ADVANCES IN NUMERICAL-SIMULATION OF PULSATING COMBUSTION AT ENEL

This paper concerns the simulation of unsteady combustion processes. I t gathers up-to-date results of simulation activities, part of a long- term research program between the Italian National Institute for Elect ric Power (ENEL) and the University of Padova. The software used is 'F luent'; the hardware a Var 8530 and a Cray X-MP/14. The paper is divid ed in two major sections: one dealing with combustion instabilities, t he second with pulse combustion. In the first section, of the several known forms of combustion instabilities, our attention is directed tow ard the investigation of one caused by fluctuations in turbulent mixin g rate. Assuming that the combustion rate is controlled by mixing of t he chemical species (and not by chemical kinetics), we show that the f luctuations of velocity which accompany acoustic waves produce fluctua tions in turbulence and then fluctuations in reaction rate. We also sh ow that mean flow plays a key role in determining the sign of these fl uctuations and therefore the occurrence or otherwise of instability: f or a tube-like combustor, instability takes place if the mean Row move s from an acoustic open-end to an acoustic closed-end (or if the flame is in the first half of an open/open tube). The second section of the paper presents some preliminary simulations of the operation of a mec hanical-valve pulsating combustor, aimed at modeling a Lennox unit for air heating. Three two-dimensional axis-symmetrical models were consi dered, containing more and more details of the geometry near the inlet s. While acoustic vibrations were always easily captured (the predicte d frequency is in good agreement with the actual one), tuning of combu stion with them was more difficult to reproduce. In fact, only the thi rd model, with two fully separated inlets for air and propane, shows r easonable behavior. Analyses suggest that correct modeling of the way the reactants mix together (both macro- and micro-scale mixing) is the key for correct prediction of combustion delay (because the time requ ired for mixing is by far the most important delay term).