Bubble Dynamics In Microchannels Research Articles

Flow boiling heat transfer characteristics of two-phase flow in microchannels

A set of experimental platforms with widths of 0.5, 1.0, 1.5, and 2.0 mm was established to explore the mechanism of flow boiling bubble dynamics in microchannels, focusing on heat transfer characteristics, pressure loss, and two-phase flow pattern identification. Bubble flow, restricted bubble flow, and dry area were observed in all four channels. The appearance of flow pattern was related to flow rate and channel width. Under the condition of the same channel width, the initial heat flux of subcooled boiling gradually increased with increase in flow rate, and this change trend was close to the linear trend. Under the same flow rate, the initial heat flux of subcooled boiling increased with decrease in channel width. This condition was due to the faster flow rate of the working medium in the narrow channel, resulting in decrease of heating time. The increase in bubble generation frequency directly led to the increase in the wall heat transfer coefficient and the decrease in the bubble separation diameter. Mathematical analysis showed that under the condition of small flow, reduction of channel size led to reduction of the total wall heat transfer coefficient. In this condition, reduction of channel size cannot enhance heat transfer. With increasing volume flow rate, the range of hydrodynamic control area increased and the index decreased. When the flow rate was large, the total heat transfer coefficient increased greatly with the decrease in channel size. The theoretical values were in good agreement with the experimental data.

Bubble dynamics in microchannel: An overview of the state-of-the-art

The inception of the boiling, in a pool or flow boiling, is the formation of the vapor bubble at an active nucleation site that plays a crucial role in the boiling process and it becomes critical and unfolds many facets when channel size reduces to submicron. The detailed knowledge of the bubble dynamics is helpful in establishing the thermal and hydraulic flow behavior in the microchannel. In the current paper, bubble dynamics that include bubble nucleation at the nucleation site, its growth, departure, and motion along the flow in a microchannel(s) are discussed in detail. Different models developed for critical cavity radius favorable for bubble nucleation are compiled and observe that models exhibit large deviation. The bubble growth models are compiled and concluded that the development of a more generalize bubble growth model is necessary that would be capable of accounting for inertia controlled and thermal diffusion controlled regions. Bubbles at nucleation sites in a microchannel grow under the influence of various forces such as surface tension, inertia, shear, gravitational and evaporation momentum. Parametric analysis of these forces reckoned that the threshold between macro- to microchannel could be identify through critical analysis of such forces. Eventually, the possible impact of the various factors such as operating conditions, geometrical parameters, thermophysical properties of fluid on bubble dynamics in microchannel has been reported.

Bubble dynamics in microchannels: inertial and capillary migration forces

This work focuses on the dynamics of a train of unconfined bubbles flowing in a microchannel. We investigate the transverse position of the train of bubbles, its velocity and the associated pressure drop when flowing in a microchannel, depending on the internal forces due to viscosity, inertia and capillarity. Despite the small scales of the system, the inertial migration force plays a crucial role in determining the transverse equilibrium position of the bubbles. Besides inertia and viscosity, other effects may also affect the transverse migration of bubbles, such as the Marangoni surface stresses and the surface deformability. We look at the influence of surfactants in the limit of infinite Marangoni effect, which yields a rigid bubble interface. The resulting migration force may balance external body forces, if present, such as buoyancy, centrifugal or magnetic ones. This balance not only determines the transverse position of the bubbles but, consequently, the surrounding flow structure, which can be determinant for any mass/heat transfer process involved. Finally, we look at the influence of the bubble deformation on the equilibrium position and compare it with the inertial migration force at the centred position, explaining the stable or unstable character of this position accordingly. A systematic study of the influence of the parameters, such as the bubble size, uniform body force, Reynolds and capillary numbers, has been carried out using numerical simulations based on the finite element method, solving the full steady Navier–Stokes equations and their asymptotic counterparts for the limits of small Reynolds and/or capillary numbers.

Simulation of bubble dynamics in a microchannel using a front-tracking method

By using a front-tracking approach for moving boundaries, whose surface properties are solved in terms of an immersed-boundary method, the dynamics of bubble transport in a microchannel is computationally studied. This methodology allows the simulation of a liquid–gas system with a realistically large density ratio. In accordance with the pressure-driven inlet/outlet condition generally encountered in experiments, a projection method enforcing incompressibility is implemented as the solution scheme. The approach is then applied to two unique problems of bubble dynamics in microchannels. The first is concerned with bubble transport in a channel with sudden contraction. A bubble slug is placed in the microchannel embedded with a pair of side blocks. In the flow driven by pressure difference, the bubble slug would pass through or be stuck by the blocks, depending on the variations of Reynolds number and Weber number. With such variations of key parameters, furthermore, the bubble slug may deform in different manners. In the second part, the ascending dynamics of multiple bubbles is investigated, specifically regarding their interactions. It is found that in the confined space of small scale, the behaviors of bubbles are constrained by the walls, and not much interaction between bubbles is observable particularly when the flow is dominated by an imposed pressure difference. If the channel is sufficiently wide, for a pair of rising bubbles which are lined in a row, substantial interactions between the bubbles and the walls are observed. By changing the dimensionless parameters of rising bubbles and the channel width, variations in bubble shape, rising trajectory, and the wakes behind bubbles are discussed based on such essential mechanisms as the interplay of vortices and nonuniform pressure field. Moreover, a third bubble is inserted at the center and the flow structure is analyzed.

Experimental Techniques for Bubble Dynamics Analysis in Microchannels: A Review

Experimental studies employing advanced measurement techniques have played an important role in the advancement of two-phase microfluidic systems. In particular, flow visualization is very helpful in understanding the physics of two-phase phenomenon in microdevices. The objective of this article is to provide a brief but inclusive review of the available methods for studying bubble dynamics in microchannels and to introduce prior studies, which developed these techniques or utilized them for a particular microchannel application. The majority of experimental techniques used for characterizing two-phase flow in microchannels employs high-speed imaging and requires direct optical access to the flow. Such methods include conventional brightfield microscopy, fluorescent microscopy, confocal scanning laser microscopy, and micro particle image velocimetry (micro-PIV). The application of these methods, as well as magnetic resonance imaging (MRI) and some novel techniques employing nonintrusive sensors, to multiphase microfluidic systems is presented in this review.

Computational fluid dynamics (CFD) software tools for microfluidic applications – A case study

This paper reports on an exemplary study of the performance of commercial computational fluid dynamic (CFD) software programs when applied as engineering tool for microfluidic applications. Four commercial finite volume codes (CFD-ACE+, CFX, Flow-3D and Fluent) have been evaluated by performing CFD-simulations of typical microfluidic engineering problems being relevant for a large variety of lab-on-a-chip (LOAC) applications. Following problems are considered as examples: multi lamination by a split and recombine mixer, flow patterning on a rotating platform (sometimes termed “lab-on-a-disk”), bubble dynamics in micro channels and the so called TopSpot ® droplet generator for micro array printing. Hereby mainly the capability of the software programs to deal with free surface flows including surface tension and flow patterning of two fluids has been studied. In all investigated programs the free surfaces are treated by the volume-of-fluid (VOF) method and flow patterning is visualised with a scalar marker method. The study assesses the simulation results obtained by the different programs for the mentioned application cases in terms of consistency of results, computational speed and comparison with experimental data if available.

Bubble dynamics in microchannels. Part II: two parallel microchannels

The present work investigates experimentally the bubble dynamics in two parallel trapezoidal microchannels with a hydraulic diameter of 47.7 μm for both channels. The fabrication process of the two parallel microchannels employs a silicon bulk micromachining and anodic bounding process. The results of this study demonstrate that the bubble growth and departure is generally similar to that in a single microchannel, i.e., bubbles, in general, grow linearly with time and their departure is governed by surface tension and drag due to bulk two-phase flow. For the two low mass flow rates, the growth of bubble in slug flow is also investigated. It is found that the bubble grows in the axial direction both forward and backward with its length increases exponentially due to evaporation of the thin liquid film between the bubble and heating wall. However, the coefficient of exponent is much smaller than that caused by evaporation due to the limitation effect of liquid pressure around the bubble.

Bubble dynamics in microchannels. Part I: single microchannel

The present work explores experimentally bubble dynamics in a single trapezoid microchannel with a hydraulic diameter of 41.3 μm. The fabrication process of the microchannel employs a silicon bulk micromachining and anodic bounding process. Bubble nucleation, growth, departure size, and frequency are observed using a high speed digital camera and analyzed by the Image-Pro. The results of the study indicates that the bubble nucleation in the microchannel may be predicted from the classical model with microsized cavities and the bubble typically grows with a constant rate from 0.13 to 7.08 μm/ms. Some cases demonstrate an extraordinarily high growth rate from 72.8 to 95.2 μm/ms. The size of bubble departure from the microchannel wall is found to be governed by surface tension and drag of bulk flow and may be fairly correlated by a modified form of Levy equation. The bubble frequency in the microchannel is comparable to that in an ordinary sized channel. The traditional form of frequency–departure-diameter relationship seems to be inexistent in the microchannel of this study.