Temperature fields during the development of autoignition in a rapid compression machine.

Abstract

Temperature and concentration fields have been investigated in the cylindrical combustion chamber of a rapid compression machine (RCM) by schlieren photography, chemiluminescent imaging and planar laser induced fluorescence of acetone and of formaldehyde in a 2-dimensional sheet across the diameter. The timescale of particular interest was up to 10 ms after the piston has stopped. Experiments were performed in non-reactive and reactive conditions. Acetone was seeded in non-reactive mixtures. Combustion was studied first in a system containing di-tert-butyl peroxide vapour in the presence of oxygen. The decomposition of di-tert-butyl peroxide generates methyl radicals, which are then oxidised if oxygen is present. The overall reaction is exothermic and is characteristic of a conventional thermal ignition. In addition, chemiluminescence, resulting from CH2O*, accompanies the oxidation process. The combustion of n-pentane was then investigated at compressed gas temperatures that spanned the range in which there is a negative temperature dependence of the overall reaction rate, typically 750-850 K. The response to thermal feedback in this more complex thermokinetic system can be the opposite of the "thermal runaway" that accompanies di-tert-butyl peroxide combustion. The purpose of making comparisons between these two types of systems was to show how the temperature field generated in the RCM is modified in different ways by the interaction with the chemistry and to discuss the implications of this for the spatial development of spontaneous ignition. As the piston of the RCM moves it shears gas off the walls of the chamber. This probably creates a roll-up vortex, but more importantly it also collects gas from the walls and moves it across the cylinder head pushing it forward into a plug at the centre. Thus, soon after the end of compression there is an adiabatically heated gas which extends virtually to the wall, but this incorporates a plug of colder gas at its core. Diffusive transport will occur, but the timescale is relatively slow, and the effect hardly shows until at least 10 ms post-compression. The consequence of "thermal runaway" on a timescale that is compatible with the development of this temperature field is that the reaction rate in the adiabatically compressed toroidal region accelerates faster than in the core, and goes to completion first. A somewhat similar pattern emerges during n-pentane combustion when the initial condition is set at the lower end of the negative temperature dependent range. By contrast, at adiabatically compressed gas temperatures close to the upper end of the negative temperature dependent region, the reaction rate in the cooler core develops faster than that in the surrounding zone, and the temperature difference is rapidly smoothed out. This does not lead to spatial homogeneity in all respects, however, because different rates and extents of reaction generate different concentrations of intermediates. This stratification has implications for the eventual spatial evolution of spontaneous ignition.