Experimental, analytical, and computational study of natural convection in asymmetrically-heated vertical shafts

Abstract

During building fires, buoyant smoke is rapidly transported through elevator and ventilation shafts, posing a threat to the safety of occupants on higher floors. Therefore, smoke dynamics in vertical shafts are of interest to fire and building safety engineers. Herein, analytical solutions describing pure natural-convection laminar flows in asymmetrically-heated vertical shafts are introduced and discussed as a model situation. While the analytical solution is limited to laminar flow at relatively low Rayleigh numbers, a practically-relevant turbulent flow is explored experimentally, with a lab-scale 1-m-high shaft, and numerically, with Fire Dynamics Simulator (FDS) software in the same scale. With supplied heating powers of 100-400 W at the left-hand side wall or/and bottom, a circulatory 2D or/and 3D heat-induced convection flow (natural convection) sets in within the shaft. Air near the heated left wall rises due to buoyancy, while air near the cooler right wall descends, yielding a sine-wave-shaped velocity profile with no-slip boundary conditions imposed at both walls. While this sine-wave velocity profile is clearly observed in laminar flow, the buoyancy-driven layers are relatively thin in the corresponding turbulent flow. The temperature profile practically linearly decreases from the left to the right wall for both laminar and turbulent flows. The overall physical insights provided by the laminar analytical solution are consistent with observations in the turbulent flow. The differences between laminar and turbulent flows are investigated and discussed by qualitatively comparing the analytical solutions to the experimental and numerical data.