Experimental study on high-pressure constant-volume combustion of ultra-lean methane-air mixtures under different ignition delay times

Low-concentration coal mine gas is typically characterized by low combustion rate and significant heat losses. Studies on ultra-lean methane-air combustion under high-pressure constant-volume and turbulent conditions remain limited. This study experimentally investigates turbulent combustion characteristics and heat losses under elevated initial pressures, where turbulence intensity is regulated by adjusting the ignition delay time t d . Results reveal that varying t d non–monotonically affects both flame development and rapid combustion stages. While turbulence plays a primary role in shortening the rapid combustion duration, its intensification does not monotonically reduce heat losses, since the flow field influences both the convective heat transfer coefficient and the temperature difference between combustion products and chamber walls. At t d = 1 s, weak turbulence attenuates the promoting effect of temperature rise on flame development, with the most pronounced effect observed at P 0 = 0.5 MPa and 0.7 MPa. Further turbulence intensification yields a greater reduction in combustion duration than that resulting from the combined effect of temperature rise and weak turbulence. Within initial pressures of 0.3–0.7 MPa, the coupled effects of temperature rise and turbulence reduce the combustion duration by 20.6%–54.0% and increase P max by 10.3%–19.2% relative to quiescent combustion. An initial pressure of 0.6 MPa is found to minimize heat losses for the 6 vol% methane–air mixture.