Influence of particle properties on convective heat transfer of nanofluids

© 2017 An experimental study is performed in order to examine how particle properties such as size and thermal conductivity affect the convection heat transfer of nanofluids. For this purpose, we prepare and study self-synthesized water-based nanofluids with different kinds of particles: polystyrene, SiO 2 , Al 2 O 3 and micelles. Concentrations of the nanofluids vary in the range of 0.1–1.8 vol-% and particle sizes between 8 and 58 nm. Full-scale convective heat transfer experiments are carried out using an annular tube heat exchanger with the Reynolds numbers varying in the range of 1000–11000. The pressure losses are also taken into account in the analysis in order to assess the feasibility of the nanofluids for practical forced convection heat transfer applications. The fluids are thoroughly characterized: viscosities, thermal conductivities, densities, particle size distributions, shapes and zeta potentials are all determined experimentally. In many previous studies, anomalous enhancement in convective heat transfer is observed based on comparison of the Nusselt numbers with equal Reynolds numbers. Also in this work, the nanofluids exhibit Nusselt numbers higher than water when compared on this basis. However, this comparison neglects the impact of differences in the Prandtl numbers, and therefore the altered thermal properties of nanofluids are not properly taken into account. In this study, no difference in Nusselt numbers is observed when the Prandtl number is properly considered in the analysis. All nanofluids performed as the Gnielinski correlation predicts, and the widely reported anomalous convective heat transfer enhancement was not observed with any nanoparticle types. Instead, we show that the convection heat transfer behavior of nanofluids can be explained through the altered thermal properties alone. However, addition of any type of nanoparticles was observed to change the fluid properties in an unfavorable manner: the viscosity increases significantly, while only moderate enhancement in the thermal conductivity is obtained. The more viscous nanofluids reach lower Reynolds numbers than water with equal pumping powers resulting in lower heat transfer coefficients. However, the increase in viscosity, and therefore also the deterioration of the convective heat transfer, is less pronounced for the nanofluids with smaller particle size indicating that small particle size is preferable for convective heat transfer applications

Facile preparation of concentrated silver and copper heat transfer nanocolloids

Concentrated, yet stable silver- and copper-in-water nanocolloids are prepared using a novel method combining formation of a metal ammine complex and use of a strong NaBH4 reductant. Maximum solid contents of the stable silver and copper nanofluids are 2000 and 5000 ppm (reported as mass fractions), respectively. The metallic nanoparticles are reduced in micellar microreactors, favoring formation of small nanoparticles. Use of stable metal ammine complexes ([Ag(NH3)2]+ and [Cu(NH3)4(H2O)2]2+) as metal ion sources prevent the formation of sparingly-soluble metal salts and thus, aid the nanocolloid synthesis. Several different stabilizers and combinations of them are tested for nanofluid synthesis: anionic sodium dodecyl sulfate, polymeric polyvinylpyrrolidone, sodium citrate, nonionic sorbitan trioleate and polysorbate 20. The particle sizes and size distributions are studied using dynamic laser scattering and transmission electron microscopy. Stability of the nanofluids is assessed by zeta potential measurements, repetitive particle size measurements and visual observations. The average particle sizes of the silver and copper nanofluids with optimized surfactants are < 20 nm and ~40 nm, respectively, and the fluids with optimized stabilizer compositions are stable over the storing period of a month. Specific heat and thermal conductivities of the fluids are measured using differential scanning calorimetry and modified transient plane source technique (TCi Thermal conductivity analyzer), respectively. In addition, the nanofluid viscosities are measured in order to assess the usability of the nanofluids in convective heat transfer. The chemistry of stabilizers is found to have a significant impact on the viscosity of nanofluids. Commonly used polymeric polyvinylpyrrolidone stabilizer produces viscous fluids, whereas the viscosities of the fluids stabilized with small size surfactants are close to that of water.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

On the applicability of discrete dipole approximation for plasmonic particles

It has been recognized that the commonly used discrete dipole approximation (DDA) for calculating the optical properties of plasmonic materials may exhibit slow convergence for a certain region of the complex refractive index. In this work we investigate the quantitative accuracy of DDA for particles of different shapes, with silver as the plasmonic material. As expected, the accuracy and convergence of the method as a function of the number of dipoles is relatively good for solid spheres and rounded cubes whose size is of the same order as the wavelength of the localized surface plasmon resonance in silver. However, we find that for solid particles much smaller than the resonance wavelength, and for silver-silica core-shell particles in particular, DDA does not converge to the correct limit even for 10(6) dipoles. We also find that the slow convergence tends to be accompanied by strong, discretization dependent oscillations in the particle's internal electric field. We demonstrate that the main factor behind the slow convergence of the DDA is due to inaccuracies in the plasmonic resonances of the dipoles at the surface of the particles. (C) 2015 Elsevier Ltd. All rights reserved.Peer reviewe

Uudet faasimuutosmateriaalikompositiot lämpövarasto- ja lämmönsiirtosovelluksiin

Efficient energy storage and transfer is essential in order to meet the continuously increasing need of energy in a sustainable manner. Thermal energy covers a large part of the energy use: in Finland, the share of heating, ventilation and air conditioning is 22% of the total energy use. This dissertation focuses on the material development of heat transfer fluids (HTFs) and long-term thermal energy storages (LTES) based on phase change materials (PCMs). Many recent publications have demonstrated that nanofluids, dispersions of nanoscale particles and HTF, have higher thermal conductivity and improved convective heat transfer performance as compared to conventional HTFs. However, the results are contradictory and in some publications these abnormalities are not observed at all. This dissertation researches novel type of nanofluid: suspensions of nanoscale PCM particles. In PCM suspensions, both the latent heat of the PCM and the sensible heat of the fluid are exploited for heat transfer. PCM nanofluids are produced with high- and low-energy emulsification methods. The heterogeneity, viscosity and specific heat affect most the thermal properties and convective heat transfer performance of the fluids. No anomalies in the convective heat transfer of the PCM nanofluids are observed if the heterogeneity and changed material properties are taken into account. Efficient LTES that is able to store heat for several months or even for years would be highly beneficial, particularly in cold-climate countries. In principle, thermal energy can be stored long-term almost lossless in supercooled PCMs. The stored heat is released from the supercooled PCM on demand by crystallization. So far, the metastability of the supercooled state has prevented the use of PCMs in large-scale LTES: supercooled melts are always prone to spontaneous crystallization that dissipates the stored heat. Two novel LTES materials with modified supercooling and crystallization properties are developed in this dissertation. Both innovations are based on a supercooling polyol PCM and a polymer additive. The first material is a microstructured polyol-polystyrene composite that is prepared by a new method utilizing polymerization of high internal phase emulsion. The novel microstructured erythritol crystallized more efficiently and in a more reproducible manner than the bulk polyol. The other LTES innovation of this thesis is a glass-forming mixture of a polyol PCM and cross-linked polyelectrolyte. The strong intermolecular forces of the mixture restrain the crystallization of PCM on cooling. Instead, the PCM supercools and vitrifies. The new material releases the stored heat by cold-crystallization. For a first time, heat can be stored long-term in supercooled PCM as low temperatures as desired without the risk of spontaneous crystallization, and released from the material by a slight heating.Tehokas energian varastointi ja siirto ovat keskeisessä asemassa, jotta kasvava energiantarve voidaan täyttää ympäristöystävällisesti. Lämpöenergia kattaa suuren osan käyttöenergiasta: Suomessa lämmitys ja ilmanvaihto kattaa 22 % kokonaisenergiankulutuksesta. Tässä väitöskirjassa kehitetään uusia lämmönsiirto- ja lämpövarastomateriaaleja perustuen faasimuutosmateriaaleihin (engl. phase change material, PCM). Useiden tutkimusten mukaan nanonesteillä, lämmönsiirtonesteiden ja nanopartikkelien suspensioilla, on havaittu olevan korkeampi lämmönjohtavuus ja parempi lämmönsiirtokyky kuin perinteisillä lämmönsiirtonesteillä. Alan tutkimustulokset ovat kuitenkin ristiriitaisia, ja joissakin tutkimuksissa nanonesteillä ei ole havaittu lainkaan poikkeavia lämmönsiirto-ominaisuuksia. Tämä väitöskirja keskittyy uuteen nanonestetyyppiin: PCM nanopartikkelisuspensioihin. PCM nesteiden lämmönsiirto perustuu sekä PCM:n latenttilämpöön että suspension tuntuvaan lämpöön. Väitöskirjassa valmistetaan PCM nanonesteitä korkea- ja matalaenergian emulgointimenetelmillä. Nanonesteiden heterogeenisyys, viskositeetti ja lämpökapasiteetti vaikuttavat eniten nesteiden konvektiiviseen lämmönsiirtotehokkuuteen. Nanonesteiden johtavuudessa tai lämmönsiirtokyvyssä ei havaita poikkeavuuksia, kun nesteiden heterogeenisyys ja muuttuneet aineominaisuudet huomoidaan analyysissä. Tehokkaassa pitkäaikaisessa lämpövarastossa varastointiperiodi voi olla useita kuukausia, jopa vuosia.Tehokas pitkäaikainen lämpövarasto olisi erittäin tärkeä innovaatio erityisesti kylmän ilmaston maissa. Lämpöenergiaa voidaan varastoida pitkäaikaisesti lähes häviöttömästi alijäähtyneeseen PCM:n. Varastoitu energia vapautetaan kiteyttämällä alijäähtynyt PCM. Alijäähtyneen tilan metastabiilisuus on estänyt tähän asti ilmiön hyödyntämisen suurikokoisissa lämpövarastoissa: alijäähtynyt PCM on aina altis spontaanille kiteytymiselle, joka sattuessaan vapauttaa varastoidun energian. Tässä väitöskirjassa kehitetään kaksi uutta materiaalia pitkäaikaiseen lämmönvarastointiin. Materiaali-innovaatiot perustuvat alijäähtyvään sokerialkoholi PCM:n ja polymeerilisäaineeseen, joka muokkaa PCM:n alijäähtymis- ja kiteytymisominaisuuksia. Ensimmäinen materiaali on mikrorakenteinen polyoli-polystyreenikomposiitti, joka valmistetaan uudella menetelmällä hyödyntäen polyoli-styreeni emulsion polymerointia. Uusi mikrorakenteinen erytritoli PCM kiteytyy tehokkaammin ja toistettavammin kuin bulkki PCM. Toinen tämän työn lämpövarastoinnovaatio on lasittuva polyoli-polyelektrolyyttiseos. Kiteytymisen sijaan PCM alijäähtyy ja lasittuu jäähdytyksessä. Materiaali vapauttaa varastoimansa lämpöenergian kylmäkiteytymällä. Ensimmäistä kertaa lämpöenergiaa voidaan varastoida pitkäaikaisesti ilman spontaanin kiteytymisen riskiä alijäähtyneeseen PCM:n kuinka alhaisessa lämpötilassa tahansa, ja purkaa amorfisesta materiaalista pienellä lämmityksellä

Kalsiumoksalaatin ja bariumsulfaatin saostuminen paperi- ja öljyteollisuudessa - Dynaamisen kapillaarisaostumismenetelmän validointi

Scaling causes problems in the industry due to precipitation of undesired, sparingly-soluble salts onto process surfaces, generally onto pipelines or heat transfer equipment. Scaling of inorganic salts is a major problem in several industries, including paper and oil industry. Barium sulfate and calcium oxalate are among the most challenging scales in pulp and paper mills. Barium sulfate is also a troublesome scale in oil field reservoirs. Scales are generally treated with anionic, polymeric scale inhibitors that prevent/retard scale formation or dissolve the scale formed. The inhibition performance of polymeric antiscalants is based on attractive forces between cations in the salt and anionic, functional groups of scale inhibitor. Literature part of this study presents theory of crystallization, scaling in paper and oil industries, the most common scale inhibitor chemistries and inhibition mechanisms. Barium sulfate and calcium oxalate scaling and scale inhibition are studied by dynamic tube blocking method and dynamic test method setups for comparative scale inhibitor performance tests are presented in the experimental part of this study. The measurement conditions were optimized to correspond scaling in applications, yet the measurements were kept quick and easy to perform. Furthermore, effects of applications major parameters were studied. Dynamic tube blocking method was qualitatively validated and differences between the method and o generally used scale inhibition testing procedure, static jar tests, were compared.Saostuminen aiheuttaa ongelmia teollisuudessa, kun niukkaliukoisia suoloja saostuu prosessilaitteiden pinnoille, kuten putkilinjoihin ja lämmönsiirtimiin. Epäorgaanisten suolojen saostuminen on suuri ongelma usealla teollisuuden alalla, kuten paperi- ja öljyteollisuudessa. Bariumsulfaatti ja kalsiumoksalaatti kuuluvat hankalimpiin saostumiin sellu- ja paperiteollisuudessa. Bariumsulfaatin saostuminen on vakava ongelma myös öljykentillä. Saostumista estetään useimmiten anionisilla, polymeerisillä saostumisenestoaineilla, jotka estavät/hidastavat saostumista tai liuottavat jo muodostuneen saostuman. Polymeeristen saostumisenestoaineiden tehokkuus perustuu attraktiivisiin voimiin suolan kationien ja saostumisenestoaineiden anionisten, funktionaalisten ryhmien välillä. Työn kirjallisuusosa esittelee kiteytymisen teoriaa, saostumisongelmia paperi- ja öljyteollisuudessa sekä yleisimpien saostumisenestoaineiden kemiaa ja inhibitio-mekanismeja. Kokeellisessa osassa tutkitaan BaSO4:n ja CaC2O 4:n saostumista ja saostumisenestoa polymeerisillä saostumisenestoaineilla dynaamisella menetelmällä, jossa saostuminen tapahtuu kapillaarissa ja kehitetään dynaamisia testimenetelmiä saostumisenestoaineiden tehokkuuden vertailuun. Mittausolosuhteet optimoitiin vastaamaan saostumista sovelluksissa, mutta toisaalta mittaukset haluttiin pitää nopeina ja yksinkertaisina suorittaa. Lisäksi tutkittiin sovelluksien yleisimpien parametrien vaikutusta mittaustuloksiin. Dynaaminen kapillaarisaostumismenetelmä validoitiin kvalitatiivisesti ja mittauksia verrattiin yleisesti käytettyyn testimenetelmään, staattisiin purkkitesteihin

Novel microstructured polyol-polystyrene composites for seasonal heat storage

We propose a robust route to prepare novel supercooling microstructured phase change materials (PCMs) suitable for seasonal thermal energy storage (STES) or heat protection applications. Two supercooling polyols, erythritol and xylitol, are successfully prepared as novel microencapsulated PCM-polystyrene composites with polyol mass fractions of 62 wt% and 67 wt%, respectively, and average void diameter of ~50 μm. Thermal properties of the composites and bulk polyols are studied thoroughly with differential scanning calorimetry (DSC) and thermal conductivity analyzer. Significant differences in heat storage properties of microstructured and bulk PCM are observed. The heat release of microstructured erythritol is more controlled than that of bulk PCM, making the novel microengineered PCMs particularly interesting for STES. In the case of bulk PCM, the heat release may occur spontaneously due to crystallization by surface roughnesses or impurities, whereas these factors have only little impact on the crystallization of microstructured erythritol, making the novel composite more reliable for long-term heat storage purposes. In addition, microstructured polyol-polystyrene composites show anomalous enhancement in the specific heat as compared to bulk polyols. This enhancement may originate from strong polyol-surfactant interactions in the composites.Peer reviewe

Cold-crystallizing erythritol-polyelectrolyte

Renewable energy usage would benefit from efficient and high-capacity long-term heat storage material. However, these types of material solutions still lack reliable and durable operation on bulk level. Previously, we showed that cold-crystallizing material (CCM), which consists of erythritol in cross-linked polymer matrix, stored heat for a long-term period in a milligram scale by supercooling stably and preventing undesired crystallization during storage. Crystallization of CCM can be triggered efficiently by re-heating the material (i.e. cold-crystallization). Supercooling and cold-crystallization are stochastic phenomena which manifest in a way that the properties in bulk scale often deviate from the microscale. In this work, we scale up CCM to a bulk size of 160 g, and analyze its supercooling and crystallization characteristics for long-term heat storage. In order to identify the impact of the scale-up on the tested compositions and to discover optimal storage conditions, CCM samples are maintained in storage mode at constant temperature between 0 and 10 °C and up to 97 days. To this end, the thermal chamber measurement procedure estimates the heat release of CCM samples based on the measured temperature data and the one-dimensional transient heat conduction model. Results indicate that the heat release in cold-crystallization is over 70% of the melting heat. This heat can be stored without reduction for at least 97 days, demonstrating the reliable performance of long-term heat storage. Analysing the thermal properties of CCM compositions indicates a maximum volumetric storage capacity of 250 MJ/m3 and excellent properties for further heat storage applications.Peer reviewe

Thermal properties and convective heat transfer of phase changing paraffin nanofluids

Fluids containing micro-sized solid-liquid phase changing particles have been proposed to be promising candidates as future heat transfer fluids. In addition, smaller nano-sized particles have been claimed to enhance the heat transfer performance of fluids even if the phase change is not exploited. For the first time, we conduct full scale convection heat transfer measurements combining these two consepts. Three water-based paraffin mixture nanofluids with particle mass fractions of 5–10% are prepared and measured with an annular tube heat exchanger with Reynolds numbers varying in the range of 700–11000. In addition, the fluids are characterized: latent heats, specific heats, viscosities, thermal conductivities, densities and particle size distributions are all determined experimentally. In agreement with previous studies of solid-particle nanofluids and nanoemulsions, also the phase changing nanofluids are found to exhibit Nusselt numbers clearly higher (up to ∼60% in the turbulent regime) than water when compared on the basis of equal Reynolds numbers. However, the differences in Prandtl numbers are shown to explain these deviations in Nusselt numbers. Indeed, the well-known Gnielinski correlation is able to explain the results and thus, significant anomalies in the convection heat transfer caused by neither the melting of the phase change material nor the presence of the nanoparticles are observed. However, the nanofluids have systematically slightly higher Nusselt numbers than the correlation would predict, but the deviations are within the accuracy of the correlation (10%). When compared by using equal pumping powers, the nanofluids exhibit heat transfer performance poorer than that of water. The positive impact of the latent heat is outweighed by the negative effects of the increased viscosity and decreased specific heat