Soot formation characteristics of diffusion flames of methane doped with toluene and n-heptane at elevated pressures

Abstract

Laminar co-flow diffusion flames of methane doped with n-heptane and toluene were studied experimentally to assess the sooting characteristics of two liquid fuels with increasing pressure. Experiments were conducted in the high-pressure combustion chamber that had been used previously for high-pressure soot formation studies in laminar diffusion flames. Either toluene or n-heptane was added to the methane such that 7.5% of the total carbon would be from the liquid fuel so that the results could be used to infer the pressure dependence of sooting propensities of the two liquid fuels. Pressure range was from atmospheric to 8 atm for methane and methane+n-heptane flames, whereas for methane+toluene mixture it was from atmospheric to 6 atm. A constant carbon mass flow rate of 0.41 mg/s for the three fuels was maintained at all pressures to have tractable measurements. Visible flame heights, as marked by the luminous soot radiation, were constant at all pressures except for methane at 1 atm. Variation of the maximum soot volume fractions, maximum soot yields, and the line-of-sight averaged soot temperatures of the three flames, pure methane, toluene-doped methane, and n-heptane-doped methane, with pressure were evaluated from soot spectral emission measurements which were collected line-of-sight but converted to radially-resolved values by using an Abel type inversion algorithm assuming axisymmetry of the laminar diffusion flames. Maximum soot volume fractions and maximum soot yields in n-heptane- and toluene-doped flames showed the higher sooting propensity of toluene in comparison to n-heptane at elevated pressures. Sooting propensity, in terms of both maximum soot yield and maximum soot volume fraction, of the methane+toulene flame displayed a relatively weaker dependence on pressure as compared to those of methane and methane+n-heptane mixture.

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.