Quantitative evaluation of the effects of poly-vinyl alcohol on supercooling phenomena of water

The effect of poly-vinyl alcohols (PVAs) on the supercooling of water was investigated experimentally for varying sample volumes and degrees of polymerization and saponification. A statistical analysis was carried out on the experimental results and a quantitative evaluation of the inhibition effect of PVA on release of the supercooled state was attempted by introducing a coefficient representing the inhibition effect of PVA. Experimental results and the values calculated from the statistical analysis agreed well and it was found the statistical analysis could be applied to the freezing of supercooled water containing PVA. Moreover, the effects of the PVA on the release of the supercooled state were evaluated quantitatively, using the statistical analysis. The effect of the degree of polymerization is insignificant but it was found that the inhibition effect of PVA increased with increasing degrees of saponification.     

Pressure effect on the release of supercooled water with dissolved air

It is proposed that vapor has been elucidated to having relation to the release of supercooled water.In this study, considering that the vapor has a relation to dissolved air, a supercooling experiment is performed to test tubes of pure water with three patterns of initial dissolved oxygen (DO_i = 4.1, 7.5, and 12.6 mg L⁻¹), i.e. dissolved air. The initial DO in each test tube is kept to the value as constant as possible during supercooling. And the pressure effect on supercooling release of pure water is observed by measuring supercooling degree and by visualizing bubbles enclosed to the ice after full crystallization of the pure water.From this study, it appeared that the factors such as cooling rate, initial DO and pressure of pure water are related to the release of supercooling (supercooling degree and supercooling time). Moreover, the initial DO was confirmed to the confined bubble in fully frozen ice of pure water.     

The effect of supercooling on ice structure in tuna meat observed by using X-ray computed tomography

Several studies have reported that freezing a homogeneous food such as soy bean curd with deep supercooling (supercooled freezing) results in the formation of many particle ice and homogenous ice structure. However, ice crystal morphology may be affected by the cellular structure of the food. In this study, the ice crystal structure in tuna meat, a cellular food, frozen by the supercooled freezing method was investigated by X-ray computed tomography and compared with ice structures in tuna meat frozen by conventional freezing methods. The results showed that rod-like ice crystals grew parallel with the myofibers, and inhomogeneous ice structures formed in tuna meat frozen by the supercooled freezing method regardless of the degree of supercooling, in contrast to the ice structure in frozen soy bean curds. These ice crystals linked with each other to form rod-like ice structures due to mobility limitations imposed by the cellular structure.     

Effects of electrode materials on freezing of supercooled water in electric freeze control

In order to clarify effects of electric charge on freezing of supercooled water, experiments were carried out with various kinds of electrodes in supercooled water. Water sample was kept in a test tube and cooled down at a constant cooling rate. When the water sample was maintained under a supercooling state, an electric charge was applied to the water sample with a small electric current. The degree of supercooling was measured continuously. Then the degree of supercooling at freezing was determined. Six kinds of materials were used for electrodes. Those materials were Aluminum, Copper, Argentum, Aurum, Platinum and Carbon. It was found that the effects of electric charge were distinct according to the material used for electrodes. The degree of supercooling at freezing was the lowest in the case of Aluminum. On the other hand, the highest value of the degree of supercooling at freezing was obtained in the case of carbon. The reason for the difference in the degree of supercooling at freezing by six materials was discussed.     

有关因素对冰浆制取过程中过冷度影响的实验研究

实验研究了搅拌速度、液浴温度、添加醇类物质和成核剂等因素对冰浆形成过程中过冷度及结冰特征的影响。结果表明,在制冰浆过程中,对水溶液进行搅拌有利于提高水过冷现象的稳定性。在搅拌速度0~100r/min时,不容易出现过冷现象而直接结冰;过冷度随搅拌速度增大而增大,在搅拌速度为200r/min时达到最大值4.35℃,之后过冷度急剧下降。在液浴温度降低情况下水过冷度先明显变大,随后变化不明显。添加乙二醇后,有效的改善了冰晶的细腻程度,在低浓度区(乙二醇浓度6%),过冷度呈小幅上升趋势;在中高浓度区(乙二醇浓度6%),过冷度随着乙二醇浓度的增加急剧的减小。添加成核剂后过冷度明显下降,过冷解除时间提前,与硫酸铁和二氧化硅相比,硼砂成核性较差,但是形成的冰晶具有较好的特性。     

Study on anisotropy of growth rate of ice crystal in supercooled water

The purpose of this research was to investigate the ice growth of a single crystal in three dimensions. Three-dimensional pattern of ice crystal growth in supercooled water was observed using Mach–Zehnder spectro-interferometer. Temperature was varied from −0.3 to −1.6 °C. It was found that the ice crystal began to grow as a single crystal at the tip of the capillary tube and propagated freely in supercooled water. Time variation of the shape of dendrite on a–c plane was obtained. It was found that half parabola fits the shape very closely, and the coefficient of squared term, a, of a quadratic function was calculated. The coefficient, a varied in time but at quasi steady state it was found to be depending mostly upon the degree of supercooling. Furthermore, the growth velocity in c-axis at the flat surface was calculated from the thickness measured. It was found that the velocity in c-axis is independent of the degree of supercooling but depends upon time, in other words, the thickness in c-axis.     

Effect of bubble nuclei on freezing of supercooled water

In order to clarify effects of bubble nuclei on freezing of supercooled water, various kinds of experiments were carried out with invisible sizes of bubbles in supercooled water. Water samples were kept in a test tube and cooled down at a constant cooling rate until the water solidified. The degree of supercooling at freezing was then measured. Two kinds of water surfaces were applied. One was exposure to the atmosphere, and the other was covered with silicone oil. Three kinds of pressure conditions were applied. The first type was atmospheric pressure. The second type was compression up to 6.0 atm. The third type was evacuated down to 0.02 atm. Two holding time periods before starting the experiments were applied. One was 30 min and the other was 24 h. It was found that the degree of supercooling at freezing is high in the case of the free surface compared to the one with oil–water surface. The reason suggested was that the bubbles in the water can be released from the surface in the case of normal atmospheric exposure and trapped at the oil–water interface in the case of water covered with oil. Hence, it was clarified that the freezing of supercooled water is affected by the existence of bubble nuclei.     

Active control of supercooling degree using surfactant (In system with solid–liquid interface)

Supercooling reduces the COP of ice formation systems. So, control of supercooling dissolution is useful and would bring numerous benefits. In a previous report, the authors conducted an experiment to control supercooling dissolution in a mixture with a hydrophilic surfactant and water in a glass test tube which had the liquid–gas(air) interface, and it was possible to actively control average supercooling degree by varying the surfactant concentrations. The average supercooling degrees' tendencies below and over a critical micelle concentration, calculated based on the variation in surface tensions, were quite different. However, since the critical micelle concentration is a physical quantity concerned only with liquid–gas interface, it is essential to investigate phenomena in a system with a solid–liquid interface. So, in a syringe with a system having only a solid–liquid interface, possibility of active control of average supercooling degree by variation in surfactant concentrations is investigated.     

Effects of shapes of electrodes on freezing of supercooled water in electric freeze control

In order to clarify the effects of electric charge on freezing of supercooled water, experiments were carried out. Two kinds of shapes were used for tips of electrodes. One was the sharp end surface. The other was the flat end surface. Aluminum was selected as the material. Water sample was kept in a test tube and cooled down at a constant cooling rate. When the water sample was maintained under a supercooling state, an electric charge was applied to the water sample with a small electric current. The degree of supercooling and the electric current were measured, continuously. Then the degree of supercooling at freezing was determined. It was found that the effects of electric charge were distinct according to the shapes of electrodes. The degree of supercooling at freezing was the lower in the case of flat end surface of the anode than that in the case of sharp end surface of the anode.     

Study on formation of high performance ice slurry by W/O emulsion in ice storage (discussion on characteristics of propagation of supercooling dissolution)

A W/O type emulsion was developed as a new thermal material for ice storage. The water contents of the emulsions were 70, 80 and 90 vol%, and silicone oil was used. An amino group modified silicone oil with 0.9 vol% was used as a surface-active agent. The freezing points of those emulsions were 0 °C. However, due to the emulsion structure, the propagation rate of supercooling dissolution for each emulsion was very slow. Therefore, the propagation rate and maximum supercooling degree were estimated using probability, varying the water content of the emulsion, the method of the ice nucleus charging, and the size and number of ice nuclei. In addition, the influence of various parameters on the propagation rate and maximum supercooling degree was clarified.     

真空搅拌法流态冰制备特性的实验研究

本文设计了一套真空搅拌法流态冰制取装置,对冰浆的生成全过程进行了观测,通过实验研究了乙二醇添加剂、溶液体积、搅拌速度等因素对冰浆形成过程过冷状态的影响。结果表明:真空状态下冰浆的制取需经历溶液蒸发、沸腾、过冷及冰晶生成这四个阶段;冰晶生成瞬间系统的压力会发生突增,压力的升值即过冷结晶压差可用于表征过冷程度的高低;平均过冷结晶压差随乙二醇添加剂浓度的增大而减小,当乙二醇浓度从3%提高至6%时,过冷结晶压差从58.9 Pa降至49.4 Pa,降低了16.1%;溶液体积的增大会使平均过冷结晶压差减小,40 m L溶液的平均过冷结晶压差较20 m L溶液的平均过冷结晶压差下降了12.9%;搅拌速度对过冷程度几乎无影响。     

过冷水动态冰晶制取的实验研究

本文提供了一种过冷水动态制取冰晶的实验装置,着重介绍了系统的设计过程以及各部分的技术要求.为保证系统稳定运行,本实验装置采取了一系列措施,包括冰晶消除、过冷解除以及防冻结策略等.实验采用自来水、纯水、6%的乙二醇溶液作为试样,在不同的冷却速率、速度以及壁面条件下进行了对比实验,并给出了定性分析.     

Continuous ice slurry formation using a functional fluid for ice storage

A functional fluid composed of an oil–water mixture with an additive is transformed into an ice slurry by cooling while stirring. This paper describes a new continuous ice slurry formation method. Experiments were carried out by varying conditions such as the supply time of functional fluid, the stirrer torque, brine temperature and degree of supercooling. As a result, the characteristics of the ice formation and recovery processes were clarified. It was found that the ice particles gradually became uniform in size and spherical, and grew to 3.5 mm in diameter during about 10 h. The factors influencing the size of formed ice particles were discussed because the larger ice particles were expected to melt more rapidly. The ice particle size was found to increase with decreasing degree of supercooling and cooling rate, and with increasing stirrer wing diameter.     

Formation of high performance ice slurry by W/O emulsion in ice storage (effective method to propagate supercooling dissolution)

This study focused ice slurry formation in an ice storage system using W/O emulsions with 70 and 80% water contents. Emulsions consisted of a silicone oil–water mixture with a small amount of amino-group-modified silicone oil additive. Ice slurry was formed by cooling the emulsion without ice adhesion to the cooling wall, as water in the emulsion did not directly contact the cooling wall. As the structure of W/O emulsion slowed the propagation rate of supercooling dissolution, voltage and ultrasonic wave were applied to the W/O emulsion to propagate dissolution more quickly and decrease maximum supercooling degree, respectively. Thus, the effects of voltage and ultrasonic wave applications on propagation rate were clarified.     

超声波对水的过冷度影响的实验研究

对水的过冷度的研究在冰蓄冷和低温生物等许多领域都有重要意义。通过引起压力场的波动，超声波能够对水的过冷结冰过程产生影响。经对800kHz超声波作用下，水的过冷度进行实验研究，发现超声波能够大幅度降低水的过冷度。超声的这种作用与空化现象有关。     

Study on the supercooling degree and nucleation behavior of water-based graphene oxide nanofluids PCM

This article aimed to study the supercooling degree and nucleation behavior of nanofluids phase change material. The nanofluids were prepared by adding small fraction of graphene oxide nanosheets in deionized water without any dispersants. The supercooling degree of nanofluids with different concentrations was tested experimentally. The results show that the supercooling degree can be reduced by 69.1%, and the nucleation was started in advance, shorting the time by 90.7%. Theoretical analysis based on the heterogeneous nucleation theory indicates that ice crystal nucleus cannot grow on the thickness surface of the graphene oxide nanosheet, while it can grow on the top or bottom surface of the nanosheet only when the supercooling degree ΔT and the nanosheets size D meet the formulaD⋅ΔT≥4.2×10⁻⁸. Our study implies that graphene oxide nanofluids have the potential to be used as PCMs in cold storage applications because of their low supercooling degree and rapid nucleation behavior.     

Laboratory experiments on frazil ice growth in supercooled water

Presented herein are the results of a laboratory investigation of the influence of turbulence and water temperature on the nature and rate of frazil ice growth in a turbulent body of supercooled water. The results indicate that the rate and the quantity of frazil ice growth increase with both increasing turbulence intensity and with decreasing water temperature at the instant of seeding, when a small fragment of ice is placed in the supercooled water. The turbulence characteristics of a flow affect the rate of frazil-ice growth by governing the temperature to which the flow can be supercooled; by influencing heat transfer from the frazil ice to surrounding water; and by promoting secondary nucleation, crystal, platelet and floc fracture, thereby increasing the number of nucleation sites available for further frazil ice growth.Larger frazil ice platelets, beginning as single crystals then becoming laminar fusions of crystals, were observed to form in water supercooled to lower temperatures. However, platelet size generally decreased with increasing turbulence intensity, as platelets with a major diameter larger than a certain value tend to break when buffeted by turbulence eddies.The investigation of frazil ice growth involved the use of a simplified analytical model, in which the rate of frazil ice growth is related to temperature rise of a turbulent volume of water due to the release of latent heat of fusion of liquid water to ice. Experiments conducted in a turbulence jar with a heated, vertically oscillating grid served both to guide and to calibrate the analytical model as well as to afford insights into frazil ice growth.     

Modelling the freezing of butter

Butter is a water-in-oil emulsion so its behaviour during freezing is very different from that of most food products, for which water forms a continuous phase. The release of latent heat during freezing is controlled as much by the rate of crystallization of water in each of the water droplets as by the rate of heat transfer. Measurements of the freezing of butter show that the release of latent heat from the freezing water depends on the degree of supercooling, which, in turn, depends on the cooling medium temperature, the size of the butter item, the packaging and the type of butter. Four modelling approaches were tested against the experimental data collected for a 25 kg block of butter. A “sensible heat only model” accurately predicted the butter temperature until temperatures at which water freezing becomes significant were reached. An “equilibrium thermal properties model” predicted a temperature plateau near the initial freezing point of the butter in a manner that was inconsistent with the measured data. A third model used a stochastic approach to ice nucleation based on supercooling using classical homogeneous nucleation theory. The predicted temperatures showed that supercooling-driven nucleation alone is not sufficient to predict the freezing behaviour of butter. A fourth approach took account of time-dependent nucleation and ice crystal growth kinetics using classical Avrami crystallization theory. The relationship between the ice crystal growth rate and the supersaturation was assumed to be linear. The model predicted the experimental data accurately, particularly by predicting the slow rebound in the temperature following supercooling that is found when freezing butter under some conditions.     

Properties of hydrogels synthesized by freezing and thawing aqueous polyvinyl alcohol solutions and their applications

The hydrogel synthesized by freezing a polyvinyl alcohol (PVA) aqueous solution and thawing it slowly has high water content, excellent mechanical properties of high tensile strength, elongation and good shape recovery by elasticity. The PVA used had a degree of polymerization of 2500 and a degree of saponification of 99.5 mol%. The solution was obtained by dissolving 7.5 g of PVA in 80 g of water, this was frozen at –50°C for 3 h and then warmed up to room temperature over 10h. This freezing–thawing process was repeated once again and a hydrogel was synthesized. The hydrogel had a water content of about 90 wt%. Its tensile strength was 0.6 MPa and the elongation at break was 130%. The shape of the hydrogel which was deformed by an external force recovered in a short time when it was released from the force. This recovery had good persistence and repeatability. Applying these properties a strain sensor and a gas pressure sensor were tested. Furthermore, a PVA hydrogel rod containing polyacrylic acid was used as a bending actuator. This hydrogel had the ability to deform when direct current was applied.     

The experimental study on freezing of supercooled water using metallic surface

Freezing of supercooled water on a metallic plate was studied, experimentally. First of all, a gold plated surface was selected as a metallic surface because the surface has very little change in the characteristics of the surface against time. The experiments on freezing under various cooling rates were carried out and the probability of freezing per unit surface area per unit time interval was calculated. It was found that the probability was independent of the cooling rate. Secondly, in order to clarify the effect of oxidation on freezing of supercooled water, an electrolytically polished copper surface was selected and a time variation of the probability of freezing was investigated. A large number of experimental data is required to obtain an accurate value for the probability, but it is impossible to perform it before the characteristics of the surface changes. Hence, in order to cover a wide range of the degree of supercooling at freezing in a short period of time, experiments were carried out under various cooling rates. It was found that the oxidation of the surface restrained the supercooled water on the surface from freezing. By comparing the results with the one for a gold plated surface, a parameter was obtained to express the characteristics of the surface.