Application Of Laser Doppler Velocimetry (ldv) To Study The Structure Of Gravity Currents Under Fire Conditions

Abstract

Gravity currents are of considerable safety importance primarily because of their role in the spread and transport of smoke and hot gases in building fires. Despite recent progress in the field, relatively little is known about the structure of gravity currents under conditions pertinent to building fires. The present investigation is an attempt to address this shortcoming by studying the turbulent structure of gravity currents. For this purpose, a series of experiments was conducted in a rectangular tank with turbulent, sub-critical underflows. Laser-Doppler Velocimetry was employed to quantify the velocity field and associated turbulent flow parameters. Experimental results indicated that the mean flow within the head region primarily consisted of an undiluted large single vortex which rapidly mixed with the ambient flow in the wake region. Cases with isothermal wall boundary conditions showed three-dimensional effects whereas those with adiabatic walls exhibited two-dimensional behaviour. Turbulence was found to be highly heterogeneous and its distribution was governed by the location of large eddies. While all components of turbulence kinetic energy showed minima in the regions where velocity was maximum (i.e. low fluid shear), they reached their maximum in the shear layer at the upper boundary of the flow.