Study on lifted flame stabilization under different background pressures

A numerical experimental investigation is presented for a steady methane lifted flame and a nonreaction jet flow in a co-flow of hot combustion products from lean premixed air-hydrogen combustion. The main objective has been to analyze the dependence of methane jet flame stability on the background pressure: a pressurized vitiated co-flow burner (PVCB) has been used to study the methane lifted flame and nonreaction jet flow under different background pressures (1–1.5 bars). The lifted flame is characterized by a liftoff height, which has been measured with a high-speed camera, and a central jet flow defined by the jet velocity, which has been measured by means of a high-sensitivity Schlieren imaging system. The experimental results show that the liftoff height decreases for an increment in the background pressure (from 1 to 1.5 bar at 1073 K) and in the co-flow temperature (from 1058 K to 1118 K at 1 bar). The standard deviation of the liftoff height also reduces for an increase in either the background pressure or the co-flow temperature, which indicates that the liftoff height is more stable at higher background pressures and co-flow temperatures. As far as the experimental tests on the nonreaction jet flow is concerned, the jet velocity becomes extinct faster as the background pressure rises, which is consistent with the decrease in the liftoff height as the background pressure grows. The evolution of the jet velocity has been proved to be another important factor that affects the liftoff height under different background pressures (physical factor), in addition to the fuel autoignition delay (chemical factor). The simulation data led with a Reynolds-averaged Navier–Stokes (RANS)/probability density function (PDF) model show that an increment in the background pressure makes the temperatures increase and induces a brighter yellow part of lifted flame, which leads to more soot production. This proves that the flame is not completely premixed. On the other hand, the Schlieren images of the non-reaction jet flow highlight that the flame is partially premixed, since the edge of the jet is not well defined, as the jet penetration increases with time. The liftoff height values of the flame in the numerical simulations were found to be generally higher than those measured in the corresponding experiments. This discrepancy was caused by an appreciable radiation heat loss at the thermocouple. A correlation was therefore developed for the thermocouple temperature measurement in order to correct the inaccuracy.