Effect of bend on premixed flame dynamics in a closed duct

Abstract

The premixed flame dynamics and pressure build-up in a closed duct with a 90° bend are experimentally and numerically investigated, and compared to a previous analytical theory. Emphasis is placed on the effect of the bend on the flame propagation, especially on the detailed flame front evolution. The results show that the finger flame is curved along the bend. A noticeable tulip flame is produced in the horizontal straight section as the upper parts of the indented flame nearly catches up with the lower parts. The lower parts tend to dominate the flame propagation after the complete formation of the tulip flame. The tulip flame disappears with the vanishing of the upper parts near the end of combustion. The flame dynamics in the experiment is reasonably reproduced by the numerical simulations, especially with isothermal wall. It is found that the full formation of the tulip flame is accompanied by a second drastic flame deceleration arising from the rapid reduction of the surface area of the outer flame front near the outer sidewall. It is revealed that the takes on the appearance of a “trough” shape after the flame touches the outer walls. The flame is still concaved from the center toward the burnt gas after the disappearance of the tulip upper parts. The pressure rise is closely related to the flame behavior. The heat transfer through the duct walls has a significant influence on the combustion dynamics. Both the flame tip location and pressure rise in the later stages are greater in the case of adiabatic duct walls than in the experiment. The theoretical analysis demonstrates that the flame propagation mechanism in the early stages in the curved duct coincides with that in a straight duct despite the presence of the bend. However, the flame behavior after its contact with the duct sidewall and tulip formation differs from those in a straight duct under the effects of the bend.