功率超声对小麦面粉可溶性组分形成溶液过冷度的影响

应用自行设计的超声辅助冷冻循环系统,研究了频率为25kHz、不同电功率的超声对小麦面粉可溶性组分形成溶液过冷度的影响.研究结果表明,功率超声能显著地降低小麦面粉可溶性组分形成溶液中冰结晶的过冷度,促进冰晶晶核的形成.随着超声电功率逐渐增加,小麦面粉可溶性组分形成溶液过冷度逐渐减小,即成核温度随着超声电功率的增加而增加.研究结果有助于揭示超声强化食品冷冻过程中,溶解有多种无机与有机化合物的食品复杂溶液的冰结晶机理.     

氧化石墨烯纳米流体分散性及过冷特性研究

以质量分数为0.05%的氧化石墨烯纳米流体为实验对象,测量了不同超声分散时间下氧化石墨烯纳米流体的粒度和Zeta电位分布。同时测定了降温过程中氧化石墨烯纳米流体的过冷度和相变时间,探讨了超声时间对氧化石墨烯纳米流体分散性和过冷特性的影响。实验结果表明,超声分散时间越长,分散性能越好,但在150min后,粒径和Zeta电位大小趋于稳定;且超声分散时间为150min时,过冷度较小,相变时间较短,分别为5.2℃和4870s。     

超声振幅对激光熔注镁合金凝固组织的研究

采用CA法建立了镁合金晶粒生长模型,研究了超声振动作用下镁合金熔体的晶粒生长。结果表明,随着超声振幅的增大,熔体过冷度呈幂函数增大,熔体在过冷度很小时,基本不形核,达到一定值时,形核率急剧增加。当振幅达到10μm时,超声振动提高的过冷度达到32.23K,镁合金熔体形核率为未施加超声的353倍。超声振动的引入增大了合金熔体的过冷度,提高了熔体形核率,凝固组织平均晶粒尺寸减小,组织分布更均匀。模拟结果与试验结果相符,说明该模拟可以指导超声振动在辅助激光熔注工业生产中的应用。     

超声波对水的过冷度影响的实验研究

对水的过冷度的研究在冰蓄冷和低温生物等许多领域都有重要意义。通过引起压力场的波动，超声波能够对水的过冷结冰过程产生影响。经对800kHz超声波作用下，水的过冷度进行实验研究，发现超声波能够大幅度降低水的过冷度。超声的这种作用与空化现象有关。     

相变材料的过冷现象及其抑制方法的研究进展

过冷是材料液-固相变过程中为提供离子扩散、晶体生长及晶面扩大所需能量而产生的一种亚稳态。过冷是结晶过程的推动力,但大的过冷度导致相变材料结晶温度降低、结晶时间延迟,使得储存的潜热不能及时释放,储-放热温度不匹配,降低了热能利用效率。过冷度大已成为限制相变储热技术规模化应用的重要影响因素之一。大量实验结果表明:过冷现象与熔体晶核的生成与长大速率、环境温度、接触面的粗糙程度、熔体温度等因素有关,但其产生的内在机制尚不明确,影响规律和调控手段仍需借助实验探索。目前,主要采用外加添加剂或在胶囊化、流体化过程中加入添加剂的添加晶种法,以及通过搅拌、超声振荡和鼓泡等外部刺激的动力学成核法诱发过冷液体结晶。利用外加成核剂、纳米颗粒以及部分未熔化母相晶体作为晶核诱发非均匀成核是抑制过冷现象最常用、最有效的方法。为提高添加剂的分散性和抑制水合无机盐类相变材料的相分离现象,在添加成核剂的同时往往需要加入一定量的增稠剂,但成核剂和增稠剂的添加量需要优化。胶囊化可以改变相变材料的结晶特性,目前的研究一致认为在胶囊化前添加成核剂有利于改善相变材料的过冷度。动力学成核常采用的方法是超声振荡法,其通过空化作用使晶体持续破碎并与熔体混合而提高晶核的分散性和加速结晶过程。在超声的同时添加纳米颗粒也有利于抑制过冷度。本文简述了过冷现象和典型的过冷曲线,分析了影响过冷度的因素,并着重介绍了外加添加剂法、胶囊化法、功能流体法和超声振荡法等抑制过冷度的方法。     

深过冷Ni80.3B19.7合金的再辉和非规则共晶的形成

采用熔融玻璃净化结合气体保护的方法,使Ni80 3B19 7过共晶合金获得了407 K的大过冷度,研究了其在不同过冷度下快速凝固过程中的再辉行为.结果表明,Ni80 3B19.7过共晶合金在0～112 K过冷度范围内无明显再辉,在112～323 K过冷度范围内,其再辉曲线表现为两个再辉峰,而在323～407 K过冷度范围内,其再辉曲线为一个再辉峰.初生固相含量的随着过冷度的增大而增大,导致一次再辉度随着过冷度的增大而增大.深过冷Ni80 3B19.7合金凝固组织中非规则共晶的形成,归因于共晶两相在快速凝固阶段以自由枝晶的形式进行的非耦合生长和再辉后的慢速凝固阶段两相枝晶所发生的形态上的转变.     

Fe-B合金非平衡凝固过程的DSC研究

采用DSC技术研究了Fe-B合金冷却速率及试样纯度对其非平衡凝固过程的影响,得到了不同实验条件下过冷度和转变分数的演化结果,并从经典形核理论、非等温结晶动力学等方面进行了分析和讨论.实验结果表明,随冷却速率的增加,过冷度逐渐增大,最大可达409K;在同一冷却速率下,熔体的过冷度随试样纯度的提高也显著增大,并逐渐趋向于一定值.此外,整个非平衡凝固转变时间随过冷度的增大而不断缩短.     

过冷度对Fe-Ni-P-B软磁合金凝固组织的影响

采用玻璃包覆法(fluxing)提纯和在不同温度下保温,获得了Fe40Ni40P14B6合金熔体的凝固组织,研究了过冷度对凝固组织的影响.结果表明,随着过冷度的增大,Fe40Ni40P14B6的凝固组织从亚共晶转变为共晶组织,晶粒尺寸明显减小.当过冷度超过某一临界值时,合金熔体发生Spinodal分解,形成网状结构的凝固组织并使晶粒显著细化,达到纳米尺度.在深过冷条件下,可获得块体纳米晶凝固组织.     

以有限元分析法对CdZnTe晶体的垂直布里奇曼法生长的组分过冷模拟计算

本文采用有限元分析法,应用软件COMSOL,对Cd0.9Zn0.1Te (CZT)晶体垂直布里奇曼生长法进行模拟计算.为了研究固液界面过冷度的影响,通过改进传统的晶体生长模型只考虑热传导和热对流,加上了固液界面前沿过冷度的计算模型,组分过冷临界边界条件约束模型,研究分析了不同温度梯度和生长速率生长CZT晶体的曲率过冷度...     

GH4169高温合金过冷度对其摩擦磨损性能的影响

目前,鲜见关于过冷度对GH4169高温合金摩擦磨损性能的研究报道。采用复合熔盐净化法对GH4169高温合金进行过冷处理。采用光学显微镜分析过冷合金凝固组织的演化规律,采用硬度计测量其硬度;利用球-盘干摩擦磨损试验研究了不同过冷度下GH4169高温合金的摩擦系数和磨损机理,采用扫描电镜及能谱仪分析其磨损形貌及成分。结果表明:在小过冷度范围内(10 K≤△T63 K),摩擦系数随着过冷度的增大先增大后减小;在中等过冷度范围内(63 K≤△T90 K),摩擦系数随过冷度的增大而增大;在大过冷度范围内(90 K≤△T250 K),摩擦系数随过冷度的增大而减小;过冷GH4169高温合金的磨损机制也分为3个阶段,前期以磨粒磨损为主,中期由接触疲劳磨损向黏着磨损转变,后期为黏着磨损和氧化磨损的竞争机制。     

Supercooling Water in Cylindrical Capsules

An experimental apparatus was developed to investigate the supercooling phenomenon of water inside cylindrical capsules used for a cold storage process. The coolant is a water–alcohol mixture controlled by a constant temperature bath (CTB). Temperatures varying with time are measured inside and outside the capsule. Cylinders with an internal diameter and thickness of 45 and 1.5 mm, respectively, were made from four different materials: acrylic, PVC, brass, and aluminum. The supercooling period of the water and the nucleation temperature were investigated for different coolant temperatures. The supercooling and nucleation probabilities are shown as a function of the coolant temperature for the four different materials. Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

烟气超声流量计时间测量准确度校准方法研究

大口径气体超声波流量计广泛应用于烟道排放监测和工业控制等领域。超声波流量计的传播时间测量准确度校准是非实流校准的重要环节,通过建立超声波流量计时间测量准确度校准装置,实现对大口径气体超声流量计传播时间的非实流校准。该文首先提出3种不同的超声流量计传播时间校准方法,分析不同方法的影响因素,其次通过改变探头间距离,对不同探头间距时超声波流量计传播时间的测量误差进行校准。试验结果表明:超声波流量计传播时间的测量误差随距离变化,并确定利用标准声速对超声波流量计传播时间进行修正的方法更为准确,测量结果不确定度为0.2%。     

基于收缩流动的气体超声流量计声道稀疏化及测量方法

为了提高低声道数超声流量计的准确度水平,在传统超声流量计的基础上设计了一种具有收缩流动结构的低声道数超声流量计。采用数值模拟研究了渐缩管中收缩流动的流场特征,确定了渐缩管的几何参数及超声探头的安装方式。通过空气实流实验,研究了单声道及双声道在收缩流动条件下流量计量的基本特性。在流量范围27~432m3/h,管径范围100~150mm进行了实验验证。结果表明:收缩流动的数值模拟结果与理论模型相吻合,当渐缩管的收缩比由2增大为6,被测流场均匀区占比显著增大;当收缩比固定不变时,随着流量的增大,边界层厚度显著降低。通过实流实验比较不同结构和配置超声流量计的测量结果,发现单声道收缩管超声流量计的准确度水平显著优于传统单声道气体超声流量计,与传统双声道气体超声流量计的准确度水平相当。     

Active control of supercooling degree using surfactant (In system with solid–liquid interface)

Supercooling reduces the COP of ice formation systems. So, control of supercooling dissolution is useful and would bring numerous benefits. In a previous report, the authors conducted an experiment to control supercooling dissolution in a mixture with a hydrophilic surfactant and water in a glass test tube which had the liquid–gas(air) interface, and it was possible to actively control average supercooling degree by varying the surfactant concentrations. The average supercooling degrees' tendencies below and over a critical micelle concentration, calculated based on the variation in surface tensions, were quite different. However, since the critical micelle concentration is a physical quantity concerned only with liquid–gas interface, it is essential to investigate phenomena in a system with a solid–liquid interface. So, in a syringe with a system having only a solid–liquid interface, possibility of active control of average supercooling degree by variation in surfactant concentrations is investigated.     

R404A和R407C在水平强化管外的凝结换热实验研究

实验研究了近共沸制冷工质R404A与非共沸制冷工质R407C在水平强化换热管管外的凝结换热性能。采用"Wilson图解法"对实验数据进行处理。结果表明:对于R404A和R407C,强化管外的凝结换热系数随着壁面过冷度的增加而增大,呈现出与纯工质冷凝时不同的变化趋势,这主要是近共沸或非共沸工质凝结过程中,某些组分的凝结会遇到其它组分的凝结气膜热阻所造成的;随着过冷度增加,易挥发组分开始凝结,气膜变薄,冷凝传热系数增大。R407C在强化换热管管外的凝结换热系数比R404A要小70%左右,这是由于R407C的温度滑移较R404A要大,管外形成的凝结扩散气膜造成的影响更大。R407C在高热流密度工况下的换热效果提升明显,故应尽量工作在高热流密度区域。     

Formation of high performance ice slurry by W/O emulsion in ice storage (effective method to propagate supercooling dissolution)

This study focused ice slurry formation in an ice storage system using W/O emulsions with 70 and 80% water contents. Emulsions consisted of a silicone oil–water mixture with a small amount of amino-group-modified silicone oil additive. Ice slurry was formed by cooling the emulsion without ice adhesion to the cooling wall, as water in the emulsion did not directly contact the cooling wall. As the structure of W/O emulsion slowed the propagation rate of supercooling dissolution, voltage and ultrasonic wave were applied to the W/O emulsion to propagate dissolution more quickly and decrease maximum supercooling degree, respectively. Thus, the effects of voltage and ultrasonic wave applications on propagation rate were clarified.     

应用超声技术的燃烧气体温度测量方法的研究

高温气体瞬时温度的测量一直是温度测量中的难点，主要原因是温度高，压强大，温度变化速度快。传统的温度测量方法，无法实现温度的快速准确测量。在理想气体中，超声波的传播速度与气体绝对温度的平方根成正比，利用超声波频率差的测量测得气体介质的温度。基于此，本文提出了一种超声温度测量方法并实现了测量系统，同时解决了系统探头的安装、高速计数和数据的二次脉冲锁存问题。     

Ultrasonic techniques for imaging and measurements in molten aluminum

In order to achieve net shape forming, processing of aluminum (Al) in the molten state is often necessary. However, few sensors and techniques have been reported in the literature due to difficulties associated with molten Al, such as high temperature, corrosiveness, and opaqueness. In this paper, development of ultrasonic techniques for imaging and measurements in molten Al using buffer rods operated at 10 MHz is presented. The probing end of the buffer rod, having a flat surface or an ultrasonic lens, was immersed into molten Al while the other end with an ultrasonic transducer was air-cooled to room temperature. An ultrasonic image of a character "N", engraved on a stainless steel plate immersed in molten Al, and its corrosion have been observed at 780 degrees C using the focused probe in ultrasonic pulse-echo mode. Because cleanliness of molten Al is crucial for part manufacturing and recycling in Al processing, inclusion detection experiments also were carried out using the nonfocused probe in pitch-catch and pulse-echo modes. Backscattered ultrasonic signals from manually added silicon carbide particles, with an average diameter of 50 microm, in molten Al have been successfully observed at 780 degrees C. For optimal image quality, the spatial resolution of the focused probe was crucial, and the high signal-to-noise ratio of the nonfocused probe was the prime factor responsible for the inclusion detection sensitivity using backscattered ultrasonic signals. In addition, it was found that ultrasound could provide an alternative method for evaluating the degree of wetting between a solid material and a molten metal. Our experimental results showed that there was no ultrasonic coupling at the interface between an alumina rod and molten Al up to 1000 degrees C; therefore, no wetting existed at this interface. Also because ultrasonic velocity in alumina is temperature dependent, this rod proved to be able to be used as an in-line temperature monitoring sensor under 1000 degrees C in molten Al.     

Reduction of supercooling of water by TiO2 nanoparticles as observed using differential scanning calorimeter

Aqueous dispersions of Titania nanoparticles were cooled using differential scanning calorimetry (DSC). The nanoparticles reduce the supercooling of water by acceleration of nucleation. The experimental results demonstrated the important role in water crystallisation by the internal surface of the sample holder, and that stable nanofluids are required for the measurements to investigate the effect of nanoparticles on freezing. But for dispersions at low concentrations, such as 0.05?wt% and 0.1?wt%, the starting temperatures of freezing were found to be unusual at the cooling rates of 3°C/min and 4.5°C/min, namely they were even lower than that of the deionised (DI) water. It is found that surface controlled nucleation (SCN) is dominant at low cooling rates, while volumetric controlled nucleation (VCN) is dominant at high cooling rates.