低压气泡雾化的实验及数值模拟研究

为了进一步研究混合腔内部流场和参数最佳组合的两相流装置,设计了6 种不同结构的两相流雾化喷嘴,并实验测定雾滴粒径、雾滴轴向速度、雾化角和雾通量等相关雾化特性参数。在实验基础上,采用Fluent 模拟软件对混合腔内部的速度流场、压力流场和气液体积分数进行模拟分析,对实验结果进行补充与验证。得到雾化特性和灭火效果兼优的最优喷嘴的工作参数为：液体工作压力0.4 MPa,液体质量流量为60.0 kg/h,气体工作压力0.6 MPa,气体质量流量为0.1 kg/h,喷孔直径0.8 mm,注气孔数32 个。     

Characterization of hydraulic fluid spray combustion

The combustion intensity of hydraulic fluids and mineral oil, methanol, ethanol, and heptane, ejected vertically up ward through a pressure-jet hollow cone nozzle and stabilized by a ring burner, has been characterized in terms of heat release rates. A relationship has been established between the chemical heat release rate, fluid exit velocity, and chemical heat of combustion. Mineral oil, along with some organic esters, has the highest combustion intensity as indicated by heat release rate, followed by esters (organic and phosphates), heptane, water-in-oil emulsion, ethanol, methanol, and polyglycol-in-water. Variations in combustion intensities in hydraulic fluids are found to be due to variations in the chemical structures and additives.The efficiency of combustion is found to be sensitive to fluid exit velocity.The radiative fraction of the efficiency of combustion for phosphate esters is found to be the highest (0.38–0.40), followed by mineral oil (0.36), organic esters (0.28–0.35), water-in-oil emulsion (0.27–0.28), and polyglycol-in-water (0.12–0.25). The radiative fraction of the efficiency of combustion for ethanol and heptane spray fires is found to be less than for the pool fires. For methanol spray fire, radiative fraction of the efficiency of combustion is found to be about the same as for the pool fire.The visible flame length of hydraulic fluid spray fires varies with the chemical heat release rate to the power of 0.6 for both hollow and solid cone nozzles.     

熄灭森林阴燃火的细水雾喷头设计与仿真

森林大火后的残余地表余火及地下阴燃火是引起二次森林火灾的主要原因,为防止发生二次森林火灾,清理森林余火和阴燃火是必要的.细水雾作为哈龙灭火剂的最佳替代品,被广泛应用在消防领域.本文设计一种适合熄灭森林余火和阴燃火的撞击分流高压细水雾喷头,采用Fluent进行喷头流域的仿真,分析喷孔孔径、喷孔长度和喷孔喷雾锥角对喷出细水...     

Model scale tunnel tests with water spray

A model scale study (1:23) was carried out in order to improve the basic understanding of water spray systems in longitudinal tunnel flow. The water spray system consisted of commercially available axial-flow hollow cone nozzles. Tests with both a deluge system made of 12 nozzles placed directly above the fire source and a water curtain system consisting of four nozzles placed either downstream or upstream of the fire source were carried out. A wood crib was used to simulate the fire source, which was designed to correspond to a HGV (heavy goods vehicle) fire load in large scale. A second wood crib was used as a target pile and was placed downstream the ignited wood crib. The parameters varied were the water flow rate and water pressure, the longitudinal ventilation rate and the arrangement of the nozzle system. Possible fire spread between wood cribs, with a free distance corresponding to 15 m in large scale, was investigated.     

船用细水雾灭火喷头结构优化与分析

根据液体的雾化和灭火机理,对细水雾的液滴形成和破碎过程、影响液体雾化的因素、多种船用细水雾喷头结构的优劣进行分析,得到细水雾雾化对喷头的基本要求。设计船用喷头采用螺旋内流道,使液体在室内进行强烈的旋转,以提高水雾动能,加强离心雾化效果,增强灭火能力。运用STAR-CCM+对直射式喷头和改进的离心喷头进行对比分析,得到流场以及流场内有关面上的压力、速度、流动迹线以及液相水的体积分数。结果表明,螺旋流道的离心结构喷头可以增大喷雾半径、扩大保护范围、提升灭火效果。     

单、双喷头细水雾对正庚烷池火灾的抑制作用

为了研究单、双喷头细水雾抑灭正庚烷池火灾的效能和机理,在半体积飞机模拟货舱中开展了单、双喷头细水雾雾滴粒径测试和抑灭20 cm 正庚烷池火灾的实验研究。结果表明,双喷头细水雾协同工作会导致雾滴之间相互碰撞发生二次破碎,有助于雾化效果的提升。通过对燃料表面温度、火焰区平均温度和舱内氧气浓度的测量和计算,对比分析了单、双喷头细水雾抑灭火的主导机理。结果表明,单喷头细水雾灭火的平均时间为283.14 s,耗水量约为3.54 L,燃料表面冷却是其抑灭火的主导机理。双喷头细水雾灭火的平均时间为212.22 s,耗水量约为5.31L,火焰冷却是其抑灭火的主导机理。     

大空间空调中射流理论的适用性问题研究

以大空间空调设计中广泛采用的圆形喷口为例,对比分析了传统半经验射流理论计算结果与CFD模拟结果。研究结果表明,模拟得到的多喷口水平并排射流的射流轴心弯曲及轴心风速衰减均较快;实际建筑空间结构、内部发热条件及喷口布置形式对射流的流场有较大影响;空间温度场分布并非仅与射流的射程有关,还受空间整体气流状况影响。     

Submerged rectangular air jets as a particulate barrier

In several technical applications, it is convenient to carry out physical separation between two neighboring zones with different physical characteristics (such as temperature or contaminant concentration), using a two dimensional gaseous jet, often called air curtain. In this paper, a submerged rectangular air jet, simulating an air curtain, has been investigated in order to infer the capability to reduce the transfer of dusty air between a “clean” zone and a contaminated environment (dusty zone). The investigated jet was generated by means of a rectangular nozzle with a discharge area of 0.02 square meters, while the dust has been simulated using atomized distilled water in a cloud of small droplets, sprayed transversally against the air curtain. Experiments have been performed running the air curtain at Reynolds Number(Re) ranging from 4500 to 25,500 while the Sauter Mean Diameter (SMD) of atomized water was equal to 6 microns. The water spray has been characterized using a TSI single component fiber optics Phase Döppler Particle Analyzer (PDPA), and the air curtain has been investigated by means of a Particle Image Velocimetry (PIV) technique. The amount, size distribution and velocity of the droplet cloud able to cross the air curtain was measured, obtaining as the main result, the dependence of the amount of droplets passing the barrier as a function of the Re of the air curtain. For a Re as high as 25,500, the curtain is able to interrupt the amount of surviving droplets, i.e. all the dust is rejected.     

地下车库诱导通风射流及风口的计算

理论分析了地下车库诱导通风的流场,提出了诱导送风口的布置方法,阐述了诱导射流参数及风口尺寸的计算方法,并给出了计算实例和设计参考数据。     

固定式压缩空气泡沫系统单喷头喷洒特性及灭火效能

对5 种典型结构喷头进行了单喷头喷洒压缩空气泡沫和纯水试验,对喷洒介质在底面上的密度分布以及喷洒的泡沫25%析液时间进行比较分析。研究发现,喷头结构类型和流动参数都会影响泡沫析液特性,喷头内部旋芯会使泡沫稳定性降低;下垂式水喷头的底面分布均匀性较好,而扰流喷头和两种七孔喷头的底面分布均匀性相对较差,且下垂式水喷头和扰流喷头在更高的压力和流量条件下的底面分布均匀性降低;同时,压缩空气泡沫系统的喷头选型可以在一定程度上参考相应喷头的喷水特性。选用最优性能参数喷头,建立火灾模型,开展固定式压缩空气泡沫系统全尺寸变压器热油火灾试验,验证喷头与固定式系统的适配性。     

A numerical study of the interaction of water spray with a fire plume

Water spray-based fire extinguishing equipment such as sprinklers has been widely used in fire suppression and control. However, the fire extinguishing mechanism in such devices is not well understood due to the complexity of the physical and chemical interactions between water spray and fire plume. Currently, quantitative approaches (e.g. numerical modeling) to estimate the performance and effectiveness of water spray systems have not been developed to a stage where they can be used to optimize the design for different operating environments and types of fire. In the present work, a numerical simulation approach is introduced to provide a quantitative analysis of the complex interactions occurring between water spray and fire plume. The effects of several important factors (namely water spray pattern, water droplet size and water spray flow rate) on the fire suppression mechanism are investigated. The simulations show that the water spray with solid cone pattern and finer water droplet size is more effective in extinguishing fires than the one with hollow cone pattern and coarse water droplet size. To suppress a fire, the water spray flow rate has to be more than a certain critical value. However, using too high water spray flow rate does not increase fire suppression efficiency but only leads to increased operational cost because of the excess water flow rate. In the current paper, the principles of fire suppression with water spray are also discussed, which are useful in designing more effective water spray fire suppression systems.     

气液流量对低压双流体细水雾雾场特性的影响

由于对民机货舱中不同气液流量的低压双流体细水雾研究较少,且对单个喷头雾通量和雾动量的定量、定性分析不够完善,还需设计开展实验进行更深入的研究。参考美国联邦航空管理局（简称“FAA”）制定的相关标准,设计并搭建飞机货舱低压双流体细水雾实验平台,主要包括FAA全尺寸飞机货舱、低压双流体细水雾系统。采用的喷头与常规的喷头有所不同,喷出的细水雾是一个圆形平面,具有优良的弥散性和滞空性。实验中采用马尔文粒径分析仪结合Spraytec软件测量不同气液流量下的细水雾雾滴粒径,发现当气体流量从250 L/min增加至350 L/min、液体流量保持0.5 L/min不变时,细水雾雾滴粒径从130 μm降低至95 μm;当液体流量分别为0.75 L/min和1.0 L/min时,粒径大小分别从161 μm降至110 μm,从201 μm降至142 μm。根据美国防火协会的标准,采用量杯收集法测量得到不同气液流量下的雾通量,发现单独增加气体流量或液体流量,其雾通量都会增加。当气体流量为350 L/min、液体流量为1.0 L/min时,雾通量达到最大值,为0.255 L/（min·m2）。利用粒子图像测速仪的高速摄像机拍摄喷雾照片,测量雾滴速度,结果显示,在3种不同的液体流量工况下,随着气体流量从250 L/min增加至350 L/min,对应的雾滴速度均近似以0.04的增长率上升,最小值为8.5 m/s,最大值为16.0 m/s。在假设细水雾雾滴形状为球状时,雾动量关系式由雾滴质量与速度的乘积得到。选取雾滴速度最大值代入公式,得到单个雾滴在不同气液流量下的雾动量变化曲线,发现当气体流量为250 L/min、液体流量为1.0 L/min时,单个雾滴动量达到最大值,为3.6×10-8 kg·m/s。通过实验研究不同气液流量对低压双流体细水雾雾场特性的影响,得出以下结论：当液体流量不变,气体流量从250 L/min以25 L/min的变化率增加至350 L/min时,雾滴粒径和雾动量逐渐减小,而雾滴速度和雾通量逐渐增加;当气体流量不变,液体流量从0.5 L/min以0.25 L/min的变化率增加至1.0 L/min时,雾滴粒径、雾动量和雾通量均逐渐增大,而雾滴速度逐渐减小。对于单个雾滴,雾动量取决于雾滴粒径的大小;而对于喷头喷出的所有雾滴,雾滴速度决定了整体雾动量的大小。今后课题将进行细水雾灭火实验,探究达到最佳灭火效果的气液流量。结论可用于飞机货舱细水雾灭火系统的设计与改进,为飞机防火系统一些参数的设置提供了实验和理论基础。     

A review on the applied techniques of exhaled airflow and droplets characterization

In the last two decades, multidisciplinary research teams worked on developing a comprehensive understanding of the transmission mechanisms of airborne diseases. This article reviews the experimental studies on the characterization of the exhaled airflow and the droplets, comparing the measured parameters, the advantages, and the limitations of each technique. To characterize the airflow field, the global flow-field techniques—high-speed photography, schlieren photography, and PIV—are applied to visualize the shape and propagation of the exhaled airflow and its interaction with the ambient air, while the pointwise measurements provide quantitative measurements of the velocity, flow rate, humidity and temperature at a single point in the flow field. For the exhaled droplets, intrusive techniques are used to characterize the size distribution and concentration of the droplets' dry residues while non-intrusive techniques can measure the droplet size and velocity at different locations in the flow field. The evolution of droplets' size and velocity away from the source has not yet been thoroughly experimentally investigated. Besides, there is a lack of information about the temperature and humidity fields composed by the interaction of the exhaled airflow and the ambient air.     

Buoyant jet in a ventilated room: Velocity field,temperature field and airflow patterns analysed with three different whole-field methods

The instantaneous velocity field and temperature field were measured and the airflow patterns visualised close to a diffuser for displacement ventilation. Since the low-velocity diffuser was located above the floor and the inlet air temperature was below the room temperature, the flow was governed by both momentum and buoyancy forces. The data were recorded with whole-field measuring techniques, particle streak velocimetry (PSV), particle image velocimetry (PIV) and infrared thermography (IR), in conjunction with a low thermal mass screen. The environment is very complex, supply of buoyant air with a commercial supply terminal with 20 nozzles pointing in different directions, which makes it difficult to use point-measuring techniques or computational fluid dynamics (CFD). The aim was twofold: (a) to explore what kind of information can be derived from whole-field measurement techniques in this context and (b) to investigate the trajectory of the flow discharged into the room and the entrainment of the flow.     

PIV measurements for a complex topographic terrain

Particle image velocimetry (PIV) measurements are performed on a 1:3000 scale model of complex topographic area. Three distinct topographic features are investigated: a valley, a ridge and a 3D hill.PIV performs well in capturing two-dimensional mean and turbulent velocity fields both horizontally and vertically over extended areas with an approx. 5 m full-scale resolution. The results show that while for the valley and hill cases the flow is relatively well correlated over large spatial extends, the ridge flow is strongly influenced by both local and major topographies.The PIV measurements compare well with hot-wire-based mean velocity profiles for the three cases. Moreover, the turbulence intensity profiles match well for flow regions without recirculation. The ridge wake region shows discrepancies between the two techniques, which are attributed to the complexity of the flow in this region and limitations of both techniques.     

Interaction between water spray and smoke in a fire event in a confined and mechanically ventilated enclosure

This work deals with the interaction between water droplet flows and smoke in a fire event in a confined and ventilated enclosure. The objective is to identify the specific effect of water spray in the specific environment of a confined and ventilated enclosure. The study is based on 17 large-scale fire tests performed in one room of 165 m³ ventilated at a renewal rate of 15.4 h⁻¹. The fire source is a propane gas burner with a heat release rate of between 140 and 290 kW. The water spray system consists of two Deluge nozzles with a nozzle coefficient of 26 l/min/bar^0.5. The test parameters are the fire heat release rate, the water flow rate, from 50 to 124 l/min, and the activation time. The study focuses on three topics, the interaction of the droplets with the smoke, the droplet evaporation process and the energy transferred to the droplets. The water spray significantly modifies the smoke stratification by mixing and cooling the gas phase. The rate of droplet evaporation has been determined from the water mass balance and is of the same order of magnitude as the rate of water vapor production by the combustion reaction. Heat transfer from the smoke to the droplets has been investigated using the energy balance equation. For a fire scenario in a confined and ventilated enclosure, the energy released by the fire is mainly transferred to the walls and extracted by the ventilation network. In the event of water spray activation, a significant share, up to 65%, is transferred to the droplet flows.     

一种旋流式喷嘴的雾化实验研究

细水雾喷嘴是细水雾灭火系统的关键部件。本文主要采用激光多普勒测试仪（LDV）和相位多普勒粒子测量仪（PDPA）对一种旋流式喷嘴进行雾化实验研究。通过实验得到了不同测试压力下,喷嘴的流量、流量特性系数K、雾化锥角θ、水平射程、索特尔平均粒径（SMD）和粒子轴向速度的分布情况,最后得到了这几个参数随测试压力的变化关系。所测试的细水雾灭火喷头具有较大的轴向动量和径向动量,有利于扑灭深位火灾和扩大灭火范围。     

Effect of initial conditions of modeled PDFs on droplet characteristics for coalescing and evaporating turbulent water spray used in fire suppression applications

Parametric studies were conducted for a coalescing and evaporating turbulent water spray using a stochastic separated flow technique that includes submodels for droplet dynamics, heat and mass transfer, and droplet–droplet binary collisions. While the initial droplet size distribution, in general, is not known due to the difficulty in the optical access to the nozzle exit region, the size distribution is modeled using the analytical PDFs (probability density functions) such as log-normal, Rosin–Rammler, Gaussian, and Nukiyama–Tanasawa distribution model. Standard deviation of the PDFs is varied and their effects on droplet size and speed distribution in the downstream are reported. The arithmetic mean droplet size at the nozzle exit, which is used as input for simulations, was extrapolated using the existing experimental data obtained at downstream locations.     

正交试验法分析喷水室热湿交换效率影响因素

根据某织布车间喷水室可调控的三个主要影响其热湿交换效率的因素:风速、蒸汽管和喷雾风机的喷水管的开关、喷排布置,分别在冬季和过渡季节安排正交试验,得到三种因素水平的最佳组合及关键因素:冬季时,喷排组合是影响其效率的关键因素,低风速、喷蒸汽、喷雾开、离心喷排和对喷撞击流喷排同时运行为喷水室最佳运行工况;过渡季节时,喷雾的开关是影响其效率的关键因素,其中低风速、喷雾开、对喷撞击流喷排单独运行为喷水室最佳运行工况.     

A numerical study of water-mist suppression of large scale compartment fires

The focus of this paper is on simulating water mist suppression of fires in large enclosures. A two-continuum formulation is used in which the gas phase and the water-mist are both described by equations of the Eulerian form. The water-mist model is coupled with previously developed codes based on the multi- block Chimera technique for simulating fires. Computations are performed to understand the various physical processes that occur during the interaction of water-mist and fires in large enclosures. Droplet sectional density contours and velocity vectors are used to track the movement of water-mist and to identify the regions of the fire compartment where the droplets evaporate and absorb energy. Parametric studies are performed to optimize various water-mist injection characteristics for maximum suppression. The effects of droplet diameter, mist injection velocity, injection density, nozzle locations and injection orientation on mist entrainment and flame suppression are quantified. Numerical results indicate that for similar injection parameters such as mist injection density, injection velocity and droplet diameter, the time for suppression was smallest for the top injection configuration. Water-mist injection through the side walls, the front and rear walls and through the floor were found to be less efficient than the top injection configuration. These results are compared with our earlier predictions on water-mist suppression of small scale methanol pool fires and other experimental studies.