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Soutenance de thèse G. OZTARLIK

23 octobre

Numerical and Experimental Investigations of Combustion Instabilities of
Swirled Premixed Methane-Air Flames With Hydrogen Addition

Soutenance de thèse Gorkem OZTARLIK

Mardi 20 novembre - Amphithéâtre Nougaro

Abstract :

In this work, hydrogen assisted (hydrogen enrichment and piloting) swirl stabilizedflames are studied experimentally via MIRADAS experiment. First of all, static stability characteristics, such as flame lengths and flame attachment characteristics are studied via CH* chemiluminescence flame images and cases with hydrogen piloting, methane piloting and hydrogen enrichment are compared to the reference case of perfectly premixed methane-air combustion for a wide range of equivalence ratios and bulk velocities. It is found out that hydrogen piloting is the most efficient method to attach the flames and extend operating ranges of the combustion chamber.

Next the dynamic stability characteristics of the setup is studied experimentally via stability maps and it is shown that injection of a very small portion of the thermal power worth of hydrogen results in a more stable system and an extension in the stable operating points in the stability maps, meaning safer overall operation. Hydrogen enrichment and methane piloting are also explored, and it is demonstrated that these methods are not effective in changing stability maps, stability maps are not effected.

Subsequently, the forced flame responses are studied experimentally and it is shown that hydrogen piloting and hydrogen enrichment causes a drop in the global time delay of the flame transfer function. With hydrogen piloting, there is a global drop in the flame transfer function gain, however for hydrogen enriched cases, the gain is increased. For methane piloted cases, there is a global reduction in the flame transfer function gain, however the time delay is not affected.

Consequently, to explore why and how the global flame transfer function is changed with different injection strategies, forced flame images are studied. It is shown that the changes in flame transfer function is caused by the competition behavior between the local heat release responses for hydrogen piloted cases. Simply put, there is a phase difference between the local responses near the injection tube and the flame edges, causing a « pull-back » effect, which in turn causes a drop in the flame transfer function gain.

Next the effect of different injection strategies on the hydrogen are investigated. It is demonstrated that adding hydrogen, in pilot injection or hydrogen enrichment configuration, causes a drop in CO2 emissions for the same thermal power. Piloting strategies cause a slight increase in NOx emissions, however results show that an optimization is possible to obtain flames that are stable, low CO2 and low NOx.

Finally, LES calculations and their comparisons with experimental results are presented. The capability of LES calculations in predicting flame responses is demonstrated and it is shown that the flame responses originate from the interactions of the vortices that are formed as a result of acoustic pulsations and the flames. Flames are wrapped around these vortices which increase the flame surface area. Further down the forcing cycle, the rolled up portions of the flames start touching the combustion chamber walls and gets quenched which causes a loss of flame surface area. These changes in flame surface area result in a fluctuating heat release rate, consisting the flame response.

Jury :

  • Franck NICOUD Professeur d’Universit´e Pr´esident du Jury
  • Thierry POINSOT Directeur de recherche Membre du Jury
  • Laurent SELLE Charg´e de Recherche Membre du Jury
  • Bénédicte CUENOT Charg´e de Recherche Membre du Jury
  • Thierry SCHULLER Professeur d’Universit´e Invit´e ´