【作者】 蒋夫花；

【导师】 邓先和；

【作者基本信息】 华南理工大学 ， 化学工程， 2011， 博士

【摘要】 管壳式换热器具有机械密封性好、承压能力强等特点,是动力、能源、冶金、硫酸工业等行业的关键通用设备。换热器热流体与冷流体的出口温度比例α(αT_H2 T_L2)可表征换热器的换热深度。当热流体的出口温度低于冷流体的出口温度即α＜1时,换热器进入深度换热状态。工业中有一些场合需要冷热流体作深度换热,例如在硫酸生产转化系统中SO₂ /SO₃气体的换热,在乙烯或炼油系统中的冷热油料换热等。近些年来随着硫酸工业生产规模的扩大,转化工序中换热设备也越来越大型化。换热器大型化过程中,长径比（L/D）锐减的同时也伴随出现了深度换热难以实现的问题。在超大型换热器中采用壳程多通道结构是改善深度换热受限的有效方法,但其内部速度分布尚不清楚,本论文将对此进行研究。壳程多通道管壳式换热器相当于一个由若干个长宽比为L/W的并列分置管壳式换热器组成的换热器网络。在壳程多通道超大型管壳式换热器中,深度换热之所以得以实现是由于并列分置管束管壳式换热器的L/W远大于超大型管壳式换热器L/D,因此研究L/W与深度换热的关系很有必要。在本论文中,根据几何相似原理抽取壳程多通道超大型管壳式换热器中具有代表性的并列分置管束单元流路区域进行数值模拟研究,运用大型计算流体力学商业软件FLUENT研究了长宽比对并列分置管束单元流路区域模型的深度换热性能的影响,给出了并列分置管束单元流路区域的速度场分布,由于并列分置管束单元流路区域与壳程多通道大型管壳式换热器的壳程流路具有极高的相似性,所得结果对了解壳程多通道超大型管壳式换热器的壳程流路有意义。壳程多通道超大型管壳式换热器中可看作是由并列分置管束单元流路区域模型组成的换热器网络。本论文根据并列分置管束单元流路区域模型制造了5个并列分置管束管壳式换热器。运用实验与数值模拟相结合的方法对5个并列分置管束管壳式换热器的传热与流阻性能进行了研究。实验结果与数值模拟结果具有很好的一致性。建立了并列分置管束管壳式换热器实验平台,对实验装置与测试系统的可靠性进行了系统分析,得到了不同L/W的管壳式换热器的总传热系数,总结了L/W对管壳式换热器深度换热性能的影响规律。换热器的深度换热性能与长宽比L/W密切关联,随着换热器L/W的减小,换热器壳程流场分布越来越不均匀,换热器性能下降剧烈并且壳程压降急剧增大。在管壳程平均流速均为10 m·s^-1时,冷热流体在长宽比L/W≥4.62的并列分置管束管壳式换热器可以进行深度换热,而在长宽比L/W≤3.08的并列分置管束管壳式换热器中不能产生深度换热。以温差场均匀性原则为指导,从管程与壳程温差场的协同关系角度分析了换热器性能随L W减小而下降的机理。将并列分置管束管壳式换热器划分为若干个微元,每个微元均可看作一个子换热器,在子换热器中,冷热流体的特征（平均）温度分别为t h和t c,所以对于每一个子换热器都存在着一个冷热流体温度差H ,从而在整个换热器中形成冷热流体的温差场H（x,y,z）=t_h（x,y,z）-t_c（x,y,z）冷热流体温度场间的搭配,即温差场的特性本质决定了换热器的性能。冷热流体的温度场都是空间的函数,它们的函数形式越接近时,它们的协同就越好。在本论文中将并列分置管束管壳式换热器分为若干个子元素,通过数值模拟的方法将其内部温差场以二维图形式直观显现、计算了5个并列分置管束管壳式换热器的温差均匀性因子,定量表述了其温差场的均匀程度,经过分析得出结论:换热器深度换热性能随L/W的减小而下降是因为换热器温差场的均匀性随L/W的减小而下降,这与过增元院士提出的温差场均匀性原则是相符合的。换热效率不仅取决于冷热流体的进口温度差和传热单元数,还取决于冷热流体的流动形式（顺流、叉流、逆流）。逆流换热器之所以具有最高的换热效率是因为它的传热温差场最均匀,即同等条件下,错流、顺流换热器相对于逆流换热器温差场的不均匀程度反映了错流、逆流换热器中传热温差相对于逆流换热器传热温差的损失程度。在本论文中采用流路分析的方法对管壳式换热器壳程进出口折流区域的传热温差进行研究,并比较了不同换热深度条件下折流较纯逆流换热的传热温差损失。经数学分析知,为确保换热器能发生深度换热,应使换热器传热温差较纯逆流的偏移量小于5%,这可以通过控制折流区域面积占总传热面积的比例小于0.6/R_1a,c来实现, R_1a,c为临界点逆流冷流体出口、进口温差与平均温差之比。壳程多通道管壳式换热器已应用于工业生产系统中,换热效率较高,取得了很大的经济效益。本论文采用流道分区方法对壳程多通道管壳式换热器进行简化,在实验室条件下完成了其壳程流路分析,给出了其内部速度分布;通过实验和数值分析的方法研究了壳程多通道管壳式换热器的子换热器——并列分置管束管壳式换热器的L/W与其深度换热性能的关系,给出了L/W在何种范围内的并列分置管束管壳式换热器能发生深度换热;以温差场均匀性原则为指导进行了机理分析;采用流路分析的方法对不同换热深度条件下壳程折流区域较逆流的传热温差损失进行研究,给出了折流区域占总壳程面积的合理比例,这为壳程多通道超大型管壳式换热器的设计提供了理论依据,在工业应用中具有参考价值。更多还原

【Abstract】 Shell-and-tube heat exchangers （STHXs） are the most important general equipments widely used in power, energy, metallurgy, chemical industries, etc. In STHXs, the ratio, ? （outlet temperature of hot fluid to that of cold one） indicates heat exchange depth. When outlet temperature of hot fluid is lower than that of cold fluid, ? ? 1 deep heat transfer is achieved. Deep heat transfer state is meaningful in some industries; for example, in sulphuric acid manufacture transforming system, ethylene production system, and distillation system. In sulphuric acid industry, STHXs have a trend to be super large as the production scale is getting larger. L/D （ratio of length to diameter） of super large STHXs is getting smaller because the length of STHXs （L） is unchanged but the diameter of STHXs （D） is getting larger. However, deep heat transfer is getting more and more uneasy to be achieved as L/D is smaller and smaller. Setting multi-parallel-channel structure （MPC） in the shell side of super large STHXs is a way to solve the problem. Super large STHX with MPC in the shell side can be seen as a heat exchanger network composed of many parallel-tube-bundle STHXs. In super large STHXs with MPC in the shell side, deep heat transfer can be achieved because of L/W of parallel-tube-bundle STHX is large though L/D of super large STHXs is small. It is necessary to study on L/W and deep heat transfer characteristics of parallel-tube-bundle STHXs. Super large STHXs with MPC in the shell side have been applied in industries, but velocity field in the shell side is unclear. In this thesis, velocity field in the shell side of super large STHXs with MPC also will be studied.In this thesis, super large STHXs with MPC in the shell side can be divided into a lot of parallel-tube-bundle by adding clapboards in the shell side. Based on geometric similar principle, a classical parallel-tube-bundle flow path in the super large STHXs with MPC in the shell side is studied by numerical analysis to get the velocity field in the shell side. As it is similar to the flow path in the shell side of super large STHXs with MPC in the shell side, the research results are meaningful to show the velocity field in STHXs with MPC in the shell side. The influence of L/W of parallel-tube-bundle flow path to deep heat transfer characteristic is also studied by computational fluid dynamics commercial software FLUENT.Super large STHXs with MPC in the shell side can be seemed as a heat exchanger network composed of a lot of parallel-tube-bundle flow path models. Each parallel-tube-bundle flow path model can be seemed as a parallel-tube-bundle STHX. Five parallel-tube-bundle STHXs are made according to the parallel-tube-bundle flow path model. Experimental and numerical study on the five STHXs with different L/W has been done to reveal the influence of L/W on fluid distribution, heat transfer and resistance performances in the shell side. The experimental platform is set up and reliability analysis is carried out on the equipments and test system. Hot air flow in the tube side and cold air flow in the shell side of parallel-tube-bundle STHXs. Numerical results have a good agreement with experimental results. Inlet temperature of hot air in five STHXs is the same, also is the inlet temperature of cold air. The total heat transfer coefficients are obtained. The influence of L/W on deep heat transfer performance in STHXs is summarized. The results show that in the condition of average velocity is 10 m·s^-1 both in shell and tube side, velocity distribution in shell side is more and more uneven with L/W decreasing. At the same time, heat transfer performances decline sharply and pressure drop in shell side increases dramatically with L/W decreasing. Deep heat transfer can be achieved in parallel-tube-bundle STHXs when L/W≥4.62. But in parallel-tube-bundle STHXs with L/W≤3.08, deep heat transfer performance can’t be achieved any more.Temperature field uniformity principle is used to analysis the mechanism of heat transfer performance decline with L/W decreasing. A parallel-tube-bundle STHX can be divided into a lot of sub-elements. This is equivalent to take a whole heat exchanger as a heat exchanger network composed of several sub-STHXs. In sub-STHXs, there is a characteristic hot fluid temperature （ t_h） and a characteristic cold fluid temperature （ t_c）, and their difference is named local characteristic temperature difference （ H ）. The aggregate of these local characteristic temperature differences forms a temperature difference field （in short: TDF） of the STHX H（x,y,z）=t_h（x,y,z）-t_c（x,y,z） Heat transfer performance of STHXs is determined by the synergy between hot fluid temperature field and cold fluid temperature field, that is, temperature difference field. Hot and cold fluid temperature is a function of space. The closer of their functional form, the better synergy they are. In this thesis, parallel-tube-bundle STHXs are divided into a lot of sub elements. By this method, the internal temperature difference field is shown in the form of 2D figures. In order to quantitatively describe the uniformity of temperature difference field in a STHX, a factor named the uniformity factor of TDF is defined. After analysis, conclusions are obtained: the inherent reason why heat transfer efficiency decreases with L/W decreasing is that the uniformity of temperature difference field decreases with L/W decreasing. It is consistent with the principle of uniformity of temperature difference field proposed by GUO.Heat transfer efficiency depends not only on numbers of transfer units and temperature differences between both the inlet t and outlet temperature of hot fluid and cold fluid, but also on the fluid flow pattern （co-current flow, counter-current flow, cross flow）. Countercurrent heat exchanger has the highest heat transfer efficiency depends on the most uniform field of heat transfer temperature difference compared to co-current and cross heat exchangers. At the same condition, compared with TDF in the counter-current flow pattern, the degree of uneven of TDF in cross flow or co-current flow reflects the loss of temperature difference. In this thesis, heat transfer temperature difference in baffle flow pattern is compared with that in counter flow pattern in terms of flow path distribution analyses. The deviation of heat transfer temperature difference between the two flow patterns have been analyzed at differentα. In order to make the whole heat transfer temperature difference in STHXs lose under 5% forα<1, the ratio of baffle flow area to the whole area of heat exchangers should be smaller than 0.6 / R_1a,c and R_1a,c is R₁ at critical point of deep heat transfer in countercurrent flow pattern.Super large STHXs with MPC in the shell side have been applied in industrial production systems, and have made great economic benefits by the higher efficiency. In this thesis, velocity field in the shell side of super large STHXs with MPC in the shell side is studies based on geometric similar principle. Five parallel-tube-bundle STHXs have been made to do experimental study on the influence of L/W to deep heat transfer characteristic. The range at with L/W deep heat transfer can be achieved is obtained based on the experimental and numerical study. The mechanism of heat transfer performance decreases with L/W decreasing is analyzed on the basis of the uniformity principle of temperature difference field. Based on flow path analysis research, the deviation of heat transfer temperature difference between baffle flow area and counter flow area for differentαis given. In order to achieve deep heat transfer, the reasonable ratio of baffle flow area to the whole heat transfer area has been given. Research work in this thesis provides a theoretical reference for design of super large STHXs with MPC in the shell side, and is useful in industrial applications.更多还原