Two-Phase Flow Characteristics

Understanding the differences in two-phase flow characteristics between conventional size channels and micro-channels is also important for designing mini- or micro-heat exchangers, since the flow characteristics will affect the phase change heat transfer. 

However, our current knowledge is still limited and in reality only a small number of literature sources are available. One of the questions is whether the two-phase flow patterns in small size channels are different from those encountered in ordin- 

Fluid Flow, Heat Transfer and Boiling in Micro-Channels Springer 2009 

For adiabatic, steady-state, and developed gas-liquid two-phase flow in a smooth pipe, assuming immiscible and incompressible phases, the essential variables are pu, pG, Pl, Pg, cr, dh, g, 9, Uls, and Uas, where subscripts L and G represent liquid and gas (or vapor), respectively, p is the density, p is the viscosity, cr is the surface tension, dh is the channel hydraulic diameter, 9 is the channel angle of inclination with respect to the gravity force, or the contact angle, g is the acceleration due to gravity, and Uls and Ugs are the liquid and gas superficial velocities, respectively. The independent dimensionless parameters can be chosen as Ap/pu (where Ap = Pl-Pg), and 

Zhao and Rezkallah (1993), Rezkallah (1996), and more recently Lowe and Rezkallah (1999) developed two-phase flow transition models for micro-gravity channel flows based on liquid and gas Weber numbers. Zhao and Rezkallah (1993) suggested Wees 1 as the upper boundary for the surface tension-dominated zone, and Wees 20 as the lower boundary for the inertia-dominated zone. 

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Chung PM-Y, Kawaji M (2004) The effect of channel diameter on adiabatic two-phase flow characteristics in micro-channels. Int J Multiphase Flow 30 735-761 Colgan E (2005) A practical implementation of silicon microchannel coolers for high power chips. 

Two-phase flow characteristics of capillaries are known to be significantly different from the characteristics of larger channels, and consequently the existing vast literature associated with the phenomenology of change-of-phase heat transfer and two-phase flow hydrodynamic processes generally do not apply to capillaries. 

Comparison of Gas-Liquid Two-Phase Flow Characteristics Between Conventional Size Channels and Micro-Channels... 

Me et al. (2006) addressed the differences in gas-liquid two-phase flow characteristics that occur in conventional size channels and micro-channels by examining the two-phase flow pattern, interfacial wave, void fraction and friction pressure drop data obtained in circular and rectangular channels with a hydraulic diameter ranging from 50 pm to 6.0 mm. 

The experimental data obtained in conventional size channels and micro-channels with diameters between 100 pm and 6.0 mm are examined to further elucidate and understand the differences in two-phase flow characteristics between the microchannels and conventional size channels. Since two separate sets of experiments have been conducted using air and water in acrylic channels with diameters between 500 pm and 6.0 mm, and nitrogen gas-water in fused silica channels with diameters between 50 and 500 pm, the authors refer to the former channels as conventional size channels, and the latter channels as micro-channels for convenience. Two different inlet sections were covered in micro-channel experiments, a gradually reducing section and a T-junction. 

Ide H, Kawahara A, Kawaji M (2006) Comparison of gas-liquid two-phase flow characteristics between mini-channels and micro-channels. In Proceedings of 13th International Heat Transfer Conference, Sydney Convention and Exibition Centre, Sydney, Australia, 13-18 August 2006, MPH-51... 

For sufficiently large heat flux to mass flow ratios, the nucleation mechanism predominates and the heat transfer becomes independent of the two-phase flow characteristics of the system. Thus at large values of the boiling number, the heat transfer coefficients are virtually independent of the Lockhart-Martinelli parameter, Xn. 

During vaporization or condensation, thermal and hydraulic performances depend essentially on the two-phase flow structure. Furthermore, as very often in industrial processes, the heat exchanger operates in thermosyphon or under natural circulation knowledge of the pressure drop and liquid holdup is of major importance. But there is very little information on two-phase flow characteristics in compact geometries. 

Margat L, Thonon B, Tadrist L. Heat transfer and two-phase flow characteristics during convective boiling in a corrugated channel In Shah R.K., ed. Compact Heat Exchangers for the Process Industry. Begell House, 1997 323-329. 

Winkelmann D, Thonon B, Auracher H, Bontemps A. Two-phase flow characteristics of a corrugated channel. Proceedings of the 20th International Congress of Refrigeration, IIR/IIF, Sydney, 1999. 

Waelchli, S. Rudolf von Rohr, P. 2006. Two-phase flow characteristics in gas—liquid microreactors. International journal of multiphase flow, 32, 791-806. 

The effect of channel diameter on adiabatic two-phase flow characteristics in microchannels. Int. J. Multiphase Flow,... 

Waelchli S and Rohr PRV (2006), Two-phase flow characteristics in gas-liquid microreactors , Int J Multiphas Flow, 32,791-806. 

Cho, S. and Wang, Y. Two-phase flow characteristics in a gas flow channel of polymer electrolyte fuel cells 1 carbon paper diffusion media, submitted. 

In this chapter, we summarize our general review of gas-water two-phase flow characteristics in the surf zone. There are several measurement methods for the aeration in the surf zone, such as in situ measurements, video or photographic measurements, laser measurements, and acoustic measurements. There is no one appropriate method to measure the volume of air and bubbles in the surf zone. The spatial and temporal characteristics of void fraction are summarized, and the bubble size distribution is described with simple theories. Furthermore, the gas transfer prediction method is presented as a function of the bottom slope and wave characteristics for the surf zone waves. 

As shown by different groups one can in general distinguish between different two-phase flow characteristics inside a micro-chaimel. " One can imagine that the flow characteristics have a strong impact on the transport properties inside the... 

Different two phase flow characteristics in a channel, (a) Bubbly flow, (b) Wedging flow, (c) Slug flow. 

Xu JH, Luo GS, Chen GG, Tan B. 2005. Mass transfer performance and two-phase flow characteristic in membrane dispersion mini-extractor. J Membr Sci 249 75-81. 

In PEFC porous media, the pore size distribution and surface properties are tailored to achieve the desired two-phase flow characteristics. Typically, highly hydrophobic PTFE is added to the naturally hydrophilic diffusion media structure. The result is a mixed hydrophobic-hydrophilic surface and internal structure. Figure 5.30 shows a microscopic view of a wet diffusion media material. 

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