【作者】 王伟平；

【导师】 郑津洋；

【作者基本信息】 浙江大学 ， 化工过程机械， 2014， 博士

【摘要】 大型冷箱是化工换热工艺中的重大核心设备,可应用于超大型空分、百万吨级乙烯、大型LNG液化以及大型煤制油等生产装置。由于面临装置大型化与工作条件极端化的发展趋势,冷箱内的流体均配与传热优化已成为其发展的主要技术瓶颈。在浙江省重大科技专项“大型石化冷箱多相流均配与传热优化的关键技术研究”(项目编号：2010C11020)的资助下,本文主要针对大型冷箱内板翅式换热器及其配管系统开展流体均配特性与传热优化研究,探讨了不同工况、结构及介质物性对其内流体分布与流阻的影响,研究了板翅式换热器及其配管系统内工况、流体动力学特性以及传热性能之间的相互耦合作用,并分析了流体分配不均对单台及多台并联换热器组能耗与换热效率的影响,最后提出了相应的板翅式换热器及其配管系统结构优化设计方案,旨在提高大型冷箱开发效率与产品性能,对实际大型冷箱设计、加工及制造具有一定指导意义。本文主要研究内容与创新如下：(1)结合多孔介质理论,构建了基于多孔介质的冷箱换热器及其配管系统数值计算模型,通过模拟计算获得了多孔翅片与板翅式换热器对应的分布阻力系数。针对此模型与初步模拟结果,设计并搭建了冷箱换热器及其配管系统流体分配测试台,主要创新在于提出了基于多元并测结构的板翅式换热器两相分离与测量方法,适用于分析不同工况条件下(如流速、气液比)对冷箱换热器及其配管系统内流体分布与流阻的影响。(2)分析了进口雷诺数、通道翅片及介质粘度对板翅式换热器内流体分布与流阻特性的影响,并获得了板翅式换热器内流体分配不均匀度与压降、进口雷诺数的关联式,开展了流场与温度场耦合作用下板翅式换热器流体分布失稳效应研究。研究表明,当介质粘度与温度间呈负指数关系且在特定雷诺数范围内,由传热引起的介质粘度变化将导致板翅式换热器内流体分布出现失稳。在此基础上,提出了基于通道阻力修正的板翅式换热器流体均配结构优化方案,经模拟结合试验论证,该方案优于传统在封头内添加挡板的方法。(3)研究了稳态与瞬态下多单元并联换热器组的配管系统内流场与压力场分布特性,分析了板翅式换热器与支管结构分别对U型与Z型配管内流体分布和流阻的影响,并揭示了配管系统内的流体均配自适应动平衡效应;深入开展了递增、递减以及脉动等变工况作用下冷箱配管系统内流体瞬态动力学特性。在此基础上,提出了基于动平衡效应及二分法的冷箱配管系统流体均配结构优化方案,经论证,提出的改进方案可较大改善配管系统内流体分布,并且不增加运行能耗。(4)构建了流体分配不均对板翅式换热器工作性能影响的理论模型,并研究了流体分配不均对单台及多台并联换热器组传热性能及能耗的影响；鉴于板翅式换热器传热性能与能耗间的矛盾关系,提出了单台及多台并联换热器传热性能与能耗的评估分析方法。研究表明,对于传热单元数(Ntu)较大的板翅式换热器,流体分配不均程度对其传热效率的影响较为敏感。随着传热单元数、热负荷以及进口雷诺数的增加,单台及多台并联板翅式换热器组最佳工作性能对应的压降值趋于增大。更多还原

【Abstract】 As one of the core equipment in the heat exchange process, large-scale cold box is generally applied in air separation, megaton ethylene production, natural gas and coal liquefaction, etc. Given the growing size of the cold box and extreme operating conditions, flow distribution and heat transfer optimization have become the technical "bottleneck" in the development of the cold box. Under the support of the Major Science and Technology Project of Zhejiang Province "Research on the key technologies of the multiphase flow distribution and heat transfer optimization for the large-scale petrochemical cold box"(No.201OC11020), the flow distribution characteristics and heat transfer optimization of the plate-fin heat exchanger (PFHE) and its manifold piping system in the large-scale cold box were studied in this paper. Effects of operating conditions, structural factors and fluid properties on flow distribution and pressure drop of the PFHE and manifold structure were analyzed. The mutual coupling effects of the operating condition, flow hydrodynamic characteristic and heat transfer performance of the studied structures were investigated. Meanwhile, the influence of the flow nonuniformity on heat transfer efficiency and energy consumption of a single PFHE and multiple parallel PFHEs were further studied. Based on the results, corresponding optimization strategies were proposed in order to effectively improve the efficiency and performance of the cold box, which has certain guiding significance for the design, processing and manufacturing of the large-scale cold box. The main research content and innovations are as follows:(1) Numerical models of the PFHE and its manifold system in cold box were established by using the porous media theory, resistance coefficients of the PFHE and its fins were obtained. Based on the models, an experimental platform for measuring flow distribution of the PFHE and its manifold system was designed and implemented. A multiparallel-based flow separation and measurement method for two-phase flow in the PFHE was proposed. Effects of various operating conditions (velocity and gas-liquid ratio) on flow distribution and pressure drop of the PFHE and related manifold system were experimentally studied.(2) Effects of the inlet Reynolds number (Re), channel fins, and fluid dynamic viscosity on flow distribution and pressure drop of the PFHE were analyzed. Based on the results, a correlation among the flow distribution standard deviation parameter (STD), pressure drop and Reynolds numbers was obtained. Meanwhile, the velocity-temperature coupling effects on the flow distribution instability of the PFHE were particularly studied. It was found that when the fluid viscosity has negative exponential relationship with the fluid temperature, the flow distribution instability phenomenon, which was caused by the variation of the fluid viscosity due to heat transfer, was observed at a certain range of Re. Finally, a channel resistance-based optimization strategy for improving flow distribution of the PFHE was proposed, which is proved to be better than the traditional strategy by adding baffle inside the header of the PFHE.(3) Velocity and pressure distribution in the manifold system of the cold box were studied under steady-state and transient conditions. Meanwhile, effects of the PFHE and branch sizes on flow distribution and pressure drop of the manifold structure were investigated, and a phenomenon termed as adaptive dynamic balance phenomenon was observed. Transient hydrodynamics characteristics of the U-type and Z-type manifold structure were further studied under the increment, decrement and pulsation operating conditions. Based on the results, two novel strategies, named dynamic balance-based strategy and the dichotomy-based strategy, for the attainment of flow uniformity were proposed. It was verified that the strategies can greatly improve flow distribution of the studied manifold structure, and meanwhile cause negligible pressure drop variation.(4) A theoretical model was developed for determining the performance deterioration of the PFHE due to flow nonuniformity effect. Based on the model, the effects of the uneven flow distribution on the heat transfer performance and energy consumption of a single PFHE and multiple parallel PFHEs were studied. Given the contradictory relationship between the heat transfer performance and energy consumption, an analysis method to assess the heat transfer performance and energy consumption of a single PFHE and multiple parallel PFHEs was proposed. It was found that for the PFHE with higher number of transfer units (Ntu), its heat transfer performance is sensitive to the flow distribution. As the Ntu, heat load and Re increase, corresponding pressure drop of the optimum performance for a single PFHE and multiple parallel PFHEs tends to increase.更多还原