Scaling Thermo-Acoustic Characteristics of LP and LPP Swirl Flames

Abstract

One of the main objectives of combustion research in field of gas turbine application during the last decades was and still is the reduction of pollutant emissions. The most promising technology to reduce these pollutants turned out to be Lean Premixed (LP) and Lean Premixed Pre-Vaporized (LPP) combustion. However, serious problems concerning combustion-driven instabilities occurred with the implementation of the LP/LPP-concept. Today, prediction and systematic suppression of self-sustained combustion instabilities is an issue still unsolved, due to incomplete understanding of the physical feedback mechanism and the lack of models for dynamic flame response, i.e. frequency dependent characteristics of LP/LPP swirl flames. In that context, the purpose of the current paper is the establishment of a physical model to describe frequency dependent flame dynamics concerning burning velocity of steady-state premixed flames. Derived from that basic understanding, scaling laws for the prediction of unstable operation conditions will be established in dependence on main operation parameters such as thermal load, mixture temperature, air equivalence ratio and especially of fuel and operating pressure. Therefore, a new swirl-burner has been designed, offering the feasibility to choose the type of fuel, to adjust the swirl number for main and pilot burner and the burner exit geometry steplessly and to vary preheating temperatures, air equivalence ratios and thermal loads in a range of industrial relevance for gas turbine applications. To establish a periodical modulation of the mixture mass flow of the main L(P)P flame at the burner outlet sinusoidally in-time with well-defined frequencies and amplitudes, a pulsating unit was used. Using a mixing/ pre-vaporizing unit to create a time-independent and spatial homogeneous mixture of natural gas/ kerosene vapor and combustion air at the burner outlet, flame transfer functions of LP- and LPP swirl flames depending on main operating parameters were determined. The discussed results then lead to stability map for a given combustion system depending on the main operation parameters based on the knowledge of only one fully-described parameter combination leading to an instable condition. Based on this scaling procedure and confirmed by further experimental work the prediction of stability limits depending especially on the type of fuel, the swirl number and the operating pressure will be possible.

Determination and Scaling of Thermo Acoustic Characteristics of Premixed Flames

“…17 over x 0,c to x 0,h and dividing by U 0,c yields an expression for effective velocity as (18) Assuming a linear dependency of the jet expansion on the temperature ratio across the flame with a proportionality constant C u one obtains (19) A value of the C u = 0.08 is obtained from the PIV data presented in Fig. 13 [14] the mean convective time delay is directly proportional to the ratio of the distance at which the highest reaction occurs (≈ L f ) and the effective transport velocity (U eff ,c ) which is derived from the burner exit velocity and the temperature ratio between reactants and products. Once we have the suitable flame length model (Eqn.…”

“…Given reliable and effective scaling rules as a function of thermal power, equivalence ratio and preheat temperature the dynamic flame parameters could then be determined at all points of interest for the stability analysis. Such scaling laws for the prediction of thermo acoustic characteristics in dependence of the main operating parameters for premixed flames have been proposed by Lohrman and Büchner, 2004 and Russ et al, 2007 [13,14]. Their analysis is based on a characteristic mean time delay model and using a critical Strouhal number scaling.…”