Abstract: The former theory of single window overflow fire is not suitable for multi window, and there are obvious differences between them. In order to explore the phenomenon and mechanism of multi window overflow fire, two building models were established based on FDS, and two parallel windows were set up on the facade wall. The influence of fire source power (HRR) and window spacing (D) on the behavior of overflow fire in parallel double windows, indoor air flow rate, height of overflow flame (Hef) and superstructure facades were studied. The results showed that in the ventilation control fire, the air mass flow rate coefficient k decreases with the increase of D, and finally tends to be stable. In the fuel control fire, the larger the HRR, the greater the k. Under the same HRR, with the increase of D, k increases at the beginning and then decreases, and then tends to be stable. Under the same D, the bigger the HRR, the higher the Hef. Under the same HRR, with the decrease of D, Hef decreases at the beginning and then increases. That is, there is a critical transition distance D*, and the larger the HRR, the greater the D*. H ˉef /ζ1 = cQ* 2/5 ex is only applicable to single window or relatively independent window with little influence among multiple windows. In the range of D≤1 m, the heat receiving area of facade increases with the increase of D.

Key words: ejected flame behavior, flame fuse, indoor fire, ejected flame plume height