Slug-micro flow

Cas/Liquid Micro Flow Dispersive Mixers Generating Slug and Annular Patterns... 

The effect of fluid properties and wetting properties of the channel wall on the flow regime and flow pattern transition was studied [72]. For lower hydrophilidty of the channel surface, the transition boundaries of the slug flow to the slug-annular flow and the slug-annular flow to the annular flow shift to lower Red Rei Figure 9.16). The effect of wettability on micro-fluidics in the surface tension-dominated zone was more apparent than that in the inertia-dominated zone. Both surface tension and viscosity of the liquid are influencing the flow pattern transitions. 

How patterns in conventional size channels deviate significantly from those in micro-channels. Slug and annular flow constitute dominant flow patterns in con-... 

In the study by Qu et al. (2004), experiments were conducted with adiabatic nitrogen-water two-phase flow in a rectangular micro-channel. The bubbly, stratified and churn flow patterns commonly encountered in macro-channels were never observed in the study. No water droplets were observed in the nitrogen bubble, nor were any nitrogen bubbles present in the water slugs. 

For all flow conditions tested in that study, a bubbly flow pattern with bubbles much smaller than the channel diameter (100 pm) was never observed. While liquid-only flows (or liquid slugs) containing small spherical bubbles were not observed, small droplets were observed inside gas core flows. Furthermore, no stratified flow occurred in the micro-channel as reported in previous studies of two-phase flow patterns in channels with a diameter close to 1 mm (Damianides and Westwater 1988 Fukano and Kariyasaki 1993 Triplett et al. 1999a Zhao and Bi 2001a). 

The flow patterns (expansion of the bubbly, slug and annular regions of flow) affect the local pressure drop, as well as the pressure oscillations in micro-channels (Kandlikar et al. 2001 Wu and Cheng 2003a,b, 2004 Qu and Mudawar 2003 Hetsroni et al. 2005 Lee and Mudawar 2005a). 

Evaporative two-phase flow in a heated micro-channel resembles a two-phase slug flow with distinct domains of liquid and vapor. These domains are divided by the infinitely thin evaporating front, which propagates relatively to the fluid with a velocity u f equal (numerically) to the linear rate of liquid evaporation. In the frame of reference associated with micro-channel walls, the velocity of the evaporation front is... 

When symmetry is given, there is no need to consider the flow in a micro device or even a micro channel as a whole, but favorably one can refer to only a small part of it and regard this as a true functional region. A first-approach description for slug flow, according to the above consideration, may therefore refer to the repeti-... 

This class of hybrid components comprises chip micro-reactor devices, as described in Section 4.1.3, connected to conventional tubing. This may be H PLC tubing which sometimes has as small internals as micro channels themselves. The main function of the tubing is to provide longer residence times. Sometimes, flow through the tube produces characteristic flow patterns such as in slug-flow tube reactors. Chip-tube micro reactors are typical examples of multi-scale architecture (assembly of components of hybrid origin). 

P 53] Before operation, a start-up time of about 10 min was applied to stabilize pressure in the chip micro reactor ([R 6]) [20]. As a result, a stable flow pattern was achieved. The reactant solutions were filled into vials. Slugs from the reactant solutions were introduced sequentially into the micro chip reactor with the autosampler and propelled through the chip with methanol as driving solvent. The flow rates were set to 1 pi min The slug volume was reduced to 2.5 pi. 

The chip micro reactor ([R 6]) was only one part of a complex serial-screening apparatus [20]. This automated system consists of an autosampler (CTC-HTS Pal system) which introduces the reactant solutions in the chip via capillaries. A pumping system (p-HPLC-CEC System) serves for fluid motion by hydro dynamic-driven flow. A dilution system [Jasco PU-15(5)] is used for slug dilution on-chip. The detection system was a Jasco UV-1575 and analysis was carried out by LC/MS (Agilent 1100 series capLC-Waters Micromass ZQ). All components were on-line and self-configured. 

Figure 5.6 Flow pattern map for a gas/liquid flow regime in micro channels. Annular flow wavy annular flow (WA) wavy annular-dry flow (WAD) slug flow bubbly flow annular-dry flow (AD). Transition lines for nitrogen/acetonitrile flows in a triangular channel (224 pm) (solid line). Transition lines for air/water flows in triangular channels (1.097 mm) (dashed lines). Region 2 presents flow conditions in the dual-channel reactor ( ), with the acetonitrile/nitrogen system between the limits of channeling (I) and partially dried walls (III). Flow conditions in rectangular channels for a 32-channel reactor (150 pm) (T) and singlechannel reactor (500 pm) (A) [13].

A hydrodynamic characterization of the micro reactor is given in [12], A flow-pattem map reveals the existence of dispersed flow, annular flow, slug-dispersed... 

Benchmarking of the micro reactors themselves - slug flow vs. falling film... 

GL 18] [R 6b] [P 18] Using a stoichiometric amount of hydrogen and operating in the slug-flow mode, it was shown that the yield of a micro reactor exceeds that of a mini fixed-bed reactor (LHSV = 60 h ) [61]. A maximum yield of 30% was obtained for the micro reactor for the range of pressure investigated (10-35 bar). 

In advance, comparative fixed-bed measurements were undertaken. It was ensured that the performance of a plug-flow operation with both flows having the same direction is superior to trickle-bed operation, using counter-flow instead. The plug flow was assumed to model the slug-flow behavior in the micro reactor. 

Tracey, M. C., Cox, T. I., Davis, J. B., Microfluidic mixer employing temporally interleaved liqiuid slugs and parabolic flow, in Ramsey, J. M., van den Berg, A. (Eds.), Micro Total Analysis Systems, Kluwer, Dordrecht, 2001,141-141. 

A flow-pattern map was derived for nitrogen/acetonitrile flows in the dual-channel micro reactor [274]. Bubbly, slug, churn and annular flows as well as wavy annular and wavy annular-dry flows with smaller region of stability were found (see Figure 4.35). 

The high-pressure and temperature micromodel system has been used in this study to investigate the formation, flow behavior and stability of foams. Micromodel etching patterns were made from binary images of rock thin sections and from other designs for a comparison of pore effects. These experiments show how simultaneous injection of gas and surfactant solution can give better sweep efficiency on a micro-scale in comparison to slug injection. 

The heat transfer mechanisms that are active in boiling in micro-channels can be summarized as follows (i) in bubbly flow, nucleate boiling and liquid convection would appear to be dominant, (ii) in slug flow, the thin film evaporation of the liquid film trapped between the bubble and the wall and convection to the liquid and vapor slugs between two successive bubbles are the most important heat transfer mechanisms, also in terms of their relative residence times, (iii) in annular flow, laminar or turbulent convective evaporation across the liquid film should be dominant, and (iv) in mist flow, vapor phase heat transfer with droplet impingement will be the primary mode of heat transfer. For those interested, a large number of two-phase videos for micro-channel flows from numerous laboratories can be seen in the e-book of Thome [22]. 

Thome et al. [8] developed a phenomenological method to describe heat transfer for a purely convective micro-channel slug flow (no nucleate boiling), based principally on the following assumptions ... 

Current experimentation on micro-channel two-phase flows has provided some evidence of the heat transfer mechanisms that govern the micro-scale flow boiling process (i) at low vapor qualities, when bubbly flow is the dominant flow pattern, thermal transport is primarily associated to nucleate boiling, (ii) at intermediate vapor qualities, with the intermittent passage of elongated bubbles and slugs of liquid, heat is transferred by single phase... 

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