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1.
《Advanced Powder Technology》2021,32(8):3063-3074
The objective of the present numerical study is to investigate the heat transfer enhancement, entropy generation, and thermal performance of turbulent nanofluids inside double-pipe heat exchangers equipped with novel perforated cylindrical turbulators. Effects of inflow velocity, CuO nanoparticles volume fraction and perforated index are evaluated on the Nusselt number, friction loss, thermal performance factor (η), and viscous irreversibilities of the double-pipe heat exchangers. The newly proposed perforated turbulators with CuO nanopowder with ϕ = 1.5% provide the thermal performance of η = 1.931, which is considerably higher than the other previous studies. The results show that raising PI reduces the turbulent kinetic energy, especially in outer regions of the cylindrical turbulator. The jet formation near the walls and the perforations is the primary physical reason for this. The viscous entropy generation is increased up to 153.0% by increasing the Re number from 6,000 to 17,000 for PI = 8% and DR = 0.7. Thermal boundary layer disruption is the primary physical reason for heat transfer enhancement.  相似文献   
2.
Three-dimensional Bödewadt flow (fluid rotates at a large enough distance from the stationary plate) of carbon nanomaterial is examined. Single walled and multi walled CNTs are dissolved in water and gasoline oil baseliquids. Darcy-Forchheimer porous medium is considered. Stationary disk is further stretched linearly in radial direction. Heat transfer effect is examined in presence of radiation and convection. Effect of viscous dissipation is accounted. Entropy generation rate is studied. By using adequate transformation (von Kármán relations), the flow field equations (PDEs) are transmitted into ODEs. Solutions to these ODEs are constructed via implementation of shooting method (bvp4c). In addition to Entropy generation rate, Bejan number, heat transfer rate (Nusselt number), skin friction and temperature of fluid are examined through involved physical parameters. Axial component of velocity intensifies with increment in nanoparticles volume fraction and ratio of stretching rate to angular velocity parameter while it decays with higher porosity parameter. Higher nanoparticles volume fraction and porosity parameter lead to decay in radial as well as tangential component of velocity. However it enhances with higher ratio of stretching rate to angular velocity parameter. Temperature of fluid directly varies with higher ratio of stretching rate to angular velocity parameter, radiation parameter, Eckert number, Biot number and nanoparticles volume fraction. Rate of Entropy generation is reduced with higher estimations of porosity parameter, nanoparticles volume fraction and radiation parameter. Skin friction coefficient decays with higher porosity parameter and ratio of stretching rate to angular velocity parameter. Intensification in porosity parameter, nanoparticles volume fraction and Biot number leads to higher Nusselt number. Prominent impact is shown by multiple-walled CNTs with gasoline oil basefluid than single-walled CNTs with water basefluid.  相似文献   
3.
We focused on the work function of metals as an index of ion binding property between metals and hydrogen and analyzed the relation between the standard entropy change of metal hydrides and the work function of metals (alloys). The standard entropy change of saline hydrides and metallic hydrides roughly increased with the work function. It was indicated that hydrogen storage alloys consist of metal elements A and B whose work functions are small and large, respectively. The standard entropy changes of alloy-based metal hydrides with different composition ratio ΔS0 linearly increased with the work functions of the alloys Wc. Wc also linearly increased with the bulk modulus Bc. Then, it was found that ΔS0 was proportional to the standard heat of formation ΔH0 because dWc/dBc, dΔS0/dWc and dΔH0/dBc are constant.  相似文献   
4.
In this paper, we analyzed squeezing in the information entropy, quantum state fidelity, and qubit-qubit entanglement in a time-dependent system. The proposed model consists of two qubits that interact with a two-mode electromagnetic field under the dissipation effect. An analytical solution is calculated by considering the constants for the equations of motion. The effect of the general form of the time-dependent for qubit-field coupling and the dissipation term on the temporal behavior of the qubit-qubit entanglement, quantum state fidelity, entropy, and variance squeezing are examined. It is shown that the intervals of entanglement caused more squeezing for the case of considering the time-dependent parameters. Additionally, the entanglement between the qubits became more substantial for the case of time dependence. Fidelity and negativity rapidly reached the minimum values by increasing the effect of the dissipation parameter. Moreover, the amount of variance squeezing and the amplitude of the oscillations decreased considerably when the time dependence increased, but the fluctuations increased substantially. We show the relation between entropy and variance squeezing in the presence and absence of the dissipation parameter during the interaction period. This result enables new parameters to control the degree of entanglement and squeezing, especially in quantum communication.  相似文献   
5.
Transient combustion of a single biomass particle in preheated oxygen and nitrogen atmospheres with varying concentration of oxygen is investigated numerically. The simulations are rigorously validated against the existing experimental data. The unsteady temperature and species concentration fields are calculated in the course of transient burning process and the subsequent diffusion of the combustion products into the surrounding gases. These numerical results are further post processed to reveal the temporal rates of unsteady entropy generation by chemical and transport mechanisms in the gaseous phase of the reactive system. The spatio-temporal evolutions of the temperature, major chemical species including CO, CO2, O2, H2 and H2O, and also the local entropy generations are presented. It is shown that the homogenous combustion of the products of devolatilisation process dominates the temperature and chemical species fields at low concentrations of oxygen. Yet, by oxygen enriching of the atmosphere the post-ignition heterogeneous reactions become increasingly more influential. Analysis of the total entropy generation shows that the chemical entropy is the most significant source of irreversibility and is generated chiefly by the ignition of volatiles. However, thermal entropy continues to be produced well after termination of the particle life time through diffusion of the hot gases. It also indicates that increasing the molar concentration of oxygen above 21% results in considerable increase in the chemical and thermal entropy generation. Nonetheless, further oxygen enrichment has only modest effects upon the thermodynamic irreversibilities of the system.  相似文献   
6.
A general flux equation for multicomponent diffusion in oxide melts is presented. An explicit method was developed to calculate the gradients of single-ion activities from those of oxides with the constraints of local equilibrium and electroneutrality. This resolves ambiguity in quantifying the thermochemical driving force for ionic diffusion. A model equation for multicomponent ionic diffusion was derived within the framework of non-equilibrium thermodynamics by de Groot and Mazur. The proposed model takes empirically measurable quantities as input variables, so the diffusion calculations are consistent with thermochemical data, as furnished by the CALPHAD (CALculation of PHAse Diagrams) method, as well as ionic mobility measurements. Although the model is derived for oxides, it can be applied to diffusion in other concentrated liquid electrolytes, such as chloride and fluoride melts. Formulas for multicomponent ionic diffusion in various reference frames are presented with respect to mole fraction.  相似文献   
7.
The heat transfer and flow characteristics of MWCNT-R141b nanorefrigerant with different mass fractions have been studied through experiments. Experimental results were compared with existing correlations. A two-step method was used to prepare the nanorefrigerants. Span-80 was used as surfactant with an average particle diameter of 20 nm. Transmittance method was used to evaluate the stability of nanorefrigerants. Results showed that the stability of MWCNT-R141b nanorefrigerant, which is the added dispersant, was good during the experiments. The 0.3 wt% MWCNT-R141b nanorefrigerants had optimal heat transfer enhancement effects compared with pure refrigerants. The maximum Nusselt number increased by 40%. The specific pressure drop of nanorefrigerant increased as the Reynolds number (Re) increased, and the specific pressure drop of the pure refrigerant was minimum, which is similar to R141b.  相似文献   
8.
Isobaric heat capacities of β-Si3N4 and γ-Si3N4 were measured at temperatures between 1.8 and 309.9 K with a thermal relaxation method. The measured heat capacities of γ-Si3N4 are smaller than those of β-Si3N4 in this temperature range. Using these data, we determined the standard entropies of β-Si3N4 and γ-Si3N4 to be 62.30 J·mol−1 K−1 and 51.79 J·mol−1 K−1, respectively. The equilibrium phase boundary between β-Si3N4 and γ-Si3N4 was calculated using these values and thermodynamic parameters reported in previous studies. The obtained equilibrium phase transition pressure at 2000 K is 11.4 GPa. It is lower than the experimental pressures at which γ-Si3N4 was synthesized in previous studies. The calculated Clapeyron slope at this temperature is 0.6 MPa K−1, which is consistent with those of theoretical studies.  相似文献   
9.
研究了预装式变电站主要设备的关键状态参量,建立了配电变压器状态评价模型,利用状态评价模型进行了实例分析,给出了分析结果。  相似文献   
10.
针对目前煤尘爆炸危险性评价研究较少、各评价因素选择偏差较大的问题,通过分析煤尘爆炸特性,提出以煤尘可爆性危险、煤尘爆炸浓度危险、点火源危险3个必要条件为一级指标,煤质变异程度、挥发分、水分、灰分等14个二级指标,以层次分析法确定各指标的主观权重,以熵权法确定各指标的客观权重,利用乘法归一化方法对各指标的组合权重进行计算,构建集对分析-组合赋权煤尘爆炸危险性评价模型,并将该模型与已较成熟的改进FAHP模型、模糊事故树模型、灰色聚类评估体系、未确知测度评价模型一同应用于济宁某矿的煤尘爆炸危险性评价过程中,结果显示:集对分析-组合赋权评价模型、改进FAHP模型、灰色聚类评估体系评价结果为一般危险等级,模糊事故树模型的评价结果为较安全等级,集对分析-组合赋权煤尘爆炸危险性评价模型评价结果与改进FAHP模型、灰色聚类评估体系、未确知测度评价模型的评价结果一致,该模型对煤尘爆炸危险的评价具有较好的适用性。  相似文献   
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