Instabilities In Microchannels Research Articles

Wetting and lubricating film instabilities in microchannels

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Pressure effects on flow boiling instabilities in parallel microchannels

The effects of pressure on flow boiling instabilities in microchannels were experimentally studied. Experiments were conducted using water in 223 μm hydraulic diameter microchannels with mass fluxes ranging from 86 to 520 kg/m 2 s and pressures ranging from 50 to 205 kPa. Onset of flow oscillation, critical heat flux (CHF) conditions, local transient temperature measurements along with flow boiling visualization were obtained and studied. System pressure was found to significantly affect flow instabilities. For high pressure, it was observed that boiling instabilities were significantly delayed and CHF was extended to high mass qualities. Local temperature measurements also revealed lower magnitudes and higher frequencies of oscillations at high system pressures.

Flow Boiling Instabilities in Microchannels and Means for Mitigation by Reentrant Cavities

The ability of reentrant cavities to suppress flow boiling oscillations and instabilities in microchannels was experimentally studied. Suppression mechanisms were proposed and discussed with respect to various instability modes previously identified in microchannels. It was found that structured surfaces formed inside channel walls can assist mitigating the rapid bubble growth instability, which dominates many systems utilizing flow boiling in microchannels. This, in turn, delayed the parallel channel instability and the critical heat flux (CHF) condition. Experiments were conducted using three types of 200×253μm2 parallel microchannel devices: with reentrant cavity surface, with interconnected reentrant cavity surface, and with plain surface. The onset of nucleate boiling, CHF condition, and local temperature measurements were obtained and compared in order to study and identify flow boiling instability.

Experimental Study and Model on Critical Heat Flux of Refrigerant-123 and Water in Microchannels

The present work is aimed toward understanding the effect of flow boiling stability on critical heat flux (CHF) with Refrigerant 123 (R-123) and water in microchannel passages. Experimental data and theoretical model to predict the CHF are the focus of this work. The experimental test section has six parallel microchannels, with each having a cross-sectional area of 1054×157μm2. The effect of flow instabilities in microchannels is investigated using flow restrictors at the inlet of each microchannel to stabilize the flow boiling process and avoid the backflow phenomena. This technique resulted in successfully stabilizing the flow boiling process. The present experimental CHF results are found to correlate best with existing correlations to overall mean absolute errors (MAEs) of 33.9% and 14.3% with R-123 and water, respectively, when using a macroscale rectangular equation by Katto (1981, “General Features of CHF of Forced Convection Boiling in Uniformly Heated Rectangular Channels,” Int. J. Heat Mass Transfer, 24, pp. 1413–1419). A theoretical analysis of flow boiling phenomena revealed that the ratio of evaporation momentum to surface tension forces is an important parameter. A theoretical CHF model is proposed using these underlying forces to represent CHF mechanism in microchannels, and its correlation agrees with the experimental data with MAE of 2.5%.

Electrohydrodynamic instabilities in microchannels with time periodic forcing

In microfluidic applications it has been observed that flows with spatial gradients in electrical conductivity are unstable under the application of sufficiently strong electric fields. These electrohydrodynamic instabilities can drive a nonlinear flow despite the low Reynolds number. Such flows hold promise as a simple mechanism for mixing fluids. In this work, the effect of a time periodic electric field on the instability is explored. The case where an electric field is applied across a diffuse interface of two fluids with varying electrical conductivity is considered. Frequency-dependent behavior is found only in the regime where the instability growth rates are very slow and cannot outpace mixing by molecular diffusion. Improving mixing by modulation of the electric body force is not a viable strategy in this geometry.

Direct numerical simulation of electrohydrodynamic flow instabilities in microchannels

Abstact Fluids with gradients in electrical conductivity subjected to strong electric fields can undergo violent instability. In flows where molecular diffusion is slow, high conductivity gradients can be maintained beyond the initial instability and become a driver for chaotic flow. Despite the low Reynolds number in microchannel applications, molecular diffusion occurs slow enough that such flows are possible. This work investigates a class of electrically driven, conductivity gradient flow instabilities in microchannels that display turbulent characteristics at low Reynolds number. Flows far from the influence of solid boundaries and flows confined between parallel plates are considered. Electroosmotic flow at solid boundaries is also considered and found to have a dramatic impact on the flow. A variety of configurations and assumptions are studied using direct numerical simulation.

Condensation flow patterns in silicon microchannels

A simultaneous visualization and measurement experiment was carried out to investigate condensation flow patterns of steam flowing through an array of trapezoidal silicon microchannels, having a hydraulic diameter of 82.8 μm and a length of 30 mm. The degassed and deionized water steam flowing in the microchannels was cooled by flowing water of 8 °C from the bottom. The silicon microchannels were covered with a thin transparent pyrex glass from the top which enabled the visualization of flow patterns. Experiments were performed at different inlet pressures ranging from 4.15 × 10 5 Pa to 1.25 × 10 5 Pa (with corresponding mass fluxes decreasing from 47.5 g/cm 2 s to 19.3 g/cm 2 s) while the outlet pressure was maintained at a value of 10 5 Pa. Different condensation flow patterns such as fully droplet flow, droplet/annular/injection/slug-bubbly flow, annular/injection/slug-bubbly flow, and fully slug-bubbly flow were observed in the microchannels. At a given inlet pressure and mass flux, the flow pattern depended on both the location and time. Of particular interest is that the vapor injection flow, consisting of a series of bubble growth and detachment activities, appeared and disappeared periodically. During the disappearance period of injection flow, the slug-bubbly flow at downstream changed to the single-phase liquid flow due to the reversed flow of outlet condensate, while the annular flow at upstream changed to the vapor flow due to the effect of incoming vapor. Therefore, two-phase flow and single-phase flow appeared alternatively in the microchannels, causing large fluctuations of wall temperatures as well as other measurements. It was also found that the occurrence of vapor injection flow moved from the outlet toward the inlet as the mass flux was decreased. The vapor injection flow and its induced condensation instabilities in microchannels are reported here for the first time.

An instability effect on two-phase heat transfer for subcooled water flowing under conditions of zero gravity : Papell, S. Stephen, Lewis Research Center, Cleveland, Ohio, NASA TN D-2259, November 1964, 11 p., graphs, charts and illus.

The present work has been undertaken with an aim to address major challenges during two-phase flow boiling in microchannels. A comprehensive review of literatures has been presented that highlights the problems of two-phase flow boiling and its suppression techniques. Emphasis has been given to the recent literatures addressing the issues of flow boiling instabilities in microchannels. The broad aspects that have been covered are overview of instabilities, their causes, consequences and suppression techniques. Major attention has been paid to highlight the promising solutions that have been investigated to mitigate the instabilities. Study revealed that inception of instabilities is inherent during flow boiling. Nevertheless, it is more sensitive to trigger-out in microchannels compare to macro or conventional channels. Researchers have common agreement over the causes of instabilities and their consequences such as pressure and temperature fluctuations, severe unstable flow boiling, flow reversal, early CHF (critical heat flux) and dry out. It has been identified that instabilities during flow boiling depends on mass flow rate, flow regimes, bubble dynamics, inlet subcooling, inlet compressibility and coolant properties. Study also revealed that in the recent year’s more attention has been paid to overcome the instability and issue has been addressed simultaneously with heat transfer enhancement studies. In order to suppress the instability various techniques have been investigated however, geometrical modification has been prominently considered to overcome the instability phenomenon.