【作者】 刘益民；

【导师】 廖光煊； 秦俊；

【作者基本信息】 中国科学技术大学 ， 安全技术及工程， 2008， 博士

【摘要】 雾通量是影响细水雾灭火性能的关键参数之一。本文在原有雾通量定义的基础上,将能够直接与燃料和火焰作用的细水雾通量定义为有效雾通量。细水雾灭火过程是一个多种机理同时作用的、非定常、非稳态的过程。为了对细水雾的灭火机理进行定量研究,将火灾中常见的扩散火分成了燃料区、反应区和卷吸区(供氧区)三个部分。将能够在这三个区作用的有效雾通量分为燃料区有效雾通量、反应区有效雾通量和卷吸区有效雾通量,对有效雾通量的分类可以把燃料冷却、氧气冷却、燃料稀释和氧气稀释等作用机理分开,便于更好的对这些灭火机理进行定量分析。粒径小于50μm的细水雾具有类似于气体灭火剂的特性,它们的粒径范围相对稳定,雾通量、动量以及单位体积内的水雾总量等参数很容易得到控制,使用这种细水雾抑制火焰可以将瞬息万变的灭火过程转换为稳态或准稳态过程。因此,本文将这部分细水雾独立出来,定义为超细水雾,而粒径大于50μm的细水雾则定义为常规细水雾。在调研了超细水雾发生方法及机理的基础上,研制了适合于本文实验要求的、可用于灭火的超细水雾发生系统。使用电子天平和激光多普勒测速仪等设备对超细水雾与常规细水雾的特性进行了测量,并将超细水雾与常规细水雾的特性进行了对比分析。超细水雾粒径小、速度低,无法穿透火焰反应面进入反应区,因此,使用低速超细水雾抑制火焰时,细水雾只能在火焰对空气的卷吸作用下到达火焰附近,通过汽化吸热以及稀释氧气的机理实现灭火,这样就排除了其他灭火机理的影响。由于超细水雾到达火焰反应面之前就全部汽化,因此在超细水雾与火焰反应而之间存在一个没有液滴的包络面,将这个包络面的厚度假设为无穷大之后,就可以通过扩散火空气卷吸量的计算方法求出包络面内火焰卷吸的(空气-水蒸气)混合气的体积,由于超细水雾汽化前后的质量并没有变,所以通过混合气的体积可以进一步推算出不同尺寸火焰卷吸超细水雾的量。于是,这些超细水雾从火焰反应区吸收的热就能得到定量,同时超细水雾汽化对氧气的稀释作用也可以定量分析。为了验证卷吸区有效雾通量计算方法的可靠性,建立了杯形燃烧器实验台,通过实验验证了这种理论的准确性。前人进行细水雾灭火有效性实验时使用的通常是常规细水雾,这些雾滴粒径和动量均较大的细水雾在灭火时,雾滴会很快穿过羽流与火焰到达燃料表面,仅有少量细水雾完全汽化,因此前人在冷态情况下测量的雾通量可以近似等于本文中的燃料区有效雾通量,测量时通常使用收集法和积分法,测量值均为一段时间内的平均,测量基于雾通量定常的假设之上。但在灭火过程中,尤其是细水雾施加初期,燃料区有效雾通量却存在不稳定的变化过程。为了对有效雾通量进行实时测量,在试管(烧杯)收集法的基础上进行改进,建立了电子天平实时称重系统和数据采集系统,实验验证了称重法有效雾通量测量的可行性。对比不同高度下测量的有效雾通量与假设雾场雾流密度均匀分布条件下推导的有效雾通量随喷头高度变化的曲线,可以看出本文给出的雾通量计算模型具有较高的精确度。在测量了燃料区有效雾通量的基础上,对影响细水雾灭火性能的各种参数进行了分类,将雾滴粒径、雾滴速度、动量、动能等参数定义为直接参数,而将喷头高度、喷头特性参数、细水雾系统压力、通风条件等参数定义为间接参数,因为间接参数通过影响直接参数来影响细水雾灭火性能。通过常规细水雾扑灭正庚烷与柴油火的实验,在直接参数的因子分析基础上研究了直接参数与灭火性能的相关性。结果表明,在本文研究的这些直接参数中,雾场最前端细水雾速度与稳定喷射时细水雾的平均速度是两个决定细水雾是否能够迅速将火焰抑制,并最终扑灭火焰的参数。这个结果表明在设计常规细水雾灭火系统的时候,应该优先保证细水雾具有足够的速度。为了定量研究反应区有效雾通量对燃烧的影响,独立分析反应区细水雾冷却火焰、稀释燃料的灭火机理,建立了另一种杯形燃烧器实验台,在燃烧器的气体燃料通道中加入了雾通量可控的超细水雾发生系统,超细水雾跟随燃料运动到燃烧器的出口,作用于燃烧反应。实验测量了冷态条件下的有效雾通量之后,通过热态实验研究不同有效通量的超细水雾对火焰燃烧的影响,定量分析了反应区有效雾通量的灭火机理,得到了反应区有效雾通量灭火的临界值。更多还原

【Abstract】 Water mist flux is one of the key factors that affect the suppression efficiency. In order to focus on the water mist which has direct influence on the fuel and flame, the flux made up by these water mists is effective flux. The process of water mist fire suppression contains couples of mechanisms at the same time, and the process is characterized as an un-constant and un-stable process. To investigate the effective flux’s suppression mechanism quantitatively, the common seen buoyant flame which often occurs in fires is divided into three regions: the fuel-zone, the reaction-zone and the entrainment-zone. Consequently, the effective flux is consisted of fuel-zone effective flux, reaction-zone effective flux and entrainment-zone effective flux. This kind of division can separate the suppression mechanisms like fuel cooling, oxygen cooling, fuel dilution and oxygen dilution, make the quantitative analysis easier.Water mist with the diameter less than 50 microns has the characters similar to the gas extinguishant. Its diameter is steady-going, the flux, momentum and the water in unit volume can easily controlled. Using this kind of water mist to suppress flame can transform the fire suppression process to a relatively stable process. So this kind of water mist is picked up and defined as Super-fine Water Mist, and water mist with the diameter lager than 50 microns is defined as Regular Water Mist. On the basis of investigations on the methods and mechanisms to produce Super-fine Water Mist, a Super-fine Water Mist generator system is established; using electronic balance, the Laser Doppler Velocimeter and Adaptive Phase/Doppler Velocimeter (LDV/APV) system, the characters of Super-fine Water Mist were measured, and compared with those of Regular Water Mist.Super-fine Water Mist can’t perforate the flame surface into the reaction-zone because of the diameter and low velocity, when Super-fine Water Mist is used to suppress flame, it can only be entrained by the flame, suppress the flame by means of heat absorption and dilution of the oxygen. Because Super-fine Water Mist becomes vapor before coming to the flame surface, there is a none-liquid zone between the flame and water mist. Suppose that the none-liquid zone can extend to far away, the calculation for buoyant flame entrainment with air can be used to calculate the air-vapor mixture volume, in this way, the Super-fine Water Mist entrained into the entrainment-zone is calculated. Consequently, the heat absorbed by the water mist and the oxygen dilution degree can also be determined. In order to verify the reliability of the calculation, a cup-burner system was established, experiments results confirmed the veracity.Former researchers mainly use ordinary water mist in the fire, suppression experiments, this kind of water mist can penetrate the plume and flame quickly and reach the fuel surface because of the large diameter and high velocity, only little proportion of water mist changes to gas phase. Therefore, former measurement results of water mist flux are nearly the same as the fuel-zone effective water mist flux. People use collect-method and integral-method to measure fuel-zone effective flux, the flux measured is the average of a period time, based on the supposition that the flux is constant. But, in fact, the flux is unstable, especially at the beginning of the spray. In order to measure the flux in real-time, the collect-method is upgraded to an electronic balance together with a real-time data acquisition system. The feasibility of this method was verified by experiments. The data acquired in the experiments have a good coordinate with the results calculated on the supposition that the flux density of the water mist flux field is uniform. Therefore, a water mist flux calculation method is established.After the fuel-zone effective flux is measured, the factors that affect the suppression efficiency are divided into two groups; mist diameter, mist velocity, momentum and kinetic energy are one group, named as direct factors. Height of sprinkler, and the K factor, the operation pressure and the ventilation condition are the other group, named as in-direct factors. The factors in the latter group are called in-direct factors, because they don’t affect the suppression process directly, but affect the factors in the former group, while the former group can affect the suppression directly. The correlativity of the direct factors and the suppression efficiency is analyzed by the suppression experiments of heptanes-fire and diesel fires. There comes the conclusion that the water mist velocity has the largest correlativity with the suppression efficiency, this result suggests that velocity should be considered prior to other factors of the water mist in the water mist system designs.In order to investigate influence of the effective flux on the combustion, analyze water mist suppression mechanism of flame cooling and fuel dilution in the reaction-zone separately, another cup-burner experimental apparatus was set up, in this burner, a flux controlled Super-fine Water Mist generator system is added into the water mist transmit process, with the force of the fuel, water mist generated is taken to the opening of the burner. Effective flux was measured before the fire ignited, after that, effects of different flux on the flame combustion was investigated; the suppression mechanism of reaction-zone effective flux is analyzed. Threshold flux for the fire extinguishment was obtained by experiments.更多还原