Abstract
Models for premixed combustion in lean burn natural gas engines usually assume that combustion takes place in thin reaction zones (flamelets). The validity of this assumption has been tested for a typical HD engine. For this the burning velocity of lean methane air mixtures has been measured with the constant-volume technique using a combustion bomb. The resulting laminar burning velocity data have been correlated with pressure, unburned temperature and equivalence ratio. Further, measurements have been conducted in a single-cylinder, optically accessible engine. Using PIV, the flow field in the combustion chamber has been determined as well as turbulence levels and –scales. Mie scattering has been used to visualize the shape of the zone with burned products. Finally a two-zones heat release model, using the measured cylinder pressure signal as input was used to calculate the instantaneous mass burning rate. The combined results of these investigations confirm the flamelet assumption. They further illustrate the complex shape of the apparent turbulent flame surface, something usually neglected in 1-D phenomenological combustion models.
KEYWORDS – Optical HD engine, Laser diagnostics, Premixed mixture, Dilution limit, Combustion regime