scholarly journals Primary instability and subsequent dynamics of liquid film flow along periodic corrugations

2013 ◽  
Author(s):  
Zhehui Cao
Keyword(s):  
Liquid Film ◽  
Vertical Wall ◽  
Liquid Films ◽  
Wave Instability ◽  
Global Mode ◽  
Flat Substrate ◽  
Wide Range ◽  
Inclination Angles ◽  
Parametric Effects ◽  
Primary Instability

This work is an experimental study of gravity-driven liquid film flowalong corrugated substrates. Thin liquid films appear in importantindustrial, environmental and biomedical flows. The modifications,resulting from the substitution of the flat substrate on which the filmflows by one with topography, are presently not well understood. Inparticular, there is little experimental evidence about the effect ofperiodic corrugations on the primary instability of the flow, and therelevant theoretical predictions appears contradictory. This problemis of significant interest, because of the potential for instability controlin such flows through the development of appropriately tailoredsubstrates.Systematic experiments were performed with water-glycerol solutions,in two flow facilities spanning a wide range of inclination angles. Substrateswith different shapes (sinusoidal and rectangular) and variouswavelengths and amplitudes were examined. The temporal variationof liquid film thickness was measured by conductance probes at multiplelocations along the flow. A photographic technique was developedto record the spatial variation of the free surface in the streamwisedirection. Using these techniques, the primary instability thresholdwas reliably determined, and the characteristics of traveling waves inthe unstable regime were recorded.A major finding of the thesis is that, for intermediate and high inclinations,steep enough corrugations trigger a new instability modeof finite wavelength, while they delay significantly the occurrence ofthe classical, convective, long-wave instability. The new mode is ashort, traveling wave, which is highly regular and persistently twodimensional,and appears to be a global mode. Transition from longtoshort-wave instability is observed to occur with increasing inclinationangle, and the exact location of the transition varies with thecorrugation shape. The parametric effects of liquid viscosity and ofcorrugation wavelength and amplitude are examined, and the importantspecial case of a vertical wall is investigated using a cylindricalflow geometry. Finally, an unexpected oscillatory instability is documented,specific to steep, rectangular substrates, which appears andintensifies when decreasing the liquid flow rate. It is speculated thatthis counter-intuitive behaviour is related to the specific separationcharacteristics of the rectangular substrate, and is triggered when thebackflow velocity exceeds a percentage of the forward stream.

Experimental evidence for a short-wave global mode in film flow along periodic corrugations

2013 ◽  
Vol 718 ◽  
pp. 304-320 ◽  
Author(s):  
Z. Cao ◽  
M. Vlachogiannis ◽  
V. Bontozoglou
Keyword(s):  
Film Flow ◽  
Order Effect ◽  
Short Wave ◽  
Travelling Wave ◽  
Wave Mode ◽  
Global Mode ◽  
Flat Substrate ◽  
Wide Range ◽  
Liquid Film Flow ◽  
Primary Instability

AbstractThe primary instability of liquid film flow along periodically corrugated substrates is studied experimentally. Two different wall shapes, of the same wavelength and height, are tested for a wide range of inclinations. It is found that, beyond a specific inclination, a new instability mode occurs before the classical, convective, long-wave one. This is a short, travelling wave, which is highly regular and persistently two-dimensional, and appears to be a global mode. The exact shape of the corrugations has a leading-order effect on the inclination at which the new mode appears and on its wavelength at inception. Compared with the behaviour of film flow on a flat substrate, the presently tested periodic walls are found to delay very significantly, but each one to a different extent, the onset of the primary instability. This delay increases with inclination, and presents a distinct discontinuity when transition from the long- to the short-wave mode takes place.

scholarly journals Motion and Evaporation of Shear-Driven Liquid Films in Turbulent Gases

1991 ◽  
Author(s):  
S. Wittig ◽  
J. Himmelsbach ◽  
B. Noll ◽  
H. J. Feld ◽  
W. Samenfink
Keyword(s):  
Shear Stress ◽  
Liquid Film ◽  
Gas Phase ◽  
Film Flow ◽  
Phase Interface ◽  
Liquid Films ◽  
Numerical Code ◽  
Optical Instrument ◽  
Turbulent Characteristics ◽  
Wide Range

Detailed measurements of wavy liquid films driven by the shear stress of turbulent air flow are obtained for different air temperatures, air velocities and flow rates of the liquid. The experimental conditions are chosen from characteristic data of liquid film flow in prefilming airblast atomizers and film vaporization employing combustors. For the measurement of the local film thickness and film velocity a new optical instrument — based on the light absorption of the liquid — has been developed, which can be used at high temperatures with evaporation. The measured data of the gas phase and the liquid film are compared with the results of a numerical code using a laminar as well as a turbulent model for the film flow and a standard numerical finite volume code for the gas phase. The results utilizing the two models for the liquid film show that the film exhibits laminar rather then turbulent characteristics under a wide range of flow conditions. This is of considerable interest when heat is transferred across the film by heating or cooling of the wall. With this information the optical instrument can also be used to determine the local shear stress of the gas phase at the phase interface. Using time averaged values for the thickness, the velocity and the roughness of the film the code leads to relatively accurate predictions of the interaction of the liquid film with the gas phase.

Motion and Evaporation of Shear-Driven Liquid Films in Turbulent Gases

1992 ◽  
Vol 114 (2) ◽  
pp. 395-400 ◽  
Author(s):  
S. Wittig ◽  
J. Himmelsbach ◽  
B. Noll ◽  
H. J. Feld ◽  
W. Samenfink
Keyword(s):  
Shear Stress ◽  
Liquid Film ◽  
Gas Phase ◽  
Film Flow ◽  
Phase Interface ◽  
Liquid Films ◽  
Numerical Code ◽  
Optical Instrument ◽  
Turbulent Characteristics ◽  
Wide Range

Detailed measurements of wavy liquid films driven by the shear stress of turbulent air flow are obtained for different air temperatures, air velocities, and flow rates of the liquid. The experimental conditions are chosen from characteristic data of liquid film flow in prefilming airblast atomizers and film vaporization employing combustors. For the measurement of the local film thickness and film velocity a new optical instrument—based on the light absorption of the liquid—has been developed, which can be used at high temperatures with evaporation. The measured data of the gas phase and the liquid film are compared with the results of a numerical code using a laminar as well as a turbulent model for the film flow and a standard numerical finite volume code for the gas phase. The results utilizing the two models for the liquid film show that the film exhibits laminar rather than turbulent characteristics under a wide range of flow conditions. This is of considerable interest when heat is transferred across the film by heating or cooling of the wall. With this information the optical instrument can also be used to determine the local shear stress of the gas phase at the phase interface. Using time-averaged values for the thickness, the velocity, and the roughness of the film, the code leads to relatively accurate predictions of the interaction of the liquid film with the gas phase.

Flow Patterns and CHF in a Locally Heated Liquid Film Shear-Driven in a Minichannel

2010 ◽  
Author(s):  
D. V. Zaitsev ◽  
O. A. Kabov
Keyword(s):  
Heat Flux ◽  
Liquid Film ◽  
Gas Flow ◽  
Liquid Films ◽  
Vapor Flow ◽  
High Heat Flux ◽  
Film Rupture ◽  
Space Applications ◽  
Flow Rates ◽  
Wide Range

Thin and very thin (less than 10 μm) liquid films driven by a forced gas/vapor flow (stratified or annular flows), i.e. shear-driven liquid films in a narrow channel is a promising candidate for the thermal management of advanced semiconductor devices in earth and space applications. Development of such technology requires significant advances in fundamental research, since the stability of joint flow of locally heated liquid film and gas is a rather complex problem. The paper focuses on the recent progress that has been achieved by the authors through conducting experiments. Experiments with water in flat channels with height of H = 1.2–2.0 mm (mini-scale) show that a liquid film driven by the action of a gas flow is stable in a wide range of liquid/gas flow rates. Map of isothermal flow regime was plotted and the length of smooth region was measured. Even for sufficiently high gas flow rates an important thermocapillary effect on film dynamics occurs. Scenario of film rupture differs widely for different flow regimes. It is found that the critical heat flux for a shear driven film can be 10 times higher than that for a falling liquid film, and exceeds 400 W/cm2 in experiments with water for moderate liquid flow rates. This fact makes use of shear-driven liquid films promising in high heat flux chip cooling applications.

Microgap Cooling Technique Based on Evaporation of Thin Liquid Films

2009 ◽  
Author(s):  
D. V. Zaitsev ◽  
O. A. Kabov
Keyword(s):  
Heat Flux ◽  
Liquid Film ◽  
Gas Flow ◽  
Liquid Films ◽  
Vapor Flow ◽  
High Heat Flux ◽  
Film Rupture ◽  
Space Applications ◽  
Flow Rates ◽  
Wide Range

Thin and very thin (less than 10 μm) liquid films driven by a forced gas/vapor flow (stratified or annular flows), i.e. shear-driven liquid films in a narrow channel is a promising candidate for the thermal management of advanced semiconductor devices in earth and space applications. Development of such technology requires significant advances in fundamental research, since the stability of joint flow of locally heated liquid film and gas is a rather complex problem. The paper focuses on the recent progress that has been achieved by the authors through conducting experiments. Experiments with water in flat channels with height of H = 1.2–2.0 mm show that a liquid film driven by the action of a gas flow is stable in a wide range of liquid/gas flow rates. Map of isothermal flow regime was plotted and the length of smooth region was measured. Even for sufficiently high gas flow rates an important thermocapillary effect on film dynamics occurs. Scenario of film rupture differs widely for different flow regimes. It is found that the critical heat flux for a shear driven film can be 10 times higher than that for a falling liquid film, and exceeds 400 W/cm2 in experiments with water for moderate liquid flow rates. This fact makes use of shear-driven liquid films promising in high heat flux chip cooling applications.

scholarly journals Visualization of Surface Acoustic Waves in Thin Liquid Films

2016 ◽  
Vol 6 (1) ◽  
Author(s):  
R. W. Rambach ◽  
J. Taiber ◽  
C. M. L. Scheck ◽  
C. Meyer ◽  
J. Reboud ◽  
...  
Keyword(s):  
Liquid Film ◽  
Acoustic Waves ◽  
Low Cost ◽  
Thin Liquid Film ◽  
Surface Acoustic Waves ◽  
Liquid Films ◽  
Theoretical Description ◽  
Propagation Path ◽  
Microfluidic Channels ◽  
Potential Applications

Abstract We demonstrate that the propagation path of a surface acoustic wave (SAW), excited with an interdigitated transducer (IDT), can be visualized using a thin liquid film dispensed onto a lithium niobate (LiNbO3) substrate. The practical advantages of this visualization method are its rapid and simple implementation, with many potential applications including in characterising acoustic pumping within microfluidic channels. It also enables low-cost characterisation of IDT designs thereby allowing the determination of anisotropy and orientation of the piezoelectric substrate without the requirement for sophisticated and expensive equipment. Here, we show that the optical visibility of the sound path critically depends on the physical properties of the liquid film and identify heptane and methanol as most contrast rich solvents for visualization of SAW. We also provide a detailed theoretical description of this effect.

Dynamics of Developing Laminar Non-Newtonian Falling Liquid Films With Free Surface

1978 ◽  
Vol 45 (1) ◽  
pp. 19-24 ◽  
Author(s):  
V. Narayanamurthy ◽  
P. K. Sarma
Keyword(s):  
Liquid Film ◽  
Power Law ◽  
Mathematical Formulation ◽  
Liquid Films ◽  
Interfacial Shear ◽  
Newtonian Liquid ◽  
Power Law Model ◽  
Falling Liquid Film ◽  
Falling Liquid Films ◽  
Special Case

The dynamics of accelerating, laminar non-Newtonian falling liquid film is analytically solved taking into account the interfacial shear offered by the quiescent gas adjacent to the liquid film under adiabatic conditions of both the phases. The results indicate that the thickness of the liquid film for the assumed power law model of the shear deformation versus the shear stress is influenced by the index n, the modified form of (Fr/Re). The mathematical formulation of the present analysis enables to treat the problem as a general type from which the special case for Newtonian liquid films can be derived by equating the index in the power law to unity.

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