Axisymmetric background-oriented schlieren measurement for cylindrical combustion flow in micro-rocket torches

A high-precision background-oriented schlieren (BOS) measurement technique was developed to analyze the density field of gases ejected from a micro-rocket torch in scramjet combustors. The BOS method was enhanced using an axisymmetric model and inverse Abel transforms, thereby allowing accurate density measurements in complex flow fields and addressing the limitations of traditional BOS applications. Validation tests confirmed the accuracy and stability of the BOS analysis, demonstrating its robustness against experimental noise and reliable density reconstruction. Experiments with varying equivalence ratios revealed significant effects on the ejected gas-density distribution and spread. Under fuel-rich conditions, the combustion temperature and molecular interactions increase, resulting in a lower density and greater jet expansion. This effect produced a clear boundary between the ejected gas and the surrounding atmosphere, whereas lean combustion conditions displayed more diffuse gas boundaries and instability, owing to the lower hydrogen concentration and combustion temperature. These observed behaviors were linked to Lewis number effects, where higher values under fuel-rich conditions enhanced combustion stability and maintained a more defined gas boundary. To further improve the accuracy of the density analysis, this study applied a modified Gladstone–Dale constant based on the in-flame gas composition. The BOS approach was optimized for angled fields of view and extended measurement areas. The results demonstrated that the BOS technique, particularly with the integration of axisymmetric modeling, provides an effective approach for capturing intricate flow structures in combustion, including spatial density gradients along both radial and axial directions.