Effects of swirl on the heating process of a central gas stream in a tubular flame.

• Swirl tubular flame burner yields a wider operation limit than the slit burner without rotation. • Heating rate enhances with swirl number via increase of heat transfer area or turbulence. • Turbulent mixing under cold flow evolves to flow laminarization under combustion condition. • Propose a formula employing swirl number to accurately predict the heating rate. This study experimentally investigated the heating process of an axially induced cold gas stream in a swirl tubular flame established through tangential fuel/air pre-mixture injection, aiming to clarify the effects of swirl motion on the heat and mass transfer enhancement. Tubular flame burners with geometry swirl numbers (S) of 0.35 to 1.4 were employed in addition to a conventional slit-type burner to systematically study the flame structure, stability, temperature distribution and flow field. The results show that in comparison to the slit burner without rotation, the swirl tubular flame burner provides a much wider range of stable flame from lean to rich limits even under large flow rates. The heating rate of the inner cold stream by the outer burned gas stream increases as the swirl number raises. Cold flow visualization illustrates that the interface between the two streams is rolled up and instability appears at the interface for the larger swirl numbers of S = 0.7 and 1.4, resulting in enhancement of mixing rate. At large flow rates the two streams are well mixed under cold flow condition, however, they are separated and ordered under combustion condition in the upstream, probably resulted from the burned gas expansion outside the central gas stream. While their mixing rapidly enhances as S raises to beyond 0.7. A simple theoretical analysis illustrates that the enhancement of the heating rate with a rotational motion can be quantitatively evaluated by replacing the numeric unity in Eq. (1) with the value of S + 1. The enhancement seems to be through an increase of heat transfer area at low rotational velocities, or increases of heat and mass transfer rates at high rotational velocities. [ABSTRACT FROM AUTHOR]