THERMODYNAMIC ANALYSIS OF DRYING FOODS

Thermodynamic functions computed for food systems showing hysteresis indicate considerable differences. There appears to exist no qualitative agreement between the values obtained. Large differences in values between the adsorption an desorp-tion branches of isotherms make the absolute magnitude of the entropy and enthalpy changes uncertain. The desorption iso-steric heat was found to decrease with temperature while the adsorption branch of the isotherm for a few foods were found to exhibit temperature reversal effect giving higher values at low temperatures. Such trend may indicate either transition in the form of binding energy or the effect of temperature on structural alterations. In designing drying equipment, the isosteric heat of desorption will provide “a worst case” analysis as the true isosteric heat will lie somewhere between the adsorption and desorption isosteric heat. Any overcalculation in designing equipment by using the desorption branch of hysteresis data can only be in favor of increased throughput capability. The use of isosteric heat in freeze drying and in air drying calculations were undertaken and found to rise at a faster rate than the energy expended in moisture removal.     

Irreversible Circulation of Fluctuation in Reactor Noise

Irreversible circulation of fluctuation, α , is introduced as a new variable for the analysis of reactor noise in the normal case, which further develops our previous formulation based on the system size expansion method. It is shown that α—considered in conjunction with the variance σ—provides useful information about reactor noise, apart from the data we usually obtain on power spectral density. The relations holding between α and the conventionally used variables are given for the case of steady state. The present formalism is applied to a non-linear system with three degrees of freedom (total neutron number, fuel energy and coolant energy), to examine numerically the behavior of the fluctuations. The stability is illustrated as a phase diagram in a reactorparameter space. It is shown that the so-called soft-and hard-mode instabilities can be distinguished by observing α It is also demonstrated that appropriate processing of such quantities as α and σ will provide advance information on instabilities in power reactors.     

Analysis of Eddy‐Current Losses in Solid Iron Under dc‐Biased Magnetization Considering Minor Hysteresis Loops

In designing solid‐pole synchronous machines, it is important to take into account the surface eddy‐current losses in the field pole cores. In this study, a solid‐iron ring specimen is adopted as an approximate model of solid‐pole surfaces in order to conduct a fundamental study of the surface losses. The influence of the minor hysteresis loops on the losses under dc‐biased magnetization is investigated. The losses in the ring under dc‐biased magnetization are measured and analyzed by finite element analysis (FEA). The losses computed by the FEA considering minor loops are nearly the same as those measured. In contrast, the eddy‐current losses computed by FEA without considering minor loops are considerably inaccurate because the incremental permeability is overestimated and the skin depth is underestimated. It is important to consider hysteresis for the accurate calculation of the surface eddy‐current losses under dc‐biased magnetization.     

Cu添加对La0.7Ce0.3Fe11.54-xCuxMn0.16Si1.3合金及其氢化物磁性能的影响

La(Fe, Si)₁₃系合金是具有一级相变大磁热效应的磁制冷合金,被认为是极具应用前景的磁热效应材料之一。本文采用高频感应悬浮炉制备了添加不同Cu元素比例的La_0.7Ce_0.3Fe_11.54-xCu_xMn_0.16Si_1.3(x=0, 0.05, 0.1, 0.15)合金。并利用粉末XRD衍射仪,扫描电镜(SEM)对合金的相组成、微观组织结构进行了研究,采用多功能振动样品磁强计VersaLab对合金的磁性能进行了分析。添加Cu元素后,合金的居里温度提高,但氢化后添加Cu的合金居里温度反而偏低。随着Cu元素提高磁热性能下降,但La_0.7Ce_0.3Fe_11.44Cu_0.1Mn_0.16Si_1.3H_1.68合金的最大等温磁熵变仍高达8.5 J/kg.K(0 ~ 2 T),相对制冷能力RCP提高(118 J/kg),磁滞明显降低。     

室内二维弥散试验拖尾现象研究

针对传统二维弥散试验理论模型解析解不能描述拖尾现象的问题,建立了指数衰减模型来模拟二维弥散试验中C-t曲线浓度下降段并描述拖尾现象,通过室内二维弥散试验来进行验证,发现模拟效果较好,通过改变水力梯度和浓度来研究指数衰减模型相关参数α、β的意义及变化规律,研究结果表明α为污染物浓度随时间变化的速率,β为投源孔初始浓度与测孔峰值浓度的比值。污染物浓度越大,α值越小,污染物衰减速度越慢,拖尾时间越长;水力梯度越大,α值越大,污染物衰减速度越快,拖尾时间越短,而β值与流速无关;距离投源孔越远,α值越小,污染物衰减速度越慢,拖尾时间越长,而β值越来越大,C-t曲线浓度峰值越来越小。     

基于 Preisach 模型的磁致伸缩位移传感器迟滞补偿方法

迟滞是磁致伸缩位移传感器测量误差的主要来源之一,减小迟滞可以显著提高传感器的测量精度。根据铁磁材料的磁滞特性分析了位移迟滞的形成过程。针对磁致伸缩位移传感器在短行程中仍存在较大迟滞的问题,将传感器整体视为迟滞系统并通过Preisach模型描述其输入输出关系,然后在此基础上提出了一种软件层面上的迟滞补偿方法,将迟滞曲线分段线性化以计算迭代初始值并根据稳态误差确定收敛值的搜索范围。为解决理论模型在磁环转向后的一小段距离内与实际偏差较大的问题,在转向点附近对迭代过程中的收敛条件进行调整,有效提高了补偿方法在该区域内的准确性。实验结果表明,该方法所需的迭代次数较少,且经过补偿后,传感器的迟滞降低至原先的1/3左右,非线性度也得到了小幅度提升。在不改动现有传感器结构的前提下,该方法可快速、有效地降低传感器的位移迟滞,为磁致伸缩位移传感器的迟滞补偿提供了一种新的方案。     

航天服充气压状态下的关节阻力矩特性研究

航天服为航天员在太空环境提供基本生命保障,同时还需具有较好的活动性能以便开展作业。航天服工作时需充气压构成真空防护,而气压会导致较大关节阻力矩,对着服者活动产生很大影响。为研究航天服充气压状态下的关节阻力矩特性,研制了一种能够消除重力、牵引轨迹误差等因素干扰的测试平台,并对航天服肘关节、膝关节、髋关节活动阻力矩进行测试,分析了关节阻力矩与航天服余压及关节活动速度的关系。在测试数据的基础上,采用改进型数值实现方法及线性插值方法建立了基于Preisach迟滞模型的航天服关节角度与关节阻力矩数学模型,并在MATLAB/Simulink中进行模型预测仿真计算,验证了所提出的改进型迟滞模型能实现对航天服关节阻力矩的精确预测,为航天服关节活动性能的改善提供了理论参考。     

动态土水特征曲线滞后模型研究

土水特征曲线是描述非饱和土力学特性必不可少的方程,它的最大特点就是存在滞后效应,尽管目前已经建立了很多的土水特征曲线滞后模型,但这些模型都是针对平衡条件建立的。现有的理论和试验研究已经证实,土水特征曲线存在动态效应,与静态土水特征曲线相比,相同的饱和度时,动态土水特征曲线的吸力在干燥过程中要更大,而湿化过程中则更低。为了描述动态条件下土水特征曲线,首先从热力学的基础理论出发,探讨了动态效应的理论基础,并从熵增不等式中得到了动态的土水特征曲线方程。基于所得方程,利用边界面塑性理论,建立了可以预测任意扫描线的土水特征曲线模型。通过与已有的试验结果进行对比,表明模型可以很好地预测土体在干燥和湿化不同过程中的扫描线变化。所得结果可以用于解释一些实际问题,比如降水引起的沉降和降雨诱发滑坡等。     

Reduced dielectric loss and strain hysteresis in (0.97−x)BiScO3–xPbTiO3–0.03Pb(Mn1/3Nb2/3)O3 piezoelectric ceramics

(0.97−x)BiScO₃–xPbTiO₃–0.03Pb(Mn_1/3Nb_2/3)O₃ (BS–PT–PMnN) ceramics were prepared by solid-state reaction method. X-ray diffraction analysis revealed that the morphotropic phase boundary (MPB) of BS–PT–PMnN ceramics located near x=0.58. The high Curie temperature (437 °C), large piezoelectric constant (300 pC/N), low dielectric loss (0.015) and small strain hysteresis (20%) were obtained for the MPB composition. Its dielectric loss and strain hysteresis were reduced down to one third and half those of pure BiScO₃–PbTiO₃ (BS–PT) ceramics, respectively. In addition, the maximum vibration velocity of the BS–PT–PMnN ceramics was 0.85 ms⁻¹, much superior to those of BS–PT (0.15 ms⁻¹) and modified PZT (0.4 ms⁻¹) ceramics. These results indicated BS–PT–PMnN ceramics were promising candidates for high temperature piezoelectric applications.     

集磁式OCS磁特性随温度变化的研究

温漂问题是制约集磁式光学电流传感器实用化的瓶颈之一,目前研究集中在光学器件温度特性分析与补偿方面,缺乏对传感头磁特性随温度变化的分析。实际应用中磁导率和气隙长度随温度变化,使气隙磁场强度改变并影响测量准确度,因此有必要对传感头磁特性随温度变化情况进行分析。本文分析了铁磁材料磁滞回线随温度变化的情况,并在J-A模型的基础上建立带温度系数的磁滞模型;提出热膨胀系数和磁致伸缩系数的近似表达式,修正了热膨胀和磁致伸缩对气隙长度的影响;最后建立集磁环附加温度参数和气隙参数的磁滞模型,分析磁特性随温度变化情况。基于该模型提出一种具有稳定温度特性的传感头设计方法,以铁基非晶合金为例进行了仿真和实验验证,二者结果吻合,证明了模型和设计方法的有效性。