Interaction between a fire and a spray

• Main Author: Schwille, John A
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2004
• Subjects: Mechanical engineering
• ISBN: 9780496798414; 0496798413

The interaction between a fire and spray is a key factor in the success of fire suppression by fire sprinklers. Previous work in this area has been mostly limited to actual delivered density (ADD) experiments, which provide practical measures of sprinkler performance but little insight into the interaction between the fire and the spray, and computational simulations, which have proven to be somewhat inaccurate but provide important details of the flow field. This thesis is a detailed study of the interaction between a spray and a flame. A simple model was developed that includes the momentum of a uniform downward droplet spray in conservation equations describing the width, velocity and buoyancy in a fire plume. The analysis shows that the droplets slow the plume velocity and widen the plume. For any combination of droplet velocity, droplet size, droplet concentration, and fire size, the model predicts a boundary between the spray and the flame. Along with the theoretical work, experiments were performed with fire sizes ranging from 5 to 50 kW. Spray sources, either spray nozzles or a fire sprinkler, were located directly above the fire. Measurements of the thermal plume with IR Thermography show that the thermal plume decreases in height and widens with increasing spray strength. Droplet velocities measured with Particle Image Velocimetry show that the downward velocities of the droplets in the spray slow with increasing fire size. Droplet tracking indicates droplets are pushed aside and prevented from entering the plume. From droplet velocity and sizes measured with Phase Doppler Interferometry, the height above the fire at which droplets move slower than their terminal velocities in still air is identified as a boundary between the downward-moving spray-entrained air and the upward-moving buoyant plume. The data from all the experiments is scaled using the nondimensional ratio of the spray drag to the plume momentum. Collapse of the data for the height of the boundary between the plume and the spray when plotted as a function of this parameter suggests that this parameter generalizes the interaction between the spray and the flame.