Freezing processes in high-pressure domains

Experimental freezing of water in high-pressure domain is studied considering temperature reduction (TRF) as well as high-pressure-assisted freezing (HPAF). The most important advantage of HPAF is that the whole volume of the sample is subcooled when an expansion is made, so a rapid and uniform nucleation and growth of ice crystals are produced. In this work through mathematical modelling the amount of ice appearing instantaneously in the latter freezing, is predicted.     

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.     

Ice-slurry production using direct contact heat transfer

An ice slurry generation system was developed using direct contact heat transfer between water and the coolant, Fluroinert FC-84. The location of the coolant nozzle is an important design consideration to avoid clogging due to freezing of water. An ice fraction of up to about 40 percent was obtained with the nozzle located at the bottom of the ice slurry tank and the jet directed upwards into the water. Two simplified model were developed to extract the heat transfer coefficient between the coolant drops and the water. The first model requires as input the average drop diameter and the residence time while the second model uses the measured drop diameter distribution. The estimated heat transfer coefficients are much smaller than those computed using single-sphere correlations.     

新型流态冰蒸发制冷循环及制冰量性能分析

介绍了蒸发过冷水制冰原理及系统流程,分析了相应的蒸发制冷循环,对单位质量(1k)干空气制冰量性能及影响因素进行了研究并建立了相关数学模型,计算结果显示,水的初始温度越高单位质量干空气制冰量越低,而单位质量干空气制冰量相同时水温越高对应的蒸发水量则越大,同时随着空气出口相对湿度增大,单位质量干空气制冰量也随之增加.另外,...     

Study on continuous ice slurry formation using functional fluid for ice storage – Discussion of optimal operating conditions

A functional fluid was made by adding a small amount of additive to a water–silicone-oil mixture with 90 vol% water content, and the functional fluid was transformed into an ice slurry by cooling while stirring. The new ice formation system, which authors proposed for ice storage based on the results of previous studies, demonstrated that the ice slurry could be formed continuously for 10 h. In the current paper, experiments were carried out, varying operating conditions, and an optimal operating condition was determined to improve performance of the present system still more. From the experimental results, the conditions necessary to increase the amount of recovery ice were clarified. The time-dependencies of the shape and size of formed ice particles were also shown. Moreover, the reason why the freezing temperature of the functional fluid rose due to repetition of ice formation was clarified, and its measure was discussed. The present study then found that it was possible to form and recover a larger amount of ice than in previous attempts, given the rise in freezing temperature.     

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.     

Formation of gas hydrate or ice by direct-contact evaporation of CFC alternatives

Experiments have been made on cool storage by evaporation of HFC-134a (CH₂FCF₃) or HCFC-123 (CHCl₂CF₃) brought into direct contact with water in a crystallizer, which was incorporated into a vapour-compression refrigerator loop. The degree of supercooling before the inception of gas-hydrate formation with HFC-134a was found to be reduced by the addition of powdery alumina or zinc or the addition of a surfactant to the water, while the addition of Pseudomonas fluorescens, a strain of ice-nucleating bacteria, showed no effect. The use of HCFC-123 instead of HFC-134a resulted in the formation of slush ice only; no sign of gas-hydrate formation was recognized. The reason for this is considered to lie in the molecular size of HCFC-123.     

Shape identification for water–ice interface within the cylindrical capsule in cold storage system by inverse heat transfer method

In this study, the inverse heat transfer method is applied to shape identification for the ice layer within the cylindrical capsule in cold storage system. The approach is constructed by combining the curvilinear grid generation scheme, the direct problem solver, the conjugate gradient optimization method, and the redistribution method. According to the practical condition of freezing ice, shape identification for the water–ice interface based on the data of the outer surface temperature is attempted. Results show that the profile of the water–ice interface is possible to be identified by using the inverse heat transfer approach and the accuracy of the ice shape identification is dependent on the uncertainty of the outer surface temperature data, the Biot number, the thickness of the ice layer, and the geometric configuration as well.     

Freezing of a water droplet due to evaporation—heat transfer dominating the evaporation–freezing phenomena and the effect of boiling on freezing characteristics

One of the authors has proposed a novel transport/storage system for the waste cold from the gasification process of liquefied natural gas (LNG), which consists of an evaporator, a cold trap, and a pipeline. In order to estimate the performance of this system, one should know the pressure in the evaporator, in which evaporation–freezing of a PCM occurs, and in the cold trap, as well as the pressure drop of the pipeline due to the flow of low pressure vapor of the PCM. In this paper, the cooling/freezing phenomena of a water droplet due to evaporation in an evacuated chamber was experimentally examined, and the heat transfer dominating the evaporation-freezing phenomena was investigated in order to estimate the pressure in the evaporator. From the results, it was shown that the water droplet in the evacuated cell is effectively cooled by the evaporation of water itself, and is frozen within a few seconds through a remarkable supercooling state, and that the cooling rate of the water droplets were dominated by heat transfer within the droplet under the abrupt evacuation condition. The later result means that, in order to obtain an ice particle by evaporation–freezing, the surroundings of the water droplet should be evacuated at the pressure as low as the saturate pressure of water at the maximum supercooling temperature of the droplet.     

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.     

Numerical analysis of coupled water,heat and stress in saturated freezing soil

Clapeyron equation can be applied in freezing soil to describe the relationship among temperature, water pressure and ice pressure when ice and water coexist in phase equilibrium. The mathematical deduction shows that the driving force that makes the unfrozen water in soil moves from high temperature area to low temperature area is determined by gravity, temperature and pore pressure. Upon proposing the concept of separating void ratio as a judge criterion for the formation of ice lenses, adjusting the hydraulic conductivity to describe the unfrozen water gathering at the front of ice lenses and the growth of ice lens, a mathematical model of coupled water, heat and stress is established. A typical process of coupled water, heat and stress that happens in a saturated freezing soil column is simulated by COMSOL Multiphysics simulation software. The amount of frost heave is calculated, and the result of simulation gives the distribution bar graph of ice lenses and distribution curves of temperature, equivalent water content and pore pressure, and shows how they change.     

Direct contact ultrasound assisted freezing of mushroom (Agaricus bisporus): Growth and size distribution of ice crystals

To reduce the size of ice crystals in mushroom (Agaricus bisporus) contact ultrasound (300 W, 20 kHz) was applied during freezing and frozen storage. Stereomicroscopy was used to observe the ice crystal morphology, and DSC and NMR spectroscopy were performed to evaluate the water states in the samples. Results indicated that ultrasound irradiation initiated the nucleation of ice and reduced the mean size of ice crystals during freezing and frozen storage, and therefore improved the frozen product quality compared to the control samples. Most of the ice crystals in the ultrasound assisted frozen (UAF) samples were in the size range of 0–80 microns while that for the control samples were in the size range of 50–180 microns. SEM photos also proved that due to the application of ultrasound, the sizes of the ice crystals was reduced. This micro-scale information on the documentation of ice crystals will assist in understanding the ice crystal growth phenomena in an ultrasound assisted freezing process.     

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.     

充冷过程中冷板内特性研究

针对机械冷板冷藏车冷板充冷时间长的问题,根据热量守恒原理建立了冷板内共晶冰冻结过程的数学模型,并对影响冷板充冷过程的关键因素进行了讨论,用准静态方法对蒸发盘管外共晶冰的形成过程进行了数值计算.计算结果表明,随着盘管周围共晶冰厚度的增加,共晶冰冻结缓慢;降低蒸发温度,减少冷板外热负荷,可以明显减少共晶冰的冻结时间;冻结过程中,冷板内的逐时蓄冷量基本不变.     

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.     

The modelling of the freezing process in fine-grained porous media: Application to the frost heave estimation

The solution of the moving-boundary problem, related to heat- and mass-transfer processes in freezing, fine-grained, porous media under phase-transition conditions is presented. It is assumed that a freezing zone, characterized by a wide temperature range of phase transitions, is formed. Therefore a three-zone model is developed. The preservation of the term ∂L/∂t(L is the ice content) in the system of equations has made it possible to determine the ice distribution within the frozen and the freezing zones. For loamy soils the dependence of the freezing process on the characteristic parameters, the Stefan and Lewis numbers, was analyzed. It was found that increasing the enthalpy of phase transition, i.e., decreasing the Stefan number Ste, resulted in diminution of the frozen zone but, at the same time, the ice content within this zone increased. Intensification of the migration process, i.e., increasing the Lewis number Le, also led to diminution of the frozen zone, in which the ice content and, consequently, the total moisture (including ice) were increased. For large Lewis numbers the freezing zone was observed to decrease. When the water migration process is absent (Le = 0), the calculations, which were based on the described model show that in the course of freezing the redistribution takes place only between moisture and ice contents. The total moisture remains constant and equal to the initial water content. The theoretical conceptions and results derived from the analytical solution are in agreement with experimental findings. The presented model predicts the freezing process in porous media and satisfactorily reflects observed phenomena. The utilization of the considered problem solution to the prediction of the frost heave phenomenon in soils freezing processes shown that the calculated frost heave curve matches the experimental results very closely indicating that the model can well reproduce the frost heaving process associated with the freezing. Propagation of the freezing front in the test is predicted the experimental results with reasonable accuracy.     

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.     

Three-dimensional measurement of ice crystals in frozen dilute solution

A Micro-Slicer Image Processing System (MSIPS) has been applied to observe the ice crystal structures formed in frozen dilute solutions. Several characteristic parameters were also proposed to investigate the three-dimensional (3-D) morphology and distribution of ice crystals, based on their reconstructed images obtained by multi-slicing a frozen sample with the thickness of 5 μm. The values of characteristic parameters were determined for the sample images with the dimension of 530×700×1000 μm. The 3-D morphology of ice crystals was found to be a bundle of continuous or dendrite columns at any freezing condition. The equivalent diameter of ice crystals were in the range of 73–169 μm, and decreased exponentially with increasing freezing rate at the copper cooling plate temperature of −20 to −80 °C. At the T_cp −40 °C, the volumes of ice crystals were in the range of 4.6×10⁴ μm³ to 3.3×10⁷ μm³, and 36 ice columns were counted in the 3-D image.     

Experimental investigation of a novel multifunction heat pipe solid sorption icemaker for fishing boats using CaCl2/activated carbon compound–ammonia

The performance of a solid sorption icemaker is investigated. CaCl₂/activated carbon was used as compound adsorbent and ammonia was employed as adsorbate. The influence of operating conditions (cooling water temperature, mass recovery and heat pipe heat recovery, etc.) on the mass of ice, SCP (specific cooling power) and COP (coefficient of performance) was experimentally assessed. At the desorption temperature of 126 °C, cooling water temperature of 22 °C, ice produced temperature of −7.5 °C, 40 s of mass recovery and 2 min of heat pipe heat recovery, the mass of ice, SCP and COP values are 17.6 kg/h, 369.1 W/kg and 0.2, respectively.     

Atmospheric icing in a coastal mountainous terrain. Measurements and numerical simulations, a case study

Icing on structures occurs as rime ice, clear ice or wet snow deposit. Reliable forecasts of duration and intensity of this icing require prognoses of standard meteorological parameters, in addition to more specific parameters such as the density (ρ_LWC) of cloud Liquid Water Content (LWC). Icing conditions on the mountain Brosviksåta (723 m a.s.l., 61° 2′ N, 5° 9′ E), on the western coast of Norway were investigated from March 21–24, 2003. A non-rotating vertical steel rod mounted on a scale was used to measure the accumulated ice load. Air temperature, relative humidity and wind were measured at three levels along the mountain slope. The maximum build-up of ice, in this case study, was measured to 4.5 kg on a 1 m high 0.14 m diameter rod. Comparison of measured ice-growth rate and calculated ρ_LWC gave a correlation coefficient of 0.85.A mesoscale atmospheric model (MM5) has also been tested at a high horizontal resolution (1 km) in order to evaluate its ability to reproduce weather conditions where freezing occurs. Comparison from the direct measurements and calculations, with results from MM5, gave 58% of the measured accumulated ice growth. Further studies of real-time cases on a real-time system at a coarser model resolution will reveal its capability for forecasting freezing events.