Impact of the Flame-Holder Heat-Transfer Characteristics on the Onset of Combustion Instability

Abstract

In this article, we investigate the impact of heat transfer between the flame and the flame-holder on the dynamic stability characteristics of a 50-kW backward-facing step combustor. We conducted a series of tests where two backward step blocks were used, made of ceramic and stainless steel, whose thermal conductivities are 1.06 and 12 W/m/K, respectively. Stability characteristics of the two flame-holder materials were examined using measurements of the dynamic pressure and flame chemiluminescence over a range of operating conditions. Results show that with the ceramic flameholder, the onset of instability is significantly delayed in time and, for certain operating conditions, disappears altogether, whereas with the higher conductivity material, the combustor becomes increasingly unstable over a range of operating conditions. We explain these trends using the heat flux through the flame-holder and the change in the burning velocity near the step wall. Results suggest a potential approach using low-thermal-conductivity material near the flame-holder as passive dynamics suppression methods.

Keywords:

• Combustion instability
• Flame-holder
• Flame speed
• Heat transfer
• Thermal conductivity

ACKNOWLEDGMENTS

The authors acknowledge the King Abdullah University of Science and Technology for their support of this research. This work was funded by KAUST Grant No. KUS-110-010-01.

Notes

¹While the cutoff operating time (7 min) in the transient tests is large enough compared to the time scale associated with the onset of the instability as discussed in Section 3.4.2, we note that it is arbitrarily chosen based on practical considerations for a large number of repeated tests and thereby has no physical significance.

²See Altay et al. (2009) for the corresponding flame chemiluminescence images in each operating regime, and Hong et al. (2013b) for the corresponding particle image velocimetry (PIV) data.

³The naming of these operating regimes (e.g., “low” and “high”) are based on the relative magnitude of the frequencies, while they both correspond to the longitudinal acoustic modes.

⁴The consumption speed is defined as , where q ^′′′ is the volumetric heat-release rate, c _p is the specific heat of the mixture, y is the coordinate normal to the flame, ρ _u is the unburned mixture density, and T _u and T _b are the unburned and burned gas temperatures, respectively.

⁵The oscillation frequency increases, e.g., from ∼70 Hz at T _in = 300 K to ∼90 Hz at T _in = 500 K. This is because the acoustic frequency depends on the speed of sound, which varies as the square root of the gas temperature; . See Hong et al. (2013a) for detailed acoustic analysis.

⁶The bias of the error bars in turn indicates that the inception times in most cases are close to their average values, showing the consistency in the repeated tests.

⁷The Biot number defined as hL _c /k _s is estimated to be ∼0.158 (<1), where h, L _c , and k _s denote the heat convection coefficient, characteristic length scale of a solid block, and thermal conductivity of a solid block, respectively. The temperature inside the step block is regarded as uniform in this example as an approximation, while spatial distribution of the temperature field is presented later in this section.

⁸This is expected based on the estimated Biot numbers, ∼0.16 and ∼1.79 for the stainless steel and the ceramic blocks, respectively. See also footnote 7.