Initiation and Stabilization of cool flames in a diesel fueled vaporizer

Abstract In this paper results of experimental investigations of initiation and stabilization of cool flames at ambient pressure in a technical reactor are presented. The reaction is stabilized in a tubular glass reactor which permits optical access. The fuel is injected into the reactor using a common swirl atomizer. Preheated air serves as oxidant. It is shown, that cool flames are stabilizing in discrete regions of the reactor. The range of stability of the reaction is investigated by changing the boundary conditions, in particular the air ratio, the wall temperature and the inlet temperature of the flow. It is observed, that initiation and extinction of the cool flame reaction, motivated by a change of the inlet flow temperature, reveal a characteristic hysteresis curve. Depending on the reactor temperature, both events are accompanied by transient reaction phenomena. ∗ Corresponding author: d.diarra@owi-aachen.deProceedings of the European Combustion Meeting 2007 Introduction In times of rapidly decreasing energy resources the efficient use of hydrocarbons in combustion systems is mandatory. Not only does this mean that demands for low emissions and high efficiency have to be met, but also that processes and equipment has to be designed, which is flexible enough to use various types of fuels, meeting the concerns of regional and economical boundary conditions. The ability of using different fuels will be one of the most important features of future combustion devices. Proper mixture preparation is crucial for making efficient use of both, liquid and gaseous hydrocarbons in technical application. However, mixture preparation for liquid fuels requires additional steps, making the process more sophisticated. In modern combustion systems liquid fuels are atomized and evaporated before being mixed with the oxidant. To guarantee complete conversion of technical fuels (e.g. diesel or heating oil) to the gas phase, temperatures up to 350 °C are needed. An important characteristic of fuel vapour–air mixtures at these temperatures is the occurrence of exothermal reactions, which are referred to as cool flames in the literature. Phenomena of cool flame behaviour have been studied extensively on a laboratory scale using both, closed and continuous flow reactors. Varying the thermal boundary conditions, reaction phenomena like oscillatory cool flames and multi stage ignitions have been observed and a lot of work has been done to throw a light on the complex thermo kinetic interactions of low temperature chemistry. Just recently cool flames were stabilized in technical reactors were they were found to enhance the process of mixture preparation for liquid hydrocarbon fuels. In so called “cool flame vaporizers” the mild exothermicity of cool flames is used to support vaporization of fuel spray to achieve a homogeneous fuel-air mixture. Reactors of different types have been successfully used for premixed combustion and reforming application [1]. Design of technical systems differs noticeably from the design of laboratory systems, which are essentially implementing well stirred type of reactors and for which a large amount of data can be found in the literature. Various phenomena have been observed experimentally for the low temperature oxidation of hydrocarbons in CSTR-reactors, including steady state behavior but also transient behavior like ignition and oscillatory cool flames [2]. Similar phenomena may occur in a technical system where cool flames are stabilized. However, under technical aspects it is obvious, that dynamic behavior of cool flames may represent modes of reactor instability and therefore are not wanted.