初始水分含量对油纸绝缘热老化特性的影响

为探讨初始水分含量对变压器油纸绝缘老化过程的影响机制,设计了绝缘纸初始水分质量分数分别为1%、3%、5%的矿物油-纸绝缘试品在90°C下的加速热老化试验,定期取样测量了绝缘纸聚合度、油纸绝缘水分质量分数、油中糠醛质量浓度、油中酸值等特征参量。然后,在分子模拟软件内建立了不同含水量的矿物油分子、糠醛分子、水合氢离子模型,通过分子动力学模拟技术计算了糠醛分子及水合氢离子与不同水分含量油分子模型之间的扩散系数、相互作用能、径向分布函数。实验结果表明:绝缘纸和矿物油中水分含量在老化过程中均是波动的,油中糠醛和酸值在老化过程中的变化趋势未与绝缘纸聚合度的下降规律相对应,受系统水分含量的影响较大。分子模拟结果表明:水分的增加显著降低了糠醛及小分子酸的扩散能力,并与糠醛及小分子酸形成稳定的氢键;同时,水分与糠醛及小分子酸的极性作用改变了彼此的相互作用能。实验数据与分子模拟结果相互验证,为深入研究油纸绝缘的老化机理及诊断提供了理论参考。

热压初始水分对重载纤维模塑材料性能的影响

分别以杨木高得率浆(HYP)、杨木化学浆(CP)、废纸板浆(WPP)为原料,研究热压初始水分含量(IMC)对重载纤维模塑材料性能的影响。依照Herzberg染色法和造纸原料化学分析的标准比较了纤维原料纤维种类和化学成分的异同,采用XPS、XRD和SEM表征手段系统比较了IMC对重载纤维模塑材料的化学组分、聚集态结构以及微观形貌的影响,同时考察了IMC对重载纤维模塑材料物理力学性能的影响。实验结果表明,零压干燥时,IMC增加导致WPP模塑材料密度与力学强度降低。在热压条件下,IMC的提高,促进胶黏性聚合物的生成,有助于改善HYP和CP模塑材料的结晶指数,利于纤维细胞腔压实和纤维间的紧密交织;IMC的提高,能够显著提高HYP、CP和WPP纤维模塑材料的密度和力学强度,IMC由10%增加到60%时,HYP、CP和WPP模塑材料的密度提高18.6%、23.8%和8.0%,拉伸强度提高119.74%、173.2%和168.3%,弯曲强度提高134.9%、140.2%和125.0%。

初始水分含量与温度对油炸香蕉片贮藏稳定性的影响

为研究油炸香蕉片在不同贮藏条件下的贮藏特性,采用静态平衡将样品水分稳定到高、低两种初始水分含量,分别贮藏于0、25、37℃的避光环境,通过测定过氧化值、酸值、茴香胺值和介电常数等指标评价油炸香蕉片贮藏期间的氧化状况。结果表明,相同的贮藏温度和时间条件下,高初始水分含量油炸香蕉片的过氧化值、酸值、茴香胺值、介电常数均高于低初始水分含量的样品,降低油炸香蕉片的初始水分含量可以提高其贮藏稳定性。高初始水分含量油炸香蕉片的过氧化值随贮藏时间的延长而升高,低初始水分含量样品达到峰值后逐步降低;样品的酸值、茴香胺值在贮藏期间均升高;高初始水分含量样品介电常数在贮藏期间变化不大,低初始水分含量样品介电常数随贮藏时间的延长,整体呈上升趋势。

不同水分含量鲜腐竹冷藏过程中品质变化研究

目的探究不同初始水分含量下鲜腐竹冷藏过程中品质的变化规律,确定适宜鲜腐竹冷藏的初始水分含量。方法将鲜腐竹干燥至水分含量分别为50%、45%、40%后,4℃冷藏。利用色差计、质构仪(texture profile analysis,TPA)、扫描电镜(scanning electron microscope,SEM)、低场核磁共振(low-field nuclear magnetic resonance,LF-NMR)测定鲜腐竹的色泽、质构特性、微观结构和水分分布状态;采用化学分析手段测定鲜腐竹的可溶性肽含量、过氧化值、蒸煮损失率,并对鲜腐竹进行感官评分和菌落总数测定,探究冷藏过程中鲜腐竹的品质变化规律以及水分含量对鲜腐竹冷藏品质的影响机制。结果随着冷藏时间的延长,鲜腐竹的L^(*)呈下降趋势,a^(*)、b^(*)呈增加趋势,鲜腐竹感官品质劣变,蒸煮损失率、可溶性肽含量、菌落总数增加。初始水分含量下降,腐竹色泽L^(*)降低,a^(*)增加;40%和45%水分含量的腐竹,冷藏过程中的硬度、胶黏性、咀嚼性较高,45%水分的腐竹弹性最大,且质构特性更为稳定;冷藏过程中,鲜腐竹感官品质劣变,降低水分含量至45%和40%时,可使鲜腐竹在第8 d时感官评分在60分以上,其对感官品质中的质地和气味改善最大;40%的初始水分含量,腐竹的蒸煮损失率最低,但与45%的水分含量不具有显著性差异(P<0.05);降低水分含量,可延缓鲜腐竹最大过氧化值出现的时间,降低蛋白质网络结构的破坏程度。冷藏过程中存在明显的水分迁移现象,鲜腐竹的硬度、咀嚼性、弹性、胶黏性受可溶性肽含量影响最大,与其呈极显著负相关(P<0.01)。结论初始水分含量显著影响鲜腐竹的冷藏品质,降低水分含量对于延长鲜腐竹保质期具有重要意义。

储存条件对玉米胚及其毛油中黄曲霉毒素B_1含量和品质的影响

将不同初始水分(5%、9%、12%、18%)的玉米胚在25℃、45%相对湿度的条件下进行储存,取不同储存时间的玉米胚样品检测其粗脂肪、粗蛋白含量及黄曲霉毒素B1(AFB1)含量,并提取玉米毛油检测其酸值、过氧化值、色泽及AFB1含量,研究储存条件对玉米胚及其毛油中AFB1含量和品质的影响。结果表明,在30 d的储存时间内,初始水分为5%和9%的玉米胚及其毛油的主要质量指标和AFB1含量均无明显变化,但毛油酸值(KOH)分别增加了1.31 mg/g和3.60 mg/g;初始水分为12%和18%的玉米胚,粗脂肪含量分别下降了5.05个百分点和6.09个百分点,毛油酸值(KOH)分别增加了25.89 mg/g和61.69 mg/g,色泽明显加深,玉米胚及其毛油中AFB1含量随储存时间的延长逐渐增加,至30 d时,玉米胚中AFB1含量分别达到3.192μg/kg和7.528μg/kg,对应的毛油中AFB1含量分别达到1.134μg/kg和2.128μg/kg。但所有玉米胚样品及其毛油样品中AFB1含量均未超过国家限量标准(20μg/kg)。

树莓果片微波膨化机理

为了揭示微波加热树莓果片的体积膨胀机理,对树莓果片微波膨化特性进行研究,根据电磁场、传热场、结构力学及稀物质传递理论,建立了树莓果片微波膨化四场耦合模型,选取微波强度和初始水分质量分数作为影响参数,得到了膨化过程中树莓果片的温度分布、水分质量分数及膨胀变形的变化规律,并与实验结果进行对比分析。结果表明:微波加热条件下,果片内部的水分蒸发使得果片内产生较高的压力,推动果片膨胀,果片表面水分的蒸发使得果片发生收缩行为,膨胀和收缩这两种相反的趋势最终决定树莓果片的体积变化。果片的温度分布主要由微波穿入果片的渗透深度决定,随着微波加热时间的延长,果片的水分质量分数逐渐降低。温度的升高和水分质量分数的降低导致树莓果片弹性模量的增大,弹性模量的变化影响着果片体积的膨胀。当微波强度为20~40 W/g、果片初始水分质量分数为26%时,膨化后树莓果片的品质较好,且膨化率也比较高,最大膨化率可达到3.91。

不同储藏条件下玉米挥发性成分研究

为通过挥发性物质变化来探究玉米储藏期间品质变化,并找出相关性特征挥发物,为玉米安全储藏提供参考。本实验模拟不同仓储条件,通过电子鼻(E-NOSE)和顶空固相微萃取-气质联用(GC-MS)对玉米挥发性物质进行检测,结合主成分分析法和样品得分图对结果进行分析。结果表明,随储藏周期变化,不同水分、温度条件下玉米样品,雷达图有明显区别,对应传感器显示15%高水分样品比低水分样品更易出现霉味。高温高水分样品在储藏3个月后在主成分分析图上与其余样品有明显区分,表明气味变化较大。气质检测中共检出醇类物质15种,芳香烃25种,醛类23种,酸酯类59种,酮类18种,烷烃53种,烯烃22种,杂环类27种。贡献率较大的是酸酯类、芳香烃和醇类物质。氧气浓度较低时,酸酯类物质挥发量减少,氧气浓度对醇类、芳香烃等影响较小,且只在浓度极低(2%~5%)时,2,6-二叔丁基对甲酚和1-氯二氟甲氧基-4-硝基-苯挥发量会增加。在低水分条件下苯甲醇挥发量较大;alpha-戊基-gama-丁内酯、壬酸和烯醛类物质在低温低水分条件下挥发量较大;酰胺类物质则在高温高水分条件下产生较多;香兰素在高温高水分下挥发量较小;不饱和烯烃与烯醇类物质都随着储藏时间的推移而增加。电子鼻和GC-MS能有效对不同储藏条件下的玉米样品进行区分,亦可通过特定挥发物质的多少来判别玉米品质的好坏,水分与温度两个条件对挥发物质影响较大,氧气浓度影响较小,当水分含量低于13%,温度低于20℃时在常压下储藏就能有效防止玉米品质劣变。

Isothermal diffusion of water vapor in unsaturated soils based on Fick’s second law

In arid regions, water vapor diffusion predominates the total water migration in unsaturated soil, which significantly influences agriculture and engineering applications. With the aim of revealing the diffusion mechanism of water vapor in unsaturated soil, a water vapor migration test device was developed to conduct the water vapor migration indoor test. The test results demonstrate that the characteristics of water vapor diffusion in unsaturated soil conformed to Fick’s second law. A mathematical model for water vapor diffusion under isothermal conditions in unsaturated soil was established based on Fick’s law. Factors including the initial moisture content gradient, initial moisture content distribution, soil type and temperature that affect the water vapor diffusion coefficient were analyzed. The results show that there was good agreement between the moisture content calculated by the mathematical model and obtained by the indoor experiment. The vapor diffusion coefficient increased with increasing initial moisture content gradient and temperature. When the initial moisture content gradient is constant, the vapor diffusion coefficient increases with the increase of matrix suction ratio in dry and wet soil section. The effect of soil type on the water vapor diffusion coefficient was complex, as both the moisture content and soil particle sizes affected the water vapor diffusion.

Drying of Fish Sardines in Oman Using Solar Tunnel Dryers

This paper deals with the design, construction and performance evaluation procedure of a solar tunnel dryer in drying fish. A 12 meter long and 2 meter width half-circled tunnel was designed and constructed to dry about 50-100 kg of freshly harvested fishes per batch. The half of the tunnel base was used as the flat plate air heating solar collector and the remaining half as a dryer. The drying air was forced from the collector region （north side） to the drying region （south side） of the half circled tunnel where the product is to be dried. The drying temperature could be easily raised by some 5-30 ℃ above the ambient temperature inside the tunnel at an air velocity of approximately 0.2 m/sec. The test was conducted with 51.5 kg freshly harvested sardines （hall-load） with initial moisture content of 66.5% （wet-basis） to analyze the performance of the dryer. The fishes were dried to a final average moisture content of 15.5% （wet-basis） within three days （30 hours）. It was possible to reach the moisture content level for safe storage within less than three days （30 hrs） with solar tunnel dryer and 7 days in open air natural sun drying. The improvement in the quality of fishes in terms of color, brightness, flavor, and taste and food value was distinctly recognized.

Numerical investigation on freeze-drying of aqueous material frozen with pre-built pores

Freeze-drying of the initially porous frozen material with pre-built pores from liquid material was found experimentally to save drying time by over 30% with an initial saturation being 0.28 compared with the conventional operation with the initial saturation being 1, using mannitol as the solid material. In order to understand the mass and heat transfer phenomena of this novel process, a two-dimensional mathematical model of coupled mass and heat transfer was derived with reference to the cylindrical coordinate system. Three adsorption–desorption equilibrium relationships between the vapour pressure and saturation value namely, power-law, Redhead's style and Kelvin's style equation, were tested. Kelvin's style in exponential form of adsorption equilibrium relation gave an excellent agreement between the model prediction and experimental measurement when the equation parameter, γ, of 5000 was applied. Analyses of temperature and ice saturation profiles show that additional heat needs to be supplied to increase the sample temperature in order to promote the desorption process. Simulation also shows that there is a threshold initial porosity after which the drying time decreased with the increase in the initial porosity. Enhanced freeze-drying is expected to be achieved by simultaneously enhancing mass and heat transfer of the process.

Experimental Study on the Dam-Break Hydrographs at the Gate Location

When studying the dam-break flow phenomenon,the basic hydrodynamic features of the dam-break flow at the gate location should be verified primarily.In this study,laboratory experiments were performed in a rectangular and horizontal flume with the same initial water head setting on the dry and wet downstream bed conditions.Water surface elevation was extracted through image analysis and validated by comparing with the data measured using a wave gauge.Temporal variation of the water surface elevation at the gate location,quantified in terms of high-speed video recorded images,can be divided into three stages,the sharp decreasing stage,the relatively steady stage,and the gradually decreasing stage.Applicability of several classic analytical solutions of the dam-break problem at the gate location was validated using present experimental data.Ritter's solution is effective for the dry bed condition while Stoker's solution could be applied to the wet bed case,and both are only applicable during the steady stage.Lin' solution reproduces the gate-site hydrographs well during both the relatively steady and the gradually decreasing stages,especially for the condition under which the down-upstream water depth ratio is smaller than 0.138.