Phenomenological Model and Experimental Comparisons on Static Foam Drainage for Fire Fighting Foams

This paper is concerned with the development of a phenomenological model for drainage from static foams used in standard fire-foam qualification tests for low expansion ratio commercially available foams. The fact that operational foam heights (30 mm) are much smaller than foam drainage apparatus heights (200 mm) has been the inspiration to determine the height dependence of static drainage. This is done by constructing a model of foam drainage based on momentum flux balance and conducting experiments with an apparatus with foam drainage through a fuel layer. The results show a linear relationship of quarter drainage time with the height consistent with the theoretical expectations. The constants are related to viscosity and liquid film thickness. Microscopic examination on bubble movement and the pictures are used to infer that the bubble size distributions between three commercial foams are not distinctively different and so are the film thicknesses. It is argued that the strong dependence on quarter drainage time on the film thickness can be consistent with the experimental results only if the variation of these thicknesses between different foams is not significantly large. Assuming a constant film thickness, the constants of the relationship between quarter drainage time and height are obtained from the experimental data. The constants derived from the experimental data show dependences in which lower concentration foams have a behavior different from those with higher concentrations beyond known influences of viscosity and surface tension. The need for longer duration drainage as a qualifying measure is argued to be important to correlate with fire extinction behavior.