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International Journal of Thermal Sciences 82 (2014) 111e121

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International Journal of Thermal Sciences

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Effect of silicon nanorod length on horizontal nanostructured platesin pool boiling heat transfer with water

Ebru Demir a, Turker Izci a, Arif Sinan Alagoz b, Tansel Karabacak b, Ali Kosar a,*aMechatronics Engineering Program, Sabanci University, Tuzla, 34956 Istanbul, TurkeybDepartment of Applied Science, University of Arkansas at Little Rock, Little Rock, AR 72204, USA

a r t i c l e i n f o

Article history:Received 19 April 2013Received in revised form20 March 2014Accepted 26 March 2014Available online

Keywords:Pool boilingNanostructured surfacesNanorod length effectBoiling heat transfer enhancementSi nanorods

* Corresponding author. Tel.: þ90 216 4839621; faxE-mail address: kosara@sabanciuniv.edu (A. Kosar

http://dx.doi.org/10.1016/j.ijthermalsci.2014.03.0151290-0729/� 2014 Elsevier Masson SAS. All rights res

a b s t r a c t

An experimental study was conducted to investigate the effect of nanorod length on pool boiling heattransfer of water from nanostructured surfaces. Three nanostructured plates featuring Si nanorods ofdiameter 850 nm and of three different lengths (900, 1800 and 3200 nm), which were etched throughsingle crystal p-type silicon wafers using metal-assisted chemical etching (MaCE), were utilized toenhance pool boiling heat transfer. Nanostructured plates were placed on the bottom of a heatedaluminum pool inside a Plexiglas container filled with water, and the surface temperatures of nano-structured plates were measured at increasing heat flux values to quantify boiling heat transfer en-hancements, which were obtained with changing the surface morphology using nanostructured plates ofdifferent Si nanorod lengths. The experiments were repeated using a plain surface Si plate. A visuali-zation study on bubble formation and release from individual plates was conducted using a high speedcamera to bolster the experimental results. Compared to the plain Si surface, boiling heat transfer en-hancements up to 254% and 120% were attained using the shortest and longest nanorod configurations,respectively.

� 2014 Elsevier Masson SAS. All rights reserved.

1. Introduction

With the miniaturization of electronic devices, the functionalityof such devices greatly increased due to the improvements in theirpackaging. Recent developments inmicrosystems technologymadeit possible for electronic devices to have a continuous increase intheir computational powers with continuous reduction in theirsize. While benefitting from miniaturization process, heat dissi-pated per unit area by such devices significantly increased. There-fore, the development of more effective and equally miniaturizedcooling systems became a priority in order to preserve the func-tionality and stability of such devices.

Conventional methods such as using air cooled systems andeven their improved versions with fin arrays started to fail as theheat removal problems became more demanding. As Upadhye andKandlikar [1] stated, air is not preferred for microchannel flows dueto its low heat transfer coefficient and low specific heat. Due to thesuperior heat removal characteristics of many fluids (water, re-frigerants) compared to air, a paradigm shift in cooling applications

: þ90 216 4839550.).

erved.

became inevitable so that using such fluids as coolants became apopular trend. Mudawar [2] nominated Fluorinets FC-87, PF-5052and FC-72 as the most promising Fluorinets in electronic coolingdue to their low saturation temperatures of 32.0, 50.0 and 56.6 �Crespectively. Cardenas and Narayanan [3] performed jet impinge-ment boiling on heated Cu surface using FC-72. Nguyen et al. [4]found that a heat transfer coefficient increase as much as 40% uti-lizing Al2O3 nanoparticle-water mixture could be obtainedcompared to the base fluid, namely water.

Still, some advanced electronic systems demanding high heatremoval rates rendered single phase liquid cooling applicationsinsufficient [5]. In order to achieve higher efficiency in miniatur-ized cooling systems, focus of this particular research area shiftedtowards cooling applications benefiting from phase-change, suchas jet-impingement [3,6,7], flow boiling in micro-channels [8,9],and pool boiling [5,10,11]. Conducted experiments repeatedlyshowed that two-phase cooling systems yielded better resultscompared to their single-phase cooling counterparts. Mudawar [2]reported a reduction in device temperature rise with boilingcompared to single-phase cooling. Although boiling instabilities[12,13] and high pressure losses [14,15] pose disadvantages oversingle-phase cooling, boiling heat transfer is considered as aneffective heat removal mechanism since lower temperature rise in

Nomenclature

A total heated areah heat transfer coefficientP power input to the systemq00 constant heat fluxTs surface temperatureTth thermocouple temperature readingTi initial temperature of the liquid poolTsat saturation temperature of the liquidQloss heat lossRAl thermal resistance of aluminumRtg thermal resistance of thermal greaseRSi thermal resistance of Si PlateRtot total thermal resistance

E. Demir et al. / International Journal of Thermal Sciences 82 (2014) 111e121112

surface temperatures is evident with applied heat flux underboiling conditions.

Even though boiling applications are not limited to poolboiling, pool boiling is one of the most popular heat removalmechanisms and is being studied by many researchers. Anothersignificant topic is the effect of nanoparticles and nanostructuredsurfaces on boiling heat transfer characteristics of cooling sys-tems so that it became a rising trend in the heat transfer com-munity to couple these two methods to attain better heatremoval performances. You et al. [16] obtained a dramatic in-crease around 200% in pool boiling Critical Heat Flux (CHF)compared to pure water by employing nanofluids. Park and Jung[17] reported an increase up to 28.7% in boiling heat transfercoefficients of R22 and water when carbon nanotubes (CNTs)were employed. Ahn et al. [18] observed a 40% increase in CHFusing multiwalled carbon nanotubes (CNTs). Xiao et al. [19]emphasized on reduced wall superheat for boiling inception asanother advantage of using CNTs in pool boiling applications.Sesen et al. [20] reported up to 400% enhancement in pool boilingheat transfer coefficients using nanostructured plate with Cunanorods. The abovementioned studies include just some ofmany experiments, which proved that nanofluids [21e25] andnanostructured surfaces [20,26e29] are very compatible withpool boiling applications and provide a significant enhancementin heat removal performance.

It was repeatedly shown that the heat transfer coefficients andCHF significantly increase when nanostructured plates are utilizedin pool boiling applications. Even though the effects of nanofluidson boiling heat transfer coefficients are still controversial [4,16,30],the increase in CHF was widely reported for nanofluids.

As an advantage of nanostructured surfaces, dramatic re-ductions in boiling inception temperatures [16,21e30] and capa-bility of such surfaces in decreasing the contact angle andincreasing wettability in boiling applications [16,27,28,30,31] havebeen reported in the literature.

Boiling heat transfer performance enhancement attained usingnanostructured surfaces are often attributed to 1) increased bubblerelease frequency [5,26,32], 2) increased nucleation site density[26,33,34], 3) enhanced surface wettability [35e38], 4) enhancedheat transfer area [38,39] introduced by nanostructures. Moreover,overall enhancement achieved through nanostructures is alsodependent on many variables -such as the dimensions of the in-dividual nanorods, porosity of the nanostructured surface, tiltingangle and distribution of the nanorods on the surface (e.g. randomor periodically aligned nanorods) e and these variables must beindividually studied in order to have a better understanding of theenhancement mechanisms.

There are several studies in the literature reporting nanorodlength effect on boiling heat transfer performance of the system.Lu et al. [39], Im et al. [40] and Yao et al. [37] conducted poolboiling experiments on Cu and Si nanorods of different lengths.However, in none of these studies, nanorod length is singled outas the only variable. Lu et al. [39] observed controversial trends inboiling curves of Si nanorods with different lengths. They re-ported that the shortest nanorod configuration (16 mm nanorods)yielded the lowest wall superheat at a fixed heat flux value,whereas the wall superheat increased with increasing nanorodlength for other samples (122, 59 and 32 mm long nanorods). Theyattributed this behavior to a change in the number of large cav-ities resulting from the fabrication process. Similarly, in the studyconducted by Im et al. [40], the authors observed a decreasingwall superheat with increasing nanorod length (1, 2, 4, and 8 mmlong nanorods). The nanostructures used in the study of Yao et al.[37] were not uniform in diameter, length and array spacing, andthe authors used the average nanorod height as the mainparameter so that cavity size and surface roughness were left asdependent parameters.

The novelty of this study lies on its separation of the effect ofvarying nanorod length on heat removal performance in poolboiling. For this, three nanostructured plates, each featuring Sinanorods of different lengths and having the same diameter andspacing were tested, and the results were compared to the mea-surements obtained from a plain surface Si plate. Surface temper-atures were recorded for each of the plates with increasing heatflux values. Furthermore, emerging bubbles from individual plateswere visualized with a high speed camera system, and the bubblerelease frequency from the plates could be obtained.

2. Nanostructure fabrication

Single crystal p-type (100) oriented silicon wafers at resistivity1e100 U cm were cleaned by standard RCA-I cleaning procedure.Samples were dipped into ammonium hydroxide and hydrogenperoxide solution (NH4OH, 30% v. : H2O2, 30% v. : H2O ¼ 1 : 1 : 5) at80 �C for 15 min, rinsed with deionized water and dried with ni-trogen gas. Following the cleaning step, single layer hexagonallyclose-packed polystyrene (PS) nanospheres were deposited ontosamples through convectional self-assembly method [41] andslightly etched in oxygen plasma. Plasma etching decreased nano-sphere diameter from 1010 nm to 850 nm and formed a hexagonalpattern of isolated nanospheres. These non-closely packed nano-spheres were used as shadow mask for gold film deposition of50 nm thickness where gold atoms filled the gaps among nano-spheres. This step was followed by nanosphere lift-off by ultra-sonicating samples in toluene for 1 min, which left a honeycombpatterned gold mesh layer on the silicon substrate. After thepatterning process, samples were immersed into room tempera-ture hydrofluoric acid e hydrogen peroxide solution (HF, 50% v.:H2O2, 30% v. : H2O ¼ 4 : 1 : 5). Silicon underneath the gold layeretched and formedwell-ordered single crystalline silicon nanorods.Samples were etched for 40 s, 80 s and 160 s in order obtain Si rodsof 900, 1800 and 3200 nm lengths, respectively. Finally, gold layerwas removed by etching with potassium iodine (KI) solution for3 min (Fig. 1). In this metal-assisted chemical etching (MaCE)procedure, silicon nanorod diameter is defined by the reducednanosphere diameter and nanorod separation by initial nanospherediameter, while nanorod length is set by etching time. The effects ofsilicon wafer crystal orientation, etching solution concentration,and etching time on nanorod morphology were discussed else-where [42].

In determining the length of Si nanorods, the aim wasdoubling them to produce samples of different length scales (e.g.

Fig. 1. Top-view and crossectional view Scanning Electron Microscope (SEM) images of single crystalline silicon nanorods a) After 160 s etching (3200 nm rods) b) After 80 s etching(1800 nm rods) c) After 40 s etching (900 nm rods).

E. Demir et al. / International Journal of Thermal Sciences 82 (2014) 111e121 113

900 nm, 1800 nm, 3600 nm,.so on). However, etching is anonequilibrium process, since solution tries to penetrate betweenthe nanorods while gas bubbles are escaping from the etchinginterface. Therefore, due to such effects, the authors ended upwith 3200 nm long rods instead of 3600 nm ones. On the otherhand, the 3200 nm long rods are still significantly longer than the900 nm long rods, which help revealing the effects of the nanorodlength.

3. Experimental setup and procedure

3.1. Experimental apparatus

The experimental setup is demonstrated in Fig. 2. An aluminumbase of dimensions 6 cm � 6 cm featuring four cylindrical groovesthat house four cartridge heaters (Isitel Cartridge Heaters), each oflength of 31.25 mm and of diameter of 6.25 mm is produced. Apart of the aluminum base housing the heaters is surrounded withair gaps on all sides to minimize heat losses. On the surface of thealuminum base lies a square shaped groove of a depth of 4 mmand of size 20 mm � 20 mm. The nanostructured plates to betested are positioned at the bottom of this groove. A Plexiglascontainer (please refer to Fig. 2c) for dimensions) with an opening

of size 20 mm � 20 mm at its bottom is closely fitted to thealuminum base and sealed with liquid gasket in order to preventany leakage. The opening at the bottom of the Plexiglas containerfeatures extensions to cover the side walls of the square groove onthe aluminum base so that the heat transfer from the sides of thegroove is prevented. Thus, heat flux is applied only from thebottom. The depth of the opening is 4 mm so that the nano-stuctured plate is confined in a 20 mm � 20 mm � 8 mm space,which lies beneath the bulk fluid. The heat generated by the car-tridge heaters is delivered to the nanostructured plate of size19 mm � 19 mm, which is placed on the bottom of the pool. Theheaters provide constant heat flux to the system. The total resis-tance of the heaters that are connected in parallel is 65 U, andeach heater alone is capable of proving power output as high as225 W. The heaters in the heating block are closely packed.Computer simulations for the same heating configurations wereperformed using COMSOL� Multiphysics 3.5a commercial CFDsoftware in order to obtain heat flux variation on the top surface ofthe heating block. Rectangular mesh elements were used with thenumber of degrees of freedom of about 100,000. Heat flux dis-tribution over the top surface was deduced. According to thesimulation results, the heat flux variation over the projectedheated area was within �30% along the top surface.

Fig. 2. Experimental setup a) Aluminum base housing the heaters and a pool (white substance is thermal grease) b) Heated section of the aluminum base showing heater mountingc) 3D representation of the whole setup with Plexiglas container and the aluminum base.

E. Demir et al. / International Journal of Thermal Sciences 82 (2014) 111e121114

The heaters and the nanostructured plate are treated with highquality silicone thermal grease in order to minimize thermal con-tact resistances and heat losses. The peak to peak roughness of Sibottom surface of the samples is typically below 1 mm, while thepeak to peak roughness of the plates used in our study is keptbelow 1 mm. As a result, no direct contact between the Si plates andaluminum block is expected. It is also emphasized that the sameprocedure was consistently repeated for all the samples tested inthis study. To measure the thickness of the thermal grease, samplesof test plates, which are attached to the polished aluminum samplewith thermal grease, were carefully analyzed using zoom-in imagesof cross sections provided by light microscopy. The thickness of theapplied thermal grease is measured to be 0.05 mm with an un-certainty of �5%. The thickness of the resulting layer well overlapswith those reported in datasheets [43,44]. The water container isopen to the atmosphere, and the water level is kept constant. Onethermocouple (w76 mm thin) is placed between the nano-structured plate and the bottom of the pool (in the thermal grease)to record surface temperatures, whereas one thermocouple issecured under the base, and two others are placed on the heaters.To estimate heat losses, electrical power is applied to the test sec-tion after evacuating de-ionized water from the test loop. Once thetemperatures recorded from thermocouples of the test sectionbecome steady, the temperature difference between the ambientand test setup is recorded as a function the corresponding power.Thus, the power versus temperature difference profile could beconstructed. This profile is used to calculate the heat loss associatedwith applied powers for each experimental data point.

3.2. Experimental procedure

After the experimental setup was prepared as explained, surfacetemperatures were measured (through OMEGA 5TC-TT K-Typethermocouples) as a function of input power calculated usingvoltage and current readings on the power supply (AMETEK Sor-ensen XHR Series Programmable DC Power Supply). Surface tem-peratures were recorded for various constant heat flux values.Temperature data were acquired with the aid of a workstation in-tegrated to the data acquisition system (NI-SCXI 1102 module, PCI-

6036E card) at a rate of 100 data points per second. These datapoints were exported using data acquisition software LabView�

and then averaged usingMS Visual Studio. The thermocouples werecalibrated using the calibration tool, which is included in LabView�

Data Acquisition System, for accurate temperature measurements.The Thermocouple Compensation Feature in this tool generates acorrection polynomial that effectively compensates for sensor andsystem inaccuracy through the entire measurement. In addition,the thermocouples were carefully calibrated against a precisethermometer.

Constant voltage was applied to the ends of the cartridgeheaters (Isitel Cartridge Heaters) providing constant heat flux to thesurface. Heat flux values were in a suitable range covering bothsingle phase and boiling heat transfer conditions. The experimentswere conducted for each of the four plates (three of them featuringSi nanorods of different lengths and one being the plain Si surfacecontrol sample). The experiments were repeated for three times,each time using a new nanostructured plate with the same surfacemorphology, so that it was made sure that the data was repro-ducible. It was observed that no significant difference was present,and the average of three measurements was taken as the corre-sponding data point.

The experimental data was collected when temperature fluc-tuations were under 0.1 K so that there was no significant tem-perature change with time. The results were compared tocharacterize the effects of nanostructures on boiling heat transferperformance. In order to determine heat losses accurately, a secondthermocouple was placed underneath the heated base.

3.3. Data reduction

The heat flux provided to the system, q00, was obtained from:

q00 ¼ ðP � QlossÞ=A (1)

where P is the power input, Qloss is the heat loss, and A is the heatedarea of the plate. In order to determine Qloss, surface temperaturerise vs. heat flux profiles were obtained from the thermocoupleswhen the system was heated in the absence of the working fluid.

Table 1Uncertainty analysis results.

Uncertainty Error

Power (P) �0.15 WSurface area (A) �0.08%Thermocouple reading (Tth) �0.1 �CThermal resistance (Rtot) �5%Heat flux (q00) �3.5%Heat transfer coefficient (h) �8.2%

E. Demir et al. / International Journal of Thermal Sciences 82 (2014) 111e121 115

The surface temperatures were calculated by considering ther-mal contact resistances from the thermocouple to the surface of thenanostructured plate as:

Ts ¼ Tth � q00*Rtot (2)

where Tth is the thermocouple temperature reading. Rtot is the totalthermal resistance from the thermocouples to the surface of thenanostructured plate and was calculated as:

Rtot ¼ RAl þ Rtg þ RSi (3)

where RAl is the thermal resistance of aluminum between heaterand the bottom of the pool, Rtg is the thermal resistance of thermalgrease between the nanostructured plate and the bottom of thepool and RSi is the thermal resistance of the Si plate. The average ofthemeasured surface temperatureswas taken to obtain the averageof the surface temperature readings from thermocouple Tth. Theboiling heat transfer coefficient, h, was then calculated as:

h ¼ q00=ðTs � TsatÞ (4)

where Ts is the surface temperature and Tsat is the saturationtemperature of the fluid. Tsat was replaced with Ti (initial temper-ature of the liquid pool) while calculating single phase heat transfercoefficients.

3.4. Uncertainty analysis

The uncertainties of the measured values are given in Table 1and were derived from the manufacturer’s specification sheetwhile the uncertainties of the derived parameters were obtainedusing the propagation of uncertainty method developed by Klineand McClintock [45].

3.5. Contact angle measurements

Water drop contact angle measurements of the nanostructuredplates used in the experiments were obtained using DSA (DropShape Analysis) as shown in Fig. 3. Contact angle values were foundto be 47.7�, 33�, 32� and 31.8� for plain Si surface, and nano-structured surface with 3200, 1800 and 900 nm Si nanorods,respectively. It was observed that the plain surface had the highestcontact angle, whereas amongst the nanostructured plates, the

Fig. 3. DSA images showing contact angle views for four plates a) Nanorod configuration ofNanorod configuration of nanorod length of 900 nm, d) Plain surface.

contact angle tends to increase, as the length of the nanorodsincreased. However, since the contact angle difference between thenanostructured plates is small to be considered as a parameter,these figures are only significant to emphasize on the reduction incontact angle in the presence of nanorods compared to the plainsurface. Since the contact angle difference is minor in the testedplates, there will be only minor effects of the contact angle in thisstudy. The authors included contact angle measurements just forthe completeness of this study.

3.6. Visualization study

Bubble visualization study is conducted using a Phantom V310Monochrome high speed camera to provide support for theexperimental results. Software provided by Vision Research isused to control the frame rate, exposure time, resolution andrecording time. Bubble growth and release dynamics of each plateused in this study is observed via high speed image recording at2800 fps, 100 ms exposure time and 1280 � 800 resolution. Im-ages from each plate are recorded for 103 s for an applied heatflux of 20 W cm�2. Applied heat flux value is specifically chosen toobserve the performance of the plates. The performance assess-ment of different nanostructured plates is based on heat transfercoefficients at fixed heat flux values. The discussion about theresults is provided according to their performance at a fixed heatflux provided to the configuration so that performanceenhancement with nanostructures could be revealed. Therefore, itwas found more appropriate to compare images from the visu-alization study at a fixed heat flux to justify the presenteddiscussion. However, the authors also agree that for a compre-hensive visualization study, which will be performed as a futurestudy, the comparison of images taken at the same superheatwould be more meaningful and will definitely be considered asfuture work.

In order to zoom-in on the plates from the side, a secondexperimental setup made of glass is prepared. A 19 mm � 19 mmaluminum base housing four cartridge heaters of a length of19.25 mm and of a diameter of 4.5 mm is constructed. Thealuminum base is surrounded with glass walls with a height of20 cm to produce a pool like structure. The setup is carefully sealedwith liquid gasket to prevent any leakage. The plates to be testedare placed on the surface of the aluminum base and at the bottomof the pool after being treatedwith thermal grease. The results fromthe analysis are presented in Section 4.1.

4. Results and discussion

4.1. Results of the visualization study

In order to determine the average bubble release frequency fromthe plates, the number of bubbles emerging from the tested platesare counted and averaged over time. Average bubble departurediameter is determined by measuring the diameter of each bubbleindividually and averaging the sum of all diameters over the total

nanorod length of 3200 nm, b) Nanorod configuration of nanorod length of 1800 nm, c)

Table 2Results obtained from the data in the visualization study.

Surface Bubble releasefrequency

Average bubbledeparture diameter

Si with 900 nm long nanorods 51 Hz 1.6 mmSi with 1800 nm long nanorods 45 Hz 1.9 mmSi with 3200 nm long nanorods 23 Hz 2.8 mmPlain Si 12 Hz 1.3 mm

E. Demir et al. / International Journal of Thermal Sciences 82 (2014) 111e121116

number of emerging bubbles. The results from this analysis arepresented in Table 2. Due to the long recording time, the authorsbelieve that the data is reliable, since a cycle of bubble growth andrelease has an order of magnitude of milliseconds and such a longrecording time covers a great number of growth and release cycles.Selected image sequences obtained from the plates used in thisstudy are presented in Figs. 4e8.

The nucleation from a seemingly smooth surface initiates fromsmall cracks or crevices on the surface. The reason behind this isthe capability of such surface defects entrapping small gas bub-bles. These very small gas bubbles are called “nuclei”, which tendto grow very fast and be released from the surface when sub-jected to a relatively small amount of heat flux compared to theheat flux required to initiate boiling on a smooth surface. Nano-rods on the nanostructured plates serve as such cracks or creviceson the surface: They entrap small gas bubbles that grow very fastwhen the surface is heated. When the surface is covered withsuch nanorods, the number of cavities that entrap gas bubbles(nucleation sites) is increased, and therefore, the number ofnucleation sites in nanostructured plates is higher compared totheir smooth surface counterparts. In this study, visualizationstudies are performed to provide visual support. As a result of theexistence of more nucleation sites, the number of emerging

Fig. 4. Images from the plate with 90

bubbles generated is higher. Furthermore, the tendency of suchnanostructured surfaces to generate new bubbles is more so thatthe bubble release frequency also increases (Table 2). The bubbledeparture diameter depends on the interaction of adjacent bub-bles (Table 2).

Fig. 4a)ef) presents a selected image sequence showingemerging bubbles from the nanostructured plate with the shortestnanorods, namely 900 nm long nanorods. As can be observed fromthe images, at 20 W cm�2 heat flux (at 5.5 K wall superheat) bub-bles emerge individually from the surface of the nanostructuredplate. However, at the same heat flux value (at 11 K wall superheat),it is observed that the bubbles emerging from the nanostructuredplate with the longest nanorods (3200 nm) are merging laterallybefore leaving the surface as shown in Fig. 5a)eh). This kind ofbubble growth mechanism has two disadvantages. Firstly, an in-dividual bubble needs to merge with others to leave the surface,which delays bubble release and decreases the bubble release fre-quency. Secondly, the bubbles lingering on the surface of the platedecrease the heat transfer rate from the heated plate to the bulkfluid, since they act as an insulation layer. Hence, at the same heatflux, these two nanostructured plates yield different wall superheatvalues.

Fig. 6a)ef) show emerging bubbles from the nanostructuredplatewith a nanorod length of 1800 nm. At the same heat flux value(20 W cm�2, at 7 K wall superheat), the nanostructure withnanorods of medium length results in features of emerging bubblesobtained from nanostructured plates with both the shortest andlongest nanorods. Accordingly, bubbles merging laterally beforebeing released from surface coexist with individual bubbles leavingthe surface.

A comparison of bubble generation performance of the nano-structured plates with the plain Si surface proves the advantage ofnanostructured surfaces. Fig. 7a)ef) display the poorer bubble

0 nm long nanorods (shortest).

Fig. 5. Images from the plate with 3200 nm long nanorods (longest).

E. Demir et al. / International Journal of Thermal Sciences 82 (2014) 111e121 117

generation performance on the plain Si plate, despite the fact thatthe wall superheat exceeds 15 K in this case. The plain Si plateproduces bubbles much less frequently, and the bubbles seem tobe emerging from only certain sites on the plate, suggesting thatsome imperfections on the surface might be serving as nucleationsites.

The visualization study conducted on three different nano-structured plates and the plain Si control sample used in this studyyields the following observations:

� The nanostructured plates increase the number of the nucle-ation sites significantly compared to the plain surface.

� At lower wall superheat values, the bubbles are released fromthe surface individually and frequently. However, at higher wallsuperheats, bubbles are not released from the surface beforemerging laterally. Therefore, the waiting time increases andbubble release frequency decreases at higher wall superheatvalues.

� As the nanorod length increases, the bubble release frequencyfrom the heated plate decreases, whereas the average diameterof the emerging bubbles increases. The increase in bubble de-parture diameter is observed to be due to the lateral merging ofindividual bubbles residing on the surface before beingreleased.

Lateral merging of the bubbles is observed on all three nano-structured plates to some extent. Since the spacing between

individual nanorods is too small to serve as single nucleation sites,very small bubbles emerging from the gaps cannot directly breakfree from the surface. These small bubbles merge, and thus, adja-cent gaps in a group serve as a single nucleation site (Fig. 8aeb).However, a distinction should be made between the lateral merg-ing of bubbles on the nanostructured plate with the longestnanorods mentioned before and this initial merging to create activenucleate sites. First of all, the merging of small bubbles created byadjacent gaps takes place, while the bubble diameters are verysmall. Secondly, after the initial merging, the gaps in group canserve as single nucleation sites and produce an individual bigbubble that could depart from the surface without merging withany other, depending on the type of the plate. The bubbles on thenanostructured plate with the longest nanorods go through a sec-ond phase of lateral merging, in which much bigger bubbles formand the bubble release frequency decreases. Subsequent scenariosfor bubble generation are included in Fig. 9.

4.2. Heat transfer

The experimental data was gathered as explained in the previ-ous sections. Based on the experimental data, surface temperaturedata points are plotted against various constant heat flux valuesbetween 0.3 and 23W cm�2 for all four plates in Fig. 10. It should benoted that CHF experiments are not conducted in the current study,and therefore, the corresponding maximum heat flux values do notrepresent CHF. The nanostructured plate with Si nanorods of length

Fig. 6. Images from the plate with 1800 nm long nanorods (medium length).

E. Demir et al. / International Journal of Thermal Sciences 82 (2014) 111e121118

900 nm (Si NR 900 nm) has the best improvement compared to theplain Si surface control sample (Plain Si), where the surface tem-perature at the boiling inception remains very close to 100 �C forthis configuration. The other two nanostructured plates with1800 nm and 3200 nm long Si nanorods (Si NR 1800 nm and Si NR3200 nm in the legends) show the same trend and enhance heattransfer compared to the plain Si surface, however with a slight

Fig. 7. Images from th

increase in surface temperatures at the same heat flux compared tothe configuration with shorter nanorods.

Overall enhancement in boiling heat transfer compared to theplain surface can be related to the pin-fin [35,38] effect of thenanorods, which is the main heat transfer enhancement mecha-nism utilized by all three types of the nanostructured surfaces usedin this study. The resulting increased heat transfer surface area

e Plain Si surface.

Fig. 8. Adjacent gaps serving as a single nucleation site: a) Two adjacent bubbles, b)Bubbles merge and an individual bubble leaves the surface at the same time: Thismerging may result in individually released bubbles depending on the type of theplate.

Fig. 10. Wall Superheat vs. heat flux profile for all test samples.

E. Demir et al. / International Journal of Thermal Sciences 82 (2014) 111e121 119

available to remove heat from the surface generates a more effec-tive cooling system. However, as shown in the visualization study,the change in nanorod length influences the efficiency of other heattransfer enhancement mechanisms resulting from the use ofnanostructured surfaces, such as bubble release frequency, bubbledeparture diameter, which are related to enhanced nucleateboiling.

It is widely studied and proven that such nanostructured sur-faces have the ability of increasing the nucleation site density byoffering more active nucleation sites due to the contributions ofmultiple nanostructures and bubble generation frequency from thesurface [5,26,32]. The bubble visualization study conducted by Liet al. [26] reported an increase in bubble release frequency and alsoan increase in nucleation site density at low wall superheats. Thevisualization study (presented in the previous section) on thenanostructured plates and the plain Si plate confirms that nano-structured plates significantly enhance the bubble release fre-quency from the surfaces compared to the plain surface. In the lightof the results of the visualization study, it can be seen that thenanostructured plate with shortest nanorods of 900 nm has thehighest bubble release frequency amongst all the plates. Further-more, the nanostructured plate with the longest nanorods of3200 nm results in a higher bubble release frequency twice asmuch compared to the plain Si surface, even though it generates thesmallest bubble release frequency amongst the three

Fig. 9. Bubble generation scenarios.

nanostructured plates. Therefore, it can be stated that although allof the nanostructured plates utilize the pin-fin effect, the distinc-tion between them becomes apparent due to the change in theirbubble release performance (frequency, diameter etc.), which isaffected by the change in the nanorod length.

It can be stated that the nanorod spacing is not enough to pro-vide the critical cavity mouth radius for activation at the low wallsuperheats observed during the experiments. However, multiplenanorods act together to offer more active nucleation sites, and thecombined spacings among multiple nanorods could exceed thecritical cavity size. The visualization study allows obtainingsequential images and proves this statement. Similar trends wereobserved by other researchers. Chen et al. [38] obtained more than100% increase in boiling heat transfer coefficients using Si and Cunanowire arrays.

Boiling heat transfer coefficients of all four plates are plottedagainst various heat fluxes in Fig. 11. As expected from surfacetemperature profiles, where the shortest nanorod configurationyielded the lowest surface temperatures, the nanostructured platewith the shortest nanorods results in the greatest enhancementamong all the configurations relative to the plain surface Si plate.On average, boiling heat transfer coefficients of the nanostructuredplate with 900 nm long nanorods are about 3.5 times as much asboiling heat transfer coefficients of the plain Si surface plate. Eventhe worst case observed using a nanostructured plate (with

Fig. 11. Boiling heat transfer coefficients vs. heat flux profile for all test samples.

Fig. 12. The SEM image views of nanorods taken after the experiments.

E. Demir et al. / International Journal of Thermal Sciences 82 (2014) 111e121120

nanorods of the longest nanorod length) yields a significantenhancement in heat transfer coefficients. Accordingly, the nano-structured platewith 3200 nm long Si nanorods results in about 2.2times higher heat transfer coefficients on average compared to theplain Si surface plate. One possible reason for this trend can beattributed to the significant increase in bubble release frequencyfrom the surface through the utilization of nanostructures. Asproven by the visualization study presented in the previous section,all of the nanostructured plate configurations have significantlyhigher bubble release frequencies compared to the plain Si plateand thus they dramatically increase the heat transfer from theheated surface to the bulk fluid. Increasing the bubble release fre-quency allows the quick replenishment of fluid to the nucleationsites. Moreover, the existence of nanostructures also yields anincreased active nucleation site density and enhanced heat transferarea as discussed earlier. The poorer performance of the nano-structured plate with the longest nanorods is attributed to itsdifferent bubble growth and release mechanism, which delaysbubble release from the surface. In this case, bubbles merge later-ally and reside on the surface for longer time, and this way, they actas an insulative layer.

The results reveal the potential of nanostructured surfaces toenhance boiling heat transfer. Accordingly, the nanostructureconfiguration in terms of nanorod height, length, spacing, andarrangement could be optimized, which could lead to furtherboiling heat transfer enhancements.

Our experimental results regarding boiling heat transfercontradict to the previous experimental results found in the liter-ature. As presented in the introduction section, Yao et al. [37] andIm et al. [40] observed a decrease in wall superheat with increasingnanorod length at fixed heat flux values. However, there are majordifferences between the nanostructured plates used in the presentstudy and the ones used in the abovementioned studies. Thenanostructured plates used in this study have nanorods of uniformlength, spacing and diameter so that the nanorod length effectcould be filtered out. Moreover, the nanorod lengths in this studyare different from the previous studies. In this study, the experi-ments are conducted on rather shorter nanorods.

4.3. Structural integrity of the nanostructured plates

Scanning Electron Microscope (SEM) images of nanostructuredplates obtained after being used in the experiments are shown inFig. 12. The images confirm that the nanostructures are still solidand stable after the experiments. Neither fracture nor failure ofindividual nanorods is observed. There are no significant structuralchanges except for the nanorods being slightly rounded at the tops.

5. Conclusions

This study proves positive effects of utilizing nanostructuredplates on pool boiling heat transfer and reveals the effect of

nanorod length on boiling heat transfer performance. Significantenhancement in heat transfer coefficients in both single-phase (upto 14%) and boiling regions (up to 254%) is achieved using nano-structured plates with Si nanorods. Due to the increased activenucleation site density, surface wettability and heat transfer area,nanostructured surfaces could enhance boiling heat transfer per-formance. Furthermore, as proven in the visualization study, it isseen that nanostructured surfaces significantly increase the bubblerelease frequency. It is observed that as the length of the nanorodsincreases, the enhancement in heat removal performance de-teriorates with the decrease in the bubble release frequency.Further enhancements with optimization studies using importantnanorod parameters (nanorod size, spacing and length) arepossible so that performance enhancements could become com-parable to the existing commercially available enhanced surfaces.Moreover, SEM images of the plates after the tests are obtained inorder to prove the durability of the nanostructures under stringentoperating conditions of this study.

Under the light of the presented data, it is safe to confirm thatnanostructured plates featuring Si nanorods have the potential ofmaking a significant contribution to pool boiling heat transfer, andthe enhancements in heat transfer coefficients in both single-phaseand boiling regions are very promising. Si nanorods can replaceconventional nanostructures in applications benefitting from su-perior electronic properties of Si. It is also worth of mentioning thatthe nanostructure deposition technique presented in this paperallows isolating the nanorod length effect on boiling heat transfersince the production of nanorods with uniform spacing, length anddiameter is possible with the proposed method (Metal-assistedChemical Etching).

Acknowledgments

The authors would like to thank the UALR NanotechnologyCenter, Sabanci University Nanotechnology Research and Applica-tion Center (SUNUM) and Dr. Fumiya Watanabe for the continuedsupport in performing SEM measurements. This work is supportedby TUBITAK (The Scientific & Technological Research Council ofTurkey) Support Program for Scientific and Technological ResearchProjects Grants, 107M514 and 111M007, and TUBA (Turkish Acad-emy of Science) Outstanding Young Investigator Support Program.

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