A USM-Θ two-phase turbulence model for simulating dense gas-particle flows

A second-order moment two-phase turbulence model for simulating dense gas-particle flows （USM-Θ model）, combining the unified second-order moment twophase turbulence model for dilute gas-particle flows with the kinetic theory of particle collision, is proposed. The interaction between gas and particle turbulence is simulated using the transport equation of two-phase velocity correlation with a two-time-scale dissipation closure. The proposed model is applied to simulate dense gas-particle flows in a horizontal channel and a downer. Simulation results and their comparison with experimental results show that the model accounting for both anisotropic particle turbulence and particle-particle collision is obviously better than models accounting for only particle turbulence or only particle-particle collision. The USM-Θ model is also better than the k-ε-kp-Θ model and the k-ε-kp-εp-Θ model in that the first model can simulate the redistribution of anisotropic particle Reynolds stress components due to inter-particle collision, whereas the second and third models cannot.

Numerical simulation of the effect of void fraction and inlet velocity on two-phase turbulence in bubble-liquid flows

There are contradicted opinions on whether bubbles enhance or reduce the liquid turbulence. In this paper, the effect of void fraction and inlet velocity on the bubble-liquid two-phase turbulence of the multiple bubble-liquid jets in a two-dimensional channel is studied by using the two-phase second-order moment turbulence model. The results confirm the phenomena observed in experiments and reported in references that at a low void fraction and low inlet velocities the bubbles enhance the liquid turbulence, whereas at a high void fraction and high inlet velocities the bubbles reduce the liquid turbulence.

Studies on gas turbulence and particle fluctuation in dense gas-particle flows

Particle fluctuation and gas turbulence in dense gas-particle flows are less studied due to complexity of the phenomena. In the present study, simulations of gas turbulent flows passing over a single particle are carried out first by using RANS modeling with a Reynolds stress equation turbulence model and sufficiently fine grids, and then by using LES. The turbulence enhancement by the particle wake effect is studied under various particle sizes and relative gas velocities, and the turbulence enhancement is found proportional to the particle diameter and the square of velocity. Based on the above results, a turbulence enhancement model for the particle-wake effect is proposed and is incorporated as a sub-model into a comprehensive two-phase flow model, which is then used to simulate dilute gas-particle flows in a horizontal channel. The simulation results show that the predicted gas turbulence by using the present model accounting for the particle wake effect is obviously in better agreement with the experimental results than the prediction given by the model not accounting for the wake effect. Finally, the proposed model is incorporated into another two-phase flow model to simulate dense gasparticle flows in a downer. The results show that the particle wake effect not only enhances the gas turbulence, but also amplifies the particle fluctuation.

A two-scale second-order moment two-phase turbulence model for simulating dense gas-particle flows

A two-scale second-order moment two-phase turbulence model accounting for inter-particle collision is developed, based on the concepts of particle large-scale fluctuation due to turbulence and particle small-scale fluctuation due to collision and through a unified treatment of these two kinds of fluctuations. The proposed model is used to simulate gas-particle flows in a channel and in a downer. Simulation results are in agreement with the experimental results reported in references and are near the results obtained using the sin- gle-scale second-order moment two-phase turbulence model superposed with a particle collision model （USM-θ model） in most regions.

Wake-up effect in Hf_(0.4)Zr_(0.6)O_(2) ferroelectric thin-film capacitors under a cycling electric field

We examined the wake-up effect in a Ti N/Hf_(0.4)Zr_(0.6)O_(2)/TiN structure.The increased polarization was affected by the cumulative duration of a switched electric field and the single application time of the field during each switching cycle.The space-charge-limited current was stable,indicating that the trap density did not change during the wake-up.The effective charge density in the space-charge region was extracted from capacitance-voltage curves,which demonstrated an increase in free charges at the interface.Based on changing characteristics in these properties,the wake-up effect can be attributed to the redistribution of oxygen vacancies under the electric field.

Investigation on the passivation,band alignment,gate charge,and mobility degradation of the Ge MOSFET with a GeO/Al;O;gate stack by ozone oxidation

Ge has been an alternative channel material for the performance enhancement of complementary metal-oxide-semiconductor(CMOS)technology applications because of its high carrier mobility and superior compatibility with Si CMOS technology.The gate structure plays a key role on the electrical property.In this paper,the property of Ge MOSFET with Al_(2)O_(3)/GeO_(x)/Ge stack by ozone oxidation is reviewed.The GeO_(x)passivation mechanism by ozone oxidation and band align-ment of Al2O3/GeO_(x)/Ge stack is described.In addition,the charge distribution in the gate stack and remote Coulomb scatter-ing on carrier mobility is also presented.The surface passivation is mainly attributed to the high oxidation state of Ge.The en-ergy band alignment is well explained by the gap state theory.The charge distribution is quantitatively characterized and it is found that the gate charges make a great degradation on carrier mobility.These investigations help to provide an impressive un-derstanding and a possible instructive method to improve the performance of Ge devices.

Developing theory of probability density function for stochastic modeling of turbulent gas-particle flows

Turbulent gas-particle flows are studied by a kinetic description using a prob- ability density function （PDF）. Unlike other investigators deriving the particle Reynolds stress equations using the PDF equations, the particle PDF transport equations are di- rectly solved either using a finite-difference method for two-dimensional （2D） problems or using a Monte-Carlo （MC） method for three-dimensional （3D） problems. The proposed differential stress model together with the PDF （DSM-PDF） is used to simulate turbulent swirling gas-particle flows. The simulation results are compared with the experimental results and the second-order moment （SOM） two-phase modeling results. All of these simulation results are in agreement with the experimental results, implying that the PDF approach validates the SOM two-phase turbulence modeling. The PDF model with the SOM-MC method is used to simulate evaporating gas-droplet flows, and the simulation results are in good agreement with the experimental results.

Research on Community Fresh O2O Purchase Intention Based on Customer Perceived Value

Based on the theory of customer perceived value,this paper makes an empirical analysis on the purchase intention of community fresh O2O,and proposes the customer perceived value concept including result,program,service,social and emotional value.At the same time,the intermediary variable of consumption attitude is put forward and the theoretical model constructed.It is verified that perceived value significantly influences purchase intention and consumption attitude,and plays an intermediary role between them by regression analysis method.This detailed study on the mechanism of perceived value influencing purchase intention through consumption attitude is of great guidance value.

Measurement and simulation of swirling coal combustion

Particle image velocimetry （PIV）, thermocouples and flue gas analyzer are used to study swirling coal combustion and NO formation under different secondary-air ratios. Eulerian-Lagrangian large-eddy sim-ulation （LES） using the Smagorinsky-Lilly sub-grid scale stress model, presumed-PDF fast chemistry and eddy-break-up （EBU） gas combustion models, particle devolatilization and particle combustion models, are simultaneously used to simulate swirling coal combustion. Statistical LES results are validated by measurement results. Instantaneous LES results show that the coherent structures for swirling coal com- bustion are stronger than those for swirling gas combustion. Particles are shown to concentrate along the periphery of the coherent structures. Combustion flame is located in the high vorticity and high par-ticle concentration zones. Measurement shows that secondary-air ratios have little effect on final NO formation at the exit of the combustor.

Studies on theory and modeling of droplet and spray combustion in China: a review

Combustion phenomena were discovered still in far ancient time of China.From the 50’s of the last century,owing to the fast development of energy and power,aeronautical and astronautical,chemical and metallurgical engineering,combustion theory started to be studied in China.The Chinese scientists studied the theory of ignition,laminar fame propagation,droplet combustion,and spray combustion.Later,from the 80’s of the last century,numerical modeling of combustion started to be studied in China,including turbulence modeling,turbulent combustion modeling,two-phase turbulence modeling and two-phase combustion modeling,in the approaches of Reynolds Navier–Stokes(RANS)modeling,large-eddy simulation(LES),and direct numerical simulation(DNS)of combustion.Due to the limitation of a paper size,this paper gives only a review of studies on theory and modeling of droplet and spray combustion in China.

Two-phase turbulence models for simulating dense gas-particle flows

The two-fluid model is widely adopted in simulations of dense gas-particle flows in engineering facili- ties. Present two-phase turbulence models for two-fluid modeling are isotropic. However, turbulence in actual gas-particle flows is not isotropic. Moreover, in these models the two-phase velocity correlation is closed using dimensional analysis, leading to discrepancies between the numerical results, theoretical analysis and experiments. To rectify this problem, some two-phase turbulence models were proposed by the authors and are applied to simulate dense gas-particle flows in downers, risers, and horizontal channels; Experimental results validate the simulation results. Among these models the USM-O and the two-scale USM models are shown to give a better account of both anisotropic particle turbulence and particle-particle collision using the transport equation model for the two-phase velocity correlation.