2D Fringe Probing of Liquid Film Dynamics of a Plug Bubble in a Micropipe

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. Introduction Recent progress in microand nano-technologies has yielded a rapidly emerging area in studying fluid dynamics and heat transfer in microchannels, which have promising applications in the lab-ona-chip devices, μTAS, DNA separation and analysis, electrokinetic micro pumps, micro heat pipes, microfuel cells and micro fluidic research, etc. Many important new technologies involve multiphase flows in mini/micro channels. Examples include a high efficient gas-liquid-solid micro fluidics hydrogenation reactor (Kobayashi et al. 2004), a promising technique utilizing air-bubble method of catheter locking with anticoagulant at the catheter tip and bactericidal properties at the catheter hub for hemodialysis access (Moore and Twardowski, 2003), prevention of gas embolism damage in surgery caused by intravascular air bubbles carried in the bloodstream (Branger and Eckmann 1999, Ayyaswamy 2004), manipulating microbubbles by ultrasonic waves (Yamakoshi 2001), cooling of electronics devices using mini/micro heat pipes (Cotter, 1984, Groll et al., 1998, Kang and Huang, 2002, Berre et al. 2003 and Groll and Khandekar 2004). Research on interfacial film dynamics, heat and mass transfer, and the dynamics characteristics of plugs/slugs in mini/microchannels are important in understanding and optimizing aforementioned processes and applications. The physical behavior of multiphase flows in mini/microchannels is 14th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2008 2 complex because it involves interfacial transport phenomena coupled with the dynamics of thin liquid, the disjoining pressure of the phases, stability and phase change at such an interface in microscale fluids (Kobayashi et al. 2004, Moore and Twardowski, 2003, Branger and Eckmann 1999, Ayyaswamy 2004). In the direct methanol fuel cells (DMFC), the behavior of generated CO2 plugs have significant impact on the mass transfer coefficient of the fuel, which limits the performance of DMFC. A particularly interesting phenomenon associated with multiphase flows in mini/microchannels is, therefore, the interfacial film dynamics caused by the liquid surface tension, the surface energy of the wall, the flow and plug/slug velocities, the gas property and the geometry of the groove structures. Progress in understanding the interfacial film dynamics, heat and mass transfer and fluid flows of plug/slug bubbles in a mini/microchannel has relied on experimental investigation due to the complexity of the process. Since the interfacial film thickness between a plug/slug bubble in a mini/microfluidic device has length scales of 1-100μm, traditional flow diagnostic tools cannot be used. Most measurements in two-phase microfluidic devices have been limited to the qualitative visualization techniques or bulk property measurements of two-phase flows. Recently, advances in micro-resolution particle image velocimetry (PIV) have become available for microflow field measurements (Meihart et al. 1999, Tian and Qiu 2002). Many techniques have been developed to measure the liquid film thickness, such as optical method (Xishi Wang, Huihe Qiu, 2005; Wali M. Nozhat, 1997; S. C. M. Yu, C. P. Tso and R. Liew, 1996), microwave method (R.P. Roy, J. Ku, B.Kaufman, J. Shukla, 1986), ultrasonic method (T. Kamei, A. Serizawa, 1998), electrical impedance method (Tohru Fukano, 1998), conductimetry method (Patrice Tisné, Louis Doubliez, Fethi Aloui, 2004) and capacitance method (Mustafa R. Özgü, John C. Chen, Nikolai Eberhardt, 1973; Billy W. Marshall, William G. Tiederman, 1972). However, these methods obtain the local film thickness information and can not measure the whole film thickness profile at the same time. In this paper, based on the previous research results, the extended measurement method of the whole film thickness profile is presented. The approach is based on the spatial fringe scattering method, where the spatial frequency of scattered fringes is a function of liquid film thickness. In the improved method, the laser sheet is used instead of the laser beam, and the width of laser sheet can cover the whole bubble, which can implement the profile measurement of liquid film thickness. Methodology Based on the method proposed by Wang and Qiu [1], the film thickness measurement method is further extended. Figure 1 shows the schematic of a moving bubble in a capillary pipe where the advancing and receding contact angles are different at the front and the end of the bubble. When two incident laser sheets form a cross-sectional area along the tube, parallel interference fringes will be formed in the intersection area. The fringes are focused onto the bubble surface and scattered from the interface between gas and liquid. Capturing the image of the scattered fringes by using a high speed CCD camera, the spatial frequencies of the received fringe image along the tube can be measured. Due to the change of the curvature of the interface between gas and liquid with the film thickness, the reflecting fringes will shift and extend. The motion of the bubble can make the spatial frequency of the fringes change. According to Wang and Qiu (2005) the measured spatial frequency is a function of the film thickness at that location being measured [1]. 14th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2008