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Measurement,Modelling and Analysis of Residence Time Distribution Characteristics in a Continuous Hydrothermal Reactor

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Journal of Thermal Science 2024年第4期33卷 1301-1311页

作者： LI Yi ZHAI Binjiang WANG Junying WANG Weizuo JIN Hui State Key Laboratory of Multiphase Flow in Power Engineering Xi’an Jiaotong UniversityXi’an710049China

Understanding the residence time distribution(RTD)of a continuous hydrothermal reactor is of great significance to improve product quality and reaction *** this work,an on-line measurement system is attached to a continuous reactor to investigate the characteristics of *** approach that can accurately fit and describe the experimental measured RTD curve by finding characteristic values is proposed for analysis and *** RTD curves of three experiment groups are measured and the characteristic values are *** show that increasing total flow rate and extending effective reactor length have inverse effect on average residence time,but they both cause the reactor to approach a plug flow reactor and improve the materials *** branch flow rate fraction has no significant effect on RTD characteristics in the scope of the present work except the weak negative correlation with the average residence ***,the natural convection stirring effect can also increase the average residence time,especially when the forced flow is *** analysis reveals that it is necessary to consider the matching of natural convection,forced flow and reactor size to control RTD when designing the hydrothermal reactor and working conditions.

关键词： continuous reactor residence time distribution modelling

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On revised mixture fraction describing multi-stream mixing

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Combustion and Flame 2025年 277卷

作者： Ziqi Wang Weijie Zhang Hai Huang Jinhua Wang Zuohua Huang State Key Laboratory of Multiphase Flow in Power Engineering Xi’an Jiaotong University Xi’an 710049 PR China

Multi-stream mixing is common in practice but cannot be modeled using the traditional Bilger mixture fraction defined for only two streams. This work was intended to define the multi-stream mixture fractions that can be adopted in flamelet-tabulated combustion models to simulate the multi-stream mixing and combustion. These mixture fractions are expressed as linear combinations of species mass fractions like the Bilger mixture fraction. The present definition is special as the new mixture fractions are nonconservative and can include the preferential diffusion (PD) effects which were often ignored before. Meanwhile, it can be appliable for any number of inlet streams. The multi-stream mixture fractions are derived based on some basic conserved scalars, like the element mass fractions, enthalpy, and transported scalars. Governing equations for the multi-stream mixture fractions are also presented.

关键词： Volume fraction

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Effect of temperature on Lu’an bituminous char structure evolution in pyrolysis and combustion

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Frontiers in Energy 2021年第1期15卷 14-25页

作者： Yandi ZHANG Yinhe LIU Xiaoli DUAN Yao ZHOU Xiaoqian LIU Shijin XU State Key Laboratory of Multiphase Flow in Power Engineering Xi'an Jiaotong UniversityXi'an 710049China

In the process of pyrolysis and combustion of coal particles, coal structure evolution will be affected by the ash behavior, which will further affect the char reactivity, especially in the ash melting temperature zone. Lu’an bituminous char and ash samples were prepared at the N_(2) and air atmospheres respectively across ash melting temperature. A scanning electron microscope (SEM) was used to observe the morphology of char and ash. The specific surface area (SSA) analyzer and thermogravimetric analyzer were respectively adopted to obtain the pore structure characteristics of the coal chars and combustion parameters. Besides, an X-ray diffractometer (XRD) was applied to investigate the graphitization degree of coal chars prepared at different pyrolysis temperatures. The SEM results indicated that the number density and physical dimension of ash spheres exuded from the char particles both gradually increased with the increasing temperature, thus the coalescence of ash spheres could be observed obviously above 1.00℃. Some flocculent materials appeared on the surface of the char particles at 1.00℃, and it could be speculated that β-Si_(3)N_(4) was generated in the pyrolysis process under N_(2). The SSA of the chars decreased with the increasing pyrolysis temperature. Inside the char particles, the micropore area and its proportion in the SSA also declined as the pyrolysis temperature increased. Furthermore, the constantly increasing pyrolysis temperature also caused the reactivity of char decrease, which is consistent with the results obtained by XRD. The higher combustion temperature resulted in the lower porosity and more fragments of the ash.

关键词： bituminous char pyrolysis ash structure evolution reactivity

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Laminar Flame Structure-Dependent Exergy Destruction Behavior at Auto-Ignition Time Scale:A Case Study of Dimethyl Ether(DME)

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Journal of Thermal Science 2024年第4期33卷 1257-1271页

作者： WU Honghuan HUANG Wenlin ZHAO Hao SUN Wuchuan HUANG Zuohua ZHANG Yingjia State Key Laboratory of Multiphase Flow in Power Engineering Xi’an Jiaotong UniversityXi’an710049China

Hybrid deflagration/auto-ignition flame structures coexist in the combustion of advanced *** exergy destruction caused by different irreversible processes under varied flame regimes is thus important for understanding engine *** this study,the flame propagation modes for the premixed DME/air mixtures are numerically investigated under engine-relevant *** entropy generation and exergy destruction characteristics are compared under different flame *** reveal that as the typical premixed flame transition towards auto-ignition front,the exergy destruction from heat conduction and species mass diffusion gradually vanish and are dominated by chemical *** distributions of temperature and species mole fraction in the flame domain are analyzed to clarify the exergy destruction behaviors caused by heat conduction and mass ***,by dividing the DME oxidation process into four stages,the main reaction channels that contribute to the increase in exergy destruction from chemical reaction have been *** is found that the production and consumption of CH_(2)O and HO_(2) radical dominate the exergy destruction behavior during DME *** this basis,the kinetic mechanism of low-temperature chemistry causing greater exergy destruction is ***,low-temperature chemistry leads to significant exergy destruction due to(a)the large irreversibility of itself and(b)(mainly)the enhancement of H_(2)O_(2)loop reactions by low-temperature reaction *** the reduction of combustion irreversibility is promising to be achieved by reasonably regulating the fuel oxidation path.

关键词： exergy destruction flame structures entropy generation low-temperature chemistry DME

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Simulation Study on Hydrogen-Heating-power Poly-Generation System based on Solar Driven Supercritical Water Biomass Gasification with Compressed Gas Products as an Energy Storage System

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Journal of Thermal Science 2020年第2期29卷 365-377页

作者： JIN Hui WANG Cui FAN Chao State Key Laboratory of Multiphase Flow in Power Engineering Xi'an Jiaotong UniversityXi'an 710049China

Supercritical water gasification driven by solar energy is a promising way for clean utilization of biomass with high moisture content, but direct discharge of liquid residual causes energy waste and decreases energy efficiency. To reduce energy waste, a poly-generation system for hydrogen-rich gas production coupling heat supply and power generation based on supercritical water gasification of biomass driven by concentrated solar energy was established in this paper, which also provided a novel energy storage method to overcome the shortcomings of solar discontinuity. Thermodynamic model of the system was proposed and life cycle assessment(LCA) of the system was conducted. Influence of different parameters(temperature of 600℃ to 800℃, outlet temperature of heat exchanger of 42℃ to 56℃, biomass slurry concentration of 5% to 6.5%) on the gasification performance, energy and exergy efficiency, energy distribution and global warming potential(GWP) was discussed. The results indicated that hydrogen yield increased as gasification temperature increased since free radical reaction was enhanced which gas production reaction was classified into. Molar fraction of hydrogen increased as gasification temperature increased and reached 65.6% at 750℃. Energy and exergy efficiency of the system reached 74.84% and 34.87% at 700℃ and 600℃ respectively and that of gas production was 1..1.% at 650℃, which was the highest. Increasing reaction temperature and decreasing biomass slurry concentration were effective ways to decrease GWP. Optimal operating parameter was reaction temperature of 650℃, outlet temperature of heat exchanger of 50℃ and biomass concentration of 5%.

关键词： biomass supercritical water gasification poly-generation system thermodynamics analysis energy storage life cycle assessment

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Effects of physical properties of supercritical water on coarse graining of particle cluster

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Particuology 2023年第11期82卷 166-178页

作者： Xiaoyu Li Huibo Wang Yi Li Hui Jin State Key Laboratory of Multiphase Flow in Power Engineering Xi'an Jiaotong UniversityXi'an710049China

The coarse graining of particle cluster is of great significance to the study of a fluidized bed. The effects of variations in the physical properties of supercritical water on the coarse graining of particle cluster are investigated in this work. The drag coefficient distributions of the particle cluster are not influenced by the physical properties. However, the physical properties have effects on the values of drag coefficient. The effects of physical properties are weaker in the case of large particle concentrations. Furthermore, the physical properties lead to that the effect of particle cluster wake on the drag of downstream particles being significantly different from that of constant property flow. The variation trend of drag of coarse graining particle is consistent with that of isolated particle. The physical properties lead to significant differences in the values of drag. In this paper, the dominance of the effects of physical properties in a variety of cases is confirmed. Finally, a physical properties effect model is developed accordingly.

关键词： Supercritical water Variations in physical properties Fluidized bed Coarse grain method Particle drag

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Drag coefficient and volume fraction of bubbles in a supercritical water fluidized bed

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Particuology 2021年第4期19卷 127-138页

作者： Hao Wang Youjun Lu State Key Laboratory of Multiphase Flow in Power Engineering Xi’an Jiaotong UniversityXi’an 710049China

The supercritical water fluidized bed(SCWFB)is a recently introduced reactor for biomass gasification that does not release *** groups of Geldart B-type quartz sands with different particle sizes were fluidized at a system pressure of 20-27 MPa and system temperature of 41.-570℃.A series of experiments were performed for determining the drag coefficient and volume fraction of *** effects of the particles’size,superficial velocity,system pressure,and temperature on the drag coefficient and volume fraction are *** addition,a correlation between experimental and computed values is demonstrated for both the drag coefficient and volume fraction in *** relative error of the correlation is within±30%.The results of this study provide significant guidance for the scaling-up design of SCWFBs and for the development of supercritical water gasification technology.

关键词： Supercritical water fluidized bed Bubbles Drag coefficient Volume fraction

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Discrete particle simulation of mixed sand transport

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Particuology 2012年第2期10卷 221-228页

作者： Fengjiun Xiao Lieiin Guo Debiao Li Yueshe Wang State Key Laboratory of Multiphase Flow in Power Engineering Xi'an Jiaotong UniversityXi'an 710049China

An Eulerian/Lagrangian numerical simulation is performed on mixed sand transport. Volume averaged Navier-Stokes equations are solved to calculate gas motion, and particle motion is calculated using Newton＇s equation, involving a hard sphere model to describe particle-to-particle and particle-to-wall collisions. The influence of wall characteristics, size distribution of sand particles and boundary layer depth on vertical distribution of sand mass flux and particle mean horizontal velocity is analyzed, suggesting that all these three factors affect sand transport at different levels. In all cases, for small size groups, sand mass flux first increases with height and then decreases while for large size groups, it decreases exponen- tially with height and for middle size groups the behavior is in-between. The mean horizontal velocity for all size groups well fits experimental data, that is, increasing logarithmically with height in the middle height region. Wall characteristics greatly affects particle to wall collision and makes the fiat bed similar to a Gobi surface and the rough bed similar to a sandy surface. Particle size distribution largely affects the sand mass flux and the highest heights they can reach especially for larger particles.

关键词： Mixed sand transportHard sphere modelSand mass fluxParticle horizontal mean velocity

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Numerical simulation of the drag and heat-transfer characteristics around and through a porous particle based on the lattice Boltzmann method

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Particuology 2021年第5期19卷 99-107页

作者： Mingyue Zhang Qiuyang Zhao Zujie Huang Lei Chen Hui Jin State Key Laboratory of Multiphase Flow in Power Engineering Xi’an Jiaotong UniversityXi’an 710049China

Porous particle flow is universal in nature and ***,in previous numerical simulations,porous particles have usually been assumed to be *** is necessary to study the flow and heat-transfer characteristics around porous particles because they are greatly different from those of impermeable *** this study,two-dimensional steady flow and heat transfer around and through a porous particle with a constant temperature placed in a cold fluid were numerically *** effects of the Reynolds number(Re)and Darcy number(Da)on the flow and heat-transfer characteristics were investigated in *** investigated ranges of the parameters were 1.≤Re≤40 and 1.^(−6)≤Da≤1.^(−2).It is sophisticated to simulate porous particles with traditional simulation methods because of their complicated ***,the lattice Boltzmann method was used to solve the generalized macroscopic governing equations because of its *** drag coefficient decreased with increasing Re or Da,but the decrease was not prominent in the range 1.^(−6)≤Da≤1.^(−4).The heat-transfer efficiency of the front surface was much stronger than that of the rear *** heat-transfer efficiency between the particle and the fluid increased with increasing Re or ***,for 1.^(−6)≤Da≤1.^(−4),the increase was not prominent and the heat-transfer enhancement ratio was slightly larger than ***,the effect of Da became more prominent at larger *** addition,new correlations for the drag coefficient and surface-averaged Nusselt number were obtained based on the simulated results.

关键词： Porous particle Darcy-Brinkman-Forchheimer model flow and heat-transfer characteristics Lattice Boltzmann method Correlation

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Molecular sieving through a graphene nanopore： non-equilibrium molecular dynamics simulation

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Science Bulletin 2017年第8期62卷 554-562页

作者： Chengzhen Sun Bofeng Bai State Key Laboratory of Multiphase Flow in Power Engineering Xi'an Jiaotong University Xi'an 710049 China

Two-dimensional graphene nanopores have shown great promise as ultra-permeable molecular sieves based on their size-sieving effects. We design a nitrogen/hydrogen modified graphene nanopore and conduct a transient non-equilibrium molecular dynamics simulation on its molecular sieving effects. The distinct time-varying molecular crossing numbers show that this special nanopore can efficiently sieve CO_2 and H_2S molecules from CH_4 molecules with high selectivity. By analyzing the molecular structure and pore functionalization-related molecular orientation and permeable zone in the nanopore, density distribution in the molecular adsorption layer on the graphene surface, as well as other features, the molecular sieving mechanisms of graphene nanopores are revealed. Finally, several implications on the design of highly-efficient graphene nanopores, especially for determining the porosity and chemical functionalization, as gas separation membranes are summarized based on the identified phenomena and mechanisms.

关键词： Graphene nanopore Molecular sieve Molecular dynamics Gas separation membrane

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