Particle residence time distribution and axial dispersion coefficient in a pressurized circulating fluidized bed by using multiphase particle-in-cell simulation

The particle residence time distribution(RTD)and axial dispersion coefficient are key parameters for the design and operation of a pressurized circulating fluidized bed(PCFB).In this study,the effects of pressure(0.1-0.6 MPa),fluidizing gas velocity(2-7 m·s^(-1)),and solid circulation rate(10-90 kg·m^(-2)·s^(-1))on particle RTD and axial dispersion coefficient in a PCFB are numerically investigated based on the multiphase particle-in-cell(MP-PIC)method.The details of the gas-solid flow behaviors of PCFB are revealed.Based on the gas-solid flow pattern,the particles tend to move more orderly under elevated pressures.With an increase in either fluidizing gas velocity or solid circulation rate,the mean residence time of particles decreases while the axial dispersion coefficient increases.With an increase in pressure,the core-annulus flow is strengthened,which leads to a wider shape of the particle RTD curve and a larger mean particle residence time.The back-mixing of particles increases with increasing pressure,resulting in an increase in the axial dispersion coefficient.

Residence time distribution of high viscosity fluids falling film flow down outside of industrial-scale vertical wavy wall: Experimental investigation and CFD prediction

The flow behavior of gravity-driven falling film of non-conductive high viscosity polymer fluids on an industrial-scale vertical wavy wall was investigated in terms of film thickness and residence time distribution by numerical simulation and experiment.Falling film flow of high viscosity fluids was found to be steady on a vertical wavy wall in the presence of the large film thickness.The comparison between numerical simulation and experiment for the film thickness both in crest and trough of wavy wall showed good agreement.The simulation results of average residence time of falling film flow with different viscous fluids were also consistent with the experimental results.This work provides the initial insights of how to evaluate and optimize the falling film flow system of polymer fluid.

Distribution, residence time, autotrophic production, and heterotrophic removal of DOP in the Mirs Bay, northern South China Sea

The importance of dissolved organic phosphorus(DOP) as a potential nutrient source for primary producers in marine systems has been recognized for up to eight decades, but currently, the understanding of the biogeochemistry of DOP is in its infancy. In the present study, monthly data between 2000 and 2014 were used to analyze the temporal and spatial distributions of DOP in the Mir Bay, the northern South China Sea. The DOP residence time(TDOP) was also investigated using a simple regression analysis in combination with chlorophyll a(Chl a) measurements while excess DOP(ΔDOP), produced by the biogeochemical processes of autotrophic production and heterotrophic removal, was determined using a two-component mixing mass-balance model in combination with salinity measurements. The results showed that the DOP concentration was(0.017±0.010) mg/L higher in the surface-water compared with the bottom-water and higher in the inner Tolo Harbour and waters adjacent to Shatoujiao compared with the main zone of the bay. Although seasonal changes and annual variability in the DOP were small, the surface DOP concentration was higher in the wet season(April–September)than in the dry season(October–March) due to the impacts of seaward discharges and atmospheric deposition into the bay. Measurement and regression results showed that the DOP release rate from phytoplankton production was about 1.83(gP)/(gChl a) and the TDOP was about 7 d, which implied that the DOP cycle in the bay was rapid. The ΔDOP was calculated from the model to be about 0.000 mg/L in the main zone of the bay and about 0.002 mg/L in the inner Tolo Harbour and waters adjacent to Shaotoujiao, suggesting that the autotrophic production of DOP was almost balanced by the heterotrophic removal in the main zone of the bay and dominated in the inner Tolo Harbour and waters adjacent to Shaotoujiao. In conclusion, the Mirs Bay is very productive and fairly heterotrophic.

Modeling on Residence Time Distribution in Subsurface Flow Constructed Wetlands by Multi Flow Dispersion Model

As an important design factor for constructed wetlands,hydraulic retention time and its distribution will affect the treatment performance.Instantaneously injected sodium chloride tracers were used to obtain residence time distributions of the lab scale subsurface flow constructed wetland.Considering the presence of trailing and multiple peaks of the tracer breakthrough curve,the multi flow dispersion model(MFDM)was used to fit the experimental tracer breakthrough curves.According to the residual sum of squares and comparison between the experimental values and simulated values of the tracer concentration,MFDM could fit the residence time distribution(RTD)curve satisfactorily,the results of which also reflected the layered structure of wetland cells,thus to give reference for application of MFDM to the same kind of subsurface flow constructed wetlands.

RESIDENCE TIME DISTRIBUTION STUDIES WITH TRACER EXPERIMENTS:IMPULSE INJECTION VERSUS STEP CHANGE OF FEED

1 INTRODUCTIONThe popularly used axial dispersion model is mathematically expressed by aone-dimensional partial differential

ON THE RESPONSE FUNCTION AND RESIDENCE TIME DISTRIBUTION OF THE AXIAL DISPERSION MODEL FOR TUBULAR REACTORS

In view of confusion on the residence time distribution and transfer fucntions for dispersion models, the solution of the model subject to possible combinations of closed and open type boundaries at the ends of a chemical reactor is investigated systematically. In particular, some new results are obtained for the closed-open and open-closed types of reactors. It is found that the interchange of boundary conditions for these reactors is not immaterial. In several cases, caution is necessary to distinguish whether the transfer function is applied to the concentration or the flux. When the dispersive flux is notable, measurement techniques must be carefully selected to match the kind of transfer functions derived.

RESIDENCE TIME DISTRIBUTION IN AN OPEN REACTOR:EFFECT OF BACKGROUND REACTANT ON TRACER DISPERSION

A new approach is presented for deriving the residence time distribution(RTD)in open-type reactors with exclusion of residence in the fore section.Inasmuch as failure to predict the con-version of chemical reactions has been evidenced in many occasions,numerical solution of tracerdispersion in the presence of background concentration gradient of reactant is given to demonstratethe strong effect of background reactant on the true RTD a reactant molecule experiences.Withinthe error of computation,the conversion of a first-order chemical reaction under steady state is shownto be equal in both closed and open reactors,despite difference in relevant

THE RESIDENCE TIME DISTRIBUTION FOR MULTIFLOW SYSTEM

In this paper,the superposition rule of the residence time distribution functions for the general systemhaving multiple inlet and outlet streams has been described and proved rigorously.For the cascade ves-sels system where the processed material in separate stages may be nonideally mixed in various degrees andthe volumes of separate stages may not be equal,the overall residence time distribution function E(t)and eachE(t)of the flow systems have been derived.The applications of these results to various flow systems havebeen discussed.

Gas Residence Time Distributions in a Spouted Bed

In a spouted bed of 80mm in ID and 1700mm in height, the gas residence time distributions at different radial positions in both spout and annular area were measured with five different kinds of particles as spouting material, air as spouting gas, and hydrogen as tracer. The effects of superficial gas velocity, operating pressure, particle size and its category on gas residence time distribution were discussed. It was found that the gas velocity profile in spout was more uniform than that in annulus. It could be concluded that the gas flow in the spout could be treated as a plug-flow, while that in the annulus inhibited a strong non-ideal flow behavior. Increasing the superficial gas velocity and decreasing the operating pressure, the particle density and its size gave rise to spouting disturbance, thus the measured tracer concentrations vs. time curves fluctuated. The variances of residence time distribution curves could be taken as a measure of the gas fluctuation degree.

Residence Time Distribution at Laminar Pulsatile Flow in a Straight Pipe

This paper deals with the problem of theoretical identification of the residence time distribution （RTD） characteristics of a straight pipe at laminar pulsatile flow, if tracer diffusion can be neglected. This situation is typical for micro-apparatuses （e.g. fluidic element） and also for flow in large arteries. Residence time distribution based on velocity profiles at pulsatile flow of a Newtonian liquid in a rigid pipe will be derived theoretically and compared with the well known results for a constant flow rate E（τ） = τ-^2/2τ^3 at τ 〉 τ^-/2, where E （τ） is differential distribution, x is residence time and τ^- is the mean residence time. The following part of the paper deals stimulus response experimental techniques using tracers. The principal problem related to laminar and convection dominated pulsatile flows is discussed： Can the impulse response also be identified with the actual residence time distribution in the case of variable flow？ The general answer is no, and differences between RTD and impulse responses are evaluated as a function of the frequency and amplitude of pulsatile flows.

Numerical simulation of flow field and residence time of nanoparticles in a 1000-ton industrial multi-jet combustion reactor

In this work,by establishing a three-dimensional physical model of a 1000-ton industrial multi-jet combustion reactor,a hexahedral structured grid was used to discretize the model.Combined with realizable k–εmodel,eddy-dissipation-concept,discrete-ordinate radiation model,hydrogen 19-step detailed reaction mechanism,air age user-defined-function,velocity field,temperature field,concentration field and gas arrival time in the reactor were numerically simulated.The Euler–Lagrange method combined with the discrete-phase-model was used to reveal the flow characteristics of particles in the reactor,and based on this,the effects of the reactor aspect ratios,central jet gas velocity and particle size on the flow field characteristics and particle back-mixing degree in the reactor were investigated.The results show that with the decrease of aspect ratio in the combustion reactors,the velocity and temperature attenuation in the reactor are intensified,the vortex phenomenon is aggravated,and the residence time distribution of nanoparticles is more dispersed.With the increase in the central jet gas velocities in reactors,the vortex lengthens along the axis,the turbulence intensity increases,and the residence time of particles decreases.The back-mixing degree and residence time of particles in the reactor also decrease with the increase in particle size.The simulation results can provide reference for the structural regulation of nanoparticles and the structural design of combustion reactor in the process of gas combustion synthesis.

An Estimation Method for Kill Probability Based on Random Residence Time Series

Relations between statistical residence time series and effective shooting are analyzed in accordance with the properties of the random residence time of maneuver targets crossing shot area in a given time. An estimation method for kill probability is proposed, which solves the probability of number of residence times satisfied effective shooting in given time. Some expressions and their approximate formulae of kill probability are derived, under known the distribution of residence time series. Theoretical analysis and simulation results show that this method is suitable for evaluating the hit ability of fire system for maneuver targets in random shooting.

基于RTDS的有源配电网暂态实时仿真与分析

有源配电网是智能电网中集成、管理分布式发电及储能设备、需求侧响应资源的重要平台,包括各种智能终端、控制及保护装置的研发与测试将对实时仿真提出更高要求。考虑有源配电网实时仿真的建模需求,文中基于实时仿真平台RTDS完成了含光伏、燃料电池等分布式电源的有源配电网暂态实时仿真建模,并给出了电力电子装置、分布式电源及控制器等特殊模型的实时仿真算法。研究了有源配电网在环境条件变化、系统故障及功率指令变化等各种场景下的动态过程,通过与离线仿真软件PSCAD/EMTDC的对比,验证了实时仿真模型的正确性,对系统资源消耗等分析,更为后续硬件在回路仿真(HIL)等研究中奠定了基础。

RTD型磁通门传感器检测系统设计

为了对外界静磁场进行检测,设计了滞留时间差(RTD)型磁通门传感器的检测系统。通过对检测系统信号调理电路、数字存储单元以及数据处理算法等方面的设计优化,提高了检测系统的检测精密度和准确度。在电磁屏蔽室中,利用赫姆霍兹线圈给定外磁场,对所设计的检测系统进行测试,实验结果表明:时间差波动小于±0.1μs,在±5×104n T范围内线性度误差为±0.21%,灵敏度为22.4 s/T,多次测量标准差不大于0.03,适合用于对静磁场的检测。

长径比对SK静态混合器RTD影响的实验研究

对两长径比AR=1和AR=1.5的SK型静态混合器进行停留时间分布(RTD)实验研究。各截面的出峰时间和峰值表明,在相同截面上,随长径比减小停留时间分布曲线峰值变大,峰域变窄;长径比越小E(θ)曲线拖尾现象越不明显,流动趋于理想流动。采用先前平均停留时间的计算公式对实验结果进行计算,模拟结果与实验结果有较好的一致性,平均误差为14%。σθ2的分析表明,AR=1的值比AR=1.5的值平均小60%,表明AR=1的RTD分布越集中,混合系统内返混少,流动趋于活塞流,与E(θ)的分析结果相一致。

食品挤压过程的分析方法——RTD分析、流变分析

挤压过程的分析方法分为6种,着重介绍RTD分析和流变分析2种方法。首先介绍了停留时间分布(RTD)的测定方法、参数求解以及模拟分析,然后论述了流变特性的测定方法以及流变模型的建立。最后指出了今后的研究方向。

管式振荡流反应器的流动模式研究(Ⅰ)PIV和RTD实验研究

采用粒子成像可视化(PIV)技术研究了管式振荡流反应器(OFR)内的流场形态和混合特点,并采用脉冲进样法测定了OFR在不同振荡条件下的停留时间分布函效。实验结果表明,OFR的混合特性十分复杂,并随振荡强度的变化呈现出不同的特征。振荡强度较低时,振荡使得OFR径向混合加强,减少了滞留区, 流体的流动趋于平推流;振荡强度较高时,腔室内布满数目、尺寸和位置不断变化的漩涡,使每个腔室趋向于全混,腔室间的返混增大。实验数据与多级串联全混釜模型的比较结果显示,OFR的混合特性远非简单流动模式模型所能表征。

PABR的RTD实验CFD模拟研究

为解决传统的流动和传质特性研究只能通过实验方法获得一些经验性关系的局限性问题,采用计算流体动力学(CFD:Computational Fluid Dynamic)技术对微生态周期切换式厌氧系统(PABR:Periodic Anaerobic Baffled Reactor)进行了停留时间分布(RTD:Residence Time Distribution)实验的CFD模拟,以期为PABR构型改进、工况模拟、风险预测与评估等工作的开展提供节省人力物力的实施方案。利用COMSOL平台对PABR内水体的SOLID WORKS三维建模进行网格划分,进而使用单向流层流模块、稀物质传递模块及瞬态求解器,根据PABR试验装置实物RTD的边界条件进行模拟。应用Matlab对其实物与CFD模拟的标准化曲线分别进行归一化积分求解,判定其相似程度在可接受范围。最后利用COMSOL后处理技术可视化示踪剂在PABR内的流动和扩散过程,从而实现人机交互。

Residence time distribution of wood chips in a semi-industrial multiple hearth furnace using RFID tracers Author links open overlay panel

In continuous biomass torrefaction plants,the products'yields,composition and homogeneity highly depend on the residence time of particles.A characterization of particle residence time distribution(RTD)was therefore carried out in an industrial-scale multiple hearth furnace on poplar wood chips using radio frequency identification tracers.The effects of operating conditions,namely,mass flow rate of biomass,shaft speed of the rabbling system and interdental length on the RTD were studied.The increase of shaft speed and mass flow rate reduces particles’mean residence time.Lowering the length between two successive teeth also increases the bed speed.Uncontrollable biomass accumulation(also called“bulldozing”)was observed during several tests.This phenomenon is favored by a high mass flow rate of resources,a small interdental length between the teeth and a low shaft speed.RTD measurements were compared to the axial dispersion model.For all tests,the Peclet number is ranging between 20 and 62,indicating that the multiple hearth furnace cannot be modelled as an ideal plug flow reactor.

Numerical analysis on the transport properties and residence time distribution of ribbon biomass particles in a riser reactor based on CFD-DEM approach

A bended ribbon biomass particle model was developed to explore the dynamic transport properties inside a riser reactor.Residence time distribution(RTD)of the particles was analyzed by using the Eulerian-Lagrange method.The effects of sampling height,particle density,particle size and gas-to-solid mass ratio on RTD were investigated.The coupled Computational Fluid Dynamics and Discrete Element Method(CFD-DEM)model was verified firstly by experimental data on pressure drop and residence time distribution density function.The simulation results demonstrated that the ribbon biomass particles display a typical annular-core spatial distribution during transportation.The RTD of particles exhibit an approximate single-peaked normal distribution.The mean residence time(MRT)can reach up to 0.7 s when the particle density is 1200 kg/m^(3).Particle with higher density has longer mean residence time.The flow patterns are closer to plug flow if particle length over 12 mm.The particle flow pattern is not sensitive to changes in particle density and size,while the gas-to-material mass ratio has a significant impact on it.