Effects of spacing distance and heat release rate on flame merging and burning characteristics of two ultra-long thin line fires

In this paper, the flame merging and burning characteristics of two ultra-long thin line fires (length 1000 mm, width 3 mm) with aspect ratio of 333:1 are studied at different spacing distances ( S = 2, 4, 6, 10, 12, 18, 20, 26, 30, 36, 40 and 50 cm) and heat release rates ( Q ˙ = 19.4, 25.7, 32.7, 38.9 and 43.6 kW). The two parallel burners are fueled by propane. The values of S / L fs , S = 0 = 0.8 for the critical merging position and S / W = 1000 for the critical interaction position are both correspondingly larger. It can be considered as that, the larger aspect ratio will promote the flame merging interaction and increase flame merging probability P m .The flame structure shows the “U” shape on the lengthwise direction, and the side flame height L fs and the middle flame height L fm are firstly introduced in order to reveal the three-dimensional effect. A segmental correlation of dimensionless flame height L fs / L fm with S / Z c is well developed, with the critical value S / Z c = 4 . A side flame height L fs prediction model is also derived theoretically, which shows a good agreement with the experiments value. Meanwhile, it is found that, the flame inclination angle θ increases with S ,due to the competition of air entrainment in the additional region. A side flame length Z fs model based on θ and L fs is proposed, which will extend the flame height model. This work provides the essential knowledge on the side detailed flame structure including the important parameters of L fs , θ and Z fs related with S and Q ˙ .