Intensification of mass transfer from liquid to capillary wall by Taylor vortices in minichannels, bubble velocity and pressure drop

Gas–Liquid Two-Phase Flow and Heat Transfer without Phase Change in Microfluidic Heat Exchanger

This work presents an experimental study of the possibility of intensifying in microfluidic heat exchangers (MFHE) by creating a two-phase segmented flow (gas–liquid). Measurements of convective heat transfer were carried out using an MFHE, consisting of six channels 1 × 1 mm. Experimental studies have shown that segmented flow makes it possible to increase the Nusselt number of a laminar flow in MFHE up to 1.67 and reduce thermal resistance up to 1.7 times compared to single-phase flow. At the same time, it was found that the intensification of heat exchange by a two-phase flow is observed only for the range of the volume fraction of gas from 10 to 30%. In addition, the calculation of the thermal performance criterion, including both thermal and hydraulic parameters (friction factor), also confirmed the promise of using the Taylor segmented flow as a method for single-phase heat transfer intensifying in microchannels.

Isolated Taylor Bubbles in Co-Current with Shear Thinning CMC Solutions in Microchannels—A Numerical Study

Slug flow is a multiphase flow pattern characterized by the occurrence of long gas bubbles (Taylor bubbles) separated by liquid slugs. This multiphase flow regime is present in many and diversified natural and industrial processes, at macro and microscales, such as in eruption of volcanic magmas, oil recovery from pre-salt regions, micro heat exchangers, and small-sized refrigerating systems. Previous studies in the literature have been mostly focused on tubular gas bubbles flowing in Newtonian liquids. In this work, results from several numerical simulations of tubular gas bubbles flowing in a shear thinning liquid in microchannels are reported. To simulate the shear thinning behavior, carboxymethylcellulose (CMC) solutions with different concentrations were considered. The results are compared with data from bubbles flowing in Newtonian liquids in identical geometric and dynamic conditions. The numerical work was carried out in computational fluid dynamics (CFD) package Ansys Fluent (release 16.2.0) employing the volume of fluid (VOF) methodology to track the volume fraction of each phase and the continuum surface force (CSF) model to insert the surface tension effects. The flow patterns, the viscosity distribution in the liquid, the liquid film thickness between the bubble and the wall, and the bubbles shape are analyzed for a wide range of shear rates. In general, the flow patterns are similar to those in Newtonian liquids, but in the film, where a high viscosity region is observed, the thickness is smaller. Bubble velocities are smaller for the non-Newtonian cases.

Process intensification in chemical engineering: general trends and Russian contribution

Minimization of the costs with simultaneous increase in the raw materials and energy use efficiency is a challenge for the modern world. One of the most effective tools to solve this task is the use of process intensification (PI), first proposed by Ramshaw C. The incentive for process intensification, Proceedings, 1st Intl. Conf. Proc. Intensif. for Chem. Ind., 18, BHR Group, London, 1995, p. 1. and then extended by Stankiewicz AI, Moulijn JA. Process intensification: transforming chemical engineering. Chem Eng Prog 2000: 22–34. In the presented review, some principles of PI in chemical engineering and their application for wide variety of processes is discussed. The role of the Russian scientist with a research background is carried out in other countries.

Wide Range Simulation Study of Taylor Bubbles in Circular Milli and Microchannels

A deep knowledge of the hydrodynamics of two-phase flow in millichannels and microchannels is relevant to the design and control of micro structured equipment. While there is plenty of work published in this area, there is a lack of studies over a wide range of dimensionless numbers and some factors have not been properly addressed, such as the role of the Reynolds number, the features of recirculation regions in the liquid slug and the liquid film development length. Therefore, a wide range parametric study of isolated gas Taylor bubbles flowing in co-current with liquid in circular milli- and microchannels is presented, in a wide range of Capillary (CaB) (0.01–2) and Reynolds numbers (ReB) (0.01–700). The shape and velocity of the bubbles are, together with the flow patterns in the flowing liquid, analyzed and compared with numerical and experimental correlations available in the literature. For low values of CaB, the streamlines (moving reference frame (MRF)) in the liquid slug show semi-infinite recirculations occupying a large portion of the cross-section of the channel. The mean velocity of the fluid moving inside the external envelope of the semi-infinite streamlines is equal to the bubble velocity. For high values of CaB, there are no recirculations and the bubble is moving faster or at least at the velocity of the liquid in the center of the tube; this flow pattern is often called bypass flow. The results also indicate that the liquid film surrounding the bubbles is for low CaB and ReB numbers almost stagnant, and its thickness accurately estimated with existing correlations. The stagnant film hypothesis developed provides an accurate approach to estimate the velocity of the bubble, in particular for low values of CaB. The asymptotic behavior of the studied parameters enables the extrapolation of data for CaB lower than 0.01. In addition to the simulations of isolated bubbles, simulations with two consecutive bubbles were also carried out; coalescence was only observed in very specific conditions. The results obtained in this study are directly applicable to co-current slug flow in milli- and microchannels for 0.1 < ReB < 1000 and 0.02 < CaB < 2.