Bubble Dynamics and Heat Transfer During Pool Boiling on Wettability Patterned Surfaces

Surfaces with spatial wettability patterns have been proven to enhance heat transfer coefficient and critical heat flux in pool boiling. To understand the physical mechanism behind this phenomenon and obtain the correlation among some critical parameters (bubble departure frequency, bubble size, nucleation site density, surface tension), pool boiling experiments were conducted. A Pyrex glass with a layer of indium-tin-oxide was used as the substrate. Hydrophobic patterns will serve as nucleation sites. Experiments were conducted in deionized water under atmospheric pressure at a relatively low heat flux. The processes of nucleation, growth, and departure of individual bubbles were visualized by using a high speed camera through the bottom of the heater surface. It has been found that the patterned surface performed the best in heat transfer for subcooled pool boiling when compared with hydrophilic and hydrophobic surfaces. The nucleation site density of the biphilic surface was much higher, when compared with that of the homogeneous surface. The individual bubbles always nucleate on the edge of the hydrophobic and hydrophilic area, and then move onto the hydrophobic pattern. Most of the individual bubbles detach from the wettability patterned surface in the diameter range from 300 µm to 450 µm (around 77.3%). The bubble departure periods scatter in the range from 80 ms to 1500 ms.     

Pool boiling heat transfer of water and nanofluid outside the surface with higher roughness and different wettability

In order to investigate the effect of surface wettability on the pool boiling heat transfer, nucleate pool boiling experiments were conducted with deionized water and silica based nanofluid. A higher surface roughness value in the range of 3.9 ~ 6.0μm was tested. The contact angle was from 4.7° to 153°, and heat flux was from 30kW/m² to 300kW/m². Experimental results showed that hydrophilicity diminish the boiling heat transfer of silica nanofluid on the surfaces with higher roughness. As the increment of nanofluid mass concentration from 0.025% to 0.1%, a further reduction of heat transfer coefficient was observed. For the super hydrophobic surface with higher roughness (contact angle 153.0°), boiling heat transfer was enhanced at heat flux less than 93 kW/m², and then the heat transfer degraded at higher heat flux.     

Contribution of thin-film evaporation during flow boiling inside microchannels

Flow boiling through microchannels is characterized by nucleation and growth of vapor bubbles that fill the entire channel cross-sectional area. As the bubbles nucleate and grow inside the microchannel, a thin film of liquid or a microlayer gets trapped between the bubbles and the channel walls. The heat transfer mechanism present at the channel walls during flow boiling is studied numerically. It is then compared to the heat transfer mechanisms present during nucleate pool boiling and in a moving evaporating meniscus. Increasing contact angle improved wall heat transfer in case of nucleate boiling and moving evaporating meniscus but not in the case of flow boiling inside a microchannel. It is shown that the thermal and the flow fields present inside the microchannel around a bubble are fundamentally different as compared to nucleate pool boiling or in a moving evaporating meniscus. It is explained why thin-film evaporation is the dominant heat transfer mechanism and is responsible for creating an apparent nucleate boiling effect inside a microchannel.     

Appearance of a low superheat “quasi-Leidenfrost” regime for boiling on superhydrophobic surfaces

Pool boiling experiments were performed with degassed water on stainless steel substrates with different surface topographies and wettabilities. Boiling curves and visual observations of the boiling process have been carried out. The onset of nucleate boiling (ONB) has been measured and the influence of roughness and wettability has been quantified. Boiling curve shape is different between the hydrophilic and the superhydrophobic cases; superhydrophobic surfaces reaching the ONB heat flux at a lower superheat and presenting a "quasi-Leidenfrost" regime, without showing the typical boiling curve. Bubbles are easier to form on superhydrophobic surfaces, therefore the nucleation temperature is smaller, and bubbles are larger and stable. The ONB appears after less than 5 K of superheat on superhydrophobic surfaces, while on hydrophilic surfaces, with the same surface roughness, the superheat is above 7 K. Furthermore, superhydrophobic samples with a different roughness present the same boiling curve, meaning that, when the contact angle exceeds a certain value, the wettability has a predominant role on the surface roughness.     

Molecular dynamics study of wettability and pitch effects on maximum critical heat flux in evaporation and pool boiling heat transfer

Molecular dynamics simulations were employed to investigate the effects of wettability (contact angle) and pitch on nanoscale evaporation and pool boiling heat transfer of a liquid argon thin film on a horizontal copper substrate topped with cubic nano-pillars. The liquid–solid potential was incrementally altered to vary the contact angle between hydrophilic (～0°) and hydrophobic (～127°), and the pitch (distance between nano-pillars) was varied between 21.7 and 106.6?Å to observe the resultant effect on boiling heat transfer enhancement. For each contact angle, the superheat was gradually increased to initiate nucleate boiling and eventually pass the critical heat flux (CHF) into the film boiling regime. The CHF increases with pitch, and tends to decrease substantially with increasing contact angle. A maximum overall heat flux of 1.59?×?10⁸?W/m² occurs at the largest pitch investigated (106.6?Å), and as the contact angle increases the superheat required to reach the CHF condition also increases. Finally, in certain cases of small pitch and large contact angle, the liquid film was seen to transition to a Cassie–Baxter state, which greatly hindered heat transfer.     

Effects of Surface Wettability on Rapid Boiling and Bubble Nucleation: A Molecular Dynamics Study

Molecular dynamics simulation is conducted to study the effects of surface wettability on rapid boiling and bubble nucleation over smooth surface. The simple L-J liquid is heated by smooth metal surface with different conditions of wettability in cuboid simulation box. The results show that surface wettability has significant impact on phase transition of liquid film. When the heating temperature is 200 K, the rapid boiling occurs above strongly hydrophilic and weakly hydrophilic surfaces; however, only slow evaporation phenomenon occurs above weakly hydrophobic surface within 2.5-ns simulation time. The reason is that the interaction between argon and platinum atoms is stronger over hydrophilic surface, which has higher efficiency in heat transfer. Furthermore, based on the difference of surface wettability in heat transfer efficiency, the surface with nonuniform wettability is constructed, and the central region is more hydrophilic than surrounding region. The growing process of bubble nucleus can be completely observed above the more hydrophilic region.     

Nucleate boiling in two types of vertical narrow channels

To explore the mechanism of boiling bubble dynamics in narrow channels, we investigate 2-mm wide I- and Z-shaped channels. The influence of wall contact angle on bubble generation and growth is studied using numerical simulation. The relationships between different channel shapes and the pressure drop are also examined, taking into account the effects of gravity, surface tension, and wall adhesion. The wall contact angle imposes considerable influence over the morphology of bubbles. The smaller the wall contact angle, the rounder the bubbles, and the less time the bubbles take to depart from the wall. Otherwise, the bubbles experience more difficulty in departure. Variations in the contact angle also affect the heat transfer coefficient. The greater the wall contact angle, the larger the bubble-covered area. Therefore, wall thermal resistance increases, bubble nucleation is suppressed, and the heat transfer coefficient is lowered. The role of surface tension in boiling heat transfer is considerably more important than that of gravity in narrow channels. The generation of bubbles dramatically disturbs the boundary layer, and the bubble bottom micro-layer can enhance heat transfer. The heat transfer coefficient of Z-shaped channels is larger than that of the I-shaped type, and the pressure drop of the former is clearly higher.     

Numerical Simulation of Bubbles Deformation,Flow, and Coalescence in a Microchannel Under Pseudo-Nucleation Conditions

This paper reports the results of numerical study on bubbles deformation, flow, and coalescence under pseudo-nucleate boiling conditions in horizontal mini-/microchannels. The numerical simulation, which is based on the multiphase model of volume of fluid method, aims to study the corresponding flow behaviors of nucleate bubbles generated from the tube walls in mini-/microchannels so as to understand the effect of confined surfaces/walls on nucleate bubbles and heat transfer. Under the pseudo- or quasi-nucleate boiling condition, superheated small vapor bubbles are injected at the wall to ensure that the bubbles generation is under a similar condition of real nucleation. The numerical study examined the fluid mechanics of bubble motion with heat transfer, but the mass transfer across the bubble–liquid interface is not simulated in the present work.     

Boiling Feature on a Super Water-Repellent Surface

The boiling feature on a super water-repellent (SWR) surface has been studied. The SWR surface has a coating layer of fine particles of nickel and PTFE. Its contact angle to water is 152°in room temperature. The heat transfer surface is facing upward, and the diameter of the heated section is 17 mm. The boiling feature of this surface is completely different from that of usual surfaces. The stable film boiling occurs in very small superheating, and there is no nucleate boiling region. The bubbles generated on the surface coalesce into a vapor film without departing from the surface. The stable vapor film exists even at a surface temperature below the saturation temperature.     

Nucleate Boiling Enhancements on Porous Graphite and Microporous and Macro–Finned Copper Surfaces

Enhancements in nucleate boiling heat removal with dielectric liquids, by increasing either the bubbles nucleation sites density and/or the wetted surface area, are desirable for immersion cooling of high-power computer chips. This article presents the results of recent investigations of nucleate boiling enhancement of FC-72, HFE-7100, and PF-5060 dielectric liquids on porous graphite, copper microporous surfaces, and copper surfaces with square corner pins, 3 mm × 3 mm in cross-section and 2, 3, and 5 mm tall. All surfaces have a footprint measuring 10 × 10 mm. These investigations examined the effects of liquid subcooling up to 30 K and surface inclination, from upward-facing to downward-facing, on nucleate boiling heat transfer coefficient and critical heat flux. Natural convection of dielectric liquids for cooling chips while in the stand-by mode, at a surface average heat flux <20 kW/m², is also investigated for the different surfaces.     

Boiling Heat Transfer Performance in a Spiraling Radial Inflow Microchannel Cold Plate

ABSTRACT

This study presents an experimental exploration of flow boiling heat transfer in a spiraling radial inflow microchannel heat sink. The effect of surface wettability, fluid subcooling, and mass fluxes are considered. The design of the heat sink provides an inward radial swirl flow between parallel, coaxial disks that form a microchannel of 300 microns. The channel is heated on one side, while the opposite side is essentially adiabatic to simulate a heat sink scenario for electronics cooling. To explore the effects of varying surface wetting, experiments were conducted with two different heated surfaces. One was a clean, machined copper surface and the other was a surface coated with zinc oxide nanostructures that are superhydrophilic. During boiling, increased wettability resulted in quicker rewetting and smaller bubble departure diameter, as indicated by reduced temperature oscillations during boiling, and achieving higher maximum heat flux without dryout. The highest heat transfer coefficients were seen in fully developed boiling with low subcooling levels as a result of heat transfer being dominated by nucleate boiling. The highest heat fluxes achieved were during partial subcooled flow boiling at 300 W/cm² with an average surface temperature of 134° Celsius. Recommendations for electronics cooling applications are also discussed.     

An analysis of surface-microstructures effects on heterogeneous nucleation in pool boiling

The interaction of surface microstructures and wettability effects on heterogeneous nucleation in pool boiling is analyzed in this paper based on the changes of free energy and availability. It is shown that the bubble is most easily formed on a concave surface in comparison with a convex surface or a plane surface at the same wettability and the same wall temperature. It is found that the effect of microstructures greatly enhances nucleation of bubbles when the curvature radius of these microstructures is in the range of 5–100 times less than the bubble radius. Larger than this limit, the surface roughness effect is negligible and the wettability effect predominates. Closed form analytical solutions for the critical radius and change in availability are obtained for the special case of homogeneous nucleation where no wall temperature gradient exists on surfaces with microstructures. Under this simplified assumption, it is found that the microstructures have no effect on critical nucleation radius and their effect on the change in availability is underestimated.     

Boiling Enhancement on Nanostructured Surfaces with Engineered Variations in Wettability and Thermal Conductivity

ABSTRACT

This work provides fundamental insights into the underlying mechanisms of pool boiling enhancement using a variety of different engineered surface designs. Specifically, the effects of nanostructured coatings, surfaces with mixed wettability, and surfaces with in-plane variations in thermal conductivity are investigated. The positive and negative impacts of each design on the onset of nucleate boiling, heat transfer coefficient, bubble dynamics and the ebullition cycle, as well as critical heat flux have been characterized. It is seen that while several techniques enhance one element of the boiling process, they can degrade others. This analysis has led to the design, fabrication, and characterization of complex heterogeneous surfaces by combining multiple engineered surface techniques. Nanostructured surfaces with variations in substrate thermal conductivity have been shown to increase critical heat flux by a factor of 2.6× as compared to bare copper substrates. In addition, nanostructured surfaces with engineered variations in substrate conductivity as well as surface wettability have been shown to increase heat transfer coefficient by more than a factor of 10×.     

Physical mechanisms of heat transfer during single bubble nucleate boiling of FC-72 under saturation conditions-I. Experimental investigation

This paper is the first of a two-part study concerning the dynamics of heat transfer during nucleation process of saturated FC-72 liquid. Experimental results discussed in this paper provide new physical insight on the nature of heat transfer events at the nucleation site during the nucleate boiling process. The thermal field underneath a bubble during the boiling of FC-72 was measured with a spatial resolution of 22--40 μm. The time period of activation, area of influence, and magnitude of three different mechanisms of heat transfer active at the nucleation site were determined. These mechanisms consisted of: (1) microlayer evaporation following the rapid bubble expansion, (2) transient conduction due to rewetting of the surface during bubble departure, and (3) microconvection in the region external to the bubble/surface contact area. The area of influence of the transient conduction mechanism was found to be limited to the bubble/surface contact area, with most of the heat transfer occurring prior to the bubble detachment from the surface. The microconvection heat transfer mechanism was localized primarily outside the contact area and was found to be steady in nature. All three mechanisms of heat transfer were found to make significant contributions to the total surface heat transfer. The second part of this study provides the theoretical analysis of the results.     

Enhancement of nucleate boiling on composite surfaces

A composite heating surface composed of materials with different thermal conductivities can be expected to enhance heat transfer in nucleate boiling. This is because the end surface, with higher conductivity, will attain a higher temperature and as a result will serve to provide preferential nucleation sites. To confirm this idea, several composite surfaces were fabricated by uniaxially imbedding thin copper cylinders in the heat flow direction on a stainless steel circular plate 30 mm in diameter and 5 mm thick. The imbedded copper cylinders ranged from 1 mm to 4 mm in diameter and one to 77 in number. The heat transfer performance of these composite surfaces was investigated for pool boiling of saturated water at atmospheric pressure. It was confirmed that the copper cylinder surfaces exposed to water functioned as local hot spots to initiate preferential nucleate boiling, leading to higher boiling heat transfer coefficients than those on a homogeneous stainless steel surface. The measured void fraction above the heating surface verified intensive bubble generation on the surface of the copper cylinders. This situation continued up to a certain heat flux level and was then followed by nucleation on the mother surface of stainless steel around the copper cylinders. A numerical analysis of heat conduction within a composite wall simulated the temperature distribution within the wall and the variation in surface heat flux at the time of boiling incipience. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(3): 216–228, 1998     

Design,synthesis, and characterization of hybrid micro-nano surface coatings for enhanced heat transfer applications

Metal particles coating is extensively used for surface coating a wide range of application including thermal management of electronics, concentrating photovoltaics, sensors and nuclear power plants. Both micro and nano-scale surfaces have been proven to show an enhanced two-phase heat transfer performance by varying surface properties like area, wettability, and roughness. To combine the unique features of both micro and nano-scale surface coatings, this study presents the design, synthesis, and characterization of new hybrid micro-nano scale surface coating by a new two steps approach. Five different types of surfaces; namely, plain nanocoated (PNC), uniform micro-porous (UMP), uniform hybrid micro-nano porous (UHMNP), 2-D modulated microporous (MMP) and modulated hybrid micro-nano (MHMNP) surfaces were fabricated. A new two steps approach of hot-pressing followed by nucleate boiling is used for the fabrication of these surfaces. Successful coating of hybrid micro-nano scale coating was achieved. Considering the critical surface properties of micro and nanoscale coatings, new hybrid micro-nano surfaces have been characterized for SEM, wettability, roughness test. The comparative analysis of these new hybrid coating is also performed with micro coated and uncoated surfaces. With the coating of nanoparticles, the average roughness of PNC surface increased by 4.67 times and that of hybrid micro-nano particle surface by 2.3 times. The deposition of nanoparticles resulted in an increase in contact angle for PNC surface, while the contact angle of hybrid micro-nano surfaces decreases from 126.4° to 82.1°.     

A Study on Enhancement of Boiling Heat Transfer by Mixed-Wettability Surface

ABSTRACT

The enhancement of pool boiling heat transfer in FC-72 on a novel mixed-wettability surface was experimentally investigated. On the mixed-wettability surface, the micro-pin-finned area and the smooth area were distributed in the form of fractal by using micromaching method (dry etching method). From the comparison with the smooth surface and the micro-pin-finned surface, the mixed-wettability surface could efficiently enhance the heat transfer performance in the nucleate boiling region, and the critical heat flux was also efficiently improved. From the boiling experiment result, it is discovered that a larger heat transfer area does not always lead to a better heat transfer performance. From the peculiar boiling phenomenon of the novel surface, it can be observed that large number of nucleation sites are formed in the micro-pin-finned area, and the small bubbles grow, collide, merge and move rapidly to the nearby smooth channel. When the bubble grows large enough, it will departure quickly under the effect of channel pressure. It can be concluded that the mixed-wettability surface can not only guarantee sufficient nucleation sites, but also facilitate the departure of bubbles and enhance the bubbles' interaction.     

A Photographic study of subcooled flow boiling burnout at high heat flux and velocity

The present paper reports the results of a visualization study of the burnout in subcooled flow boiling of water, with square cross section annular geometry (formed by a central heater rod contained in a duct characterized by a square cross section). The coolant velocity is in the range 3–10 m/s. High speed movies of flow pattern in subcooled flow boiling of water from the onset of nucleate boiling up to physical burnout of the heater are recorded. From video images (single frames taken with a stroboscope light and an exposure time of 1 μs), the following general behaviour of vapour bubbles was observed: when the rate of bubble generation is increasing, with bubbles growing in the superheated layer close to the heating wall, their coalescence produces a type of elongated bubble called vapour blanket. One of the main features of the vapour blanket is that it is rooted to the nucleation site on the heated surface. Bubble dimensions are given as a function of thermal-hydraulic tested conditions for the whole range of velocity until the burnout region. A qualitative analysis of the behaviour of four stainless steel heater wires with different macroscopic surface finishes is also presented, showing the importance of this parameter on the dynamics of the bubbles and on the critical heat flux.     

Surface wettability change during pool boiling of nanofluids and its effect on critical heat flux

The pool boiling characteristics of dilute dispersions of alumina, zirconia and silica nanoparticles in water were studied. Consistently with other nanofluid studies, it was found that a significant enhancement in critical heat flux (CHF) can be achieved at modest nanoparticle concentrations (<0.1% by volume). Buildup of a porous layer of nanoparticles on the heater surface occurred during nucleate boiling. This layer significantly improves the surface wettability, as shown by a reduction of the static contact angle on the nanofluid-boiled surfaces compared with the pure-water-boiled surfaces. A review of the prevalent CHF theories has established the nexus between CHF enhancement and surface wettability changes caused by nanoparticle deposition. This represents a first important step towards identification of a plausible mechanism for boiling CHF enhancement in nanofluids.