Buoyancy and inertial force on oscillations of thermal-induced convective flow across a vent

Natural vents are commonly installed in buildings for smoke control. Air motion is induced by buoyancy of the thermal sources inside the building. Hot smoke is expected to be exhausted out of the vent. However, directions of air flowing across the vent might be oscillating under some conditions. The ratio B of buoyancy to inertial force defined by the Grashof number over the square of the Reynolds number is the key parameter in determining airflow oscillations.     

用Tenax管吸附检测室内空气中苯的实验研究

进行了用Tenax管吸附检测室内空气中苯的实验,对实验结果进行了分析,研究表明:在一定的条件下用Tenax管吸附按GB 50325-2001(2006年版)规范附录B规定检测室内空气中的苯是可行的.     

供暖系统管段阻力和流量关系的研究及编程计算

分析了供暖管路阻力和流量的关系式△P=SQ2的适用范围,提出这一关系式不适用于室内供暖管路系统.通过与电路的类比,分别提出适于计算大型室外供暖管路和室内供暖管路的有效算法.将编程计算的方法应用于室内同程式供暖管路,得到各管段阻力的变化对管段流量和系统总流量的影响规律.     

General expressions for the calculation of air flow and heat transfer rates in tall ventilation cavities

Solar heated tall ventilation cavities including solar chimneys are used to enhance natural ventilation of buildings. A validated CFD model was used to predict the buoyancy-driven air flow and heat transfer rates in vertical ventilation cavities with various combinations of heat distribution on two vertical walls ranging from symmetrical to fully asymmetrical heating. The natural ventilation rate and heat transfer rate have been found to vary with the total heat input, heat distribution on the cavity walls, cavity width and height and inlet opening position. General expressions for these variables have been obtained and presented in non-dimensional terms, Nusselt number, Reynolds number, Rayleigh number and aspect ratio (H/b), as Nu = f(Ra, H/b) and Nu = f(Ra, Re) or Re = f(Ra, Nu), for natural ventilation design.     

空调水系统中定压差控制水力平衡措施相关问题探讨(二)——电动调节阀实际流量特性的改善

以《空调水系统中定压差控制水力平衡措施相关问题探讨(一)——自力式压差平衡阀的选型》一文中对四类典型空调水系统末端电动调节阀所设自力式压差平衡阀的选型结果为基础,分析了定压差控制水力平衡措施对电动调节阀实际流量特性的改善程度。对本文中算例而言,采用定压差控制水力平衡措施可在较大程度上改善电动调节阀的实际流量特性,但受自力式压差平衡阀产品流量系数的限制,电动调节阀在小开度工况下的实际流量特性仍偏离于理论流量特性。     

长距离输水管线管道试压方法的探讨

结合GB50268—2008给水排水管道工程施工及验收规范,对长距离输水管线管道试压方法进行了探讨,以球墨铸铁管为例,分析了新型式水压试验方法的必要性,总结得出了该施工工艺的优点,以期指导输水管线施工。     

火源—竖井间距对自然通风隧道火灾烟气流动特征的影响

通过数值计算,研究顶部开口自然通风隧道火灾火源–竖井间距对烟气流动特征与竖井排烟效率的影响。考虑因素有火源–竖井间距、竖井断面尺寸。结果表明：随着火源–竖井间距的增大,竖井前方来流烟气的质量流量增大,且竖井的排烟效率逐渐降低,竖井内空气卷吸量减少;当火源–竖井间距较小时,竖井更有利于排出更多的热量,竖井后方的温度降低幅度更大,烟气可以被控制在更小的范围内。此外,随着竖井截面尺寸的增大,竖井的排烟效率增加,且增大竖井的宽度更有利于增加竖井的排烟量。因此建议当相邻竖井的间距较大时,可适当增加竖井的截面尺寸和竖井高度。     

Effect of filter collection efficiency on the clean air delivery rate in an air cleaner

The performance of an air cleaner is evaluated by the clean air delivery rate (CADR), which is defined as the measure of the delivery of contaminant-free air. Herein, we conducted comparative analyses of various particulate air filters with various collection efficiencies. We installed each filter in identical commercial air cleaners to determine the effects of the collection efficiency on the CADR. Three different filters (E11, E12, and H13 classes) were prepared to determine the effects of the filter collection efficiency and pressure drop on the air cleaner performance (ie, the CADR). Based on experimental data, filters E11 and E12 had similar CADRs and flow rates. However, filter H13, which had the highest collection efficiency and the lowest flow rate, had the lowest CADR. This indicates that even if a filter with higher collection efficiency is installed in an air cleaner, the larger pressure drop causes a reduction in the air flow rate. The CADR value is widely distributed for a flow rate range for commercially available models; however, the collection efficiencies for most air cleaners on the market lie in a narrow range. Therefore, the flow rate has the most direct impact on the performance of a commercial air cleaner.     

Numerical simulation of transient flow development in a naturally ventilated room

Numerical simulations were conducted to model the transient flow development in a naturally ventilated space containing a centrally located localized source of heat. The simulations were compared with a series of small-scale laboratory experiments and existing theoretical models. The aim of the work was to benchmark CFD models for time-dependent buoyancy-driven natural ventilation against previously published experimental results and theoretical models. The simulations agree well with experimental results during the initial development of the room stratification. The CFD results accurately predict the maximum depth of the hot buoyant layer at the top of the room as well as the steady-state interface height which separates the warm upper buoyant layer from the cooler air below. The simulations also predict well the time taken for the buoyant upper layer to reach its maximum depth. However, at longer times the results diverge. This may be due to thermal diffusion and mixing at the interface between the upper and lower layers due to the inflow from the floor level vents.     

Simulation of pipe progression in a levee foundation with coupled seepage and pipe flow domains

A very large percentage of piping cases have been brought about by internal erosion, which is the primary cause of dam failures. This study developed a numerical model to simulate the pipe progression in a levee foundation by analyzing the inception and transportation of erodible particles from the soil fabric. An approach that considers the turbulent flow in an erodible pipe and the seepage flow in the remaining area of a levee foundation is employed to capture the main hydraulic characteristics of piping. The mechanical analysis of individual erodible particles is considered to quantify the critical condition for particle inception in an erodible pipe. In addition, physical piping model tests are numerically simulated to examine the proposed approach. The simulation demonstrates that the flow in a pipe can progress backward from downstream to upstream when the upstream water head reaches a critical value. Furthermore, the function mechanism of a cut-off wall can be explained by this model. The results have revealed that this model can reproduce the experimental data, such as the critical water head and the progression time, which are obtained from the physical model. The relationship between the depth of a suspended cut-off wall and the critical water head is obtained; this relationship facilitates the practical design of the critical depth of a cut-off wall for a given water head.     

计量供热小区变流量测试与分析

从不同的空间尺度和时间尺度对北方某分户计量按热量收费的住宅小区进行了变流量测试与分析,测试结果发现,用户流量变化差别很大,日最大相对流量变化范围为0～100%;从用户、楼到换热站的相对流量变化幅度逐渐减小;换热站的流量变化存在高低不同频率的波动,1 h内的相对流量变化幅度小于2.5%,1 d内小于5%,一个控制周期内小于10%,整个供暖季的流量变化明显受压差控制的影响,呈阶跃性变化;水泵功耗与压差设定值有关.     

火灾时隧道内烟流流动状态试验研究

通过大比例火灾模型试验,研究火灾时隧道内烟流流动状态、烟流速度变化以及通风对烟流流动状态的影响。试验模型隧道长100m,内径1.8m。火源采用燃烧床盛放油料模拟,试验中设定了A、B、C三个火灾规模用以模拟实际隧道火灾场景。试验结果表明,点火后,隧道内火区、火区下游烟流速度在2～8min内增加很快,明显大于点火前风速,且其增幅随通风风速、火灾规模的不同而变化。同时,随着火势的逐渐减弱隧道内烟流速度也逐渐减小,并趋于初始风速。试验结果建议对于一般的限制或禁止油罐车通行的隧道,火灾时,隧道内应尽快建立起2～3m/s的纵向风流以抑止烟气的逆流。     

楔形射流特性的实验研究

通过实验分析了平面楔形射流在不同的楔角及边壁条件下形成的流场特性。研究表明，无论是有边壁的半受限楔形射流，还是无边壁的自由楔形射流，都存在一个临界楔角，偏离此临界角，射流所形成的分散射流都将失稳而成为向上或向下的贴附射流。     

Enhancing nitrification in vertical flow constructed wetland utilizing a passive air pump

Nitrification of simulated secondary effluent solution using a variant of the vertical flow bed constructed wetland was studied. In this system oxygen required for the nitrification process is supplied by a passive air pump. The passive air pump is based on a fill and draw cycle, where oxygen depleted air is removed from the system while fresh air is introduced. Each volume of effluent drained is displaced by an equal volume of fresh air. Spatial and temporal oxygen distribution in the system as a function of drained effluent volume was investigated. Experimental results of nitrification and the corresponding oxygen consumption show good agreement with theoretical calculations based on physico–chemical considerations. Observations show that while oxygen distribution within the media was found to be non-uniform at the beginning of each cycle, it approached more uniform distribution with time. The latter resulted mainly from diffusion of oxygen in the gaseous phase.     

高层建筑管道安装中空气预检的应用

结合具体的工程概况，分析了管道空气预检的根据与标准，对管道空气预检试压方法的施工工艺以及优点作了阐述，并提出施工中应注意的问题，从而确保高层建筑管道的安装。     

高大厂房热风供暖气流组织实验研究

利用模型了热风供暖系统的气流组织,分析了实验范围内圆形风口等温与非等温射流轴心速度的衰减,找出射流流量,热气流上飘,热射流上下两部分流量的变化规律,认为实验结果可供高厂房热风供暖设计计算参考。     

Temperature- and wind-induced air flow patterns in a staircase. Computer modelling and experimental verification

The typical infiltration load for a residential building has been found to range from one-third to one-half of the total space conditioning load. However, most infiltration measurements have been made on single-family houses. Information about the role of infiltration in the energy consumption of large buildings is limited. Furthermore, the prediction of infiltration rates in high-rise buildings is a complex problem. The forces that drive this flow result from the superposition of wind pressure on the faces of the building and the stack effect across the height of the building. Infiltration models have shown the latter effect to be significant in single-family residences, particular in colder climates and, consequently, the stack effect is even greater in high-rise buildings. For this work, we performed tracer gas and fan pressurization measurements on a 30 m tall University of California dormitory in order to determine the importance of both wind and stack effect upon infiltration. Measured pressure and tracer gas distributions were compared with those from a predictive infiltration computer model for high-rise buildings. To study the influence of the air flow pattern around the building, this model uses various wind velocity profiles characteristic of urban areas and different sets of surface pressure coefficients derived from wind tunnel experiments.     

Thermal performance of the solar parabolic trough collector at different flow rates: an experimental study

Currently, the most capable thermal systems based on the solar energy are the concentrating collectors, which are essentially finding applications in power generation and process industries. In the present study, thermal performance of the parabolic tough collector (PTC) is investigated experimentally at different flow rates of working fluid. Mass flow rate is one of the key parameters influencing its performance. Here, PTC is constructed as a simple structure having a non-evacuated tube and tested in tracking and south-facing modes utilising water as the working fluid. The performances in terms of water temperature rise, useful heat gain, collector efficiency are evaluated with and without utilising glazing on the receiver. Results revealed that performance of collector chiefly depends upon the mass flow rate and no considerable change is found when the flow rate of water is more than 0.024?kg/s. Furthermore, small-sized PTC offers slight better performance in the south-facing than the tracking mode.