Modeling Suppression of a Liquid Pool Flame by Aqueous Foams

Cedrick Ngalande1, James W Fleming, and Ramagopal AnanthNaval Research Laboratory

Washington, DC 20375

1NRL/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 interfaceHeat from flame to pool = vaporization energy

Hmy

T

um

Ty

H fg

is mass flux is thermal conductivity of heptane vapor

is the latent heat of vaporization is temperature gradient

Where,

u is burning rate

Two Approaches to Predict Burn Rates

Approach 1: assume a steady state burning rate distribution across the pool1. 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 modeling

Approach2: predicts burning rates of at every computational cell on pool surface4. Compares well with experimental data5. 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 100 0C 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