Study on the atomizing mechanisms and the fire extinguishing efficiency of pneumatic-atomizing water mist generated by the steam atomizing medium
Student thesis: Doctoral Thesis
The generating method and the efficiency of fire extinction are the most important issues that should be treated firstly in the research and application of water mist fire suppression system. And it is with great theoretical and realistic significances to develop new generating methods and application areas to water mist fire suppression technology. However, some problems have been faced nowadays in the processes of the development and application of water mist fire suppression technology, such as the short flow-length, the poor mobility, the low droplet momentum, the low mist flux density and so on. Therefore, much more works should be carried out to solve these problems. The previous studies reveal that the pneumatic-atomizing (twin-fluid) water mist system can discharge mist flow in a long distance. Therefore, the present study is conducting works on pneumatic-atomizing water mist technology. The contributions and works of the present studies are summarized as follows: 1) The generating method and atomizing mechanism of a pneumatic-atomizing water mist with longer flow-length and larger mist flux density were developed in the thesis. Firstly, a new atomizer with adjustable annular orifice was designed to generate pneumatic-atomizing water mist with long flow-length and large mist flux density. The gas and liquid was mixing in the way of 'Y' type. Secondly, the water mist fire suppression system was set based on the systems designs of water supply and gas supply. The steam was taken as the gaseous atomizing medium to study the promotion of the inert gas in fire extinguishing. And the water mist with good spray performance was then generated. The water mist spray generated was steady and symmetrical. The flow-length was about 10m. Thirdly, the calculation of the GLR value and the condensation of the steam are measured to study the flow characteristics of the atomizer. Fourthly, the atomizing mechanism of the pneumatic-atomizing atomizer was studied. The interactions between liquid sheet rings or liquid sheet ring and liquid sheet were studied with a high speed camera. And the reasons to the formation of the 'tree and root' structure were explained firstly. 2) The spray performance of the pneumatic-atomizing water mist generated with a new designed atomizer was measured. Firstly, the temperature distribution of the water mist with different GLR value was measured due to the higher-temperature property of the gaseous atomizing medium. Secondly, the mean droplet diameter of water mist with different GLR value have been calculated based on the binary analysis of the images which were captured by a high-speed camera with a frequency of 2000 fps. Thirdly, the droplet velocity was measured based on the DPIV methodology. And the two dimensional velocity vector field of water mist with different GLR was rebuilt. Fourthly, the cone angle of the spray was measured based on the binary analysis of the images. Fifthly, the mist flux was measured with a device of 'collecting tubes' in case of water mist with different GLR value. 3) Experiments have been conducted to pool fire extinguishing with water mist in test case of local application and total-flood application. Firstly, the experimental setup was set for fire suppression in local application. And the heat release rate of the ethanol, gasoline and diesel in different oil pan was calculated. Secondly, the experimental results were studied. Different kind types of the fuel were compared to study the effects of the fuel. And the typical processes of pool fire extinguished with water mist in local application were also studied based on the pictures captured with a digital video. And then the dominative mechanisms for extinguishing pool fire with pneumatic-atomizing water mist in local application were studied. The lifetime of the droplet with various diameters in case of different flame temperature was calculated. Thirdly, the experimental setup for fire suppression in total-flooding application was set. And different kind types of the fuel were compared to study the effects of the fuel. And the effects of the water mist with different GLR values were also studied. And then the dominative mechanisms for fire extinction with water mist in enclosed space were studied. 4) The flame expansion phenomenon of pool fire extinguishing with steam jet and water mist was studied. Firstly, the existence of a fuel vapor rich core is a prerequisite for observing a flame expansion phenomenon caused by an impinging steam jet with sufficient momentum. This is also the reason responsible for the phenomenon that no significant flame expansion phenomenon was observed in Class A fire. The axial length of the fuel vapor rich core of a medium size pool fire is Lc = 1/5Lf (Lf is the visible flame height). Secondly, the root cause responsible for the flame expansion is the fuel vapor convection after impacting with the steam jet. An expanded fire ball is formed due to the burning of the convected fuel vapor, which gives rise to a flame expansion phenomenon. Thirdly, the flame expansion phenomenon could be divided to three typical stages: the flame height-decreasing stage, the fire-expanding stage and the burn out stage. The flame height-decreasing stage is the stage in which the steam jet impacts the flame and reaches the fuel vapor rich core. The fire-expanding stage is the stage in which the fuel vapor is convected and burnt as a fire ball after the steam impacting the fuel vapor rich core. The fire is then immediately suppressed for lacking of fuel in the burn out stage. The lacking of fuel is the key for the transient nature of the flame expansion phenomenon. Fourthly, the experimental results reveal that, the expanded flame shape was very close to the theoretical analysis proposed and the typical three stages are also be observed in the experiments. Fifthly, the quantitative features of the flame expansion phenomenon, the expansion ratio of the flame width φl, the expansion ratio of the flame volume φv and the expansion ratio of the flame radiation φr, are defined to characterize the level of the expansion. Sixthly, experiments have been conducted to confirm the quantitative features.
- Fire extinction, Atomization