Experimental Study Of Foam Generated By Compressed Air Foam System

The paper describes a newly developed high performance compressed air foam system (CAFS). The construction of the experimental CAFS includes design of the power train, selection of pump and air compressor and other parts of the system.The experimental results indicate the superior viscous, durable, wetting and fire resistant properties generated by CAFS against fire. The data of foam surface tension display an obvious decrease by 43-64% compared with pure water. The viscosity coefficient of foam is more than twenty times larger than water. As well as the fire resistant time is larger than water by 9 to 18 times. The durable time and fire resistant time of foam is dependent with gas fraction and bubble diameter etc.Apart from the ingredient of foam concentrate, mixing factors are critical to foam characters. The mixing way of compressed air and foam solution, mixing area as well as mixing pressure was found to be the key elements, which influence foam quality.In order to study the rheology of fire-fighting foam, which is similar to some special fluid, a specially designed pendulum system with rectangular bob is used to measure yield stress. The equilibrium angle away from vertical under very low shear rate strongly prove the existence of the hypothesis of the yield stress of fire foam which has been related to solid object. The yield stress can be easily calculated with the equilibrium angle precisely measured by a protractor. Some curves and equations were obtained to demonstrate the change of the yield stress as gas fraction, drainage time, bubble size increased. Disperse of some data implies the lack of independence of the factors, there is still some uncertainty about the mechanism of fire foam yield stress. It's impossible to provide a perfect mathematical model of yield stress of foam with the equipment that we have.Another important property, which is the decline of the viscosity as the shear rate increased, is also proved to be existed based on strong support. Three rotators with different diameters under four rotate velocities show the same tendency. A valuable mathematical model is constructed to describe the rheology performance and predict the flowing behavior of fire foam occurring in open area. The origination of the model will be of great importance to the application of foam generated by CAFS in forest fire suppression both in fire direct attack and acting as a firebreak.Some recommendations are also presented to construct CAFS used in forest fire suppression.