E. A. Chinnov

Jet formation in gravitational flow of a heated wavy liquid film

Jet formation was studied in the region of two-dimensional and three-dimensional waves in a heated liquid film flowing down a vertical surface. Jet-to-jet spacings were measured versus the film Reynolds number and the heat flow density. Three-dimensional waves on the film surface were formed naturally or by artificial perturbations. In addition to the thermocapillary mechanism of jet formation, a thermocapillary–wavy mechanism was found to exist.

Two-phase flow in short rectangular mini-channel

Recent developments in electronics and medicine generate an interest in capillary hydrodynamics and heat transfer in Microsystems. Microsystems also are very important for aerospace industry. In the present paper we focus upon experiments in the short channel oriented horizontally with the width 40 mm, height 1 mm and length 80 mm. New flow regimes are observed and the two-phase flow pattern map is created. The obtained results are compared with the data available in the literature.

Marangoni effect on wave structure in liquid films

Liquid films are encountered in space systems as well as in numerous industrial processes and everyday life. The present work is a part of the preparation of the SAFIR experiment of the European Space Agency onboard the International Space Station. Wave characteristics of the water film flow over a vertical or an inclined plate with a heater have been studied. The fluorescence method was used to measure local instantaneous film thickness. In addition to measure wave characteristics eight capacitance sensors were located as a line across the flow. The process of rivulet formation in a heated film was registered. Two zones of the heat flux effect on liquid film deformation were distinguished. At low heat fluxes, the film flow is hardly deformed. At high heat fluxes the thermocapillary forces provide formation of rivulets and a thin film between them. The measured values of the distance between rivulets depend slightly on the plate inclination angle. It was found that for a positive temperature gradient along the flow, heating may increase the wave amplitude because thermocapillary forces are directed from the valley to the crest of the wave. This effect was shown in the interrivulet zone, where relative wave amplitude and Marangoni number increase with a rise of the heat flux density.

Evolution of the temperature field at the three-dimensional wave front in a heated liquid film

An experimental study of the transformation of three-dimensional waves into thermocapillarywave structures in the flow of water film on a vertical plate with a heater was carried out using the fluorescent thickness measurement technique and a high-speed infrared recording technique in order to measure the temperature field on the liquid film surface. It was shown that the temperature disturbances occur in the residual layer of the wave film. Then, as the film flows along the heater, temperature inhomogeneities appear transverse to the flow at the three-dimensional wave front; and these inhomogeneities cause deformation of the liquid film and formation of rivulet due to the action of thermocapillary forces.

Experimental Study of the Wave Flow of a Liquid Film on a Heated Surface

The wave flow of a water film over the surface of a vertical plate with a 150×150-mm heater has been experimentally studied. The action of heat flux on the wave flow of the liquid film is manifested by the formation of periodic flowing rivulets separated by thin film regions. The thickness of the film between rivulets was measured using a fiber optical reflection probe. As the heat flux grows, the average film thickness h continuously decreases. However, when the thickness reaches h≈0.5 h0, where h0 is the value given by the Nusselt formula for a laminar liquid film, the film exhibits spontaneous rupture. It was found that, as the local flow rate decreases, the wave amplitude in the region between rivulets drops more rapidly than expected according to the laws of “cold hydrodynamics.”

Integrated measurement of the wave characteristics of heated film of liquid by the capacitance and fluorescence methods

Measuring the wave characteristics of a film of liquid by different methods enables one to improve the reliability of data and obtain new information about the process being studied. Along with the data obtained using the capacitance method already employed for the investigation of the field and dynamics of thickness of a nonisothermal film of liquid, results of measurement by the fluorescence method are given in this paper. The quantitative characteristics of nonisothermal film flow, obtained for the first time using the fluorescence method, are compared with the data obtained using an eight-channel capacitive thickness gage. The capabilities of each method are analyzed from the standpoint of measuring the characteristics of nonisothermal film of liquid in the heater region. It is experimentally demonstrated that the data obtained by the capacitance and fluorescence methods agree well with each other. This enables one to assume that both methods of measurement produce results which adequately accurately describe the real wave pattern of flow of nonisothermal film of liquid. The error of measurement of the thickness of nonisothermal film of liquid by the fluorescence and capacitance methods is at least 3%. The spatial resolution of the capacitance method is 0.5–1 mm, and that of the fluorescence method—0.1 mm.

The wave characteristics of a nonisothermal film of liquid under conditions of jets forming on its surface

An eight-channel capacitive sensor is used for the first time, which enables one to investigate the dynamics of three-dimensional wave flows and the variation of the transverse profile of a nonisothermal film of liquid during formation of jets. Measurements are performed of the wave characteristics of the flow of a film of water on a vertical plate with a heater 150 × 150 mm in size. During the heating of falling liquid, the thermocapillary forces cause the formation of jets and of a thin film between them. The film thickness and wave amplitude in the interjet region decrease with increasing heat flux. Two ranges of the effect of the heat flux on the characteristics of wave flow are identified. Under conditions of low heat fluxes, the film flow hardly differs from isothermal. Under significant heat loads, an intensive formation of jets occurs. Three-dimensional waves propagate over the jet crests, where the film thickness and wave amplitude increase with increasing heat flux. In the interjet region of the film being heated, the average relative amplitude of waves increases with decreasing average thickness, and in the isothermal region this amplitude decreases. Comparison of the obtained results with experimental data for isothermal film reveals that the values of relative amplitude differ significantly in the interjet region at high densities of heat fluxes. Transverse temperature gradients cause a decrease in the liquid film thickness, and longitudinal gradients cause an increase in the relative amplitude of waves compared to isothermal flows. In the end, this leads to the emergence of dry spots and breakdown of film. The relative amplitude of waves on the jet surface decreases with increasing heat flux; this is true of isothermal film flows.

Two-phase flow in a horizontal rectangular microchannel

Two-phase flow in a rectangular short horizontal channel 200 μm high was studied experimentally. The use of the fluorescent method made it possible to reveal flow of liquid in the channel and to determine its characteristics quantitatively. The existence of the regime of separate (stratified) flow is established. Based on analysis of previous investigations and newly obtained data, it is shown that a change in the height of the horizontal channel has a substantial effect on the boundaries between the regimes. The region of the churn regime increases with decreasing thickness of the channel.

Control over the formation of structures in a heated liquid film

An experimental investigation is performed of the impact made by artificial disturbances on the formation of structures under conditions of flow of water film on a vertical plate with a heater. The fluorescent method is used for measuring the film thickness. It is found that artificial disturbances on the surface of liquid film may cause a significant variation of the spacing between jets, and the scenario of evolution of wave pattern depends on their intensity. Artificial disturbances on the film surface cause the spacing between jets to decrease from values corresponding to the thermocapillary-wave mode of formation of jets to values close to those of spacings typical of the thermocapillary mode. In so doing, the spacing between jets may be varied at higher values of Reynolds number than is the case in the thermocapillary mode. It is demonstrated that it is possible to control the formation of structures on the surface of a heated film of liquid, and the conditions of maximal effect of artificial disturbances are determined.

Two-phase flow in short horizontal rectangular microchannels with a height of 300 μm

The two-phase flow in a narrow short horizontal channel with a rectangular cross section is studied experimentally. The channel has a width of 10, 20, or 30 mm and a height of 300 μm. The specifics of formation of such two-phase flows are investigated. It is demonstrated that the regions of bubble and churn flow regimes grow and constrain the region of jet flow as the channel gets wider. The boundaries of the regions of annular and stratified flow regimes remain almost unaltered.