Experimental investigation of ultrasonic water injection effects on laminar flame characteristics of propane-air mixtures

This study experimentally investigates the effects of ultrafine water fog (mean droplet diameter < 5 µm), generated via an ultrasonic fogging system, on the combustion dynamics of laminar premixed propane-air flames. By integrating advanced CH* chemiluminescence imaging, the present work quantifies how water injection ratios (WIR: 0–10%) alter the laminar flame speed, adiabatic temperature, heat release rate, and flame thickness across equivalence ratios (ϕ = 0.9–1.3). Results reveal that water injection reduces laminar flame speed by up to 36% near stoichiometric conditions (ϕ = 1.07, WIR = 10%), primarily due to the thermal cooling effect (97–90% contribution). Flame thickness decreases by 21% at WIR = 10% and ϕ = 1.3, while CH* chemiluminescence intensity declines sharply, indicating suppressed radical formation. Furthermore, water injection advances the onset of flame tip opening, a critical hydrodynamic instability marker, reducing the threshold equivalence ratio from ϕ = 1.41 (dry) to ϕ = 1.3 at WIR of 10%. The impact of water injections on the premixed laminar flame under rich conditions demonstrates water's combined effect of chemical and thermal influences, altering flame stability and characteristics, and offering valuable insights for optimizing wet combustion in advanced gas turbine systems.