Rheological properties of solutions of fluorine-free foams

Abstract

NFPA and EN design standards for systems of firefighting foams suggest applying the Hazen-Williams formula to predict the pressure drop associated with pumping of foam solutions. The empirical Hazen-Williams formula pertains only to water at room temperature, which is a Newtonian fluid. In this contribution, we examine the rheology of six foam solutions, comprising four alcohol-resistant fluorine-free foams (AR-FfreeF), one FfreeF and one AR-AFFF (alcohol-resistant aqueous film-forming foam). We reveal that two solutions are weak gel-like viscoelastic solids, one is a viscoelastic solid displaying high elasticity, and three are viscoelastic fluids. All solutions exhibit significant shear thinning. We associate these non-Newtonian properties of the solutions mainly with the presence of polysaccharides in their chemical make-up. The pressure-drop calculations suggest that the Hazen-Williams formula should not be used for solutions of FfreeF that display semi-dilute entangled and concentrated abundance of polysaccharides. However, we also demonstrate that, the Hazen-Williams formula approximates the results of the formulation of the AR-AFFF, but only in the turbulent regime. Finally, we flag that premixes of AR-FfreeF may display aging on the time scale of weeks, suggesting the inclusion of this property in foam standards.