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921.
In order to overcome the inconvenience of manual bubble counting, a bubble counter based on photoelectric technique aiming for automatically detecting and measuring minute gas leakage of cryogenic valves is proposed. Experiments have been conducted on a self-built apparatus, testing the performance with different gas inlet strategies (bottom gas-inlet strategy and side gas-inlet strategy) and the influence of gas pipe length (0, 1, 2, 4, 6, 8, 10 m) and leakage rate (around 10, 20, 30, 40 bubbles/min) on first bubble time and bubble rate. A buffer of 110 cm^3 is inserted between leakage source and gas pipe to simulate the downstream cavum adjacent to the valve clack. Based on analyzing the experimental data, experiential parameters have also been summarized to guide leakage detection and measurement for engineering applications. A practical system has already been successfully applied in a cryogenic testing apparatus for cryogenic valves. 相似文献
922.
This paper describes a Least Squares (LS) channel estimation scheme for MIMO OFDM systems based on time-domain training sequence. We first compute the minimum mean square error (MSE) of the LS channel estimation, and then derive the optimal criteria of the training sequence with respect to the minimum MSE. It is shown that optimal time-domain training sequence should satisfy two criteria. First, the autocorrelation of the sequence transmitted from the same antenna is an impulse function in a region longer than the channel maximum delay. Second, the cross-correlation between sequences transmitted from different antennas is zero in this region. Simulation results show that the estimator using optimal time-domain training sequences has better performance than that using optimal frequency training sequence at low signal-to-noise ratio (SNR). To reduce the training overhead, a suboptimal training sequence is also proposed. Comparing with optimal training sequence, it has low computation complexity and high transmission efficiency at the expense of little performance degradation. 相似文献
923.
This paper presents the argument that education and the teaching profession have been saturated by a new form of risk consciousness and risk consideration. The aim is to shed light on this issue and present a number of empirical examples and questions of interest in educational research. Furthermore, the paper presents some of the central theories regarding risk and an attempt is made to relate these theories to an epistemological framework. The article also emphasizes the problems and issues that arise from the way teachers think, experience and manage risk in their daily work, and examines how this affects their didactic intentions and, in the long run, also the pupils’ learning potential. It is the intention of the authors, in the scientific as well as in the professional discussion, to reintroduce the positive connections between teaching and risk zones, as well as between professional development and risk taking. 相似文献
924.
Colby Hall Garrett J. Roberts Eunsoo Cho Lisa V. McCulley Megan Carroll Sharon Vaughn 《Educational Psychology Review》2017,29(4):763-794
This meta-analysis synthesizes the last two decades of experimental and quasi-experimental research on reading instruction across academic contexts (e.g., social studies, science, mathematics, English language arts) for English learners (ELs) in grades 4 through 8, to determine (a) the overall effectiveness of reading instruction for upper elementary and middle school students who are ELs and (b) how the magnitude of the effect varies based on student, instructional, and study characteristics. The analysis included a total of 11 studies with 46 individual effect sizes and yielded a mean effect size of g?=?0.35 across all (i.e., standardized and unstandardized) reading measures, g?=?0.01 across standardized reading measures, and g?=?0.43 across unstandardized reading measures. For all reading, unstandardized reading, all vocabulary, and unstandardized vocabulary measures, results suggest that higher quality studies tended to have smaller effects, and these effects were even more evident for unstandardized measures (i.e., one unit increase in study quality was associated with decreased effects: g?=?0.21, g?=?0.30, g?=?0.24, g?=?0.30, respectively). For all comprehension measures, effects were larger for instruction that included both vocabulary and comprehension (g?=?0.39) than for instruction that focused on vocabulary alone (g?=?0.08). Results suggest the benefit of developing and refining high-impact approaches to reading instruction for ELs that can be delivered across content areas and grades. 相似文献
925.
Application of neural network in the study of combustion rate of natural gas/diesel dual fuel engine
In order to predict and improve the performance of natural gas/diesel dual fuel engine (DFE), a combustion rate model based
on forward neural network was built to study the combustion process of the DFE. The effect of the operating parameters on
combustion rate was also studied by means of this model. The study showed that the predicted results were good agreement with
the experimental data. It was proved that the developed combustion rate model could be used to successfully predict and optimize
the combustion process of dual fuel engine.
Project supported by National Lab. for Automotive Engine and Safety, Tsinghua University, China 相似文献
926.
927.
3D printing technology is a powerful educational tool that can promote integrative STEM education by connecting engineering, technology, and applications of science concepts. Yet, research on the integration of 3D printing technology in formal educational contexts is extremely limited. This study engaged preservice elementary teachers (N?=?42) in a 3D Printing Science Project that modeled a science experiment in the elementary classroom on why things float or sink using 3D printed boats. The goal was to explore how collaborative 3D printing inquiry-based learning experiences affected preservice teachers’ science teaching self-efficacy beliefs, anxiety toward teaching science, interest in science, perceived competence in K-3 technology and engineering science standards, and science content knowledge. The 3D printing project intervention significantly decreased participants’ science teaching anxiety and improved their science teaching efficacy, science interest, and perceived competence in K-3 technological and engineering design science standards. Moreover, an analysis of students’ project reflections and boat designs provided an insight into their collaborative 3D modeling design experiences. The study makes a contribution to the scarce body of knowledge on how teacher preparation programs can utilize 3D printing technology as a means of preparing prospective teachers to implement the recently adopted engineering and technology standards in K-12 science education. 相似文献
928.
Sourav Das 《Resonance》2003,8(10):76-84
An analysis of the Madelung constant of the sodium chloride crystal lattice has been made, in an effort to supplement information
available in undergraduate textbooks. A method of approximation of the constant using the personal computer is presented.
An erratum to this article is available at . 相似文献
929.
Ivy Kidron 《Educational Studies in Mathematics》2008,69(3):197-216
I investigate the contributions of three theoretical frameworks to a research process and the complementary role played by
each. First, I describe the essence of each theory and then follow the analysis of their specific influence on the research
process. The research process is on the conceptualization of the notion of limit by means of the discrete continuous interplay.
I investigate the influence of the different perspectives on the research process and realize that the different theoretical
approaches intertwine. Moreover, I realize that the research study demanded the contribution of more than one theoretical
approach to the research process and that the differences between the frameworks could serve as a basis for complementarities. 相似文献
930.
Exemplifying definitions: a case of a square 总被引:1,自引:1,他引:0
In this study we utilize the notion of learner-generated examples, suggesting that examples generated by students mirror their
understanding of particular mathematical concepts. In particular, we explore examples generated by a group of prospective
secondary school teachers for a definition of a square. Our framework for analysis includes the categories of accessibility
and correctness, richness, and generality. Results shed light on participants’ understanding of what a mathematical definition
should entail and, moreover, contrast their pedagogical preferences with mathematical considerations. 相似文献