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.     

Freezing due to direct contact heat transfer including sublimation

From the energy saving viewpoint, the utilization of LNG cold is very important in the refrigeration industry concerning the low temperature region. In this paper, as a basic study of the freezing due to direct contact, including evaporation, solid–liquid direct contact heat transfer associated with sublimation has been investigated using the dry ice in water experimentally and theoretically. Based on a two-layers-model composed of CO₂ vapor and the bulk water around the dry ice, the velocity and temperature fields in two layers was calculated numerically and the calculated results for the freezing condition of the bulk water were compared with the experimental results.     

Prediction of freezing and thawing times for foods — a review

This communication presents a review of methods to predict the freezing and thawing times of foods. A bibliography of over 400 references was used.     

Freezing time and rate of cauliflower florets under different freezing conditions

In this study, the freezing time and rate of 1 cm³ cauliflower floret samples were determined under different freezing conditions in an air blast freezer. Four different air temperatures (−20, −25, −30 and −35°C) and six different air velocities (70, 131, 189, 244, 280 and 293 m min⁻¹) were applied in the freezer, and the freezing rate and time of cauliflower pieces were determined under each condition. The freezing time of cauliflower samples frozen with cold air at −20°C and 280 m min⁻¹ was similar to that of samples frozen with cold air at −35°C and 70 m min⁻¹. When the velocity of air was increased from 70 m min⁻¹ to 293 m min⁻¹, the freezing time was approximately halved.     

Freezing phenomena of supercooled water under impacts of ultrasonic waves

In order to clarify effects of ultrasonic waves on supercoold water, ultrasonic waves were applied to supercooled water. Frequency of the ultrasonic waves applied was 45 kHz, and the intensities of the waves were 0.13 and 0.28 W cm⁻², respectively. Four cases of experimental conditions were selected; a case of a free surface, a case of an oil-water surface, a case of a free surface with a dipped metal bar and a case of an oil–water surface with a dipped metal bar. For each experimental condition, water was cooled at a constant cooling rate and the ultrasonic wave was applied from underneath until the water in a test tube solidified. It was found that in the case of 0.28 W cm⁻², the ultrasonic wave had distinct effects on freezing of supercooled water for all experimental conditions. On the other hand, in the case of 0.13 W cm⁻², the ultrasonic wave had an effect on freezing of supercooled water only under existence of a free surface and a metal bar.     

Freezing of strawberry pulp in large containers: experimental determination and prediction of freezing times

Strawberry pulp packed in bulk in containers of various shapes and of large sizes (10–2001) was frozen in a tunnel at −35°C. Time-temperature curves and freezing times for heat transfer in one, two and three dimensions were obtained. Experimental freezing times were compared with predicted values given by approximate methods based on two different equations for the calculation of freezing times in slabs, which in turn used three different methods for the calculation of shape factors. The predicted results obtained in this work confirmed that both the simplified prediction methods of freezing times and the shape factors used were sufficiently accurate for the design of freezing equipments for these types of product and working conditions.     

Flow boiling heat transfer characteristics of CO2 at low temperatures

This paper presents an overview of the flow boiling heat transfer characteristics and the special thermo-physical properties of CO₂ at low temperatures (down to −30 °C). Subsequently, the boiling heat transfer of CO₂ at low temperatures is experimentally investigated in a horizontal tube with inner diameter of 4.57 mm. Due to the large surface tension, the boiling heat transfer coefficient of CO₂ is found to be much lower at low temperatures but it increases with vapour quality (until dryout), which is contrary to the trend at high temperatures around 0 °C. None of the empirical correlations from open literature were able to predict the boiling heat transfer coefficient for CO₂ in good agreement with the experimental data, suggesting the need for further research in this area.     

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.     

Experimental study and modelling of the crystallization of a water droplet

Modelling the crystallisation of a water droplet into a cold humid airflow is the first step in modelling the behaviour of water droplets sprayed out of a snow gun. This modelling, which is based on an experimental study, deals with the behaviour of the droplet which is transformed successively from the supercooled liquid phase to the liquid–solid phase and then the solid phase. These three transformations are brought about by various exchanges: heat transfer with conduction and convection as well as mass transfer with evaporation and sublimation. The supercooling phenomenon is naturally observed during experiments and taken into account in the modelling.     

Flow boiling heat transfer to carbon dioxide: general prediction method

An updated version of the Kattan–Thome–Favrat flow pattern based, flow boiling heat transfer model for horizontal tubes has been developed specifically for CO₂. Because CO₂ has a low critical temperature and hence high evaporating pressures compared to our previous database, it was found necessary to first correct the nucleate pool boiling correlation to better describe CO₂ at high reduced pressures and secondly to include a boiling suppression factor on the nucleate boiling heat transfer coefficient to capture the trends in the flow boiling data. The new method predicts 73% of the CO₂ database (404 data points) to within ±20% and 86% to within ±30% over the vapor quality range of 2–91%. The database covers five tube diameters from 0.79 to 10.06 mm, mass velocities from 85 to 1440 kg m⁻² s⁻¹, heat fluxes from 5 to 36 kW m⁻², saturation temperatures from −25 °C to +25 °C and saturation pressures from 1.7 to 6.4 MPa (reduced pressures up to 0.87).     

Nucleate boiling heat transfer coefficients of pure halogenated refrigerants

Nuclate pool boiling heat transfer coefficients (HTCs) of HCFC123, CFC11, HCFC142b, HFC134a, CFC12, HCFC22, HFC125 and HFC32 on a horizontal smooth tube of 19.0 mm outside diameter have been measured. The experimental apparatus was specially designed to accomodate high vapor pressure refrigerants such as HFC32 and HFC125 with a sight glass. A cartridge heater was used to generate uniform heat flux on the tube. Data were taken in the order of decreasing heat flux from 80 to 10 kW m⁻² with an interval of 10 kW m⁻² in the pool of 7 °C. Test results showed that HTCs of HFC125 and HFC32 were 50–70% higher than those of HCFC22 while HTCs of HCFC123 and HFC134a were similar to those of CFC11 and CFC12 respectively. It was also found that nucleate boiling heat transfer correlations available in the literature were not good for certain alternative refrigerants such as HFC32 and HCFC142b. Hence, a new correlation was developed by a regression analysis taking into account the variation of the exponent to the heat flux term as a function of reduced pressure and some other properties. The new correlation showed a good agreement with all measured data including those of new refrigerants of significantly varying vapor pressures with a mean deviation of less than 7%.     

Continuous formation of slurry ice by cooling water–oil emulsion in a tube

Water-silicone oil emulsion with an additive, (C₂H₅O)₃SiC₃H₆NH₂, was examined as a heat storage material. A spiral tube used as a heat exchanger was immersed in a low temperature bath and the emulsion was circulated in the tube to make ice continuously. Ice was separated from the ice–liquid suspension in an outlet tank. The amount of formed ice, the temperatures of the inlet and the outlet of the heat exchanger, and the temperatures in the tube wall were measured and the overall heat transfer coefficient and the heat flux through the tube were calculated. Experiments were carried out, varying the flow rate, the temperature of cooling brine, and the thickness of tube wall. The condition under which slurry ice was formed continuously without adhesion of ice to the cooling wall was clarified. Though decrease in the thermal resistance of the tube increased the rate of ice formation or raised the brine temperature, it narrowed the range of the flow rate and of the brine temperature in which slurry ice was formed continuously.     

Theoretical and experimental freezing times of strawberries

Strawberries were frozen at different air velocities in an air blast freezer at −30°C. The freezing time was taken as the time required to lower the temperature at the geometric centre of the samples to −10°C. The freezing times measured in the experiments were compared with the values calculated using Plank's equation. The freezing times calculated by Plank's equation were fround to be higher than those found experimentally at any given air velocity. Freezing time decreased with increasing air velocity. This is attributed to the increase in heat transfer coefficient at increased air velocities.     

Measurement and prediction of freezing times of vacuum canned Pacific shrimp

Freshly cooked Pacific shrimp, vacuum packed in cans fitted with monitoring instruments, were frozen under industrial conditions. Surface heat transfer coefficients were determined experimentally using aluminium cyclinders. Freezing times were measured and also calculated using Plank's equation and the formulae of Cleland and Earle. The investigation indicated that the vacuum level had little effect on the freezing time and that freezing time essentially doubled when cans were enclosed in a commercial carton. For the prediction of freezing times, Cleland and Earle's method was found to be more accurate than Plank's for the tests performed, with predicted values within ± 10% of measured values.     

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.     

Testing the computer assisted solution of the electrical analogy in a heat transfer process with a phase change which has an analytical solution

A new method has been developed for the computer assisted solution of electrical analogies. The method, relatively simple in comparison with other methods, allows the study of heat transfer problems, including the change of phase. The aim of this work has been to apply this technique to the study of a heat transfer problem with change of phase, which has an analytical solution, in order to test its validity. The results obtained by both methods, analytical and analogical, are compared, and they are observed to agree satisfactorily. It makes it possible to establish that the technique developed is suitable for the study of processes with a phase change present.     

Use of a droplet nucleation analyzer in the study of water freezing kinetics under the influence of ultrasound waves

In this report we present a new instrument (a droplet nucleation analyzer) to be used in the study of the influence of ultrasonic waves on the freezing of pure water. This influence can be of great interest in the cryopreservation of biological material. Two different types of experiments have been carried out. In the first set of experiments, ultrasound waves were used during the cooling process. In the second set, ultrasound was applied prior to the cooling process, trying to quantitatively reproduce some experiments in which the ice nucleation temperature of water was successfully decreased. A theoretical discussion of the results is also presented.     

Development of a slug flow absorber working with ammonia–water mixture: part II—data reduction model for local heat and mass transfer characterization

This study deals with a data reduction model for clarifying experimental results of a counter-current slug flow absorber, working with ammonia–water mixture, for significantly low solution flow rate conditions. The data reduction model to obtain the local heat and mass transfer coefficient on the liquid side is proposed by using the drift flux model to analyze the flow characteristics. The control volume method and heat and mass transfer analogy are employed to solve the combined heat and mass transfer problem. As a result, it is found that the local heat and mass transfer coefficient on the liquid side of the absorber is greatly influenced by the flow pattern. The heat and mass transfer coefficient at the frost flow region is higher than that at the slug flow region due to flow disturbance and random fluctuation. The solution flow rate and gas flow rate have influence on the local heat and mass transfer coefficient at the frost flow region. However, it is insignificant at the slug flow region.     

Thermal design calculations for food freezing equipment--past, present and future

The literature on methods for thermal design of food freezing equipment is reviewed with emphasis on two questions: what do those who design, build and commission freezers most need from researchers in terms of improved design calculation methods, and what are the most limiting factors in determining whether a particular freezer design will satisfactorily freeze the product at the required throughput rate? Freezing time prediction methods have been significantly improved over the last two decades and are now infrequently the factor most limiting accurate design. There is a much greater need for more accurate thermophysical properties and better information on heat transfer coefficients for a variety of practical situations. The failure of many industrial freezers to deliver the design conditions to product in all parts of the freezer is also important. Future research should address the full range of factors limiting accurate freezer design, which may mean less emphasis on freezing time prediction.

Résumé

La littérature concernant la conception thermique des congélateurs industriels est passée en revue et l'accent est mis sur deux aspects: qu'est ce que les concepteurs, les fabricants et les installateurs attendent le plus des chercheurs en termes d'amélioration des méthodes de calculs de conception, et quels sont les facteurs dans la conception d'un congélateur qui exercent la plus grande influence sur la performance, c'est à dire qui déterminent si une conception donnée va permettre de congeler un produit à la vitesse voulue? Pendant les deux dernières décennies, on a amélioré de façon significative les méthodes qui prédisent le temps de congélation, et de nos jours, ces méthodes sont rarement le facteur qui limite le plus la conception précise. On a nettement plus besoin de précision sur les propriétés thermophysiques et les coefficients de transfert de chaleur pour différentes situations. Il est important de savoir que beaucoup de congélateurs industriels n'arrivent pas à reproduire les performances prévues lors de leur conception en tout point de l'appareil. Dans l'avenir, la recherche devrait être axée sur tous les facteurs limitant la conception précise des congélateurs, quitte à mettre moins l'accent sur la prédiction du temps de congélation.     

Surface heat transfer coefficients to stationary spherical particles in an experimental unit for hydrofluidisation freezing of individual foods

Convection heat transfer to spherical particles inside a hydrofluidisation freezing unit was investigated. The unit contained a food tank with a perforated bottom plate to create agitating jets. An aqueous solution of 30% ethanol+20% glucose was used as the refrigeration medium in a temperature range of −20 to 0 °C and flow rates from 5 to 15 l min⁻¹. The lumped capacitance method was applied on cooling profiles of aluminium spheres of 5–50 mm to obtain surface heat transfer coefficients. Coefficients were within a range of 154–1548 W m⁻² °C⁻¹, and depended on diameter, flow rate, refrigeration temperature and fluid agitation level. The agitation due to jets was accounted for by means of an agitation Reynolds number in a Nusselt correlation A large variability of measured surface heat transfer coefficients was observed. This could be attributed to non-constant flow and turbulence fields in the refrigeration medium. The value of the heat transfer coefficient was compared to values determined on strawberries.