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The experimental study on flow characteristics of the turbulent falling film flowing along a vertical Perspex plate was carried out using ultrasonic Doppler velocimetry (UDV, a non-intrusive technique) at 1.85 × 104 ≤ Reynolds number of the liquid film (Rel ) ≤ 3.69 × 104. The instantaneous velocity of the turbulent falling film at different Rel was measured to investigate the velocity distribution, fluctuation range, turbulence intensity, and the flow behavior in falling films. The results showed that the increase of Rel reduced the length of the development region and accelerated turbulent falling films to a steady flow state. There were mainly two reasons for the change of the velocity gradient in films: the amplitude of interfacial waves and the collision of the liquid that has different velocity directions. When the falling films were away from the inlet, the mass transfer within falling films may be dominated by the lateral movement rather than Rel .

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