As aircraft engines trend towards higher bypass ratios, the relative importance of combustion noise has seen a recent increase. To study and mitigate combustion noise, it is important to understand the different sources and generation mechanisms of sound within a combustor. It was proposed that partial coherence methods using chemiluminescence measurements could provide insight into the contribution of direct combustion noise compared to indirect noise. This requires a priori knowledge of the coherence between the unsteady heat release rate and direct combustion noise. Theory suggests that for a farfield observer and acoustically compact source, the coherence between globally integrated heat release and combustion noise is unity for a direct noise dominant system. However, no such measurements are available in literature. In fact, the very few literature that does report coherence show values well less than unity. The primary goal of this work is to bridge this gap between theory and literature through both analytical and experimental studies on the heat release - direct noise coherence. 

An analytical model based on the solution of the wave equation was developed to study nearfield and acoustical noncompactness effects on the heat release - pressure coherence. The governing parameters of coherence were identified: i) the acoustical compactness which is the relative magnitude of the flame dimension to the flame length or width, ii) the observer position relative to the flame, iii) the spatiotemporal statistics of the unsteady heat release, and iv) the flame geometry. Model findings were experimentally verified by measurements taken from a turbulent premixed methane - air Bunsen burner setup. Explicit measurements of unity coherence in the acoustically compact limit were obtained, and noncompactness effects due to the distributed monopole nature of the flame were demonstrated. Results were extended to a ducted configuration, where the effect of reflections on coherence are presented. 

Overall this work fills an important gap in the combustion noise literature, where a clear understanding of the coherence between the unsteady heat release and direct combustion noise is established. Results contribute to further study of core noise by enabling chemiluminescence as a diagnostic tool, while deepening fundamental understanding of noise generation mechanisms.