Convective Flows Under Conditions Applicable to Fires in High Rise Buildings

Abstract

This study was an attempt to analyze convective flow patterns under conditions applicable to fires in high rise buildings when natural convection alone is the most important driving force. The primary aim of this investigation was to consider the turbulent flow in vertical shafts caused by hot gases entering the bottom of the shaft which is already filled with a cooler (denser) gas, and hence creating an unstable density field. The small scale model used to study this problem consisted of a vertical tube of dense fluid placed in an infinite (less dense) fluid environment. General scaling laws were developed for the variation of density with time for the flow set up in the model. Also, an analytical model was developed to account for the observed mixing rates in this simple configuration. In the analysis, the diffusion equation was solved and found to be in agreement with the small scale model. The mixing coefficient for this unstable system is See abstract for formula where the constant, 0.28, was determined empirically, and the fundamental time scale that characterizes the mixing in the vertical column is See abstract for formula Later, the above techniques were used to investigate in a more superficial way the effects on the mixing rates of geometric variations, limited external environments, changes in the internal geometry of the shafts, gas density ration, and heat transfer.