Asea Brown Boveri Ltd. (ABB) Award 2009 - Robert Etienne

"For his remarkable experimental Ph.D. research involving the conception and construction of a unique burner to produce unstrained diffusion flames and the first comprehensive study of their instabilities advancing the understanding of dynamic extinction."

Experimental investigation of unstrained diffusion flames and their instabilities.

In this thesis, thermal-diffusive instabilities are studied experimentally in diffusion flames. The novel species injector of a recently developed research burner, consisting of an array of hypodermic needles, which allows to produce quasi one-dimensional unstrained diffusion flames has been improved. It is used in a new symmetric design with fuel and oxidizer injected through needle arrays which allows to independently choose both the magnitude and direction of the bulk flow through the flame.

The thermal-diffusive instabilities observed close to extinction are investigated experimentally and mapped as a function of the Lewis numbers of the reactants. The use of a mixture of two inerts (helium and CO_2) allows for the effect of a wide range of Lewis numbers to be studied. A cellular flame structure is observed in hydrogen flames when the Lewis numbers is relatively low with a typical cell size between 7 and 15 mm. The cell size is found to scale linearly with the diffusion length, in good agreementwith theoretical predictions. When the Lewis number is increased by using a higher helium content in the dilution mixture, the instabilities observed are planar intensity pulsation. The pulsating frequencies measured are in the 0.7to 11 Hz range and were found to scale linearly with a diffusion frequency defined as U^2/Dth multiplied by the square root of the Damk¨ohler number. The experimental results presented here are the first observations of thermal-diffusive instabilities in such a low-strain flame. They constitute a unique dataset that can be used to quantitatively validate theoreticalmodels on diffusion flame stability developed in the simplified onedimensional configuration.