Soot formation characteristics of diffusion flames of methane doped with toluene and n-heptane at elevated pressures
Introduction
The sensitivity of soot processes to elevated pressures is important since most practical combustion devices operate at high pressures. The intensity of combustion (or heat release per unit volume) scales approximately with the square of the operating pressure, thus the footprint of the combustion engine becomes smaller as the operating pressure is increased for a required power output. On the other hand, the rate determining chemical reactions involved in combustion, including the various soot processes, are intrinsically nonlinear, and as a result the sensitivity of combustion events to pressure changes are not usually monotonic [1]. Therefore, it is not trivial to scale information gathered from measurements at atmospheric flames to high-pressure combustion.
For liquid fuels, we still rely on smoke point of the fuel or sooting index for practical applications due to a lack of full understanding of the effects of the various operating conditions on the soot formation process [2], [3]. Earlier efforts to link the smoke point and sooting tendency of liquid fuels to chemical structure of the fuel were successful [4], [5], and they provided scaling information for further studies on sooting propensities of hydrocarbons, see e.g., [6].
Information on soot formation processes in laminar diffusion flames at higher pressures is limited to ethylene [7], [8], [9], methane [7], [10], [11], ethane [12], [13] and propane flames [14]. Data available on the sooting behavior of liquid fuels in tractable laminar diffusion flames at pressures above atmospheric are very limited; most data are at atmospheric pressure. A few studies with liquid fuels at pressures above atmospheric has been reported recently [3], [15], [16], [17], [18]. The effects of small amounts of m-xylene (up to 5% of fuel carbon coming from m-xylene as a perturbation to a base flame) on aromatic species and soot were studied in a nitrogen-diluted ethylene flame between 1 and 5 atm, Mensch et al. [3]. Their results indicate that the observed increase in soot and aromatic species are about first order with respect to amount of m-xylene added to the flame [3]. Karatas et al. [15] reported detailed measurements of soot volume fraction and temperature field in n-heptane diffusion flames diluted by nitrogen at pressures up to 7 atm. Comparing n-heptane results to ethylene flames, similarly diluted with nitrogen, revealed that n-heptane flame’s soot yield is higher than that of ethylene at pressures above atmospheric [15]. Mouis et al. [16] investigated changes in soot volume fraction resulting from the addition of a JP-8 surrogate and each of its components to a nitrogen-diluted, ethylene co-flow diffusion flame between 1 and 5 atm. Pre-vaporized liquid fuel was added at two different levels: 2.5% and 5% of the total carbon flow rate. The linear behavior between the amount of carbon from the liquid fuel and the soot volume fraction suggests that the liquid fuel is not changing the base ethylene flame substantially [16]. Zhou et al. [17] studied the sooting behavior of n-heptane between 1 and 3 atm and inferred a pressure dependence of soot volume fraction similar to the one reported in [15]. However, the flame height, measured by luminescence and LII is reduced by 10% and 13%, respectively, from 1 atm to 3 atm. This indicates that the mass flow rate of n-heptane was not kept constant; so that the observed changes in sooting characteristics cannot be attributed to pressure change alone.
In this work we selected two liquid hydrocarbons, one paraffinic and one aromatic. Methane was used as the base fuel. Each liquid fuel was added to the base methane such that 7.5% of the carbon is provided by either n-heptane or toluene. Rationale for selecting methane as the base fuel is similar to the work reported by McEnally and Pfefferle [19] who doped methane flame with toluene such that it would account for 1.5% of the total fuel carbon flux in atmospheric diffusion flames. The main objective of the work reported here was to investigate the sooting behavior of co-flow methane laminar diffusion flames doped with n-heptane or toluene at pressures above atmospheric. Soot and temperature measurements in these flames at pressures from atmospheric to 8 atm are presented and discussed.
Section snippets
Experimental methodology
The laminar diffusion flame burner and the high-pressure combustion chamber used in this work have been described previously in detail [1], [9], [11], [12], [13], [15]. A brief description, summarizing the essential features of the experimental set up, will be given here. The combustion chamber was designed to sustain pressures up to 110 atm and its internal diameter and height are 24 and 60 cm, respectively, Fig. 1. Optical access into the chamber is provided by three ports installed at 0°,
Results and discussion
As documented in the literature, the laminar co-flow diffusion flame shape changes with pressure and the characteristic flame cross-sectional area at a given location on the flame centerline scales with the inverse of pressure [1], [12]. Still pictures of flames at various pressures, depicted in Figs. 2–4, display shapes similar to those observed previously at elevated pressures. In pure methane flames at atmospheric pressure, measurable soot concentrations are very low, however presence of
Conclusions
Effects of pressure on sooting propensities of two liquid hydrocarbons with the same number of carbon atoms but different chemical structures, namely n-heptane and toluene, were studied in laminar diffusion flames at high pressures. Liquid hydrocarbons were added to the base fuel methane in amounts to have 7.5% of total carbon to be contributed from either toluene or n-heptane. Pressure range was from atmospheric to 8 atm for methane and methane+n-heptane flames, whereas for methane+toluene
Acknowledgments
The financial support was provided by the Natural Sciences and Engineering Research Council of Canada through discovery (251116-2012) and strategic project (STPGP 430362-12) grants, and by the BioFuelNet Canada (7B_Gulder).
References (33)
- et al.
Soot formation in high pressure laminar diffusion flames
Prog. Energy Combust. Sci.
(2012) - et al.
The effects of molecular structure on soot formation II. Diffusion flames
Combust. Flame
(1985) - et al.
Sooting characteristics of surrogates for jet fuels
Combust. Flame
(2010) Influence of hydrocarbon fuel structural constitution and flame temperature on soot formation in laminar diffsuion flames
Combust. Flame
(1989)- et al.
Measurements of the soot volume field in laminar diffusion flames at elevated pressures
Combust. Flame
(2005) - et al.
Experimental study of soot and temperature field structure of laminar co-flow ethylene–air diffusion flames with nitrogen dilution at elevated pressures
Combust. Flame
(2011) - et al.
Dependence of sooting characteristics and temperature field of co-flow laminar pure and nitrogen-diluted ethylene-air diffusion flames on pressure
Combust. Flame
(2015) - et al.
Soot concentration and temperature measurements in co-annular, nonpremixed CH4/air laminar diffusion flames at pressures up to 4 MPa
Combust. Flame
(2005) - et al.
Soot formation and temperature field structure in co-flow laminar diffusion flames at pressures from 10 to 60 atmospheres
Proc. Combust. Inst.
(2009) - et al.
Soot formation in laminar ethane diffusion flames at pressures from 0.2 to 3.3 MPa
Proc. Combust. Inst.
(2011)
Unified behaviour of maximum soot yields of methane, ethane, and propane laminar diffusion flames at high pressures
Combust. Flame
Soot formation and temperature field structure in laminar propane-air diffusion flames at elevated pressures
Combust. Flame
Sooting behaviour of n-heptane laminar diffusion flames at high pressures
Combust. Flame
Effects of a jp-8 surrogate and its components on soot in laminar, N2-diluted ethylene co-flow diffusion flames from 1 to 5 atm
Combust. Flame
Measurements of sooting tendency in laminar diffusion flames of n-heptane at elevated pressure
Combust. Flame
Investigation of the effect of molecular structure on sooting tendency in laminar diffusion flames at elevated pressure
Combust. Flame
Cited by (40)
Comparative study of soot properties and pressure sensitivity in n-dodecane and decalin laminar flames
2023, Journal of Aerosol ScienceCombustion chemistry of aromatic hydrocarbons
2023, Progress in Energy and Combustion ScienceEffects of pressure on laminar flame characteristics of C1-C3 alkanes: A review
2023, Fuel Processing TechnologySoot and PAH formation in high pressure spray pyrolysis of gasoline and diesel fuels
2022, Combustion and Flame