II-5
RIVULET WAVES
S.V. ALEKSEENKO and D.M. MARKOVICH
Siberian Branch of Russian Academy of Science
When an inclined cylinder is irrigated by a liquid jet the liquid collects on its lower side and moves in the form of a rivulet. This type of flow regimes exists in recently developed condensers and heat exchangers with inclined tubes.
The appearance of nonlinear waves caused by the flow instability is the characteristic feature of rivulet flow [1]. Similar to the case of falling liquid films [2], the waves affect essentially the hydrodynamics of the rivulet and should apparently have a strong action on the heat and mass transfer. Experimental study of the wavy motion of a rivulets on the flat surfaces is rather complex. The trouble lies in the difficulty of organizing a stable (with constant characteristics) flow. Due to the existence of a contact line and, correspondingly, a contact angle hysteresis [3], the rivulet moves usually in the form of a nonuniform nonstationary snake. For this reason, it is impossible to obtain unambiguous experimental data. The use of inclined cylinder as the basis for rivulet motion allows to make stable the rivulet flow. Method of superimposed oscillations gives us the possibility to realise the repeatable and well determined wave patterns. This paper presents the comprehensive experimental data on rivulet waves in wide range of conditions.
The scheme of the experiment is shown in Figure. As a test section the stainless steel tube and rough glass tube (outer diameters D of 20 mm and 19 mm) are used. The length of tubes is 1 m. Liquid is supplied to the test section in the form of a jet issuing from a convergent nozzle with a diameter, d, equal to 1-3 mm. The nozzle-to-tube distance, d , was varied from 0 to 10 mm. The incline of cylinder, , changed from 2 to 15Ў. Different media are used as working fluids: ethanol, water-glycerine solutions and some kinds of fluorocarbons whose peculiarity is high density and low surface tension.
The local film thickness and phase velocity of waves were determined with the aid of shadow method. For measurement of wall shear stress the electrodiffusional method was applied [4].
The distributions of the rivulet thickness along the test section, the length of a smooth zone, and the liquid ejection were measured in relation to the flow rate and inclination angle. The rivulet width was found to be constant and independent on the flow rate.
Both the natural and excited waves were investigated during the experiments. The data on natural waves characteristics are in satisfactory agreement with the calculations for maximum growth waves [5].
It is found that in many flow regimes the rivulet and film wave profiles are identical and the regularities of wave propagation are qualitatively the same. This similarity was not obvious because of principal distinctions between two kinds of flow. The film waves are strictly two-dimensional and the film is uniform in transversal direction. Contrary, the rivulet waves are significantly three-dimensional and the cross-section of a rivulet is nonuniform and represents a hanging droplet (see Figure).
New regularities are found in comparison with thin films. Foe example, the stationary waves of triangular profile and multi-hump solitary waves were observed.
REFERENCES