Instantaneous hydrodynamics of a laminar wavy liquid film

Abstract

Experiments have been conducted to investigate instantaneous hydrodynamic characteristics of laminar falling films of 20 cS silicone oil on an inclined plate using a photochromic dye activation technique and high-speed video photography over a Reynolds number range of 11–220. Instantaneous velocity profiles across a wavy laminar film and axial profiles of film thickness have been measured simultaneously and statistically analyzed. Experimental data indicated that the time-averaged mean and maximum velocity data are significantly over-predicted by Nusselt's theory, while the time-averaged film thickness data are slightly under-predicted. The instantaneous velocity profiles examined in different regions of the wavy laminar film showed that in the smooth substrate film region, the velocity profiles matched Nusselt theory, however, under the large waves they were substantially smaller than Nusselt's predictions for films of the same thickness. Based on the measured velocity profiles, approximately 30–40% of liquid mass was estimated to be transported by the large waves. The instantaneous wall shear stress values were close to the gravity force per unit area in the substrate film, but much smaller under the large waves. The temporal variations of wall shear stress and film thickness showed that the peaks in wall shear stress more often preceded the film thickness peaks, suggesting that the liquid in the wave region experiences acceleration, while in the non-wavy region both acceleration and deceleration are experienced.