Fuel wall film effects on premixed flame propagation, quenching and emission

Abstract

<jats:p> The formation of fuel wall film is a primary cause for efficiency loss and emissions of unburnt hydrocarbons and particulate matters in direct injection engines, especially during cold start. When a premixed flame propagates toward a wall film of liquid fuel, flame structure and propagation could be fundamentally affected by the vaporization flux and the induced thermal and concentration stratifications. It is, therefore, of both fundamental and practical significance to investigate the consequent effect of a wall film on flame quenching. In this work, the interaction of a laminar premixed flame and a fuel wall film has been studied based on one-dimensional direct numerical simulation with detailed chemistry and transport. The mass and energy balance at the wall film interface have been implemented as boundary condition to resolve vaporization. Parametric studies are further conducted with various initial temperatures of 600–800 K, pressures of 7–15 atm, fuel film and wall temperatures of 300–400 K. By comparing the cases with an isothermal dry wall, it is found that the existence of a wall film always promotes flame quenching and causes more emissions. Although quenching distance can vary significantly among conditions, the local equivalence ratio at quenching is largely constant, suggesting the dominant effects of rich mixture and rich flammability limit. By further comparing constant volume and constant pressure conditions, it is observed that pressure and boiling point variation dominate the vaporization boundary layer development and flame quenching, which further suggests that increased pressure during compression stroke in engines can significantly suppress film vaporization. Emissions of unburnt hydrocarbon, soot precursor and low-temperature products before and after flame quenching are also investigated in detail. The results lead to useful insights on the interaction of flame propagation and wall film in well-controlled simplified configurations and shed light on the development of wall film models in three-dimensional in-cylinder combustion simulation. </jats:p>

Experimental Investigations on Fuel Spray and Impingement for Gasoline Direct Injection Engines

“…Recently, Schulz et al [20] found that the time for film evaporation duration increases at elevated ambient pressure, implying that high ambient pressure has a significant effect on inhibiting evaporation of fuel film. This founding is confirmed by Kim et al [21] from their single droplet experiment and Tao et al [22] from the computational fluid dynamics (CFD)results. Furthermore, the results that film mass under P amb = 0.74 MPa is larger than that under P amb = 0.15 MPa can also demonstrate that high ambient pressure inhibits the fuel evaporation.…”

Prediction of gaseous pollutant emissions from a spark-ignition direct-injection engine with gas-exchange simulation

“…Fuel-film (5%-45%): film of liquid fuel deriving from direct injection, which does not burn completely because of the local lack of oxygen. 37 Combustion chamber deposits (;10%): they can adsorb and desorb fuel as the oil-film. Blow-by: leakage of gases through the piston ringpack into the crankcase, but the quantification is quite challenging.…”

“…Fuel-film HC-source can have also a relevance for DI engines. 37 However, its contribution is strongly dependent on 3D effect bound to fuel injection and fuel-film formation, highly variable with operating conditions. For this reason, the modeling of the fuel-film contribution requires likely a high calibration effort within 0D/1D and it is of difficult generalization and transferability to other engines.…”

Impact of gasoline direct injection fuel films on exhaust soot production in a model experiment

“…On the other hand, there are few experimental studies that analyze the fuel film interaction with a premixed flame in detail. The numerical results presented by Desoutter et al 21 and Tao et al 22 determined that flame quenching was controlled by chemistry and the vaporization boundary, respectively. Desoutter et al 21 found that the heat fluxes from the flame front to the wall were much smaller when compared to a dry wall, leading to a marginal evaporation of the fuel film.…”