【作者】 迪玛（Dmitriy S.Likhachev）；

【导师】 李凤臣；

【作者基本信息】 哈尔滨工业大学 ， 工程热物理， 2013， 博士

【摘要】 对淡水持续增长的需求促进了海水淡化技术的不断发展。目前全球约95%的淡水是采用热方法和渗透膜技术这两大技术生产的。然而，这两种方法均存在一定的问题，热方法受限于结垢和传热系数，渗透膜技术则受限于渗透膜的污染和回收系数。大量研究一直致力于通过各种方法对热方法和渗透膜技术进行改进，如采用废热方法以及水处理初期时减少水垢和污垢方法等。然而，现代海水淡化技术则利用较新的物理现象来达到减少水垢、污垢、材料强度、能源消耗、环境影响、劳动力和技术支持等目的。本文主要对海水淡化的热方法进行研究和改进。本文首先对流体在固体壁面沸腾传热的过程进行了分析，认为过热流体在壁面处生成汽泡主要是由于汽泡内部的蒸汽压力大于环境流体压力。在圆管内部流动沸腾传热过程中，热流密度的增大使得汽液两相的流速增加。当传热不依赖于壁面的热流密度时，沸腾过程仅存在于对流区域，这就使得壁面处液体沸腾换热系数的热流指数值减少。因此，对海水淡化热方法的进一步改进需要更高的传热速率。通过大量的调研，发现超空化过程能在一定程度上很好地解决上述问题。早在30年前，Machinski就将锥形超空泡发生器引入到海水淡化的过程中，通过在超空泡区域内加入真空系统抽取超空泡内的低压蒸汽来获取淡化水。超空泡的汽液交界面处的蒸发过程与壁面处液体沸腾过程类似，其传热系数均由热流密度决定。然而，在超空泡蒸发过程中，增加液体的流速能使得超空泡的形态增大，同时也使得超空泡汽液界面的蒸发传热系数增大。因此，对应超空泡过程中液体蒸发换热系数的热流指数值也会相应升高。然而，将超空泡方法应用到工业中的海水净化过程中却遇到了一些问题。这种方法需要使用连续不断的大流量的过冷热源水循环通过包含级联配合的蒸发冷凝模块系统，这个系统的制造需要大量金属，而且超空泡的体积与水的总体积之比非常小，同时该系统采用的是能量密集型的水泵再循环系统，容量控制也十分复杂。为了解决这些问题，本文采用一种新型的旋转超空泡蒸发器（RSCE）来对海水的淡化过程进行研究。在超空泡发生过程中，水和蒸汽之间的高温和较大的速度梯度可以维持快速的蒸发。通常，在两相交界面上只允许水蒸气和溶解在水中的气体通过交界面进入超空泡内。然而，在远离空化器的后方发生的超空泡溃灭将产生非定常的回射流，其液滴会被夹带到抽取的蒸汽中并降低脱盐后的淡水质量。此外，在没有进行蒸汽抽取的情况下，超空泡蒸发器中的蒸汽生产率等于旋转超空泡内部的蒸汽发生率减去由于超空泡尾部脉动和尾涡引起的蒸汽损失，超空泡表面连续的纵向和径向振荡以及超空泡内部的低压将引起回射流并夹带蒸汽进入尾部区域。空化器的旋转运动引起的叶片末梢和轮毂的旋涡也会导致蒸汽的损失。因此，对于海水淡化应用，如何减少这些副作用从而有效回收最纯净蒸汽也是一个研究热点。为了达到以上所述的目标，本文自主研发了RSCE实验装置，对海水淡化新的热方法即旋转超空化蒸发法进行了研究和改进。RSCE的核心部件是具有特殊形状的高速旋转的空化器，用于生成超空泡和抽取的超空泡内的蒸汽。在设计空化器时需要解决以下问题：1）形成最大尺寸和体积超空泡的同时，确保操作的安全且不会造成气蚀破坏；2）蒸汽抽取的开口定位在超空化叶轮叶片的出口边缘上，以确保获得最高纯度的蒸汽。针对第一个问题，设计了具有两个叶片的超空泡旋转叶轮。每个叶片具有变化的出口边缘厚度，从而保证能够生成具有最大尺寸的超空泡，同时沿叶片长度方向在不同半径上均为安全尺寸。通过使用MathCAD-11.0b编程求解得到叶轮的尺寸比例，并根据对楔形空化器流动实验的初步分析实现了对这种叶轮的设计。两个叶片进口边的对称线穿过旋转轴线，并与旋转轴线垂直。由于不同半径处流动的速度矢量均垂直于叶片的进口边缘，因此可以通过楔形空化器流动的计算来进一步优化旋转叶轮。针对第二个问题，通过数值模拟软件对本文设计的叶轮进行模拟来预测其性能。为了得到较为准确的数值模拟结果，本文首先对网格、求解器的选择以及超空泡模拟方法进行了分析，并确定了数值模拟方法。通过大量的数值模拟，分析了蒸汽在空间中的分布、流态和回射流，从而确定了蒸汽抽取开口的合理位置。在数值模拟的基础上，设计和制造了数值模拟中较优工况的超空泡旋转叶轮并进行实验研究，通过多因素实验法分析了RSCE的实际性能。实验中，将真空系统通过中空轴连接至叶片的开口处来获得超空泡产生过程中的蒸汽。整个真空系统由旋风分离器、辅助容器、蒸汽冷凝器、真空泵和管道组成，以实现分离后的液滴和冷凝蒸汽的收集。为了达到叶轮5430转/分的转速，本文还设计了一个能自定义传动比的增速行星齿轮来与转速1440转/分电动机进行连接。由于叶轮处于高速旋转过程中，轴承密封的实现也是使得实验顺利进行的关键因素。RSCE实验系统还包括温度、压力和盐度传感器，以及用于实验记录的高速摄像系统。冷源水（25℃）为质量浓度3.5%的NaCl水溶液。本文选择了Box-Wilson统计方法来对实验工况进行最优设计，由于Box-Wilson统计方法能够通过一个最优的实验设计方法来使得在最少的实验次数的前提下获得统计上有效的结果，且允许将收集到原始仪器的数据和实验照片代入解析回归方程进行处理。因此本文选用这种方法来对实验次数进行最优化设计。在实验数据处理方面，自行设计了计算方法对实验获得的图像和其他数据进行处理，并基于软件，分别采用具有10个和6个参考半径的径向网格将叶片分成9和5等份，以更准确和方便地获得沿着叶片半径方向超空泡长度的分布。为了进一步验证实验的准确性，将由解析回归方程得到的转速和蒸汽发生率对叶片半径方向上超空泡长度分布的影响与数值模拟软件计算得到的结果以及经验公式进行了对比。在分析结果之前，本文对与RSCE研究直接相关的现有的前人研究结果进行了综述。超空泡是由流动惯性引起的高速蒸汽所产生，同时，由于超空泡内的蒸汽压力可能远低于平衡压力，从而进一步增加了超空泡内蒸汽的生成率。较长的超空泡显著地削弱了卷吸效应，同时也带来了更大的蒸发界面。由于在蒸发过程中是直接从源水中吸收潜热的，因此没有传统换热表面的热阻，从而具有较高的热质交换效率。理论上来说，两相交界面上的边界层可用于无水垢传热和无污垢矿物质去除，因此，超空化蒸发方法可以消除水蒸发过程中由固体导热壁面传热引起的低能量密度、水垢和污垢。通过采用ANSYS CFX-13.0软件进行数值模拟，计算获得了超空泡和周围流场的三维定常结构。基于模拟结果，选择叶片出口边缘区域作为蒸汽抽取的开口，此处的水蒸汽分数达到最大。此外，强制抽取蒸汽的模拟研究揭示了超空泡中蒸汽的不同流动状态可以减小蒸汽损失区域的体积。多因素实验结果揭示了冷凝液的盐度、超空泡内部蒸汽的温度以及超空泡的尺寸与蒸汽抽取率和叶轮转速之间的关系。较低叶轮转速下的实验结果表明该类型叶片形状能产生超出预期的超空化效果。同时，实验中也获得了较好的蒸汽抽取效果，表明蒸汽抽取开口位置设计是合理的。随后，基于实验结果，通过采用统计学上有效的回归方程获得了超空泡形态与转速和蒸汽抽取速率之间的经验关系。RSCE具有所有固定式超空化锥形蒸发器所具备的优点，同时也能在最小的工作容积内产生连续的蒸发过程。由于本实验设计的RSCE用于在大气压力下海水中运行，因而该装置只需要耐腐蚀金属制成的驱动器，对壳体的要求较低。此外，实验过程中也无需用到水质预处理的耗材。因此该RSCE系统具有很好的经济性及应用前景。本研究的理论贡献包括以下方面：1）提出了具有出口边厚度随半径变化的楔形叶片的超空泡旋转叶轮，因此能够在一个旋转平面内控制超空泡的尺寸；2）证明了通过楔形空化器形成的超空泡尺寸的经验公式来设计超空泡旋转叶轮的合理性；3）提出了控制旋转超空泡的尺寸的设想，研究了从超空泡中抽取蒸汽时的流动特性和热物性参数；4）提出利用旋转超空泡的最大空化研究部分相容性液体的混合、以及液体中含有固体颗粒时的混合效果，分别获得稳定和优良的乳化液和悬浮液；5）在从超空泡抽取蒸汽的过程中观察了稳定流动中的旋转空化器上叶片末端和轮毂处的旋涡。6）当空泡中的压强与通道内的蒸汽压强相平衡时，观察了两个叶片处生成的旋转超空化现象。本研究的实用创新性包括以下方面：1）设计的超空泡旋转叶轮，在旋转平面内能形成有最大体积的超空泡，同时对叶轮的气蚀破坏程度最小；2）设计的高速空心轴可以实现对介质的抽取或注射，并采用压紧螺栓和夹紧垫圈来实现轴的延伸，从而能够平衡和可靠地安装不同的旋转叶轮；3）采用MathCAD编写算法计算了旋转叶轮的形状及其形成的超空泡的尺寸；4）根据超空泡旋转蒸发器形成的超空泡的参数，对其数学模型采用ANSYS CFX-13.0软件进行了数值求解，验证了实验数据；5）设计和制造了高速旋转的超空化装置以及用于蒸汽抽取的真空系统；采用测量仪器监测了蒸汽温度、冷凝水盐度、蒸汽抽取率和抽取的真空压力；采用高速摄像机实现了旋转超空泡的可视化。本研究的理论价值包括以下方面：1）以回归方程的形式确定了超空泡旋转蒸发器的水动力学特性和热物性之间统计学上有效的经验关系；2）根据现有的大规模工业设备和最新的研究文献，提出了具有前景的海水淡化方法和技术；3）描述了Rayleigh-Plesset空化模型对超空泡动力学特性和热物性的建模具有更高的精度；4）进行了多因素极值实验，从而得出了在旋转超空泡中所观察到现象的回归方程。本研究的实用价值的研究包括以下方面：1）旋转空化器在工业应用中的运用，如海水淡化和脱氧；2）在初步提出的旋转空化器中，采用低温泵来实现流动的冷却和初始旋转流的适用性；3）采用旋转空化器生产高度均一、稳定且良好分布的高品质悬浮液和乳化液，可应用于热电、化学工程和建筑材料工程。更多还原

【Abstract】 Growing demand in the fresh water has led to development of both conventional and modern seawater desalination methods. The thermal and membrane methods are today’s two major technologies covering about95%of worldwide fresh water production. However, the former is limited by the scale formation and thermal transfer coefficient; and the later by the membrane fouling and the recovery factor. Research and development has been continuously improving these major technologies, with the goal to develop the desalination method, which can use the waste heat, and also maintain the scaling and fouling free operation only supported by primary water treatment. The modern desalination methods use the previously unemployed physical phenomena to reduce scaling, fouling, material intensity, energy consumption, environmental effects, labour and technical support. We are interested in research and improving of the thermal desalination method.We have analyzed the boiling of liquid during the heat transfer from the wall, the process of overheating of a liquid to generate steam as a result of the phase change due to increase of the vapour pressure inside the bubble above the ambient liquid pressure. In case of liquid boiling inside the pipe, growth of heat-flux density increases the velocity of two-phase flow. This limits boiling to persist only in its convective regime, when heat transfer is not depended on heat-flux density on the wall. This decreases the exponent value of heat-flux in coefficient of heat transfer for liquid boiling on the wall. Therefore, the process with higher heat transfer rate is required for further improvement of the thermal desalination method. We have surveyed the literature on the subject and found that the process of supercavitation meets our requirements.Machinski30years ago has introduced the stationary supercavitating cone evaporator with ability to create a relatively stable steam generating interface, so called supercavity, between liquid water flow and the low pressure steam volume, which can be connected to vacuum system for steam extraction. The heat transfer coefficient of evaporation from the surface of supercavity also depends on the heat-flux density; the same as for boiling of liquid. However, during supercavitating evaporation, increase of flow velocity results in growth of the steam generating surface of supercavity, keeping heat transfer rate to rise. Therefore, the exponent value of heat-flux in coefficient of heat transfer for liquid evaporation during supercavitation will be comparatively high. However, the industrial application of this evaporator for water desalination requires continuous high-volume recirculation of the subcooled hot source water through the entire system of the cascading matching evaporation-condensation modules. This scheme is metal-intensive and the supercavity volume to the water bulk volume ratio is very small; it also uses energy-intensive pump recirculation system; and the capacity control is complicated. Therefore, we have made efforts into research and development of the device, called rotational supercavitating evaporator (RSCE) to eliminate these shortcomings.During supercavitation the high temperature and velocity gradients between water and steam maintain the rapid evaporation. The interphase boundary only let water steam and dissolved gases pass inside the supercavity. However, supercavity collapsing far behind the cavitator produces unsteady backward jet of the source water, and its droplets can be entrained during extraction of steam, thus reducing the quality of the desalinated water. In addition, without extraction of steam, the steam production rate of the supercavitating evaporator is equal to steam generation rate inside the supercavityless the steam loss through its pulsating end and trailing vortices. Continuous longitudinal and diametrical oscillations of the supercavity surface and low pressure inside the supercavity cause whirling backward water jet, which entrains steam downstream through the tail region. In RSCE, the steam is also lost through the tip and hub vortices generated by rotational movement of the cavitator. Therefore, for desalination application, the research of the ways to reduce these side effects for effective retrieving of the most pure steam is topical.Achieving above-stated goals we have researched the new thermal desalination method–rotational supercavitating evaporation–on the experimental facility designed from scratch.The major component of RSCE is the specially shaped high-speed rotating cavitator. This cavitator have been designed for both generation of supercavity, and extraction of steam from the supercavity volume. These requirements have raised the following problems:1) formation of the supercavity with maximum dimensions and volume, which ensures safety operation without cavitation damage;2) positioning of the steam extraction openings on the supercavitating impeller blade’s exit edge that ensures highest purity of steam extracted.The first problem, has led to development of the rotational supercavitating impeller with two blades. Each blade has the alternating thickness of the exit edge for generation of the supercavity with maximum, but safe dimensions on each radius along the blade’s length. This design has required programming of a solver in MathCAD-11.0b to get the proportions of the impeller, and has been based on preliminary analysis of experiments with flowing wedge-shaped cavitators. Symmetric alignment of the entrance edges of the two blades in line passing through the rotation axis, and laying in plane, perpendicular to the rotation axis. This technique has allowed use of calculations, made for flowing wedge-shaped cavitators, for designing of the rotational impeller; because flow velocity vectors are also perpendicular to the entrance edges of two blades on each radii.To find out the solution of the second problem, the predicted performance of newly designed impeller has been verified by industrial standard numerical simulation software. The preliminary mesh study, careful choice of the solver, and use of the best practices for supercavitation simulation have been combined to get the most accurate results. The spatial distribution of the steam, its flow regime, and backward water jet action has been analysed for reasonable positioning of the openings for steam extraction. To find out the accuracy of the above stated idealized calculations, we have designed and manufactured the experimental facility for experimental investigation of the rotational supercavitating impeller. The real performance of the RSCE have been tested during the multiply factor experiment. The extraction of steam requires connection of the steam volume of supercavity with the vacuum system through the specially designed hollow shaft. The vacuum system consists of cyclone separator, auxiliary vessels, steam condenser, vacuum pump, and piping for collecting of separated water droplets and condensed steam for analysis. The major effort has been put into reliable and leakless bearing assembly working with angular velocity of5430rpm. We have designed a custom multiplying planetary gear to achieve this high rotation speed while driving the shaft by the standard1440rpm electric motor.The experimental facility has been equipped with temperature, pressure and salinity sensors; and high-speed camera for photography. The cold (25°C) source water salinity has been35ppt totally dissolved solids. We have chosen the Box-Wilson’s statistical method for experimental planning, because it requires a minimum number of experiments and gives statistically valid results; and allows processing of the collected instrumentation indications and the data from photography, into analytical regression equations. Manual image analysis and automatic processing of the experimental data have been done by the in-house designed algorithms. The photography of blades has been software-based divided into9and5equal segments by the radial grid giving10and6reference radii respectively. The reading of distribution of the supercavity length along blade’s radii has been made more convenient and accurate depending of this grid.For drawing the valid conclusions, we have compared the results about influence of the rotation speed and steam extraction rate at the distribution of the supercavity length on the blade’s radii obtained by analytical regression equations, with the results calculated on the industrial standard numerical simulation software; and empirical equations.Prior to reporting of the results, we would like to present the following relevant information revealed during the literature survey that have been confirmed studying the RSCE. The supercavitation is caused by flow inertia and, during the high rate of steam extraction the steam pressure inside the supercavity may be much lower than equilibrium pressure, thus increasing the rate of steam generation. The longer supercavity considerably reduces the entraining effect and also gives larger evaporation surface. The higher heat-mass exchange is obtained by eliminating thermal resistance induced by conventional heat transfer surface, because evaporation process takes the latent heat directly from the source water. Literally, the two-phase boundary layer both operates as the scale free thermal transfer and fouling free demineralising mediums. Therefore, supercavitating evaporation method eliminates the low energy intensity, scaling and fouling associated with the heat transfer through the solid heat-conducting wall for evaporation of water.The solutions obtained in ANSYS CFX-13.0environment, has revealed the3-D steady structure of the supercavity and the ambient flow. Basing on this data, the area of the blade’s exit edge, where the water steam fraction has a maximum, has been considered for location of steam extraction openings. In addition, simulation of forced steam extraction has revealed the different flowing patterns of steam inside the supercavity that reduce the volume of the lost partitions of steam.The multiply factor experiments has revealed the salinity of the condensate; the temperature of steam inside the supercavity; and dependence of the shape of supercavity; on the rate of steam extraction and rotation speed of impeller. The shape of impeller, and the expected supercavitating effects it generates, has been confirmed by experimental results surpassing our expectations–at the much lower rotation speed. The design of the steam extraction openings has been approved by satisfactory performance during steam evacuation. The empirical dependencies of the shape of supercavity on rotating speed and the rate of steam extraction has been obtained in form of statistically valid regression equations.RSCE has all the advantages proper for stationary supercavitating cone evaporator, but also naturally maintains continuous evaporation process within a minimal working volume. Designed for operation with water under atmospheric pressure, this device requires only actuator made of incorrodible metal, while casing uses much less expensive materials. Also there is no need for the consumables for pretreatment of water during operation.Theory contribution of the research includes the following statements:1) the idea of the rotating supercavitating impeller with the wedge-shaped blades which exit edge has an alternating thickness along radii for control of the dimension of supercavity in a plane of rotation; 2) applicability proof of the empirical formula for calculation of the supercavity dimensions, generated by flowing wedge-shaped cavitator, for designing of the rotational supercavitating impeller;3) the idea to control the rotational supercavity dimensions; hydrodynamic and thermal-physical parameters of the flow by extraction of steam from supercavity;4) the idea to use the rotational supercavity with maximum cavitation influence for mixing of liquids of partial miscibility; and solid particles with liquids, for production of stable and fine emulsions and suspensions respectively;5) observation of tip and hub vortices on the rotational supercavitator for steady flow and during the steam extraction from the supercavity;6) observation of the rotational supercavities generated by two blades, while the pressure inside the volumes has been equilibrated by the steam extraction channel.Practical novelty of the research includes the following statements:1) design of the rotational supercavitating impeller, which develops supercavity with maximum possible volume in a plane of rotation, while inducing minimum cavitation damage to the impeller;2) design of the high-speed hollow shaft allowing either extraction or injection of a medium, and possessing a shaft extension with a hold-down bolt and a clamping washer for balanced and reliable mounting of the different rotating impellers;3) in-house algorithm for calculation of the rotational impeller’s shape and dimensions of the supercavity it generates written in MathCAD-11.0b;4) numerical solution of the mathematical model made in ANSYS CFX-13.0based on the parameters of supercavity formed in RSCE for verification of experimental data;5) design and manufacturing of the high-speed rotational supercavitating facility with vacuum system for steam extraction; measurement instrumentation for monitoring of steam temperature, condensate salinity, steam extraction rate, and vacuum pressure of extraction; and high-speed photography for visualizing of the rotational supercavity.Theoretical value of the research includes the following statements:1) formulation of the statistically valid empiric dependencies between hydrodynamic and thermal-physical characteristics of rotational supercavitating evaporator in form of regression equations;2) suggestion of the most promising desalination methods and technologies basing on the review of the large-scale industrial facilities and state-of-the-art research and development publications;3) demonstration of higher accuracy of the Rayleigh-Plesset cavitation model for modeling of the hydrodynamic and thermal-physical characteristics of the supercavity; 4) planning of the multiply factor extremal experiment, and handling of experimental data to derive a regression equations of processes observed during the rotational supercavitation.Practical value of the research includes the following statements:1) use of rotational cavitator in industrial applications such as desalination and deaeration;2) availability of the proposed rotational cavitator as the first stage of a cryogenic pump for cooling and preliminary swirling of the flow;3) availability of the proposed rotational cavitator for production of the highly uniform, stable and fine suspensions and emulsions with improved qualities for the thermal power, chemical, and construction material engineering.更多还原