Experimental Investigation On Fire Suppression Mechanisms Of Water Mist Based On The Water Mist Flux Analysis

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.