Thermal and Rheological Properties of Water-based

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o investigate the effect of temperature and nanop tep method using only commercially available nanopa hat larger than usually used in such experiments. Main.
Physics Procedia Volume 75, 2015, Pages 1458–1467 20th International Conference on Magnetism

Thermal and rheological properties of water-based ferrofluids and their applicability as quenching media Josip Župan1 and Marijana Majić Renjo1 1

Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Zagreb, Croatia [email protected], [email protected]

Abstract Water-based ferrofluids present a new energy transfer fluid with tunable properties. Previous research has shown the increase in thermal conductivity of water-based nanofluids with the addition of iron oxide. Such increased thermal properties show great potential for use in heat transfer. In this paper, several nanofluids were prepared with two step method. Iron (II, III) oxide nanoparticles with average paerticle size less than 50 nm were added to deionized water in following concentration: 0.01, 0.1, 0.5 and 1 g/L. Their thermal and rheological properties were measured at 20, 40 and 60 °C. Results showed increase in thermal conductivity and viscosity with increase in the addition of nanoparticles at all three temperature levels. The biggest increase was observed at 20°C. For this research, all of the prepared nanofluids were tested as immersion quenching liquid according to ISO 9950 standard. Besides still conditions, quenching experiments were conducted under the magnetic field at two levels, 500 and 1000 Gauss. The magnetic field effect was least present at 60°C with almost no influence on the cooling curve and cooling rates. At lower temperature levels quenching under the magnetic field shortened the full film boiling phase and increased the maximum cooling rate. Keywords: Ferrofluid, thermal conductivity, viscosity, quenching.

1 Introduction Nanofluids represent a novel energy transport fluid based on their improved properties compared to base fluids. Nanofluids are colloidal suspensions of stably dispersed nanoparticles in base fluids. The term was established at Argonne National Laboratory as a part of the Advanced Fluids Program investigating means to enhance thermal conductivity of base fluids (Das et al. 2008). Investigations determined that for heat transfer applications, the average size of particles should be less than 100 nm. With particles less than 100 nm in diameter, nanofluids exhibit enhanced thermal properties (Godson et al. 2010). There are papers showing that thermal conductivity of nanofluids depends on several 1458

Selection and peer-review under responsibility of the Scientific Programme Committee of ICM 2015 c The Authors. Published by Elsevier B.V. 

doi:10.1016/j.phpro.2015.12.166

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ˇ J. Zupan and M. Maji´c Renjo

factors: nanoparticle volume fraction, size and morphology, additives such as surfactants, pH value of the nanofluid, temperature and nature of base fluid, nanoparticle and base fluid material (Philip & Shima 2012). It is important to notice that different research groups obtained very different results of thermal conductivity of prepared nanofluids, leading to a discussion whether there is a valid theoretical model to predict and describe thermal conductivity of nanofluids. Parallel to intensive research in the field of heat transfer and application of nanofluids as new transport fluids, major concern was the viscosity of the newly developed fluids. This is due to the fact that viscosity represents internal resistance of a fluid to flow, an important property for all heat transfer applications involving fluids. Viscosity directly relates to the pumping power and in laminar flow pressure drop is proportional to the viscosity. A study found that temperature, nanoparticle size and shape, as well as volume fraction have significant effects on the viscosity of nanofluids (Mahbubul et al. 2012). Results show increase of viscosity with nanoparticle volume fraction increase. Also, it was found that there is no existing model or correlation that is capable of precise prediction of the viscosity enhancement with respect to volume fractions. Contrary to the nanoparticle volume fraction effect, viscosity of nanofluids decreases as temperature increases. Quenching is still one of the most widely used industrial heat treatment processes. In order to achieve the desired physical properties of the quench-hardened work piece, it is necessary to control the cooling rate at all phases of quenching. Different cooling rates are applied in order to achieve specific properties. There are some basic differences between the cooling rates for various liquid quenchants (Liščić et al. 2010). Most commonly these media are water and water-based solutions, polymer solutions, quenching oils, molten-salt baths, fluidized beds and compressed gases. In order to achieve a specific cooling rate, ferrofluid was investigated as a novel immersion quenching medium. First experiments were conducted in order to investigate the influence of magnetic field on contact angle in bubble boiling (Chigarev 1984). Boiling stage is one of the most important parts of quenching process so the impact on this stage can affect mechanical properties of quenched workpiece. Magnetic field causes smaller diameter bubbles to depart the surface, leading to the increase in heat transfer. Also, there was an investigation on the possibilities of controlled quenching in a magnetic liquid (Mirkin et al. 1993). Quenching experiments with various concentrations of dispersed magnetic particles of magnetite in magnetic fields of various strengths showed that it is possible to control the course of phase transformations in the quenched steels. Special attention has to be given to the quenching arrangement. Studies have shown that the position of quenched workpiece against the magnetic field can influence the heat transfer (Bashtovoi et al. 1993). Cooling curves in the magnetic field parallel to the cylindrical sample axis stabilized a heat-insulating vapour film, so that the heat transfer conditions deteriorate. Magnetic field perpendicular to the sample axes led to higher heat transfer coefficient in all stages of quenching which resulted in higher achieved surface hardness. Objective of this paper is to investigate if prepared ferrofluids exhibit extraordinary thermal properties under the influence of the magnetic field. Enhanced thermal properties mean that ferrofluids in magnetic field show potential to be used as liquid quenchants. Experiments were conducted in order to determine quenching properties of prepared nanofluids in selected magnetic field.

2 Experiments The selected method of nanofluid preparation consisted of adding nanoparticles to the base fluid, followed by ultrasonic homogenization for 90 minutes. Ultrasonic bath type BRANSONIC 220, with the frequency 50 kHz and power 120 W, was used to homogenize the nanofluid as well. For the preparation of nanofluids, iron (II, III) oxide particles produced by Sigma Aldrich, China were used. The average size of the nanoparticles was less than 50 nm. No dispersants, surfactants or activating agents were used. Pure magnetite nanoparticles were added to the base fluid, deionised water, and 1459

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homogenized. The sonified colloid c was stable and during a period of time there were w no visible signs of particle agglomeration or o sedimentation as shown in Figure 1. Differentt concentrations of nanoparticles were added to the t base fluid. The tested concentrations were 0.01, 0.1, 0.5 and 1 gram of nanoparticles per litre of baase fluid to see the effect of different concentratio ons on thermal and rheological properties as well w as their applicability as quenching fluid. Thhree levels of bath temperature were investigated. For the first set of experiments, bath temperature was w set at 20°C±1°C, the second temperature invesstigated was 40°C and the last set temperature was 600°C. In all cases, the base fluid and nanoparticless were first homogenized and after that heated or coooled to the defined temperature.

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c) d) Figure 1 Sedimentation test results for specified nanofluids a) immediately after sonification, b) one hour after sonification, c) 24 hours after sonification and d) 48 hours after sonificattion In this study, the therm mal conductivity was measured using the KD2 Pro Thermal Properties Analyzer (Decagon Devices,, USA), which is based on the transient hot wire meethod. The KD2 Pro consists of a handheld contrroller and sensors that should be inserted into the medium. m The singleneedle sensor with 1.3mm diameter d and 60mm length was used (KS-1). The sen nsor integrates in its interior a heating element andd a thermo resistor and it is connected to a microproceessor for controlling and conducting the measurem ments. The nanofluid sample was held in a cylindriccal plastic container with 35mm diameter and 900mm height. In addition to KD2 Pro system, a therm mostat bath LAUDA ECO RE 415 was used to maintain different temperatures of nanofluids durinng the measurement process. For every measurem ment, the plastic vessel containing the nanofluid was pllaced in a thermostat bath to reach the bath tempeerature and to ensure temperature equilibrium beforee each measurement was kept inside the bath for 20 2 min. The experimental set up is shown in Figure 2a. Rheological characterizattion included recording of flow curves and describingg obtained results by appropriate model. Flow curvves show dependence of shear stress and viscosity on shear rate. After the homogenization, all suspensiions were subjected to rheological measurements, onne at the time. Eight millilitres of each prepared ssuspension was used to measure rheological propertiies on the rotational viscometer Brookfield DV-IIII Ultra, USA, with accompanying software Rheocalc. For testing of water and water-based nanofluids ultra u low viscosity adapter was used, as shown in Fig gure 2b. The testing temperature was held constannt at each of the selected temperature levels: 20, 40 an nd 60°C. The results are given for viscosity at a shhear rate of 100 s-1. All quenching experimennts were conducted in accordance with the ISO 99500 standard. Cooling characteristics of water and water-based w nanofluids were measured and evaluatedd. Iron (II, III) oxide particle mass content, quench hant temperature and magnetic field strength in question have been varied to find the best suitable com mbination. All of the experiments were conducted in one litre beaker provided with the IVF SmarrtQuench system (Kristoffersen et al. 2014) shown in n Figure 2c. For the

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investigation of the magnnetic field effect, electromagnet was constructed according to our specifications by Končar - Ellectrical Engineering Institute Inc., Zagreb.

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b) c) Figure 2 Equipment used in characterization of nanofluids: a) Decagon KD D2 Pro Thermal Properties Analyzer, b) Broookfield DV-III Ultra rotational viscometer and c) ivf Sm martQuench system for quenchinng experiments in accordance with the ISO 9950 standard.

Described electromagnet has the ability to generate magnetic field up to 1200 G. When quenching experiments in magnetic fieldd were conducted, a glass container with the liquid qu uenchant was placed inside the electromagnet to induce i the magnetic field of desired strength, as show wn in Figure 3. The standard test probe was usedd in all experiments, in compliance with the internaational standard ISO 9950. The probe material was Inconel 600 and the used thermocouple wass standard Type K (NiCr/NiAl), 1.5 mm diametter. Inconel 600 is a paramagnetic material above 92 K. K Probe was heated to 850°C and placed in the centre of the glass container which is also the centtre of the generated magnetic field. The magneticc field was induced a minute before the heated probe was w immersed in the quenchant. The magnetic fieeld strength at the position of the temperature probe was w measured using Hall probe. Since particle deposition on the probe surface during quenchiing was previously experienced, and knowing thhat this deposition changes the surface wettability (K Kim et al. 2010), the probe was cleaned with sandd paper after each measurement. This ensured that the t condition of the probe surface has a minimal effect e on the results of the experiment.

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b) Figure 3 Experimental sett up for quecnhing with a) specified electromagnet and d b) probe position

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3 Results The aim of this paper waas to investigate the effect of temperature and nanopaarticle concentration on thermal and rheological properties p of ferrofluids. Reason for such research is the importance of thermal conductivity and visscosity in heat transfer problems. Ferrofluids present a novel approach to heat transfer enhancement duue to the fact that their properties are easily tunable witth external magnetic field. Ferrofluids show extraoordinary increase in thermal conductivity under the magnetic m field. All of this makes them applicable as a a medium for immersion quenching process. Their greatest g advantage is the possibility to control the process p through the alteration of properties with magnnetic field.

3.1 Thermal conducctivity Main reason for the deveelopment of nanofluids was to increase heat transfer capabilities of base fluids. The addition of mettallic or oxide nanoparticles, whose thermal condu uctivity is order of magnitude or two higher thhan the one of base fluid, increases thermal propertties of the prepared nanofluid. Different research hers have obtained various results, from dramatic inncrease to values of thermal conductivity lower thhan the base fluid (Keblinski et al. 2008). Some of th he proposed theories for the increase in propertiees include Brownian motion, aggregation of nanoparrticles, nanolayer of ordered liquid molecules at nanoparticle-liquid interface and other (Lee et al. 2010). 2 Some of the investigations of iron oxide nanofluids n show that thermal conductivity increases with the increase of temperature, while the relativ ve conductivity is almost constant (Yu et al. 2010). Results R also show the enhancement of the thermal conductivity of the nanofluid as a function of voluume fraction of iron oxide nanoparticles. The higghest investigated volume fraction of nanoparticles was w 1% which led to increase in thermal conductivvity of 34%.

Figure 4 Thermal propertiees of ferrofluids and Hamilton-Crosser model: a) relative theermal conductivity vs temperaturee and b) relative thermal conductivity vs nanoparticle contennt.

In this paper thermal connductivity measurements were done without the influeence of the magnetic field. The intention was too investigate the effect of temperature and nanoparticle addition for nanofluids prepared in two-sttep method using only commercially available nanopaarticles. The selected nanoparticles size is somewhhat larger than usually used in such experiments. Main problem with larger nanoparticles is agglomeratioon and sedimentation. Since no surfactants or activatinng agents were used in the preparation of nanofluuids, the measurements were conducted immediately after a the preparation in order to avoid nanoparticle aggregation and sedimentation. Sedimentation testts in Figure 1 show that such prepared nanofluidds are stabile long enough to conduct the experimentts. The results show some expected trends. Therm mal conductivity increases with the increase of nanoparticle content at all

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temperature levels. The highhest increase is visible at 20°C for nanofluid with 1 g/L of iron oxide reaching 37% higher values than deionised water. This also represents an anomallous result, since all other values are within 15% 1 increase. When comparing thermal conducttivity for different temperatures, almost all of thhe measurements show increase wheatear the observeed fluid is deionised water or nanofluids. Only thee nanofluid with nanoparticle content of 0.5 g/L reachhes maximum values of thermal conductivity at 40°C. Thermal conductivity for almost all of the investig gated nanofluids and selected temperatures are in line l with results from other researchers (Abareshi et al. a 2010; Gavili et al. 2012; Krichler & Odenbach 22013; Nkurikiyimfura 2011; Nkurikiyimfura et al. 201 13a; Syam Sundar et al. 2013; Tsai et al. 2009), as well as in line with main models that describe thhermal conductivity change in nanofluids, such as Hamilton-Crosser, Wasp or Bruggeman model. Figure F 4 shows the change of relative thermal coonductivity with different nanoparticle content and testt temperature.

3.2 Viscosity Viscosity is another impoortant property of the fluid when dealing with the heatt transfer, or even as important as thermal conduuctivity in engineering systems involving fluid flow w. Literature review shows that most of the researrch is dealing with the effect of the volume concentrattion of nanoparticles on the viscosity of nanofluidss (Mahbubul et al. 2012). Experimental results of dyynamic viscosity for water-based ferrofluids at differeent temperatures and various nanoparticle content show decrease of viscosity with the increase of tempeerature. This is valid for ferrofluids with all of thhe tested nanoparticle contents as well as base fluidd. As expected, the highest increase in viscosity is for ferrofluid with 1g/L nanoparticles at 20°C. Com mpared to base fluid, deionised water, the increaase is about 40%. Figure 5 shows the change of viscosity with the temperature and the nanopartticle content for all tested samples.

Figure 5 Rheological properties p of ferrofluids: a) viscosity vs temperature an nd b) viscosity vs nanoparticle content.

3.3 Quenching Quenching in ferrofluids under the influence of a magnetic field presents an opption for controllable heat extraction. The experimeents showed that nanofluids exhibit higher heat transfeer coefficient (HTC) and better thermal conductivvity compared to the base fluid, but also a shorter fulll-film boiling phase. When it comes to quenchinng in ferrofluids under the influence of the magneticc field, some of the achieved results so far have already a been explained. Research has shown that the vapour v layer size can be controlled by the presencee of the magnetic field when cooling in a magnetic fluid fl (Gogosov et al. 1990). The analysis of sectio ons of steel specimens cooled in the magnetic fluid, shows that the steel 1463

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structure essentially changes as the outer magnetic field strength varies. Other researchers have found that quenching time in ferroofluids is longer compared to that of deionised watter and it might be explained by the effects of thhe surface activators and nanoparticles on the viscosityy of the fluid (Habibi T same research showed that as a result of the magnetic field Khoshmehr et al. 2014). The implementation, ferrofluid booiling heat flux increases by 50% in all phases comppared with ferrofluid boiling without implementingg magnetic field. In this paper quenching experiments e according to ISO 9950 standard have beeen performed using ferrofluids with different nannoparticle content and at three temperature levels. As a reference, cooling curve of deionised water at 20, 40 and 60°C was recorded. For all of the tested nanofluids, cooling curves with and without mag gnetic field were recorded. Figure 6 shows some of the t recorded cooling curves and cooling rate channges for some specific cases. As quenchant at 20°C C shows the highest cooling rate and shortest fuull film boiling stage duration, the effect of nanopaarticle addition and magnetic field implementatioon was up to 10% increase in maximum cooling ratee. This was the case for all cooling curves recordeed at 20°C – all levels of nanoparticle content and mag gnetic field strength. First set of experiments condducted was without the presence of magnetic field, only o to see how the addition of nanoparticles affeects cooling curve and cooling rate. For ferrofluids at 40°C 4 there is similar behaviour to nanofluids withh other types of nanoparticles (Župan et al. 2012) as shown in Figure 6c. As the nanoparticle content increases, i the maximum cooling rate increases as welll and there is shorter full film boiling stage. The coonvective heat transfer is slightly higher for ferrofluidds than for deionised water. The differences are smaller s when the quenchant temperature is set at 60 0°C. The maximum cooling rate increase is less thhan 3% and full film boiling stage lasts almost the sam me, regardless of the nanoparticle content. When magnetic m field is applied, cooling rate increases and there is a change in the cooling curve. At 40°C and a magnetic field strength 500G the effect is most visible v for ferrofluid with the highest nanoparticlee content, 1g/L. The increase in maximum cooling ratee is around 20% and full film boiling stage is shhorter. The magnetic field also influenced the last stage s of the cooling process when convective heat transfer is dominant. This was in accordance with h results from other researchers dealing with convective heat transfer in ferrofluids under the t magnetic field (Nkurikiyimfura et al. 2013b)).

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f) Figure 6 Cooling curvess (solid lines) and cooling rate (dashed lines) for selectted conditions: a) quenchant at 20°C and magnetic m field 500G, b) quenchant at 20°C and magnettic field 1000G, c) quenchant at 40°C and no maagnetic field, d) quenchant at 60°C and no magnetic fiield, e) quenchant at 40°C and magnetic field 500G and f) quenchant at 60°C and magnetic fieeld 1000G.

When magnetic field streength is increased to 1000G, heat transfer dynamics off the same nanofluid deteriorate and in this case thhe maximum increase is reached with 0.5g/L of nanopparticles added to the base fluid. All of the coolingg curves recorded at this magnetic field strength are alm most identical to the cooling curve of base fluidd, deionised water. We can conclude that 1000G is above a point of saturation when thermal propperties of ferrofluids decrease due to large agglomeratiion of nanoparticles. This effect has been explaineed by other researchers (Philip et al. 2007). When queenchant temperature is set at 60°C, nanoparticle adddition and magnetic field proved to have little or no effect e on the cooling curve. The shape of the coooling curve compared to the base fluid didn’t changge and there was no expected effect on the durattion of the full film boiling phase. With lower mag gnetic field strength maximum cooling rate did inncrease with nanoparticle content but the increase was insignificant. When higher magnetic field strenggth was applied cooling curve properties and coolinng rate at all stages remained almost the same.

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4 Conclusions The following conclusions can be made from the obtained results: • • •

• • •

Thermal conductivity of ferrofluids increases with the increase in nanoparticle content at all temperature levels. Thermal conductivity for almost all of the tested nanofluids is in line with results from other researchers as well as in line with main models that describe thermal conductivity change in nanofluids, such as Hamilton-Crosser, Wasp or Bruggeman model. Experimental results of dynamic viscosity for water-based ferrofluids at different temperatures and various nanoparticle content show decrease of viscosity with the increase of temperature. This is valid for ferrofluids with all of the tested nanoparticle contents as well as base fluid. Ferrofluids without the presence of magnetic field at 40°C show similar behaviour to nanofluids with other types of nanoparticles – as the nanoparticle content increases, the maximum cooling rate increases and there is shorter full film boiling stage. When magnetic field is applied, cooling rate increases and there is a change in the cooling curve. The highest increase in maximum cooling rate is around 20% for ferrofluid with 1g/L of nanoparticles at 40°C and full film boiling stage is shorter. The magnetic field also influenced the last stage of the cooling process when convective heat transfer is dominant.

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