非屏蔽门地铁车站站台空调系统冷量有效利用率的数值分析

针对非屏蔽门闭式地铁站台,来自区间隧道的活塞风将影响站台空调送风射流,从而影响站台冷量的分布。在此两股气流相互的耦合基础上,提出了站台空调冷量有效利用率的概念。采用计算流体力学(CFD)方法,建立闭式地铁站台三维几何模型,模拟得到了列车在区间匀速运行、减速进站、停站和加速离站4个阶段的地铁站台1.5 m高度水平面的速度场和温度场,并以此计算分析了相应车况下,站台空调冷量有效利用率。由变因素分析,得到冷量利用率受空调送风温度和送风口数量影响最显著。     

On the numerical study of contaminant particle concentration in indoor airflow

The application of three turbulence models—standard k–ε, re-normalization group (RNG) k–ε and RNG-based large eddy simulation (LES) model—to simulate indoor contaminant particle dispersion and concentration distribution in a model room has been investigated. The measured air phase velocity data obtained by Posner et al. [Energy and Buildings 2003;35:515–26], are used to validate the simulation results. All the three turbulence model predictions have shown to be in good agreement with the experimental data. The RNG-based LES model has shown to yield the best agreement. The flow of contaminant particles (with diameters of 1 and 10 μm) is simulated within the indoor airflow environment of the model room. Comparing the three turbulence models for particle flow predictions, the RNG-based LES model through better accommodating unsteady low-Reynolds-number (LRN) turbulent flow structure has shown to provide more realistic particle dispersion and concentration distribution than the other two conventional turbulence models. As the experimental approach to access indoor contaminant particle concentration can be rather expensive and unable to provide the required detailed information, the LES prediction can be effectively employed to validate the widely used k–ε models that are commonly applied in many building simulation investigations.     

新建地铁隧道内活塞风温度变化理论分析

列车在地铁隧道中运行时,会产生大量的热,一部分被隧道内岩土层吸收,其它部分散失在空气中,随列车活塞风带入站台。本文假设在新建单线隧道,一列车行驶周期内,对隧道内活塞风温度变化规律进行理论分析。隧道内列车散热假设为移动热源,将隧道区间内的空气流动简化成一维管流,活塞风与隧道壁面发生对流换热,根据隧道内空气的热平衡,建立简单的流固耦合模型。简化后得到新建地铁区间隧道活塞风温度变化数学模型,并给出其数值计算方法,借鉴上海某地铁的参数,利用MATLAB软件计算并绘出整个过程中隧道内活塞风温度变化曲线,隧道内活塞风温度下降约1.9 ℃。分析发现隧道内的岩土层温度、隧道长度和列车速度等影响隧道内温度分布和温度变化幅度。     

Influences of the train-wind and air-curtain to reduce the particle concentration inside a subway tunnel

Subways are used widely for public transportation in major cities and require efficient ventilation systems to maintain indoor air quality in the subway tunnel. A subway tunnel was investigated numerically and experimentally to reduce the particle concentration in subway tunnels. The subway tunnel is 54-m long, 1.65-m high, and 2.5-m wide. The subway tunnel is one-quarter scale of a real subway tunnel. The tunnel has two U-type mechanical ventilation shafts. The steady three-dimensional airflow in the tunnel was analyzed using ANSYS CFX software to solve the Reynolds-averaged Navier–Stokes equations. The airflow in the tunnel and shafts was observed numerically using the train-wind and air-curtain. The effects of the train-wind, air-curtain, and electric precipitator were examined experimentally. The ventilation performance in the subway tunnel was observed with respect to the particle concentration in the tunnel. The numerical results suggest proper operating conditions for experimental analysis of the particle concentration. The average velocity of the airflow increases in the shaft when the velocity of the air-curtain increases. The particle concentration at the dust monitoring device after ventilation shaft 1 was reduced significantly in the tunnel when the air-curtain and train-wind were operated.     

The effect of ceiling configurations on indoor air motion and ventilation flow rates

The purpose of this paper is to evaluate the effects of a building parameter, namely ceiling configuration, on indoor natural ventilation. The computational fluid dynamics (CFD) code Phoenics was used with the RNG k–? turbulence model to study wind motion and ventilation flow rates inside the building. All the CFD boundary conditions were described. The simulation results were first validated by wind tunnel experiment results in detail, and then used to compare rooms with various ceiling configurations in different cases. The simulation results generated matched the experimental results confirming the accuracy of the RNG k–? turbulence model to successfully predict indoor wind motion for this study. Our main results reveal that ceiling configurations have certain effects on indoor airflow and ventilation flow rates although these effects are fairly minor.     

Experimental and numerical study of a full scale ventilated enclosure: Comparison of four two equations closure turbulence models

Full-scale experimental and computational fluid dynamics (CFD) methods are used to investigate the velocity and temperature fields in a mechanically ventilated enclosure. Detailed airflow fields are measured in three cases of ventilation air temperature: an isothermal case, a hot case and a cold case. The ventilation system creates an axisymmetric jet which is developing near the ceiling. The experimental data are used to test four two equations turbulence models: a k–ε realizable model, a k–ε RNG model, a k–ω model and a k–ω SST model. It is found that, even if the models can predict reasonably the hot and isothermal cases global values of temperature and velocity, none of the models is reliable concerning the cold case. Moreover, a detailed analysis of the jet shows that none of the models is able to predict the exact experimental velocity and temperature fields.     

Mixing characteristics in a ventilated room with non-isothermal ceiling air supply

A two-dimensional turbulence k–ε model is used to predict distribution of air velocity, temperature and turbulence kinetic energy in an air-conditioned room using ceiling air supply. Mixing characteristics of the airflow are analyzed under different air supply velocities and temperatures. A modified Archimedes number is correlated with the parameters characterizing heat transfer, ventilation system, and turbulence kinetic energy of room air flow. Significant correlations have been shown. It is found that the linear ceiling air supply air-conditioning system with a high level return air provides excellent air mixing across a wide range of Archimedes numbers. The results are useful for air-conditioning design and thermal comfort study.     

A computational model to calculate the flow-induced pressure fluctuations on buildings

A computational model to predict the flow-induced pressure fluctuation on bluff bodies is presented. Unlike direct and large-eddy simulation, the present model employs a stochastic model to generate plausible velocity fluctuations (synthetic turbulence) that satisfy the mean turbulent quantities such as turbulent kinetic energy (k) and dissipation energy rate (ε). This model has three main components: (1) prediction of mean flow quantities by solving the 3D Navier-Stokes equations using the standard k-ε model with Kato and Launder modifications; (2) generating a synthetic turbulent velocity field using a stochastic model and finally (3) solving the Poisson equation that governs the pressure fluctuations field. Flow around the low-rise building at Texas Tech was analyzed using the developed model. Two different wind angles of attack are considered for the analysis. Results obtained using the developed model are compared with wind tunnel and field measurements. The computed rms values for pressure fluctuations show good agreement with the experimental results.     

Numerical investigation of the unsteady flow characteristics of human body thermal plume

Human thermal plume is quite important to the study of airflow organization in the indoor environment, especially in the micro-environment research such as personalized ventilation, infectious disease transmission through air, etc. In order to investigate the unsteady fluctuation of the thermal plume around human body, a series of transient numerical simulations are conducted in this study. Numerical simulation based on 9.7 million grids and 0.02 s time step is performed to obtain the detail quantitative data of flow field. The obvious fluctuation and separation are captured in the upper flow region of human body based on the high resolution grids. The maximum time-averaged velocity of the thermal plume is found to be 0.25 m/s while the maximum fluctuate velocity is about 0.07 m/s. The further analysis of frequency spectrum shows that the thermal plume around the body is mainly dominated by the low frequency fluctuation which is lower than 1 Hz and the principal frequency is around 0.1 Hz. In order to overcome the drawback of the high computation cost for application of the engineering simulation, a new numerical simulation method combining a modified k–ε turbulence model and coarse grids is presented. This modified k–ε model can reduce the calculation error of Reynolds stress in the flow region of natural convection through redefining the turbulence viscosity coefficient segmentally and avoid a high numerical viscosity appeared due to the central difference scheme. It can reasonably predict the general fluctuation velocity and the frequency distribution during simulation process in coarse grids and show a huge potential to be applied to the engineering application.     

Effects of vent shaft location on the ventilation performance in a subway tunnel

A computational analysis of a ventilation system in a subway tunnel is carried out by solving 3D Reynolds-averaged Navier–Stokes equations for train-induced unsteady flow using the sharp interface method as the model for the moving boundary of an immersed solid. The ventilation performance is evaluated depending on the location of the vent shaft by analyzing the ventilating flow rate through the shaft and the feature of the flow field in the subway tunnel around the shaft. It is found that the optimum location of the vent shaft with respect to maximizing ventilation performance lies near the station.     

An evaluation exercise of a wind pressure distribution model

The wind pressure distribution (WPD) around a building is an important parameter for multi-zone airflow simulation. The input is usually in the form of pressure coefficients (C_p). The values of C_p are traditionally derived based on wind tunnel studies. However, since it is not possible to always conduct wind tunnel tests to obtain the C_p values, alternative approaches have been suggested. One approach involves the use of statistical regression analysis of data obtained from wind tunnel studies. In this paper, C_p values of a shophouse building under investigation are generated using wind tunnel, a WPD model, which is based on the regression analysis of wind tunnel data as well as based on wall averaged values from published data. Using the C_p values obtained from the wind tunnel as the reference, the accuracy of the C_p values generated by the WPD model and the wall averaged values are analysed and discussed. The effects of such accuracy on the computed air flow rates and age of air for the building using a multi-zone network airflow model are also considered.     

Simulation of mean flow and turbulence over a 2D building array using high-resolution CFD and a distributed drag force approach

A numerical simulation has been performed of the disturbed flow through and over a two-dimensional array of rectangular buildings immersed in a neutrally stratified deep rough-walled turbulent boundary-layer flow. The model used for the simulation was the steady-state Reynolds-averaged Navier-Stokes equations with linear and non-linear eddy viscosity formulations for the Reynolds stresses. The eddy viscosity was determined using a high-Reynolds number form of the k-ε turbulence-closure model with the boundary conditions at the wall obtained with a standard wall-function approach. The resulting system of partial differential equations was solved using the SIMPLE algorithm in conjunction with a non-orthogonal, colocated, cell-centered, finite volume procedure. The predictive capabilities of the high-resolution computational fluid dynamics (CFD) simulations of urban flow are validated against a very detailed and comprehensive wind tunnel data set. Vertical profiles of the mean streamwise velocity and the turbulence kinetic energy are presented and compared to those measured in the wind tunnel simulation.It is found that the performance of all the turbulence models investigated is generally good—most of the qualitative features in the disturbed turbulent flow field through and over the building array are correctly reproduced. The quantitative agreement is also fairly good (especially for the mean velocity field). Overall, the non-linear k-ε model gave the best performance among four different turbulence closure models examined. The turbulence energy levels within the street canyons and in the exit region downstream of the last building were underestimated by all four turbulence closure models. This appears to contradict the ‘stagnation point anomaly’ associated with the standard k-ε model which is a result of the excessive turbulence energy production due to normal straining. A possible explanation for this is the inability of the present models to account properly for the effects of secondary strains on the turbulence and/or for the effects of large-scale flapping of the strong shear layer at the canopy top.The results of the high-resolution CFD simulations have been used to diagnose values of the drag coefficient to be used in a distributed drag force representation of the obstacles in the array. Comparisons of the measured spatially-averaged time-mean mean velocity and turbulence kinetic energy in the array with predictions of the disturbed flow using the distributed drag force approach have been made.     

Numerical study on the mean velocity distribution law of air backflow and the effective interaction length of airflow in forced ventilated tunnels

The effective interaction length of airflow (L_e) is the key factor in the determination of the distance between an air duct nozzle and the working face in dead-end tunnels. At present, the current calculation methods of L_e consider only the tunnel cross section dimension and are therefore not suitable for large underground cavern groups. Because air backflow is the direct promoting factor in the removal of pollutants and the construction specifications in underground space require a minimum wind speed, this paper proposed that L_e is the distance between the nozzle and the critical cross section, where the mean velocity of the air backflow equals the minimum wind speed required. For this purpose, a computational study was conducted to investigate the flow pattern of air backflow in ventilated tunnels. Semi-empirical equations of the mean velocity distribution law of air backflow were derived based on the numerical results. By solving the equations, the location of the critical cross section can then be acquired. Accordingly, the value of L_e could be obtained. The proposed method is related to the tunnel width, the tunnel height, the air duct diameter and the efflux velocity. Thus, it is more comprehensive and applicable compared to the current methods.     

Effects of the solid curtains on natural ventilation performance in a subway tunnel

Solid curtains can be installed in subway tunnels for the promotion of air ventilation in ventilation ducts in association with the piston effect caused by a running train. With an aim to analyze the effects of solid curtains on duct ventilation performance in a subway tunnel, the current study adopts the tunnel and subway train geometries which are exactly the same as those in a previous model tunnel experiment, but newly incorporates two ventilation ducts connected vertically to the tunnel ceiling and two solid curtains placed at an upstream position of a duct near the tunnel inlet and at a downstream position of another duct near the tunnel outlet, respectively. A three-dimensional CFD model adopting the dynamic layering method for tracking the motion of a train, which was validated against the reported model tunnel experiment in a previous study, is employed to predict the train-induced unsteady airflows in the subway tunnel and in the ducts. The numerical results reveal that the duct ventilation performance in a subway tunnel strongly depends on the operation of the solid curtains. The suction mass flow of the air through the duct near the tunnel inlet and the exhaust mass flow of the air through the duct near the tunnel outlet are increased considerably in the case with the solid curtains in comparison with those in the case without the solid curtains.     

Numerical simulation of rooftop ventilator flow

Rotating rooftop turbine ventilators are cost effective environmental friendly natural ventilation devices, which are used to extract airflow from a building to improve air quality and comfort. A CFD study using the standard k-? turbulence model with multiple reference frame (MRF) meshing technique was employed to explore the suitability of numerical approach in modelling various features of a ventilator flow. The initial CFD results were validated against wind tunnel data obtained for a commercial ventilator on a simulated inclined rooftop configuration conducted at the aerodynamic laboratory of University of New South Wales. The numerical studies were then extended to examine both the internal and the external flows associated with the ventilator at different wind speeds and to quantify the performance of a rotating ventilator in terms of air extraction rate. The trend observed appeared to be in good agreement with published data suggesting that application of numerical simulation is feasible as a cost effective tool in the future design, development and performance analysis of rotating wind driven ventilation device.     

Numerical studies on air flow around a cube

In this paper, air approach flow moving towards a cube will be studied using computational fluid dynamics (CFD). The Reynolds Averaging of Navier-Stokes (RANS) equation types of k-ε turbulence model are used. Some RANS predicted results are compared with different upstream air speeds. Flow separation at the corner above the top of the cube, level of separation and reattachment are investigated. Reference is made to the experimental data on wind tunnels reported in the literature.A method similar to ‘recirculation bubble promoter’ is used for different approach flow speed distributions. Problems encountered in numerical simulations due to the sharp corner are discussed with a view to obtaining better prediction on recirculation flow in regions above the top of the cube. Correlations between the turbulent kinetic energy above the cube and the recirculation bubble size are derived for different distributions of approach flow speed.By limiting the longitudinal velocities in the first cell adjacent to the sharp edge of the cube or rib, and making good use of the wall functions at the intersection cells of the velocity components, positions of maximum turbulent kinetic energy and the flow separation and reattachment can be predicted by a standard k-ε model. The results agree with those obtained in the experiments.     

某地铁站负荷模拟计算案例分析

本文首先总结了地铁站空调通风系统的负荷模拟计算与一般公共建筑相比主要有三点不同。针对这些不同,本文通过建立计算流体力学(CFD)场模型与STESS网络模型的方法,分别对屏蔽门渗透风、隧道活塞风、隧道热环境进行模拟计算与分析,获得了相应结果,并将此结果作为负荷模拟计算的输入;同时在考虑了地下围护传热的基础上建立地铁站DeST模型,设定了合适的计算参数,计算出地铁站全年逐时负荷,为设计计算和空调选型提供了有效设计依据和相关节能建议。     

Experimental and numerical analyses of train-induced unsteady tunnel flow in subway

To analyze the unsteady three-dimensional flow in the subway tunnel caused by the passage of a train, both experimental and computational studies have been conducted. The experimental analysis of train-induced unsteady flow is conducted on a 1/20 scale model tunnel and the pressure and air velocity variations with time are presented. The three-dimensional unsteady numerical analysis using the sharp interface method for the moving boundary of an immersed solid was carried out for the same geometric configurations as the experimental analysis. The predicted numerical model results show good agreement with the experimental data.     

Numerical prediction of gas concentrations and fluctuations above a triangular hill within a turbulent boundary layer

The objective of the present work is to propose a numerical and statistical approach, using computational fluid dynamics, for the study of the atmospheric pollutant dispersion. Modifications in the standard k-ε turbulence model and additional equations for the calculation of the variance of concentration are introduced to enhance the prediction of the flow field and scalar quantities. The flow field, the mean concentration and the variance of a flow over a two-dimensional triangular hill, with a finite-size point pollutant source, are calculated by a finite volume code and compared with published experimental results. A modified low Reynolds k-ε turbulence model was employed in this work, using the constant of the k-ε model C_μ=0.03 to take into account the inactive atmospheric turbulence. The numerical results for the velocity profiles and the position of the reattachment point are in good agreement with the experimental results. The results for the mean and the variance of the concentration are also in good agreement with experimental results from the literature.