| 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. |
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