Heat Mass Transfer (2012) 48:267–274 DOI 10.1007/s00231-011-0883-8
ORIGINAL
The bubble fossil record: insight into boiling nucleation using nanofluid pool-boiling David Huitink • Elvis Efren Dominguez Ontiveros Yassin Hassan
•
Received: 7 September 2010 / Accepted: 3 August 2011 / Published online: 13 August 2011 Ó Springer-Verlag 2011
Abstract Subcooled pool boiling of Al2O3/water nanofluid (0.1 vol%) was investigated. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used to observe surface features of the wire heater where nanoparticles had deposited. A layer of aggregated alumina particles collected on the heated surface, where evidence of fluid shear associated with bubble nucleation and departure was ‘‘fossilized’’ in the fluidized nano-porous surface coating. These structures contain evidence of the fluid forces present in the microlayer prior to departure and provide a unique understanding of boiling phenomena. A unique mode of heat transfer was identified in nanofluid pool boiling. Abbreviation C Heat capacity CHF Critical heat flux dNP Nanoparticle diameter EDS Energy dispersive X-ray spectroscopy N Number of nucleation sites per area per time PIV Particle image velocimetry pzc Point of zero charge 00 q conv Heat flux due to convective heat transfer q00 latent Heat flux due to latent heat exchange q00 NP Heat flux resulting from the removal of the adhered nanoparticles
D. Huitink Y. Hassan (&) Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123, USA e-mail:
[email protected] E. E. D. Ontiveros Y. Hassan Department of Nuclear Engineering, Texas A&M University, College Station, TX 77843-3133, USA
q00 quench rbubble SEM t Twall
Heat flux due to quenching of nucleation site after bubble departure Bubble radius Scanning electron microscope Thickness of nanoparticle layer [given as a function of rbubble: t(rbubble)] Wall temperature
Greek symbols h Contact angle q Density u Void fraction Subscripts b Buoyancy nf Nanofluid NP Subscript referring to nanoparticles r Surface tension
1 Introduction In the past decade, nanofluids have garnered interest for enhancing heat transfer in thermal management systems. These colloidal solvents containing dispersed nanometer (*10–100 nm) sized particles [1] have displayed many desirable features for use as a heat transfer medium. The primary reasoning behind the development of these fluids is derived from the fact that stable colloidal suspensions can be achieved in contrast to earlier attempts using unstable suspensions containing micron-sized particles. Furthermore, the high surface to volume ratio of nanoparticles was expected to enhance their thermal conductivity at low concentrations, which in many cases was also experimentally observed [2–4]. Nanofluids have also exhibited single-phase heat transfer enhancement, although
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often viscosity and pumping requirements are significantly increased [5–7]. Additionally, investigations considering two-phase (pool boiling) convective heat transfer have also reported the effects of nanofluids. In one example, the critical heat flux (CHF) was reported to increase by 300% during pool boiling of nanofluids while the required wall superheat for a given heat flux was also found to increase for nanofluids compared to the pure dispersing fluid [8, 9]. Despite these amazing results, there are contradicting reports in the literature that document a reduction in the pool boiling heat transfer coefficient for nanofluids [10, 11]. Trisaksri and Wongwises observed degradation of nucleate boiling performance when considering TiO2 suspended in R-141b at 0.03 and 0.05 vol%, but saw little difference with 0.01 vol%, even when fluid pressure was varied [12]. Yet when considering refrigerant based nanofluids containing carbon nanotubes 1% by volume, pool boiling enhancements were observed [13, 14]. The observed enhancement is possibly due to increase in the number of nucleation sites, the size of the nucleation cavities and the associated nucleation site density arising from the precipitation of the nano-particles. Nanofluids are also known to have different wetting behavior which could affect the boiling performance based on the nanoparticles’ hydrophobicity [15, 16]. Furthermore, enhanced fouling of the heater surface due to rapid precipitation of the nano-particles could also cause degradation of the pool boiling performance as noted by Liu and Liao [17]. Other reports have indicated similar phenomena, but have offered that the combination of wettability of the nanoparticles in combination with their surface precipitation during boiling leads to either enhancement or degradation in CHF [16, 18]. As a counter point, enhancements similar to that made by nanofluids could also be triggered by surface modifications without nanofluids [16]. Furthermore, Buongiorno et al. reiterated the surface effects of nanofluid influence, and also reported differences in hot-spot dynamic behavior, indicating that the nanofluids were not simply changing the surface properties, but also affecting some of the phenomena normally observed in nucleate boiling [18]. Moreover, a number of different experiments have observed the deposition of nanoparticles onto a heated surface as chronicled in a recent review on pool booling of nanofluids [19], which most certainly result in altered surface conditions. Whether one of these modes (fluid property enhancement, wetting behavior, or surface modifications) dominates the boiling behavior is not well understood, although contributions from each can be expected to affect twophase behavior. In order to further understand the nature of boiling phenomena in the presence of nanofluids, a surface
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investigation of wire boiling in a sub-cooled pool was investigated here.
2 Experiment description 2.1 Pool boiling Alumina (c-Al2O3) nanoparticles (NEI) were dispersed in water without any surfactant agent at a volume concentration of 0.1 vol%. A 250 lm diameter stainless steel wire was utilized for a boiling surface immersed in the nanofluid, whose bulk fluid temperature was maintained between 70 and 80°C with an auxiliary heater. Prior to boiling, the auxiliary heater was used to degas the fluid. More details concerning the configuration were previously reported [20]. During boiling, high speed, high resolution images were obtained at the boiling surface for use with particle image velocimetry (PIV). The fluid was seeded with 6–9 lm diameter polystyrene particles—having similar density to the fluid—for tracking the fluid behavior near the surface of the wire. To maintain visibility for imaging, boiling was maintained at a relatively low heat flux (30 kW/m2). Results of the PIV analysis are reported elsewhere [20], which showed increased velocity fluctuations in the nanofluid tests over plain water boiling indicating enhanced convective ability. After reaching a steady-state boiling process, the PIV imaging was conducted for *10 min; after which the power supply was disconnected and the wire was removed from the nanoparticle suspension and air dried. 2.2 Ex situ microscopy To evaluate the surface after boiling, the wire was carefully divided into *1 cm lengths for mounting in the scanning electron microscope (SEM). Samples were taken from the heated portion of the wire, including near both the positive and negative poles, and from an unheated portion of the wire that was also submerged in the nanofluid during the experiment. For comparison, a clean, unused portion of the wire was also evaluated. Being careful not to disturb the wire surfaces, the samples were mounted onto SEM sample stages using conductive carbon tape. The samples were characterized in a FEI Quanta 600 FE-SEM equipped with an Oxford INCA Energy Dispersive X-ray Spectrometer (EDS), using a stage capable of up to 70° of tilt. The SEM operated at vacuum pressures less than 5E-5 torr with electron beam energy operated at either 10 or 20 kV, depending on the level of sample charging present. The beam energy was held constant at 20 kV for all EDS analyses. A working distance of 10 mm was maintained between the sample and the backscatter detector for
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imaging with zero tilt, but increased to 40 mm to allow room for the stage tilt when required.
3 Results and discussion A SEM visual comparison taken at low magnification of three different wire samples is shown in Fig. 1. A clean, unused wire appears in Fig. 1a, b depicts an unheated wire immersed in the nanofluid during the boiling experiment, and Fig. 1c shows a portion of the heated wire where
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boiling took place. Both images in Fig. 1a and b appear quite similar, whereas the wire used in boiling (Fig. 1c) has a significant amount of material adhered to the surface. The left-hand surface of the wire in the image does not exhibit this layer of material due to mechanical removal by contact with the forceps used to handle the sample (the removed portion(s) can be seen in the corner of the image). This area shares features with the other two samples; but more importantly, it allows the thickness of this external coating to be determined. Based on the edges of the removed material, approximately six microns of deposited material
Fig. 1 Comparison of SS wire samples: a unused, b immersed in nanofluid but unheated, and c heated surface for boiling. Inset images depicting higher magnification of surface
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Fig. 2 Nucleation Site SEM Images. a Profile taken at 70.6° tilt. b Top view at 0° tilt. c Higher magnification at center of nucleation site
exists on the heated portion of the wire, which is not seen in either the unused wire or the unheated wire immersed in the experimental apparatus. Energy dispersive X-ray spectroscopy (EDS, not shown) on the immersed and unheated wire revealed only the typical elemental composition of the stainless steel (Fe, Cr, Ni), whereas Al and O are the primary detected elements on the heated portion. This corresponds well to the SEM observations, since the penetration depth of EDS is approximately 5 lm. Even so, a closer examination (inset images) revealed the presence of alumina particles on the surfaces of both the unheated and heated portion, of predominantly 50–100 nm diameter (see Fig. 2). Van der Waals attraction between the surface of the stainless steel wire and alumina nanoparticles in the nanofluid results in a thin coating (less than 500 nm) over the entire submerged wire; however, the presence of resistive heating produced a nanoparticle coating more than 10 times the thickness of the van der Waals layer. The electrostatic potential created by the current passing through the heating wire produces this extra attraction by which such a thick layer of nanoparticles are adhered to the surface. This particle electrodeposition has been reported in the case of alumina nanoparticles on a charged surface by Tang et al. [21], as a result of a electrochemical surface phenomena known as the isoelectric point. This ‘‘point’’ is sometimes referred to as the point of zero charge (pzc), which occurs at a specified pH for a given oxidized surface,
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where neither an excess of H? or OH- ions are present on the surface of the material. The pH of the nanofluid was determined to be 4.3 for which a positive surface charge potential can be attributed to Al2O3 according to the following generalized equation (Eq. 1) [22], E 0:06½pzc pH
ð1Þ
which is adapted from the Nernst equation [23] in electrochemistry. In this equation, E is the surface charge potential, pzc is the ‘‘point of zero charge’’ relative to the solution pH, for which Al2O3 nanoparticles is reported to be 9.6 [21]. In fact, the heater element was observed to have much thicker deposition of nanoparticles near the negative terminal of the circuit than near the positive terminal. The interesting fact of this attraction is that it is relatively weak for individual (or even clusters) of nanoparticles since their surface area is so small. This fact explains why the amount of nanoparticle deposition can be highly dependent on the size of the particles present in the suspension, as noted by Milanova and Kumar [24] since small changes in diameter at the nanoscale result in large surface area variations. Because of the relatively weak attractive force, the nanoparticles formed a fluidized layer of the surface that is subject to fluid shear and flow patterns. Subsequently, the fluid phenomena occurring at the surface of the wire has been ‘‘fossilized’’ into the layer of nanoparticles, allowing for a ‘‘post-humous’’ or ex situ boiling evaluation.
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As pictured in Fig. 2, the surface of the heated wire was littered with ‘‘craters’’ as evidence of locations of bubble nucleation. These ‘‘fossilized’’ nucleation sites exhibit a central raised protrusion from the layer of nanoparticles, surrounded by a region of excavated particles spreading radially, occasionally with stepped height. The feature in Fig. 2 was viewed from directly above (Fig. 2b, c) and rotated 70.6° with respect to the electron beam (Fig. 2a) using the SEM’s tilt stage to obtain an oblique view. This feature is seen to have approximately 5 lm of removed material surrounding the central protrusion which is slightly taller than the nanoparticle coating in the region. A similar effect is also seen in Fig. 3, which depicts a separate feature viewed at an oblique angle using the SEM’s tilt feature, at an angle of 45.8°. Here the center peak is nearly 8 lm tall where the surrounding removed area is only a few microns deep. In Fig. 2b and c, ‘‘cracks’’ or separations in the nanoparticle layer can be seen immediately next to the center peak. EDS of the peak, the removed area,
Fig. 3 Profile of Alternate Nucleation Site (45.8° tilt). Inset image: Shadowgraph of departing bubble
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and the surrounding area revealed that all areas were comprised of the alumina nanoparticles, with the excavated region indicating a layer thinner than the penetration depth of the EDS scan (i.e. stainless steel components detected). Also presented in Fig. 3 is an in situ shadowgraph image of a bubble having just departed the surface of the wire. Its size is approximately 400 lm in diameter, which is an order of magnitude larger than the various nucleation sites observed to have diameters of 40–60 lm. Another interesting feature preserved on the surface of the heater was the proximity of adjacent nucleation sites as shown in Fig. 4. In Fig. 4a, two similar sized features are found roughly 1.5 diameters of the feature apart. In this particular case, it appears that the upper left site produced a bubble first, followed by the lower right shortly after, evidenced by the presence of debris (likely from the lower region) on top of the upper feature. Since the electrodeposition and shear removal of the nanoparticle layer on the wire is a dynamic process, only the nucleation events immediately prior to switching off the power supply were preserved on the wire surface, indicating that the time period between these events is short. Furthermore, in Fig. 4b, it appears that either two nucleation sites have merged during the nucleation process, or surface conditions at this particular location resulted in an irregular shape at the bubble-surface interface. The liquid microlayer present during bubble growth has received much attention for its high heat flux, as studied by several researchers [25–27]. By its very nature, in order for this microlayer to maintain its stability during bubble growth, it must either maintain a superheated state without vaporizing (indicating increased local pressure) or sustain large shear forces due to mixed convective currents supplying liquid to this region, or a combination of the two. From the observations made on the boiling surface, it is apparent that the fluid shear mode is a dominant feature of the microlayer phenomena. As evidenced by the nucleation site ‘‘fossils,’’ as the bubble expands around the vapor
Fig. 4 Adjacent nucleation sites of similar size (left) and different size (right)
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generation site, the bubble grows while liquid is supplied through the porous structure of the coating. This liquid flux may increase the deposition of particles onto the electrodeposited layer, however, for low volume concentrations and heat fluxes such as in this experiment, the contribution is insignificant. Alternatively, in instances of boiling at or near CHF, this contribution would increase, resulting in more significant deposition of particles onto the heater surface, even in circumstances where there is not electrodeposition. At any rate, this flow pattern helps to explain the local ‘‘cool spots’’ observed around the nucleation sites [28]. Furthermore, at the time of departure, the liquid–vapor interface on the boiling surface rapidly collapses inwardly to enclose the vapor bubble, as observed with the high speed camera. During this collapse, which was observed in one case to last less than 0.3 ms, the liquid–vapor boundary which is defined by the surface tension between the liquid and the surface must close at an equally great rate. This the inward ‘‘slap’’ of the liquid–vapor interface at the boiling surface certainly shears the surface toward the center of the nucleation site, causing a buildup in the center protrusion that can be taller than the original nanoparticle layer. This fluid shear is proportional to the bubble size, since the buoyant force of the vapor must overcome the surface tension, leading to the shear that initiates bubble departure, as illustrated in Fig. 5. In this diagram, Fb is the buoyant force of the vapor bubble, Fr is the surface tension force acting along the perimeter of the vapor channel that holds the bubble in contact with the boiling surface until the normal component (FN) is exceeded by the bubble’s buoyancy. Furthermore, depending on the contact angle (h) of the liquid–vapor interface at the surface, the magnitude of FN and the shear force (Fs) at the surface can exist in varying proportions. An evaluation of the shadowgraphy video data used in conjunction with the PIV analysis showed the contact angles of the bubbles to be somewhat larger than that seen on the plain wire surface (*70° for the nanofluid versus *60° in water) indicating differing contact at the heated surface. An examination of this behavior reveals the significant effect of contact angle, as observed by many other researchers [18, 29, 30], by the contributions of the normal and shear forces at the bubble-surface interface. In this case, where the surface is comprised of somewhat loosely deposited nanoparticles, the situation becomes slightly more complex. The critical buoyant force for bubble departure will now depend rather on the adhesion of the nanoparticles to the surface, resulting in the removal of a superficial layer of the porous coating. As a result, the nucleation dynamics during boiling can be dramatically altered based on the adhesion properties of the nanoparticles to the boiling surface as well as the contact angle of the vapor–liquid interface with the coated surface. The movement of this porous coating may very well be the
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Fig. 5 Schematic overlaid on nucleation site illustrating forces acting on bubble. Enlarged diagram showing the effect of the contact angle on the magnitude of shear and normal forces, leading to different bubble sizes at departure
explanation of the decrease in boiling incipience noted in a recent review of nanofluid pool boiling experiments [19]. Ultimately, the combination of the shear across the surface and the upsetting of the surface coating at the time of bubble departure results in the formation of the structures observed in this paper. The evidence of removed surface material introduces a novel mode of heat transfer in which the particles expelled from the surface act as energy carriers into the bulk fluid. When precipitated (or more likely adsorbed by surface attraction) onto the wire surface, the nanofluids can be assumed to take on the wall temperature (Twall) of the wire. As a bubble nucleates and rapidly expands, it shears the surface and expels the nanoparticles back into suspension, carrying with them the energy stored while adhered to the wire. In other words, the boiling heat flux now becomes represented by Eq. 2. q00boiling ¼ q00latent þ q00conv þ q00quench þ q00NP
ð2Þ
Here q00 latent is the flux resulting from the latent heat removal by vaporization, q00 conv is the convection term,
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q00 quench is the quenching experienced when the nucleation site is reflooded [31]—which has also been shown to be enhanced by the presence of nanoparticles by Kim et al. [32]—and q00 NP is the flux resulting from the removal of the adhered nanoparticles. This term can be represented as Eq. 3: q00NP ¼ N qNP CNP Twall Z 2/rbubble tðrbubble Þ ½1 uðdNP Þdrbubble
ð3Þ
where N is the number of nucleation sites per heated area per time, and CNP and qNP are the specific heat capacity and the density of the nanoparticles respectively, which for this study would be that of Al2O3. Furthermore, t(rbubble) is the thickness of the adhered nanoparticle layer—which is represented as a function of the bubble radius (rbubble). u(dNP) is the void fraction (i.e. nanoparticle solid volume per unit volume) in the nanoparticle layer, which is a function of the nanoparticle diameters (dNP). Since the deposited nanoparticle layer is also a function of the material interaction between heated surface and the nanoparticles as well as the degree of adhesion (i.e. the required shear to remove the layer), further research would be required to adequately predict this deposition phenomena. However, this expulsion of surface material helps to explain how localized hot spots at the nucleation sites are notably cooler, as observed by Buogiorno et al. [18]. Furthermore, the expulsion of the solid mass from the surface helps to explain the increased vorticity in the fluid field surrounding the bubble as measured by PIV measurements [20]. Furthermore, the adhesion and expulsion of the nanoparticles would also depend on the surface condition of the heater, for which relative sizes of roughness parameters to the NP sizes would affect the surface tension required to release the particles, as alluded to by Wen et al. in their investigation of alumina nanofluid boiling from textured surfaces [33]. Although, this would seem to imply that the overall heat transfer is enhanced, the unfortunate result of having a coating of nanoparticles on the surface, particularly insulating particles such as alumina, is that the wall temperature is reduced as compared to a clean conductive heater surface, as noted by Liu and Liao [17]. As a result, the q00 latent, q00 conv, and q00 quench terms are decreased, and the moderate gain of the q00 NP is not sufficient to enhance the overall flux. In fact, using the sizes of the craters seen in this experiment, an estimation of the heat removal per nucleation site is on the order of 1 lJ per bubble, which is approximately 1% of the total heat removed by a 0.5 mm diameter bubble’s latent heat. If the nanoparticles were higher or equal conductivity to the heater, then the effective heat transfer would be quite similar to the results seen in modulated porous coatings as noted by Liter and Kaviani [34].
The elucidation of this behavior is useful in the design of boiling water reactors in nuclear energy industry which faces many difficulties associated with the deposit of ‘‘crud’’ onto fuel rods. Much like the deposit of nanoparticles presented in this work, the deposits in the reactors can significantly alter their boiling performance [35–37] By understanding the nature of boiling on surfaces with time dependent surfaces may increase the performance and reliability of such systems through better design.
4 Conclusion It is well known that the presence of nanoparticles in solution and on the surface of a heating element affects boiling performance, but in this study, a more explicit understanding of the mechanisms involved with nanofluid pool booling was revealed. Because of electrochemical reactions at the surface of nanoparticles, charging on the nanoparticles results in deposition on the electric heater, resulting in morphological changes at the boiling surface. These changes intrinsically affect boiling behavior, however, the fluid-like behavior of the nanoparticle coating create a dynamic interaction between the fluid behavior and the surface’s role in nucleating bubbles. Consequently, evidence of this behavior was captured using electron microscopy, leading to an increased understanding of the fluid dynamics of bubble departure, and the nature of nanofluid enhancements/degradations under pool boiling conditions. Acknowledgments Special thanks are due to Dr. Tom Stephens of the Texas A&M University Microscopy Imaging Center, who assisted with the SEM and EDS operation. The FE-SEM acquisition was supported by the NSF grant DBI-0116835, the Vice President for Research Office, and the Texas Engineering Experiment Station. The authors also graciously acknowledge the assistance of Stephen Fortenberry and Rebecca in measurements with regard to pH and contact angles. Further support was provided by the Texas Engineering Experiment Station and the NSF GRFP.
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