【作者】 郑毅；

【导师】 马学虎；

【作者基本信息】 大连理工大学 ， 化学工程， 2018， 博士

【摘要】 薄液膜蒸发换热,具有传热温差小、热流密度高、传热系数高、均温性好、结构简单和动力消耗低等优点,被大量应用在石油、化工、制药、海水淡化、航天等领域。其中,水平管降液膜蒸发技术因具有传热效率高、耐结垢以及可利用低品位余热等优点,广泛应用于大型工业生产中,是解决能源消耗大、排放污染物严重等问题的一种高效换热技术。近年来,随着对于环保和节能要求日益提高,利用低流量、小温差实现高效换热已经越来越受到学者的重视。低喷淋密度下的水平管降液膜蒸发,是一个伴随中液滴与液膜相互作用、相互影响的分区特征显著的复杂的界面演化过程。因此,深入认识降液膜蒸发过程中的液滴和液膜的分区特性以及其对传热特性的影响,对进一步探究降液膜蒸发的低喷淋密度传热控制机理和高效传热强化技术的开发具有十分重要的意义。为此本文聚焦于低喷淋密度下水平管降液膜蒸发全过程,通过理论分析,获得低喷淋密度下影响蒸发传热性能的关键因素;结合力平衡模型建立了水平管降液膜最小喷淋密度理论模型;并通过引入部分润湿表面的液膜厚度计算公式,提出低喷淋密度下利用亲水改造实现蒸发传热性能强化的新方法;利用氧化刻蚀法制备了两种润湿性不同的亲水表面:超亲水与一般亲水表面,观察了不同亲水表面的管间流型,测定了最小喷淋密度,最后通过引入了修正因子获得了与实验结果吻合良好的最小喷淋密度关系式。通过数值模拟,建立了普适的超亲水表面三维模型降液膜模型,实现了间断液滴和连续液液柱的准确再现。着重分析了超亲水表面滴状流和柱状流的铺展特性和涟漪波动特性,揭示了马鞍形液膜和相邻液柱撞击区的“相互作用环”形成原因;考察了喷淋Re数对液膜厚度和铺展速度的影响,发现喷淋Re数对滴状流液膜厚度和铺展特性影响不大,而对柱状流影响显著;探讨了三维液膜的轴向和周向液膜厚度演化规律,结果表明在轴向上柱状流液膜呈波谷-波峰-波谷的分布形式,液膜厚度在中间汇聚区取得极值,在整个圆周角变化范围内,液膜厚度都几乎出现了翻倍现象。利用高精度红热像仪对低喷淋密度下超亲水表面水平管降液膜表面的温度分区和波动特性进行了研究,分析了喷淋密度、初始液膜温度以及加热功率对不同流型表面液膜温度分布的影响。结果表明,超亲水表面滴状流液膜温度呈现周期性分布,分为撞击区和非撞击区,撞击区和非撞击位置并不固定,存在明显变化周期,非撞击区液膜温度基本不变,撞击区液膜温度升温明显,且温升主要发生在铺展液膜达到最大后,撞击区内部存在高温环状结构,其比主体温度高0.3-0.6K左右;柱状流液膜温度呈现均匀分布,在相邻液柱交汇区存在明显的低温区分界线,较主体低0.2K左右;处于中间的过渡流型具有两种温度分布—均匀分布和周期分布共存。换热管表面的温度波动周期随喷淋密度和初始液膜温度的升高降低,波动强度随喷淋密度的升高存在一个显著下降区间,其对应流型从滴状流到滴柱过渡流的转换。结合红外示踪技术和统计学分析研究了超亲水表面低喷淋密度多液滴间的空间传递规律。发现液滴的产生并不是随机的,而是存在一定周期性,同时这种典型的周期性并不出现在全部范围内,只是在少数几个特殊流量才存在,重复单元数目随流量的增加而降低;利用能量最小理论和统计学分析获得了形成液滴周期性出现的原因,并给出了相关预测模型,与实验结果吻合良好,并基于此构建非稳态流动下的液膜厚度分布图。最后设计并搭建了水平管降液膜蒸发口流实验平台,考察了润湿性对水平管降液膜蒸发传热的作用机制,研究了喷淋密度、入量、入口温度和饱和温度等对传热性能的影响。结果表明,在低喷淋密度下超亲水表面传热系数高于亲水表面和光滑铜管,且在较宽的范围内具有良好的耐受性,最大约为光管表面的4倍,从而利用超亲水表面实现低喷淋密度下的传热强化,为新型强化换热表面的设计和开发提供了理论指导和实验基础。更多还原

【Abstract】 Thin film evaporation heat transfer process is widely used in petrochemical,light industry,energy,aerospace,electronics and many other fields,due to the advantages of small heat transfer temperature difference,high heat flux density,high heat and mass transfer coefficient,good temperature uniformity,simple structure and low power consumption,.The horizontal tube liquid film evaporation technology is widely used in large-scale industrial production due to its advantages of high heat transfer efficiency,anti-fouling,and easily to use the low-grade residual heat and elimination non-condense gas,etc.It is a high-efficiency heat exchange technology that solves the problems of large energy consumption and serious emission of pollutants.In recent years,with the increasing demand for environmental protection and energy conservation,the use of low flow rates and small temperature differences to achieve efficient heat transfer technology has increasingly attracted the attention of scholars.The evaporation process of the falling film process on the horizontal tube at low spray density is a complex and complicated interface evolution process with the partition features that interact with and interact with each other in the liquid droplet.Therefore,a deeper understanding of the partition characteristics of droplets and liquid films and their effect on heat transfer characteristics during the evaporation process of the falling film has significance influence on further explore the low spray density heat transfer control mechanism and efficient heat transfer enhancement technology for the falling film evaporation process.This article focuses on the falling film evaporation process on horizontal tube under low spray density.Through theoretical analysis,the key factors affecting the evaporation heat transfer performance at low spray densities are obtained.The theoretical model of the minimum spray surface density of the iquid film falling down in the horizontal tube was established by using the force balance model.Moreover,the enhanced method of heat transfer deterioration under low spray density was obtained by introducing the liquid film thickness formula on partially wetted surfaces.Thus,two kinds of hydrophilic surfaces with different wettability are prepared by oxide etching method.The film flow patterns on the tubes with different wettability were observed and the minimum spray density was determined.Additionally,a minimum spray density model of the downhole liquid film with good agreement with the experimental results was obtained by introducing a correction factor.A three-dimensional model was established in numerical simulation for both of drop model and jet model to accurate presence of intermittent droplets and continuous liquid jets in experiments for superhydrophilic surface.The spreading characteristics and ripple fluctuation characteristics of drop model and jet model were emphatically analyzed.The cause of the saddle-shaped film and the formation of the "interaction ring" at the impingement zone for adjacent liquid jets were revealed.The effect of Re on film thickness and spreading velocity was also investigated.It was found that Re had little effect on the thickness and spreading characteristics of the liquid film in droplet model but had a significant effect on the jet model.The evolution of the liquid film thickness in the axial and circumferential directions was discussed.The results show that the liquid film was in the form of a valley-peak-valley distribution for the entire axial direction.The film was thickest at the middle convergence area and nearly double times thicker than the other locations.A high-resolution thermal imaging camera and a high-speed camera were used to study the temperature partition and fluctuation characteristics of the falling film process at low spray density.The effects of spray density,initial liquid film temperature,and heating power on the temperature distribution of liquid film under different flow patterns were analyzed.The result showed that the film temperature in droplet model presented a periodic distribution.The temperature of the liquid film in the impact area rised significantly,while non-impact area was basically unchanged.At the same time,the impact area and the non-impact location were not been fixed,there were changes in a periodic.A high-temperature annular structure had formed in the impact area,which was about 0.3-0.6K higher than the body area temperature and the temperature rise of that region occurd when the spreading liquid film reached the maximum.The temperature of the liquid film in the jet model was uniformly distributed.There was a clear low-temperature boundary between adjacent liquid column junctions,which was about 0.2K lower than the region temperature.The transitional flow is between droplet and jet model provided two kinds of temperature distributions-uniform distribution and periodic distribution.The temperature fluctuation periodic of the heat exchange tube decreased with the increasing of the spray density and the initial film temperature.The intensity of the fluctuation significant decreased as the increasing of spray density.That corresponded to the conversion of the flow pattern from the droplet model to the droplet/jet model.Combined with infrared tracing technique and statistical analysis,the multiple droplets spacing transfer rule under low spray density on superhydrophilic surface was studied.It was found that the production of droplets was not random,but there was a certain periodicity.This typical periodicity did not appear in the entire range,but only existed in a few special flows and the number of repeat units decreased as the flow increases.The energy minimization theory and statistical analysis were used to obtain the reasons for the periodic appearance of the droplet.A correlation prediction model was given and it was in good agreement with the experimental results.Based on this,the distribution of averaged liquid film thickness under for droplet model was constructed.Finally,a horizontal tube falling film evaporation experiment platform was designed and built.The surface wettability on evaporation heat transfer effect of the horizontal tube falling film process was investigated.The effects of spray density,inlet flow,inlet temperature and saturation temperature on the heat transfer performance were investigated.The result shows that the heat transfer coefficient for superhydrophilic tube was higher than that of the hydrophilic tube and the smooth copper tube under lower spray density and offered a better heat patience over a wide range of temperature.The maximum heat transfer coefficient was approximately 3 times higher than that of the smooth surface,thus,heat transfer enhancement at low spray density was achieved.It provides theoretical guidance and experimental basis for the design and development of new enhanced heat transfer surfaces.更多还原