Modeling Suppression of a Liquid Pool Flame by Aqueous Foams Cedrick Ngalande 1, James W Fleming, and Ramagopal Ananth Naval Research Laboratory Washington, DC NRL/NRC Postdoctoral Associate

Introduction Aqueous foams are used for suppression of fires Suppression mechanisms by which foam suppresses fire are unknown We are developing a FLUENT based computational model to understand suppression of flames by aqueous foams

Heptane Cup Burner Flame Heptane Liquid Pool

Pool Fire Modeling Solving Navier Stokes equations using FLUENT Assumptions made in the model: – Fuel is isothermal; the entire liquid pool is at the boiling point – Effects of transport in the liquid are negligible for predicting flame suppression dynamics – Soot-less laminar flame; radiation negligible for the small size pool – Container effects negligible Energy balance across liquid-gas interface Heat from flame to pool = vaporization energy is mass flux is thermal conductivity of heptane vapor is the latent heat of vaporizationis temperature gradient Where, is burning rate

Two Approaches to Predict Burn Rates Approach 1: assume a steady state burning rate distribution across the pool 1.Burning rate is computed for only one point ; the average burning rate=local burning rate 2.Not computationally expensive and predicts experimental burning rates 3.Not good for foam extinguishment modelin g Approach2: predicts burning rates of at every computational cell on pool surface 1.Compares well with experimental data 2.Good for foam extinguishment modeling and will be used in the task

Approaches Compared With Data

Dry Foam For liquid pool fires, low expansion foams such as aqueous film forming foam (AFFF) are used. Expansion ratio=volume of foam/volume of liquid = 5 to 7 In this study, we are simulating a high expansion (HiEx) foam, which is a very dry foam, expansion ratio=1000 HiEx is typically used for Class A fire-fighting inside a ship. We conducted large scale HiEx foam tests on liquid pool fires in combination with Class A fire inside hanger bay and found to be quite effective. A HiEx model for liquid pool fires will help understand the mechanisms.

Dynamics of Liquid Pool Flame Extinction with Foam Velocity 8 cm/s Flame is suppressed by foam evaporative cooling, water vapor diluting oxygen, and smothering, which is physical blocking of air entrainment into the flame The foam evaporates along the C isotherm and does not spread on the pool because the foam is very dry A low expansion foam is expected to spread over the fuel surface unlike HiEx

Predicted Average Burn Rates During Extinction with Foam Burn rate decreases as the aqueous foam comes into contact with the flame

Burning Rates For Foam Application Rates, 10cm/s And 6cm/s

Burning Rates For Foam Application Rates, 4cm/s And 2.5cm/s

Predicted Local Burning Rate Distribution on Pool Surface During Foam Extinction Foam application was 2.5 cm/s

Conclusions We have developed a model for liquid pool flame in a cup burner and predicted burn rates Reasonable agreement of model burn rates with experimental data Developed a foam extinction model for heptane flame and was able predict effects of foam application rates on burn rates for high expansion (HiEx) foams The model includes only gas phase mechanisms of cooling, smothering, oxygen dilution effects. Future work will include foam-pool interactions that play critical role in low expansion foam (AFFF) extinction