Internal entrainment effects on high intensity distributed combustion using non-intrusive diagnostics

Abstract

h i g h l i g h t sExamined the role of entrainment on distributed combustion under different conditions. N 2 and CO 2 are used to simulate entrained combustion gases from within the combustor. Local equivalence ratio distribution obtained through OH ⁄ and CH ⁄ chemiluminescence. NO chemiluminescence significantly reduced under distributed combustion conditions. Reaction distribution demonstrated at 15% O 2 and lower, with almost invisible flame. a b s t r a c tHigh intensity colorless distributed combustion (CDC) provides high efficiency combustion with stable operation and ultra-low emissions. The role of internal entrainment of hot reactive gases requires further investigation in order to obtain minimum requirements for distributed combustion. In this paper, the impact of internal entrainment of reactive gases on the flame behavior and structure is investigated with focus on fostering distributed combustion. A mixture of nitrogen and carbon dioxide was introduced to the air stream prior to mixing with the fuel to simulate the recirculated product gases from within the combustor. Increase dilution with nitrogen or carbon dioxide increased the reaction zone volume to result in uniform distribution of CH ⁄ and OH ⁄ chemiluminescence signal and uniform equivalence ratio (measured optically). These conditions replaced the normally present blue flame with a uniform almost invisible faint bluish flame. The increased entrainment also decreased NO chemiluminescence significantly for the same amounts of fuel burned. The chemiluminescence data suggested that lowering oxygen concentration from 21% to 15% resulted in improved distributed combustion conditions with the reaction volume occupying most of the combustor. These conditions provide the minimum entrainment requirement and reduction of oxygen concentration for achieving distributed combustion. Results obtained at different equivalence ratios and entrained gas temperatures showed similar behavior at oxygen concentration of 15%. The reaction distribution was further enhanced at lower oxygen concentration ($11%) with further reduction in pollutants emissions.

Towards colorless distributed combustion regime

“…Table 1 summarizes the operating conditions for each of the experimental cases investigated herein while figure 3 shows the flame images for these cases with a filter to minimize the impact of the seeding. It is noteworthy that the change did not only occur to the flame shape as seen here and in previous publication [33], but also on the pollutants emission, thermal field uniformity, and local equivalence ratio distribution as shown in previous investigations [15,33,34]. The fuel flow rate was kept constant for all the experiments to result in a constant thermal load.…”

Acoustic and heat release signatures for swirl assisted distributed combustion

“…The experiments were performed using a swirl burner fueled with methane in three different combinations for oxidants (methane-air, air-methane with N 2 /CO 2 dilution, and oxygen-methane with CO 2 dilution). Details of this swirl burner can be found elsewhere [25]. To simulate product gas entrainment and lower oxygen concentration in the mixture prior to ignition, different amounts of N 2 -CO 2 mixture were added to the air upstream of the burner.…”

“…To simulate product gas entrainment and lower oxygen concentration in the mixture prior to ignition, different amounts of N 2 -CO 2 mixture were added to the air upstream of the burner. The flow control setup can be found in the literature along with measurement accuracy on data [25]. Nitrogen and carbon dioxide were mixed in a ratio of 90% N 2 -10% CO 2 by volume simulating product gases near stoichiometric conditions.…”

“…Nitrogen and carbon dioxide were mixed in a ratio of 90% N 2 -10% CO 2 by volume simulating product gases near stoichiometric conditions. Further details on this selection can be found in the literature [25]. The flow field for this swirler has been characterized under both reacting and non-reacting flow relating to the cases studied herein.…”

“…CDC investigations have demonstrated significant emissions reduction under different configurations [7,8,21,22] and fuel flexibility [23,24]. The required conditions to achieve CDC and the subsequent uniform local equivalence ratio and thermal fields within the reaction zone has been described in the literature; distinguishing distributed combustion from swirl assisted flames under the same flow conditions [25,26]. However, to date, there has been no effort to quantify any fluctuations in the flame as there were no noticeable instabilities under distributed combustion condition.…”