Lean-Blow-Out Simulation of Natural Gas Fueled, Premixed Turbulent Jet Flame Arrays With LES and FGM-Modeling

To ensure compliance with stricter regulations on exhaust gas emissions, new industrial burner concepts are being investigated. One of these concepts is the matrix burner, consisting of an array of premixed, non-swirling jet flames. For the design of such burners, the prediction of fundamental burner properties is mandatory. One of these essential quantities is the lean blowout limit (LBO), which has already been investigated experimentally. This study investigates the possibility of numerical LBO prediction using a tabulated chemistry approach in combination with Large-Eddy-Simulation turbulence modeling. In contrast to conventional swirl burners, the numerical description of blowout events of multi jet flames has not yet been studied in detail. Lean blowout simulations have therefore been conducted for multiple nozzle variants, varying in their diameter and global dump ratio for a variety of operating conditions, showing their general applicability. A procedure to induce LBO is introduced where a stepwise increase in total mass flow is applied. LBO is determined based on the temporal progress of the mean reaction rate. A comparison with measurements shows good agreement and demonstrates that the procedure developed here is an efficient way to predict LBO values. Further investigations focused on the flame behavior when approaching LBO. The flame shape shows a drastic change from single jet flames (stable conditions) to a joint conical flame approaching LBO, which increases in length for increasing inlet velocity, showing the importance of jet interaction at LBO

Determination of a correlation for predicting lean blow off limits of gaseous fueled, premixed turbulent jet flame arrays enclosed in a hexagonal dump combustor

Combustion of natural gas with air in gas turbines is a key technology for efficient provision of electric energy and heat. More stringent regulations regarding the emission of pollutants, such as NOx emissions, are necessitating research on technologies to reduce NOx formation during the combustion process. One technical approach onto the reduction of NOx-formation during combustion is fuel-lean premixed combustion. Current lean combustion concepts applied in stationary gas turbine combustors rely on flame stabilization through recirculation of hot flue gas using swirling flows. Swirl stabilized flames may be prone to combustion instabilities especially in lean premixed arrangements. Therefore, another approach is followed in the present study. In this concept, a matrix of turbulent lean premixed jet flames in a dump combustor is applied. The matrix burner consists of a nozzle with an array of circular channels in a hexagonal arrangement and a combustion chamber with a hexagonal cross section. In order to develop an appropriate burner design based on this concept, the experimental determination and theoretical evaluation of the lean blow out limit using different nozzles and operating conditions were conducted in this work in order to quantify the influence of different parameters on the flame stability. The varied geometric parameters are the diameter of the circular channels in the burner matrix as well as the ratio of the free cross section area of the nozzle to the cross section are of the combustion chamber, the combustor area dump ratio. The lean blow limit was determined at different preheating temperatures and flow velocities. The results show that the velocity at the LBO limit increases with increasing channel diameter, area combustor dump ratio and preheating temperature. The experimental results of three matrix burner are correlated in terms of a critical Damkoehler number and it is shown through experimental validation, that the Damkoehler number correlation derived is capable of predicting the LBO of a scaled matrix burner

Numerische Untersuchung der Flammenform eines vorgemischten, unverdrallten Multijetbrenners

Um die immer strikter werdenden Vorschriften bezüglich der Abgasemissionen zu erfüllen, ist es notwendig, neue Brennerkonzepte zu erforschen und genauer zu verstehen. Eines dieser neuen Brennerkonzepte ist der Matrixbrenner, der eine Gruppe von nicht-verdrallten Strahlflammen erzeugt. Zur Auslegung solcher Brenner sind Kenntnisse über fundamentale Brennereigenschaften notwendig. Vergangene Untersuchungen berichteten bereits in Abhängigkeit der Betriebsbedingungen über eine signifikante Änderung der Flammenform, die Gegenstand der aktuellen Arbeit ist. Zu diesem Zweck werden unterschiedliche Matrixbrenner unter Änderung der Betriebsbedingungen mithilfe eines Grobstruktur- und tabellierten Chemie-Ansatzes numerisch simuliert. Zunächst werden die beiden Flammenformen genauer charakterisiert und verglichen und ein Algorithmus zur Identifikation der Flammenform vorgestellt. Weiterhin wird ein Korrelationsansatz zur Vorhersage des Flammenumschlagpunktes auf Basis des Peclet-Kriteriums vorgeschlagen