Assessing students' conceptual understanding using an online three‐tier diagnostic test

When solving a mathematical problem, students who do not have sufficient conceptual understanding may perform poorly and exhibit misconceptions. This study was aimed to examine students' conceptual understanding and significant misconceptions when solving number sense‐related problems. An online three‐tier diagnostic test was administered to 125 fifth‐grade students with varied socio‐economic backgrounds in Hong Kong. Only 14.40% of the students exhibited high performance with high confidence, indicating that these students had a profound conceptual understanding of number sense. In addition, the majority of the students (66.40%) did not demonstrate number sense; these students exhibited several significant misconceptions and could solve the questions only by using a rule‐based method or guessing. Accordingly, most students performed unsatisfactorily on number sense‐related problems. This study is imperative in identifying early predictors and provides information for further compatible interventions in the teaching and learning of number sense in Hong Kong in particular and worldwide in general.     

Use of nets treated with food-grade coatings on dry-cured ham to control Tyrophagus putrescentiae infestations without impacting sensory properties

Dry-cured hams may become infested with the ham mite Tyrophagus putrescentiae during aging. Food-grade coatings have been developed to control mite infestations as a potential alternative to replace methyl bromide, but dipping or spraying these coatings requires additional labor and processing steps. Nets composed of polyester, a polyester/cotton blend and cotton were infused with coatings that consisted of propylene glycol, and either propylene glycol alginate and carrageenan or xanthan gum. Results indicated that the polyester/cotton blend and cotton nets that contained propylene glycol were effective at controlling mite infestations in assays with ham cubes covered in nets. The polyester nets slowed mite growth but were not effective at controlling mite reproduction due to low absorbance of the coatings. Polyester/cotton blend nets treated with coatings on whole hams were not different from control hams with respect to flavor, texture and moistness. Mite infestation tests on whole hams indicated that coated nets with greater stitch densities were effective at controlling mite growth. Future research will include the optimization of coating formulations and scaled-up testing in dry-cured ham plants.     

高效80MW等级亚临界凝汽式汽轮机结构特点分析

对哈汽公司新开发的高效80MW等级亚临界凝汽式汽轮机的总体布置和结构特点进行介绍,分析了该型汽轮机的独特性和先进性,以期为同类型汽轮机的设计提供参考和借鉴,帮助行业相关技术人员快速了解其总体布置和结构特点。     

Electrospinning of nanofibers: Potentials and perspectives for active food packaging

Electrospun nanofibers with structural and functional advantages have drawn much attention due to their potential applications for active food packaging. The traditional role of food packaging is just storage containers for food products. The changes of retailing practice and consumer demand promote the development of active packaging to improve the safety, quality, and shelf life of the packaged foods. To develop the technique of electrospinning for active food packaging, electrospun nanofibers have been covalently or non‐covalently functionalized for loading diverse bioactive compounds including antimicrobial agents, antioxidant agents, oxygen scavengers, carbon dioxide emitters, and ethylene scavengers. The aim of this review is to present a concise but comprehensive summary on the progress of electrospinning techniques for active food packaging. Emphasis is placed on the tunability of the electrospinning technique, which achieves the modification of fiber composition, orientation, and architecture. Efforts are also made to provide functionalized strategies of electrospun polymeric nanofibers for food packaging application. Furthermore, the existing limitations and prospects for developing electrospinning in food packaging area are discussed.     

Opportunities and challenges for thermally driven hydrogen production using reverse electrodialysis system

Ongoing and emerging renewable energy technologies mainly produce electric energy and intermittent power. As the energy economy relies on banking energy, there is a rising need for chemically stored energy. We propose heat driven reverse electrodialysis (RED) technology with ammonium bicarbonate (AmB) as salt for producing hydrogen. The study provides the authors’ perspective on the commercial feasibility of AmB RED for low grade waste heat (333 K–413 K) to electricity conversion system. This is to our best of knowledge the only existing study to evaluate levelized cost of energy of a RED system for hydrogen production. The economic assessment includes a parametric study, and a scenario analysis of AmB RED system for hydrogen production. The impact of various parameters including membrane cost, membrane lifetime, cost of heating, inter-membrane distance and residence time are studied. The results from the economic study suggests, RED system with membrane cost less than 2.86 €/m², membrane life more than 7 years and a production rate of 1.19 mol/m²/h or more are necessary for RED to be economically competitive with the current renewable technologies for hydrogen production. Further, salt solubility, residence time and inter-membrane distance were found to have impact on levelized cost of hydrogen, LCH. In the present state, use of ammonium bicarbonate in RED system for hydrogen production is uneconomical. This may be attributed to high membrane cost, low (0.72 mol/m²/h) hydrogen production rate and large (1,281,436 m²) membrane area requirements. There are three scenarios presented the present scenario, market scenario and future scenario. From the scenario analysis, it is clear that membrane cost and membrane life in present scenario controls the levelized cost of hydrogen. In market scenario and future scenario the hydrogen production rate (which depends on membrane properties, inter-membrane distance etc.), the cost of regeneration system and the cost of heating controls the levelized cost of hydrogen. For a thermally driven RED system to be economically feasible, the membrane cost not more than 20 €/m²; hydrogen production rate of 3.7 mol/m²/h or higher and cost of heating not more than 0.03 €/kWh for low grade waste heat to hydrogen production.     

解决17型车钩钩体尾部裂纹措施

本文论述了17型车钩钩体生产过程的工艺改进,介绍了17型车钩钩体尾部裂纹产生的原因及采取的措施。通过工艺上采用保温暗冒口提高凝固过程补缩性,同时采用空心销孔芯提高芯子的退让性,并加强车钩钩体生产过程质量控制等措施,成功地解决了17型车钩钩体尾部裂纹问题,产品废品率由4.3%降到0.23%.     

Unsteady free convection of second‐grade nanofluid with a new time‐space fractional heat conduction

The Caputo and Caputo–Fabrizio derivative are applied to study a second‐grade nanofluid over a vertical plate. A comparative analysis is presented to study the unsteady free convection of a second‐grade nanofluid with a new time–space fractional heat conduction. The governing equations with mixed time–space fractional derivatives are non‐dimensionalized and solved numerically, and a comparison between the Caputo and the Caputo–Fabrizio models is made. It is found that the temperature is higher for the Caputo–Fabrizio fractional model than the Caputo model, but the higher velocity only exists near the vertical plate for the Caputo–Fabrizio model than the Caputo model. Moreover, the velocity for the Caputo model will exceed the Caputo–Fabrizio model as y evolves.     

630℃超超临界二次再热塔式锅炉热力特性与壁温耦合计算研究

630℃超超临界火电技术作为下一代超超临界发电技术,对于煤炭资源的节约、发电机组的经济性以及环境改善,都显示了优越性。目前630℃超超临界锅炉的热力计算还缺乏相关研究。本文以某1000MW超超临界二次再热塔式锅炉布置方案为例,将锅炉管道、炉膛、高温受热面作为串联管路计算,在工质侧将热力计算与管路计算进行耦合建立数值分析模型。该计算模型能够同时得到比实现水动力和热力计算更精细的结果,对后续700℃工程持续优化提升做好技术储备。     

355 MPa级转向架构架用钢连续冷却转变行为

高速动车组转向架构架对高速动车组的运行品质、行车安全起到至关重要的作用。为向355 MPa级转向架构架用钢的工业试制提供指导，在Gleeble-3800热模拟试验机上进行了热模拟试验，结合热膨胀法和金相-硬度法，绘制了355 MPa级转向架构架用钢动态连续冷却转变曲线，研究了试验钢在连续冷却条件下的显微组织演变规律。结果表明，冷速小于1 ℃/s时，其组织主要为多边形铁素体和珠光体；冷速大于5 ℃/s时，贝氏体类组织快速增加；冷速达到50 ℃/s时，为板条贝氏体组织。因此，对于355 MPa级转向架构架用钢，冷速控制在1~5 ℃/s较为适宜，其组织主要由比例适中软硬性相结合的铁素体、珠光体和贝氏体构成，可以获得优良的强韧性匹配。     

中老年肾癌患者内脏型肥胖与肿瘤分级的关系

目的探讨中老年肾癌患者内脏型肥胖与肿瘤病理学分级的相关性。方法回顾性分析2009年1月至2014年9月北京大学人民医院278例经病理证实的中老年(年龄≥50岁)肾癌患者资料。采用术前脐平面CT平扫进行内脏脂肪、皮下脂肪和总脂肪含量测定。内脏型肥胖以内脏脂肪占总脂肪的百分比表示。应用Logistic回归分析方法进行统计学分析,评估临床常用指标及肥胖评估指标与高级别肾癌风险肿瘤分级的相关性。结果 278例肾癌患者中,29例(10.43%)为高级别肿瘤。与低级别肿瘤组相比,高级别肿瘤组内脏型肥胖患者比例较高[(47.80±8.33)%比(43.24±10.24)%,P=0.022],肿瘤直径较大[(5.42±2.99)cm比(4.11±2.27)cm,P=0.021],但体质量指数、总脂肪含量、内脏脂肪含量和皮下脂肪含量在两组间无明显差异。Logistic回归分析结果提示,内脏型肥胖与较高的肿瘤分级相关(OR=1.045,95%CI:1.002~1.090,P=0.042)。亚组分析结果显示,在进展性肾癌和肿瘤直径较大(>4 cm)的病例中,内脏型肥胖与肿瘤分级相关(OR=1.131,95%CI:1.017~1.256,P=0.023;OR=1.061,95%CI:1.005~1.121,P=0.032),而在局限性肾癌和肿瘤直径较小(≤4 cm)的病例中未观察到该现象。结论在中老年肾癌患者中,尤其是进展性肾癌和肿瘤直径较大的患者中,内脏型肥胖与肿瘤分级相关,内脏型肥胖可能是高级别肾癌的危险因素。