https://orcid.org/0000-0002-7737-6087
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ABSTRACT
This paper presents experimental studies on flame appearance and static flame stability characteristics of inverse jet flame (IJF) using a circumferentially arranged fuel port (CAFP) burner by following two different methods, namely, Protocol I and Protocol II. In the case of Protocol I, experiments are conducted for a fixed air flow rate, by varying fuel flow rate whereas, in the case of Protocol II, by varying air flow rate for a fixed fuel flow rate. In Protocol I, the flame is lifted off from the burner rim unlike in Protocol II. Moreover, the flame base becomes unstable at higher fuel flow rate before the occurrence of flame liftoff for Protocol I. Momentum flux ratio (MFR) and overall equivalence ratio parameters are considered to obtain insights on the phenomenon with variation of flame liftoff height in CAFP-based IJF burner. An attempt has also been made to analyze the hysteresis effect on the flame liftoff. In contrast to the normal jet diffusion flame, the hysteresis effect is observed at lower MFR, and is absent particularly when the base flame gets strongly anchored to the burner rim for higher MFR. On the other hand, in Protocol II, with an increase in air velocity, the inverse jet diffusion flame is no longer diffusion controlled, rather mixing controlled. In addition, the flame blowoff for Protocol I is attributed to occurrence of leaner overall equivalence ratio obtained by reduction of fuel flow rate whereas global strain rate governs flame blowout for Protocol II. In addition to flame blowout and liftoff, the main flame for Protocol II experiences local extinction and re-ignition at certain conditions with increase in the global strain rate at certain threshold value. In addition, semi-empirical correlations based on Peclet number for main flame extinction, flame blowout and flame blowoff are proposed in the present study which can be employed for designing CAFP-based IJF-based combustor system.
NOMENCLATURE
| CAFP | = | Circumferentially arranged fuel port |
| CoA | = | Co-axial |
| de | = | Equivalent diameter |
| Fr | = | Froude number |
| HL | = | Mean flame liftoff height |
| IJF | = | Inverse jet flame |
| MFR | = | Momentum flux ratio |
| NJDF | = | Normal jet diffusion flame |
| Pe | = | Peclet number |
| Re | = | Reynolds number |
| Va | = | Average air velocity |
| Vf | = | Average fuel velocity |
| VRAF | = | Air–fuel velocity ratio |
| Ve | = | Relative jet velocity between average air–fuel velocities |
| Vfc | = | Critical fuel velocity |
| VfL | = | Threshold fuel velocity |
| Zst | = | Stoichiometric mixture fraction |
| ρa | = | Density of air |
| ρf | = | Density of fuel |
| α | = | Thermal diffusivity |
| ϕ | = | Overall equivalence ratio |