Effects of pressure on turbulent flame structure and pollutant emissions in hydrogen-containing micro-mixing flames

Annular micro-mixing (MM) nozzles can stabilize flames with low pollutant emissions in gas turbines owing to the small characteristic scale and the presence of central recirculation zones (CRZs), outer recirculation zones (ORZs), and flame interaction zones (FIZs). This study quantified the turbulent flame structure and pollutant emissions of the annular MM nozzles at different inlet air total pressures ( P ₃) and hydrogen blending ratios ( χ _H2). As P ₃ increases from 0.2 MPa to 0.6 MPa ( χ _H2 = 100%), flame centroid shifts upstream by 39.4% and flame brush thickness decreases by 50.1%, indicating that the flame-shape evolution is primarily axial compaction. The flame surface density ( Σ ) increases with peak Σ progressively concentrated in ORZs and FIZs, indicating that at elevated pressures, the flame surfaces develop numerous fine-scale wrinkles, enhancing flame anchoring in ORZs and flame interaction. The quantification of instantaneous relative fold rate demonstrates the flame-surface wrinkling intensifies by 24.1%. As χ _H2 increases from 25% to 100% ( P ₃ = 0.3 MPa), temporal fluctuations of the flame-surface position intensify. The wrinkling increases by 9.1%. The best-fit exponents ( k ) from the power-law fits of NO emission versus P ₃ remain below 1, indicating that NO emission increases initially and then approaches a plateau with increasing P ₃. As χ _H2 increases, k rises from 0.70 to 0.97, implying that this tendency weakens at higher χ _H2. For χ _H2 of 25%–75%, P ₃ = 0.4 MPa is the turning point at which NO emission approaches a plateau. NO emission decays exponentially as flame centroid shifts downstream, implying that controlling flame centroid helps achieve near-zero NO emission.