# Experimental Study of Coflow Propane--Air Laminar Diffusion Flames at Subatmospheric Pressures
## 1 Introduction  
Soot formation within flames is an important factor that impacts practical combustion systems as well as unwanted fires. Soot particles associated with strong radiation energy affects the performance and durability of combustion equipment and promote fire spread. Soot emissions from the combustion of fuels and fires have been widely recognized as hazards to the environment and human health [1, 2]. Fully understanding the mechanism of soot formation in flames is critical to predict and control soot emissions in combustion devices and fires.  
As an important environmental condition, the pressure effects on soot formation are of significance [3, 4]. Numerous experimental and numerical studies have been conducted on laminar diffusion flames at various pressures. Laminar diffusion flames are widely adopted in the studies of soot formation for the reasons of reliability and relative ease of use in spatial and temporal measurements. In this study, the soot volume fraction (\(f_{g}\)) was measured to evaluate the pressure effect on soot formation. Power law relationships have generally been observed between pressure and \(f_{v}\). [PERSON] and [PERSON] studied coflow ethylene-air laminar diffusion flames at pressures from 1 to 10 atm and observed that the maximum integrated soot volume fraction is proportional to \(p^{n}\), where \(n\) is \(1.2\pm 0.1\). An almost identical result was reported by [PERSON] and [PERSON] at pressures up to 16 atm. [PERSON] et al. continued studying the flames fueled by simple aliphatic fuels such as methane [5, 6], ethane [7] and propane [8] at elevated pressures and showed that the exponent in the pressure scaling law between the maximum integrated soot volume fraction and pressure changes at different pressure ranges. The exponent decreases at higher pressure ranges for all kinds of fuels. The exponent is also influenced by the fuel type, and fuels withlarger molecular weight (\(M\)) are more sensitive to pressure. [PERSON] et al. employed the Raman spectroscopy method to investigate the soot originating from the ethylene laminar diffusion flames in the pressure range from 1 to ca. 12 bar and found that soot maturity increases with pressure [9]. Based on serial measurements [10], a universal empirical equation was presented to describe the dependence between the maximum soot yield and pressure. [PERSON] et al. studied soot formation using a time-resolved LII method and found a relationship between the measured soot diameter and pressure [11]. Except for the flames of pure fuel, the flames of the fuel diluted with nitrogen or carbon dioxide were measured [12; 13], and it was found that carbon dioxide and nitrogen can suppress soot formation at atmospheric and higher pressures. Most of the studies investigating the pressure effects on soot formation were conducted at pressures higher than the atmospheric pressure; studies at subatmospheric pressures are not frequently observed [14; 15; 16]. Soot formation study at low pressures is important to understand the combustion processes at high altitudes [17; 18; 19].  
In this study, soot formation at low pressures ranging from 30 to 100 kPa was examined for coflow propane-air laminar diffusion flames. Two different air-to-fuel velocity ratios were investigated to observe their influence on the sooting tendency. Propane was selected to mimic large-molecule hydrocarbons. Two-dimensional (2D) distributions of the relative soot volume fraction were measured using laser-induced incandescence (LII) [20].