2D fringe probing of liquid film dynamics of a plug bubble in a micropipe

Abstract

Research on bubble dynamics and heat transfer in micropipe two-phase flows is an important area in micro electronic cooling, microfluidics and non-invasive laser/ultrasound microsurgery. For example, a pulsating heat pipe (PHP) is a very effective heat transfer device which made of a relatively long and thin sealed pipe containing both phases of the working fluid. The inner diameter of the pipe must be sufficiently small so that vapor bubbles can grow to vapor plugs in the tube. The optical diagnostic technique developed by Wang and Qiu (2005) has been further extended to probe 2D interfacial film thickness in micro capillary two-phase flows. Similar to single point measurements, the spatial frequencies from the multi-scattering measured by CCD camera are used to determine the film thickness but whole fringe image is divided into N columns for determining the special frequencies. The spatial frequency of each image column can be used to calculate the interfacial film thickness of the segment of the plug bubble. By integrating all the measured film thickness along the bubble, the interfacial film surround the whole bubble can be determined. The very fine parallel fringes are projected onto the liquid/gas-bubble interface. The scattered fringe pattern can be imaged at a proper orientation angle where the spatial frequency of the fringe pattern can be measured. To determine the spatial frequency variations during the plug/slug pulsating, a highly accurate signal processing technique for continuously evaluation of signal phases utilizing a modified fast Fourier transform algorithm was used. Through geometrical optics approach, the curvature of the cross-section of a plug can be derived from the spatial frequency of the scattering pattern on the screen. Capillary tubes, with inner diameters of 1.0mm and 0.3mm, and a length of 65mm, are used. A gas plug bubble, 5mm~20mm long, is introduced and moves through the testing part of tube, which is filled with water as the working fluid. The interference fringes produced by two incident laser sheets are scattered from the interface between gas and water, and captured by high speed camera at the speed of up to 2000 frames per second. The experimental results show that the improved method can obtain the liquid film thickness profile at the different time and can be used to analyze the status of plug bubble movement in a micropipe.

Reconstruction of wave features in wind-driven water film flow using ultrasonic pulse-echo technique

“…Taylor's hypothesis assumes that the time-space transformation is linear: , , 0 R r R r U (10) which can be further transformed to relate the time-correlation and space-correlation: 0, , 0 R R U (11) Therefore, the spatial distribution can be constructed simply using Eq. (12). U dy dt (12) where U is the mean flow velocity.…”

“…Over the years, various experimental techniques have been developed to measure thin liquid film flows [10][11][12][13][14][15][16]. Among these techniques, fluorescence imaging based, stereoscopic imaging based, density based, and structured light projection techniques are most frequently used.…”

Interfacial Flows of Contact Line Dynamics and Liquid Displacement in a Circular Microchannel

“…The dynamics of interfacial film displacement are often controlled by the capillary number, and usually include events of how the liquid-gas interface evolves, depending on flow conditions and fluid properties [7][8][9][10][11][12]. However, the processes of liquid film formation, development and breakup are complex which involve the interfacial tension between fluid-fluid and fluid-solid, and the capillary induced instability of thin liquid film breakup.…”

Measurements of interfacial dynamics of gas–liquid displacement in a capillary

“…Qiu et al [10], Wang andand Qiu [11] developed an optical method that can measure the interfacial film thickness utilizing fringe scattering technique. Ji and Qiu [12] further extended this method to 2D utilizing a laser sheet. Some other optical methods, based on counting the pixels of liquid film thickness, are too simple to get high resolution, for example, Ong and Thome [13], Schubring et al [14].…”