Understanding the thermo-acoustic stability in burners
Vertika Saxena defended her PhD thesis at the Department of Mechanical Engineering on December 5th.
Thermo-acoustic instability in burners with flames poses a critical challenge for achieving stable and efficient combustion systems. The PhD research of Vertika Saxena addresses the innovative problem of determining a burner's quality factor in relation to its stability under variable acoustic conditions. Employing a probabilistic framework and leveraging both experimental and numerical methods, the study explores the impact of acoustic embeddings鈥攄efined by reflection coefficients at system boundaries鈥攐n burner stability. Key contributions include the development of a groundbreaking experimental setup to manipulate boundary conditions and the use of a low-order network model to replicate realistic physical boundaries. The findings unveil stability-enhancing design insights, such as the benefits of larger perforation areas, and identify the properties that define burners with superior stability across diverse applications.
This research tackles a recently identified problem: evaluating the quality factor of burners with flames in relation to their thermo-acoustic instability properties. The approach employs a probabilistic framework to study burners placed in systems with varying acoustic conditions, described using reflection coefficients at upstream and downstream boundaries. Variability in acoustic embeddings is achieved experimentally and through low-order network modeling.
Experimental approach
A novel experimental setup was constructed to impose desired reflection coefficients at system boundaries. Acoustic mufflers were used to achieve a controlled magnitude range of reflection coefficients, while a telescopic tube enabled phase variation upstream. Tests on different burners revealed stability maps that identified favorable boundary conditions for stable operation and classified unstable regimes based on the presence and number of unstable frequencies.
Numerical approach
Low-order network models expanded the experimental findings by simulating diverse boundary conditions. Monte-Carlo simulations introduced randomness in reflection coefficient magnitudes and phases, exploring a broader range of possible configurations. Reflection coefficients were modeled both as frequency-independent constants and as frequency-dependent functions derived from strictly positive real functions. The study showed that flames with shorter time delays were more stable, while peak gain frequencies above critical phase crossings induced multiple unstable modes.
Application to boiler design
The study also addressed burner stability in systems with predefined geometries, such as boilers. A network model simulated acoustic variations by altering duct lengths and diameters while maintaining realistic boundary conditions. Statistical analysis identified optimal geometric configurations for stable operation, providing practical insights for combustion system design.
Generic system-level correlations
The research of Saxena introduces a comprehensive method to assess the quality factor of burners with flames under varying acoustic conditions. By combining experimental and numerical approaches, it establishes several generic system-level correlations between the flame transfer function properties and the flame stability, contributing valuable insights for designing stable and efficient combustion systems.
Title of PhD thesis: . Supervisors: Prof. Philip de Goey, Prof. Ines Lopez Arteaga, and Dr. Victor Kornilov.