Experimental and Theoretical Study of Rack Storage Fires

Abstract: A theoretical and experimental study of rack storage fires and responsiveness of sprinklers is presented. Free-burn tests with non-combustible and combustible material were carried out in reduced scale with verification in large scale. Formulas for in-rack flame height, excess gas temperature, gas velocity and heat flux to storage walls are provided. The formulas include overall heat release rate, vertical flue width, height above the floor, height of virtual origin and sootiness of fuel. They can be used to predict activation times of in-rack sprinklers and it is possible to directly incorporate them into engineering models designed to predict fire growth in storage geometries.The storage arrangement is important for the initial flame spread and fire growth rate. The reduced scale study shows that the initial fire growth rate decreases with increasing vertical gaps (flues) and that the vertical and lateral flame spread rate increase when the lateral flue height increases. The fire growth rate of rack storages is usually described by a power law dependence on time to the third power. The large scale test shows, however, that the initial fire growth rate is better described by an exponential function.The present work provides measurements of the heat flux distribution at the surface of four square steel towers representing an idealisation of a rack storage at reduced scale. Three gaseous fuels, carbon monoxide (CO), propane (C3H8), and propylene (C3H6) were supplied from a circular gas burner at the floor. The fuels were chosen to cover a wide range of sooting tendencies leading to distinctly different flame heat fluxes. The differences are surprisingly large. For the same overall fire heat release rate the peak heat flux from C3H8 flames is twice that from CO flames, whereas the peak heat flux from C3H6 is 2.8 times greater than from CO flames. The heat fluxes were measured by thermocouples spot-welded onto the backside of the exposed steel tower sheets. The measuring technique was found to be simple, accurate and rugged in addition to being inexpensive.