Analysis of length effect on thermodynamic characteristics in a Z-shaped evaporating flameholder

In this paper, the length effects on thermodynamic characteristics of a Z-shaped evaporating flameholder are studied to optimize the ignition and flame-holding performance. The characteristics of ignition kernel, flamelet propagation, and flame stability under the operating conditions of turbine-based combined cycle combustors are experimentally investigated. The numerical flow fields are performed to explain the experimental results. Furthermore, the droplet distribution and flame equivalence ratio are acquired using image-processing to interpret the length effects. The results indicate that optimizing the structure can achieve a better matching of flow pattern and fuel mist, and a more uniform spatial distribution of flame equivalence ratio, attributed to the improved fuel-air mixing. The optimization of the L/H ratio can reduce the ignition delay time and increase the premixed degree during the ignition process, thereby owning the superior ignition and blowout performance within the wide-range operation. Changing the L/H ratio alters the double-reaction-zone morphology, and the contribution of the second reaction zone to sustain flame rises with the reduced L/H ratio. The optimal structure can consequently decrease the lean ignition and blowout equivalence ratio by 29.3% and 39.0%, respectively. The results provide a theoretical basis for the optimization of pilot-ignition structure employed in afterburner/ramjet combustors.