Research On The Rheological Properties And The Hydrodynamic Calculating Method Of Fire-fighting Foams

Foams have the advantages of low density,good heat shield protecting and high fire extinguishing efficiency,which make them more attention in the field of firefighting,foams are mainly applied in putting out liquid combustible and solid combustibles fire.The rheological properties of foams are the important basis of hydrodynamic calculation method,the research on hydrodynamic calculation method provides technical support for engineering application.The evolution process and drainage properties of aqueous film-forming foam(AFFF)over time has been researched.The drainage rate is divided into three stages: increasing stage(0-200 s),dropping stage(200-600 s)and stabilizing stage(600 s-).The bubble shape remains spherical or ellipsoidal,and equivalent diameter increases at increasing stage;the polyhedron foams begin to appear at dropping stage;the sphere(or ellipsoid)foams almost become polyhedron at stabilizing stage.It illustrates that the drainage is mainly affected by bubble structure and shape.An apparent drainage rate predicted model was proposed by using weighting method,on account of foams formed by two phase of gas and liquid.The rheological properties of AFFF with the change of time has been researched.The results showed that AFFF is a kind of viscoelastic fluid,at a given constant shear rate,apparent viscosity increased time before 600 s,and then remained almost unchanged,it illustrates that the apparent viscosity of foams are mainly affected by bubble shape,topology structure,physical properties of foam liquid and gas fraction.Foams showed shear thinning and could be well fitted by Herschel-Bulkley model,the relative change rate of apparent viscosity quickly decreased with the increase of shear rate,thus the research of apparent viscosity is divided into two sections: one section,foams showed non-Newtonian fluid at low shear rate;the other section,the foams approximated to Newtonian fluid at high shear rate.At low shear rate,the HerschelBulkley modified model,relating to get time,shear rate and foam type,was obtained by considering the drainage influence.The transportation of compressed air foam(CAF)in pipe were simulated by Fluent 15.0 software.Simulation results show that the pressure drop of horizontal straight pipe can be well computed by the Spalar-Allmaras model,and the relative error is no more than 10%,more simulations were done to observe the influence of velocity and diameter on pressure drop,it found that the pressure drop raise with the increase of inlet velocity,frictional resistance coefficient were obtained by Darcy's equation,a relation between frictional resistance coefficient and Reynolds number was fitted.The pressure drop of horizontal bent pipe were computed by the Spalar-Allmaras model and Standard kepsilon,respectively,and the relative error is almost no more than 40%.The simulation effect of standard k-epsilon is relatively good when bending radius is lesser,otherwise,the Spalar-Allmaras model is relatively good.The method of CAF maybe have a guiding significance on the engineering application of hydrodynamic calculation.