Correction to "New Measurements of the Apparent Thermal Conductivity of Nanofluids and Investigation of Their Heat Transfer Capabilities"

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Performance evaluation on an air-cooled heat exchanger for alumina nanofluid under laminar flow

This study analyzes the characteristics of alumina (Al2O3)/water nanofluid to determine the feasibility of its application in an air-cooled heat exchanger for heat dissipation for PEMFC or electronic chip cooling. The experimental sample was Al2O3/water nanofluid produced by the direct synthesis method at three different concentrations (0.5, 1.0, and 1.5 wt.%). The experiments in this study measured the thermal conductivity and viscosity of nanofluid with weight fractions and sample temperatures (20-60°C), and then used the nanofluid in an actual air-cooled heat exchanger to assess its heat exchange capacity and pressure drop under laminar flow. Experimental results show that the nanofluid has a higher heat exchange capacity than water, and a higher concentration of nanoparticles provides an even better ratio of the heat exchange. The maximum enhanced ratio of heat exchange and pressure drop for all the experimental parameters in this study was about 39% and 5.6%, respectively. In addition to nanoparticle concentration, the temperature and mass flow rates of the working fluid can affect the enhanced ratio of heat exchange and pressure drop of nanofluid. The cross-section aspect ratio of tube in the heat exchanger is another important factor to be taken into consideration

Review of thermo-physical properties, wetting and heat transfer characteristics of nanofluids and their applicability in industrial quench heat treatment

The success of quenching process during industrial heat treatment mainly depends on the heat transfer characteristics of the quenching medium. In the case of quenching, the scope for redesigning the system or operational parameters for enhancing the heat transfer is very much limited and the emphasis should be on designing quench media with enhanced heat transfer characteristics. Recent studies on nanofluids have shown that these fluids offer improved wetting and heat transfer characteristics. Further water-based nanofluids are environment friendly as compared to mineral oil quench media. These potential advantages have led to the development of nanofluid-based quench media for heat treatment practices. In this article, thermo-physical properties, wetting and boiling heat transfer characteristics of nanofluids are reviewed and discussed. The unique thermal and heat transfer characteristics of nanofluids would be extremely useful for exploiting them as quench media for industrial heat treatment

Intensification of heat exchanger performance utilizing nanofluids

Heat exchangers are widely utilized in different thermal systems for diverse industrial aspects. The selection of HEx depends on the thermal efficiency, operating load, size, flexibility in operation, compatibility with working fluids, better temperature and flow controls, and comparatively low capital and maintenance costs. Heat transfer intensification of heat exchangers can be fulfilled using passive, active, or combined approaches. Utilizing nanofluids as working fluids for heat exchangers have evolved recently. The performance of heat exchangers employed different nanofluids depends mainly on the characteristics and improvement of thermophysical properties. Regarding the unique behavior of different nanofluids, researchers have attended noteworthy progress. The current study reviews and summarizes the recent implementations carried out on utilizing nanofluids in different types of heat exchangers, including plate heat exchangers, double-pipe heat exchangers, shell and tube heat exchangers, and cross-flow heat exchangers. The results showed that nanofluids with enhanced thermal conductivity, although accompanied by a considerable decrease in the heat capacity and raising viscosity, has resulted in performance enhancement of different heat exchangers types. So, the performance evaluation criterion that combines the thermal enhancement and increases the pumping power for any type of heat exchangers is requisite to evaluate the overall performance properly. The challenges and opportunities for future work of heat transfer and fluid flow for different types of heat exchangers utilizing nanofluids are discussed and presented

A review on the heat and mass transfer phenomena in nanofluid coolants with special focus on automotive applications

Engineered suspensions of nanosized particles (nanofluids) are characterized by superior thermal properties. Due to the increasing need for ultrahigh performance cooling in many industries, nanofluids have been widely investigated as next-generation coolants. However, the multiscale nature of nanofluids implies nontrivial relations between their design characteristics and the resulting thermo-physical properties, which are far from being fully understood. This pronounced sensitivity is the main reason for some contradictory results among both experimental evidence and theoretical considerations presented in the literature. In this Review, the role of fundamental heat and mass transfer mechanisms governing thermo-physical properties of nanofluids is assessed, from both experimental and theoretical point of view. Starting from the characteristic nanoscale transport phenomena occurring at the particle-fluid interface, a comprehensive review of the influence of geometrical (particle shape, size and volume concentration), physical (temperature) and chemical (particle material, pH and surfactant concentration in the base fluid) parameters on the nanofluid properties was carried out. Particular focus was devoted to highlight the advantages of using nanofluids as coolants for automotive heat exchangers, and a number of design guidelines was suggested for balancing thermal conductivity and viscosity enhancement in nanofluids. This Review may contribute to a more rational design of the thermo-physical properties of particle suspensions, therefore easing the translation of nanofluid technology from small-scale research laboratories to large-scale industrial applications

Study of compact heat exchangers: two-phase flow characteristics - heat transfer using nanofluids

The scope of the present thesis is to enhance the performance of compact heat exchangers by studying both the flooding phenomenon, when they are used as reflux condensers, and the possibility of employing nanofluids as working fluids. More specifically experiments were conducted: · in inclined small diameter tubes with the intention to study the flooding phenomenon and to elucidate the effect of various parameters, i.e., the diameter, the inclination angle, the surface tension and viscosity of the liquid, on the flooding onset and · in two compact heat exchangers using a typical nanofluid as cooling liquid, in order to evaluate its efficacy. The results show that during counter-current flow the liquid layer characteristics are affected by the velocities of the two phases and that the onset of flooding can be attributed to the interaction of the waves formed on the interface with the gas phase. In general, three regions have been observed on the flooding curve. New correlations have been proposed for the prediction of the flooding velocities in inclined small diameter tubes in the two main regions of the flooding curve, as well as for the transition point between them. The correlations are expected to be useful for estimating the range of operating conditions in equipment where the flow passages are characterized by equivalent diameters less than 10 mm and two-phase counter-current flow is present. The study of nanofluids has shown that their characteristics depend on many factors and cannot be easily predicted. Their performance in a heat exchanger depends both on the type of flow and on all their thermophysical properties. In micro-scale equipment with increased thermal duties, where also volume is a matter, and especially in laminar flow, the use of a nanofluid instead of a conventional fluid seems advantageous, the only disadvantages so far being its high price and the potential instability of the suspension. When the heat exchanging equipment operates under conditions that promote turbulence, the use of nanofluids is beneficial if and only if the increase in their thermal conductivity is accompanied by a marginal increase in viscosity. Thus, the substitution of conventional fluids by nanofluids seems inauspicious in industrial heat exchangers, where large volumes of nanofluids are involved and turbulent flow is usually developed.Η παρούσα διατριβή έχει ως στόχο τη βελτίωση της αποδοτικότητας των συμπαγών εναλλακτών θερμότητας μελετώντας αφενός το φαινόμενο της πλημμύρισης, όταν χρησιμοποιούνται ως συμπυκνωτές αντιρροής, και αφετέρου τη δυνατότητα χρήσης νανορευστών ως βοηθητικών ρευστών. Συγκεκριμένα διεξήχθησαν πειράματα: · σε κεκλιμένους αγωγούς μικρής διαμέτρου, ώστε να μελετηθεί το φαινόμενο της πλημμύρισης και να αποσαφηνισθεί η επίδραση της διαμέτρου και της κλίσης του αγωγού καθώς και του ιξώδους και της επιφανειακής τάσης της υγρής φάσης, και · σε δύο τύπους συμπαγών εναλλακτών θερμότητας με ένα τυπικό νανο­ρευ­στό ως ψυκτικό μέσο, με σκοπό να αξιολογηθεί η ενεργειακή τους απόδοση. Τα αποτελέσματα έδειξαν ότι τα χαρακτηριστικά της υγρής στιβάδας κατά τη διφασική αντιρροή επηρεάζονται από τις ταχύτητες των δύο φάσεων και η έναρξη της πλημμύρισης αποδίδεται στην αλληλεπίδραση των κυμάτων και της αέριας φάσης. Παρατηρήθηκε ότι οι καμπύλες πλημμύρισης χαρακτηρίζονται γενικά από τρεις διαφορετικές περιοχές. Για τις δύο βασικές περιοχές διατυπώθηκαν νέοι συσχετισμοί για την πρόβλεψη της πλημμύρισης σε κεκλιμένους αγωγούς μικρής διατομής, καθώς και για τον προσδιορισμό της μετάβασης μεταξύ τους. Οι συσχετισμοί αναμένεται ότι θα είναι χρήσιμοι στον προσδιορισμό του εύρους των λειτουργικών συνθηκών για εξοπλισμό με κανάλια μικρών διαστάσεων, όπου υπάρχει αντιρροή υγρής και αέριας φάσης. Η επισταμένη μελέτη των νανορευστών έδειξε ότι τα χαρακτηριστικά τους εξαρτώνται από πολλούς παράγοντες και δεν είναι εύκολο να προβλεφθούν. Η απόδοσή τους σε έναν εναλλάκτη εξαρτάται τόσο από το πεδίο ροής που διαμορφώνεται όσο και από το συνδυασμό όλων των θερμοφυσικών τους ιδιοτήτων. Η χρήση των νανορευστών μπορεί να βελτιώσει την ενεργειακή απόδοση εναλλακτών μικρών διαστάσεων, όπου ο συνολικός όγκος του εξοπλισμού είναι βασικό μέλημα του σχεδιαστή. Η ροή που διαμορφώνεται σε τέτοιου είδους συσκευές δεν είναι έντονα τυρβώδης και, όπως φάνηκε από τα πειράματα, η παροχή του νανορευστού για δεδομένο θερμικό καθήκον μπορεί να είναι χαμηλότερη από αυτή του νερού, μειώνοντας παράλληλα την απαίτηση σε αντλητική ισχύ. Βασικά μειονεκτήματα αποτελούν το αυξημένο κόστος προμήθειας των νανορευστών και ο περιορισμένος χρόνος ζωής τους, λόγω κατακάθισης των σωματιδίων. Τέλος, με βάση τα μέχρι τώρα δεδομένα δεν ενδείκνυται η χρήση νανορευστών σε βιομηχανικούς εναλλάκτες, όπου απαιτούνται μεγάλες ποσότητες βοηθητικών ρευστών και επικρατεί έντονα τυρβώδης ροή