Non-monotonic liftoff height behaviors in laminar nonpremixed coflow jet flames of DME with ambient temperature variation

Liftoff height behaviors are experimentally investigated in nonpremixed coflow jets of dimethyl ether (DME) diluted with nitrogen, by varying initial fuel mole fraction (X_F,0), jet velocity (U₀ ≤ 1.0 m/s), and initial temperature (up to T₀ = 500 K). As jet velocity increases, three distinct liftoff height (H_L) behaviors are observed depending on X_F,0; monotonically-increasing H_L for small X_F,0, monotonically-decreasing H_L for large X_F,0, and decreasing and then increasing H_L (U-shaped behavior) for intermediate X_F,0. Such characteristics of exhibiting all three types of behaviors for a specified fuel and monotonically decreasing behavior have not been previously observed. For relatively-large jet velocities at small X_F,0, where the jet momentum is sufficiently larger than buoyancy, the liftoff height increases with jet velocity. The monotonically-decreasing H_L behavior for large X_F,0 is attributed to strong buoyancy. As the jet velocity decreases, a blowout occurs for all X_F,0 tested. In the increasing liftoff height regime with jet velocity, the normalized liftoff height is characterized in terms of U₀ scaled by laminar burning velocity (S_L⁰), while in the decreasing regime, in terms of the Richardson number. The blowout velocities are characterized in terms of the density difference between the fuel and burnt gas, which have distinct ranges among the regimes, emphasizing the role of the density difference on the three liftoff height behaviors.