Freezing Characteristics Research Articles

Temperature Evolution and Strain Characteristics of Liquid Nitrogen Freezing and Thawing of Different Coal Ranks

Temperature Evolution and Strain Characteristics of Liquid Nitrogen Freezing and Thawing of Different Coal Ranks

Freezing characteristics and relative permittivity of rice flour gel in pulsed electric field assisted freezing

This study investigated the effect of pulsed electric field assisted freezing treatment on the freezing characteristics of rice flour gel under output voltages varying from 0 to 25 kV. The results indicated that by applying a pulsed electric field, the phase change time decreased. Scanning electron microscopy images indicated that pulsed electric field treatment led to the formation of rounder and smaller ice crystals. For further understanding and quantifying the interaction between rice flour gel and a pulsed electric field, the relative permittivity of rice flour gel with and without the addition of salt was measured between 100 and 3100 kHz and −20 and 20 °C. Relative permittivity increased with decreasing frequency or increasing temperature, and sharp variation was observed during the phase transition period. In addition, salt was proved to be an effective additive for increasing relative permittivity.

Effect of different additives on freezing characteristics and stability of GnP-aqueous-based PCM for cold thermal storage

Effect of different additives on freezing characteristics and stability of GnP-aqueous-based PCM for cold thermal storage

Freezing characteristics of deposited water droplets on hydrophilic and hydrophobic cold surfaces

Freezing phenomena of water droplets on metal surfaces widely exist in nature and industry. While the freezing of a sessile water droplet has been extensively studied, here we report an experimental and thoeritical investigation of the freezing characteristics of a deposited water droplet on cold hydrophilic and hydrophobic aluminum surfaces and also compare them with those of the sessile droplet. The results show that the deposited droplet starts to freeze from the liquid-solid interface after the spreading contact line is arrested by the cold surface and the recalescence stage observed during the freezing of a (supercooled) sessile water droplet does not appear. Furthermore, under our experimental conditions of surface temperature and wettability, the frozen shapes of the deposited droplets on the hydrophilic and hydrophobic surfaces are only dependent on the surface temperature while those of the sessile droplets unnoticeably change with the surface temperature but significantly vary with the surface wettability. Additionally, we present a scaling analysis to describe the temporal evolution of freezing front and the relationship between freezing time and surface temperature for the deposited droplet.

A combined experimental and computational study of phase-change dynamics and flow inside a sessile water droplet freezing due to interfacial heat transfer

This study experimentally and numerically investigates the freezing characteristics and fluid dynamics of millimetre-sized sessile water droplets submerged in silicone oil at sub-zero temperatures under free convection. Individual water droplets were cooled to sub-zero temperatures (260-270 K) via interfacial heat transfer between the two liquid phases, in an approach different to studies in the literature where the cooling is done either from the solid substrate or from a low-temperature gas phase (such as air) surrounding the droplets. Laser-induced fluorescence was employed to perform spatiotemporally-resolved measurements of the phase distribution (from which interface distributions, freezing fronts, and rates were extracted). The particle image velocimetry was used to generate information on the velocity fields inside the liquid droplets. The experimental data are complemented by computational fluid dynamics (CFD) simulations, which showed acceptable qualitative and quantitative agreement with the experimental results. The experimental and simulation results indicated that prior to the initiation of freezing, two counteracting recirculation zones are generated in the central plane of the droplets, one on either side of the centreline, leading to a net upward flow at the edges and a downward flow in the centre due to the natural convection driven by internal temperature gradients. The nucleation sites appear on the external regions of the recirculation structures (which are locations with higher shear). Once freezing starts, the natural circulation patterns are suppressed, and instead, a sole downwards flow dominates, which is the result of the freezing layer suppressing the water phase. CFD results demonstrated a relatively wide temperature and pressure distribution in the water droplet at the beginning of the freezing stage, which gradually diminishes as the freezing process proceeds. The effect of droplet size and oil temperature on the freezing rates were investigated parametrically. The finding showed that when the droplet size doubles, the time required for the droplet to completely freeze extended by almost an order of magnitude. In addition, lower oil temperatures appeared to modify the normalised freezing rate, with sharper exponential growth rates and reduced initial and final stages at higher oil temperatures.

Cold start behavior and freeze characteristics of a polymer electrolyte membrane fuel cell

AbstractVehicle applications require efficient cold start ability and durability of polymer electrolyte membrane fuel cells (PEMFCs). In this study, various self‐cold start strategies including purging the PEMFC at shutdown and using galvanostatic operation at startup are proposed. The cold start characteristics from −5°C of a single cell are experimentally investigated in situ on a laboratory scale. The amount of cumulated charge transfer density, corresponding to the amount of product water, is used as an index to quantify the cold start capability. Gas purge at shutdown before freeze is found to facilitate the PEMFC cold start, although the improvement is relatively small compared with other methods such as gradually increasing the current during startup. Microscopic studies of the membrane electrode assembly (MEA) after cold start failure are conducted to determine material degradation due to ice formation.

Freeze concentrated apple juice maintains its flavor

Concentrated juices are sources of alcoholic drinks. Juice concentration may be achieved using different methods, such as freezing or heating. High temperatures in the process of juice concentration damage heat-sensitive components, such as aromatic compounds. Although the freezing process of juice concentration has been studied, analyses have been inadequate, particularly in addressing flavors. Therefore, we investigated the characteristics of freezing and heating during apple juice concentration in the context of flavor. We found that a total of 97 compounds were found in fresh juice, and freeze-concentrated juice retained 57 of these compounds. Interestingly, freezing led to the generation of 37 flavor compounds. Furthermore, people had difficultly differentiating between intact and frozen concentrated juice. The ratios were almost same between those who correctly identified (28%) and those who incorrectly identified fresh and reconstituted freeze-concentrated juice (25%). We discuss the mechanisms of flavor generation on freezing concentration with regard to the increases in enzymatic activity or other causes. Our study showed that the methods of juice concentration that utilize freezing retain flavor better. These data will benefit juice concentration processes of apples and other fruits in the future.

A methodology to establish freezing characteristics of partially saturated sands

The thermo-hydro-mechanical (THM) properties of frozen soils depend primarily on the parameters like volumetric ice content, unfrozen water content, dry density, mineralogy, temperature etc. However, establishing the influence of ice/unfrozen water content on the abovementioned properties of frozen soils becomes a challenging task owing to the dependency of ice/unfrozen water content on temperature. The relationship between unfrozen water content and temperature is conventionally referred to as the ‘Soil Freezing Characteristics Curve’, SFCC. With this in view, an easy and efficient capacitance-based technique has been utilized to determine the volumetric ice content of standard sands of varying initial volumetric moisture content (=6%,12%,18%,25 % and 41%) by exposing them from room temperature to −15°C in a controlled manner. It was found that the SFCCs obtained from these experiments depend upon the initial volumetric moisture content of the soils. Further, existing analytical SFCC models were employed to fit the experimental results. It was found that the model parameters are highly dependent upon the initial volumetric moisture content, and models are incapable of including the effect of initial water content on the SFCC.

Comparison of freezing and hydration characteristics for porous media

AbstractThe processes of freezing–thawing and hydration–dissociation change the content of liquid water that coexists with ice or hydrate in porous media, such as frozen soils and hydrate‐bearing sediments, changing their physicomechanical properties. In this study, a generalized phase equilibrium equation is presented for both frozen soils and hydrate‐bearing sediments by considering the capillary and osmotic pressures. The liquid water content is related to temperature depression, plotted as the soil freezing characteristic curve (SFCC) or the soil hydration characteristic curve (SHCC), by combining the generalized phase equilibrium equation and the soil‐water characteristic curve (SWCC). From the SFCC or the SHCC, the phase equilibrium surface can be calculated in the space of temperature, pressure, and liquid water content. The proposed generalized phase equilibrium equation and the model of SFCC and SHCC can help to estimate the physicomechanical properties that depend on the fraction of the liquid or solid phase in porous media. Finally, the SHCC is employed to analyze the dissociation of hydrate‐bearing sediments using various methods.

A hysteresis model for the unfrozen liquid content in freezing porous media

The description of the freezing characteristics of porous media is one of the most conspicuous ingredients in flow and heat transport models that involve freezing and thawing processes. Unfrozen liquid content (ULC) shows strong hysteresis during freezing and thawing cycles in different types of soils and other porous media. We discuss the possible mechanisms of hysteresis in porous media and develop a numerical model for the unfrozen liquid content that is capable of describing the hysteresis phenomenon in freezing and thawing cycles. We present a coupled finite element model as the framework for the numerical simulation of fluid flow and heat transport in partially frozen porous media. The implementation aspects of the ULC model as well as its integration into numerical codes are discussed in detail. We investigate the potential impact of the hysteresis phenomenon on the numerical simulation of transport processes in porous media through benchmark examples and validate the behavior of the model against available laboratory measurement data.

Changes of Water/Ice Morphological, Thermodynamic, and Mechanical Parameters During the Freezing Process

To reduce ice adhesion hazards, optimize or develop the anti/de-icing methods, it is necessary to understand the change of freezing parameters during the freezing process, such as thermodynamic, morphological, and mechanical parameters. The present study investigates the freezing characteristics by purpose-built devices to describe the freezing process quantitatively. Morphological parameters were calculated the reverse engineering. The results showed that the inner temperature and morphology of water droplet were obviously changed, and the freezing process could be mainly divided into three stages: initial and spreading, freezing, and steady-state. Moreover, an experimental apparatus that measured the phase swelling force was built on investigating the freezing process of water from the mechanical aspect. It was found that the swelling force generated from the freezing process of 2473 mm3 water could reach 46.38 N. The generation process of swelling force could also be separated into three stages: non-expansive stage, increasing stage, and stable stage. The formation stage of swelling force was similar to that of ice. Combining the measured expansion force with the calculated freezing parameters based on the observed test, the freezing process of water could be better understood. The study would help researchers and engineers understand the freezing process and provide some freezing characteristics parameters for the anti/de-icing research.

Rehydration characteristics of vacuum freeze- and hot air-dried garlic slices

This work aimed to study the rehydration characteristics of vacuum freeze- (VFD) and hot air-dried (HAD) garlic slices and explain the rehydration mechanism. It was observed that VFD samples had significantly faster water absorption rates than HAD samples. However, VFD samples had lower final moisture contents than the HAD samples. Additionally, the effect slice thickness on rehydration characteristics of samples was lower than that of drying methods. The contact angle and microstructure analysis of dried samples revealed that the strong water wettability and the porous structure contributed to the faster water absorption rate of VFD samples. Moreover, we found and confirmed, for the first time, that numerous air bubbles trapped in the VFD samples hindered the absorption of water. This resulted in a lower water holding capacity in VFD samples compared with HAD samples, which had less air bubbles.

Effects of magnetic field on freezing characters of carrot, potato and broccoli

In order to study the effects of magnetic field on the freezing characteristics of fruits and vegetables, carrot, potato and broccoli were frozen without and with a magnetic field of intensities 4.6 Gs, 9.2 Gs, 18 Gs and 36 Gs, while recording the temperature changes. The microstructure and temperature change of carrot during freezing were analyzed in detail. The results show that the magnetic field has effects on the freezing process of carrot strips, characterized by a reduction of the phase transition time and a characteristic diversity of the ice crystal formation zone. Crystallization characteristics are similar in the samples without and with a magnetic field of low intensities (4.6 Gs and 9.2 Gs), which show as a non-homogeneous phase transition process in the carrot sample, and the tissue changed showing a sunken, cell morphology variation and increased intercellular space. The freezing is almost homogeneous under a magnetic field of intensities 18 Gs and 36 Gs, then results in a slightly change of cell morphology. In addition, the effects of magnetic field on fruits and vegetables are varying with their species.

Study on dynamics and freezing behaviors of water droplet on superhydrophobic aluminum surface

In this work, a kind of superhydrophobic aluminum surface (treated-Al-SA) is fabricated by chemical etching using hydrochloric acid and hydrogen peroxide, and then treated by stearic acid. The surface of treated-Al-SA has typical hierarchical structure, which hold its durable superhydrophobicity under water droplet impact test, bending test, temperature test, humidity test and corrosive liquids test. Moreover, treated-Al-SA show excellent self-cleaning property at the different tilt angle under room temperature. Results on the dynamic behavior of water droplet is indicated the good anti-adhesion and superhydrophobicity of the treated-Al-SA. The whole freezing process of the water droplets on the surface of untreated-Al and treated-Al-SA is observed. The freezing time of treated-Al-SA is 59 s, which is longer than that of untreated-Al. It is estimated that the different anti-freezing behavior is caused by the thermal conductivity between the water droplet and plate, which is provided an insight into freezing characteristics of water droplet. This study is offered the novel opinion of the superhydrophobic aluminum surface produced by chemical etching, which may help the application of structured aluminum surface.

Experiment on the freezing temperature of silty clay modified by fly ash and crumb rubber

Silty clay modified by fly ash and crumb rubber has been proven to have potential as a subgrade filling in frozen areas due to its steady strength after freeze–thaw cycles and outstanding thermal insulation. To understand the freezing characteristics of this novel modified soil, its freezing temperature was measured as an evaluation index. An improved method using a Beckman thermometer was adopted to obtain the freezing temperatures from freezing curves. The freezing curves of silty clay, fly ash soil (silty clay modified by fly ash) and the modified soil (silty clay modified by fly ash and crumb rubber) with different water contents were determined. The influence of fly ash and crumb rubber on the freezing temperature of silty clay was investigated. The results indicated that the characteristics of the freezing curves of the test soils differed slightly, particularly in terms of the freezing temperatures and freezing rates. Fly ash can be applied to decrease the freezing temperature of silty clay, whereas crumb rubber increases the freezing temperature slightly. Three empirical equations without considering the effect of fly ash and crumb rubber were developed to calculate the freezing temperatures of the test soils.

Studies on the performance of ground source heat pump affected by soil freezing under groundwater seepage

Groundwater seepage and soil freezing will affect heat transfer ability in soil and the operation performance of ground source heat pump system (GSHPS) in cold regions during heating period. In this paper, a full-scale dynamic simulation platform for the ground heat exchangers (GHEs) performance under typical geological conditions is established, and the soil freezing characteristics and dynamic performance of a GSHPS at different seepage velocities are studied. This realizes the real-time coupling between the temperatures at the inlet and outlet of the buried pipes and the building loads, which is more suitable for actual operation of GSHPS. The results show that with the increase of seepage velocity, the soil freeze later and thaw earlier, and the freezing shape and size of the area around the borehole will also change. When seepage velocity is 0, 0.1, 0.5 and 1.0 m/d, the outlet temperature of GHEs with soil freezing factor is higher than that without freezing and the maximum difference between them is 2.68, 2.41, 1.40 and 0.95 K, and the minimum heating coefficient of performance (COP) with freezing or not are 2.24/2.00, 2.43/2.22, 2.82/2.74 and 3.04/2.99, respectively. Therefore, whether soil freezing is taken into account or not, the difference is getting smaller, and the influence of soil freezing on system performance weakens gradually with seepage velocity increasing. The influence of groundwater seepage and soil freezing on soil temperature field and system performance are quantified, which provide a reference for the design and operation of GSHPS.

Effect of precooling conditions on the ice nucleation temperature and freezing characteristics of semisolid matrices

Abstract This study investigated the effects of precooling conditions on the ice nucleation phenomena of water and semisolid matrices (i.e., gelatin and agar gels). For water, the precooling rate was affected by the ice nucleation temperature, while the difference in the ice nucleation temperature was 0.9 °C even if the precooling rate was changed by 10-fold. For a semisolid matrix, the impact of the precooling rate on the ice nucleation temperature was clearly observed when a constant temperature was applied, whereas this relationship was diminished under stepwise cooling conditions with large temperature intervals (2–8 °C). Linear precooling could be achieved by applying stepwise cooling with a 0.5 °C temperature interval. Ice nucleation could be manually obtained at varying temperatures, and the results indicated that the nucleation temperature affected the phase transition time and the size of the ice crystals for completely frozen gelatin gel. Consequently, this study demonstrated a possible application of precooling conditions in the design of a feasible and novel freezing technique.

The Effect of Cross-linking Efficiency of Drug-Loaded Novel Freeze Gelated Chitosan Templates for Periodontal Tissue Regeneration

Innovative strategies for periodontal regeneration have been the focus of research clusters across the globe for decades. In order to overcome the drawbacks of currently available options, investigators have suggested a novel concept of functionally graded membrane (FGM) templates with different structural and morphological gradients. Chitosan (CH) has been used in the past for similar purpose. However, the composite formulation of composite and tetracycline when cross-linked with glutaraldehyde have received little attention. Therefore, the purpose of the study was to investigate the drug loading and release characteristics of novel freeze gelated chitosan templates at different percentages of glutaraldehyde. These were cross-linked with 0.1 and 1% glutaraldehyde and loaded with doxycycline hyclate. The electron micrographs depicted porous morphology of neat templates. After cross-linking, these templates showed compressed ultrastructures. Computerized tomography analysis showed that the templates had 88 to 92% porosity with average pore diameter decreased from 78 to 44.9 μm with increasing concentration. Fourier transform infrared spectroscopy showed alterations in the glycosidic segment of chitosan fingerprint region which after drug loading showed a dominant doxycycline spectral composite profile. Interestingly, swelling profile was not affected by cross-linking either at 0.1 and 1% glutaraldehyde and template showed a swelling ratio of 80%, which gained equilibrium after 15 min. The drug release pattern also showed a 40 μg/mL of release after 24 h. These doxycycline-loaded templates show their tendency to be used in a functionally graded membrane facing the defect site.

Transmittance freezing of a random frozen photons beam in a turbulent ocean

By proposing a statistical averaging control function of the light intensity longitudinal profile and deriving the second moment of random complex weighting factors of Bessel-Gaussian beams, we derive the transmittance of a random frozen photons beam and analyze the freezing evolution characteristics of a random frozen photons beam as the parameters of the beam and channel of a turbulent ocean. It is concluded that the freezing characteristics of transmittance can be effectively improved by choosing beam parameters, such as, a larger number of superposition sub beams, a larger beam waist and a smaller quantum number of orbital angular momentum of the vortex mode. However, channel parameters, such as “equivalent temperature structure constant”, dissipation rate of the mean-squared temperature, dissipation rate of kinetic energy per unit mass of fluid, and the ratio of temperature and salinity can only affect the transmittance of a random frozen photons beam without changing the transmittance freezing characteristics. In addition, the influences of the inner and outer scales of turbulence on the transmittance freezing characteristics can be ignored.

Effect of low temperature on the aging characteristics of a potato starch gel

In this work, the freezing curve of a potato starch gel with different concentrations was determined. The water migration, texture, microstructure and gelatinization of a potato starch gel with 8% starch concentration were studied during aging. The results showed that the freezing characteristics of the potato starch gel with different starch concentrations were quite different. NMR results showed that the relaxation time and proportion of water with different existing states (T21, T22 and T23) in the potato starch gel varied significantly under different aging temperatures. Under different aging temperatures, the texture characteristics and the gel strength of the starch gel were significantly different. The water retention of the gel was better under aging temperatures of 3 °C and −3 °C than for other gel samples. SEM and C-cell results showed that under aging temperatures of 3 °C and 0 °C, the formation of a gel network structure was accelerated, and the gel was relatively firm, with small and uniform pores and a larger pore area and number. The rapid viscosity analysis results showed that the peak viscosity, breakdown and setback of the vacuum freeze-dried gel powder changed differently under the aging temperatures.