The present paper reports the results of a visualisation study of the burnout in subcooled flow boiling of water, with square cross section annular geometry (formed by a central heater rod contained in a duct characterised by a square cross section). The coolant velocity is in the range 3 -10 m/s. High speed movies of flow pattern in subcooled flow boiling of water from onset of nucleate boiling up to physical burnout of the heater are recorded. From video images (single frames were taken with a stroboscope light and an exposure time of 1 μs) the following general behaviour of vapour bubbles was observed: when the rate of bubble generation is increasing, with bubbles growing in the superheated layer close to the heating wall, their coalescence produces a sort of elongated bubble called vapour blanket. One of the main features of the vapour blanket is that it is rooted to the nucleation site on the heated surface. Bubble dimensions are given as a function of thermal-hydraulic tested conditions for the whole range of velocity until the burnout region. A qualitative analysis of the behaviour of three test section with different macroscopic surface finiture is also presented, showing the importance of this parameter on the dynamics of the bubbles and on the critical heat flux.