This study demonstrates how or whether an inlet vent rate affects flame suppression behaviour in highly confined and mechanically ventilated installations which are a typical hazard in the nuclear safety area. The primary objective of the present work is to provide a detailed experimental study on fire suppression phenomena via water mist in a buoyant, turbulent diffusion flame representing the key characteristics of a realistic fire. From the present work, it is found that water mist application in a hot confined environment allows to achieve maximum droplet evaporation in the smoke layer near the ceiling. As a result, the mist can't be transported to the fire source during the fire-water interaction period. Reduction in flame-to-wall radiation heat transfer via cooling of smoke layer by water mist over liquid surface does not conduct to suppression of a confined fully developed fire. Through the analysis on HRR (Heat Release Rate), gas temperature, heat flux and chemical species, it is concluded that in a hot environment, a consecutive efficient cooling of the smoke layer thanks to a strong droplet evaporation leads to the fire suppression only with ACPH (Air Change Per Hour) below 28 via lack of oxygen.