Experimental and Modeling Study of Wall Film Effect on Combustion Characteristics of Premixed Flame in a Constant Volume Combustion Bomb

The primary objective of the present study was to investigate the impact of wall film on the combustion characteristics of premixed flames in internal combustion engines through the joint experimental and numerical techniques. The interaction between the premixed methane-air flame and n-dodecane film attached to the wall of a constant volume combustion bomb was experimentally examined. The flame propagation processes, as well as pressure evolution were quantitatively characterized. Then, computational fluid dynamic (CFD) simulation was performed incorporating the combustion chemistry model. To enable efficient simulation of the chemically reacting flow in engine chambers, a simplified modeling approach based on a two-step reaction scheme was developed. A compact reaction model for the selected model fuel n-dodecane was constructed and reduced to include 35 chemical species and 180 reactions. The flame propagation process of the premixed flame and its interaction with dry and wet walls was studied. The results showed that the propagation of the premixed flame could be divided into four stages, and the existence of the slit structure increased the instability of the flame structure in the near-wall region. The wall film tended to promote emissions, producing more unburned hydrocarbons, soot precursors and aldehydes.

Reduced Combustion Mechanism for Fire with Light Alcohols

The need for sustainable energy has incentivized the use of alternative fuels such as light alcohols. In this work, reduced chemistry mechanisms for the prediction of fires (pool fire, tank fire, and flash fire) for two primary alcohols—methanol and ethanol—were developed, aiming to integrate the detailed kinetic model into the computational fluid dynamics (CFD) model. The model accommodates either the pure reactants and products or other intermediates, including soot precursors (C2H2, C2H4, and C3H3), which were identified via sensitivity and reaction path analyses. The developed reduced mechanism was adopted to predict the burning behavior in a 3D domain and for the estimation of the product distribution. The agreement between the experimental data from the literature and estimations resulting from the analysis performed in this work demonstrates the successful application of this method for the integration of kinetic mechanisms and CFD models, opening to an accurate evaluation of safety scenarios and allowing for the proper design of storage and transportation systems involving light alcohols.

Ultrafine particulate matter in methane-air premixed flames with oxygen enrichment

A complementary computational and experimental study is carried out on the formation of ultrafine particulate matter in premixed laminar methane air flames. Specifically, soot formation is examined in premixed stretch-stabilized flames to observe soot inception and growth at relatively high flame temperatures common to oxygen enriched applications. Particle size distribution functions (PSDF) measured by mobility sizing show clear trends as the equivalence ratio increases from Φ = 2.2 to Φ = 2.4. For a given equivalence ratio, the measured distribution decreases in median mobility particle size as the maximum flame temperature increases from approximately 1950 K to 2050 K. The median mobility particle size is 20 nm or less for all flame conditions studied. The volume fraction decreases with increasing flame temperature for all equivalence ratio conditions. The Φ = 2.2 condition is close to the soot inception limit and both number density and volume fraction decrease monotonically with increasing flame temperature. The higher equivalence ratio conditions show a peak in number density at 2000 K which may indicate competing soot inception processes are optimized at this temperature. Flame structure computations are carried out using detailed gas-phase combustion chemistry of the Appel, Bockhorn, Frenklach (ABF) model to examine the connection of the observed PSDF to soot precursor chemistry. Agreement between measured and computed flame standoff distances indicates that the ABF model could provide a reasonable prediction of the flame temperature and soot precursor formation for the flames currently studied. To the first order, the trends observed in the measured PSDF could be understood in terms of computed trends for the formation of benzene, naphthalene and other soot precursors. Results of the current study inform particulate matter behavior for methane and natural gas combustion applications at elevated temperature and oxygen enriched conditions.

Ultrafine Particulate Matter in Methane-Air Premixed Flames With Oxygen Enrichment

A complementary computational and experimental study is carried out on the formation of ultrafine particulate matter in premixed laminar methane air flames. Specifically, soot formation is examined in premixed stretch-stabilized flames to observe soot inception and growth at relatively high flame temperatures common to oxygen enriched applications. Particle size distribution functions (PSDF) measured by mobility sizing show clear trends as the equivalence ratio increases from Φ = 2.2 to Φ = 2.4. For a given equivalence ratio, the measured distribution decreases in median mobility particle size as the maximum flame temperature increases from approximately 1,950–2,050 K. The median mobility particle size is 20 nm or less for all flame conditions studied. The volume fraction decreases with increasing flame temperature for all equivalence ratio conditions. The Φ = 2.2 condition is close to the soot inception limit and both number density and volume fraction decrease monotonically with increasing flame temperature. The higher equivalence ratio conditions show a peak in number density at 2,000 K which may indicate competing soot inception processes are optimized at this temperature. Flame structure computations are carried out using detailed gas-phase combustion chemistry of the Appel, Bockhorn, Frenklach (ABF) model to examine the connection of the observed PSDF to soot precursor chemistry. Agreement between measured and computed flame standoff distances indicates that the ABF model could provide a reasonable prediction of the flame temperature and soot precursor formation for the flames currently studied. To the first order, the trends observed in the measured PSDF could be understood in terms of computed trends for the formation of benzene, naphthalene and other soot precursors. Results of the current study inform particulate matter behavior for methane and natural gas combustion applications at elevated temperature and oxygen enriched conditions.

A Comparison Study of Soot Precursor and Aggregate Property Between Algae-Based Aviation Biofuel and Aviation Kerosene RP-3 in Laminar Flame

Algae-based aviation biofuel shows the potential to reduce soot emission in flight. A comparison study of soot precursor and aggregate property between algae-based biofuel and aviation kerosene RP-3 in laminar flame was conducted to investigate the reason of biofuel’s less soot formation. The soot precursors were determined by the fringe lengths of soot particles. At a typical dimensionless height DH = 0.50 of both flames, the geometric mean fringe lengths of biofuel and RP-3 are measured to be 0.67 and 0.73 nm, respectively, approximating to the size of five-ringed (A5) and seven-ringed (A7) poly-aromatic hydrocarbon, respectively. An A5 growth mechanism was then added to biofuel surrogate mechanisms for soot formation simulation. Since the carbon number component of biofuel is wide and difficult for comprehensive mechanism development, two surrogate mechanisms were developed. One is based on the C8–C16 n-alkane that covers biofuel’s main components, and the other one is based on biofuel’s average carbon number to simplify the mechanism. Meanwhile, an A7 growth mechanism was added to a popular RP-3 mechanism. The soot formation simulation with the combination mechanisms for both fuels provides a better agreement with the measured primary particle diameter and suggests that the reason for less soot production by biofuel is its less soot precursor production that weakens soot nucleation and growth. Lastly, the soot fractal dimension of biofuel is smaller than that of RP-3, indicating that biofuel has a looser soot aggregate.