A Numerical Study of Syngas Laminar Premixed Flames: Effects of Lewis Number and Flame Stretch
thesisposted on 21.10.2015, 00:00 by Cesare D'Ippolito
In the current power generation scenario, two countervailing necessities are faced daily by designers and manufacturers of combustion systems: on the one side, the increasing energy demand and the need to respond adequately and e ciently; and on the other side, the commit- ment to cleaner combustion and the ful llment of environmental measures. Among the variety of alternative fuels under development, syngas is particularly interesting, as it can be produced locally through a gasi cation process from biomass (as well as fossil fuels and coal). It also o ers the advantages of hydrogen combustion, and it is widely available. The purpose of the present work is to assess some syngas combustion characteristics for which literature is still fragmentary. The underlying aim is to provide combustion systems manufacturers with some useful results about characteristics and ranges of application, thus incentivizing the employment of this clean fuel. A computational study is carried out, by means of one- and two-dimensional CFD simulations. The physical model considers the classical coun- ter ow con guration, consisting of the impingement of two opposing jets. In particular, laminar premixed syngas ames under lean conditions are considered. The e ects of syngas composition, stretch, and preferential di usion on ame speed, structure, and extinction behavior are exam- ined. Although such issues have been broadly addressed in combustion literature for traditional fuels, ame studies of syngas are still in progress. The Lewis numbers (Le) of several syngas/air mixtures (di erent in H2/CO ratio, equiv- alence ratio, and N2 dilution) are computed. Results indicate that while Le is less than or greater than unity for lean and rich mixtures, respectively, the mixture transport properties are predominantly characterized by H2 rather than by CO. The combined e ects of non-equidi usion and ame stretch lead to a modi cation in the ame speed and structure. Since the premixed ames in a counter ow con guration are posi- tively stretched, the e ect of stretch leads to higher burning rate for mixtures with Le < 1, i.e., lean syngas ames, and lower burning rate for mixtures with Le > 1, i.e., rich syngas ames. In this sense, syngas ames exhibit an analogous behavior to hydrogen ames. Flame stability is assessed, nding that rich syngas ames are cellularly stable, while lean syngas ames are unstable, as a consequence of the Lewis number e ect. Flame extinction limits of lean syngas ames are calculated. Extinction occurs at either too high or too low global strain rate. In the rst case, it is purely induced by stretch; in the second case, gas radiation plays a crucial role. Lowly stretched (lean) ames with lower are more a ected by strain rate variations. Moreover, the mixture extinction limits can be extended by an increase in equivalence ratio. Finally, the e ect of N2 dilution on syngas ames extinction is also assessed.