Soot Surface Growth in Laminar Hydrocarbon/Air Diffusion Flames

Abstract

The structure and soot surface growth properties of round laminar jet diffusion flames were studied experimentally. Measurements were made along the axes of ethylene-, propylene- traversedthe flames when moving to-and-fromthe flame axeswasestablished in two ways: (1) sampling in the region prior to the onsetof soot formation along the axesof the flames (but where there was a well-developedannularsoot-containingregion nearthe edge of the region exhibiting yellow soot luminosity) properly indicatedthat no soot aggregates, that could have beendepositedas the samplerpassedthrough the annularsoot-containingregion, werepresent on the samplerfor the samplingconditionsusedduring the presentstudy, and (2) the sampled sootexhibitedonly nearly constant primary sootparticle diameters at eachflame conditionalong the axis (e.g., the standarddeviationsof primary particle diameterswere lessthan 10%)rather than the widely-varying primary soot particle diametersthat would be observedif off-axis soot particles were present in the sample. The experimental uncertainties(95% confidence) of primary particlediametersat a pointwereestimatedto be lessthan10%.Streamwisegas velocities were measuredusing laser velocimetry. The experimental uncertainties(95%confidence)of thesemeasurements werelessthan5%. Fig. 1 is that after an initial rapid increase of primary soot particle diameters, just after the onset of soot formation along the axis of the flames, mean primary soot particle diameters subsequently decrease with increasing distance from the burner exit in spite of effects of soot surface growth throughout the remainder of the soot formation region. This behavior has been observed for all the laminar jet diffusion 10 68 flames studiedin this laboratory, _3'7andwill be discussedmore fully when flame structure is discussed.The HRTEM imagesin Fig. 1 (Flame4) illustratedin Fig. 3 (atz=82 mm),however,the flame sheetlies beyondthe regionillustrated for the acetylene-nitrogen/airflame (Flame 2) illustrated in Fig. 2 (at z=106 ram) and for the acetylene-benzene-nitrogen/air flame (Flame9) illustrated in Fig. 4 (at z=110 ram) Concentrationsof 02 either progressively increasewith increasing distancefrom the burner exit (Figs. 2 and 3) or progressively increase as the flame sheet is approached after reaching a broad minimum near the burner exit (Fig. 4) Soot Surface Growth RatesPast and present measurements were used to find soot surface growth rates for laminar premixed and diffusion flames. Major assumptions were identical to earlier work, 1"7 as follows:soot mass production is dominated by soot surface growth rather than primary soot particle nucleation, effects of diffusion (Brownian motion) and thermophoresis on soot motion are small so that soot particles convect along the axes of the flames at the local gas velocity, the density of soot is constant (taken to be 1850 kg/m 3 from Ref. 1), and the surface area available for soot growth is equivalent to constant diameter spherical primary soot particles that meet at a point.The justif...

Interactions among soot, thermal radiation, and NOx emissions in oxygen-enriched turbulent nonpremixed flames: a computational fluid dynamics modeling study

“…Cases 1 and 2 reveal the sensitivity of soot yields to the soot surface growth mechanism. Kazakov and co-workers' model [34], recommended by El-Leathy and colleagues' empirical study [35], produces a soot yield almost one order of magnitude higher than that of Appel and co-workers' [26] model. Cases 2 and 3 illustrate the sensitivity of soot yields to the steric factor.…”

“…The correlation in [26] gives steric factor values that are less than 0.4 for any temperature. The correlation was adjusted in Cases 3 and 4 so that the values of α are in the range 0.9-1.0, as suggested by El-Leathy et al [35]. In all four cases, the P 1 -gray Fig.…”

A soot particle surface reactivity model applied to a wide range of laminar ethylene/air flames

“…Experimental studies have indicated that soot surface reactivity is a function of temperature [3,5,[6][7][8][19][20][21][22][23][24][25][26]. Thus, as the first attempt to define a function for a, a comparison was made between the reference a for each flame to the corresponding peak flame temperature, and to the instantaneous temperature at the location peak soot concentration on the wings.…”

“…While initially a constant fraction of active sites was used with the kinetic soot model [1,3], it was later expressed as a function of flame temperature [20], and subsequently as a function of flame temperature and mean particle size [10]. Several studies measured the ageing parameter, a, based on the HACA surface growth scheme in laminar premixed and diffusion flames with different fuels, pressures, and flame temperatures, and proposed a temperature dependent function for the ageing parameter [23][24][25][26][27][28][29].…”

Soot Measurements in Steady and Pulsed Ethylene/Air Diffusion Flames Using Laser-Induced Incandescence

“…The difficulty of measuring soot properties in turbulent flames due to their fluctuating flow fields have motivated studies of laminar flames, which are experimentally and computationally more tractable. Since 1983, there have been papers published on two popular configurations for experimental studies of soot properties, which are the coflow and counterflow laminar flames (Kent et al 1981;Santoro et al 1983;Santoro et al 1987;Glassman 1989;Dobbins et al 1990Dobbins et al , 1998Sunderland and Faeth 1996;Kennedy et al 1996;Kang et al 1997;Zhang and Megaridis 1998;Mohammed et al 1998;Hepp and Siegmann 1998;Smooke et al 1999;Xu and Faeth 2002;El-Leathy et al 2003;D'Anna et al 2005;McCrain and Roberts 2005). The center of attention in most of these studies has been the measurement of the soot volume fraction, which is needed for estimating particle formation/growth rates.…”