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Compressed air foam (CAF) and compressed nitrogen foam (CNF) have been widely used for storage tank fire suppression, benefiting from their highly effective extinguishing performance. Mass loss rate is an important parameter to evaluate the stability of foam and an important factor to influence the suppression efficiency of foam. In this work, mass loss mechanisms including drainage and evaporation of two types of foams under a fixed radiation source were experimentally studied to reflect the foam fire suppression efficiency. It was observed that the foam drain rate is directly proportional to the foam mass, with a slight relation to the irradiance when irradiance is lower than 20 kW/m2. Once the irradiance reaches 30 kW/m2, the drain rate increases significantly. For evaporation, as expected from the established theoretical model, the evaporation rate shows a positive relationship with the irradiance level. However, there is no relationship between foam evaporation rate and the initial foam thickness. Through the experimentally measured evaporation rate, the average foam absorptivity is calculated to be approximately 0.63 ± 0.07 for both foams at an expansion ratio (ER) of 10. A comparative analysis of mass loss characteristics in the two types of foam was also undertaken. It was found that the two types of foam have distinct drain rates due to their different foam stability caused by the different expanded gas properties. However, they have a similar evaporation rate. In addition, the foam destruction rate and time to ignite the underlying fuel were measured, and a linear relationship between ignition time and initial foam height was found with a slope of 1.7 ± 0.2. The results demonstrate that CNF has a better fire suppression efficiency, which can be attributed to the better stability of the foam and the nitrogen released by broken bubbles, which inhibits ignition. The work presented here reveals the different functional mechanisms of compressed air/nitrogen foam and presents useful information that could enhance our understanding of the fire suppression effects of fire extinguishing foam. [ABSTRACT FROM AUTHOR]

Compressed air foam (CAF) is widely used as a high-performance extinguishing agent for liquid fuel fire. In this work, the effects of initial fuel surface temperatures (50–80 °C) on the drainage and evaporation of CAF were studied experimentally. It was found that at the same foam mass, the drain rate is positively correlated to the initial fuel surface temperature. Furthermore, the drainage and evaporation with ER = 5.5 is less affected by the fuel surface temperature than ER = 10. Then the foam height was recorded and it was observed when the temperature is above 60 °C, the foam height with ER = 10 reduces rapidly at the beginning, while only slight thermal expansion occurs in ER = 5.5. In addition, the cooling effect of the foam was studied. The fuel layer temperature variation could be divided into three stages: rapid decline, relatively stable and gradual rise, which may relate to the foam drainage. But in the rapid decline state, the drop in fuel temperature is mainly related to the heat transfer between the fuel and foam. In addition, for the foam with ER = 10, the rising stage starts earlier. This work improves the understanding of mass loss and suppression mechanisms of fire-extinguishing foams.