Bubble Nucleation Site Density Research Articles

Effect of graphene coating on flow boiling in a minichannel at sub-atmospheric pressures

Flow boiling in a mini-channel with graphene-coated sintered porous copper surface was investigated at sub-atmospheric pressures by analyzing bubble motion and flow patterns. A method of surface modification based on graphene coating was presented for thermal management systems operating at lower temperatures. The experiments were carried out at mass fluxes of 60 kg.m–2.s–1 and 120 kg.m–2.s–1 and sub-atmospheric pressures of 48 kPa and 68 kPa in a mini-channel with dimensions of 50 mm × 15 mm × 2 mm using HFE-7100 coolant as the working fluid. At sub-atmospheric pressures, the bubble nucleation site density decreases, and the bubble diameter increases while the bubble release frequency decreases. The existence of porous surfaces and HFE-7100 coolant with lower surface tension leads to the inception of boiling at lower wall superheats. The graphene coating promotes microlayer evaporation and consequently increases the nucleation site density by enhancing the thermal conductivity and wicking rate. Additionally, it provides stable annular flow at high heat fluxes, whereas intermittent-annular flow is observed for the untreated surface under the same condition. This enhancement results in a more pronounced increase in the boiling heat transfer for the graphene-coated surface. As a result, this study provides the high potential of graphene coatings for phase change heat transfer enhancement at sub-atmospheric pressures.

Micro/nanostructuring of metal additively manufactured aluminum alloy for enhanced pool boiling of dielectric fluids

Surface micro/nanotextures have the potential to increase bubble nucleation and promote capillary liquid wicking to enhance pool boiling heat transfer significantly. Despite past studies implementing surface micro/nanofabrication strategies to enhance boiling, the role of structure length scale and morphology on boiling heat transfer coefficient (h) and critical heat flux (CHF) of low-surface-tension dielectric fluids remains unclear. In this study, we systematically tune the surface microstructures on additively and conventionally manufactured aluminum alloys (AM AlSi10Mg and Al6061) from 1 µm to 5 µm to study the influence of microstructure length scale on boiling performance. In addition, to further understand the effect of submicron structures on boiling, nanostructures of 300 nm length were generated on a plain AM surface and hierarchically incorporated atop the microstructures. Dielectric fluid, HFE-7100, was used to investigate the effects of surface morphology on pool boiling heat transfer coefficient (h) and critical heat flux (CHF). Our results show that the 5 µm microstructured AM surface, AM-H(400)E(5), attained a CHF of 19.44 W/cm2 and a maximum heat transfer coefficient (hmax) of 2.89 W/cm2·K, which represents a reduction in CHF of 28.5 % and an enhancement in hmax of 103.8 %, as compared to the plain Al6061 surface. In addition, AM-H(400)E(5) achieved a large enhancement ratio of 2.2 as compared to the plain Al6061 at a heat flux of 20 W/cm2, and the highest h value amongst all structured surfaces, indicating its cavity size is optimal for bubble nucleation. Through the systematic tuning of micro/nanostructures length scale and morphology, this study found that the micro/nanostructures of 1 µm size and below are too small to serve as bubble nucleation sites. Additionally, micro/nanostructure wickability plays a negligible role in affecting pool boiling performances of highly wetting dielectric fluids, while surface morphology plays a dominant role in bubble nucleation to enhance boiling. Lastly, high-speed immersion microscopy was performed to understand the effects of micro/nanostructures on bubble characteristics such as bubble departure diameter and growth period. In summary, this work not only reports the first micro/nanostructured AM surfaces utilizing scalable fabrication techniques for enhanced boiling, but it also demonstrates the potential of tunning the micro/nanostructure length scale to optimize the bubble nucleation site density, resulting in significantly improved pool boiling performances.

Enhanced nucleate boiling heat transfer on bubble-induced assembly of 3D porous interconnected graphene oxide/silver nanowire hybrid network

Due to an increasing trend of size diminution, integration, and multifunctionality, the heat load on electronic devices is constantly increasing. Though nucleate boiling is regarded to be the most efficient mode of heat transfer, it still has much room for improvement in terms of heat transfer coefficient and critical heat flux. Thus, extensive research efforts have been devoted to the preparation of functional heating surfaces for enhanced boiling. This study presents nucleate boiling enhancement on a 3D porous graphene oxide/silver nanowire hybrid film. Then, boiling experiments were conducted under atmospheric pressure using a custom-developed boiling apparatus with a flat copper heating surface. Bubble-induced graphene oxide/silver nanowire hybrid deposition structures were analyzed through a series of characterizations such as Field Emission Scanning Electron Microscopy, Energy Dispersive X-Ray Spectroscopy, profilometry, and contact angle analysis to elaborate the physical mechanism behind the nucleate boiling enhancement. Among three tested concentration ratios (GO:AgNW; 0:1, 1:1, 1:5 by weight), maximum heat transfer coefficient and critical heat flux enhancement were achieved at a GO:AgNW concentration ratio of 1:5 owing to the improved surface characteristics such as surface area, bubble nucleation site density, lateral heat conduction, and capillarity. Heat transfer coefficient and critical heat flux enhancements of graphene oxide/silver nanowire hybrid surface reached 196.6% and 182.4%, while the upgrades on graphene oxide surface were 112.4% and 135.3%, respectively.

Effect of PVD-coated chromium on the subcooled flow boiling performance of nuclear reactor cladding materials

We elucidate the separate effect of a thin Cr coating deposited by physical vapor deposition (PVD) on the subcooled flow boiling performance of zircaloy-4. First, we run flow boiling experiments on prototypical zircaloy-4 surfaces mimicking the scratch pattern and surface roughness of nuclear reactor claddings. Then, we PVD-coat a 0.3 µm thick chromium layer on the same exact surface and repeat the same flow boiling investigations. All experiments are run using deionized water at atmospheric pressure, flowing on a 1 × 3 cm2 rectangular cross section channel at a rate of 1000 kg/m2/s and a subcooling of 10 K. We measure the average temperature of the boiling surface at increasing surface heat fluxes, covering a wide range of heat transfer regimes, from single-phase forced convection to the boiling crisis. We also record high-speed videos of the boiling process, which we postprocess to measure bubble nucleation site density, growth time, departure diameter and frequency. The surface analysis reveals that, while the chromium coating does not seem change the surface roughness and morphology, it improves surface wettability. However, it decreases the critical heat flux. The chromium coating causes an increase of nucleation temperature, bubble departure diameter and growth time, and a reduction of the nucleation site density. The concurrence of these observations indicates that a size reduction of the nucleation sites, conformally covered by the chromium coating, may be the cause of the boiling performance deterioration. We confirm this hypothesis repeating the same analysis on a FeCrAl sample prepared and tested using the same protocol as the zircaloy-4 sample, but with a different initial surface texture.

Boiling Heat Transfer with a Well-Ordered Microporous Architecture.

Boiling heat transfer through a porous medium offers an attractive combination of enormous liquid-vapor interfacial area and high bubble nucleation site density. In this work, we characterize the boiling performances of porous media by employing the well-ordered and highly interconnected architecture of inverse opals (IOs). The boiling characterization identifies hydrodynamic mechanisms through which structural characteristics affect the boiling performance of metallic microporous architecture by validating empirical measurements. The boiling performances can be optimized through the rational design of both the structural thicknesses and pore diameters of IOs, which demonstrate up to 336% enhancement in boiling heat-transfer coefficient (HTC) over smooth surfaces. The optimal HTC and critical heat flux occur at approximately 3-4 μm in porous structure thickness, which is manifested through the balance of liquid-vapor occupation within the spatial confinement of the IO structure. The optimization of boiling performances with varying pore diameters (0.3-1.0 μm) can be attributed to the hydraulic competitions between permeability and viscous resistance to liquid-vapor transport. This study unveils thermophysical understandings to enhance multiphase heat transfer in microporous media for ultrahigh heat flux thermal management.

Subcooled flow boiling in an expanding microgap with a hybrid microstructured surface

Flow boiling in microgap is attracting increasingly wide attention in heat transfer community due to its promising application potential for thermal management of electronic devices. In this study, an expanding microgap with a hybrid microstructured surface is proposed. It consists of a roughened inlet region followed by an array of staggered micro triangular pin-fins machined downstream. Subcooled flow boiling tests were conducted using distilled water as the working fluid with the inlet subcooling between 20 °C and 40 °C at different flow rates ranging from 80 to 480 mL/min over a heat flux range of 100.0–651.7 W/cm2. High-speed visualization was performed to explore the flow pattern transitions and the effects of operating conditions on heat transfer and pressure drop characteristics. A new flow pattern was observed in which the bubbles blanket the surface over both roughened upstream surface and the micro pin-fins downstream surface at higher flow rates and higher heat flux conditions. Combination of the increased bubble nucleation site density on the roughened surface and suppression of local dryout from the micro pin-fins due to their capillary effect jointly led to the high heat transfer performance in the bubble blanket pattern. In addition, the expanding configuration provides additional space downstream to facilitate bubble growth and two-phase flow in the direction of expansion with reduced resistance, resulting in a relatively low pressure drop in the present system.

Amelioration of the pool boiling heat transfer performance by colloidal dispersions of carbon black

Recently, carbon-based colloid materials gained popularity in boiling heat transfer (BHT) research. Among them, carbon nanotubes (CNT), functionalized CNT, graphene, graphene oxide, reduced graphene oxide, and nanodiamonds have been investigated in the literature. Most of the studies reported enhanced BHT performance. This study aimed to investigate the BHT performance of the colloidal dispersions of carbon black (CB), which has favorable intrinsic properties but it has not yet been studied in BHT research. Experimentally, stable dispersions of CB colloids were prepared in deionized water at three different weight concentrations: 0.001%, 0.005% and 0.01%. A custom-built experimental pool boiling setup with a flat copper heating surface was used to investigate the BHT performance of the CB colloids. The heat transfer coefficient (HTC) and critical heat flux (CHF) enhancements of the nanofluids increased with the increasing CB concentration and reached 190.5% and 67.8%, respectively at 0.01 wt% CB. The CHF enhancement was attributed to the decent lateral heat conduction of the CB microlayer. It prevented localized heating by spreading the surface heat in lateral directions and delayed the formation of hot/dry spots to higher heat fluxes. The HTC enhancement was due to the densely distributed small cavities on the coating layer, which increased the surface roughness by ∼9-fold. This provided a larger effective surface area for heat transfer, and a greater active bubble nucleation site density. The more hydrophobic surface rendered higher bubble departure frequency that effectively transferred the heat from the heating surface to the bulk fluid.

Enhanced bubble nucleation and liquid rewetting for highly efficient boiling heat transfer on two-level hierarchical surfaces with patterned copper nanowire arrays

Enhancing boiling heat transfer by surface modification is of critical interest for improving the efficiency of many energy systems and for addressing thermal management bottlenecks in electronics. However, the improvement of all boiling heat transfer characteristics including the critical heat flux, heat transfer coefficient and onset of nucleate boiling, usually has conflicting requirements on surface wettability and morphology. In this work, we develop a two-level hierarchical surface with patterned copper nanowire arrays for boiling heat transfer enhancement. By surrounding long nanowire arrays with short nanowires where microcavities are formed between short nanowire clusters, a novel strategy is reported to improve all the boiling heat transfer characteristics through increasing bubble nucleation site density, capillary-induced liquid rewetting, and the separation of liquid-vapor pathways. Compared to boiling heat transfer performance on the plain copper surface, a 71% higher critical heat flux, a 185% higher heat transfer coefficient as well as a 37% lower onset of nucleate boiling are demonstrated on such two-level hierarchical surfaces. In addition, we correctly predict the effect of surface structure on the boiling heat transfer performance by an analytical model. Through distinguishing the role of different structure morphologies including the improved nucleation site by microcavities, enhanced liquid wicking by nanowires, and continuous liquid supply by long nanowire arrays, we have established a comprehensive understanding on the relation between the surface structures and boiling heat transfer characteristics.

A genetic algorithm-based optimization model for pool boiling heat transfer on horizontal rod heaters at isolated bubble regime

A new optimized model which can predict the heat transfer in the nucleate boiling at isolated bubble regime is proposed for pool boiling on a horizontal rod heater. This model is developed based on the results of direct observations of the physical boiling phenomena. Boiling heat flux, wall temperature, bubble departing diameter, bubble generation frequency and bubble nucleation site density have been experimentally measured. Water and ethanol have been used as two different boiling fluids. Heating surface was made by several metals and various degrees of roughness. The mentioned model considers various mechanisms such as latent heat transfer due to micro-layer evaporation, transient conduction due to thermal boundary layer reformation, natural convection, heat transfer due to the sliding bubbles and bubble super-heating. The fractional contributions of individual mentioned heat transfer mechanisms have been calculated by genetic algorithm. The results show that at wall temperature difference more that about 3 K, bubble sliding transient conduction, non-sliding transient conduction, micro-layer evaporation, natural convection, radial forced convection and bubble super-heating have higher to lower fractional contributions respectively. The performance of the new optimized model has been verified by comparison of the existing experimental data.

Numerical investigation of nucleate pool boiling heat transfer

Multidimensional numerical simulation of the atmospheric saturated pool boiling is performed. The applied modelling and numerical methods enable a full representation of the liquid and vapour two-phase mixture behaviour on the heated surface, with included prediction of the swell level and heated wall temperature field. In this way the integral behaviour of nucleate pool boiling is simulated. The micro conditions of bubble generation at the heated wall surface are modelled by the bubble nucleation site density, the liquid wetting contact angle and the bubble grow time. The bubble nucleation sites are randomly located within zones of equal size, where the number of zones equals the nucleation site density. The conjugate heat transfer from the heated wall to the liquid is taken into account in wetted heated wall areas around bubble nucleation sites. The boiling curve relation between the heat flux and the heated wall surface temperature in excess of the saturation temperature is predicted for the pool boiling conditions reported in the literature and a good agreement is achieved with experimentally measured data. The influence of the nucleation site density on the boiling curve characteristic is confirmed. In addition, the influence of the heat flux intensity on the spatial effects of vapour generation and two-phase flow are shown, such as the increase of the swell level position and the reduced wetting of the heated wall surface by the heat flux increase.

Numerical prediction of nucleate pool boiling heat transfer coefficient under high heat fluxes

This paper presents CFD (Computational Fluid Dynamics) approach to prediction of the heat transfer coefficient for nucleate pool boiling under high heat fluxes. Three-dimensional numerical simulations of the atmospheric saturated pool boiling are performed. Mathematical modelling of pool boiling requires a treatment of vapor-liquid two-phase mixture on the macro level, as well as on the micro level, such as bubble growth and departure from the heating surface. Two-phase flow is modelled by the two-fluid model, which consists of the mass, momentum and energy conservation equations for each phase. Interface transfer processes are calculated by the closure laws. Micro level phenomena on the heating surface are modelled with the bubble nucleation site density, the bubble resistance time on the heating wall and with the certain level of randomness in the location of bubble nucleation sites. The developed model was used to determine the heat transfer coefficient and results of numerical simulations are compared with available experimental results and several empirical correlations. A considerable scattering of the predictions of the pool boiling heat transfer coefficient by experimental correlations is observed, while the numerically predicted values are within the range of results calculated by well-known Kutateladze, Mostinski, Kruzhilin and Rohsenow correlations. The presented numerical modeling approach is original regarding both the application of the two-fluid two-phase model for the determination of heat transfer coefficient in pool boiling and the defined boundary conditions at the heated wall surface.

Experimental study on the saturated pool boiling heat transfer on nano-scale modification surface

Plain nickel foil surface, three kinds of plain nickel-based chemical treatment surfaces and four kinds of nickel-based electrochemical treatment surfaces with nano-cone array structure were used in the pool boiling experiment. The quantitative effects and the effect mechanism of the surface characteristic parameters, including nano-scale roughness (Ra), solid–liquid contact angle (CA) and effective heat transfer area ratio (r: the ratio of the actual heat transfer area and its projected area) on the heat transfer coefficient (HTC) on the modification plain surfaces and nano-cone array surfaces were studied and summarized. By improving the previous proposed the HTC prediction correlation for the common plain surfaces (without considering the effect of r), a new uniform correlation is proposed and it can well predict the HTC of the heated surface with simple convex structure (with considering the effect of r). In addition, the bubble dynamic characteristics including the bubble departure diameter, the bubble departure frequency, the active bubble nucleation site density and the combined bubble were investigated by visualization research. These results proposed some mechanism support for the new predicting correlation.

Assessment of subcooled boiling wall boundary correlations for two-fluid model CFD

Abstract An assessment of the heat and mass transfer wall boundary conditions used for subcooled boiling simulations with a CFD two-fluid model has been performed. This assessment was focused on the wall heat flux partitioning model using the state-of-the-art multidimensional Freon experimental data available in the literature. Various constitutive relations used to close the vapor generation rate at the heated wall were studied in order to obtain the best suited combination(s). The current study was restricted to vertical flows through pipe and annulus geometries. Two Freon data sets from the literature were considered: the first with R12 at about 2.6 MPa pressure and the second with R113 at 269 kPa pressure. In these data sets, the bubble diameter distribution measurements across the ducts were available. Bubble diameter estimation is the largest uncertainty in the boiling two-fluid model predictions and hence using the data with known bubble sizes allowed to focus on the assessment of other parameters to model vapor generation rate at the wall, in particular bubble nucleation site density and bubble departure frequency. From the parametric simulations with different combinations of the nucleation site density and frequency models, the discrepancy in obtaining a consistent frequency model for Freon became evident. The state-of-the-art frequency model under consideration is a function of the wall temperature which is estimated using a well-known empirical nucleate boiling heat transfer coefficient correlation. The frequency model provided inaccurate predictions in cases where the nucleate boiling heat transfer coefficient was inadequate to predict the wall superheat. The frequency model is now improved by using a more recent nucleate boiling heat transfer coefficient correlation with wider applicability, i.e., Freon. The simulations were carried out using the CFD code CFX (version 12). The Freon data used here corresponds to fluid-vapor density ratios ranging from 6 to 76. This assessment validated a consistent site density-departure frequency correlation combination for subcooled boiling simulations which has not been used so far for CFD simulations and an extension of the applicability of the correlation combination to high pressure steam–water conditions due to the favorable scaling of Freon physical properties.

Effects of surface roughness on flow boiling in silicon microgap heat sinks

Understanding the influence of surface characteristics on flow boiling heat transfer behavior in microgap is necessary to enhance the performance of microgap heat sink. The influences of surface roughness on flow boiling heat transfer, pressure drop and instability in microgap heat sink are experimentally investigated. Flow boiling experiments are conducted over silicon microgap heat sink of three different microgap dimensions namely 500μm, 300μm and 200μm. The original silicon surface of surface roughness, Ra=0.6μm is modified to Ra=1.0μm and 1.6μm to examine the effect of surface finish. These studies are carried out with the inlet deionized water temperatures 91°C at two different mass fluxes, G=390kg/m2s and 650kg/m2s and imposed effective heat flux, qeff″ ranging from 0W/cm2 to 85W/cm2. High speed flow visualizations are conducted simultaneously along with experiments to explore the bubble behavior in microgap heat sink. The results of this study show that bubble nucleation site density as well as heat transfer coefficient increases with the increase of surface roughness and pressure drop is independent of surface roughness in microgap heat sink. Moreover, rougher surface maintains lower and uniform wall temperature over the heated surface. However, surface roughness has an adverse effect on the inlet pressure instability and inlet pressure fluctuation increases with increasing surface roughness at larger microgap heat sink.

Numerical investigation of boiling regime transition mechanism by a Lattice–Boltzmann model

A numerical study has been performed to investigate the hydrodynamic aspects of the pool boilingon horizontal-, vertical- and downward-facing surfaces. The FlowLab code, which is based on a Lattice–Boltzmann (LB) model of two-phase flows, is employed. Macroscopic properties, such as surface tension ( σ) and contact angle ( β), are implemented through the fluid–fluid ( G σ ) and fluid–solid ( G t ) interaction potentials. The model is found to express a linear relation between the macroscopic properties ( σ, β) and microscopic parameters ( G σ , G t ). The simulation results on bubble departure diameter appear to have the same parametric dependence as the empirical correlation. Hydrodynamic aspects of two-phase flow regime transition mechanism are investigated for different surface–coolant configurations. Results of the LB simulation clearly demonstrate that not only the bubble nucleation site density (related, e.g. to the heater surface condition and heat fluxes), but also the surface position have a profound effect on the flow regime (pool boiling) characteristics. The results of the LB simulation of hydrodynamics of two-phase flow on the horizontal surface provide the pictures quite similar to the experimental observation for saturated pool boiling. Two mechanisms of flow (boiling) regime transition on the vertical surface are predicted for the local bubble coalescence at bubble generation site and the downstream bubble coalescence. On the downward-facing surfaces, friction between bubbles and the surface wall is found to significantly enlarge the bubble size prior the bubble slip upwards. This behavior is responsible for the earlier bubble coalescence, and therefore, lowers the maximum heat removal rate, in a similar regime of nucleate boiling on a downward-facing surface.