Multiphase fluid flow and heat transfer characteristics in microchannels

The boiling flow or condensation is widely encountered in many industrial applications for both cooling as well as heating processes. Compact heat transfer devices, such as micro-heat exchangers and evaporators, are extensively used for both cooling as well as heating processes over conventional heat exchangers, such as microelectronic circuits, automobile and aerospace industries, due to high surface area to volume ratio and heat transfer rates, compactness and easy thermal control. For better design of micro- or mini-heat exchangers, a detailed specific knowledge of the multiphase flow and its properties such as the flow pattern during flow boiling, critical heat flux (CHF) and stable operation are very important. This paper provides a state of art review on boiling flow in microchannels since year 2000 till date. Flow patterns formed and the parameters influencing flow pattern transitions, during multiphase heat transfer in micro- or mini-channels, have been reviewed in detail. The flow regimes and flow pattern maps, and modeling approaches considered for boiling flow in micro-channels/devices with various challenges have been discussed. A lot of contradiction between the experimental data has been observed for the analysis of flow regimes and flow pattern maps. Further, the effect of hydrodynamics during flow boiling and CHF on heat transfer coefficient has been discussed in detail. Recently, with the advancement in measurement techniques, the heat transfer measurement technologies have been synchronized with the visualization techniques, which helped in understanding the boiling flow physics in micro- and mini-channels. Therefore, an in-depth understanding of flow patterns and regimes under boiling flow conditions in mini- and micro-channels can be used to predict the boiling heat transfer mechanism, which can be further used for developing better heat transfer models for boiling flow. Further, enhancement in heat transfer coefficient for boiling flow in microchannels, either by using complex microchannel configurations or nanocoating on the microchannel surface, have received attention recently which have been discussed and analyzed in the present review. Both micro- and mini-channels have number of applications in aerospace, refrigeration and computational systems; therefore further attention is needed for more robust and precise design

Applicability of the axial dispersion model to coiled flow inverters containing single liquid phase and segmented liquid-liquid flows

Residence time distribution (RTD) curves for coiled flow inverters (CFIs), helically coiled tubes (HCT), and straight tubes (ST) with single phase and segmented liquid-liquid flows were studied with pulse injection employing laser optical and conductivity detection. Reactor design and volume rates were chosen to cover conditions relevant for practical laboratory investigations, such as channel diameters between 0.8 mm and 3.2 mm, volume rates between 1 and 360 ml/min, 14 ≤ Re ≤ 2414, 4 ≤ De ≤ 451 and a number of up to 60 flow inversions for CFIs. RTD curves were examined in terms of moment analysis and time-domain least squares fitting. For single phase flows deviations from the axial dispersion model (ADM) were observed for ST and HCT and rationalized in terms of Fo number. For the CFIs consistency of the RTD curves with the ADM was observed over the entire range of experimental parameters due to strong chaotic mixing. For segmented liquid-liquid flow simultaneous measurements of RTD curves and slug lengths were performed. The length of the slugs decreased with increasing flow rate. The RTD curves showed good consistency with ADM throughout the experiments for all reactors investigated. Observed Bo numbers decreased at relatively low flow rates in the order CFI > HCT > ST while at relatively high flow rates Bo for HCT and CFI became similar but still higher than ST. Ca numbers indicated that mixing between the vortex and film regions probably dominated the dispersion at low flow rates, while at high flow rates dispersion was likely to be dominated by the film thickness

Liquid-liquid extraction system with microstructured coiled flow inverter and other capillary setups for single-stage extraction applications

Process intensification via miniaturization has become an attractive research field for industry and R&D especially for the production of fine chemicals and pharmaceuticals due to enhanced mass and heat transport. Fabrication of helically coiled tubular devices (HCTDs) in micro-scale can further enhance the mass and heat transfer due to the formation of secondary flow profile at laminar flow. Liquid–liquid (L–L) mass transfer performance of different microstructured HCTDs were investigated for slug flow patterns. A complete microextraction system was constructed and characterized including a T-junction (T-mixer) for slug flow generation, HCTDs as residence time units (RTUs), and a continuously working in-line phase splitter for an instantaneous phase separation. RTUs were fabricated by using fluorinated ethylene propylene (FEP) tubes (ID = 1 mm). EFCE test system, namely, n-butyl acetate/acetone/water system was chosen as an extraction system for the mass transfer characterization. The total volumetric flow rate and the volumetric flow ratio of aqueous to organic phase were varied in the range of 1–8 mL min−1 and 0.5–2.0, respectively. Effects of residence time, flow ratio, and the generation of secondary flow profile, i.e. Dean vortices on L–L mass transfer were investigated and results were compared with straight capillaries. Results revealed that a certain type of HCTD, i.e. coiled flow inverter (CFI) offers higher extraction efficiencies up to 20% in comparison to straight capillaries at constant residence times. Additionally, it was found that for slug flow patterns, Dean vortices provide enhanced L–L mass transfer compared to Taylor vortices that occur in straight capillaries. A complete, continuously operated microextraction system was developed for single-stage applications, where very small liquid hold-ups and longer residence times are required due to slower mass transfer rates