微通道内冰晶生长过程的相场模拟

为研究微通道内冰晶生长的微观机理以便提出相应的防结冰措施，建立了微通道内结冰过程的相场模型，模型中引入了实际边界条件的影响(取为第三类边界条件),相场计算中采用随机方法生成晶核。基于有限差分法对相场模型进行了数值求解，对不同边界条件影响强度(以努塞尔数Nu表征)下冰晶生长过程进行了系统研究。研究结果表明：边界对微通道内的冰晶生长影响显著，Nu越大，微通道内冰晶生长第一阶段(冰晶未扩展到边界时)结晶速率越大，使得充分结晶(定义为结晶百分数达到80%)所需总时间越短；同时应在结冰初始阶段对其进行及时干预才能有效防止结冰。     

双冷针冷冻手术中冻结相变传热的数值模拟

建立了生物组织(含正常组织与肿瘤)冻结过程相变传热的数学模型.模型中采用树状分形方法模拟血管的结构形态,同时考虑了组织在冻结过程中存在冻结区、糊状区和未冻结区三个区域.对生物组织在同时插入两根探针时冻结过程中温度场分布及冰晶生长规律进行了模拟.模拟结果表明:在冷冻初期,冰晶的生长速度较快,到了后期冰晶生长速度明显减慢;探针间距在冷冻初期对冰晶生长产生较大影响,随着冷冻时间的增加这种差别会逐渐减少.     

静电场对红细胞悬液冻结特性的影响

本文以红细胞悬液为研究模型,通过在对其进行慢速冻结的过程中引入静电场,具体研究了静电场对降温过程中红细胞悬液冻结特性的影响。实验结果表明:静电场在一定程度上改变了冰晶的形成与生长特性,抑制了晶核的形成, 使晶枝出现不对称生长,并减缓了冰晶的生长速度,这些影响随着场强的增加而逐渐加强。在较强的静电场的影响下,冰晶明显变粗,最终成为块状,细胞也不再与冰晶分离而是完全融入粗大的冰晶之中,在冻结的末期,细胞不再受到冰晶挤压,从而减少了其所受的机械性损伤。     

肿瘤冻融相变传热过程的数值模拟

本文建立了肿瘤组织多孔介质冻融相变传热的数学模型.采用显热容法研究模拟了冷热交替治疗过程中肿瘤组织中温度场分布以及冰晶增长和融化过程.研究结果表明,当冷刀进入组织以后,组织内会产生冰晶的生长.当冷刀的温度上升时,开始时在冷刀侧附近冰晶融化,出现一个冰晶融化的相界面;随着冷刀温度的上升,逐步出现两个冰晶融化的相界面.血液灌注率的存在使组织冻结速度变慢.     

壁面静止水滴冻结过程形状变化

低温表面上的液滴冻结时会形成具有尖顶的形状。针对这一现象开展了理论与实验研究,建立了新的动态曲形相界面模型用来模拟水滴冻结过程中的形状变化。模型考虑重力和成核再辉效应的影响,将冻结过程中的冰水相界面近似为球冠形曲面,并在三相点处引入动态生长角和直角关系。对壁面上20μL静止水滴进行了冻结实验,记录水滴三相点高度的演化过程,以此拟合得到了其随时间变化的关联式,基于该关联式求解理论模型,得到了水滴最终冻结形状。模拟结果与实验结果在水滴初始轮廓、成核再辉轮廓和最终冻结轮廓以及冻结时间上均吻合良好。曲面模型的计算结果表明,固液相界面上不同位置处的冻结速率不同;随着相界面向上推移,冻结速率逐渐减小。     

各向异性系数对等温结晶冰晶生长相场法模拟的影响

采用国内外描述相变微观结构的相场模型,将果汁体系视为水和溶质组成的二元系统,探究各向异性系数取值对冰晶相貌及溶质分布模拟的的影响.结果表明:随着各向异性系数的增大,冰晶的二次分枝越来越发达,其尖端生长速度变快,且尖端速度波动的幅值增大;冰晶生长速度越快,则凝固时间越短,溶质扩散层越薄.各向异性系数在0.025左右取值,较能反映冰晶实际生长形貌.     

二元合金高速定向凝固过程的相场法数值模拟

利用相场法对二元合金高速定向凝固过程进行了模拟，为简化计算，采用了温度冻结近似.模拟了定向凝固时平界面失稳、界面形态演化、溶质浓度分布以及高速绝对稳定性条件下胞晶向平面晶的转变等，得到了不同抽拉速度下胞晶间距、胞晶尖端温度以及有效溶质分配系数等参数，显示了高速生长条件下的溶质截留效应. 关键词： 定向凝固 相场法 高速绝对稳定性 溶质截留     

基于液氮喷雾式食品流态化速冻机的青刀豆速冻实验研究

速冻是用来保存食品的常用方法之一,而在食品的长期保存的过程中人们一直关心的一个问题是如何能够保持食品的外观、完整性和品质。流态化速冻是实现单体快速冻结的最有效方法之一。我所自行研制了液氮喷雾式食品流态化速冻机。本文便是基于此速冻装置的基础之上,对青刀豆进行速冻试验研究,来分析在不同的冻结条件下对物料的冻结时间、干耗和品质的影响。通过冻结青刀豆复温后的电镜照片,来分析速冻过程中冰晶生长对食品品质的影响。     

卷云冰晶粒子散射相函数的展开系数研究

冰晶粒子散射相函数的多项式展开可以应用于卷云条件下大气辐射传输模式的计算。本文利用广义球函数对表面严重粗糙的冰晶粒子相函数进行展开,并对表面严重粗糙的实心棱柱模型、实心棱柱聚合物模型和一般种类混合模型3种冰晶粒子的展开系数特性进行分析研究,数值计算结果表明了广义球函数对表面严重粗糙的冰晶粒子多项式展开的有效性。     

相场模型模拟液固界面各向异性作用下自由枝晶生长

采用自适应有限元方法求解相场模型,分别对界面能各向异性和界面动力学各向异性条件下自由枝晶生长过程进行了模拟.计算表明两种各向异性均显著影响枝晶的生长,随着各向异性的增大枝晶尖端生长速度增大,尖端半径降低. 两种各向异性对自由枝晶生长有着不同形式的影响,在界面能各向异性条件下,枝晶生长稳定性系数与各向异性系数成幂函数关系;而在动力学各向异性条件下,稳定性系数与各向异性系数成线性关系. 关键词： 界面能各向异性 动力学各向异性 自由枝晶生长 相场模型     

Antifreeze glycoproteins from the antarctic fish Dissostichus mawsoni studied by differential scanning calorimetry (DSC) in combination with nanolitre osmometry

This study investigates in detail the freezing events during cooling of solutions of various size classes of antifreeze glycoproteins. Differential scanning calorimetry and nanolitre osmometry were used to observe ice growth at temperatures within the hysteresis gap between the melting point and non-equilibrium freezing point (hysteresis freezing point) of solutions of the various sized antifreeze glycoproteins (AFGPs). The ice growth within the hysteresis gap is presumably due to both the expression of primary or near primary prism planes and also some growth at the basal plane. The binding of the AFGPs to the ice causes a particular ice crystal morphology. With the smaller AFGPs, substantial microscopic ice growth was observed in the form of a hexagonal bipyramids within the hysteresis gap.     

Freezing rate affects the survival of a short-term freezing stress in Panagrolaimus davidi, an Antarctic nematode that survives intracellular freezing

The ability of the Antarctic nematode Panagrolaimus davidi to survive a short-term freezing stress depended upon the rate of freezing of its surroundings, measured as the duration of the sample exotherm. The freezing rate increased as the sample volume and freezing temperature decreased and resulted in fewer nematodes surviving. This appears to be due to the greater risk of physical damage by ice crystal growth at high freezing rates. Once frozen the nematodes will then survive exposure to lower temperatures. The environment of the nematode is likely to produce the slow rate of freezing of its surroundings that is necessary for its survival.     

有限厚度冻冰相变过程的传热研究

1前言确定相界面的瞬态位置是求解冻冰相变问题的重要部分。目前对于有限区域相变问题还没有可靠的精确解[1]。冻冰过程最简单分析方法是忽略固液两相的显热变化，建立相界面移动的准稳态传热关系式，近似预测固一液相变位置[2-4]。本文将考虑液、固相中显热的变化，对有限厚平板冻冰过程，采用精确解与积分解相结合的方法，分析冰层冻结过程中的固一液相界面、边界热流密度和贮入固、液相内的冷量随冻冰时间的变化规律。2物理一数学模型分析该研究的问题是一块贮冷板的两侧面直接与一个低温蒸发器直接接触，使贮冷板内冻冰贮冷。通常蒸发器…     

A theoretical model for ice primary nucleation induced by acoustic cavitation

The modelling and simulation of ice nucleation triggered by acoustic cavitation was addressed in this study. The objective was to evaluate the number of nuclei generated by a single gas bubble and afterwards by a multi-bubble system as function of the acoustic pressure (ultrasound wave amplitude) and supercooling level (liquid temperature). According to our calculations, the nucleation could be initiated with moderated acoustic pressure amplitude (around one bar) even at low supercooling levels (around few degrees). These results may provide a sound basis for the control of ice crystal size and morphology which is a key issue in industrial freezing and freeze-drying processes.     

Pressure-Assisted Freezing and Thawing of Foods: A Review of Recent Studies

Phase transition phenomena take place in aqueous solutions, model foods and cellular tissues subjected to combined high pressure and low (subzero) temperature. The kinetics and mechanisms of pressure-shift ice nucleation, of type III, V or VI ice crystal formation, or of pressure-thawing are still poorly documented. Physical and chemical factors affect the ice crystal size distribution throughout sample depth. These phase transition phenomena, as well as the pressure level, influence both chemical constituents (proteins, enzymes) and structural elements (gels, emulsions, cellular tissues, micro-organisms). Recent studies indicate that: (1) in pressure-shift freezing of muscle foods, the aggregation of myofibrillar proteins and the resulting toughness due to high pressure exposition appear to offset the benefit of small ice crystal formation; (2) the cell and tissue structure of some fruits or vegetables is less disrupted by pressure-shift than air-blast freezing, but it is doubtful whether this brings improved appearance, texture or water retention; (3) high pressure-low temperature inactivation of enzymes is not sufficient to replace thermal blanching; (4) high pressure microbial inactivation is enhanced below 0 °C, but probably not enough for practical applications; (5) the benefits of pressure-thawing in terms of enhanced rate and hygiene may not compensate for increased equipment and packaging costs; (6) progress is made in modelling pressure-freezing and thawing, and in assessing the extent of pressure-shift nucleation.     

Freezing transition of interfacial water at room temperature under electric fields

The freezing of liquid water into ice was studied inside a gap of nanometer spacing under the control of electric fields and gap distance. The interfacial water underwent a sudden, reversible phase transition to ice in electric fields of 10(6) V m(-1) at room temperature. The critical field strength for the freezing transition was much weaker than that theoretically predicted for alignment of water dipoles and crystallization into polar cubic ice (>10(9) V m(-1)). This new type of freezing mechanism, occurring in weak electric fields and at room temperature, may have immediate implications for ice formation in diverse natural environments.     

The pattern of freezing of grapevine shoots during early bud growth

The pattern of freezing of two varieties of grapevine during spring bud burst was characterised using infrared thermography. All plants studied showed endogenous freezing of the stems and subsequent rapid ice spread (0.47 cm/s) analogous to ice spread in bulk water suggesting ice travel in the xylem. Barriers to ice spread were observed between stembranches and more importantly between the stem and buds. Buds froze after the stem and freezing appeared to be initiated from the stem. The lack of a fully functional xylem system is proposed as the barrier to ice spread. All buds which froze suffered complete frost kill whilst frozen stem recovered unharmed. Only 58% of the buds froze and those that did not freeze survived completely.     

An NMR protocol for determining ice crystal size distributions during freezing and pore size distributions during freeze-drying

Nuclear magnetic resonance water proton relaxometry is widely used to investigate pore size distributions and pore connectivity in brine-saturated porous rocks and construction materials. In this paper we show that, by replacing water with acetone, a similar method can be used to probe the porous structure of freeze-dried starch gels and therefore the ice crystal size distribution in frozen starch gels. The method relies on the observation that the starch surface acts as a powerful relaxation sink for acetone proton transverse magnetization so that Brownstein-Tarr theory can be used to extract the pore size distribution from the relaxation data. In addition the relaxation time distribution is found to depend on the spectrometer frequency and the Carr-Purcell-Meiboom-Gill pulse spacing, consistent with the existence of large susceptibility-induced field gradients within the pores. The potential of this approach for noninvasively measuring ice crystal size distributions during freezing and pore size distributions during freeze-drying in other food systems is discussed.