Property Characterization And Fire Extinguishing Efficiency Study Of The Multi-component Compressed Air Foam

To prevent the ozonosphere from being destroyed by Halon fire extinguishing agents, it has been an urgent task to find out those substitutes from signature of Montreal Protocol. With the advantages of high efficiency, stability, less water consuming and environment friendliness, Compressed air foam suppression method has proven itself to be of great capability to combat all kinks of massive fires, which it attracts so much attention of the researchers from all over the world. In this paper, a new foam formulation coupling of physical and chemical suppression effect was developed which shows great ability of extinguishing both class A and class B fires. Some important factors for suppression efficiency of multi-component compressed air foam were studied systematically through experimental method which is followed by the characterization of some important property parameters of the MC-CAF. In the end, foam spreading process over the combustible liquid surface was tested, and the validity examination of the simple model by B. Persson was also performed.Firstly, we set up a lab scale compressed air foam system for experiment purpose. The research show that the orientation of mixing chamber has a major impact on foam quality, and using coaxial mixing chamber with finer grains of the mixing bed usually produced better foam. The gas/liquid ratio is equal to the foam expansion ratio in a certain range, while the foam expansion ratio drops with the increasing of gas/liquid ratio beyond this range.Secondly, a clean foam agent which shows similar suppression ability to class A foam in wood crib fire as well as to aqueous film forming foam in pool fire was designed.Thirdly, foam property characterization of MC-CAF was performed. Result showed that the surface tension of foam solution decreased with concentration in all range, and a notable drop was observed from the beginning of concentration increasing at low range followed by a leveled-off trend. The surface tension dropped below 20mN/m when the concentration reached beyond 1%, and show a linear decreasing trend with temperature. The viscosity of foam solution increased almost linearly with concentration while the ratio of viscosity to concentration decreased with concentration, and the temperature increase can lead to the drop of the viscosity. It was found that a delay period exist before foam drains, and the drainage rate firstly rises to a peak and then drops slowly to zero. Some important drainage property parameters as maximum drainage rate, specific constant of drainage and drainage half-life are all closely related to the foam concentration. Increasing the concentration would cause the maximum drainage rate and the specific constant of drainage rate to decay exponentially and the half-life of drainage to increase exponentially. It was shown that lower concentration result in a quicker increasing of foam bubble size, and the initial bubble size decrease with the gas/liquid ratio and working pressure. Also, the spreading ability of MC-CAF is excellent. Below 4% of concentration, the spreading velocity of MC-CAF is a little quicker than the traditional AFFF, while beyond 4% of concentration, the spreading velocity of MC-CAF increases much faster than the AFFF. In concerns of spread volume, it first decreases with foam concentration and increases steadily with concentration beyond the value of 3% while drop begins when the concentration is beyond 9%. It was also found that the MC-CAF has an excellent ability of radiation blocking. Lower expansion ratio and thicker foam layer result in a better blocking effect and anti-burning time, while the solid combustible protecting function decays when the expansion ratio drops below 5. we studied the dynamic process of foam spread or coverage process over combustible liquid surface, and explored the mechanisms of pool fire suppression by MC-CAF. Validity examination on the simple model of foam spread over liquid surface by B. Persson was performed by comparison of model prediction and the experiment data.At last, The extinguishing test of MC-CAF on pool fire and solid combustible fire were performed though small scale experiments, disclosing the relationships between the extinguishing efficiency and some key factors as working pressure, foam solution concentration, expansion ratio, mixing chamber structure, nozzle height etc.