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991.
992.
993.
《International Journal of Hydrogen Energy》2019,44(54):28909-28918
Equilibrium hydrogen pressures over hydrogenated vanadium, i.e., crystalline VHx with , are estimated based on first-principles calculations to assess the suitability of V-based alloys for use as hydrogen storage materials. For intermediate H contents (x = 0.5 and 1) corresponding to partial site occupancies, a multistep process was used to determine the ground-state structures. First, a large number of configurations with H on tetrahedral (T) or octahedral (O) sites in bcc–type lattices were ranked in terms of energetical stability sans vibrational contributions. Next, phonon calculations were carried out for the lowest-energy structures identified in the first step and zero-point energies added to the electronic energies. Hydrogen pressures for V-VH0.5 and VH-VH2 equilibria were then calculated using these ground-state free energies and thermodynamic equations. Calculation of vibrational energies was simplified by identifying the linear relationship between H content and zero-point energy, which depends only on the type of sites occupied by H atoms. Whereas H atoms preferentially occupy T sites when vibrational terms are ignored, their inclusion leads to H atoms preferentially occupying O sites in both VH0.5 and VH at 0 K. Inclusion of vibrational energy also leads to a better match with reported experimental hydrogen pressures highlighting its importance when assessing the stabilities of different H distributions in metal lattices. The method outlined consists of a more exhaustive configurational search than has been considered previously, and should be useful for systematic investigation of a wide variety of V-based alloys. 相似文献
994.
三维混合层中湍流拟序结构对颗粒扩散的影响 总被引:1,自引:0,他引:1
对时间发展模式的三维气固两相混合层湍流拟序结构及其对不同尺寸颗粒扩散的影响进行直接数值模拟,对于气相场,应用拟谱方法(Pseudospectral method)直接求解Navier-Stokes方程组;对于颗粒场,对于Lagrangian方法跟踪半场颗粒,针对不同Stokes数的颗粒,分别模拟颗粒场在展向和流向的分布变化,进而分析流场三维大涡结构对不同颗粒扩散的影响,并引入一系列参数,着重定量描述流场展向大涡结构和流向大涡结构对不同尺寸颗粒分布的不同影响。 相似文献
995.
In the present paper second law analysis of crossflow heat exchangers has been carried out in the presence of non-uniformity of flow. This non-uniformity is modeled with the help of axial dispersion model and takes into account the back mixing and flow maldistribution. An analytical model for exergy destruction has been evaluated for the cross-flow configuration. A wide range of study of the operating parameters and non-uniform flow on exergetic behavior of crossflow heat exchangers has been carried out. The results clearly bring out not only the reason behind the maximum entropy paradox in heat exchangers but also the proper perspective of exergy destruction and the consequent optimization of crossflow heat exchangers from the second law viewpoint. 相似文献
996.
997.
In this article we analyze the behavior of plane harmonic waves in the entire space filled by a linear thermoviscoelastic material with voids. We take into account the effect of the thermal and viscous dissipation energies upon the corresponding waves and, consequently, we study the damped in time wave solutions. There are five basic waves in an isotropic and homogeneous thermoviscoelastic porous space. Two of them are shear waves, while the remaining three are dilatational waves. The shear waves are uncoupled, damped in time with decay rate depending only on the viscosity coefficients. The three dilatational waves are coupled and consist of a predominantly dilatational damped wave of Kelvin–Voigt viscoelasticity, other is predominantly a wave carrying a change in the void volume fraction and the third takes the form of a standing thermal wave whose amplitude decays exponentially with time. The explicit form of the dispersion equation is obtained in terms of the wave speed and the thermoviscoelastic homogeneous profile. Furthermore, we use numerical methods and computations to solve the secular equation for some special classes of thermoviscoelastic materials considered in literature. 相似文献
998.
Artur GUTKOWSKI 《热科学学报(英文版)》2006,(2)
Following the quantitative determination of dust cloud parameters, this study investigated the flame propagation through cornstarch dust clouds in a vertical duct of 780 mm height and 160×160 mm square cross section, and gave particular attention to the effect of turbulence on flame characteristics. The turbulence induced by dust dispersion process was measured using a particle image velocimetry (PIV) system. Upward propagating dust flames were visualized with direct light and shadow photography. The results show that a critical value of the turbulence intensity can be specified below which laminar flame propagation would be established. This transition condition is about 10 cm/s. The measured propagation speed of laminar flames appears to be in the range of 0.45-0.56 m/s, consistent with the measurements reported in the literature. For the present experimental conditions, the flame speed is little sensitive to the variations in dust concentration. Some information on the flame structure was revealed from the shadow records, showing the typical heterogeneous feature of dust combustion process. 相似文献
999.
Che-Tzu Lin 《Combustion and Flame》2007,150(3):210-219
Smoke contamination even from small fires of sensitive components in telephone central offices (TCO) and semiconductor clean rooms has caused disastrous disruptions in services and excessive financial repercussions. Containment and removal of smoke can be achieved by downward displacement ventilation, also used for other purposes in large floor area semiconductor facilities. Downward ventilation reverses the upward flow from the fire and removes the smoke and other combustion products through the floor. Small-scale model experiments under turbulent fire plume conditions investigate the physical parameters such as downward velocity, ceiling clearance, and size of the fire that control the extent of the smoke containment around the fire. The extent of smoke contamination is mapped by measuring the temperatures in several locations on the vertical axis and in the space around the fire. The downward velocities cause oscillation of the fire plume and the flames around the vertical axis. By combining the experimental data with similarity analysis, a characteristic length scale and nondimensional relations are deduced for the extent of the smoke containment and the onset of plume oscillations. These relations allow design of displacement ventilation effective to contain the smoke but not cause fire spread owing to the leaning of the flames. Previously reported discrepancies in the apparent width and behavior of the plume are resolved by examining the experimental data and assessing the suitability of steady state computational fluid dynamics to model the present flow. 相似文献
1000.
Understanding the exposure and toxic load for the interior of buildings during and following the passage of an external airborne hazard can be a critical piece of information in deciding the benefit from adopting a shelter-in-place strategy. Whilst numerical methods allow the calculation of such parameters for the general case, analytical solutions allow more rapid assessments to be made and highlight the key parameters more clearly. Analytical expressions are derived for the exposure due to the acute inhalation of toxic chemicals and the associated toxic load as a function of time, external hazard duration and building air change rate assuming a top-hat outdoor concentration profile and no indoor loss mechanism. It is shown that the internal exposure tends to the external exposure at long times for any external concentration profile. Expressions are derived for toxic loads with exponents n=m/2 where 2 ≤ m ≤ 7 is an integer to cover the range of typical values (1 ≤ n ≤ 3.5). At long times the ratio of internal to external toxic load for a top-hat outdoor concentration profile is shown to be a function of the product of the air change rate and the duration of the external hazard. 相似文献