【作者】 苏畅；

【导师】 吴双应；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2008， 硕士

【摘要】 积垢或污染是自然界中一种极为普遍的现象,也是环境中的一个重要问题之一。污垢的存在给换热设备带来了一系列的经济损失,从而成为换热设备设计、运行、维护的一个重要方面,因此,分析污垢对能量传递过程的影响就显得尤其重要。目前,对于污垢对传热过程的影响,大多认为污垢沉积在固体表面,增加了传热过程的导热热阻,然而这一观点在对污垢的沉积达到稳定后是适用的,但是在污垢的形成阶段,由于存在污垢组分的传质扩散,而传质对传热是有影响的,但这种影响在污垢的研究中常被有关文献忽略。另外,在污垢形成的初始阶段,常出现换热增强的现象,这一现象已于上个世纪被一些学者通过实验证实,对这一现象的解释,通常认为在初始阶段由于污垢表面粗糙度的变化所引起的,而由Ceylan等人的实验结果表明,污垢层的粗糙度对传热的增强并不有效,所以需要从新的角度对这一现象加以理论解释。本文引入了热质传递的薄膜理论,结合Kern和Seaton提出的渐近污垢模型,研究圆管和外掠平板流动污垢形成阶段的对流换热特性,得到了在考虑结垢传质条件下圆管和外掠平板流动对流换热系数的计算式。同时分别讨论了圆管和外掠平板流动污垢对对流换热效果作用的影响因素。并把结果与不考虑污垢的清洁换热面情形做了对比,从理论上解释了结垢初期换热增强的现象。此外,本文首次提出了圆管内临界污垢层厚度的概念,得到了临界污垢层厚度的表达式,分析了临界污垢层厚度的影响因素,并从临界污垢层厚度的角度对结垢初期换热增强的现象做了新的理论解释。另一方面,目前对换热设备最佳清洗周期的确定常采用污垢影响传热量多少的方法,然而换热设备在进行热量传递的同时,也在进行火用的传递,而从能量的质量角度来计算最佳清洗周期,目前还未见有关文献报道。本文引入了换热器的传火用有效度和传热有效度,对换热器进行了热经济性分析,研究了换热器的清洗周期,得到了换热器的总费用表达式,以此可以得到换热器的最佳清洗周期。并把结果与传统上确定的换热器最佳清洗周期的结果进行了比较,分析了换热器最佳清洗周期的影响因素,从而为工程上换热器的清洗提供参考。更多还原

【Abstract】 Depositing or contaminant is a very common phenomenon in the nature and also a very important problem in the environment. The existence of fouling can bring heat transfer equipments a series of economic losses, and has become an important aspect of design, operation and maintenance in heat transfer equipments. To investigate the impact of fouling on energy transfer process has become especially important. The existing studies for the fouling neglect the impact of mass transfer on the heat transfer process, especially for the beginning of the forming period of fouling, and only consider the increasing of thermal resistance in heat conduction process subject to fouling. However, mass transfer which can affect the heat transfer process exists at the beginning of forming period of fouling. Moreover, there is a phenomenon that the heat transfer performance at the beginning of the forming period of fouling is better than that of clean heat transfer surface, which has been proved by some experiments. And it is often explained that it is the result of the change of surface roughness due to fouling at the beginning of the forming period of fouling. But the experimental results by Ceylan proved that the duct roughness due to fouling is not effective in the augmentation of the heat transfer. So it needs to find a new approach to explain this phenomenon theoretically. As an extension of the previous works, the objective of this paper is based on the combination of thin film model reflecting heat and mass transfer synchronously with the asymptotic fouling model proposed by Kern and Seaton, to investigate the convective heat transfer characteristics through a duct/on a board at the forming period of fouling. A quantitative expression of the convective heat transfer coefficient through a duct/on a board considering fouling mass transfer process is presented. Meanwhile, the effect factors of convective heat transfer process through a duct/on a board are discussed. And the results are compared with that without fouling, which explains the phenomenon that the heat transfer performance is better than that of clean heat transfer surface at the beginning of the forming period of fouling theoretically. In addition, critical fouling layer thickness is firstly derived in this paper and its equation has been provided. The effect factors of critical fouling layer thickness are analyzed. Also, a new theoretic explanation to the enhancement heat transfer phenomenon is presented in the viewpoint of critical fouling layer thickness.On the other hand, the optimal cleaning cycle of heat exchangers has always been approached from the perspective of the first law at present. However, heat transfer is inevitably accompanied with exergy transfer in a heat exchanger. And determining the optimal cleaning cycle of heat exchangers from the perspective of energy quality has not been reported. This paper presents a comprehensive thermo-economic study of heat exchangers cleaning cycles by introducing the exergy transfer and heat transfer effectiveness. The total cost equation has been derived to determine the optimal cleaning cycle of heat exchangers. In addition, the results are compared with that based on the first law and the effect factors of the optimal cleaning cycle are discussed. The research reported in this paper provides a reference to the cleaning of heat exchangers in engineering.更多还原