Study on prevention of ice adhesion to cooling wall due to voltage impression in ice storage—discussion on possibility of attraction of oil to wall

For ice storage, one of authors has studied new ice slurry formed by cooling a water–oil mixture with stirring. When the mixture is stirred in a vessel, oil is charged by static electricity due to friction. If the vessel wall can attract charged oil, prevention of ice adhesion to the wall may be realized. Therefore, in this paper, in order to observe behavior of charged water–oil droplet or mixture in electric field by a high speed camera or video camera, two types of experiments were carried out. One was that the water–oil droplet charged by static electricity was made to fall plumb down between two electrodes with electric field or without electric field, varying the water content of droplet. The other was that a constant voltage was applied on the vessel filled with the water–oil mixture stirred. From experiments, it was confirmed that attracting force between the charged wall (electrode) and charged oil acted.     

Ice adhesion of an aqueous solution including a surfactant with stirring on cooling wall: ethylene glycol—a silane coupling agent aqueous solution

Ice adhesion to the cooling wall directly hinders continuous ice formation or system performance due to the increase of flow and thermal resistance. In order to obtain the basic characteristics of two- or three-component aqueous solutions with EG, SCA and water on freezing, the influence of component ratio and supercooling degree on those solutions was investigated through a batch type freezing process of the solutions. Moreover, the strength of ice adhesion was shown to vary by stirring power. Ice adhesion was suppressed when (1) the solution including the additive SCA at comparatively high concentration; (2) the solution had high initial concentration of EG or SCA; (3) the brine temperature was higher; (4) the supercooling degree was comparatively small in the lower concentrations. No ice adhesion occurred at the stirring power below 30 W. Furthermore, particle size of the ice slurry was smaller in higher concentrations.     

Fundamental study on adhesion of ice to cooling solid surface

Many technological troubles are caused by ice adhesion to a cooling solid wall (surface). Therefore, it is urgent to clarify a mechanism of ice adhesion.It is thought that ice adhesion to the cooling wall is governed by heat transfer and interfacial phenomena between ice and the wall. In this study, shearing stress corresponding to adhesion force per unit area to remove ice from the wall surface and some reagents' contact angles on the wall and ice were measured, varying the wall material and its surface state. Moreover, shearing work to remove ice from the wall surface and surface energies of the wall and ice were calculated by the shearing stress and contact angles, respectively. And adhesion energy at an interface between ice and the wall was also calculated by the calculated surface energies. And then, influence of heat transfer and interfacial phenomena on ice adhesion was discussed to clarify the mechanism of ice adhesion.     

Ice storage system using water–oil mixture Discussion about influence of additive on ice formation process

Ice storage is one technique for effective use of thermal energy. So, many studies on slush ice as a thermal storage material have been done. We have also been studying a suspension (slush ice) made from an oil-water mixture by cooling and stirring. From our study results, it was found that an additive having both an amino group (-NH₂) and a silanol group (-SiOH) was essential to form a suspension with high IPF without adhesion of ice to the cooling wall. Moreover, ice particles formed in the suspension were dispersed and granular, and did not stick to each other. In the present paper, we carried out experiments to clarify the characteristics of the suspension formation process. From a thermal analysis of the substance formed in the suspension by difference scanning calorimeter (DSC), it was found that the substance was not ice but a compound of ice and additive. Then, at a very small depression of freezing point (about 7°C) all water in the mixture could be frozen by using the additive.     

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.     

Study on high performance ice slurry formed by cooling emulsion in ice storage (discussion on adaptability of emulsion to thermal storage material)

This study focuses on an emulsion as a new thermal storage material for ice storage. Two types of emulsions were formed using an oil–water mixture with a small amount of additive. A silicone, light and lump oils were used. The water contents of the emulsions were 70, 80 and 90%. The additive was an amino group modified silicone oil. No depression of freezing point was observed for the emulsions because of their hydrophobic properties. In order to determine the structure of the emulsions, their electrical resistances were measured. Moreover, components of the liquids separating from the emulsions were analyzed. The results indicated that one emulsion was a W/O type emulsion, while the other was an O/W type. Finally, adaptability of the two emulsions to ice storage was discussed, it was concluded that a high performance ice slurry could be formed by the W/O type emulsion.     

Fundamental study of adhesion of ice to cooling solid surface (Discussion on influence of copper oxide layer on ice adhesion force)

In many situations, ice often adheres to a cooling solid surface, frequently causing serious accidents. It is critical to clarify the mechanism of ice adhesion to the cooling surface in order to prevent ice adhesion. In a past study, the shearing stresses of two kinds of test plates with a copper surface having the higher thermal conductivity were measured. The shearing stress corresponds to ice adhesion force. Both shearing stresses were significantly different; however, the cause remains unclear.Therefore, the present study focuses on an oxide layer as the main factor causing the difference of both shearing stresses; the influence of the oxide layer formed on shearing stress was discussed. And in the removal and reformation processes of the oxide layer, the time variation of the shearing stress was clarified. Moreover, the relationship between the state of the copper surface and the shearing stress was also clarified by surface analysis.     

Ice storage system with water–oil mixture: formation of suspension with high IPF

A new method of forming ice, which is one of the dynamic types of ice storage system, is studied. In the method a water-oil emulsion is cooled with stirring in a vessel and changed into an ice-oil and water suspension. A mixture of 10 vol% silicone-oil and 90 vol% water is emulsified with a small amount of an additive. Silane-couplers are tested as the additive and effects of the additive on ice formation process are investigated. Cooling rate is changed and vessels made of various materials are tested. It is proved that the present method has the following characteristics. Ice–oil and water suspension (slush ice) which has a good fluidity is able to be formed without adhering to the cooling surface. Ice in the suspension is granular and dispersed state and the suspension with more than 70% of ice packing factor (IPF) is also able to be formed. The suspension with the high IPF can be preserved for a long time in the granular state.     

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.