Flow Characteristics of the Vertical Turbulent Falling Film at High Reynolds Numbers

Abstract

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 .

Effect of Structural Optimization of Scrubbing Cooling Rings on Vertical Falling Film Flow Characteristics

“…With the continuous increase of axial distance, the velocity of liquid film in the stable region tends to be stable, that is, the acceleration effect of gravity is balanced with the deceleration effect caused by friction resistance and momentum loss, and V fluctuates near a stable value. 22 …”

“…It is speculated that the liquid film in the falling section of the developing region has a small acceleration effect due to the action of gravity, while the deceleration effect caused by friction resistance and the momentum loss caused by mass loss are significant, so the growth of V decreases with the increase of Z . With the continuous increase of axial distance, the velocity of liquid film in the stable region tends to be stable, that is, the acceleration effect of gravity is balanced with the deceleration effect caused by friction resistance and momentum loss, and V fluctuates near a stable value …”

“…Then, according to the time difference between the two received reflected waves and the propagation speed of the ultrasonic wave in the medium, the distance between the two interfaces can be calculated as the thickness of the liquid film. Li et al, Jayakumar et al., and Wei et al used ultrasonic technology to measure the liquid film thickness under different flow conditions, thus proving the accuracy of ultrasonic probes in measuring liquid film thickness. The detailed measurement principles and usage of the UDV are described in the literature. − …”

“…It should be pointed out that the critical Reynolds number for the transition of liquid film flow state is not constant, and it is controlled by various factors, such as the wall effect and so on. 22 …”

“…Generally, when Re l < 20–30, the liquid film follows a laminar flow, then transforms into the fluctuating laminar flow at 30–50 < Re l < 250, and finally becomes completely turbulent at Re l > 250–500. It should be pointed out that the critical Reynolds number for the transition of liquid film flow state is not constant, and it is controlled by various factors, such as the wall effect and so on …”