Impact of internal entrainment on high intensity distributed combustion

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. Low O 2 concentration in the mixture prior to ignition fostered distributed reaction. Reaction distribution demonstrated at 15% O 2 and lower, with almost invisible flame. 80-90% NO reduction demonstrated at oxygen concentration of 15% in the mixture. a b s t r a c tColorless Distributed Combustion (CDC) has shown ultra-low emissions and enhanced performance of simulated gas turbine combustors. To achieve distributed combustion, the flowfield must be tailored for desirable mixture preparation within the combustor prior to mixture ignition. Though CDC have been extensively studied using a variety of geometries, heat release intensities, and fuels, the role of internally recirculated hot reactive gases needs to be further investigated and quantified to obtain the minimum requirement of internal entrainment for achieving distributed reaction condition. In this paper, the impact of internal entrainment of product gases on flame structure and behavior is investigated with focus on fostering distributed combustion and to provide guidelines for seeking distributed combustion. To simulate the recirculated gases from within the combustor, a mixture of nitrogen and carbon dioxide is introduced to the air stream prior to mixing with fuel and combustion. Increase in the amounts of nitrogen and carbon dioxide (simulating increased recirculation) increased the reaction volume to occupy larger volume with an overall enhanced and uniform distribution as revealed from the OH ⁄ chemiluminescence intensity. At the same time, the bluish flame is replaced with a more uniform almost invisible bluish flame. The increased recirculation also decreased the NO emission significantly for the same amount of fuel burned. Lowering oxygen concentration from 21% to 15% (due to increased recirculation) resulted in 80-90% reduction in NO with no impact on CO emission with sub PPM NO emission achieved at an equivalence ratio of 0.7. The same trend was demonstrated for a range of recirculated gases temperature. The reaction distribution was significantly enhanced with ultra-low emissions for oxygen concentration lower than 16% setting a minimum recirculation requirement for distributed combustion.

Small size burner combustion stabilization by means of strong cyclonic recirculation

“…For comparison, on the same diagram, the lowest used as working conditions of some advanced, new generation burners were reported [5,11,27,28] . As it is evident, the widest range of working conditions compete to the cyclonic burner.…”

“…More specifically, in the last years several combustion modes inherently based on a strong recirculation process have been proposed and some of them found their way to commercial use. Typical examples are MILD, HiTAC and other regenerative burners (FLOX) [5][6][7][8][9] but also high intensity combustion devices have been proposed that rely on quite similar concepts [10][11][12] .…”

Thermal field investigation under distributed combustion conditions

“…Figure 11 shows the axial temperature profile for the base case of no dilution (O 2 =21%), dilution mixture at 300K (O 2 =13.8%), and two dilution cases (O 2 =13.8 and 12.38%) at diluents temperature of 600K. As the temperature of the diluents was increased, the combustor was stable at lower oxygen concentrations as opposed to the diluents at room temperature [15].…”

“…The impact of the amount of gas entrainment and recirculation on reaction distribution and pollutants emission have been investigated with focus on determining the minimum entrainment requirements for distributed reactions to occur (hot gas recirculation/oxygen concentration) [15].…”

The role of CO2 on oxy-colorless distributed combustion

“…Such low reaction rate is achieved through the low oxygen concentration and sustained by the high mixture temperature. CDC have been thoroughly investigated, demonstrating significant emissions reduction under different configurations [19,20], fuel flexibility [21,22], thermal field uniformity and noise reduction [23], with the required conditions to achieve CDC as described in detail [24].…”