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Experimental study of the effect of solid volume fraction and Reynolds number on heat transfer coefficient and pressure drop of CuO-Water nanofluid

This study presents an experimental study of the effect of solid volume fraction and Reynolds number on heat transfer coefficient and pressure drop of CuO-Water nanofluid. Pure Water and nanofluid with particle volume fractions of 0.0625%, 0.125%,
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  Experimental study of the effect of solid volume fraction and Reynoldsnumber on heat transfer coefficient and pressure drop of CuO–Waternanofluid Majid Zarringhalam a , Arash Karimipour b , Davood Toghraie a, ⇑ a Department of Mechanical Engineering, Khomeinishahr Branch, Islamic Azad University, Khomeinishahr, Iran b Department of Mechanical Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran a r t i c l e i n f o  Article history: Received 25 October 2015Received in revised form 25 March 2016Accepted 27 March 2016Available online 30 March 2016 Keywords: CuO–Water nanofluidHeat transfer coefficientPressure dropSolid volume fraction a b s t r a c t Thisstudy presentsanexperimental studyof theeffect of solidvolume fractionandReynolds numberonheat transfer coefficient and pressure drop of CuO–Water nanofluid. Pure Water and nanofluid with par-ticle volume fractions of 0.0625%, 0.125%, 0.25%, 0.5%, 1%, 1.5% and 2% are used as working fluids.Nanofluids were flowed inside a horizontal double-tube counter flow heat exchanger under turbulentflow regime. Flow Reynolds numbers of each volume fraction of nanofluid were between from 2900 to18,500 during the experiments. The Result shows that generally heat transfer coefficient of nanofluidsis higher than that of base fluid. Moreover, it is observed that heat transfer coefficient and Nusselt num-berofnanofluidsincreaseswithanincreaseinsolidvolumefractionandReynoldsnumber.Buttherateof thisincreaseinlowReynoldsnumberswasmorethanthatathighReynoldsnumbers.Themeasurementsalso show that the pressure drop of nanofluid is slightly higher than that of the base fluid and increaseswith anincrease inthe nanoparticles volume fraction. But the rateof this increase in lowReynolds num-bers was more than that at high Reynolds numbers. Therefore, it can be concluded that the effect of increasing percentage of nanoparticle in low Reynolds number of this research is stronger than that of high Reynolds number. Moreover, friction factors were calculated and compared with blasious correla-tion. Finally, in order to find the optimum condition of this nanofluid for practical applications, thermalperformance factor was defined to consider increasing Nusselt ratio besides increasing friction ratiosimultaneously. The results show that the maximum thermal performance factor of this nanofluid was1.266, which was calculated for 2% nanoparticle volume fraction at Reynolds number 3677.   2016 Elsevier Inc. All rights reserved. 1. Introduction Nowadays, cooling and heating of industrial systems with highheat flux such as nuclear fusion, microelectronics mechanicalsystem (MEMS) and micro chemical reactions are the reasons forstrong challenges among engineering designers. On the one side,convective properties of conventional fluids such as water, differ-ent types of oils and ethylene glycol play an important role inexchanging thermal energy for efficient heat exchangers; on theother hand, poor thermal properties of these fluids are an obstaclefor designers to increase heat transfer and reduce the size of heatexchangers. One of the best solutions to the aforementionedproblemis suspendingsmall amounts of nanoparticlesinto formerconventional heat fluids. This type of fluid is called nanofluid. Inrecent years, many researchers have investigated heat transfercharacteristics of different nanofluids. Experimental study of researchers shows that effects of some parameters such as solidvolume fraction, shape, size and material characteristics of nanoparticlesbesidesexperimentalconditionsplayveryimportantroles in heat transfer coefficients.In an experimental investigation, Pak and Cho [1], studied theconvective heat transfer of Al 2 O 3 –Water and TiO 2 –Water nanoflu-ids under laminar and turbulent flow regimes. They showed thatNusselt number (Nu) enhanced with increasing particle volumefraction as well as Reynolds number augmentation. They alsoreported reduction range of 3–12% of heat transfer coefficient forTiO 2 –water nanofluid with 3% volume concentration of nanoparticles.Xuan and Li [2] experimentally investigated heat transfer char-acteristics of Cu–Water nanofluid, flowing straight through a tube http://dx.doi.org/10.1016/j.expthermflusci.2016.03.0260894-1777/   2016 Elsevier Inc. All rights reserved. ⇑ Corresponding author. E-mail addresses:  st_m_zarringhalam@azad.ac.ir (M. Zarringhalam),Arashkarimipour@gmail.com (A. Karimipour), Toghraee@iaukhsh.ac.ir(D. Toghraie).Experimental Thermal and Fluid Science 76 (2016) 342–351 Contents lists available at ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs  with constant heat flux under both laminar and turbulent flowconditions, and foundthat suspendednanoparticles withdiameterbelow100nminbasefluidcanincreaseheattransfercoefficientof conventional base fluid considerably; while friction factor did notincrease remarkably during the experiment. According to theirresult, heat transfer coefficient of conventional base fluid canenhanceabout60%bysuspending2%nanoparticlevolumefractionwithout significant augmentation of pressure drop. Furthermore,theyproposedanewconvectiveheattransfercorrelationtopredictheat transfer coefficient of the nanofluid under turbulent and lam-inar flow conditions.WenandDing[3]carriedoutanexperimentalstudyontheheattransfer characteristics of Al 2 O 3 –Water nanofluid in a copper tubewith constant heat flux under laminar flow. Diameter range of nanoparticles was 27–56nm. They observed that the convectiveheat transfer increases by enhancement of nanoparticles volumeconcentrationandReynoldsnumber.Moreover,inentranceregion,heat transfer augmentation is high and decreases by increment of axial distance. They also reported that those nanoparticles candecline thermal boundary layer thickness.He et al. [4] investigated heat transfer and flow behavior of aqueous suspensions of TiO 2  nanoparticles flowing upwardthrough a vertical pipe under both laminar and turbulent flowregimes. They observed that in specified nanoparticles size andReynolds number, the convective heat transfer coefficientincreased with enhancement of volume concentration of nanopar-ticle while pressure drop did not increase remarkably. They alsoreported that heat transfer coefficient is sensible with nanoparti-cles size.Herisetal. [5,6]reportedanexperimentwhichstudiedthecon-vective heat transfer coefficient of CuO–Water and Al 2 O 3 –Waternanofluids under laminar and turbulent flow condition flowing ina circular tube exposed to saturated vapor due to fix constant walltemperature boundary condition. The result of this experimentshowed that convective heat transfer coefficient ( h ) of bothnanofluids increased with increasing Peclet number and volumeconcentration of nanoparticles. Moreover, this increase (h) inAl 2 O 3 –Water nanofluid was more than that in CuO–Waternanofluid.DangthongsukandWongwises[7]experimentallyprobednano-fluid with 2% volume concentration of suspended TiO 2  nanoparti-cles into water base fluid flowing in a double tube counter flowheat exchanger and found that the convective heat transfer coeffi-cient of nanofluid was higher than that of the water base fluid byabout 6–11% with a little penalty of pressure drop. Moreover theypublished their next experiment [8] with the same test set up andnanofluidas thatinthelast experiment[7] reportingthatdifferentthermo physical models have no strong effects on the predicationof convective heat transfer coefficient of the nanofluid, and thatcredibility and precision of experimental heat transfer coefficientis related to calibration of experimental system more thanthermo-physical models of nanofluid.Nguyen et al. [9] and Anoop et al. [10] experimentally investi- gated the effect of nanoparticle size of Al 2 O 3 –Water nanofluidandconcludedthatsmallersizeofsuspendednanoparticlesincon-ventional base fluid causes higher heat transfer coefficient thantheir larger sizes.In an experimental study, Ding et al. [11] considered the con-vective heat transfer of CNT-distilled Water nanofluid under lami-nar flow condition and concluded that the local convective heattransfer coefficient may be as a function of flow characteristics,CNT concentration and PH value. But among prementioned effec-tive parameters, pH value has the least effect on the convectiveheat transfer coefficient of the nanofluid.Chandrasekar et al. [12] experimentally studied Al 2 O 3 –Waternanofluid flowing inside a tube with and without wire coil insertsunder both laminar and turbulent flow conditions. They reportedthat the heat transfer performance of Water base fluid increasesevenwithsmallamountsofnanoparticles(0.1%volumeconcentra-tion). Furthermore, wire coil inserts with different pitch ratioscouldbeeffectiveontheheattransferperformance;whilepressuredrop was approximately as much as base liquid for givenconditions.Dangthongsuk and Wongwises [13] experimentally investi-gated heat transfer performance for the nanofluid, which containsdifferentvolumefractions(0.2–2%)ofTiO 2  nanoparticleandWaterbase fluid flowing inside a double tube counter flow heat exchan-gerunderturbulentflowcondition.Theyreportedtheheattransfercoefficient of nanofluids is higher than that of the conventionalbase fluid and increases with enhancement of Reynolds numberand volume concentration of nanoparticles. But the heat transfercoefficient of the nanofluid with 2% particle concentration wasabout14%lowerthanthatofthebasefluid;whereas,pressuredropwas higher. They also presented new correlations for the predic-tion of Nusselt number and friction factor of the nanofluid.Fotukian and Nasr Esfahani [14] experimentally investigatedturbulent convective heat transfer and pressure drop of diluteCuO–Water nanofluid inside a circular tube. Results showed thatheat transfer coefficient increased by suspending small amountsof nanoparticles to the base fluid. They reported average 25%enhancement of heat transfer coefficient and 20% pressure dropaugmentation. Nomenclature Cp  specific heat, J/kg K k  thermal conductivity, W/m K L  tube length, m Nu  Nusselt number Re  Reynolds number T   temperature, K V   mean velocity, m/s m  mass flow rate, kg/s d  tube diameter, m h  heat transfer coefficient, W/m 2 K  f   friction factor Greeks D  p  pressure drop, pa u  nanoparticles volume fraction t  viscosity, m 2 s q  density, kg/m 3 g  thermal performance factor Subscriptsbf   base fluid nf   nanofluid in  inlet out   outlet m  mean b  bulk  p  nanoparticle w  water U   uncertainty M. Zarringhalam et al./Experimental Thermal and Fluid Science 76 (2016) 342–351  343  In an experimental investigation Amrollahi et al. [15] studiedconvection heat transfer of functionalized MWNT in aqueous fluidflowing at the entrance region of horizontal tube under laminarand turbulent flow condition. They reported heat transfer coeffi-cient enhancement by about 33–40% compared to base fluid atgiven conditions.Hashemi and Akhavan-Behabadi [16] reported an experimentalstudy of heat transfer and pressure drop characteristics of CuO–base Oil nanofluid flowing in a horizontal helically coiled tubeunder constant heat flux and found that heat transfer coefficientand pressure dropincreasedby increasingnanoparticleconcentra-tion. Moreover, theyreportedthatusingahelical tube, insteadof astraighttubeisamoreeffectivewaytoincreaseheattransfercoef-ficient compared to suspending nanoparticles to the base fluid.Hojjat et al. [17] experimentally studied convective heat trans-ferofnonNewtoniannanofluidsthroughauniformlyheatedcircu-lar tube and observed that convective heat transfer increases byincreasing nanoparticles concentration and decreases with axialdistance from the entrance region.Kayhanietal. [18], inanexperimentalstudy, probedconvectiveheattransferandpressuredropofTiO 2 –Waternanofluidthroughauniformly heated horizontal circular tube. They reported that heattransfer coefficient ratio increases by increasing nanoparticles vol-umeconcentration.Butit doesnotalterwithvariationof Reynoldsnumber.Syam Sundar et al. [19] carried out an experimental investiga-tion on the effect of twisted tape insert on the heat transfer andfriction factor enhancement with Fe 3 O 4  magnetic nanofluid insidea plain tube, and concluded better heat transfer rate when the cir-cular tube contains a twisted tape insert.HemmatEsfeetal.[20]investigatedexperimentallytheconvec-tive heat transfer performance of MgO–Water nanofluid with dif-ferent volume fractions of particle under turbulent flowcondition.Theyreportedthatadditionof smallamountsofparticleinto Water base fluid can increase heat transfer. They also com-pared experimental friction factor with Blasious equation.Inthisstudy, calculationof thermal performancefactorshowedthat the nanofluid is a good choice for practical applications.Hemmat Esfe et al. [21] reported an experiment on the heattransfer characteristics and pressure drop of COOH-functionalized DWCNTs–Water nanofluid in turbulent flow atlow concentration range of particle. They observed by about 32%heat transfer enhancement with about 20% penalty of pressuredrop for 0.4% volume concentration of nanoparticles. Furthermore,the result of thermal performance factor revealed that the nano-fluid with 0.4% particle concentration is a good choice to utilizein double tube heat exchangers.Saeednia et al. [22] presented an experimental study on CuO–base oil nanofluid with different particle weight fraction flowinginside a circular tube. They reported that heat transfer coefficientincreases by with increasing nanoparticles weight fraction.Abbasian and Amani [23] published an experimental study onthe effect of diameter on heat transfer performance and pressuredrop of TiO 2 –Water nanofluid flowing in a horizontal double tubecounter flow heat exchanger. They reported that Nusselt numberdoesnotgenerallyincreasebydecreasingthediameterofnanopar-ticles. They also achieved the highest thermal performance factorwith 20nm particle size diameter.It can be noted that previous studies, did not considered differ-enthighvolumeconcentrationofCuO–Waternanofluid.Since,thisarticle analyze and compared the results of heat transfer coeffi-cient, Nusselt number, pressure drop and friction factor of eightdifferent volume fractions (0–2%) of CuO–Water nanofluid, is veryinterestingissueforfurtherstudy.Also, finalresultofthisarticleispresent by thermal performance factor which is as a reliable crite-ria to aim for utilizing this nanofluid for practical application. 2. Experiments  2.1. Experimental setup and method Anexperimentalsetupwasappliedtomeasureconvectiveheattransfercoefficientandpressuredropcharacteristicsinthepresentwork.Fig.1showsaschematicdiagramoftheexperimentalinstru-ment. The apparatus was mainly composed of two centrifugalpumps, a heat transfer test section, a counter flow heat exchanger,a flow meter, a differential pressure transmitter, a bypass valve, areservoir, and a digital data logger. The test section is designedso that a small volume of nanofluid (about 2.7L) will be sufficientto investigate the heat transfer performance of nanofluids to holddown costs. This implement is basically provided by three closedloopcycles.Thenanofluidloopiscomposedofapump,acollectiontank, a test section and a Water heat exchanger for cooling theworkingfluid.Theheatexchangertestsectionissuppliedbyadou-ble concentric tube. Two calibrated RTD-PT100 types with digitalindicators are mounted at the inlet and outlet of the test sectionto measure the temperatures of the nanofluid with an accuracyof0.1  C.Moreover,eightK-type(Chromel–Alumel)thermocouplesare assembled on the copper tube wall at equal intervals of 13cmto measure the wall temperature with an accuracy of 0.5  C. Twoplastic fittings at inlet and outlet sections of the copper tube pro-vide a thermal blockage against axial heat conduction. In accor-dance with the equation  ð L e = D  4 : 4  Re 1 = 6 Þ , the length of thetube in order to create fully developed turbulent flow at Reynoldsnumber of 29,000, is estimated to be about 15cm. The heatedlength of the test section is 111cm. Therefore, it can be concludedthat the flow becomes developed turbulent for all cases studied.The test section is protected by 7cm of fiberglass to reduce heatloss to the ambient. To minimize heat loss to the surrounding(ambient air), the test section was insulated with 7cm of fiber-glass. The test section was heated by hot Water which flows overa copper tube. The second cycle contains equipment to flow andcontrol the hot Water flowrate at the desired temperature. Atem-perature controller with PT100 sensor was utilized to control thetemperature of hot Water. Two K-type thermocouples wereinserted into the flow at the inlet and outlet of the test section tomeasure bulk temperatures of hot Water. Furthermore, a flowmeter was used to adjust the hot Water flow rate. The third cycleincluded a pump, a nanofluid heat exchanger, a bypass line, a con-densing unit and a temperature controller with a PT100 sensor.This unit controls the temperature of the nanofluid at the inletand outlet of the test section by changing the power of the con-densing unit.Moreover, two valves control the flow rate of this loop cycle atthebypasslineandinletofthetestsection.Adifferentialtransmit-terRosemount3051cd(Rosemount,Inc.USA)measuresflowstaticpressure drop of nanofluid along the test section with an accuracyof up to 0.1%. This instrument measured the pressure dropbetween the inlet and outlet of the test section during each exper-iment. The essential parameters that are measured include hotWater and nanofluid flow rate, temperatures and pressure drop.It should be mentioned that all of the thermocouples and temper-aturesensors werecalibratedbefore being insertedtothe test sec-tion. Meanwhile, an ordinary thermometer measures the ambienttemperature.  2.2. Preparation of nanofluid Preparation of nanofluids is the first key step in experimentalstudies with nanofluids. Nanofluids are not simply liquid–solidmixtures. Some special requirements are essential e.g. even andstable suspension, durable suspension, negligible agglomeration 344  M. Zarringhalam et al./Experimental Thermal and Fluid Science 76 (2016) 342–351  of particles, no chemical change of the fluid, etc. In this paper, theCuO–Water nanofluids were supplied by dispersingCuO nano par-ticles in pure Water as base liquid at ambient conditions. Thedesired particles volume concentrations dispersed in this studywere 0.000625 (0.0625%), 0.00125 (0.125%), 0.0025 (0.25%), 0.005(0.5%), 0.01(1%), 0.015 (1.5%) and 0.02 (2%). Total volume of eachnanofluids was about 6L. In this study, CuO–Water nanofluidwas supplied utilizing a two-step method. The two-step methodis extensively used in the synthesis of nanofluids considering theavailable Nano powders. In this method, firstly, nanoparticles areproduced. Secondly, they are dispersed into conventional basefluid.Duetoadeclineinagglomerationofnanoparticlesinthebasefluid, an ultrasonic vibrator is used to disperse nanoparticlesagainstlargeagglomerates.Theappliednanoparticleswithaveragediameters40nmwereproducedbyUSResearchNanomaterial,Inc.Properties of the applied nanoparticles to the base fluid have beendescribed in Table 1. In this research, nanofluids were prepared bydispersing a specific amount of CuO in pure Water by using anultrasonic vibrator (Hielscher Company, Germany) for 180min inorder to obtain a dispersed solution which is uniform and stableand break down agglomeration of the nanoparticles. No surfactantwasusedastheymayhavesomeinfluenceontheeffectivethermalconductivity of nanofluids. Then nanofluids were poured into thenanofluid reservoir immediately and tests were carried out afterabout 180min when nanofluid flows at its maximum flow rate.As presented by Nasiri et al. [24], in turbulent flow condition, par-ticlessedimentationislessimportantduetogreaterimposedshear Fig. 1.  Schematics of experimental setup.  Table 1 Properties of CuO nanoparticles with diameter of 40 nm. Purity (metal basis) 99.5+%Color BlackAPS (nm) 40Morphology Spherical M. Zarringhalam et al./Experimental Thermal and Fluid Science 76 (2016) 342–351  345  which breaks down agglomeration of particles. Hence, it can bementionedthat turbulenceof flowhelps thestabilityof nanoparti-cles during the tests. Moreover, it is important to note that duringexperiments, no sedimentation was observed even at low flowrates. As noted above, in order to achieve a proper dispersion, itis necessary to repeat mechanical mixing and ultrasonic vibration.After 16h no sedimentation was obvious in any samples of nanofluids.The typical TEM (transmission electron microscope) micro-graph of the CuO nanoparticles with nominal particle size of 40nm is shown in Fig. 2. It was observed that the CuO nanoparti-cles have spherical shapes.The XRD (X-ray diffraction) of the prepared sample are shownin Fig. 3. As shown in Fig. 3, the powder XRD was carried out with a Rigaka X-ray diffractometer by CuO K a 1  radiation in the range of 10–90  .  2.3. Data processing  Using the conservation of energy law (first law of thermody-namic),convectiveheattransfercoefficient( h )andNusseltnumber( Nu ) of inner tube can be acquired in the equation below: dT  b dx  ¼  p _ mC   p h ð T  s  T  b Þ ð 1 Þ Separating variables and integrating from the tube inlet to theoutlet, the equation is as follows: ln ð D T  0 Þ D T  i ¼   pL _ mCp 1 L Z   L 0 hdx   ð 2 Þ The following expression is used to calculate the mean heattransfer coefficient,  h ¼ 1 L Z   L 0 hdx where   h  is the average value of   h  for the entire tube. From the def-inition of the average convection heat transfer coefficient and aftersimplify, D T  0 D T  i ¼ T  s  T  b ; o T  s  T  b ; i ¼ exp    pLmC   p  h   ð 3 Þ Rearranging Eq. (3) results in:  h ¼ _ mC   p  pL ln T  s  T  b ; i T  s  T  b ; o   ð 4 Þ Bulk temperature is calculated by: T  b  ¼ T  b ; i þ T  b ; o 2  ð 5 Þ Also, Reynolds and Nusselt number are defined as follows: Re ¼ Vd m Nu ¼  hdk ð 6 Þ  2.4. Uncertainty analysis Tobesureaboutthereliabilityofexperimentresults,measuringeach data was done for twice. Moreover, for each set of the exper-iment, nanofluid was refilled.The uncertainty of the experimental results was determined bythe measurement errors of the major heat transfer parameters byusing the values presented in Table 2, and based on the methodobtained by Kline and Mcclintock [25]. This method also has beenused by Fakoor et al. [26] and Hemmat Esfe et al. [20]. Assume the followingequationtocalculatetheuncertaintyof the parameter  R . U  R  ¼ X ni ¼ 1 @  R @  V  i U  V  i  2 "# 12 ð 7 Þ where  U  R  and  U  V  i  are the uncertainties associated with theparameter  R  and independent variables ( V  i ), respectively. In addi-tion, n is the number of the independent variables. Eqs. (8)–(11)were taken into account to calculate the uncertainty of Reynoldsnumber, convective heat transfer coefficient, Nusselt number andfriction factor, U  Re  ¼  d m U  V   2 þ  V  m U  d  2 þ  Vd m 2  U  m  2 "# 12 ð 8 Þ U  h  ¼  VdC   p L U  q Ln D T  i ; s D T  o ; s  2 þ  q dC   p U  V  Ln D T  i ; s D T  o ; s  2 " þ  q VC   p L U  d Ln D T  i ; s D T  o ; s  2 þ  q VdC   p L 2  U  L Ln D T  i ; s D T  o ; s  2 þ  q VdC   p L U  D T  i ; s 1 D T  i ; s  2 þ  q VdC   p L U  D T  o ; s 1 D T  o ; s  2 # 12  ð 9 Þ U  Nu  ¼  dkU  h  2 þ  hkU  d  2 þ  hdk 2  U  k  2 "# 12 ð 10 Þ U   f   ¼  2 d q LV  2 U  D P  ! 2 þ  2 D P  q LV  2 U  d ! 2 þ  2 D Pd q 2 LV  2 U  q ! 2 þ  2 D Pd q L 2 V  2 U  L ! 2 24 þ  4 D Pd q LV  3  U  v  ! 2 35 12 ð 11 Þ After computing the uncertainty of the Reynolds and Nusseltnumbers, heat transfer coefficient and friction factor at the righthand side of Eqs. (8)–(11), the uncertainty of key parameters wascalculated. Findings are presented in Table 2. 3. Measurements of thermo physical properties of nanofluid To calculate the heat transfer coefficient for nanofluid, it is nec-essary to apply thermo-physical property models for nanofluids.Thenecessarythermo-physicalpropertiesinthispaperaredensity, Fig. 2.  Transmission electron microscopy (TEM) image of CuO nanoparticles.346  M. Zarringhalam et al./Experimental Thermal and Fluid Science 76 (2016) 342–351
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