An experiment is carried out to investigate the effects of Froude number and co-flow air on a flickering LPG/air partially premixed flame for different tube diameters (13.5, 15.3, and 19.3 mm). In this study, the constant equivalence ratio (ϕ = 2) is considered. The direct and Schlieren images, along with the chemiluminescence signal, were obtained to better understand the flame structure and frequency behaviour. Furthermore, the OH* chemiluminescence images were obtained with a lowspeed ICCD. The co-annular tube ensures that the partially premixed flame receives uniform air co-flow. The flame height increases with increasing diameters at the respective Froude number. At a tube diameter of 19.3 mm, the subharmonic frequency is detected. In contrast to 13.5 and 15.3 mm, the radially extended flame structure occurs in 19.3 mm. The natural frequency decreases with diameter while increasing with co-flow. The amplitude of natural and subharmonic frequency oscillations decreases as co-flow velocity increases. The subharmonic frequency disappears before natural oscillation suppression at critical co-flow velocity. The Schlieren images revealed that at higher co-flow velocity, the outer vortex interaction with the inner zone does not occur, and thus the subharmonic frequency does not occur. The OH* mean chemiluminescence images show an elongated flame structure with an increase in a chemical reaction at the flame front as coflow velocity increases. © 2023, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.