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Freezing Process (freezing + process)
Selected AbstractsStart Up and Freezing Processes in PEM Fuel CellsFUEL CELLS, Issue 2 2007M. Oszcipok No abstract is available for this article. [source] Frost formation on a bionic super-hydrophobic surface under natural convection conditionsHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 7 2008Yunjun Gou Abstract A bionic super-hydrophobic surface has a multiple micro-nano-binary structure (MNBS) similar to the lotus leaf surface microstructure. This kind of surface has a contact angle of water greater than 150° and a roll angle smaller than 5°. In this paper, the frost deposition phenomena on a bionic super-hydrophobic surface were observed. The surface has many micro bumps and its contact angle is 162°. The formation of water droplets, the droplet freezing process, the formation of initial frost crystals and the frost layer structure on a cold bionic super-hydrophobic surface under natural convection conditions were closely observed. The frost layer structure formed on the super-hydrophobic surface shows remarkable differences to that on a plain copper surface: the structure is weaker, looser, thin, and easily removed and most importantly, it is of a very special pattern, a pattern similar to a chrysanthemum, a frost layer structure that has not been reported before to the best of the present authors knowledge. The experimental results also show that a super-hydrophobic surface has a strong ability to restrain frost growth. The frost deposition on this bionic surface was delayed 55 minutes when compared with a plain copper surface under the conditions of a cold plate temperature of ,10.1°C, air temperature of 18.4°C, and relative humidity of 40%. A theoretical analysis was also presented to explain the observed phenomena. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(7): 412,420, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20216 [source] Freezing time calculations for various productsINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 12 2003Esmail M. A. Mokheimer Abstract This article presents a numerical simulation that estimates the freezing time for different products. In this regard, the freezing process is mathematically modelled by transient heat conduction equations that incorporate the physical properties of the three distinct regions that exist during a freezing process. These regions are namely, the solid phase region, the liquid phase region and the interface region. This model is experimentally validated and used to estimate the freezing time for three different food products, which are namely, fish balls, cherry juice and peas balls. The freezing times estimated numerically through the present model agree well with those reported in the literature and are in excellent agreement with the experimental data. Copyright © 2003 John Wiley & Sons, Ltd. [source] Estimation of apparent thermal conductivity of carrot purée during freezing using inverse problemINTERNATIONAL JOURNAL OF FOOD SCIENCE & TECHNOLOGY, Issue 7 2009Viviana Cocco Mariani Summary This article presents an inverse problem to determine the apparent thermal conductivity of carrot purée during the freezing process. The heat diffusion equation with the enthalpy model is solved to estimate the thermal conductivity. A modern meta-heuristic of evolutionary computation field called Differential Evolution (DE) is applied for the solution of inverse problem. Experiments were performed to estimate the thermal conductivity of the carrot purée as a function of temperature, using two piecewise functions. A best least square fitting between the experimental and predicted temperature curves during freezing conditions is obtained using DE. Statistical analysis are considered with Gaussian error of 0.05 and zero mean showing than the results for one piecewise function are more stable than with another piecewise function. Good agreement between the reported and estimated temperature curves was obtained. The apparent thermal conductivity was observed to decrease asymptotically with temperature in the range [,40 °C, 0 °C] and stay approximately a constant value for temperatures bigger than 0 °C. [source] State transitions and physicochemical aspects of cryoprotection and stabilization in freeze-drying of Lactobacillus rhamnosus GG (LGG)JOURNAL OF APPLIED MICROBIOLOGY, Issue 6 2008K.S. Pehkonen Abstract Aims:, The frozen and dehydrated state transitions of lactose and trehalose were determined and studied as factors affecting the stability of probiotic bacteria to understand physicochemical aspects of protection against freezing and dehydration of probiotic cultures. Methods and Results:,Lactobacillus rhamnosus GG was frozen (,22 or ,43°C), freeze-dried and stored under controlled water vapour pressure (0%, 11%, 23% and 33% relative vapour pressure) conditions. Lactose, trehalose and their mixture (1 : 1) were used as protective media. These systems were confirmed to exhibit relatively similar state transition and water plasticization behaviour in freeze-concentrated and dehydrated states as determined by differential scanning calorimetry. Ice formation and dehydrated materials were studied using cold-stage microscopy and scanning electron microscopy. Trehalose and lactose,trehalose gave the most effective protection of cell viability as observed from colony forming units after freezing, dehydration and storage. Enhanced cell viability was observed when the freezing temperature was ,43°C. Conclusions:, State transitions of protective media affect ice formation and cell viability in freeze-drying and storage. Formation of a maximally freeze-concentrated matrix with entrapped microbial cells is essential in freezing prior to freeze-drying. Freeze-drying must retain a solid amorphous state of protectant matrices. Freeze-dried matrices contain cells entrapped in the protective matrices in the freezing process. The retention of viability during storage seems to be controlled by water plasticization of the protectant matrix and possibly interactions of water with the dehydrated cells. Highest cell viability was obtained in glassy protective media. Significance and Impact of the Study:, This study shows that physicochemical properties of protective media affect the stability of dehydrated cultures. Trehalose and lactose may be used in combination, which is particularly important for the stabilization of probiotic bacteria in dairy systems. [source] Control of pore structure and size in freeze-dried collagen spongesJOURNAL OF BIOMEDICAL MATERIALS RESEARCH, Issue 4 2001Heike Schoof Abstract Because of many suitable properties, collagen sponges are used as an acellular implant or a biomaterial in the field of tissue engineering. Generally, the inner three-dimensional structure of the sponges influences the behavior of cells. To investigate this influence, it is necessary to develop a process to produce sponges with a defined, adjustable, and homogeneous pore structure. Collagen sponges can be produced by freeze-drying of collagen suspensions. The pore structure of the freeze-dried sponges mirrors the ice-crystal morphology after freezing. In industrial production, the collagen suspensions are solidified under time- and space-dependent freezing conditions, resulting in an inhomogeneous pore structure. In this investigation, unidirectional solidification was applied during the freezing process to produce collagen sponges with a homogeneous pore structure. Using this technique the entire sample can be solidified under thermally constant freezing conditions. The ice-crystal morphology and size can be adjusted by varying the solute concentration in the collagen suspension. Collagen sponges with a very uniform and defined pore structure can be produced. Furthermore, the pore size can be adjusted between 20,40 ,m. The thickness of the sponges prepared during this research was 10 mm. © 2001 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 58: 352,357, 2001 [source] Improved slice selection for R2* mapping during cryoablation with eddy current compensation,JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2008Aiming Lu PhD Abstract Purpose To improve the slice profile and image quality of R2* mapping in the iceball during cryoablation with ultrashort echo time (UTE) imaging by compensating for eddy currents induced by the selective gradient when half-pulse radiofrequency (RF) excitation is employed to achieve UTEs. Materials and Methods A method to measure both B0 and linear eddy currents simultaneously is first presented. This is done with a least-square fitting process on calibration data collected on a phantom. Eddy currents during excitation are compensated by redesigning the RF pulse and the selective gradient accordingly, while that resultant from the readout gradient are compensated for during image reconstruction. In vivo data were obtained continuously during the cryoablation experiments to calculate the R2* values in the iceball and to correlate them with the freezing process. Results Image quality degradation due to eddy currents is significantly reduced with the proposed approaches. R2* maps of iceball throughout the cryoablation experiments were achieved with improved quality. Conclusion The proposed approaches are effective for compensating eddy currents during half-pulse RF excitation as well as readout. TEs as short as 100 ,sec were obtained, allowing R2* maps to be obtained from frozen tissues with improved quality. J. Magn. Reson. Imaging 2008;28:190,198. © 2008 Wiley-Liss, Inc. [source] Convective Available Potential Energy (CAPE) in mixed phase cloud conditionsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 624 2007B. Früh Abstract An approximate but pragmatic approach is presented to define Convective Available Potential Energy (CAPE) in mixed phase cloud conditions. The underlying process calls for mixed (i.e. liquid and ice) phase parcels and assumes the liquid fraction to be a unique function of temperature. The approach is meant to represent average conditions. Differences between this and more traditional approaches are quantified and discussed for mean tropical conditions. Generally freezing increases parcel temperature and, hence, buoyancy. If freezing occurs isobarically (as was often assumed in the past), all water changes phase at a single level resulting in a discontinuity in buoyancy at that level. By contrast, the mixed phase parcel process implies a continuous phase transition in a finite range of temperatures Tfs , T , Tfe, leading to a gradual change of buoyancy with altitude and preventing any temperature inversion. The details of this gradual change depend on the choice of the specified temperature range [Tfs, Tfe]. High in the troposphere, where all water is frozen irrespective of the details, the differences between the buoyancy profiles are small (but finite). CAPE is very sensitive to the treatment of the freezing process. Isobaric freezing at a relatively high temperature (e.g. , 5 °C) in a reversible process may increase CAPE by a factor of 2 to 3, and this increase is similar in magnitude to the difference between the pseudo-adiabatic and the reversible processes for pure water parcels. Both of these processes are considered less realistic than the reversible mixed phase process with continuous freezing over a broad temperature range [Tfs, Tfe] = [,5 °C, , 40 °C]; the corresponding CAPE lies about half way between the reversible and irreversible pure water processes. For clouds with finite precipitation efficiency the effect of freezing is less pronounced than for reversible conditions. Copyright © 2007 Royal Meteorological Society [source] Solidification of binary aqueous solution cooled from aboveHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 1 2010Shigeo Kimura Abstract Freezing and melting phenomena are important in many different fields, including crystal growth, casting, metallurgy, geophysics, and oceanography. Solidification of a multi-component solution is the one often observed in nature. In order to investigate basic features of the freezing processes of binary systems, we conducted a series of laboratory experiments in a rectangular box cooled from above using aqueous NaNO3 solution. During the freezing, the solid phase always grows into many needle-like crystals called the mushy layer. We measured the growth of the mushy layer thickness, the solid fraction, the temperature, and the concentration distributions. The average solid fraction is found to increase with time in the mushy layer. This causes a slow descent of the released solute in the mushy layer and its eventual fall into the liquid region below because of gravity. We propose a one-dimensional model to explain the horizontally-averaged mushy layer growth. In the model, the estimate of a heat flux at the mushy-liquid interface due to natural convection is found essential for a correct prediction. The proposed theory predicts well the growth of the mushy-layer and the average solid fraction, once the convective heat flux is properly given. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20278 [source] Metastable States of Water and Ice during Pressure-Supported Freezing of Potato TissueBIOTECHNOLOGY PROGRESS, Issue 3 2004O. Schlüter Different ice modifications were obtained during freezing processes at several pressure levels from atmospheric pressure up to 300 MPa. In the pressure range between 210 and 240 MPa, a metastable ice I modification area was observed, as the nucleation of ice I crystals in the thermodynamically stable region of ice III was reached. A significant degree of supercooling was obtained before freezing the tissue water to ice III, which has to be considered when designing pressure-supported freezing processes. The effect of supercooling phenomenon on the phase transition time is discussed using a mathematical model based on the solution of the heat transfer governing differential equations. Phase transition and freezing times for the different freezing paths experimented are compared for the processes: freezing at atmospheric pressure, pressure-assisted freezing, and pressure-shift freezing. Different metastable states of liquid water are defined according to their process-dependent stability. [source] | ||||||||||||||||