多项式矩阵的无穷秩与广义微分算子系统的无穷行为（下）

多项式矩阵的无穷秩与广义微分算子系统的无穷行为（下）程学建，丁立刚（数学教研室）２广义微分算子多项式矩阵系统能控（观）的ＰＢＨ型条件考虑系统（１）。记定理２．１广义微分算子多项式矩阵系统（１）（１）Ｒ—能控有限；（２）脉冲能控；（３）强能控，有限及证...     

冷藏车层叠式蒸发器应用R404A与R22和R134a的比较

采用分布参数法对层叠式蒸发器建立数学模型，并对蒸发器采用R404A，R22和R134a时的换热和流动性能进行模拟比较。结果表明，在空调工况范围内，新型中低温混合制冷剂R404A具有R134a换热性能好和R22压降小的特点，能够很好地适用于冷藏车系统空调侧层叠式蒸发器。     

Leaving home,returning home: Migration as a labor market choice for Alaska Natives

We investigate determinants of the pattern of recent migration of Alaska Natives between rural homelands and urban areas. A review of the literature on moving in the North American North suggests that economic opportunities draw migrants, although such opportunities must be viewed in the context of the mixed subsistence-cash economy prevailing in rural areas of the region. Consequently, we model Native migration as a simultaneous decision with labor market participation in a mixed economy. Estimated equations explaining individual Alaska Native migration choices using the U.S. Census Public Use Microdata Sample (PUMS) are consistent with the model, and also suggest that perceived opportunities differ between women and men.The authors acknowledge financial support from the National Science Foundation, grant OPP-9521459. An earlier version of this paper was presented to the 1998 annual meeting of the Western Regional Science Association     

Flow condensation heat transfer coefficients of R22, R134a, R407C, and R410A inside plain and microfin tubes

Flow condensation heat transfer coefficients (HTCs) of R22, R134a, R407C, and R410A inside horizontal plain and microfin tubes of 9.52 mm outside diameter and 1 m length were measured at the condensation temperature of 40 °C with mass fluxes of 100, 200, and 300 kg m⁻² s⁻¹ and a heat flux of 7.7–7.9 kW m⁻². For a plain tube, HTCs of R134a and R410A were similar to those of R22 while HTCs of R407C are 11–15% lower than those of R22. For a microfin tube, HTCs of R134a were similar to those of R22 while HTCs of R407C and R410A were 23–53% and 10–21% lower than those of R22. For a plain tube, our correlation agreed well with the present data for all refrigerants exhibiting a mean deviation of 11.6%. Finally, HTCs of a microfin tube were 2–3 times higher than those of a plain tube and the heat transfer enhancement factor decreased as the mass flux increased for all refrigerants tested.     

A generalized correlation for refrigerant mass flow rate through adiabatic capillary tubes

A capillary tube is a common expansion device used in small sized refrigeration and air-conditioning systems. A generalized correlation for refrigerant flow rate in adiabatic capillary tubes is developed by implementing dimensionless parameters based on extensive experimental data for R-22, R-290, and R-407C measured in this study. Dimensionless parameters are derived from the Buckingham Pi theorem, considering the effects of tube inlet conditions, capillary tube geometry, and refrigerant properties on mass flow rate. The generalized correlation yields good agreement with the present data for R-22, R-290, and R-407C with average and standard deviations of 0.9 and 5.0%, respectively. Approximately 97% of the present data are correlated within a relative deviation of ±10%. Further assessments of the correlation are made by comparing the predictions with measured data for R-12, R-134a, R-152a, R-410A, and R-600a in the open literature. The correlation predicts the data for those five refrigerants with average and standard deviations of −0.73 and 6.16%, respectively.     

5种杀虫剂对玉米螟的药效评价

在田间用5种常用杀虫剂对玉米螟作了药效试验,试验采用“撒毒沙”和“灌心叶”两种施药方法,旨在寻求更加方便、适用、有效的防治玉米螟的药剂和方法。试验表明供试5种药剂对玉米螟均有优良的防治效果,且“撒毒沙”施药防效好于“灌心叶”施药。     

环境温度对Fe-14Mn-0.22C合金阻尼性能的影响

为了了解Fe-Mn基阻尼合金的高温阻尼性能,用真空感应电炉的方法制备了Fe-14.04Mn-0.22C合金.用倒扭摆内耗仪测试了环境温度在23～400℃变化时合金阻尼性能的变化规律,并用热机械分析仪和光学显微镜分析了合金的组织结构及其相变特点.研究结果表明,在常温条件下合金的阻尼性能随扭转应变的增加而呈线性方式增加,扭转应变在1×10-4时,对数衰减率达0.1,且没有出现峰值;随着环境温度的变化,对数衰减曲线与应变的线性关系没有发生改变,但是曲线发生了整体的上下平移;当环境温度低于200℃时,合金的阻尼性能随温度的升高而增加,但当温度达到或超过200℃后,合金的阻尼性能迅速下降,其原因在于合金发生了奥氏体转变(As为270℃),马氏体的百分含量降低,阻尼源减少.     

MCM-56分子筛的静态合成及对苯与长链烯烃烷基化反应的催化性能

以六甲基亚胺为模板剂,静态水热晶化法合成了MCM-56分子筛,通过X-射线衍射、N2等温吸附等方法对样品进行表征.当硅铝比低于20时易生成MCM-49和丝光沸石,加入少量碱能提高分子筛的相对结晶度,最佳合成温度为135～150 ℃,时间分别为3天.考察了MCM-56对苯和长链烯烃烷基化反应的催化活性,并与MCM-22进行了对比.实验表明,在最佳反应温度120 ℃时,烯烃的转化率为29.35%,烷基苯(LAB)的选择性为98.22%,MCM-56的催化活性明显低于MCM-22.     

Weak identification in the ESTAR model and a new model

Determining good parameter estimates in (exponential smooth transition autoregressive) models is known to be difficult. We show that the phenomena of getting strongly biased estimators is a consequence of the so‐called identification problem, the problem of properly distinguishing the transition function in relation to extreme parameter combinations. This happens in particular for either very small or very large values of the error term variance. Furthermore, we introduce a new alternative model – the TSTAR model – which has similar properties as the ESTAR model but reduces the effects of the identification problem. We also derive a linearity and a unit root test for this model.     

Rationally based model for evaluating the optimal refrigerant mass charge in refrigerating machines

A rationally based algorithm was proposed to evaluate the optimal mass charge into refrigerating machines. The calculated results indicate that the system performance is strongly related to the refrigerant mass charge. In the early stage of the refrigerant charge process, a sharp rise of COP is observed. The COP reaches an optimal value for a specified refrigerant charge and shows a slight drop for a further increase of refrigerant charge. Calculated results reveal similar trends to those of experimental data. The analysis of the study can well interpret the cycle transformation subject to refrigerant mass charge.