Simultaneous observation of organized density structures and the visible field in pool fires

Abstract

Organized density structures and the visible field of pool fires were observed simultaneously with a real-time holographic interferometric method. The pool diameters d varied between 1≦ d ≦10 cm and the fuels studied were methane, LNG, n -butane, n -pentane, n -hexane, n -heptane, diesel/gasoline, liquid paraffin; methanol, ethanol, propanol, cyclohexanol, glycerin; acetaldehyde, acetone, diethyl ether; acetic acid, methyl acetate; benzene. The organized structures depend on the height x above the pool rim, the pool diameter, the fuel supply rate, the equivalence ratio and the fuel type. These structures show mono-, quasi- and nonperiodic behaviour. In the height region Δ x 1 =0 to 8 cm above the fuel surface of an n -hexane pool fire, d =4.6cm, six monoperiodic subprocesses exist with one independent frequency of 3 Hz and two harmonics of 9 and 12 Hz . The independent frequencies of the first quasiperiodic process are 12, 39 and 41 Hz in the height region Δ x 2 =8 to 16 cm. The second quasiperiodic process with the two independent frequencies of 107 and 78 Hz occurs in the height region Δ x 3 =16 to 24 cm. A third quasiperiodic process with at least the two independent frequencies of 177 Hz and 192 Hz exists in the height region Δ x 4 =24 to 32 cm. For the mean frequencies f of the monoperiodic subprocesses the correlation f ¯ ( d ) = b ¯ D B ( d ) d − 0.5 ≈ 1.83 d − 0.63 , (2.0≦ d ≦2500 cm) is derived, based on the buoyant acceleration b DB of density parcels. A density structure forming concentric ring-shaped interference fringes is defined as a density parcel. With a simplified momentum equation the axial convection velocity of fire parcels, which is up to three times faster than that of the density parcels, was calculated. Instantaneous mass densities and temperatures were calculated by the Abel-inversion, including the concentration profiles of 15 stable fire gas species. From the frequency distributions of the geometric dimensions of density parcels macro-and microscales, which agree with the scales obtained from power spectra, were determined.