transfer. In virtue of the commen spiral ligament having a greater resisrance for water, we invent the grid spiral ligament. First, the article analysis3kinds principle of heat transfer using the grid spiral ligament. Second, we set up the models of commen spiral ligament and grid spiral ligament by the same structure, getting the grid spiral ligament having the better effect of heat transfer. Finally, we set up25working conditions, getting the different feature of heat transfer and pressure drop. And we point out the orderliness of heat transfer and pressure drop with the changes of Reynold number and twist ratio.(4) Based on the practical application of products and the feedback information of users and the principles include of field synergy, the article analysis systemly the condensation heat transfer mechanism of the part of beat transferr in the boiler. In order to promote the groom of condensed water and change the flow direction of steam, we set up some baffles, achieving an effective combination of structural modification and enhancement of heat transfer. At the same time, we set forth the mechanism of enhancement of heat transfer. Although this part is not the research focus of the article, it has the certain instruction significance for the condensation heat transfer of this kind of boilers.更多还原