Instabilities In Microchannels Research Articles

A Novel Asymmetric Check Microvalve for Suppressing Flow Boiling Instability in Microchannels

A Novel Asymmetric Check Microvalve for Suppressing Flow Boiling Instability in Microchannels

Simulation of the effect of the structure and location of non-closed droplet microchannel on flow/ thermal performance

In order to investigate the effects of geometric parameters and flow modes on the fluid flow morphology, resistance and heat transfer effect in the non-closed droplet pin fin array, in this manuscript, a numerical simulation has been conducted under Re = 100–700 with different pin fin heights (Hp = 0.3 mm, 0.5 mm, 0.7 mm) and pin fin densities (β = 0.046, 0.059, 0.072), with different flow directions under heat flux (10 W/cm2 – 50 W/cm2) studied as well. It is discovered that increasing the height and density of the non-closed droplet pin fin increases the interference with the fluid, resulting in a greater flow resistance phenomenon. However, the enhancement of turbulence not only exacerbates the flow instability in microchannels, but also further promotes the mixing of hot and cold fluids, making the internal temperature distribution more uniform. In which way, the effect of optimizing heat transfer performance achieves. In addition, as q > 20 W/cm2, the overall heat transfer capacity of flow direction B (the tip of the pin fin as upstream fluid direction) is better than that of flow direction A (the cavity of the pin fin upstream as upstream fluid direction). When q = 30 W/cm2, there is a process transferring from single-phase flow to two-phase flow along with the direction of working fluid flow in the microchannel. Under this condition, the local maximum heat transfer coefficient is 15,502.4 W/(m2·K) in flow direction A and 16,094.8 W/(m2·K) in flow direction B. Furthermore, the comprehensive evaluation concludes that the channel flow/thermal performance is the best with Hp = 0.7 mm, β = 0.046.

Numerical simulation of boiling behavior in vertical microchannels

High heat flux electronic devices put forward new requirements for heat dissipation, and boiling heat transfer technology is widely used because of its higher heat dissipation capacity. In this study, the volume of fluid method was employed, along with the incorporation of the Lee phase-change mass transfer model, to investigate two-phase flow and heat transfer in vertical upward rectangular microchannels. The heat flux was varied within the range of 10–40 kW/m2, while the mass flux was varied within the range of 200–600 kg/m2 s. With the increase in heat flux, bubble flow, slug flow, churn flow, and annular flow were found successively. A phase diagram was established to predict the flow pattern transition during the boiling process. When the flow pattern changes to the churn and the annular flow, the active nucleation site density increases obviously with the Boiling number (Bo). A new correlation was proposed for two-phase flow boiling heat transfer, suitable for vertical upward channels in microscale fluids. The friction factor obtained using the Darcy friction factor equation agrees well with the simulation results at a high-pressure drop. The instability in microchannels increases with the increase in heat flux, particularly in annular flow, resulting in more severe wall temperature fluctuations.

Two mathematical models of flow boiling and flow instability in rectangular expanding microchannel heat exchangers and structure optimization

Flow boiling instability in microchannels was widely studied to improve the reliability of microchannel heat exchangers. Two rectangular expanding microchannel heat exchangers have been experimentally investigated, one channel with cross-cutting and the other without cross-cutting. This study aims to optimize the heat transfer capabilities of heat exchangers. Two mathematical models containing heat transfer correlation and instability parameters have been developed. The experimental heat transfer coefficient (HTC) was used to validate them. Based on the models, the structure of the expanding microchannel was optimized, and the effect of expanding microchannel on flow instability was analyzed. As analyzed, in the models for two heat exchangers with and without cross-cutting, the mean deviations are 10.03% and 5.95%, respectively. The effect of flow instability suppression increases with the increase of the radiation angle. Moreover, the cross-cutting in expanding microchannels could further steady flow when adding fins in downstream channels. When adjusting the number of cross-cutting in cold plates from 1 to 4, the maximum increment of HTC is 58.17 kW/(m2·K). The influences of adjusting the number of fins in microchannels on the HTC are obvious for cold plates without cross-cutting. When the position of added fins is moved downstream in channels, the HTC of the heat exchanger could be increased by 26.58%. The results of this study provide some insights for inhibiting flow boiling instability and further optimizing the expanding microchannel structure to enhance heat transfer capability.

Dynamic instabilities of flow boiling in micro-channels: A review

As the heat dissipation requirements of micro-electronic component systems have increased by magnitude, scientists and engineers have shifted their focus from traditional single-phase heat transfer systems to systems that utilize phase change heat transfer. This review aims to review the literatures addressing gas-liquid two-phase flow boiling dynamic instabilities in micro-channels in theoretical and experimental aspects. Various methods to suppress instabilities were classified and summarized from the perspective of bubble dynamics. In theoretical studies, combined with bubble dynamics, the mechanism and criteria of dynamic instability are revealed. And some lumped models and models of pressure drop frequency and amplitude have been established, which are mainly for single or parallel straight micro-channels. Further theoretical studies on complex micro-channels and the suppression strategy of instabilities may be needed. In experimental studies, there is much experimental evidence for three primary dynamic instabilities called Density Wave Oscillations, Parallel Channel Instability, and Pressure Drop Oscillations in micro-channels. The effects of flow boiling instabilities on flow pattern, heat transfer coefficient, critical heat flux, and fluctuations in wall temperature and pressure were discussed. However, more work needs to be put into the identification of instabilities under different operating conditions in different micro-channel systems to avoid and suppress instability. At present, researchers tend to combine a variety of methods to suppress the occurrence of micro-channel instability.

A hydrophobic porous substrate-based vapor venting technique for mitigating flow boiling instabilities in microchannel heat sink

With the onset of nucleation, flow boiling in microchannels is susceptible to undesirable flow instabilities resulting in fluctuating wall temperature and large pressure drops. It is imperative to develop a passive technique to mitigate flow boiling instabilities and enhance the thermo-hydraulic performance of the heat sink without additional pressure drops. The present work explores a hydrophobic porous polydimethylsiloxane (PDMS) substrate-based vapor venting technique to mitigate flow boiling instabilities in microchannels. The hydrophobic porous substrate fitted at the channel exit manifold attracts the vapor plug, assists in vapor venting, and thus facilitates the easy evacuation of the bubbles from the channel. Flow boiling experiments are performed with deionized water in rectangular cross-section microchannels for a heat flux range of 10–260 W/cm2 and the coolant mass flux range of 206–335.49 kg/m2s. Heat transfer and pressure drop characteristics of the proposed configuration are compared with those of the conventional configuration. The proposed design suppresses the wall temperature oscillation and pressure oscillations. It produces a 32 % enhancement in heat transfer coefficient and a 63 % reduction in pressure drop compared to the conventional outlet configuration. The new configuration significantly reduces the bubble ebullition cycle, resulting in an efficient bubble evacuation from the channels. Further, frequent flushing of the vapor slug in the new design reduces the temperature fluctuations and enhances the heat transfer coefficient. The heat transfer augmentation is more pronounced at higher heat flux with more vapor generation rates.

Enhanced heat transfer coefficient of flow boiling in microchannels through expansion areas

Flow boiling in microchannels is a promising technique to achieve large cooling rates and high heat transfer coefficients for electronic cooling. This work reports an effective approach to enhance flow boiling heat transfer in microchannels through expansion areas. Flow boiling experiments were conducted on plain microchannels and microchannels with single and three expansion areas for comparison. Experimental results show that the flow boiling heat transfer coefficient can be effectively enhanced by up to 43.3% through adding three expansion areas in microchannels, while the pressure drop variation is within 3 kPa. Expansion areas significantly reduce the inlet temperature fluctuation, indicating the suppression of flow boiling instability in microchannels. As disclosed by the visualization of flow patterns, the improved heat transfer coefficient can be attributed to the enhanced bubble nucleation and formation of a nonuniform liquid layer near the corners of expansion areas.

A critical review on measures to suppress flow boiling instabilities in microchannels

Due to the development of the electronic components, concentrator photovoltaic, fuel cell, etc., the cooling requirement of the devices increases rapidly. Due to the higher heat transfer and compact design of the microchannel technology, it has been widely considered and investigated to solve the increasing heat flux. Boiling heat transfer is an effective way to dissipate a mass of heat using the latent heat of phase change. Hence, comparing with the single phase liquid flow, flow boiling in microchannel utilizes both advantages of micro scale effect and phase change effect, and can realize a much higher heat flux. Therefore, it becomes a critical way for super high heat dissipation. However, the procedure of the phase change and two phase flow may lead to instability, which can seriously inhibit the heat transfer performance of coolant in microchannel. To tackle with the flow boiling instability, a number of methods were proposed and reported. Therefore, in this paper, to effectively improve the flow boiling in microchannel heat exchanger, the cutting-edge control technologies in this field were reviewed, categorized and summarized, and the future trend of the research was presented.

Boiling Heat Transfer in a Micro- Channel Complex Geometry

"This prediction demonstrates an inclusive investigation on" single-phase and two- phase pressure drop properties and flows boiling" instabilities in micro-channels to resume the concepts in the boiling heat transfer tests". The predictions of "single-phase and two- phase heat transfers" in the aluminum micro-channel heat sink have been investigated". "Different heat fluxes and different mass fluxes have been applied "(each mass flow rate under numerous heats fluxes) in an aluminum parallel channel test piece is tested". "It was built up from a piece of aluminum," twenty-five mm wide by twenty-five mm long and ten mm high"."R113 working fluid temperature was constant (25°C) for all prediction tests; "Therefore the heat applies have been applied "in the ranges of (40-600 Watts) "The processes and iteration loops have reached for the prediction heat transfer properties "for three mass flow rates are (0.0125;0.015 and 0.0175 kg/s) "respectively;" and another heat transfer properties. "The aluminum micro-channels heat sink" with 0.5 mm channel height and 0.5 mm channel width "is heated via a wire electrical heater device". "The single-phase and two-phase flows are important parameters" for geometrical configuration of the aluminum micro-channels under variations heat applied". The purpose of this investigation is to explore the relationship between "prediction heat transfer coefficients and mini-scale heat sink geometrical configuration". Heat-transfer coefficients and pressure drops are investigated"." For sub-cooled and saturated boiling data acquired with Single-phase and boiling flows," The single-phase consequences are seen to be location-following, regular with a fully developed laminar flow. All the prediction boiling heat transfer coefficients are seen to have reasonably relied on as mass fraction and mass flux"."" Nevertheless, some are seen to have relied on heat flux whereas others are not. The convective boiling consisting is seen to have a boiling heat-transfer coefficient that is moderately relied on mass flow rate, gas mass fraction, and heat applied"." The prediction work presented here provides one of the first investigations into how to get single-phase" and two-phase properties" data via computer programming. In the end, "the single-phase and two-phase" heat transfer coefficients are reported" for square aluminum micro-channels heat sink under" boiling tests".

Three-dimensional viscoelastic instabilities in microchannels

Whereas the flow of simple single-phase Newtonian fluids tends to become more complex as the characteristic length scale in the problem (and hence the Reynolds number) increases, for complex elastic fluids such as dilute polymer solutions the opposite holds true. Thus small-scale, so-called ‘microfluidic’ flows of complex fluids can exhibit rich dynamics in situations where the ‘equivalent’ flow of Newtonian fluids remains linear and predictable. In the recent study of Qin et al. (J. Fluid Mech., vol. 864, 2019, R2) of the flow of a dilute polymeric fluid past a $50~\unicode[STIX]{x03BC}\text{m}$ cylinder (in a $100\times 60~\unicode[STIX]{x03BC}\text{m}$ channel), a novel 3-D holographic particle velocimetry technique reveals the underlying complexity of the flow, including inherent three-dimensionality and symmetry breaking as well as strong upstream propagation effects via elastic waves.

A REVIEW OF SINGLE-PHASE AND TWO-PHASE PRESSURE DROP CHARACTERISTICS AND FLOW BOILING INSTABILITIES IN MICROCHANNELS

This study presents a comprehensive review on single and two-phase pressure drop characteristics and flow boiling instabilities in microchannels to outline the discrepancies in the literature. The effect of mass flux, heat flux, experimental conditions, channel geometrical parameters (hydraulic diameter, aspect ratio etc.) was surveyed critically over a broad range of literature studies including the past and recent researches. Additionally, conventional and micro-scale pressure drop correlations are discussed. This study showed that the continuum theory is applicable for single-phase pressure drop applications with some considerations. Also, the two-phase pressure drop in micro-scale was found to have similar characteristics with conventional-scale channels. On the other hand, flow boiling instability is one of the important issues in microchannel heat exchangers. Flow boiling instabilities affect the system performance and may cause system failures. Therefore, unstable flow boiling conditions were identified in this paper for a reliable design of micro heat exchangers. 

Electrokinetic instability in microchannel ferrofluid/water co-flows

Electrokinetic instability refers to unstable electric field-driven disturbance to fluid flows, which can be harnessed to promote mixing for various electrokinetic microfluidic applications. This work presents a combined numerical and experimental study of electrokinetic ferrofluid/water co-flows in microchannels of various depths. Instability waves are observed at the ferrofluid and water interface when the applied DC electric field is beyond a threshold value. They are generated by the electric body force that acts on the free charge induced by the mismatch of ferrofluid and water electric conductivities. A nonlinear depth-averaged numerical model is developed to understand and simulate the interfacial electrokinetic behaviors. It considers the top and bottom channel walls’ stabilizing effects on electrokinetic flow through the depth averaging of three-dimensional transport equations in a second-order asymptotic analysis. This model is found accurate to predict both the observed electrokinetic instability patterns and the measured threshold electric fields for ferrofluids of different concentrations in shallow microchannels.

Modeling of reversal flow and pressure fluctuation in rectangular microchannel

Solving the flow instability in microchannel continues to be a topic of current research. In view of this, the current paper presents an analytical model to predict the pressure fluctuation by the analysis of bubble growth in a rectangular microchannel. To facilitate the analysis, bubble growth in the rectangular microchannel is assumed to be composed of three stages, namely, free growth, partially confined growth and fully confined growth. The interfacial velocity of bubble, being used to investigate the relationship between bubble reversal flow and pressure fluctuation, is determined by solving the conservation equations of the momentum of the liquid column coupled with the equations of the force balance at the bubble interface. The model reveals that when the length of fully confined bubble expands to some extent, the tail interface of bubble will reverse resulting in dramatically pressure increase. Additionally, the dependent factors, including Boiling number, nucleation site position, transverse shape and inlet restrictor, of pressure fluctuation are also analyzed based upon the current model, which denote that a smaller aspect ratio corresponds to a premature pressure fluctuation, and the magnitude of pressure fluctuation increases with the increasing Boiling number, decreases as the position of nucleation site moving downstream of the channel. In addition, the magnitude of pressure fluctuation decreases as the increase of pressure drop multiplier parameter, however, accompanied with the penalty of increasing pressure drop.

Unstable two-phase flow rate in micro-channels and cracks under imposed pressure difference

This paper numerically investigates two-phase flow rate instabilities in micro-channels comprising localized geometrical restrictions (i.e., a convergent–divergent duct). A numerical Lattice Boltzmann method is used to model and simulate multiphase flow with an accurate treatment of the contact angle at the triple line. A pressure difference is imposed between the inlet and outlet sections of the channel. This induces flow and allows its time evolution to be followed. Indeed, the resulting flow exhibits strong time fluctuations as it is strongly influenced by the induced viscous forces acting at solid–fluid interfaces and by pressure discontinuities sustained across the curved fluid–fluid interfaces (i.e., across the curvatures of the menisci). Flow fluctuations are investigated parametrically for wetting and non-wetting fluids and for different micro-channel geometrical irregularities. For this purpose, an equivalent dimensionless formulation is adopted. Flow fluctuations are analyzed and related to the controlling dimensionless parameters (Reynolds number, Laplace number, Bejan number, contact angle and channel constriction ratio). This allows quantification of the coupled influence of physical fluid properties (surface tension and contact angle), channel geometry, and loading conditions (imposed pressure difference) on flow evolution. Numerical results show that, under particular conditions, capillary pressure jumps sustained across the fluid bridge (owing to flow rate, contact angle, and local orientation of channel walls) entirely compensate for the imposed pressure difference. This situation results in a no-flow state, i.e., the flow becomes impossible even under an imposed pressure difference.

Effect of Running Parameters on Flow Boiling Instabilities in Microchannels.

Flow boiling instability (FBI) in microchannels is undesirable because they can induce the mechanical vibrations and disturb the heat transfer characteristics. In this study, the synchronous optical visualization experimental system was set up. The pure acetone liquid was used as the working fluid, and the parallel triangle silicon microchannel heat sink was designed as the experimental section. With the heat flux ranging from 0-450 kW/m2 the microchannel demand average pressure drop-heater length (Δp(ave)L) curve for constant low mass flux, and the demand pressure drop-mass flux (Δp(ave)G) curve for constant length on main heater surface were obtained and studied. The effect of heat flux (q = 188.28, 256.00, and 299.87 kW/m2), length of main heater surface (L = 4.5, 6.25, and 8.00 mm), and mass flux (G = 188.97, 283.45, and 377.94 kg/m2s) on pressure drops (Ap) and temperatures at the central point of the main heater surface (Twc) were experimentally studied. The results showed that, heat flux, length of the main heater surface, and mass flux were identified as the important parameters to the boiling instability process. The boiling incipience (TBI) and critical heat flux (CHF) were early induced for the lower mass flux or the main heater surface with longer length. With heat flux increasing, the pressure drops were linearly and slightly decreased in the single liquid region but increased sharply in the two phase flow region, in which the flow boiling instabilities with apparent amplitude and long period were more easily triggered at high heat flux. Moreover, the system pressure was increased with the increase of the heat flux.

Experimental investigation of electrohydrodynamic instabilities in micro channels

An electric field is applied to destabilize the interface between two Newtonian and immiscible liquids flowing in a rectangular micro channel. The liquids are pumped into the micro channel with a syringe pump and a DC electric field is applied either parallel or normal to the flat interface between these liquids. The two liquids used in the experiments are a combination of ethylene glycol, different viscosity silicone oils, castor oil, and olive oil. The onset of electrohydrodynamic instability is investigated for various parameters, including the ratios of the flow rates, and viscosities of the liquids, the width of the micro channel, and the direction of the applied electric field. The order of the voltage applied to destabilize the interface is in the range 95 and 1190 V. The results of the experiments show that an increase in the viscosity ratio and the flow rate ratio of silicone oil to ethylene glycol have a stabilizing effect. It is also found that the important parameter to determine the critical voltage is the flow rate ratio, not the individual flow rates of the liquids. Also, as the width of the micro channel increases, the critical voltage increases. Lastly, for the liquid combinations used in the experiments, the interface could not be destabilized under the influence of a parallel electric field.

Suppression of Two-Phase Flow Instabilities in Parallel Microchannels by Using Synthetic Jets

Two-phase flow instabilities in microchannels exhibit pressure and temperature fluctuations with different frequencies and amplitudes. An active way to suppress the dynamic instabilities in the boiling microchannels is to introduce synthetic jets into the channel fluid. Thus, the bubbles can be condensed before they clog the channel and expand upstream causing flow reversal. The present work experimentally investigated the effects of synthetic jets on microchannel flow boiling. An array of synthetic jets was introduced into the microchannel flow. The strength and frequency of the jets were controlled by changing the driving signals of the piezoelectric driven jet actuator. It is found that the bubbles were effectively condensed inside the jet cavity. The boiling flow reversals were notably delayed by the synthetic jets. Meanwhile, the pressure fluctuation amplitudes were suppressed to some extent. It was also observed that synthetic jets can help to uniformize the heat sink temperature distribution.

Bubble dynamics and flow boiling instabilities in microchannels

This work describes an experimental investigation of bubble dynamics and the effect on flow boiling instabilities and heat transfer in a multiple-microchannel heat sink. In this paper, results of the experimental investigation of bubble dynamics are presented and discussed. The microchannel heat sink contained 40 parallel, rectangular channels having hydraulic diameter of 194μm, and an aspect ratio of 0.549. The experiments were conducted with deionised water over a mass flux range from 71 to 204kg/m2s, and an inlet temperature of 71°C. Bubble growth rate and departure diameter under subcooled and saturated flow boiling are studied using a microscope and a high speed camera. The effects of mass flux and heat flux on bubbles growth characteristics are investigated and discussed. The results of this study demonstrate that the bubble growth rate in microchannels is different from that in macroscale channels. Confinement of microchannel and interaction between channels result in the unique bubble dynamic in microchannels.

Electrokinetic instability in microchannels

The effect of geometric confinement on electroconvective instability due to nonequilibrium electro-osmotic slip at the interface of an electrolytic fluid and charge-selective solid is studied. It is shown that the topology of the marginal stability curves and the behavior of the critical parameters depend strongly on both channel geometry and dimensionless Debye length at low voltages for sufficiently deep channels, corresponding to the Rubinstein-Zaltzman instability mechanism, but that stability is governed almost entirely by channel depth for narrow channels at higher voltages. For shallow channels, it is shown that above a transition threshold, determined by both channel depth and Debye length, the low-voltage instability is completely suppressed.

Ledinegg instability in microchannels

The static Ledinegg instability in horizontal microchannels under different flow conditions and fluids pertinent to electronics cooling was studied experimentally and numerically. Two fluids, water at sub-atmospheric pressures and refrigerant HFE-7100, were examined for a range of heat fluxes, mass fluxes, and channel hydraulic diameters. Numerical predictions from the developed pressure gradient model agree well with results from the flow boiling experiments. The model was used to quantify the susceptibility of the system to the Ledinegg instability. A parametric instability study was systematically conducted with varying system pressure, heat flux, inlet subcooling, and channel size with and without inlet restrictor. Increasing system pressure and channel diameter, reducing parallel channel number and channel length, and including an inlet restrictor can enhance the flow stability in microchannels.