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31.
针对存在严重积分纯滞后的生物发酵系统,利用Smith预估器对纯时间延迟具有补偿作用的原理,在对Smith预估器进行改进的基础上,建立了模型参考自适应串级Smith预估器温度控制模型,推导了自适应Smith预估器调节律和设计算法.仿真结果表明,改进Smith预估控制不但降低了纯时间延迟特性对控制性能产生的不利影响,也提高... 相似文献
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Methods previously proposed to calculate the effect of changes in environmental conditions were reexamined. A theoretical basis was provided for the method of adding freezing time fractions. Testing against numerical calculations was carried out over a wide range of parameters. The results showed that the method based on following the movement of the freezing front performed reasonably well, especially when Pham's equation was used to calculate freezing times. However, inaccuracies may arise when the heat transfer coefficient decreases sharply, especially for very low cooling medium temperatures or for shapes other than infinite slabs. The method of adding freezing time fractions performed well for changes in cooling medium temperature but not for changes in heat transfer coefficient. A graphical method is also provided for the general case. 相似文献
34.
A finite volume numerical code has been developed to numerically approximate the rate of ice crystal growth in a laminar falling film flowing down a cooled vertical plate. The governing energy equation contains the phase energy as the source term. Enhancement of heat transfer as a result of suspended ice crystals is accounted for in the use of effective values of thermal conductivity, viscosity, thermal diffusivity, and specific heat as function of volumetric concentration of ice crystals in the falling film. Nusselt number, overall heat transfer coefficients between the fluid and cooled plate, and ice crystal growth rate were calculated for different film thicknesses with and without axial diffusion. Nusselt number and ice crystal growth rates were found to be dependent on film thickness. Axial diffusion effects were found to be negligible for larger film thickness (large flowrate). 相似文献
35.
A one-dimensional three time level implicit finite difference computer program was developed to predict temperature profiles during the individual quick freezing of spherically shaped foods. Temperature varying thermal properties necessary for these predictions were calculated based upon properties of the unfrozen product. The centre temperatures of individual peas within a bed of peas being frozen under various conditions of fluidization were recorded. Comparison of experimental results and published data with predicted freezing curves showed good agreement. 相似文献
36.
This paper gives a monograph of freezing and quick-freezing equipment. Following a brief summary of the theory of freezing time, the author reviews the principal industrial processes and equipment in use today, classifying them according to their technological characteristics. 相似文献
37.
Freshly cooked Pacific shrimp, vacuum packed in cans fitted with monitoring instruments, were frozen under industrial conditions. Surface heat transfer coefficients were determined experimentally using aluminium cyclinders. Freezing times were measured and also calculated using Plank's equation and the formulae of Cleland and Earle. The investigation indicated that the vacuum level had little effect on the freezing time and that freezing time essentially doubled when cans were enclosed in a commercial carton. For the prediction of freezing times, Cleland and Earle's method was found to be more accurate than Plank's for the tests performed, with predicted values within ± 10% of measured values. 相似文献
38.
Chasik Park Sunil Lee Hoon Kang Yongchan Kim 《International Journal of Refrigeration》2007,30(7):1168-1175
Air-conditioners use spirally coiled capillary tubes as an expansion device to enhance compactness of the unit. However, most empirical correlations for predicting refrigerant flow rate through capillary tubes were developed for straight capillary tubes without consideration of coiled effects. The objectives of this study are to investigate the flow characteristics of the coiled capillary tubes and to develop a generalized correlation for the mass flow rate through the coiled capillary tubes. The mass flow rate of R22 through the coiled capillary tubes and straight capillary tubes was measured for various operating conditions and tube geometries. The mass flow rates of the coiled capillary tubes decreased by 5–16% more than those of the straight capillary tubes at the same operating conditions. A generalized correlation for predicting refrigerant mass flow rate through coiled capillary tubes was developed by introducing the parameter of capillary equivalent length. The present correlation showed good predictions with the present database for R22, R407C and R410A in the straight and coiled capillary tubes, yielding average and standard deviations of 0.24% and 4.4%, respectively. 相似文献
39.
对钢结构二级焊缝的无损检测抽检比例进行了分析,提出了正确理解和执行规范的方法。 相似文献
40.
With the continual growth in the use of frozen foods both in retail and in food service, there is a need to develop improved thawing methods. Current methods are often undesirably slow (still air) or are very expensive and cause uneven thawing (microwave). Air impingement technology is one possible method to improve the thawing of frozen foods. The objectives of this research were to develop a two-dimensional model for air impingement thawing frozen foods and to verify the model experimentally. Frozen products were thawed using a laboratory impingement system with a single impingement jet. A simulated meat product (Tylose gel) was used as the test material. Thawing of a Tylose disk (12.7 cm diameter, 1.98 cm thickness) with air at 6 °C without impingement required more than 12 h, while thawing under a single impingement jet took less than 3 h, over four times faster. Results from the finite difference model gave good agreement with experimental data. Moisture loss during thawing was typically over-predicted because moisture gain due to condensation was not modeled. 相似文献