Mode transition of interacting buoyant non-premixed flames

Abstract

The dynamic behavior, especially in the transition to oscillation mode (in-phase and anti-phase), of two interacting non-premixed methane-air jet flames was investigated experimentally. A well-controllable experimental system for the present purpose was constructed and key parameters; such as fuel flowrate (Q), burner diameter (d), and burner separation distance (L), were varied systematically. A well-known periodic motion of the flame was observed and the frequency monitored by thermocouples mounted adjacent to the burner exit. Time-variation of flame shape was recorded by a high speed camera associated with the optical imaging visualization. It was found that the flickering frequency was insensitive to the fuel flowrate, Q, implying that inertia played secondary role in the transition. Instead, the burner critical separation distance for the transition () varied when various burner diameters were used, confirming that the difference in distance played an important role in the transition. It was found that the critical condition could be summarized by an updated correlation as × 3~. This is slightly different from the one recently proposed by Yang et al. (2019), which was given under a narrower range of the fire scale. Accordingly, the critical condition can also be described by the critical value of the updated global parameter, such as 3 4/3 , where and denote the length ratio (/d) and Grashof number based on the burner diameter, respectively.

Similarity of dynamic behavior of buoyant single and twin jet-flame(s)

“…8(c) in the case of the twin-flame system. As we pointed out previously (Mochizuki et al, 2017;Bunkwang et al, 2020), there is a critical burner-separation distance (Lcrt) for the mode change (in-phase and anti-phase of the flame dynamics mode), in which a sudden discontinuous jump of the frequency can be found. Moreover, the anti-phase oscillation frequency is expected to be always higher than that of the in-phase.…”

“…Figure 2 displays the schematic of a twin-flame system consisting of two identical burners aligned in the same horizontal plane. This configuration is basically the same as in our previous work (Bunkwang et al, 2020), thus, only a brief description was provided herein. The variable system parameters were fuel flow rate (Q), inner burner diameter (D), and the burner-separation distance (L).…”

“…Interestingly, the mode transition occurs at the critical distance, that is, the appearance of each mode is firmly controlled by the system-controllable parameter. Recently, Yang et al (2019) and Bunkwang et al (2020) conducted an extensive investigation on the phase-transition condition, which could be summarized well by a global parameter employing the length scale ratio and the Grashof number. It is worthy to note that the in-phase oscillation was quite similar to the flame motion observed at the varicose mode in the single-flame system.…”

“…In this work, we aim to establish the similarity rule, both experimentally and numerically, which is applicable to the appearance of the periodic behaviors in the single-and twin-flame systems. In order to examine the dynamic behaviors in a precise manner, high-speed imaging with Schlieren optical system is introduced for visualization of the direct flame, as well as the hot plume boundary (Bunkwang et al, 2020). Frequency analysis is performed by using the temporal signal of the thermocouples located near the burner exit.…”