不同真空冷却方式对水煮猪肉块不同部位品质的影响

为探究不同真空冷却方式对肉块不同部位品质的影响,实验以水煮猪肉为研究对象,对比分析了普通真空冷却和浸没式真空冷却两种方法对肉块冷却效果(失重率、降温均匀性)及不同部位品质(色泽和质构)的差异。结果表明:与普通真空冷却相比,浸没式真空冷却能有效地减小肉块的失重率(P0.05);而普通真空冷却的肉块各部位降温比浸没式真空冷却更为均匀;两种冷却方式处理后肉块中部及内部的亮度值和红度值均没有显著差异(P0.05),但浸没式真空冷却的肉块外部亮度明显增加(P0.05);质构结果显示,浸没式真空冷却后,肉块中部和内部的硬度和咀嚼性均显著减小(P0.05),只有肉块外部的弹性明显减小(P0.05),中部和内部的弹性及各部位的回复性均无明显变化(P0.05)。     

真空冷却豆制品水分布状态的核磁共振分析

利用低场核磁共振及其成像技术,研究真空冷却豆制品中水分分布状态.实验测得熟食豆制品真空冷却后其中的结合水、吸附水和自由水三种状态水的迁移过程.反映结合水的T21处于相对稳定的状态,经真空冷却后,发生变化的主要是T22(吸附水)和T23(自由水),吸附水和自由水明显减少;经真空冷却后,样品水分含量减少,体现在核磁共振成像图上,就是质子的流动性降低,使弛豫信号减弱,图像变暗.     

淬火冷速对转子30CrMoNiV511钢韧性的影响

使用热处理风冷炉模拟研究了大型汽轮机转子内外径处的冷却速率对转子材料30CrMoNiV511钢的组织转变及强韧性的影响。结果表明,淬火冷却速率对转子材料的强度影响不明显,而对冲击韧性的影响显著。随着淬火冷却速率从50℃/min逐渐降低至7℃/min,淬火组织由下贝氏体逐渐向上贝氏体、粒状贝氏体及其混合组织转变,冲击韧性由66 J急剧降低到16 J;当冷却速率降低到5℃/min以下时,组织中出现先共析铁素体,且上贝氏体铁素体粗大,碳化物分布极不均匀,冲击韧性极差。不同冷却速率转子钢的微观组织及碳化物析出形态和分布是决定其冲击韧性的主要原因之一。     

淀粉基餐盒的霉变和堆肥菌填埋降解分析

目的 研究淀粉基餐盒表面微生物污染的主要影响因素以及餐盒的降解行为.方法 将淀粉基餐盒置于一定温湿度条件下储放一段时间后,利用马铃薯葡萄糖琼脂培养基对餐盒表面的微生物进行扩大培养,并利用电子显微镜对其进行镜检.同时,采用堆肥菌填埋法研究淀粉基餐盒的降解行为.结果 菌落的颜色、形态、质地与霉菌的一般特征相似,可判断为霉菌.不同淀粉基餐盒中含有的霉菌菌落数量差异较大(P<0.05),即菌落的数量大小排序为S-A>S-D>S-F>S-B>S-E>S-C.在堆肥菌填埋下,淀粉基餐盒之间降解程度的差异也较大(P<0.05),质量损失率大小排序为S-B>S-A>S-F>S-D>S-C>S-E.结论 因餐盒的淀粉含量、表面分布和直链支链淀粉比例的不同而引起的餐盒吸湿性不同是造成霉变和降解差异的重要原因;淀粉基餐盒表面淀粉分布多,降解速度快.     

利用蒸发式冷凝器改善高温环境下R410a空调器性能的研究

R410a空调器在高温环境下制冷能力显著下降.本文通过凝结水量的计算及喷淋装置的设计,利用凝结水、喷淋管、冷却盘管、冷却风机构成的蒸发式冷凝器来改善环境高温下R410a空调器的性能.     

空冷/水冷混合冷却竖管内LiBr溶液降膜吸收过程数值解

针对风冷和水冷联合冷却的竖管降膜吸收器,考虑汽液界面的阻力、变膜厚、横向对流和冷却水的冷却作用的影响,建立了降膜吸收过程中热质耦合数学模型和同心管环空内冷却水换热数学模型.计算了沿竖管内表面的液膜厚度、温度、浓度以及冷却水在混合冷却条件下的温度分布等参数.分析了冷却水进口温度、LiBr溶液Re数和PE数等参数对传热系数和吸收速率的影响.数学模型的计算结果与实验数据吻合较好.得出的结论对联合冷却吸收器的设计和优化具有指导意义.     

中厚板加速冷却和直接淬火时冷却能力研究

为了确定加速冷却或直接淬火时实现预期的冷却速率所需的对流换热系数,利用MSC.MARC有限元分析软件对Q345B中厚钢板冷却过程中温度场进行了数值模拟计算.确定了实现直接淬火条件下不同厚度(20 mm)钢板的理论极限冷却速率所需的对流换热系数为15 000 W/(m2·℃),并分析了冷却速率与对流换热系数、钢板厚度之间关系.研究表明,对于同一厚度、材质中厚钢板,其冷却速率随对流换热系数的增加而增大.超快速冷却或直接淬火时,带钢冷却速率随对流换热系数增加而显著增加;对流换热系数大于15 000 W/(m2·℃)时,厚度(30 mm)钢板的冷却速率基本不变,达到其物理极限冷却速率;换热系数增加,厚度方向上温度梯度增加.     

冷却速率对Mg68Zn28Y4自生复合材料I相形成的影响

采用两种不同冷却方式的普通铸造技术在Mg68Zn28Y4合金中制备了镁合金稳定态二十面体准晶相,分析了冷却速率对准晶相形成、分布、数量和晶粒尺寸的影响.通过扫描电镜、能谱分析仪和透射电子显微分析技术,确定了合金的组织、相成分及准晶相结构.结果表明:Mg68Zn28Y4三元合金在室温冷却过程中,准晶相直接从液相形核、长大;在铸铁模型腔内冷却时,铸件断面的温度梯度小,温度场较为平坦,沿铸型纵横截面准晶相分布均匀、晶粒尺寸约为10～15μm;在铸铁板表面冷却时,铸件断面的温度梯度较大,导致冷却速率显著影响准晶相的分布、数量及形貌.准晶相的晶粒尺寸为10～60μm不等,分布由均匀弥散到不均匀散布再到均匀.     

2.25Cr-1Mo-O.25V低合金耐热钢中贝氏体的形成机制研究

采用真空冶炼炉和可逆式轧机制备出试验材料,将其在940℃奥氏体化后分别水淬到550℃、650℃和750℃,再空冷至室温,然后采用扫描电镜和透射电镜观察材料的显微组织,发现分别主要为下贝氏体、上贝氏体和粒状贝氏体.上贝氏体组织和下贝氏体组织均以扩散机制形成,而区域冷却速率的差异是造成粒状贝氏体组织基体形貌差异的主要原因.     

大型宽幅高真空连续卷绕镀膜设备的研制

本文详细介绍了ZZL-2200/2.5大型高真空连续卷绕镀膜设备的工作原理、结构组成、性能特点和创新设计要点及推广应用前景.分析并指出影响镀膜质量、生产效益的主要因素和解决途径.重点论述了双冷却镀膜辊悬空平展镀膜技术的技术原理和结构特性,及其在解决CPP流延塑料薄膜基材表面镀制铝膜时出现亮暗条纹线难题中的显著效果.并提出独特的蒸发机构冷却和自流式蒸发舟没置的特殊设计对提高大型宽幅高真空连续卷绕镀膜设备生产效率的重要性.     

利用CFD对不同尺寸上下冲击式速冻机静压腔流场的研究

为进一步优化上下冲击式速冻机的冻结效果,本文以静压腔尺寸为4 m×1. 5 m×2 m的实体速冻机为基础,保证入口压力为190 Pa、入口体积流量为2. 64 m3/s不变,提出了4 m×1. 5 m×1. 5 m、4 m×2 m×1. 5 m、4 m×2 m×2 m、4 m×2. 5 m×1. 5 m、4 m×2. 5 m×2 m等5种不同的静压腔尺寸,通过CFD来模拟静压腔尺寸变化对于速冻机内部流场的改变,从速冻机喷嘴出口风速、钢带表面气流的矢量分布、钢带表面换热强度及换热均匀性等方面来综合分析静压腔内部换热特性。结果表明:在静压腔入口流量相同、压力不变的情况下,唯有4 m×1. 5 m×1. 5 m的出口风速与原有尺寸的出口风速相差较小,其余尺寸的出口风速略小但变化幅度不明显。而该尺寸的换热强度以及换热均匀性却远不如4 m×2. 5 m×1. 5 m、4 m×2. 5 m×2 m。此外,静压腔尺寸为4m×2 m×2 m的换热强度虽然比4 m×1. 5 m×2 m的换热强度高约4. 85%,但均匀性较差,不足以成为优选设计。4 m×2. 5 m×1. 5m与4 m×2. 5 m×2 m的换热强度达到177. 76和177. 39,比原有尺寸下钢带表面换热强度高约6. 81%和6. 59%,且均匀性也为最佳。结合上述因素,4 m×2. 5 m×1. 5 m和4 m×2. 5 m×2 m在出口风速、换热强度及均匀性方面均为5种尺寸中最优的设计。     

Freezing of a parallelepiped food product. Part 1. Experimental determination

A number of procedures are used to predict the freezing time of food. The objective of the project reported here was to test the adequacy and applicability of the various mathematical models and methods used to predict the freezing time of a small, parallelepiped food product. The approach was to compare experimental and predicted freezing times. In this paper the experimental methods and results are described; the freezing time and the thermophysical properties of both the frozen and non-frozen food were determined. Comparison between experimental and predicted freezing times will be given in a subsequent paper. The experimental conditions consisted of individually freezing the product in an air blast; the food product was french fries. The time required to lower the temperature of a french fry from 31 to −18°C in the −29°C air blast freezer used was approximately 1200 s. The mean moisture content of the fries was 73.7%. The average densities of the non-frozen and frozen fries were 1069 and 1012 kg m⁻³, respectively; the average thermal conductivities were 0.50 and 1.0 W m⁻¹ °C⁻¹; and the heat capaciti were 3420 and 1870 J kg⁻¹ °C⁻¹. The overall surface heat transfer coefficient of a parallelepiped object in the air blast freezer used for these experiments was 21.0 W m⁻² °C⁻¹. The low surface heat transfer coefficient resulted in a flat temperature profile within the fries.     

Thermal design calculations for food freezing equipment—past, present and future: Calculs thermiques pour la conception de congélateurs industriels: le passé, le présent et le futur

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.     

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.     

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.     

Investigation of heat, mass transfer and fluid flow characteristics in evaporative condensers

Heat and mass transfer and fluid flow characteristics in evaporative condensers is discussed. A complex pattern of water temperature and air enthalpy change was detected which depended upon elevation above the sump level. The results have indicated that the spray-filled space beneath the coil has a substantial effect of heat rejection in this type of apparatus. On the other hand, the effect of the upper spray nozzle zone is insignificant. It is possible to optimize the combination of various heat and mass transfer spaces, as well as the effect of extended surfaces and spray-filled spaces. Specifically, it will help to design better evaporative condensers with closely spaced staggered tubes, and to optimize the heat transfer and energy efficiency characteristics of such units.     

碳氢工质在低温保存箱中的应用分析

对-86℃低温保存箱使用的制冷剂及绝热结构进行分析,理论计算结果表明:采用HC类制冷剂(R290/R170)较采用HFC类制冷剂(R404A/R508B)的两级复叠式制冷系统的性能系数提升20%,箱体绝热性能的改善使低温保存箱的箱体换热系数降低22%,产品耗电量降低40%。测试结果表明,采用HC类制冷剂且增大保存箱真空绝热板厚度使低温保存箱的耗电量降低48%。研究结果对于低温保存箱的环保和能效升级具有借鉴意义。     

冻结装置的技术特征及分析

通过论述冻结装置的技术特征及分析影响其性能的因素,重点揭示了冻结装置存在的问题,提出了提高其效率的有效途径.并强调指出解决冻结食品"冻结机会均等"问题是速冻装置设计的技术关键.还对速冻装置的发展趋势作了概括.     

上下冲击式高效鼓风冻结装置速度场的数值模拟与验证

对上下冲击式高效鼓风冻结装置建立数学物理模型,使用CFD软件对速冻装置内静压箱和贴近网带上下方的速度场进行数值模拟。计算中将位于上下均风孔板间的网带作为多孔介质处理。研究结果表明,增设导流板可改进静压箱拐角处气流组织的均匀性;扩大冻结区两侧隔板和网带侧边形成的通道,也可以改进局部区域流场的均匀性,这种改进方法需要减小网带宽度或加大装置整体宽度。因此,要保证设备的冻结能力,则需综合考虑风速、网带尺寸等对冻结能力的共同影响。此外,网带位置会影响周围气体流场分布,网带距下方送风口距离在20 mm-30 mm之间时,可以保证风速在3 m/s左右,且均匀性较好。通过测试实验验证了模型建立的合理性及计算结果的准确性。     

Heat transfer coefficients for forced-air cooling and freezing of selected foods

To maximize the efficiency of cooling and freezing operations for foods, it is necessary to optimally design the refrigeration equipment to fit the specific requirements of the particular cooling or freezing application. The design of food refrigeration equipment requires estimation of the cooling and freezing times of foods, as well as the corresponding refrigeration loads. The accuracy of these estimates, in turn, depends upon accurate estimates of the surface heat transfer coefficient for the cooling or freezing operation. This project reviewed heat transfer data for the cooling and/or freezing of foods. A total of 777 cooling curves for 295 food items were obtained from an industrial survey and a unique iterative algorithm, utilizing the concept of ‘equivalent heat transfer dimensionality’, was developed to obtain heat transfer coefficients from these cooling curves. Nine Nusselt–Reynolds–Prandtl correlations were developed from a selection of the 777 heat transfer coefficients resulting from this algorithm, as well as 144 heat transfer coefficients for 13 food items, collected from the literature. The data and correlations resulting from this project will be used by designers of cooling and freezing systems for foods. This information will make possible a more accurate determination of cooling and freezing times and corresponding refrigeration loads. Such information is important in the design and operation of cooling and freezing facilities and will be of immediate usefulness to engineers involved in the design and operation of such systems.