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排序方式: 共有7174条查询结果,搜索用时 15 毫秒
31.
Khaled Chahine Mark Ballico John Reizes Jafar Madadnia 《International Journal of Thermophysics》2007,28(6):2118-2127
A simple quasi-2D model for the temperature distribution in a graphite tube furnace is presented. The model is used to estimate
the temperature gradients in the furnace at temperatures above which contact sensors can be used, and to assist in the redesign
of the furnace heater element to improve the temperature gradients. The Thermogage graphite tube furnace is commonly used
in many NMIs as a blackbody source for radiation thermometer calibration and as a spectral irradiance standard. Although the
design is robust, easy to operate and can change temperature rapidly, it is limited by its effective emissivity of typically
99.5–99.8%. At NMIA, the temperature gradient along the tube is assessed using thermocouples up to about 1,500°C, and the
blackbody emissivity is calculated from this. However, at higher operating temperatures (up to 2,900°C), it is impractical
to measure the gradient, and we propose to numerically model the temperature distributions used to calculate emissivity. In
another paper at this conference, the model is used to design an optimized heater tube with improved temperature gradients.
In the model presented here, the 2-D temperature distribution is simplified to separate the axial and radial temperature distributions
within the heater tube and the surrounding insulation. Literature data for the temperature dependence of the electrical and
thermal conductivities of the graphite tube were coupled to models for the thermal conductivity of the felt insulation, particularly
including the effects of allowing for a gas mixture in the insulation. Experimental measurements of the temperature profile
up to 1,500°C and radial heat fluxes up to 2,200°C were compared to the theoretical predictions of the model and good agreement
was obtained. 相似文献
32.
33.
Node cooperation in hybrid ad hoc networks 总被引:2,自引:0,他引:2
Ben Salem N. Buttyan L. Hubaux J.-P. Jakobsson M. 《Mobile Computing, IEEE Transactions on》2006,5(4):365-376
A hybrid ad hoc network is a structure-based network that is extended using multihop communications. Indeed, in this kind of network, the existence of a communication link between the mobile station and the base station is not required: A mobile station that has no direct connection with a base station can use other mobile stations as relays. Compared with conventional (single-hop) structure-based networks, this new generation can lead to a better use of the available spectrum and to a reduction of infrastructure costs. However, these benefits would vanish if the mobile nodes did not properly cooperate and forward packets for other nodes. In this paper, we propose a charging and rewarding scheme to encourage the most fundamental operation, namely packet forwarding. We use "MAC layering" to reduce the space overhead in the packets and a stream cipher encryption mechanism to provide "implicit. authentication" of the nodes involved in the communication. We analyze the robustness of our protocols against rational and malicious attacks. We show that-using our solution-collaboration is rational for selfish nodes. We also show that our protocols thwart rational attacks and detect malicious attacks. 相似文献
34.
本工作用2米401有机担体填充柱,在带四臂热导检测器的国产103型气相色谱仪上,仅8分钟将原料异丁醛、产物丙酮及水进行良好分离,并采用内标法定量,其最大平均偏差在±2%以内。方法简单、快速、准确 相似文献
35.
Creep fracture by slow crack growth is studied in a medium density polyethylene at 60 °C and 80 °C. Whereas elastic-plastic fracture mechanics load parameters fail to provide a unique temperature-independent correlation, that of the fracture mechanics for creeping solids C∗ is proved to be relevant since this parameter correlates very well with the time to failure. Correlation established on both full notched creep tensile and double edge notched tensile tests was validated on cracked gas-pipe samples tested under hydrostatic pressure, extending the use of time to failure versus C∗ diagram to predict lifetime of engineering components. 相似文献
36.
Influence of thermal boundary conditions on natural convection in a square enclosure partially heated from below 总被引:1,自引:0,他引:1
Nader Ben Cheikh Brahim Ben BeyaTaieb Lili 《International Communications in Heat and Mass Transfer》2007
Natural convection in air-filled 2D square enclosure heated with a constant source from below and cooled from above is studied numerically for a variety of thermal boundary conditions at the top and sidewalls. Simulations are performed for two kinds of lengths of the heated source, i.e., a small and a large source corresponding to 20% and 80% of the total length of the bottom wall, respectively. The Rayleigh number varied from 103 to 107. Results are presented in the form of streamline and isotherm plots as well as the variation of the Nusselt number and maximum temperature at the heat source surface. Comparisons among the different thermal configurations considered are reported. 相似文献
37.
Lung surfactant replacement has been tested clinically in recent years. In this study the outcome of 31 premature infants with moderate to severe neonatal respiratory distress syndrome (RDS) treated with surfactant was compared to that of 74 prematures with RDS treated conventionally by positive pressure ventilation and supportive care. The groups were well matched for gestational age, birthweight, sex, and Apgar scores at 1 and 5 min. Surfactant treatment resulted in a significant decrease in mortality--from 36.6% in the untreated group to 12.9% in the surfactant-treated group (P < 0.04). This improvement in survival was seen also in prematures with a birthweight < 1,000 g; in the untreated group mortality was 57.6% compared to 23.5% in the treated group (P < 0.05). The incidence of pneumothorax was lower in the treated group--42% vs. 13% (P < 0.01). Surfactant treatment resulted in a trend of more survivors without bronchopulmonary dysplasia or intraventricular hemorrhage, even though surfactant therapy did not change the incidence of either. 相似文献
38.
39.
云南绿水河二级电站高水头混流式水轮发电机组,由于受到当时的技术水平限制,自1969年设备投运后问题较多,电站进行了多次技术改造,但机组仍然存在机组效率低下、汽蚀和泥沙磨损问题,同时由于转轮的过流能力有限,远没有充分利用现有水力资源,本次电站改造的目标是提效增容,充分利用水力资源,改善汽蚀性能和解决泥沙磨损问题,保证机长期安全稳定运行,改造范围是对2号、3号和4号机组进行全面改造.电站改造采用基于CFD技术的水力优化设计方法的核心技术对电站存在的问题进行了针对性设计,给出了行之有效的全面解决方案,成功地完成了电站改造目标. 相似文献
40.
Neural Computing and Applications - Feature selection is frequently used as a preprocessing step to data mining and is attracting growing attention due to the increasing amounts of data emerging... 相似文献