Flame Flashback Critical Damkohler Number for CO2 Diluted CH4 and C3H8 Mixtures with Air

The premixed laminar flame flashback phenomenon in tubes has been known and studied for several decades. However, the effect of the CO2 dilution has not been addressed, which is relevant for assessing the safety of oil-producing facilities. Furthermore, even if numerical studies have underscored the important role played by the mixture Lewis number (Le), these lack an experimental validation. For this reason, specific studies of premixed flame flashback in laminar flows have been undertaken. The objective is to assess the flame flashback critical conditions in mixtures of hydrocarbons - methane or propane diluted by various amounts of CO2- with air. This is effected by determining the critical Damkohler number, which is computed using both experimental and numerical data. The former leads to the flashback critical velocity gradient, whereas the latter to a characteristic chemical time scale. The effect of different flame thickness definitions on the Damkohler number (Da) is examined, evidencing than an order of magnitude discrepancy may arise depending on the definition choice. For methane/air mixtures the critical Da increases with the equivalence ratio, whereas a decrease of nearly two orders of magnitude has been obtained for propane/air mixtures. The original results show that CO2 dilution increases Da only when the fuel dilution percentage is larger than 25 and 50%, for methane and propane, respectively, situations which correspond to flame extinction after flashback. The propane/ air/CO2 mixture results exhibit a Da proportional to Le 5.06 dependency which closely follows the trend computed previously, whereas the methane/air/CO2 results evidence the thermal boundary condition at the tube wall influence.

Automated summary

The phenomenon of premixed laminar flame flashback in tubes has been studied for several decades. However, the effect of CO2 dilution on flame flashback has not been addressed. This study investigates the critical conditions for flame flashback in mixtures of hydrocarbons diluted with CO2, using experimental and numerical data. The results show that the choice of flame thickness definition has a significant impact on the critical Damkohler number.