Flow Boiling In Microchannels Research Articles

Investigation of Flow Boiling in Micro-Channels: Heat Transfer, Pressure Drop and Evaluation of Existing Correlations

In this study, the flow boiling heat transfer and pressure drop characteristics of refrigerant R134a in micro-channels were experimentally investigated. The tests were performed in circular horizontal micro-channels with inner diameters of 0.5 mm and 1 mm and a heating length of 300 mm. The mass velocities varied from 500 kg/m2s to 2500 kg/m2s, and the heat fluxes varied from 15 kW/m2 to 147 kW/m2. The heat transfer coefficient (HTC) and frictional pressure drop (FPD) were measured and discussed in detail. According to the results, HTC was significantly affected by heat flux, whereas it was independent of mass velocity. Nucleate boiling was the dominant heat transfer mechanism for R134a flow boiling in the micro-channels. In comparison to the 1 mm channel, the 0.5 mm channel shows better performance in heat transfer, with a maximum increase of approximately 22 %. In addition, FPD increased with increasing mass velocity and decreasing channel diameter. Finally, several existing correlations for HTC and FPD were evaluated by comparing them with the experimental values. Tran’s correlation (1996) presented a better agreement in terms of the average HTC, while for the FPD, the model of Kim and Mudawar (2013b) showed good prediction accuracy.

Effects of Inlet Subcooling on Flow Boiling in Micro Channels - A Review

Effects of Inlet Subcooling on Flow Boiling in Micro Channels - A Review

Flow Pattern Analysis and Heat Transfer Characteristics During Subcooled Flow Boiling in a Rectangular Microchannel on ZnO Microrod Surface

Abstract The combination of microstructured surface and microchannel flow boiling is expected to solve the thermal management problems of high-heat-flux devices. In this study, the experimental investigation of subcooled flow boiling in a high aspect ratio, one-sided heating rectangular microchannel was conducted with de-ionized water as the working fluid. ZnO microrods were synthesized on the titanium surface to be used as the heated surface compared with the bare titanium surface. A facile image tool is utilized to process the flow patterns photographed by a high-speed camera, which is analyzed with the heat transfer characteristics. The flow pattern of isolated bubbly flow reveals the large number of nucleation sites formed on the microrod surface but the heat transfer performance deteriorates with increasing mass flux because of the smaller bubble area and weaker nucleation. With increasing heat flux, the flow pattern changes from isolated bubbly flow to alternating bubbly/slug flow and alternating slug/annular flow. The latter flow pattern is confirmed to bring a higher heat transfer coefficient due to the larger area of thin-film evaporation. Compared with the bare surface, a higher heat transfer coefficient is achieved on the ZnO microrod surface for up to 37% due to the more nucleate sites and strengthened convective evaporation. Therefore, this surface might be suitable for heat dissipation in the watercraft or aerospace industry considering the low density, strong intensity, and corrosion resistance of titanium.

Mitigation of Transient Fluctuations During Flow Boiling in Microchannels via Adaptive Vapor Venting

Efficient thermal management is crucial for enhancing the performance and reliability of electronic devices. In this regard, flow boiling with microchannels promises high heat dissipation capability for a nominal temperature budget as it inherently takes advantage of high latent heat of vaporization along with high surface-area-to-volume ratio. However, the promise of high heat dissipation in flow boiling microchannels is limited by two-phase flow instabilities which cause large-amplitude temperature and pressure fluctuations. The current state-of-the-art thermal management techniques for enhancing heat transfer during flow boiling in microchannels operate in steady-state condition and provide sufficient continuous cooling to address the peak thermal loads. While such approaches can manage the thermal challenge, they result in significant cooling overdesign. To eliminate the overdesign, we introduce an adaptive vapor venting strategy which continuously adjusts to the varying vapor generation across different heat fluxes to completely mitigate two-phase fluctuations. Our results demonstrate that significant transient temperature fluctuations (±5 °C) in the conventional microchannels were completely eliminated with the incorporation of the adaptive vapor venting (within the experimental uncertainty). Furthermore, our results show that this strategy has the capability to handle transience and accordingly avoid the need for cooling overdesign.

A Comparison of Finite Element and Lumped Modeling Techniques to Analyze Flow Boiling in Microchannels

This study quantitatively compares the finite element (FE) and lumped modeling techniques to analyze phase-change and multiphase flow in a microchannel evaporator. We compare the one- and two-zone models in the lumped modeling approach by considering both pressure drop and phase change in the evaporator. The study investigates the deviation in results from the lumped model relative to the FE model for various evaporator heat loads. For the one-zone model, the relative errors in predicting the average density, pressure, temperature, and vapor quality increase with the evaporator heat load. This increase is mainly due to the nonlinear variation in the heat transfer coefficient and pressure drop, which are calculated with higher inaccuracies at larger evaporator heat loads. On the other hand, the liquid-phase region and the exit vapor quality are predicted more accurately. The overall errors using one- and two-zone lumped models were less than 30% when the exit qualities do not exceed 0.5. Since strategies involving flow boiling in evaporators typically avoid high exit vapor qualities to circumvent dry-out and critical heat flux, the use of lumped models under these conditions is favorable. This approach also avoids the computational costs associated with FE models while achieving sufficient accuracy to predict the evaporator’s performance.

Enhanced Flow Boiling in Microchannels Integrated with Hierarchical Structures of Micro-Pinfin Fences and Nanowires.

Two-phase flows in microchannels have been extensively investigated due to their wide range of applications including the fluid process and thermal management in electronic devices. In this study, silicon nanowires (SiNWs) were directly integrated with all the inner microchannel surfaces including walls and micro-pinfins (μ-pinfin) to form SiNW-pinfin hierarchical structures. The objective of this study is to explore if the SiNW-decorated surfaces can further enhance flow boiling through promoting local capillary flows with a better managed pressure drop. Experimental results illustrate that the critical heat flux in the proposed SiNW-pinfin microchannels can be promoted up to 483, 71.7, and 25.5% at a mass flux of 303 kg/m2 s compared with the traditional plain-wall, SiNW-coated plain-wall, and plain-wall with inlet orifice microchannels, respectively. Moreover, the heat-transfer rate of the flow boiling can be enhanced up to 122% at a mass flux of 303 kg/m2 s compared to that of inlet orifice microchannels with an effectively managed pressure drop. The capillary-induced periodic and rapid liquid wetting is believed to be the primary enhancement mechanism.

Quasi-steady and transient study of heat transfer during sub-cooled flow boiling in a small aspect ratio microchannel

Owing to the transient nature of microchannel flow-boiling, gaining an in-depth understanding of the associated trends in time-averaged heat transfer variables as well as mechanisms of heat transfer is quite challenging. This work studies sub-cooled flow-boiling of de-ionized water in a microchannel. The micro-channel heights studied are 0.14, 0.28 and 0.42 mm while the tested mass fluxes range from 200 to 1000 kg/(m2s). Slug flow regime is observed under most of the tested conditions. Transient wetted surface heat flux and temperature as well as heat transfer coefficient are evaluated from the wall temperature data captured at a high data-logging frequency (synchronously with flow-visualizations). Heat transfer mechanisms as well as the trends observed in plots associated with time-averaged heat transfer variables are explained from the perspective of transient data correlated to the observed flow-boiling phenomena. Thin-film evaporation emerges as the prominent heat transfer mechanism. Two-way coupling between transient heat transfer variables and the boiling phenomena is evident.

Local Heat Transfer of Saturated Flow Boiling in Vertical Narrow Microchannel

Abstract An experimental study of saturated flow boiling in a high-aspect-ratio one-side-heating rectangular microchannel was conducted with de-ionized water as the working fluid. ZnO microrods with the average diameter of about 1 μm and length of about 7 μm were synthesized on the Ti wafer surface, which was used to fabricate the heated bottom surface of the microchannel. The ZnO microrod surface appeared to be hydrophobic and the capillary wetting effect on the surface was found after being wet. The heat transfer and pressure drop characteristics of saturated flow boiling in the microchannel were studied and the flow patterns were photographed with a high-speed camera. Almost all the flow patterns observed in this experiment featured the main annular flow and abrupt flush of bubbly flow. Because of the capillary wetting effect on the ZnO microrod surface, the local dryout and rewetting phenomenon did not appear in this study. However, due to the numerous nucleation sites on ZnO microrod surface, the abrupt bubble flow caused much more disruption to the liquid film of annular flow when compared to the regular silicon surface. The abrupt bubble flow flushed through the annular liquid film and caused the fluctuation and nonuniformity of the liquid film and heat transfer deterioration, which was severer in the high heat flux conditions. Otherwise, the capillary effect on the ZnO microrod surface was able to restrict the nonuniformity of the liquid film under high heat flux and low mass flux conditions; thus, the deterioration of heat transfer performances diminished.

Experimental Investigation of Heat Transfer and Flow Characteristics in Different Inlet Subcooled Flow Boiling in Microchannel

The current experimental study considered the heat transfer and pressure drop for subcooled flow boiling of deionized water in a microchannel heat sink. The heat sink consisted of a single microchannel, 300μm wide and 300μm in height with a hydraulic diameter of 300μm. The heat sink was formed of oxygen-free cooper with 72mm length and 12mm width. Experimental operation conditions spanned the heat flux 78-800 kW/m2, mass flux 1700 and 2100kg/m2.s and at 21, 31 K subcooled inlet temperature. Boiling heat transfer coefficient was measured and compared with existing correlations. Also, the experimental pressure drop was measured and compared with micro scale pressure drop correlations. The results show that higher mass flux leads to a higher boiling heat transfer coefficient and the dominant mechanism is convective boiling. Also, the experimental pressure drop decreases with increasing heat flux in a single phase region, while it increases in a two-phase region. The heat transfer coefficient increase with increasing the inlet subcooled. When compared the experimental results in the experimental condition range, it was found that an existing correlation provides satisfactory prediction of heat transfer coefficient and pressure drop.

Computational Fluid Dynamics Modeling of Flow Boiling in Microchannels With Nonuniform Heat Flux

The computational fluid dynamics (CFD) modeling of boiling phenomena has remained a challenge due to numerical limitations for accurately simulating the two-phase flow and phase-change processes. In the present investigation, a CFD approach for such analysis is described using a three-dimensional (3D) volume of fluid (VOF) model coupled with a phase-change model accounting for the interfacial mass and energy transfer. This type of modeling allows the transient analysis of flow boiling mechanisms, while providing the ability to visualize in detail temperature, phase, and pressure distributions for microscale applications with affordable computational resources. Results for a plain microchannel are validated against benchmark correlations for heat transfer (HT) coefficients and pressure drop as a function of the heat flux and mass flux. Furthermore, the model is used for the assessment of two-phase cooling in microelectronics under a realistic scenario with nonuniform heat fluxes at localized regions of a silicon microchannel, relevant to the cooling layer of 3D integrated circuit (IC) architectures. Results indicate the strong effect of two-phase flow regime evolution and vapor accumulation on HT. The effects of reduced saturation pressure, subcooling, and flow arrangement are explored in order to provide insight about the underlying physics and cooling performance.

Modeling of Dry-Out Incipience for Flow Boiling in a Circular Microchannel at a Uniform Heat Flux

Dry-out is an essential phenomenon that has been observed experimentally in both slug and annular flow regimes for flow boiling in mini and microchannels. The dry-out leads to a drastic drop in heat transfer coefficient, reversible flow and may cause a serious damage to the microchannel. Consequently, the study and prediction of this phenomenon is an essential objective for flow boiling in microchannels. The aim of this work is to develop an analytical model to predict the critical heat flux (CHF) based on the prediction of liquid film variation in annular flow regime for flow boiling in a horizontal uniformly heated circular microtube. The model is developed by applying one-dimensional (1D) separated flow model for a control volume in annular flow regime for steady, and sable saturated flow boiling. The influence of interfacial shear and inertia force on the liquid film thickness is taken into account. The effects of operating conditions, channel sizes, and working fluids on the CHF have been investigated. The model was compared with 110 CHF data points for flow boiling of various working fluids, (water, LN2, FC-72, and R134a) in single and multiple micro/minichannels with diameter ranges of (0.38≤Dh≤3.04 mm) and heated-length to diameter ratios in the range of (117.7 (117.7≤Lh/D≤470)470). Additionally, three CHF correlations developed for saturated flow boiling in a single microtube have been employed for the model validation. The model showed a good agreement with the experimental CHF data with mean absolute error (MAE) = 19.81%.

Kandlikar third number map for flow boiling in micro-channels and micro-gravity

As an extension of the recent work of Baldassari and Marengo (Baldassari C., Marengo M., Flow Boiling in Microchannels and Microgravity, Progress in Energy and Combustion Science 39 (2013) 1, pp. 1-36), this note presents Kandlikar third number (K3) map for flow boiling in microchannels and microgravity. Using several data points available in the literature, Kandlikar third number (K3) map was plotted versus the hydraulic diameter (dh) as the characteristic dimension for flow boiling in microchannels and microgravity. The ranges of the Kandlikar third number (K3), calculated using the hydraulic diameter (dh), are presented.

A Review of Computational Modelling of Flow Boiling in Microchannels

Flow boiling in microchannels has received enormous interest in academic research over the past few decades because of its importance in the thermal management of micro-structured devices. A vast number of experimental and theoretical studies have been reported and an increasing number of computational studies of microchannel flow boiling have been performed in the past few years. This article provides a review of the previous studies of flow boiling in microchannels, with a focus on the computational work. We present the governing equations, boundary conditions, numerical methods and the mathematical treatment of phase change used. Some important numerical studies are reviewed in detail and their strengths and weaknesses are presented. Finally, a summary of the current status of modelling in this area is provided.

The Analysis of Two-Phase Flow Type in the Micro-Channel Flow Boiling

This paper describes the measurement of the parameters of the experiment, the problems in the analysis of experimental data processing and experimental error. The flow pattern and pressure drop of flow boiling, heat flux and heat transfer coefficient of relations are studied by analysis of the experimental data. The test section were isolated three flow patterns, bubble-like flow, limited bubble-like flow and evaporated to dryness district. Only isolated bubble-like flow of a stream type such as channel width = 2.0mm thin channel, volume flow greater than 15.47L/h. For channel width =1.0mm thin channel, bubbly flows is limited bubbly flow changes corresponding volume flow 10.8L/h.

Nucleation and Growth Characteristics of Bubble in Subcooled Flow Boiling in Microchannel

Dynamic characteristics of micro bubble are the key for its applications. A thermal bubble is produced on the wall of microchannel with flowing fluid. Generation of bubble, including nucleation and growth, is observed and measured with different volume flow rates. The results show that nucleation becomes more difficult and instable with volume flow rate increasing. Bubble growth could be divided into three stages, while the second stage becomes shorter and even disappears as volume flow rate increasing. When volume flow rate increases suddenly, bubble shrinks, and it is more remarkable for larger volume flow rate. Furthermore it needs more time for bubble to reach and keep the minimum volume.

Numerical Investigation of Saturated Flow Boiling in Microchannels by the Lattice Boltzmann Method

Bubble formation in saturated flow boiling in 2D microchannels, generated from a microheater under constant wall heat flux or constant wall temperature conditions, is studied numerically based on a newly developed lattice Boltzmann model for liquid-vapor phase change. Simulations are carried out to study effects of inlet velocity, contact angle, and heater size on saturated flow boiling of water under constant wall heat flux conditions. Important information, such as effects of static contact angle on nucleation time and nucleation temperature, which was unable to be obtained by other numerical simulation methods, is obtained. Furthermore, effects of inlet velocity, contact angle, and superheat on nucleate boiling heat transfer in steady flow boiling of water under constant wall temperature conditions are also presented. It is found that the nucleate boiling heat transfer at the microheater is higher if the heater surface is more hydrophilic, because the superheated vapor at the hydrophilic wall has a thinner thermal boundary layer and a larger thermal conductivity.

Numerical Simulation of Bubble Growth and Heat Transfer During Flow Boiling in a Surface-Modified Microchannel

Flow boiling in a microchannel without or with surface modifications, such as fins, grooves, and cavities, has received significant attention as an effective cooling method for high-power microelectronic devices. However, a general predictive approach for the boiling process has not yet been developed because of its complexity involving the bubble dynamics coupled with boiling heat transfer in a microscale channel. In this study, direct numerical simulations for flow boiling in a surface-modified microchannel are performed by solving the conservation equations of mass, momentum, and energy in the liquid and vapor phases. The bubble surfaces are determined by a sharp-interface level-set method, which is modified to include the effect of phase change at the liquid–vapor interface and to treat the no-slip and contact-angle conditions on immersed solid surface of microstructures. This computation demonstrates that the surface-modified microchannel enhances boiling heat transfer significantly compared to a plain microchannel. The effects of various surface modifications on the bubble growth and heat transfer are investigated to find better conditions for boiling enhancement.

Influence of Triggered Seed Bubbles on Flow Boiling in Microchannels

Combined with a simple phase change model, the Volume of Fluid (VOF) method is used for simulating the seed bubble triggered boiling in a single circular microchannel with 6.5mm length. Simulation results show that the introduction of seed bubbles improves the heat transfer performance and suppresses the flow instability simultaneously. The seed bubbles are triggered into the microchannel forf=2000 Hz, passing by only about five numbers of periods, the wall superheats remarkably decreased, a complete thermal equilibrium between the liquid and vapor phases is approached, the flow instability can be controlled successfully. At the stable flow stage, the heat transfer which can reach more than 3 times the single liquid phase heat transfer dominated by the local liquid film thickness, which is the actual heat transfer length-scale in the liquid film regime. However, the liquid bridge region existing between the bubbles produces a week heat transfer mechanism.

Experimental Study on Heat Transfer Characteristics of Water and Ethanol Flow Boiling in Micro-Channel

The Heat transfer characteristics of water and ethanol flow boiling in micro-channel are studied. Flow boiling of different mass and heat flux is carried out in 61 parallel microchannel with hydro diameter of 0.293mm, and the local heat transfer coefficient (HTC) of is calculated. The results indicate that the HTC decreases with the increasing of vapor quality, and depends on heat flux. Two latest correlations are used to predict the experiment, and the results indicate that the Bond number plays an important role in the correlation.

A CRITICAL REVIEW OF RECENT INVESTIGATIONS ON FLOW PATTERN AND HEAT TRANSFER DURING FLOW BOILING IN MICRO-CHANNELS

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