Session: 04-02: Flashback and Blowoff

Paper Number: 82163

82163 - Numerical Investigation of a Coupled Blow-Off/Flashback Process in a High-Pressure Lean-Burn Combustor 

Lean-burn technology can be used to achieve high efficiency and low emissions in modern gas turbines simultaneously, due to the possibility to limit the temperature in the burnt gases. However, lean premixed conditions are prone to instabilities, which can lead to local or total flame extinction, and flashback. One particular mechanism for flashback in premixed swirl burners is that driven by the combustion induced vortex breakdown (CIVB), which occurs when, for certain flame topologies, the thermal expansion from the flame in proximity of a precessing vortex core (PVC) causes its break down, allowing the formation of a recirculation bubble that travels upstream. Although several numerical and experimental works have studied this phenomenon, the root cause of the interaction between flame and PVC is still not fully clear, which is partly due to the limitations in measurements especially at high pressure conditions, and the difficulties in numerical approaches to predict such an interaction. As a result, the phenomenon could only be investigated in simplified configurations, or with RANS methodologies, where the CIVB was enforced by a change in boundary conditions.

In this work large eddy simulations are used to investigate the CIVB driven flashback in a developmental, lean-burn, partially premixed and high-pressure combustor implementing a lean direct injector of jet-A fuel. A single sector of an annular configuration is simulated, which consists of a richer pilot flame surrounded by a leaner, main flame. A previously-developed flamelet-based approach with dynamic estimation of the model constant, validated for a similar configuration in a previous work, is used here to model the turbulence-combustion interaction due to its relatively low computational cost and the possibility to simulate a sufficiently long time window.

In stable operations, the flame stabilises in an M-shape configuration and a periodic movement of the pilot jet, with formation and destruction of a small inner recirculation bubble, is observed. The LES results predict then the CIVB flashback twice (flashback, flame extinction, re-ignition, flashback again, extinction, reignition) within a period of about 20 ms. The process in the pilot flame follows what has been already observed in other studies. The LES, however, further indicates that this process is preceded by a partial blow-out of the main flame, in turn caused by the formation of an isolated vortex in the outer recirculation region, which thus acts as precursor of the flashback. The sudden quenching of part of the flame alters the mixedness level upstream, causing first the main flame, then the pilot flame, to accelerate and initiate the CIVB before the quenched part of the main flame can re-ignite. Main and pilot flames then extinguish (numerically) as they pass their respective fuel injection point, allowing for the re-ignition due to the hot gases left. The flashback process is investigated in detail in the paper and offers an explanation for the root causes of CIVB driven flashback in realistic lean-burn systems, which can aid their development for stable operations.

Presenting Author: Alessandro Soli Loughborough University

Presenting Author Biography: Alessandro Soli is a postgraduate researcher in Aeronautical Engineering at Loughborough University. He holds a MSc degree in Thermal power from Cranfield University (2016), a MSc degree in Aerospace Engineering from Politecnico di Torino (2017), and a MRes in Gas Turbine Aerodynamics from the University of Cambridge (2018). His research interests include compressor-combustor interactions, off-design gas turbine performance, and the numerical simulation of unsteady and reacting flows.

Authors:

Ivan Langella TU Delft
Alessandro Soli Loughborough University