【作者】 王英双；

【导师】 黄素逸；

【作者基本信息】 华中科技大学 ， 工程换物理， 2011， 博士

【摘要】 换热器在诸如石油冶炼、电力、化工、过程工业以及食品等工业中是十分重要的设备。各种换热器中管壳式换热器具有许多优点,如结构可靠,技术成熟,适用范围广等优点,在工业中得到广泛应用。管壳式换热器中,支撑部件起着十分重要的作用,它不但可以支撑管束,同时对流体有扰流作用。根据壳侧流体运动的方向,管壳式换热器可以分为横向流、纵向流以及螺旋流换热器等。采取不同的折流部件,换热器的性能将会有较大的差异,而壳程的换热系数则对整个换热器的换热性能具有十分重要的作用。传统的弓形折流板换热器存在许多不足之处,如压降大,易导致流体诱导振动等缺点。为了提高弓形折流板换热器的传热性能,往往需要耗费比较大的泵功。压降和传热性能通常是相互关联的,而这两者通常对换热器的成本具有决定性作用。为了改善管壳式换热器的综合性能,国内外许多学者对各种支撑扰流部件进行了研究,其中纵流管壳式换热器由于具有较小的流动阻力和较高的传热性能,现已成为了国内外学者研究的焦点。本文在前人研究的基础上,从理论和实验两个方面研究纵流管壳式换热器的流动和传热性能;同时在核心流强化传热理论的指导下,开发研制新型的纵流式管壳式换热器,并对其性能进行研究。折流杆换热器作为一种重要的纵流式换热器,在工业中得到了广泛的应用,但是关于其传热强化机理目前尚未有统一的认识。本文通过建立折流杆换热器的数学物理模型,对折流杆换热器的传热和流动进行数值模拟研究,通过对结果分析发现,折流杆换热器的强化传热机理与核心流强化传热理论相一致;在此基础上,研究了不同形状折流杆对换热器综合性能的影响,结果表明,折流杆截面不同,对换热器的性能有较大的影响,其中方杆换热性能较强,但是流动阻力较大,圆杆换热较方杆差,但是流动阻力小,通过适当的组合,可提高换热器的综合性能。结合核心流强化传热原理,提出了支撑和扰流分离的纵流管壳式换热器强化传热思想,并设计了一种粗杆-细杆组合的折流杆换热器,采用CFD技术对其传热和流动性能进行了研究。研究结果表明,采用粗杆-细杆组合结构,对换热器起的传热性能影响不大,但可以降低流动阻力,因而换热器的综合性能较高。基于核心流强化传热原理,本文分析了折流杆换热器管束内的扰流机制,设计了一种新型的折流杆–扰流叶片组合式换热器,建立了相应的物理和数学模型,并对其传热与流动特性进行了计算模拟。结果表明,该新型换热器壳程的对流换热系数与折流杆换热器相当,但流动阻力远小于折流杆换热器,综合性能优于折流杆换热器,而且Re数越高,优势越明显。在分析折流杆换热器传热和流动特征的基础上,论文采用周期性充分发展模型,对新型折流杆换热器的性能进行了研究,并用基于热力学第一定律的评价方法和热力学第二定律方法对三种换热器的性能进行了评价。结果表明,基于热一律和热二律的评价方法在某些场合会出现不一致的结论,但这主要与评价的角度有关。花格板换热器作为一种新型的管壳式换热器,其流动和传热机理研究目前尚未有较深入的研究,论文通过CFD技术,对花格板换热器的流动和传热性能进行了研究,并将结果与传统的弓形折流板换热器进行了比较。结果表明,在相同的雷诺数下,花格板换热器的压降仅为折流板换热器的0.45倍左右,而两者的换热系数相差不大,因而花格板换热器的综合性能参数约为折流板换热器的2.2倍左右。在理论理论研究的基础上,本文对花格板换热器进行了实验研究。为了比较性能,将实验结果和相同结构参数的弓形折流板换热器进行了比较,结果表明,在相同的条件下,花格板换热器的综合性能要比弓形折流板高20%-30%。本文还对新型折流杆换热器的传热和阻力特性进行了实验研究,得出了换热和阻力特性实验准则式。本文的研究结果可以为纵流换热器的设计和实际应用提供一定的理论指导作用。更多还原

【Abstract】 Heat exchanger is a very important apparatus in many fields, such as petroleum refining, power generation, chemical engineering, process industry, food industry, etc. Among the all types of heat exchangers, shell-and-tube heat exchanger (STHX) has many advantages such as mature technique, reliable structure and widely applicability, which make it widely utilized in industrials. The baffle element plays very important roles in shell-and-tube heat exchangers, which can not only support the heat transfer tube bundles, but also disturb the fluid flow of shell side. According the direction of fluid flow of shell side, the shell-and-tube heat exchanger can be divided transverse flow, longitudinal flow and helical flow. With different flow states, the performance of the STHX will be different. The heat transfer coefficient of the shell side has a heavy impact on the performance of the heat exchanger.The traditional shell-and-tube heat exchanger with segmental baffles have many disadvantages, such as high pressure drop, low heat transfer efficiency, harmful vibration caused by the shell-side flow normal to tube bundles. When the traditional segmental baffles are used in STHX, higher pumping power is often needed to offset the higher pressure drop under the same heat load. Therefore, it is essential to develop a new type of STHX using different types of baffles to achieve higher heat transfer efficiency and lower pressure drop. Pressure drop and heat transfer are interdependent and both of them essentially influence the capital and operating costs of any heat exchange system. In order to improve the performance for shell-and-tube heat exchanger, heat exchangers with different types of baffles are developed, which have relatively higher heat transfer efficiency and relatively lower pressure drop. Therefore, theoretical and experimental sdudies are conducted to investigate the flow and heat transfer characteristics of longitudinal flow heat exchanger, and combined with the pricinple of heat transfer enhancement in the core flow, the new kinds of heat exchangers can be proposed and investigated.As one of major devices, the rod baffle heat exchangers are wildly used in various industrials, but there has no uniform recongnization for the the mechanism of heat transfer enhancement. Based on the physical and mathematical model for rod baffle heat exchangers, the fluid flow and heat tranfse characteristics are investigated, and the results present that the mechanism of heat transfer enhancement can be interpreted by the pricinple of heat transfer enhancement in the core flow. Besides, studies on defferent section configiuation of rod are carried out, and the results show that the shape of the cross section of rod has a heavy impact for flow and heat transfer characteristics. And the rod with rect shape has high heat transfer coefficient and pressure drop, whereas the rod with circle section has lower heat transdfer coefficient and flow resistance. And with the combination of rect and circle shape of rod, the heat transfer coefficient can be improved.On the basis of pricinple of heat transfer enhancement in the core flow, a new kind of rod baffle heat exchanger with thick and thin rod are proposed and investigated, and CFD technique are used to investigate the performance, and the results present that with the combined structure the heat tranfe present no significant degradation. But the flow resistance can be reduced; as a result, the overall performance can be greatly improved. Besides, a new type of shell-and-tube heat exchanger with a combination of rod and van type spoiler was designed. Corresponding mathematical and physical model on the shell side about the new type heat exchanger are established, and fluid flow and heat transfer characteristics are numerically analyzed. The simulation results showed that heat transfer coefficient of the new type of heat exchanger approximates that of rod baffle heat exchanger, but flow pressure drop is much less than the latter, which indicates that comprehensive performance of the former is superior to that of the latter. Compared with rod baffle heat exchanger, heat transfer coefficient of the heat exchanger under investigation is higher under same pressure drop, especially under the high Reynolds numbers.On the basis of the anasysis of the geometry and flow characteristic, combined with periodical full developed model, an integrated model was made to investigate the performance, and the results are analyzed according to the first low of the thermodymics and the seconde low of thermodymics. And the results show that, there would present some difference for the evaluation results.The mechanism of heat transfer enhancement of flower baffles exchanger need to be deeply investigated. The CFD technique is adopted to study the performance of flower baffle heat exchanger, and a comparision with segmental baffle heat echanger are made. The results show that, under the same Re number, the pressure drop of the flower baffle heat exchanger is 0.45 times of that of segmental baffle heat exchanger, but the heat transfer coefficients are nearly same, as results, the overall evaluation coefficiency is 2.2 times of that of the segmental buffle heat exchanger.On the basis of theoretical studies, a type of shell-and-tube heat exchanger with flowerbaffles is designed, fabricated and tested. The experimental investigation for flower baffles and segmental baffle heat exchanger are conducted, and comparisons of operation performance between the two type heat exchanger are also conducted. The results present that, under same conditions, the comprehensive performance of the new type heat exchanger is 20%~30% higher than that of the segmental baffle heat exchanger. Experimental investigation of heat transfer and pressure drop for the new type of rod baffle heat exchanger are slso conducted, and the experience relation of Nu ,Δp with Re are also obtained based on the experimental data.,The research results can serve as guardline for the design and application of longitudinal flow heat exchangers.更多还原