APPLICATION OF FIELD MODELING TECHNIQUE TO SIMULATE INTERACTION OF SPRINKLER AND FIRE-INDUCED SMOKE LAYER

The interaction between the sprinkler water spray and the free induced convective air flow is studied using the field modelling technique. A system of equations describing conservation of momentum, enthalpy and mass is used to simulate the physical picture. Solution of the proble m is divided into two parts: gas phase and liquid phase. In the gas ph ase, a two-equation kappa - epsilon model is used to account far the t urbulent effect with the solid wall boundary described by the traditio nal wall functions. Numerical finite difference method is employed to solve the system of coupled non-linear partial differential equations. The equations are firstly discretized by the Power Law scheme and the n solved using the Pressure Implicit with Splitting of Operators (PISO ) algorithm. For the liquid phase, the sprinkler water spray is descri bed by a collection of water droplets with different values of initial velocity components and diameter calculated from the Rossin-Rammler d istribution function. The motion of each droplet is described by the N ewton's Second Law with air drag and convective heat transfer from the fire induced smoke layer. This set of ordinary differential equations is solved by the fourth order Runge-Kutta method for predicting the d roplet trajectories. To simplify the physical picture and bearing in m ind that evaporative heat loss measured experimentally is small, coupl ing of the momentum and heat transfer between the smoke layer and wate r droplets is described by the Particle-Source-In-Cell method. In this way, two-phase flow analysis is avoided by taking the sprinkler water spray as a system of `hard-spheres'. Neither combustion nor water sup pression effect on the burning object is included. However, a `microsc opic' view an the resultant sprinklered fire air-flow pattern, tempera ture and droplet properties can be visualized. Macroscopic parameters such as the drag to buoyancy ratio and the amount of convective heat t ransfer are predicted.