Static and dynamic radiance structures in pool fires

Abstract

Digital image analysis of pool fire photograms and flame radiance measurements, time-averaged and instantaneous, was applied in observing static and dynamic radiance structures in the visible and infrared spectral ranges. Small-view-angle pyroelectric radiometer measurements as well as measurements of instantaneous radiances with Si- and Ge-photodiodes and of flame temperatures, concentrations of stable species and gas flow velocities were carried out. Time-averaged fields of radiances L ¯ λ ( y , x ) are defined as static equidensitometric structures consisting of symmetric W- or M-shaped core structures and elliptical plume structures. The fuel type and the pool diameter change only the geometric part of the core and plume structures. The structures L ¯ λ ( y , x ) in the visible spectrum represent in good approximation the spatial distribution of the radiances L ¯ r a d ( y , x ) in the infrared spectrum. Volumetric emission coefficient Ψ ¯ λ ( r , x ) are determined by Abel-inversion from L ¯ λ ( y , x ) . A simplified model, based on the assumptions of the optical thin limit and inhomogeneities in temperatures and species concentrations is presented to calculate Ψ ¯ λ ( r , x ) . The dynamic radiance structures are totally unsymmetric and show monoperiodic and nonperiodic properties. Formation frequencies for fire parcels, fire mushrooms, soot parcels and hot spots as well as oscillation frequencies for visible fire shapes are determined. Spatial power spectra are calculated from instantaneous radiance profiles by Fourier analysis. The macroscales determined depend strongly but the microscales only weakly on the height x above the fuel surface. It seems that the microscales reach a lower limit of 5 mm.