Tracking Blowoff Dynamics of Flames Utilizing Proper Orthogonal Decomposition

The dynamics of bluff body stabilized flames near blowoff are examined for propane-air flames comparing the effects of non-uniform fueling about the bluff body, presented previously, to those of thermoacoustic coupling with longitudinal modes in the combustor. High speed chemiluminescence imaging is used to visualize the flame as blowoff occurs for each condition. Rather than qualitatively tracking the flame’s physical response to blow off, proper orthogonal decomposition (POD) was used to quantitatively track the flame’s behavior. POD provides spatial information, from a set of bases or modes, and temporal information from mode coefficients, regarding flame emission during the blowoff process. The bases were obtained from the data of many flames with different parameters simultaneously. Bases were also obtained separately for each flame. For all cases, the method of minimum mutual information was used to optimize the number of images used in the POD algorithm to find the bases. The time scale associated with the minimum mutual information was found to be half of a recirculation zone flow time. By examining the dynamics of mode constants, time scales for physical processes were studied. A blowoff time scale and vortex propagation time scale was determined. In addition, a quantitative description of phenomena observed near blowoff such as recirculation zone burning, shear layer pinching, and acoustically driven oscillations is described. I. Introduction ANY aviation engines require flames to be stabilized in highly turbulent environments with the use of bluff bodies. Bluff bodies are utilized when the reactant mixture velocity is higher than the flame speed within the combustor or engine. The recirculation zone created in the wake of the bluff body provides hot products to the reactants in the shear layer near the bluff body. At some sufficiently high velocity, the bluff body is no longer able to stabilize a flame and blowoff occurs. The bluff body flame stability limit for a particular fuel is affected by several factors including, but not limited to, fuel stratification, bluff body size and type, and combustor dynamics including thermoacoustically coupled oscillations [1]. In some cases the bluff body is placed near the center of the engine duct with fuel injectors positioned upstream of the bluff body. In the highly turbulent environment of the engine it is not sufficient to assume that complete and symmetric mixing of the fuel occurs before it reaches the bluff body. In an attempt to develop more compact engines operating with higher inlet temperatures, the fuel injectors have been brought closer to the flame holder often being placed in the flame holder itself [2]. The shorter distance from the fuel injector to the flame decreases the chance of autoignition, but it increases the chance of incomplete mixing which can cause fueling gradients across the bluff body. Significant research has been reported on the blowoff of flames from various bluff body geometries [3-11]. Most of this work, however, has concentrated on symmetric fueling about the bluff body. Chemiluminescence is a common tool for observing flame behaviors. It is often used for qualitative descriptions of flame dynamics as in [10, 13]. Proper orthogonal decomposition (POD) is a statistical tool used to reduce data a set into only its statistically most important features. The spatial features of the data set are retained in the bases and the temporal data is retained in the coefficients. This study uses POD to deconstruct chemiluminescence data from flames with five levels of stratification and varying degrees of thermoacoustic coupling as the flames approach