Development and Evaluation of Two New Droplet Evaporation Schemes for Fire Dynamics Simulations

The evaporation of sprinkler droplets is an important phenomena in fire simulations both for heat removal from the gas and for heat removal from surfaces. The Fire Dynamics Simulator (FDS) has seen a number of different evaporation models over its history where each subsequent model has attempted to address limitations and issues with the previous model. In this paper, we address the problems of potential numerical instability and super-saturation that may occur in explicit time integration of the droplet equations. Two novel numerical approaches are developed and evaluated. The first is based on an analytical solution that relaxes the cell composition and temperature toward the equilibrium values. The second method is an implicit solution to the droplet equations. The two approaches are verified and validated using both single droplet and practical sprinkler calculations. Ultimately, the implicit approach is demed the most cost effective for practical fire simulations.