Characterization of air-water flow in mini-scale serpentine geometries

Title: Characterization of air-water flow in mini-scale serpentine geometries
Authors: Donaldson, Adam
Date: 2010
Abstract: Advancements in chemical engineering processing focused on miniaturization of reactor flow systems has led to the adoption of mixing arrangements commonly employed in single phase applications into multi-phase systems. The planar serpentine channel is one such arrangement, found in compact heat exchangers, turbines, mini/micro-reactors and fuel cells. While extensive literature is available for serpentines with single phase flow, a limited understanding of the hydrodynamics and mass transfer characteristics of two phase flow exists. The curvature-induced flow pattern transitions, pressure drop, potential enhancement of mass transfer and suitability of common multi-phase CFD approaches were studied in this work for 1 mm circular channels with serpentine configurations consisting of consecutive return bends with radii of curvature of 3 and 6 mm. The flow patterns generated by a 1 mm circular cross-flow T-junction were analyzed using high-speed imaging and compared to those observed within the serpentine geometries. The variation in flow behavior was illustrated in bubble breakup maps identifying the shifts in flow pattern transition boundaries and the onset of bubble breakup. The interactions between the interface and curvature-induced secondary flow were described by a combination of the Weber number and the characteristic length for single-phase flow in curved geometries, where critical values of WeLGLC = 3, WeLGLC = 10 and WeLGL C = 15 are proposed for the onset of deformation, intermittent breakup, and continuous breakup, respectively. Pressure drop measurements obtained for single and two-phase flow in straight and serpentine geometries were used to develop a semi-empirical separated flow model to predict two-phase friction factors. Five operating regions were identified, each having distinct friction factor dependencies on WeLG, LC and epsilonG. The transitions between these regions depended on the phase holdup and/or the critical WeLGLC observed for the onset of deformation and continuous breakup. The parametric dependency of gas-liquid inter-phase mass transfer in idealized Taylor flow was numerically investigated. The relative contributions of film and cap mass transfer at conditions suitable to industrial applications indicate that neither mechanism can be neglected when concentration polarization within the bubble film and mass transfer resistance between the slug film and bulk are accounted for. The introduction of curvature is predicted to increase overall mass transport through shear-induced deformation and secondary flow by up to 7% based on highly conservative assumptions. Finally, the diffuse interface (DI) CFD model is introduced as a suitable interface tracking technique for modeling immiscible fluids in surface tension dominated flow. Key modifications are proposed to eliminate fundamental issues of the traditional DI approach, relaxing numerical constraints responsible for limiting practical applications to highly structured 2D and axisymmetric geometries. The new methodology proposed in this thesis was implemented in OpenFOAMRTM, and validated for benchmark simulations including scalar transport, free surface flow and droplet deformation in shear.
CollectionTh├Ęses, 1910 - 2010 // Theses, 1910 - 2010
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