Physics of microstructures enhancement of thin film evaporation heat transfer in microchannels flow boiling (original) (raw)
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Microscale study of mechanisms of heat transfer during flow boiling in a microchannel
International Journal of Heat and Mass Transfer, 2015
This work examines the microscale physics of the heat transfer events in flow boiling of FC-72 in a microchannel. Experimental results discussed in this paper provide new physical insight on the nature of heat transfer events during bubbles growth and flow through the microchannel. The study is enabled through development of a device with a composite substrate that consists of a high thermal conductivity material coated by a layer of a low thermal conductivity material with embedded temperature sensors. This novel arrangement enables calculation of the local heat flux with a spatial resolution of 40-65 µm and a temporal resolution of 50 µs. The device generates isolated bubbles from a 300 nm in diameter artificial cavity fabricated at the center of a pulsed function micro-heater. Analysis of the temperature and heat flux data along with synchronized images of bubbles show that four mechanisms of heat transfer are active as a bubble grows and flows through the channel. These mechanisms of heat transfer are 1) microlayer evaporation, 2) interline evaporation, 3) transient conduction, and 4) micro-convection. Details of these mechanisms including their time period of activation and corresponding surface heat flux and heat transfer coefficient are extensively discussed. [17,23,28-30], their fundamental assumptions have not been examined due to the lack of necessary tools to probe the actual physics of the proposed mechanisms. These models are often constructed based on observations from overall trends in flow thermohydraulics characteristics as a function of externally controlled parameters (i.e. flow rate, heat flux, surface temperature, and fluid properties) as well as the observed flow regimes. The performance of
Enhancement of flow boiling heat transfer in microchannels by nano- and micro-surface treatments
Mécanique & Industries, 2011
Ce papier présente nos études sur le transfert thermique en ébullition convective dans les microcanaux dont l'objectif est de développer des systèmes de refroidissement compacts qui peuvent s'adapter aux composants de puissance miniaturisés. Les nano-et micro-structurations de surface ont été utilisées comme des techniques innovantes permettant d'améliorer la performance de transfert thermique et de retarder le phénomène d'assèchement intermittent. It a été observé que les surfaces contenant des microstructures ont de meilleurs coefficients de transfert thermique par rapport aux surfaces lisses (jusqu'à 85% d'amélioration). En particulier, en rendant cette surface structurée plus mouillante, l'assèchement intermittent a significativement retardé.
Facile Fabrication of Nanostructured Microchannels for Flow Boiling Heat Transfer Enhancement
Heat Transfer Engineering, 2018
Flow boiling in microchannels promises high heat transfer due to the combined effect of latent heat of vaporization and forced convection in confined spaces. However, flow boiling based miniaturized thermal management devices are limited due to instability induced dryout. While several efforts have been made to delay instabilities via advanced surface modification techniques, there is a need to expand the scope of applications by developing low-cost and scalable fabrication technologies for commonly used heat exchanger materials. In this paper, we use a facile and self-limiting chemical oxidation technique for fabricating sharp needle-like superhydrophilic CuO nanostructures within six parallel 500 × 250 µm 2 microchannels spread uniformly over a 1x1 cm 2 area in a copper heat sink. We demonstrate heat transfer enhancement with nanostructured microchannels (NSM) without any appreciable change either in the average pressure drop or the fluctuations in comparison to baseline plain wall microchannels (PWM). Analysis of the high-speed images was performed to attribute the enhancement with NSM to the presence of a capillarity-fed thin-film evaporation regime, which otherwise was absent in PWM. We believe that these results are encouraging and suggest that the heat sink geometry can be optimized to investigate the true potential of nanostructured microchannels.
Spatiotemporally resolved heat transfer measurements for flow boiling in microchannels
International Journal of Heat and Mass Transfer, 2015
Spatiotemporally resolved wall heat transfer measurements can provide valuable insight into the fundamental mechanisms affecting flow boiling in microchannels. Operating at the microscale, necessitates resolving changes in local and instantaneous heat transfer characteristics on the order of 100 µm and 1 kHz, respectively. Straightforward interpretation of transient temperature measurements is often challenging due to the conjugate conduction effects in the substrate, which can dampen the measured and inferred heat transfer quantities. These damping effects are described using a slip coefficient (S), which represents the fraction of the change in the local heat transfer that is registered by a sensor (negligible thermal mass) located on a given substrate. Using S, arguments are presented that the conduction patterns in the substrate are predominantly 1-D (i.e. into the substrate) at suitable spatiotemporal-scales. Building on these fundamental considerations, a numerical procedure is adopted to allow a time varying estimate of the local convective heat flux and heat transfer coefficient from transient temperature measurements. Examples of this framework are showcased with experimental results and discussions for interactions observed during flow boiling of HFE 7000 in a single microchannel (hydraulic diameter = 370 µm). At the relatively low mass flux of 200 kg/m 2-s reported in this work, liquid evaporation was found to dictate the local heat transfer trends. High rates of heat transfer were observed to accompany the growth of bubbles and evaporation of the liquid film under vapor slugs. Local dryout was routinely observed in the bubbly and slug flow regime and found to initially enhance heat transfer (i.e. at the creation and subsequent propagation of the three-phase contact line) and present near-zero heat transfer rates in the dried-out domain.
Flow boiling with deionized water in silicon (Si) microchannels was drastically enhanced in a single annular flow boiling regime enabled by superhydrophilic Si nanowire inner walls. Part I of this study focuses on characterizing enhanced flow boiling heat transfer. Part II focuses on revealing mechanisms in governing pressure drop and critical heat flux (CHF). Compared to flow boiling in plain-wall microchannels without using inlet restrictors (IRs), the average heat transfer coefficient (HTC) and CHF were enhanced by up to 326% and 317% at a mass flux of 389 kg/m 2 s, respectively. Additionally, compared with flow boiling in microchannels with IRs, HTC of flow boiling in the single annular flow was enhanced by up to 248%; while CHF in the new flow boiling regime was 6.4-25.8% lower. The maximum HTC reached 125.4 kW/m 2 K at a mass flux of 404 kg/m 2 s near the exits of microchannels. The significantly promoted nucleate boiling, induced liquid film renewal, and enhanced thin-film evaporation in the self-stabilized and single flow boiling regime are the primary reasons behind the significant heat transfer enhancements during flow boiling.
Flow Boiling of Water on Nanocoated Surfaces in a Microchannel
Journal of Heat Transfer, 2012
Experiments were performed to study the effects of surface wettability on flow boiling of water at atmospheric pressure. The test channel is a single rectangular channel 0.5 mm high, 5 mm wide and 180 mm long. The mass flux was set at 100 kg/m² s and the base heat flux varied from 30 to 80 kW/m². Water enters the test channel under subcooled conditions. The samples are silicone oxide (SiOx), titanium (Ti), diamond-like carbon (DLC) and carbon-doped silicon oxide (SiOC) surfaces with static contact angles of 26°, 49°, 63° and 103°, respectively. The results show significant impacts of surface wettability on heat transfer coefficient.
Local measurement of flow boiling in structured surface microchannels
International Journal of Heat and Mass Transfer, 2007
Experiments were conducted to investigate flow boiling in 200 lm  253 lm parallel microchannels with structured reentrant cavities. Flow morphologies, boiling inceptions, heat transfer coefficients, and critical heat fluxes were obtained and studied for mass velocities ranging from G = 83 kg/m 2 s to G = 303 kg/m 2 s and heat fluxes up to 643 W/cm 2 . Comparisons of the performance of the enhanced and plain-wall microchannels were performed. The microchannels with reentrant cavities were shown to promote nucleation of bubbles and to support significantly better reproducibility and uniformity of bubble generation. The structured surface was also shown to significantly reduce the boiling inception and to enhance the critical heat flux.
Correlation of the Flow Pattern and Flow Boiling Heat Transfer in Microchannels
Heat Transfer Engineering, 2013
Flow boiling in microchannels is characterized by the considerable influence of capillary forces and constraint effects on the flow pattern and heat transfer. In this article we utilize the features of gas-liquid flow patterns in rectangular microchannels under adiabatic conditions to explain the regularities of refrigerants flow boiling heat transfer. The flow-pattern maps for the upward and horizontal nitrogen-water flow in a microchannel with the size of 1500 × 720 µm were determined via dual-laser flow scanning and compared with corrected Mishima and Ishii prediction. Flow boiling heat transfer was studied for vertical and horizontal microchannel heat sink with similar channels using refrigerants R-21 and R-134a. The data on local heat transfer coefficients were obtained in the range of mass flux from 33 to 190 kg/m 2-s, pressure from 1.5 to 11 bar, and heat flux from 10 to 160 kW/m 2. The nucleate and convective flow boiling modes were observed for both refrigerants. It was found that heat transfer deterioration occurred for annular flow when the film thickness became small to suppress nucleate boiling. The mechanism of heat transfer deterioration was discussed and a model of heat transfer deterioration was applied to predict the experimental data.