Temperature fields during the development of combustion in a rapid compression machine

Abstract

Temperature and concentration fields have been imaged by Rayleigh scattering in one-dimension on a line and by laser induced fluorescence (LIF) of acetone in a 2-D sheet across the diameter of the cylindrical combustion chamber in a rapid compression machine. Experiments were performed in non-reactive and reactive conditions. To investigate the development of combustion, the exothermic decomposition of di-t-butyl peroxide vapor diluted by inert gas was studied. This reaction is characteristic of a conventional thermal ignition. Acetone is a major product. Inert gas mixtures, to study the temperature field in the absence of reaction, were seeded with acetone. The evidence from the experimental results supports the following interpretation. As the piston of the machine moves, it shears gas off the walls of the chamber. This probably creates a roll-up vortex, but more importantly it also collects cool gas from the walls and moves this gas across the cylinder head pushing it forward into a plug at the center. Once the piston stops, there is a stratified component at the center, which is slightly colder than the bulk of the gas, and for a short time afterwards there is very limited mixing by bulk transport of gas from one part to another, because the gas velocity is not very high. Diffusive transport will occur, but the timescale is relatively slow, and the effect hardly shows before 20 to 25 ms after the end of compression. The effect (on the combustion of di-t-butyl peroxide) of this “temperature stratification” at the core of the cylinder is that the reaction develops more slowly in the center than elsewhere. The onset of reaction in a toroidal region is shown unambiguously, and thermal runaway is initiated there. This is demonstrated by LIF measurements through the central plane of the reaction cylinder. From the study of inert mixtures seeded with acetone, it is shown also that the colder core lies just ahead of the piston crown, but it does not reach the central plane until 1 ms after the piston has stopped. Rayleigh scattering on a 1-D line in the central plane proved to be insufficiently sensitive to show the presence of the cooler zone resulting solely from the physical compression. However, the evidence for temperature stratification becomes unequivocal from Rayleigh scattering measurements made in the later stages of the peroxide decomposition. Limits of sensitivity of the scattering technique may be inferred from this. The physical characteristics of the compression are likely to be replicated in other rapid compression machines and are relevant to understanding the spatial development of autoignition in such systems, which has implications also for numerical modeling. There are rather more complicated consequences, than is the case for thermal ignition, for chain-thermal interactions which involve development through the negative temperature-dependent regimes, as occurs in combustion of the alkanes and that of other organic compounds.