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Analytical and numerical study of plug flow inside round/concentric microchannels Cao, Yadi


Plug flow can generate recirculating flow between interfaces of immiscible fluids. Depending on the phase used to segment flow, it can be gas-liquid plug flow or liquid-liquid plug flow. The recirculating flow can enhance heat transfer as compared to the continuous pipe flow, especially in the laminar regime such as the microchannel flow. The present work focuses on the flow and heat transfer of liquid plugs with low Reynolds numbers. The flow is modeled by applying Stokes simplification, and the solution is obtained by solving fourth-order partial differential equation sets. Solutions of two types of plug flow are obtained: 1) the gas-liquid plug flow in the concentric microchannel; 2) the liquid-liquid plug flow in the circular microchannel. For the gas-liquid plug flow study, the flow patterns inside the liquid phase including the volume ratio of the inner and outer vortexes, the ratio of maximum-to-minimum stream functions, the averaged recirculation flux as well as the skin friction coefficient are investigated in details. Correlations for predicting the maximum and minimum of the stream function are developed. For the liquid-liquid plug flow study, the influences of plug lengths and the viscosity ratio upon the cap vortexes and the overall skin friction coefficient are studied in details. The heat transfer of the gas-liquid plug flow in the concentric microchannel is simulated numerically in MATLAB. Three types of thermal boundary conditions are investigated. The developing process of the thermal field can be explained using a simple thermal network for each boundary condition. The influences of parameters including the plug aspect ratio, the channel inner-outer radius ratio and the Peclect number upon the thermal conductance and heat transfer enhancement to the single-phase flow are investigated systematically. Then a simplified model for the fully developed thermal field is extracted for the quick calculation need in the design work. The results obtained from about 12,000 cases form a database that can be used in the future design work of heat exchanger based upon this kind of flow.

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