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.
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