Showing papers on "Slug flow published in 2019"

Multi-Fluid VoF model assessment to simulate the horizontal air–water intermittent flow

Abstract: Numerical simulation of the plug and the slug flow regimes have been performed using the Multi-Fluid VoF and Shear Stress Transport (SST) k–ω models in a horizontal pipe with 44 mm diameter. The superficial velocities during the current study were set at 0.16, 1.64 & 3 m/s for the gas phase and 1 m/s for the liquid one. The pressure and the velocity equations were solved together, utilizing the PIMPLE algorithm in all cases of the present study. The VoF model, as well as the experimental outcomes, were applied to assess the results of Multi-Fluid VoF model. The qualitative comparison of numerical results with the experimental visualization revealed that the Multi-Fluid VoF model simulates precisely the interfacial structure of slug and plug flows, as well as the chronological formation of the slug. The length ratio of the gas slug to the liquid slug goes up with the gas superficial velocity increment; this issue increases the probability at low liquid hold-ups and decreases the higher ones. The Multi-Fluid VoF provided much more matching with the mentioned probability changes in comparison to the VoF model. The pressure drop calculated using Multi-Fluid VoF in the Lockhart–Martinelli framework illustrated a 21.8% improvement averagely in comparison to the VoF model. The obtained transitional slug velocities showed that the Multi-Fluid VoF model presented a maximum error of 8.17% versus the experimental values, while the mentioned error for the VoF model was estimated at 22.01%. Notwithstanding the mentioned advantages, the Multi-Fluid VoF model significantly increased, both the execution time of simulation as well as the associated costs.

Flow mediated metal-free PET-RAFT polymerisation for upscaled and consistent polymer production

Abstract: Four commercially available xanthene based organo-dyes, eosin Y, erythrosin B, phloxine B, and rose bengal, were investigated for their ability to catalyse PET-RAFT polymerisation under different process conditions. The dyes were shown to be efficient photocatalysts under batch and flow conditions without prior deoxygenation, however, polymerisation under flow conditions provided polymers with narrower molecular weight distributions in shorter irradiation times. Additionally, running the flow reactor under fully-continuous or semi-continuous (slug flow) operation led to large discrepancies in product quality (composition and dispersity of synthetic polymers) over the course of the reaction; using the slug flow mode drastically increased the consistency of polymer product compared to the fully-continuous process, and repressed the formation of unwanted high molecular weight products that can potentially cause reactor fouling. The superior product formation under slug flow conditions was attributed to the increase in mixing due to recirculatory fluid flow, which homogenised the residence time distributions of the reaction mixtures.

Numerical investigation on thermal characteristics and flow distribution of a parallel micro-channel separate heat pipe in data center.

Abstract: This study focuses on the flow distribution and heat transfer characteristics of the evaporation terminal of a micro-channel separate heat pipe (MCSHP) used in data center. A complete CFD model for the parallel tubes with simplified louvered fins of the evaporator structure was established to consider the thermal enhancement while reduce the computing costs. The CFD model was validated by comparing the cooling capacity and outlet temperature of MSCSHP with experiments. The transformation process from bubble flow to slug flow was visualized using VOF model. The simulations reveal that the maximum cooling capacity of the studied MCSHP system could reach 9610 W when the corresponding optimal refrigerant filling ratio is 65.27%. The generation and motion of vapor bubbles influence the height of liquid pool in the evaporation terminal, thus leaded to the change of effective heat transfer area. Moreover, it is found that configurations of inlet and outlet have great impacts on the distribution of mass flow and heat flux under low filling ratios. While under higher filling ratios, the heat flux distribution is not affected by these factors due to the flat tubes filled with two-phase refrigerant.

Liquid-liquid two-phase flow patterns in ultra-shallow straight and serpentine microchannels

Abstract: Water-butanol and water-hexane flows were visualized in ultra-shallow straight and serpentine microchannels with a cross-junction. At the inlet cross-junction, three major flow patterns including tubing/threading, dripping and jetting were mapped using the aqueous Capillary number versus the organic Weber number. Correspondingly, in the main microchannel, annular flow, slug flow and droplet flow were mapped using combined dimensionless numbers (Weber number times Ohnesorge number) of both phases. The flow pattern transitions were explained based on a force analysis, considering the phase flow rates, junction angle between the side feeding channels and the central feeding channel as well as aspect ratios. Compared to the straight microchannel, the dripping regime at the inlet junction and the slug flow occupy larger zones in serpentine microchannels because the centrifugal force tends to break up the organic annular core into slugs and droplets over the bends.