ILASS-Europe 2002 Zaragoza 9 –11 September 2002 DIESEL SPRAY EVAPORATION MODELLING IN A “COOL FLAME” ENVIRONMENT: A NOVEL APPROACH D. Kolaitis and M. Founti mfou@central.ntua.gr Laboratory of Heterogeneous Mixtures and Combustion Systems, Thermal Engineering Section, Mechanical Engineering Department, National Technical University of Athens, Heroon Polytechniou 9, Polytechnioupoli Zografou, 15780 Athens, Greece Tel. +30-10-7723605, Fax. +30-10-7723663 Abstract An innovative approach for the separation of evaporation from the combustion process can take advantage of the damped “cool flame” phenomenon. To investigate this phenomenon a “cool flame” vaporizer reactor has been numerically simulated using a computational fluid dynamics code. A novel semi-empirical approach is developed, based on experimental data to calculate the heat release due to cool flame phenomena. The model overcomes problems of high computational demand, normally required in the implementation of chemical kinetics models describing low-temperature alkane oxidation. Two test cases are computationally simulated: a “single” spray evaporation case, serving to validate the CFD code, and a “cool flame evaporation” case, used for the development of the “cool flame” model. The good agreement between experiments and predictions confirm the ability of the model to capture reasonably well the general trends observed in the experiments. Introduction Oil fired furnaces and boilers, diesel engines and gas turbines utilize liquid fuel sprays in order to increase the fuel surface area and thus accelerate the vaporization and combustion rates. Conventional liquid fuel burning technologies inject the fuel into the combustion chamber through a nozzle that atomises it, producing a spray comprising many droplets, typically the order of a few tens of microns in diameter. The droplets, subjected to the high temperatures of the combustion chamber, are evaporated and burnt in a sequential process. During this procedure, there may arise problems owing to the incomplete mixing of the fuel vapours with the combustion air. The separation of the two phenomena, namely evaporation and combustion, could lead to the alleviation of inhomogenities in the fuel vapour-air mixture. A satisfactory mixing of the gaseous mixture can be thus achieved before initialisation of the combustion process. A novel way to accomplish such “separation” is to evaporate the fuel with the use of a process based on the “cool flame” phenomenon. The cool flame evaporation is a very promising process that could prove to be more efficient compared to the conventional liquid fuel evaporation methodologies, since it allows the use of premixed combustion technologies, which are known to exhibit a wide range of advantages, like reduction in emissions of soot, NOx, CO and unburned HCs. The scope of the present work is to numerically simulate a “cool flame” vaporizer reactor, using a computational fluid dynamics (CFD) code, in order to acquire more in-depth informa- tion about the occurring physical and chemical phenomena that are involved in the process. For this purpose a novel semi-empirical approach is developed to computationally simulate the cool flame characteristics. The “Cool Flame” Phenomenon The phenomenon described as “cool flame” is essentially a low temperature oxidation process during which the fuel is partially oxidized but not burnt [1] and it is mainly observed during the autoignition process of hydrocarbon fuels. Whenever alkane fuels have to reside partially or fully mixed in an oxidizing atmosphere at high temperatures, ignition can occur in a multistage mode, subsequently following completely different schemes of oxidation. At temperatures below 500 o C, the complex chemical reactions involved result in a two- stage ignition process in which “hot” ignition is preceded by a self-quenching temperature pulse referred to as a “cool flame” [2]. During the autoignition process, the operating kinetic mechanisms change continuously according to the temperature of the air-fuel mixture. It is possible to define low and high temperature mechanisms, in which different oxidising schemes are effective. Cool flames manifest themselves in the range of temperatures where transition between low temperature and high temperature mechanisms occurs [1] and are dominated by an exothermic degenerately branched chain reaction involving one or more important long-lived intermediates. In the temperature range of cool flame occurrence, the combustion process develops with a negative temperature coefficient of the reaction rate, i.e. the cool flame process is able to self-accelerate and to self-decelerate. This ability is considered to be the main distinguishing property of cool flames, since the homogenous self-quenching ability of an autocatalytic process is a rather unique feature.