Measurements of the Combustion Characteristics of Early Premixed Flames Inside Closed-Vessel Combustion

Abstract

The behavior of the early phase of combustion in spark-ignition (SI) engines is very crucial, since cycle-by-cycle dispersion in combustion, and hence pressure development, originates during this phase. A fundamental experimental study has been undertaken to understand and establish the behavior of early quasisteady premixed laminar flames, or generally called, early flames that exist in practical SI engines. A new, specially designed, closed-volume, spherical vessel test cell with central ignition and optical access was designed, built, and used to undertake experimental measurements for laminar premixed flame growth. A novel multiple-zones mathematical model was implemented to develop a new method for calculating the key properties of early flame behavior inside a closed vessel. Laminar premixed combustion experiments were undertaken to measure, quantify, and decouple the effects of initial fuel mixture equivalence ratio (φ = 0.8 to φ = 1.4), pressure (0.5 bar, 1 bar, 2 bars, and 3.5 bars), and temperature (298 K and 425 K) on the key characteristics of early laminar flame kernel such as timing (t) and rate of change of first 5% mass fraction burned (MFB – X 5%), flame radius (R f ), flame growth rate (dR f /dt), laminar burning velocity (S u ), flame stretch (α), flame cellularity, and unburned gas temperature and pressure. The study showed that the timings of X 5% and the rate of mass fraction burned (dX/dt) had been significantly affected by the variation in equivalence ratio, initial temperature, and pressure. Flame stretch rates during early flame growth were very high but were found to decline nonlinearly to lower, steadier values after R f > 2.5 cm. The onset of cellularity or autoturbulence was found to trigger at a critical radius (R fcr ) in the laminar premixed flames causing rapid increase in its S u and dR f /dt.