Development of artificial perturbations in a nonisothermal liquid film

The effect of artificial perturbations on the formation of structures in a water film flowing over a vertical plate with a heater has been experimentally investigated. Attention has been focused on the study of the propagation of nonstationary perturbations in time immediately after their creation on the surface of the liquid film. The fluorescence method has been used to measure the film thickness field. The temperature field on the film surface has been measured by a high-speed infrared scanner. It has been shown that the passage of the wave front after the action of a perturbation system with the “most dangerous” distance between the cylinders leads to a change in the distance between the rivulets. It has been found that the intensity of introduced perturbations (cylinder diameters) and the characteristics of waves flowing to the heater significantly affect the change in the distance between the rivulets. An increase in the amplitude of the waves at the front of propagating perturbation has been detected. The coalescence of the rivulets in the lower part of the heater has been observed. It has been shown that the passage of several wave fronts is necessary for the complete rearrangement of the flow even when the intensity of perturbations is high.     

Nonstationary effect of artificial perturbations on a nonisothermal falling liquid film

The effect of nonstationary artificial perturbations on the formation of rivulets on the surface of a nonisothermal liquid film flowing down a vertical plate with a heater has been experimentally studied. The film thickness was measured using a fluorescent technique. It is demonstrated that, using periodic perturbations, it is possible to change the distance between rivulets with time. An increase in the wave amplitude at the front of a propagating perturbation has been observed. It is established that the wave amplitude growth and the liquid mass transfer across the flow lead to the washing of dry spots downstream the heater.     

Controlling the contact line behavior by introduction of artificial perturbations in a nonisothermal liquid film

The effect of periodic artificial perturbations on a heated liquid film at small Reynolds numbers is studied experimentally using a high-speed infrared camera. It is shown that the development of thermocapillary perturbations leads to movement of rivulets and affects contact line that outlines dry spot in a vertically falling heated film. It has been established that at sufficiently intense heat fluxes the artificial perturbations exert a significant effect on the structure of the flow and on the repeated wetting of dry zones.     

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.     

Thermocapillary deformation of a nonisothermal flowing liquid film

A criterion determining the onset of perturbations in front of the thermocapillary bore in a locally heated flowing liquid film is found and a formula for evaluation of the critical Reynolds number is obtained.     

Amplitudes of three-dimensional waves in a nonisothermal falling liquid film

The amplitudes of three-dimensional (3D) waves in a water film flowing down a vertical plate with a local heater have been measured. Thermocapillary forces that arise on heating lead to the formation of rivulets separated by thin-layer troughs, with 3D waves propagating over the crests of rivulets. The film thickness and 3D wave amplitudes on the heater grow with increasing heat flux density and distance downstream the flow, but the relative wave amplitude remains unchanged. In the heated regions between rivulets, the relative amplitude of waves increases with decreasing average thickness (or local Reynolds number). Analysis of results obtained for large Reynolds numbers showed that the relative amplitudes of waves in the regions between rivulets at high heat flux densities are much greater than those for small Reynolds numbers in isothermal falling films.     

Effect of thermocapillary perturbations on the wave motion in heated falling liquid film

The transformation of hydrodynamic perturbations into thermocapillary-wave structures in a locally heated water film flowing down a vertical plate has been experimentally studied using a high-speed IR imager for monitoring the temperature field and a fluorescent technique for determining the liquid film thickness. It is established that a three-dimensional (3D) front of the hydrodynamic wave acquires inhomogeneous temperature profile, which leads to a deformation of the liquid film under the action of thermocapillary forces and results in the formation of rivulets. Distances between the 3D waves and rivulets are determined as functions of the heatflux density. The experimental data are compared to the theoretical results.     

Thermocapillary effects in nonisothermal liquid film at high Reynolds numbers

The temperature and wave characteristics of the water film flowing down a vertical plate with a heater at Re = 300 are studied. The field of film thicknesses at different heat flux values was measured using the fluorescence method. The temperature field on the film surface was measured by an infrared scanner. The experimental data were obtained for variations in temperature over the liquid film surface with time during the propagation of waves. When the falling liquid is heated, the thermocapillary forces lead to formation of rivulets and a thin film between them. Averaging of measurements allowed the value of the transverse film deformation to be determined. Two mechanisms of influence of thermocapillary forces on the motion of the wavy liquid film are marked. For the first time, the exhibition of such a strong thermocapillary effects is revealed in the heated liquid film at high Reynolds numbers.     

The effect of external perturbations on the energy of temperature pulsations in a heated liquid film

The dynamics of temperature pulsations on the surface of a heated water film flowing vertically down was analyzed under the conditions of 3D wave movement for an unperturbed film and upon introduction of external perturbations with “the most dangerous wavelength.” It was established for the first time that artificial perturbations increase the amplitude and spectral energy, as well as expand the frequency range of temperature pulsations in the bottom part of the heater compared to the case without perturbation. External perturbations were found to cause an increase in the average integral energy of temperature pulsations per unit time in the bottom part of the heater and, consequently, an increase in the film resistance to rupture.     

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.     

The interaction of thermocapillary perturbations with 3D waves in a heated falling liquid film

The evolution of three-dimensional (3D) waves into thermocapillary-wave structures in a locally heated water film flowing down a vertical plate has been experimentally studied. The interaction of hydrodynamic perturbations with thermocapillary instabilities was monitored using a high-speed high-resolution IR imager. The existence of thermocapillary structures has been observed for the first time at high Reynolds numbers (Re = 150) in regime A, which is characterized by high temperature gradients on the film surface and a definite value of the ??most dangerous?? wavelength of instability that arises when the heat flux density exceeds a certain threshold. It is established that structures periodically appear in which the distance between temperature maxima is much shorter than that observed before. As the heat flux density is increased, the thermal entry length exhibits a sharp drop when thermocapillary instabilities arise in the vicinity of the upper edge of the heater.     

Self-oscillations of nonisothermal flow of viscous liquid in a channel

The process of emergence of self-oscillations of a flow of viscous liquid in a channel under conditions of heat transfer to the cold channel wall is considered. The parameters of liquid and heat transfer, at which self-oscillations may exist, are obtained, as well as the dependences of the period and amplitude of self-oscillations on the parameters of the system. The behavior of the system is investigated in the vicinity of the critical point determined in the curve of the dependence of liquid flow rate on pressure difference. A generalizing scheme of operation is given for a number of self-oscillatory systems, and it is demonstrated that their behavior in the vicinity of the critical point is described by the same equations.     

Effect of a constant electric field on the film flow of a liquid dielectric

The method and results of an experimental investigation into the effect of a constant electric field on the flow of a liquid dielectric film over a vertical surface are described.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 48, No. 3, pp. 432–436, March, 1985.     

Stretching of a plane anomalously viscous film under nonisothermal conditions

A solution is obtained for the problem of the stretching of an anomalously viscous film under nonisothermal conditions. The solution is analyzed.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 43, No. 1, pp. 62–70, July, 1982.     

Effect of out-of-plane properties of a polyimide film on the stress fields in microelectronic structures

The detailed modeling of microelectronic structures involving thin polyimide films is difficult due to the limited amount of information available on the out-of-plane polyimide properties. In the present analysis, we treat the out-of-plane mechanical properties of the polyimide as design variables and investigate their effect on the thermal stresses that develop in a one layer copper-polyimide interconnection structure using a combined finite element-design of experiment method. The interconnection structure is modeled by a three-dimensional representative cell composed of a square copper via in a thin polyimide film mounted on top of a silicon substrate. The cell is assumed to be stress free at an elevated temperature, and it is subsequently cooled uniformly. In the analysis, we obtain the initial elastic response of the cell for each combination of the out-of-plane polyimide properties called for in the experimental design, and generate response functions for the stress components at key bimaterial interfaces. Additional finite element trials are then performed over a broad temperature range, allowing for copper plasticity, to investigate the role of the out-of-plane polyimide properties when a material nonlinearity is present in the structure.     

Effects of compressibility and nonisothermal conditions on the performance of film cooling

The equations for a turbulent boundary layer are transformed in order to analyze the effects of compressibility and nonisothermal conditions on the performance of film cooling.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 19, No. 5, pp. 870–879, November, 1970.     

Numerical solution for nonisothermal motion of a viscous liquid in a two-dimensional tube

The problem of simultaneous development of velocity and temperature profiles in a two-dimensional tube is examined. The assumption is made that the liquid viscosity depends on temperature, while the other parameters are constant.     

Stability of nonisothermal flow of a viscous liquid in an annular channel

In nonisothermal flow of a viscous liquid in an annular channel between coaxial cylinders where the outer cylinder has finite dimensions and is stationary, and the inner cylinder infinitely moves along the axis, the central position of the latter is unstable. When superimposing a thermal field, principally it is possible to create as large a force as required which holds the inner cylinder exactly on center.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 63, No. 1, pp. 32–37, July, 1992.     

Dynamics of vapor film formation upon rapid superheating of liquid

The formation of a vapor film on a heater in the modes of rapid near-wall superheating of liquid at positive and negative pressures is considered. A model of this phenomenon and a technique for calculating the vaporization front velocity are proposed. Tests of this model in experiments with n-heptane at pressures from −2 to +0.2 MPa and heating rates of ∼0.5 K/μs showed a significant effect of capillary forces on the dynamics of vapor film formation.