ON COMBUSTION WITH CONDENSED PHASE PRODUCTS: APPLICATIONS TO DROPLET BURNING OF MAGNESIUM IN CARBON DIOXIDE

ABSTRACT

Liquid droplet burning where the products of combustion are in the condensed phase is studied via activation energy asymptotics to determine the effects of product condensation on bulk flame properties and flame extinction. Magnesium burning in carbon dioxide/nitrogen atmospheres is adopted for sample calculations and the results are compared to those for Mg burning when the products are gas in the phase as well as for hydrocarbon combustion. Unlike the case of hydrocarbon combustion, there is found to be a maximum flame size in response to variations in oxidizer concentration. Moreover, the flame producing condensed-phase products is closer to the droplet, has a higher fuel consumption rate, and extinguishes at a larger droplet size than its gas-phase counterpart. This behavior is understood to be a result of the flame acting as a fluid-mechanical sink when the products are in the condensed phase and the resulting enhancement in oxidizer transport. While extinction droplet size decreases monotonically with increasing oxidizer concentration for flames producing gas-phase products, there is a critical oxidizer concentration for which the flame is strongest and extinction droplet size is smallest for flames producing condensed-phase products. The effect of the Lewis number was also studied. For flames with either gas- or condensed- phase products, a smaller oxidizer Lewis number leads to a higher flame temperature, higher fuel consumption rate, smaller flame standoff distance, and a smaller droplet size at extinction. When the Lewis number of the fuel is varied, only the extinction droplet size, r_s,E, is affected. For the flame producing gas-phase products, r_s,E decreases monotonically with increasing Le_F. For the flame producing condensed-phase products, r_s E decreases with increasing L_ep when the oxidizer concentration is low, but increases with increasing L_eF when the oxidizer concentration is high. The results of this study are relevant to metal combustion and flame synthesis.

Keywords:

• metal combustion
• flame synthesis
• droplet combustion
• magneium/carbon dioxide