•This review explores potential convective heat transfer merits of nanofluids.•Both experimental and numerical findings are reviewed, including macro- and microchannels.•It is shown that heat ...transfer with nanofluids is realized mostly in inlet single-phase region.•Serious practical concerns in deploying nanofluids in cooling situations are identified.
This paper provides a comprehensive review of published literature concerning heat transfer benefits of nanofluids for both macro-channels and micro-channels. Included are both experimental and numerical findings concerning several important performance parameters, including single-phase and two-phase heat transfer coefficients, pressure drop, and critical heat flux (CHF), each being evaluated based on postulated mechanisms responsible for any performance enhancement or deterioration. The study also addresses issues important to heat transfer performance, including entropy minimization, hybrid enhancement methodologies, and nanofluid stability, as well as the roles of Brownian diffusion and thermophoresis. Published results point to appreciable enhancement in single-phase heat transfer coefficient realized in entrance region, but the enhancement subsides downstream. And, while some point to the ability of nanofluids to increase CHF, they also emphasize that this increase is limited to short duration boiling tests. Overall, studies point to many important practical problems associated with implementation of nanofluids in cooling situations, including clustering, sedimentation, and precipitation of nanoparticles, clogging of flow passages, erosion to heating surface, transient heat transfer behavior, high cost and production difficulties, lack of quality assurance, and loss of nanofluid stability above a threshold temperature.
Boiling is an effective energy‐transfer process with substantial utility in energy applications. Boiling performance is described mainly by the heat‐transfer coefficient (HTC) and critical heat flux ...(CHF). Recent efforts for the simultaneous enhancement of HTC and CHF have been limited by an intrinsic trade‐off between them—HTC enhancement requires high nucleation‐site density, which can increase bubble coalescence resulting in limited CHF enhancement. In this work, this trade‐off is overcome by designing three‐tier hierarchical structures. The bubble coalescence is minimized to enhance the CHF by defining nucleation sites with microcavities interspersed within hemi‐wicking structures. Meanwhile, the reduced nucleation‐site density is compensated for by incorporating nanostructures that promote evaporation for HTC enhancement. The hierarchical structures demonstrate the simultaneous enhancement of HTC and CHF up to 389% and 138%, respectively, compared to a smooth surface. This extreme boiling performance can lead to significant energy savings in a variety of boiling applications.
Extreme pool boiling performance is achieved by manipulating liquid–vapor transport at three length scales by engineering surface structures: 1) at millimeter scale, bubble coalescence is minimized and the complete capillary wicking is exploited with separated nucleation sites, 2) at micrometer scale, microcavities promote vapor nucleation, and 3) at nanometer scale, nanostructures extend the liquid–vapor interface for enhanced evaporation.
•The tight-coupling method is firstly developed for aircraft thermal anti-icing simulations under icing conditions.•A modified heat and mass transfer model of the runback water is established for the ...tight-coupling simulations.•The predicted anti-icing temperatures are in acceptable agreement with the experiment data, verifying the tight-coupling method.•Convective heat transfer coefficient is affected by surface temperature distribution, and the tight-coupling method can model this effect and predict more reasonable results.
Considering the influence of surface temperature distribution on air convective heat transfer coefficient, the robust tight-coupling method is firstly developed for aircraft thermal anti-icing simulations under icing conditions. To include the effects of the impinging water droplets on the conjugate heat transfer of thermal anti-icing systems, the Messinger thermodynamic model of runback water film is modified and added to the tightly coupled calculation of the external air flow and the internal solid skin heat conduction. Numerical simulations are carried out on an electro-thermal anti-icing system under both dry air and icing conditions, and the main conclusions below can be drawn. First, convective heat transfer coefficient changes slightly with surface temperature near the leading edge, but is obviously affected by temperature distribution in the downstream area. Second, the anti-icing temperature deviations between the predicted value and the experiment date are acceptable and comparable to the calculation results in the literature, verifying the feasibility and effectiveness of the tight-coupling method. Third, compared with the traditional decoupled loose-coupling method, the robust tight-coupling anti-icing method successfully captures the effect of surface temperature on convective heat transfer coefficient, and predicts higher temperature with lower drop rate on the downstream surfaces.
The knowledge of heat transfer deterioration is of great significance for the application of supercritical fluids in nuclear reactors, solar power systems, and other industrial fields. In order to ...better understand the current status, the goal of this paper is to conduct a systemic and comprehensive research review on heat transfer deterioration of supercritical carbon dioxide in vertical tubes. First, recent experiments of supercritical carbon dioxide in vertical flows are reviewed. The effect of boundary conditions such as mass flux, heat flux, tube diameter, operating pressure, and inlet temperature are compared and analyzed. Results show that the heat transfer behaviors are complex and inconsistent in different experiments. The sharp variation of the thermal properties and the induced buoyancy and acceleration effects are typically accepted explanations in existing studies. Further studies from a microscopic perspective using molecular dynamics may help to reveal the essence of the heat transfer mechanisms. Then, the identification methods to distinguish deteriorated heat transfer and normal heat transfer are summarized and assessed. A simple method with higher accuracy is also proposed, but no approach can discern the location and magnitude of heat transfer deterioration clearly. Next, the criteria of the onset of heat transfer deterioration in existing studies are reviewed. It is noticeable that the critical heat flux depends not only on mass flux, but also on tube diameter, operating pressure, and inlet temperature. Finally, an exhaustive summary of heat transfer correlations of supercritical carbon dioxide is carried out.
•Influences of boundary conditions and heat transfer mechanisms are discussed.•Identification methods for heat transfer deterioration are summarized and assessed.•A new identification method is proposed.•Criteria of the onset of heat transfer deterioration are reviewed.•Heat Transfer correlations of SCO2 are summarized.
Experiments have been conducted in a bubbling fluidized bed to determine the bed-to-tube surface heat transfer coefficient to a horizontal tube at high bed temperatures (400–950 °C). The aim was to ...study the influence of the bed temperature, the particle size, the bed material and the superficial gas velocity. A tube (do = 6 mm) through which water is flowing was placed close to the middle of the bed with a fixed bed height of 17 cm. Silica sand, crushed and beneficiated ilmenite and ground steel converter slag were used as bed materials. Air was used as fluidization gas. The mean particle diameter of the used bed materials was in the range 123–327 μm. The estimated heat transfer coefficients were compared with correlations from literature describing the convective bed-to-tube heat transfer coefficient (all of which have been derived at temperatures below 400 °C), with the addition of the radiative heat transfer contribution. The heat transfer coefficient increased with increasing bed temperature, decreasing particle size and increasing superficial gas velocity up to a certain value. These trends were observed for all bed materials although significant differences were still observed in the estimated heat transfer coefficients. Ilmenite was the material for which the highest heat transfer coefficient was estimated. The experiments verified high heat transfer coefficients (768–1858 W/(m2K)) to the outside tube surface operating at a superficial gas velocity of 0.15 m/s, which aligned very well with two of the heat transfer correlations and reasonably well with a third. The remaining three correlations were not as accurate at the examined conditions. The contribution from the convective heat transfer is significantly higher than the estimated radiative heat transfer to the tube.
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•High heat transfer coefficients (768–1858 W/(m2K)) are seen at high bed temperature.•The expected trends from heat transfer correlations are confirmed in the experiments.•Three correlations predict the heat transfer coefficient with reasonable accuracy.•The convective heat transfer dominates over the radiative heat transfer.
The heat transfer duty of heat exchangers can be improved by heat transfer enhancement techniques. In general, these techniques can be divided into two groups: active and passive techniques. The ...active techniques require external forces, e.g. electric field, acoustic or surface vibration etc. The passive techniques require fluid additives or special surface geometries. Electrohydrodynamic (EHD) techniques have been introduced as one of the types of active heat transfer enhancement techniques. This paper presents a review of research works on electrohydrodynamic heat transfer enhancement. This paper can be used as the first guideline for the researcher in using EHD techniques for heat transfer enhancement.
•Effect of channel angle on thermal performance of pin fin heat sink is reported.•Water based graphene nanoplatelets (GNPs) nanofluids are used as test fluid.•HTC for 22.5 degree heat sink is 84.30% ...higher as compared 90 degree heat sink.•Nanofluids compared to distilled water showed about 20% heat transfer enhancement.
This study reports an experimental work to examine the angle effect of pin fin heat sink channel in terms of convective heat transfer coefficient, log mean temperature difference and thermal resistance using water based graphene nanoplatelets (GNPs) nanofluids in a flow rate range of 0.25–0.75 LPM. Three heat sinks having channel angles, measured from positive x-axis, 22.5 degree, 45 degree and 90 degree are used. The volumetric concentration of GNPs particles is 9.5% and these particles consist of overlapped two-dimensional graphene layers. All heat sinks are fabricated with copper substrate, which is maintained at uniform heat flux during experimentation. Heat sink with 22.5 degree channel angle shows better thermal performance as compared to other tested heat sinks. For the same flow rate, 22.5 degree heat sink shows lowest convective thermal resistance as compared to other tested heat sinks.
•Review of experimental heat transfer studies comparing hybrid and mono nanofluids.•Analysis of hybrid and mono nanofluids for single-phase and two-phase convection.•Laminar single-phase convection: ...mono nanofluids better than hybrids (mixtures).•Laminar flow: thermal conductivity is straight related to convective heat transfer.•Turbulent single-phase convection and two-phase convection: few and divergent data.
Research on nanofluids has increased markedly in the last two decades. Initial attention has focused on conventional or mono nanofluids, dispersions of one type of solid nano-sized particles in a base fluid. Despite various challenges such as dispersion stability or increased pumping power, nanofluids have become improved working fluids for various energy applications. Among them, convective heat transfer has been the main research topic since the very beginning. Hybrid nanofluids, dispersions of two or more different nanoadditives in mixture or composite form, have received attention more recently. Research on hybrid nanofluids aims to further enhance the individual benefits of each single dispersion through potential synergistic effects between nanomaterials. Multiple experimental studies have been conducted independently analysing the convective heat transfer performance of mono or hybrid nanofluids for single-phase and two-phase convective heat transfer applications. However, there are still no general conclusions about which nanofluids, mono or hybrid, present better prospects. This review summarizes the experimental studies that jointly analyse both hybrid and mono nanofluids for these applications and the results are classified according to the heat transfer device used. Based on this criterion, three large groups of devices were noticed for single-phase convective heat transfer (tubular heat exchangers, plate heat exchangers and minichannel heat exchangers/heat sinks), while one group was identified for two-phase convective heat transfer (heat pipes). The main outcomes of these studies are summarized and critically analysed to draw general conclusions from an application point of view.
•External Convective heat transfer coefficients and radiative ones were calculated through different correlations.•Dissimilarities among the different coefficients values were investigated.•Heat flow ...meter and temperature probes were used to evaluate total heat transfer coefficients.•Convective and radiative components were distinctly assessed in two case studies.•Peculiar building features (balconies and porticos) were addressed.
In the building sector, the assessment of heat transfer processes is essential to identify the performance of structural elements. Heat transfer occurring in internal and external environments between walls and surrounding bodies is quantified by means of convective and radiative coefficients able to summarize physical phenomena. In building applications, International Standards suggest to calculate thermal transmittances by proposing specific surface thermal resistance values.
Moreover, several equations have been developed in scientific literature to compute the convective and radiative heat transfer coefficients, particularly useful in the building simulation field. For what concerns external coefficients, these correlations are generally related to the wind velocity, wind direction (windward and leeward surfaces) and surface roughness.
In this paper, the actual total external heat transfer coefficients in different case studies were obtained by measuring the physical parameters needed to define them. Peculiar geometries, i.e. balconies and porticos, were addressed, for which specific correlations are not available in literature. Convective and radiative components were distinctly assessed and, finally, actual convective heat transfer coefficients were compared with the same coefficients obtained by applying the conventional correlations available in literature and with the constant value suggested by the Standard. In addition, the convective heat transfer coefficients were evaluated by means of empirically determined correlations based on dimensionless parameters. The final goal of this work is to investigate the dissimilarities among the different coefficients values, in order to analyze applicability and limits of UNI EN ISO 6946 and of existing correlations for external heat transfer coefficients.
The analysis of the coefficients shows that the most significant differences are related to the convective heat transfer phenomena quantification. Applying the same correlations to different case studies scattered results were obtained whereas employing the dimensionless numbers correlations small oscillations in both case studies can be observed.
•Heat transfer performance of nanofluids in a plate heat exchanger is investigated.•The maximum enhancement in average Nu is 22.6% for 1.0 wt.% Fe3O4-water nanofluid.•The optimum concentration for ...thermal enhancement is 0.5 wt.% for CuO nanofluid.•Empirical formulas of experimental Nu are derived based on experimental data.•Fe3O4-water nanofluid is a promising heat transfer medium for solar energy systems.
In this paper, a corrugated plate heat exchanger in solar energy systems is used to investigate heat transfer and fluid flow characteristics of various nanofluids. By adding various nanoparticles (Al2O3-30 nm, SiC-40 nm, CuO-30 nm and Fe3O4-25 nm) into the base fluid, effects of nanofluid types and particle concentrations (0.05 wt.%, 0.1 wt.%, 0.5 wt.% and 1.0 wt.%) on the thermal performance of the plate heat exchanger are analyzed at flow rates in the range of 3–9 L/min. Results indicate that both heat transfer enhancement and pressure drop for nanofluids show significant increases compared to the base fluid. The Fe3O4-water and CuO-water nanofluids show the best and the worst thermal performances of the plate heat exchanger, respectively. When 1.0 wt.% Fe3O4-water nanofluid is used as the working fluid, compared to DI-water, the convective heat transfer coefficient is increased by 21.9%. However, an increase of 10.1% in pressure drop is obtained for the 1.0 wt.% Fe3O4-water nanofluid. Finally, empirical formulas of experimental Nusselt number are obtained based on the experimental data. A new way to predict the thermal performance for various nanofluids in heat transfer systems is provided.
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