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.     

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.     

Experimental investigation of ice slurry heat transfer in horizontal tube

Heat transfer of ice slurry flow based on ethanol–water mixture in a circular horizontal tube has been experimentally investigated. The secondary fluid was prepared by mixing ethanol and water to obtain initial alcohol concentration of 10.3% (initial freezing temperature -4.4 °C). The heat transfer tests were conducted to cover laminar and slightly turbulent flow with ice mass fraction varying from 0% to 22% depending on test performed. Measured heat transfer coefficients of ice slurry are found to be higher than those for single phase fluid, especially for laminar flow conditions and high ice mass fractions where the heat transfer is increased with a factor 2 in comparison to the single phase flow. In addition, experimentally determined heat transfer coefficients of ice slurry flow were compared to the analytical results, based on the correlation by Sieder and Tate for laminar single phase regime, by Dittus–Boelter for turbulent single phase regime and empirical correlation by Christensen and Kauffeld derived for laminar/turbulent ice slurry flow in circular horizontal tubes. It was found that the classical correlation proposed by Sieder and Tate for laminar forced convection in smooth straight circular ducts cannot be used for heat transfer prediction of ice slurry flow since it strongly underestimates measured values, while, for the turbulent flow regime the simple Dittus–Boelter relation predicts the heat transfer coefficient of ice slurry flow with high accuracy but only up to an ice mass fraction of 10% and Re_cf > 2300 regardless of imposed heat flux. For higher ice mass fractions and regardless of the flow regime, the correlation proposed by Christensen and Kauffeld gives good agreement with experimental results.     

Local and global heat transfer coefficients of a stabilised ice slurry in laminar and transitional flows

The thermal behaviour of a new two-phase secondary refrigerant has been analysed. The “stabilised ice slurry” is a suspension in a low viscosity oil of ultraporous polymeric particles filled with water. In order to determine the convective heat transfer coefficient of this secondary refrigerant with water–ice phase change, an experimental set-up was built. It allows determining the local heat transfer coefficients inside two heat exchangers, having rectangular sections (80 × 8 mm²) of 1 m length, by mean of fluxmeters located along the working section. The slurry is first cooled and frozen in one of the exchangers, then heated and melted in the other exchanger. The results obtained for laminar or transitional flows shows that the heat transfer coefficients of the ice slurry are obviously higher than the heat transfer coefficients obtained with the single-phase fluid (oil). Correlations giving the local and global Nusselt numbers, depending on the Graetz or Reynolds numbers and on the particle mass fraction, have been established.     

Prediction of ice slurry performance in a corrugated tube heat exchanger

Ice slurry performance in a concentric corrugated tube heat exchanger is experimentally studied in this work in order to compare experimental results to theoretical prediction obtained using the correlations proposed in previous papers. Once the validity of those correlations is verified, the behaviour of the studied heat exchanger is analyzed for different ice slurry flow conditions and compared to the results obtained when a heterogeneous storage is used and only carrier fluid flows through the heat exchanger. According to the performance evaluation criterion used – variation in heat transfer rate for equal pressure drop and surface area – the most remarkable conclusion obtained is that slurry improves the behaviour of the heat exchanger studied for all the cases analyzed, although the increase in heat transfer rate is always lower than 15%, being in most cases lower than 5%.     

Superheating of ice slurry in melting heat exchangers

One of the main components of an ice slurry system is the melting heat exchanger, in which ice slurry absorbs heat resulting in the melting of ice crystals. Design calculations of melting heat exchangers are mainly based on heat transfer and pressure drop data, but recent experimental studies have shown that superheating of ice slurry should also be considered. This paper presents ice slurry melting experiments with a tube-in-tube heat transfer coil. The experimental results indicate that operating conditions such as ice slurry velocity, heat flux, solute concentration, ice fraction, and ice crystal size determine the degree of superheating. The various influences are explained by considering the melting process as a two-stage process consisting of the heat transfer between wall and liquid and the combined heat and mass transfer between liquid and crystals. Bigger ice crystals and higher solute concentrations decrease the rate of the second stage and therefore increase the degree of superheating.     

Investigation into the transportation and melting of thick ice slurries in pipes

This paper presents the results of experiments and modelling carried out on ice slurries flowing in uninsulated steel pipes with a nominal diameter of 50 mm. The slurries used were formed from 4.75% NaCl aqueous solution and had ice mass fractions in the range 18–42%, with a view to the use of thick ice slurry ‘pigs’ as a pipeline clearing technique. Of particular interest was the distance over which such slurries can survive as plug-like entities, before melting reduces them to ineffective thin two-phase suspensions. The experiments showed that for small volumes of slurry, survivability is directly proportional to the quantity of slurry used, but that increasing the ice fraction has a more marked effect. A simple one-dimensional numerical model that accounts for transportation, heat transfer and melting was developed that produces reasonable predictions.     

Suppression of ice adhesion force to cooling wall due to voltage application

For ice storage, one of the authors has previously reported on the ice slurry formed by cooling water–silicone oil mixture with stirring. When the mixture is stirred in a vessel, the oil is charged due to static electricity. If the vessel can attract the charged oil, suppression of ice adhesion force to a cooling wall may be possible. In this study, therefore, a certain voltage was applied to the vessel filled with the mixture with cooling and stirring simultaneously, and water was frozen in the vessel. Then, the ice adhesion force to the cooling vessel wall was measured under a constant apparent adhesion area between the ice and cooling vessel wall. From the measurement results, optimal conditions of the oil viscosity, rotation speed and applied voltage to suppress the ice adhesion force effectively were clarified. Moreover, the factors governing suppression of the ice adhesion force were clarified.     

Measurement of thermal conductivity of ice slurry made from solution by transient line heat-source technique (analytical discussion on influence of latent heat of fusion)

We have been studying on ice slurry in a dynamic type ice storage system. The ice slurry has many good characteristics. The ice slurry can be made from a solution. When designing the ice storage system using this ice slurry, thermal conductivity of the ice slurry is essential.When thermal conductivity of the ice slurry made from a solution is measured by a transient line heat-source technique, a measurement value of thermal conductivity is affected by a latent heat of fusion of ice. Therefore, the thermal conductivity measured is apparent thermal conductivity. In this study, influences of Stefan number, initial concentration of the solution, initial solid fraction (initial IPF) and Fourier number on the thermal conductivity was analytically discussed to improve measurement accuracy of the thermal conductivity of ice slurry in the transient line heat-source technique.