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Overfire soot volume fractions and mixture fractions, flame heights, and characteristic flame residence times, were measured for turbulent acetylene, propylene, ethylene and propane diffusion flames burning in still air. Test conditions ranged from highly buoyant pool-like flames to buoyant jet flames, using three burners (with exit diameters of 5, 50, and 234 mm) and a wide range of fuel flow rates. Soot generation efficiencies (the percentage of fuel carbon converted to soot and emitted from the flame) were uniform throughout the overfire region for a given flame condition. Soot generation efficiencies increased with increasing flame residence times but tended to approach asymptotic values for residence times roughly ten times longer than residence times at the normal smoke point. Within the asymptotic region, soot volume fractions are directly related to mixing levels, analogous to the laminar flamelet concept for nonpremixed flames, which offers substantial simplifications for analysis of the continuum radiation properties of the overfire region.

Generalized state-relationship correlations giving the mass fractions of major gas species (N2, O2, fuel, CO2, H2O, CO, and H2) and temperature as functions of local fuel-equivalence ratios were studied for hydrocarbon---air diffusion flames. The data base included existing measurements in laminar methane, propane, n-heptane, acetylene, and ethylene flames burning in air (or N2/O2 mixtures) with burner configurations involving porous cylinders in crossflow, coflowing round jet flames, and flat-laminar diffusion flames. Reasonably good generalized state-relationship correlations were found for major gas species over the available data base, which included molar fuel H/C ratios in the range 1-4 and fuel-equivalence ratios in the range 10-2-102. Typical of state relationships for particular fuels, the generalized state relationships approximated thermodynamic equilibrium for fuel-lean conditions and departed from equilibrium in a relatively universal manner for near-stoichiometric and fuel-rich conditions. Temperature state-relationship correlations were also reasonably good, over the more limited available data base, in view of uncertainties concerning radiative heat losses from the test flames and thermocouples. The results should be useful for estimating the scalar properties and the infrared gas-band radiation properties of laminar and turbulent hydrocarbon/air diffusion flames--the latter in conjunction with the laminar flamelet concept.

Instantaneous soot volume fractions and temperatures were measured in the fuel-rich (underfire) region of turbulent nonpremixed acetylene, propylene, ethylene, and propane flames burning in still air. Large-scale, highly buoyant, pool-like flames were considered, having characteristic residence times greater than 250 ms and burner exit Richardson numbers greater than 18. Measurements were made using an optical probe than involved laser extinction for soot volume fractions and two-wavelength pyrometry for temperatures. Strong correlations were found between soot volume fractions and temperatures for each fuel-relatively independent burner operating conditions and position in the underfire region. This behavior is supportive of the existence of nearly universal relationships between soot volume and mixture fractions in the underfire region of turbulent nonpremixed flames having large characteristic residence times. Underfire soot is largely confined to a narrow range of mixture fractions (yielding a soot spike) and temperatures. The latter observation supports approximations of constant-temperature soot layers that have been proposed in teh past for estimates of continuum radiation from soot-containing diffusion flames.