Istanbul, Turkey b Department of Electrochemical Engineering, University of Babylon, Published online: 02 Oct 2014.

Transcription

1 This article was downloaded by: [Sabanci University] On: 03 October 2014, At: 00:34 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Nanoscale and Microscale Thermophysical Engineering Publication details, including instructions for authors and subscription information: The Effect of Nanostructure Distribution on Subcooled Boiling Heat Transfer Enhancement over Nanostructured Plates Integrated Into a Rectangular Channel Ebru Demir a, Türker İzci a, Wisam J. Khudhayer b, Arif Sinan Alagöz c, Tansel Karabacak c & Ali Koşar d a Mechatronics Engineering Program, Sabanci University, Tuzla, Istanbul, Turkey b Department of Electrochemical Engineering, University of Babylon, Hilla, Iraq c Department of Applied Science, University of Arkansas, Little Rock, Arkansas, USA d Sabanci University, Tuzla, Istanbul, Turkey Published online: 02 Oct To cite this article: Ebru Demir, Türker İzci, Wisam J. Khudhayer, Arif Sinan Alagöz, Tansel Karabacak & Ali Koşar (2014) The Effect of Nanostructure Distribution on Subcooled Boiling Heat Transfer Enhancement over Nanostructured Plates Integrated Into a Rectangular Channel, Nanoscale and Microscale Thermophysical Engineering, 18:4, , DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or

2 howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at

3 Nanoscale and Microscale Thermophysical Engineering, 18: , 2014 Copyright Taylor & Francis Group, LLC ISSN: print / online DOI: / THE EFFECT OF NANOSTRUCTURE DISTRIBUTION ON SUBCOOLED BOILING HEAT TRANSFER ENHANCEMENT OVER NANOSTRUCTURED PLATES INTEGRATED INTO A RECTANGULAR CHANNEL Ebru Demir 1, Türker İzci 1, Wisam J. Khudhayer 2,ArifSinan Alagöz 3, Tansel Karabacak 3,andAliKoşar 4 1 Mechatronics Engineering Program, Sabanci University, Tuzla, Istanbul, Turkey 2 Department of Electrochemical Engineering, University of Babylon, Hilla, Iraq 3 Department of Applied Science, University of Arkansas, Little Rock, Arkansas, USA 4 Sabanci University, Tuzla, Istanbul, Turkey In this study, subcooled flow boiling is investigated over nanostructured plates at flow rates ranging from 69 ml/min to 145 ml/min. The first configuration of the nanostructured plate includes 600-nm-long, closely packed copper nanorod arrays distributed randomly upon the surface with an average nanorod diameter of 150 nm, and the second configuration consists of a periodic structure having 600-nm-long copper (Cu) nanorods with an average nanorod diameter of 550 nm and a center-to-center nanorod separation of 1 µm. The nanorod arrays are deposited utilizing glancing angle deposition (GLAD) technique on the copper thin film ( 50 nm thick) coated on silicon wafer substrates. Dimensions of the test section, heat flux values, and flow rates are chosen to ensure that nanostructured plates remain intact along with their nanorods in their original shape and position, so that the nanostructured plates could be used for many experiments. A consistent increase (up to 30%) in heat transfer coefficients is observed on nanostructured plates compared to the Cu thin film, which is used as the control sample. However, no significant difference in the boiling heat transfer performance between the random and periodic nanorods was observed, which indicates that the distribution of nanostructures may not be very critical in achieving enhanced heat transfer. In light of the obtained promising results, channels with nanostructured surfaces are proven to be useful, particularly in applications such as cooling of small electronic devices, where conventional surface modification techniques are not applicable. KEY WORDS: nanostructured plate, subcooled flow boiling, nanorods, heat transfer enhancement INTRODUCTION One of the key issues in saving energy and achieving compact designs for smallscale mechanical and chemical devices is the enhancement of heat transfer [1]. In the design of heat exchangers for spacecrafts, automobiles, microelectromechanical devices, Manuscript received 20 December 2013; accepted 1 May Address correspondence to Ali Koşar, Sabanci University, Tuzla FENS1077 Orhanli, Istanbul, Turkey. kosara@sabanciuniv.edu Color versions of one or more of the figures in the article can be found online at 313

4 314 E. DEMIR ET AL. NOMENCLATURE A A c c p Gz H h h tp h fg k L h ṁ Nu P P Pr Q Q loss q R Al Re R plate R tg R tot T avg T f surface area of the nanostructured plate channel cross-sectional area specific heat of water Graetz number channel height heat transfer coefficient two-phase heat transfer coefficient latent heat of vaporization thermal conductivity of the fluid heated length mass flow rate of water Nusselt number power input pressure drop Prandtl number fluid flow rate heat loss heat flux thermal resistance due to aluminum Reynolds number thermal resistance of the tested plate thermal resistance due to thermal grease total thermal resistance average fluid temperature bulk fluid temperature T i T s T th u x l x th inlet temperature surface temperature thermocouple reading fluid velocity local mass quality thermocouple location Greek Symbols ν kinematic viscosity Subscripts avg average Al corresponding to aluminum c cross-sectional f corresponding to the fluid property fg fluid-to-gas h corresponding to a heat presence i inlet l local loss corresponding to loss p at constant pressure plate plate property s surface property tg corresponding to thermal grease th corresponding to thermocouple tot total tp two-phase and microprocessors, it is crucial that heat exchangers are kept compact and lightweight [2]. For the purpose of making compact and efficient heat exchangers, heat transfer enhancement with nanostructures can be considered as a futuristic candidate particularly for small sizes, where conventional surface modification techniques utilized for macroscale surfaces would not be applicable. Recently, several studies were performed to enhance convective heat transfer by enlarging the transfer surface area using extended surfaces like fins and ribs [1, 3 5]. These modifications enlarge the heat transfer surface area and provide high heat transfer rates, but their drawbacks are increased friction factor and unwanted pressure drops. For example, pressure losses caused by pin-fin structures create a significant problem in many thermofluid applications and designs [1]. Such pressure losses occur because of the additional flow resistance imposed by pin fins. To achieve positive effects on heat transfer coefficients with diminishing length scale and high heat transfer performance due to enhanced heat transfer area, nanostructured surfaces were used in more recent studies [6, 7]. The main focus of these studies is utilizing nanostructured surfaces for improving boiling heat transfer. Singh et al. obtained an increase of 30 80% in nucleate flow boiling heat flux using multiwalled carbon nanotubes and achieved even higher enhancement ( %) at boiling incipience heat flux [8].

5 SUBCOOLED BOILING HEAT TRANSFER 315 Hendricks et al. showed that enhancements up to 10 times in heat transfer coefficients and up to four times in critical heat flux are achievable using nanostructured surfaces compared to bare Al surfaces [9]. As an advantage of nanostructured surfaces, dramatic reductions in boiling inception temperatures [10 19] and capability of such surfaces in decreasing the contact angle and increasing wettability in boiling applications [13, 15, 16, 18, 20] are reported in the literature. Boiling heat transfer performance enhancement attained using nanostructured surfaces is often attributed to (1) increased bubble departure frequency [9, 21, 22], (2) increased nucleation site density [22 24], (3) enhanced surface wettability [25, 26], and (4) enhanced heat transfer area introduced by nanostructures [24, 25]. Moreover, overall enhancement achieved through nanostructures can also depend on several variables such as the dimensions of the individual nanowires, porosity of the nanostructured surface, tilting angle [27], and distribution of the nanowires on the surface (e.g., random or periodically aligned nanowires), and these variables must be individually studied in order to have a better understanding of the heat transfer enhancement mechanism. Different from the state-of-the-art, this article focuses on the effect of the configuration of nanostructures on the enhancement of boiling heat transfer so that their potential in performance enhancement could be exploited from a different perspective. For this purpose, two configurations of nanostructured plates integrated into a rectangular channel are fabricated. The first configuration consists of vertical Cu nanorods of length 600 nm and average diameter 150 nm with an average gap among the nanorods ranging from 50 to 100 nm. These nanorods are grown in a randomly distributed manner on 50-nmthick Cu thin film deposited on silicon substrates with a thickness of 400 µm. The second configuration has the similar Cu-film/Si substrate but involves periodic structures having 600-nm-long copper nanorods with an average nanorod diameter of 550 nm and a center-to-center nanorod separation of 1 µm. Heat transfer coefficients obtained from the nanostructure configurations are compared to plates with plain surface, which consist of 600-nm-thick Cu thin films deposited on silicon wafers. The effect of nanostructure configuration on boiling heat transfer is discussed. NANOSTRUCTURE DEPOSITION Vertically aligned nonperiodic and periodic Cu nanorods were deposited on (100)- oriented p-type silicon substrates using a glancing angle deposition (GLAD) technique [28] in a custom-made computer-controlled, high-vacuum sputter deposition chamber. Nonperiodic Cu nanorods are grown on flat silicon substrate due to the competitive selfassembly nature of GLAD; on the other hand, periodic Cu nanorods are grown directly on periodic silver (Ag) islands fabricated on the silicon substrate utilizing a nanosphere lithography (NSL) technique. For comparison purposes, conventional smooth Cu thinfilm samples were prepared by normal incidence deposition. Cu nanorod morphology was investigated using a JEOL 7000F scanning electron microscope (SEM) (JEOL Ltd.). To fabricate patterned substrates, silicon substrates were cleaned with an RCA I process, rinsed with deionized water, and dried with nitrogen gas. Some samples were selected for single-layer polystyrene sphere (Bangs Laboratories, Inc., Fishers, IN) coating by a convectional self-assembly method [29, 30]. These spheres were used as a shadow mask for conventional normal incidence Ag thin film by thermal evaporation approach. During the deposition, Ag atoms deposited on top of spheres and filled the gaps among them. After lifting off the spheres from the samples by ultrasonication in toluene for 3 min, well-ordered

6 316 E. DEMIR ET AL. hexagonal Ag islands on silicon substrates were obtained. More details on the NSL process can be found elsewhere [31]. Both flat and patterned Si samples were loaded into the vacuum deposition chamber together and Torr base pressure was achieved by utilizing a turbomolecular pump backed with a mechanical pump. Sputter depositions were performed by applying 200 W DC power to a 2-in. 99.5% purity Cu cathode under 2.5 mtorr ultra-high-purity argon pressure. After depositing 50-nm Cu thin film at normal incidence, vertically aligned Cu nanorods were prepared by tilting samples 87 with respect to the incoming Cu flux and rotating samples continuously at 2 rpm. Deposition rate was measured in situ using a quartz crystal microbalance and substrate temperature was kept below 85 C. Conventional smooth thin-film Cu samples were prepared at normal incidence (without tilt) for comparison. As shown in Figure 1, vertically aligned nonperiodic Cu nanorods were fabricated by GLAD on flat silicon substrates. Nanorod density was larger at the early stages of growth; as they grew longer, some of the rods stopped growing due to the shadowing effect. The average diameter of nanorods was measured as 150 nm, and the average height was measured as 600 nm. The average gap among the nanorods was 5 10 nm at early stages and reached up to 50 nm at later stages. Figure 2 shows the cross-sectional and top-view SEM images of vertically aligned periodic Cu nanorods that were fabricated by GLAD on patterned substrates. Prepatterning the substrate surface broke down the nanorod growth competition mechanism at early stages and Cu nanorods were grown directly on Ag nanoislands, which were coated with a Figure 1 (a) Top-view and (b) side-view SEM images of vertically aligned nonperiodic Cu nanorods on flat silicon substrate. Scale bars are 1 µm. Figure 2 (a) Top-view and (b) side-view SEM images of vertically aligned periodic Cu nanorods on NSL patterned Ag on silicon substrate. Scale bars are 1 µm.

7 SUBCOOLED BOILING HEAT TRANSFER 317 thin-layer Cu film (50 nm). The average diameter and length of nanorods were measured to be around 550 and 600 nm, respectively. Due to weak shadowing of nanoislands, Cu atoms were also deposited between the nanorods, forming a 300-nm rough thin film. EXPERIMENTAL SETUP AND PROCEDURE Experimental Apparatus Figure 3 depicts the test section of the experimental setup that consists of an aluminum base and a Plexiglas top. Aluminum was chosen for its high machinability and high thermal conductivity and Plexiglas is preferred for its transparency and low thermal conductivity (around 0.19 W m 1 K 1 )[32].The33mm 60 mm 8.5 mm aluminum base houses four cartridge heaters of length mm and diameter 6.25 mm, which are mounted as shown in Figure 3. The base has a 0.35-mm-deep, 20 mm 20 mm square groove etched on its surface on the outlet side, which is utilized to level the nanostructured surface with the surface of the aluminum base. Three holes are drilled into the aluminum base to secure the thermocouples at the locations shown in Figure 3. The Plexiglas top part creates a 9-mm-high and 33-mm-wide channel when attached to the aluminum base. It also carries the inlet and outlet ports that allow delivering the working fluid to and from the channel. The heat generated by cartridge heaters is delivered to the plain surface or nanostructured plates, over which water flows in a rectangular channel. The cartridge heaters are embedded into an aluminum base in parallel lines with the configurations reported in the literature [8]. Thus, heat delivered by the heaters is directly transferred to the surface of the aluminum base over the projected heated area. In order to provide uniform heat flux to the nanostructured plate above, the heaters are closely packed with a distance of only 0.25 mm. The heaters in the heating block are closely packed, ensuring a compact configuration and minimizing heat losses to the ambient. The thermal conductivity a) c) Thermocouple 1 Thermocouple 2 Thermocouple 3 d) b) Thermocouple 1 Thermocouple 3 Figure 3 Test section of the experimental setup and thermocouple locations: (a) exploded view of the experimental setup with dimensions; (b) cross section of the experimental setup with dimensions; (c) thermocouple configuration (aluminum base hidden): 1, tangent to the nanostructured surface from below; 2, tangent to the rod heater; 3, tangent to the rod heater; (d) thermocouple configuration, aluminum base visible, thermocouple 2 and the rod tangent to it are not seen from this side.

8 318 E. DEMIR ET AL. of aluminum is close to that of other high thermal conductivity metals such as copper, and the heat path from the heaters to the surface is considerable (Figure 3), although it may appear small. Computer simulations for the same heating configurations were performed using COMSOL Multiphysics 3.5, a commercial computational fluid dynamics software, a product of Comsol Inc., in order to obtain heat flux variation on the top surface of the heating block. Rectangular mesh elements were used with about 100,000 degrees of freedom. Heat flux distribution over the top surface was deduced. According to the simulation results, the heat flux variation over the projected heated area was within ±30% along the top surface. The heaters and nanostructured plate were treated with high-quality silicone thermal grease in order to minimize thermal contact resistances and heat losses. The application of thermal grease was performed using the procedure widely reported in the literature. The thickness of this layer overlapped with the thicknesses included in various datasheets [33, 34]. The peak-to-peak roughness of the Si bottom surface of the samples is typically below 1 µm, and the peak-to-peak roughness of the plates used in our study was also below 1 µm. Therefore, no direct contact between the Si plates and aluminum block was expected. It should be also noted that the same procedure was consistently repeated for all of the samples tested in this study. To measure the thickness of the thermal grease, samples of silicon plates joined to a polished aluminum sample with thermal grease were carefully analyzed using zoom-in images of cross sections provided by light microscopy. Accordingly, the thickness of the thermal grease layer was measured as 0.05 mm with an uncertainty of ±5%. The whole setup was then sealed to avoid any leakage. Experimental Procedure Deionized water with an inlet temperature of 21 C was driven through the channel using a micro gear pump (Cole Parmer, EW , Cole Parmer Company, Vernon Hills, IL) allowing precise control of the flow rate, which was also monitored using an was also monitored using an FL113 Omega rotameter (Omega Engineering Inc., Stamford, CT) integrated into the system. Two thin-wire (76-µm-thick) Omega thermocouples were placed in the holes drilled right over the surface of the heaters in the middle, one in the hole drilled under the groove that houses the nanostructured plate as shown in Figure 3 and another placed under the aluminum base for accurate measurements of the surface temperatures. A total of six thermocouples were used in this study, adding the two additional thermocouples used to monitor the inlet and outlet fluid temperatures. The temperatures and pressure values obtained from the Labview interface did not significantly change with time so that experiments could be performed after steady flow conditions were reached. The power was increased in small increments ( 5%) until wall superheats reached 30 C to avoid any damage to the nanostructure uniformity due to excessive overheating. The current/voltage, inlet pressures, and temperatures were acquired under steady-state conditions with appropriate sensors (TC-TT-K-36 Omega thermocouples, Omega Engineering Inc., Stamford, CT; PX409 Omega pressure transducer, Omega Engineering Inc., Stamford, CT; 34401A Agilent multimeter, Agilent Inc., Santa Clara, CA; FL113 Omega flowmeter, Omega Engineering Inc., Stamford, CT). This procedure was repeated for different flow rates and different plate configurations. Each test was repeated three times, and it was observed that the data were reproducible. Average experimental values of three measurements are included as experimental data in this work. To estimate heat losses, electrical power was applied to the test section after evacuating deionized water from the test loop. Once the temperature of the test section becomes steady, the temperature difference between the ambient and test section was recorded with

9 SUBCOOLED BOILING HEAT TRANSFER 319 the corresponding power so that the power versus temperature difference profile was generated to calculate the heat loss associated with each experimental data point, and a heat loss calibration curve was obtained. Heat losses varied between 6 and 15% and the average heat loss was determined to be 12%. A reduction of the data was performed while taking the heat loss for each applied heat flux value into account. Data Reduction Heat flux input, q, to the system is obtained from q = (P Q loss )/A, (1) where P is the power input supplied to the section from the beginning of heated region to the location of thermocouple, Q loss is the thermal power loss, and A is the heated area, which is taken as the surface area of the part of the aluminum base housing the heaters (33 mm 33 mm). In order to determine Q loss, surface temperature rise vs. heat flux profiles were obtained from the thermocouples when the system was heated in the absence of the working fluid. The surface temperatures were calculated by considering thermal contact resistances from the thermocouple to the surface of the nanostructured plate. T s = T th q. R tot, (2) where T th is the thermocouple temperature reading from the thermocouple placed under the nanostructured pate and R tot is the total thermal resistance from the thermocouples to the surface of the nanostructured plate. R tot is calculated by R tot = R tg + R plate, (3) where R tg is the thermal resistance caused by thermal grease applied between the aluminum base and the tested plate and R plate is the resistance of the tested plate. The average of the surface temperatures was taken to obtain the average surface temperature T s. The heat transfer coefficient, h, was then calculated by h = q /(T s T f ), (4) where T s is the surface temperature and T f is the bulk fluid temperature at the thermocouple location and is given as T f = T i + [(P Q loss )x th /ṁc p L h ], (5) where x th is thermocouple location, T f is the exit fluid temperature, ṁ is the mass flow rate, T i is the inlet fluid temperature, and c p is the specific heat of water. The Nusselt number (for the single-phase data), Nu, is extracted from Nu = h.2h/k, (6) where H is the channel height and k is the thermal conductivity of the fluid. The flow velocity, u, is expressed as

10 320 E. DEMIR ET AL. u = Q/A c, (7) where Q is the flow rate of the water and A c is the channel cross-sectional area. The Reynolds number, Re, is given as Re = u.2h/ν, (8) where ν is the kinematic viscosity of the working fluid. Because thermally developing flow conditions were present in the current study due to small heating length (3.3 cm), the Graetz number is necessary to obtain theoretical Nusselt numbers and is expressed as Gz = L h /2HRePr, (9) where L h is the heated length, which is the distance between the inlet and outlet in this study. The channel height was selected as the length scale for Nusselt and Graetz numbers to compare experimental Nusselt numbers to the theoretical Nusselt numbers recommended for developing flows between parallel plates. Because heat is provided to only one side to the channel through the plates with plain and nanostructured surfaces, flow between the parallel plates case (heated from one plate) matches with the experimental conditions in the current study. Local mass quality is deduced based on the energy balance: x l = {[( P Q loss ) xth ] /Lh [ ṁc p (T sat T i ) ]} / ( ṁh fg ), (10) where x th is thermocouple location relative to the inlet, ṁ is the mass flow rate, c p is the specific heat, T i is the inlet temperature, and h fg is the latent heat of vaporization. EES Software (F-Chart Software, Madison, WI) was used to reduce the experimental data to the desired above-mentioned parameters in the current study. Uncertainty Analysis The uncertainties in the measured values are given in Table 1 and were derived from the manufacturer s specification sheet and the uncertainties of the derived parameters were obtained using the propagation of uncertainty method developed by Kline and McClintock [35], which is one of the most commonly used uncertainty estimation methods when the Table 1 Uncertainty ranges of the presented data Uncertainty Error P ±0.15 W T th ±0.1 C T s ±0.37 C R tot ±5% Q ±1% A ±0.08% A c ±0.2% q ±3.5% h ±8.4%

11 SUBCOOLED BOILING HEAT TRANSFER 321 uncertainty in reduced data is necessary. Using this method, the uncertainty was found for each data point, and then the average was taken and considered as the uncertainty. This method provides the uncertainties in the derived parameters in the light of uncertainties of parameters in the data reduction equations and the corresponding data reduction equation. RESULTS AND DISCUSSION Single-Phase Heat Transfer The surface temperatures of the nanostructured plates in the single-phase region and single-phase heat transfer coefficients are displayed in Figures 4a and 4b. As can be seen from Figure 4a, the surface temperature becomes lower at higher flow rates for fixed heat flux as expected. Moreover, because the distance between the thermocouple location and the beginning of the heated section is rather short (1 cm), thermally developing flow conditions are present during the experiments. In the current study, the thermally developing lengths for G = 3.85, 5.98, and 8.1 kg/m 2 s are 3.5, 7.1, and cm (according to the conventional theory), respectively, and are larger than the distance between the beginning of the heated region and thermocouple location. As a result, higher heat transfer coefficients can be observed at higher flow rates due to developing flow effects. Experimental Nusselt numbers are included in Figure 4c as a function of Graetz number. Theoretical predictions suggested by Mercer et al. [36] for the flow in parallel plates with a one-sided heating are also included in Figure 4c to provide a reference for comparison between experimental and theoretical results. As seen from Figure 4c, an overlap is present between the curve including theoretical results for Pr = 0.7 and 10 and experimental values. It can be noted that multiple data points are present at a given mass flux because each of these data points was taken at a different heat flux value and surface temperature. The variation in Nusselt number shown in Figure 4 is due to resulting different thermophysical properties of the working fluid for each heat flux value at the same mass flux. As heat flux increases, corresponding heat transfer and Nusselt number values also increase because of enhanced convective effects with the reduction in viscosity. However, theroretical predictions do not capture the changes in thermophysical properties and are based on thermophysical properties at inlet conditions. In this study, Nusselt numbers at low heat fluxes correspond to the conditions closer to the inlet conditions, and there is an increasing trend in Nusselt number with heat flux, which causes the variation in Nusselt number at the same mass flux. The lower experimental values (the lowest three values) at a given mass flux correspond to the experimental conditions near the inlet condition and can be predicted by the theory within ±30%, suggesting a reasonable agreement, which can be considered as a validation of the experimental setup. Boiling Heat Transfer The acquisition of wall superheats was initiated when boiling inception was observed in the light of flow visualization, due to the transparent cover of the test section. It was observed that the nanostructured plates decreased the boiling inception temperature up to 6 C compared to the plain surface. In addition, the rise in surface temperature with applied heat flux was suppressed with the introduction of the nanostructures. This could be explained by the increase in heat transfer area and the number of active nucleate sites so that more bubbles would emerge during boiling from the nanostructured surface and promote boiling heat transfer. This can lead to effective heat removal from the surface

12 322 E. DEMIR ET AL. a) Surface Temperature [ºC] G = 3.85 kg/m2s 20 G = 5.98 kg/m2s 10 G = 8.1 kg/m2s E E E E E+04 Heat Flux [W/m 2 ] b) 1400 Heat Transfer Coefficient [W/(m 2.K)] G = 3.85 kg/m2s G = 5.98 kg/m2s 200 G = 8.1 kg/m2s E E E E+04 Heat Flux [W.m 2 ] c) Nu 30 G = 3.85 kg/m2s 25 G = 5.98 kg/m2s G = 8.1 kg/m2s 20 Pr = Pr = E Gz Figure 4 (a) Wall temperatures in the single-phase region, (b) single-phase heat transfer coefficients, and (c) Nusselt numbers.

13 SUBCOOLED BOILING HEAT TRANSFER 323 of the plate and stabilization of the surface temperature. The effect of the configuration of nanostructures (periodic or random) seems to have minor effects on the boiling curves. This implies that an optimized periodicity/randomness configuration of nanostructures might not be critical in enhancing boiling heat transfer. The wall superheat values obtained as explained were used in calculating the heat transfer coefficients, which are plotted against applied heat fluxes in Figure 5. Higher heat transfer coefficients are apparent for higher mass fluxes at fixed heat flux values. A strong dependence of heat transfer coefficients on mass flux can be observed, which is related to subcooled boiling conditions in the current study. In subcooled boiling, boiling heat transfer is a sum of single-phase and nucleate boiling effects. The single-phase part is mass flux dependent and is not negligible in the partial boiling region, whereas the nucleate boiling component is wall superheat dependent and mass flux independent and becomes dominant in the fully developed boiling region. The strong mass flux dependence in this study suggests that partial boiling plays an important role. The lack of a change in the boiling curve trend and relatively short heated length corresponding to the thermocouple location bolster this conclusion. A significant enhancement in boiling heat transfer up to 30% is remarkable with nanostructures and is in agreement with the previous experimental studies on nanostructured surfaces [31]. For pool boiling, significant enhancements in boiling heat transfer were reported in the literature [7, 28]. Vertical nanorods act as pin fins that increase the surface roughness. Moreover, they can generate more bubbles by increasing the number of active nucleation sites, which is accomplished by multiple nanorods acting together to offer more hotspots for bubble nucleation due to their interconnected configuration. Although the spacing between two adjacent nanorods might be lower than the critical active nucleate size in the theory [37], the increased effective spacing due to interactions of multiple nanorods among each other can reach the critical size, thereby offering more active nucleation sites. It can be also seen that both nanostructure configurations of random and periodic Cu nanorods are successful to augment boiling heat transfer, and the enhancements of both nanostructure configurations are close to each other. The difference between boiling heat transfer coefficients is rather small for lower flow rates, whereas the deviation of the random configuration from the periodic configuration grows with flow rate. This could be related to increased bubble frequency and bubble removal from the surface with increasing flow rate in flow boiling due to enhanced heat flux at fixed wall superheat (N s q w) [38]. Thus, more active nucleation sites are expected in the random nanostructure configuration with higher flow rates, and more bubbles would emerge. With the increasing bubble frequency and emerging bubble removal at higher flow rates, the effect of active nucleate sites grows so that heat removal is more effective at higher flow rates. As a result, the difference between boiling heat transfer coefficients obtained from the random and periodic nanostructure configurations becomes greater with flow rate. Still, the difference is not as much as the difference between the plain surface and random nanostructure configuration results. Boiling heat transfer enhancement obtained from nanostructured surfaces (Figure 6) in the current study is in agreement with the literature [8 10]. It should also be kept in mind that there is room for optimization of the nanostructure configuration in terms of parameters such as nanorod length and alignment (e.g., angle with the surface) [27], which would enable the achievement of much larger boiling heat transfer enhancements. Several studies reported the important role that nanorod length plays in heat transfer enhancement capabilities of nanostructured surfaces; however, the results still leave the optimum nanorod length as an open question [39 41].

14 324 E. DEMIR ET AL. a) 1.00E E+03 Heat Transfer Coefficient [W/(m 2.K)] Heat Transfer Coefficient [W/(m 2.K)] 8.00E E E E E E+03 Plain Configuration 2.00E+03 Random Configuration 1.00E+03 Periodic Configuration 0.00E E E E E E E E E+05 Heat Flux [W/m 2 ] b) 8.00E+03 Heat Transfer Coefficient [W/(m 2.K)] 7.00E E E E E E+03 Plain Configuration Random Configuration 1.00E+03 Periodic Configuration 0.00E E E E E E E E E+05 Heat Flux [W/m 2 ] c) 8.00E E E E E E+03 Plain Configuration 2.00E+03 Random Configuration 1.00E+03 Periodic Configuration 0.00E E E E E E E E E+05 Heat Flux [W/m 2 ] Figure 5 Boiling heat transfer coefficient as a function of heat flux: (a) G = 3.85 kg/m 2 s, (b) G = 5.98 kg/m 2 s, and (c) G = 8.1 kg/m 2 s.

15 SUBCOOLED BOILING HEAT TRANSFER 325 a) 1.3 Heat Transfer Enhancement Random Configuration Periodic Configuration E E E E E E E E E+05 Heat Flux [W/m 2 ] b) 1.3 Random Configuration Periodic Configuration E E E E E E E E+05 Heat Flux [W/m 2 ] c) 1.3 Random Configuration 1.25 Periodic Configuration Heat Transfer Enhancement Heat Transfer Enhancement E E E E E E E E+05 Heat Flux [W/m 2 ] Figure 6 Boiling heat transfer enhancements: (a) G = 3.85 kg/m 2 s, (b) G = 5.98 kg/m 2 s, and (c) G = 8.1 kg/m 2 s.

16 326 E. DEMIR ET AL. CONCLUSION In this study, subcooled flow boiling was investigated over nanostructured plates of 20 mm 20 mm integrated into a rectangular channel at different flow rates under laminar flow conditions. The first configuration of the nanostructured plate includes randomly placed 600-nm-long copper nanorod arrays with an average nanorod diameter of 150 nm, and the second configuration consists of a periodic structure with 600-nmlong copper nanorods and an average nanorod diameter of 550 nm with a center-to-center nanorod separation of 1 µm. It was observed that no damage and disturbance in the structural integrity of nanostructures was evident during the experiments, where maximum wall superheat was fixed at 30 C. The major conclusions of this study can be summarized as follows: Both nanostructure configurations of random and periodic nanorod arrays are successful to significantly enhance boiling heat transfer. A strong dependence of heat transfer coefficients on mass flux can be observed for every configuration. There is not much difference in the boiling heat transfer performance between the random and periodic nanorods, implying that an optimized configuration of nanostructures may not be critical in achieving enhanced heat transfer. However, other nanostructure geometry parameters such as nanorod height, length, spacing, and detailed arrangement might still enable the achievement of much larger boiling heat transfer enhancements. Although small, the difference between boiling heat transfer coefficients obtained from the two nanostructure configurations becomes more apparent with increasing flow rates, which shows that the random nanostructure configuration has slightly higher heat transfer coefficient values. ACKNOWLEDGMENTS The authors thank the UALR Nanotechnology Center, Sabanci University Nanotechnology Research and Application Center (SUNUM), and Dr. Fumiya Watanabe for the continued support in performing SEM measurements. FUNDING This work is supported by the TUBITAK (The Scientific & Technological Research Council of Turkey) Support Program for Scientific and Technological Research Projects (Grants 107M514 and 111M007) and the TUBA (Turkish Academy of Science) Outstanding Young Investigator Support Program. REFERENCES 1. T. Kunugi, K. Muko, and M. Shibahara, Ultrahigh Heat Transfer Enhancement Using Nanoporous Layer, Superlattices and Microstructures, Vol. 35, pp , K.M. Stone, Review of Literature on Heat Transfer Enhancement in Compact Heat Exchangers, ACRC TR-105, Air Conditioning and Refrigeration Center, University of Illinois, Urbana, IL C. Zamfirescu and M. Feidt, Cascaded Fins for Heat Transfer Enhancement, Heat Transfer Engineering, Vol. 28, No. 5, pp , 2007.

17 SUBCOOLED BOILING HEAT TRANSFER C. Zamfirescu and A. Bejan, Constructal Tree-Shaped Two-Phase Flow for Cooling a Surface, International Journal of Heat and Mass Transfer, Vol. 46, pp , W. Qu and A. Siu-Ho, Liquid Single-Phase Flow in an Array of Micro-Pin-Fins, Journal of Heat Transfer, Vol. 130, No. 12, pp , M. Sesen, W. Khudhayer, T. Karabacak, and A. Kosar, A Compact Nanostructure Integrated Pool Boiler for Microscale Cooling Applications, Micro & Nano Letters, Vol. 5, No. 4, pp , C. Li, Z. Wang, P.-I. Wang, Y. Peles, N. Koratkar, and G. Peterson, Multiscale Coupling in Copper Interfaces for Enhanced Boiling, Small, Vol. 4, No. 8, pp , N. Singh, V. Sathyamurthhy, J.A. Peterson, J. Arendt, and D. Banerjee, Flow Boiling Enhancement on a Horizontal Heater Using Carbon Nanotube Coatings, International Journal of Heat and Fluid Flow, Vol. 31, No. 2, pp , T.J. Hendricks, S. Krishnan, C. Choi, C.H. Chang, and B. Paul, Enhancement of Pool-Boiling Heat Transfer Using Nanostructured Surfaces on Aluminum and Copper, International Journal of Heat and Mass Transfer, Vol. 53, pp , S. Ujereh, T.S. Fisher, I. Mudawar, P.B. Amama, and W. Qu, Enhanced Pool Boiling Using Carbon Nanotube Arrays on a Silicon Surface, ASME International Mechanical Engineering Congress & Exposition, Orlando, FL, 5 11 November, P. Keblinski, J.A. Eastman, and D.G. Cahill, Nanofluids for Thermal Transport, Materials Today, Vol. 8, No. 6, pp , I.C. Bang and S.H. Chang, Boiling Heat Transfer Performance and Phenomena of Al 2 O 3 Water Nano-fluids from a Plain Surface in a Pool, International Journal of Heat and Mass Transfer, Vol. 48, No. 12, pp , P. Vassallo, R. Kumar, and S. D Amico, Pool Boiling Heat Transfer Experiments in Silica Water Nano-Fluids, International Journal of Heat and Mass Transfer, Vol. 47, No. 2, pp , S. Vemuri and K.J. Kim, Pool Boiling of Saturated FC-72 on Nano-porous Surface, International Communications in Heat and Mass Transfer, Vol. 32, No. 1 2, pp , H. Kim, J. Kim, and M. Kim, Experimental Study on CHF Characteristics of Water TiO 2 Nano- Fluids, Nuclear Engineering Technology, Vol. 38, No. 1, pp , H.D. Kim and M.H. Kim, Effect of Nanoparticle Deposition on Capillary Wicking that Influences the Critical Heat Flux in Nanofluids, Applied Physics Letters, Vol. 91, No. 1, pp , D. Milanova and R. Kumar, Role of Ions in Pool Boiling Heat Transfer of Pure and Silica Nanofluids, Applied Physics Letters, Vol. 87, No. 23, p , S.M. You, J.H. Kim, and K.H. Kim, Effect of Nanoparticles on Critical Heat Flux of Water in Pool Boiling Heat Transfer, Applied Physics Letters, Vol. 83, No. 16, p , H. Honda, H. Takamatsu, and J.J. Wei, Enhanced Boiling of FC-72 on Silicon Chips with Micro-Pin-Fins and Submicron-Scale Roughness, Journal of Heat Transfer, Vol. 124, No. 2, pp , J.A. Eastman, S.U.S. Choi, S. Li, W. Yu, and L.J. Thompson, Anomalously Increased Effective Thermal Conductivities of Ethylene Glycol Based Nanofluids Containing Copper Nanoparticles, Applied Physics Letters, Vol. 78, No. 6, pp , P. McHale, J. Garimella, and V. Suresh, Bubble Nucleation Characteristics in Pool Boiling of a Wetting Liquid on Smooth and Rough Surfaces, International Journal of Multiphase Flow, Vol. 36, pp , C. Li, Z. Wang, P.-I. Wang, Y. Peles, N. Koratkar, and G.P. Peterson, Nanostructured Copper Interfaces for Enhanced Boiling, Small, Vol. 4, No. 8, pp , H. Jo, S. Kim, H. Kim, J. Kim, and M.H. Kim, Nucleate Boiling Performance on Nano/Microstructures with Different Wetting Surfaces, Nanoscale Research Letters, Vol. 7, pp. 1 12, 2012.

18 328 E. DEMIR ET AL. 24. Y. Tang, B. Tang, Q. Li, J. Qing, L. Lu, and K. Chen, Pool-Boiling Enhancement by Novel Metallic Nanoporous Surface, Experimental Thermal and Fluid Science, Vol. 44, pp , Z. Yao, Y.-W. Lu, and S.G. Kandlikar, Effects of Nanowire Height on Pool Boiling Performance of Water on Silicon Chips, International Journal of Thermal Science, Vol. 50, No. 11, pp , S. Kim, H.D. Kim, H. Kim, H.S. Ahn, H. Jo, J. Kim, and M.H. Kim, Effects of Nano-fluid and Surfaces with Nano Structure on the Increase of CHF, Experimental Thermal and Fluid Science, Vol. 34, No. 4, pp , M. Sesen, E. Demir, T. Izci, W. Khudhayer, T. Karabacak, and A. Kosar, Submerged Jet Impingement Cooling Using Nanostructured Plates, International Journal of Heat and Mass Transfer, Vol. 59, pp , J.P. Singh, T. Karabacak, D.X. Ye, D.-L. Liu, C. Picu, T.-M. Lu, and G.-C. Wang, Physical Properties of Nanostructures Grown by Oblique Angle Deposition, Journal of Vacuum Science and Technology B, Vol. 23, p , B.G. Prevo, D.M. Kuncicky, and O.D. Velev, Engineered Deposition of Coatings from Nanoand Micro-particles: A Brief Review of Convective Assembly at High Volume Fraction, Colloids and Surfaces A - Physicochemical and Engineering Aspects, Vol. 311, No. 1 3, pp. 2 10, W.J. Khudhayer, A.U. Shaikh, and T. Karabacak, Periodic Pt Nanorod Arrays with Controlled Porosity for Oxygen Reduction Reaction, Nanoscience and Nanotechnology Letters, Vol. 4, No. 10, pp , A.S. Alagoz, W.J. Khudhayer, and T. Karabacak, Hydrophobicity of Teflon Coated Well-Ordered Silver Nanorod Arrays, MRS Proceedings, Boston, MA, 28 November 02 December, Plexiglas Product Description, available at: PLEXIGLAS_GS_XT_en.pdf (accessed 9 March 2014). 33. P. Bachman, The Selection and Use of Thermal Interface Material for Solid State Relay Applications, 2011, available at: pdf (accessed 12 June 2013). 34. Silicone Grease Solutions for Your Thermal Interface Needs, available at: dowcorning.com/content/publishedlit/ pdf (accessed 12 June 2013). 35. S.J. Kline and F.A. McClintock, Describing Uncertainties in Single-Sample Experiments, Mechanical Engineering, Vol. 75, p. 3 8, W.E. Mercer, W.W. Pearce, and J.E. Hitchock, Laminar Forced Convection in the Entrance Region between Parallel Flat Plates, Journal of Heat Transfer, Vol. 89, pp , J.G. Collier and J.R. Thome, Convective Boiling and Condensation, Oxford University Press, New York, R.F. Gaertner and J.W. Westwater, Population of Active Sites in Nucleate Boiling Heat Transfer, Chemical Engineering Progress Symposium Series, Vol. 56, No. 30, pp , Z. Yao, Y.-W. Lu, and S.G. Kandlikar, Effects of Nanowire Height on Pool Boiling Performance of Water on Silicon Chips, International Journal of Thermal Science, Vol. 50, pp , M.-C. Lu, R. Chen, V. Srinivasan, V.P. Carey, and A. Majumdar, Critical Heat Flux of Pool Boiling on Si Nanowire Array Coated Surfaces, International Journal of Heat and Mass Transfer, Vol. 54, pp , Y. Im, Y. Joshi, C. Dietz, and S.S. Lee, Enhanced Boiling of a Dielectric Liquid on Copper Nanowire Surfaces, International Journal of Micro-Nano Scale Transport, Vol. 1, No. 1, pp , 2010.