Thermal performance of an open thermosyphon using nanofluids for high-temperature evacuated tubular solar collectors: Part 1: Indoor experiment

An especial open thermosyphon device used in high-temperature evacuated tubular solar collectors was designed. The indoor experimental research was carried out to investigate the thermal performance of the open thermosyphon using respectively the deionized water and water-based CuO nanofluids as the working liquid. Effects of filling rate, kind of the base fluid, nanoparticle mass concentration and the operating temperature on the evaporating heat transfer characteristics in the open thermosyphon were investigated and discussed. Experiment results show the optimal filling ratio to the evaporator is 60% and the thermal performance of the open thermosyphon increase generally with the increase of the operating temperature. Substituting water-based CuO nanofluids for water as the working fluid can significantly enhance the thermal performance of the evaporator and evaporating heat transfer coefficients may increase by about 30% compared with those of deionized water. The CuO nanoparticles mass concentration has remarkable influence on the heat transfer coefficient in the evaporation section and the mass concentration of 1.2% corresponds to the optimal heat transfer enhancement.     

Thermal Physics and Critical Heat Flux Characteristics of Al2O3–H2O Nanofluids

This study investigates the influence of the thermal physics of nanofluids on the critical heat flux (CHF) of nanofluids. Thermal physics tests of nanoparticle concentrations ranged from 0 to 1 g/L. Pool boiling experiments were performed using electrically heated NiCr metal wire under atmospheric pressure. The results show that there was no obvious change for viscosity and a maximum enhancement of about 5 to 7% for thermal conductivity and surface tension with the addition of nanoparticles into pure water. Consistently with other nanofluid studies, this study found that a significant enhancement in CHF could be achieved at modest nanoparticle concentrations (<0.1 g/L by Al₂O₃ nanoparticle concentration). Compared to the CHF of pure water, an enhancement of 113% over that of nanofluids was found. Scanning electron microscope photos showed there was a nanoparticle layer formed on the heating surface for nanofluid boiling. The bubble growth was photographed by a camera. The coating layer makes the nucleation of vapor bubbles easily formed. Thus, the addition of nanoparticles has active effects on the CHF.     

Behavioral study of alumina nanoparticles in pool boiling heat transfer on a vertical surface

Experiments were carried out to investigate the pool boiling of alumina‐water nanofluid at 0.1 g/l to 0.5 g/l of distilled water, and the nucleate pool boiling heat transfer of pure water and nanofluid at different mass concentrations were compared at and above the atmospheric pressure. At atmospheric pressure, different concentrations of nanofluids display different degrees of deterioration in boiling heat transfer. The effect of pressure and concentration of nanoparticles revealed significant enhancement in heat flux and deterioration in pool boiling. The heat transfer coefficient of 0.5 g/l alumina‐water nanofluid was compared with pure water and clearly indicates deterioration. At all pressures the heat transfer coefficients of the nanofluid were lower than those of pure water. Experimental observation revealed particles coating over the heater surface and subsequent SEM inspection of the heater surface showed nanoparticles coating on the surface forming a porous layer. To substantiate the nanoparticle deposition and its effect on heat flux, investigation was done by measuring the surface roughness of the heater surface before and after the experiment. While SEM images of the heater surface revealed nanoparticle deposition, surface roughness of the heater surface confirmed it. Based on the experimental investigations it can be concluded that an optimum thickness of nanoparticles coating favors an increase in heat flux. Higher surface temperature due to the presence of nanoparticles coating results in the deterioration of boiling heat transfer. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20365     

An Experimental Investigation of Nucleate Pool Boiling Heat Transfer of Nanofluids From a Hemispherical Surface

An experimental investigation of nucleate pool boiling heat transfer is carried out using SiO₂ nanofluid in atmospheric pressure and saturated conditions. The results show that the nucleate boiling heat transfer coefficient (HTC) of the nanofluids is lower than that of deionized water, especially in high heat fluxes. In addition, the experimental results indicate that the critical heat flux (CHF) improves up to 45% with the increase of the nanoparticle volume concentration. Atomic force microscopy images from the boiling surface reveal that the nanoparticles are deposited on the heating surface during the nanofluid pool boiling experiments. It is found that the boiling HTC deteriorates as a result of the reduction in active nucleation sites and the formation of extra thermal resistance due to blocked vapor in the porous structures near the heating surface. Furthermore, the improvement of the surface wettability causes an increase in CHF. Based on the experimental investigations, it can be concluded that the changes in the properties of the boiling surface are mainly responsible for the variations in nanofluids boiling performance.     

An experimental investigation of pool boiling characteristics of alumina-water nanofluid over micro-/nanostructured surfaces

Abstract

Experiments were conducted to investigate the nucleate pool boiling heat transfer of pure water and alumina/water nanofluids on different micro- and nanostructured surfaces prepared via the thermal spray coating method. Results indicate that nanofluids boiling on all the test surfaces led to critical heat flux (CHF) values greater than that obtained for the base fluid (i.e., water). Higher roughness value, however, led to higher CHF values in boiling over the surfaces. Another finding of this study indicated that CHF values obtained with boiling on Cu-coated micro- and nanosurfaces were identical although the heat transfer coefficient (HTC) values obtained for boiling on the micro-structure surface were higher than those obtained for a nanostructured surface with almost the same roughness. A series of consecutive nanofluid boiling cycles were also performed on the aluminum-coated nanostructured surface. The CHF value obtained for water boiling on the surface undergoing repeated nanofluid boiling cycles was by 27% higher than that obtained for a clean surface although the relevant HTC values were nearly identical.     

Characteristics of Flow Boiling Heat Transfer and Pressure Drop of MWCNT–R123 Nanorefrigerant: Experimental Investigations and Correlations

In the present work, the flow boiling heat transfer characteristics and pressure drop are experimentally investigated using multiwalled carbon nanotube (MWCNT)–R123-based nanofluids flowing inside a horizontal circular tube. The effects of particle concentration, mass flux, and vapor quality on the heat transfer coefficient (HTC) and pressure drop of MWCNT–R123-based nanofluid are analyzed. Results show that flow boiling HTC and frictional pressure drop increased with nanoparticle concentration, mass flux, and vapor quality as expected. The effects of nanoparticles on the flow boiling HTC and pressure drop are quantitatively analyzed by introducing a nanoparticle impact factor. A modified correlation for predicting the flow boiling HTC of nanorefrigerants is proposed, and the proposed correlation predicts 95% of the points with a deviation of ±20%. In addition, frictional pressure drop can be predicted using the Müller-Steinhagen and Heck correlation with a mean absolute error of 13.07% if the thermophysical properties of nanofluids are substituted.     

Modification of sandblasted plate heaters using nanofluids to enhance pool boiling critical heat flux

Nanofluids are colloidal dispersions of nanoparticles in homogenous base fluids. Previous studies have shown that nanofluids can increase pool boiling critical heat flux (CHF) by forming a porous deposition on the heated surface. However, questions remain whether nanoparticles can further enhance the CHF on a passively engineered heat transfer surface, such as a sandblasted metal plate. In this study, three water-based nanofluids (diamond, zinc oxide and alumina) were used to modify sandblasted stainless steel 316 plate heaters via boiling induced deposition. The pool boiling CHF of these pre-coated heaters increased by up to 35% with respect to that of the bare, sandblasted heaters. The enhancements are highest for alumina and zinc oxide nanofluids. Detailed surface characterization of these pre-coated heaters showed different surface morphology depending on the type of nanofluids used. Additionally, the deposited nanoparticles layers were found to alter the wettability of the heaters. Contact angle measurement provided quantitative data to determine possible CHF enhancement based on existing correlations.     

Boiling characteristics of carbon nanotube suspensions under sub-atmospheric pressures

An experimental study was performed to understand the pool boiling heat transfer of deionized water-based carbon nanotube (CNT) suspensions on a flat copper surface under atmospheric and sub-atmospheric pressures. Experimental results indicate that the addition of CNT in the base liquid can apparently enhance both the heat transfer coefficient (HTC) and the critical heat flux (CHF). The pressure has great impacts on both the HTC and the CHF enhancement of CNT suspensions and the both increase significantly with the decrease of the pressure. The CNT mass concentration of CNT suspensions also has strong influences on both the HTC and the CHF of CNT suspensions and there exists an optimal CNT mass concentration corresponding to the maximum heat transfer enhancement. Besides, the effects of the surface characteristics of the heated surface on both the HTC and the CHF of CNT suspensions were also investigated.     

Thermal resistance of screen mesh wick heat pipes using the water-based Al2O3 nanofluids

In the present study, the effect of nanofluids on the thermal performance of heat pipes is experimentally investigated by testing circular screen mesh wick heat pipes using water-based Al₂O₃ nanofluids with the volume fraction of 1.0 and 3.0 Vol.%. The wall temperature distributions and the thermal resistances between the evaporator and the adiabatic sections are measured and compared with those for the heat pipe using DI water. The averaged evaporator wall temperatures of the heat pipes using the water-based Al₂O₃ nanofluids are much lower than those of the heat pipe using DI water. The thermal resistance of the heat pipe using the water-based Al₂O₃ nanofluids with the volume fraction of 3.0 Vol.% is significantly reduced by about 40% at the evaporator-adiabatic section. Also, the experimentally results implicitly show that the water-based Al₂O₃ nanofluids as the working fluid instead of DI water can enhance the maximum heat transport rate of the heat pipe. Based on the two clear evidences, we conclude that the major reason which can not only improve the maximum heat transport rate but also significantly reduce the thermal resistance of the heat pipe using nanofluids is not the enhancement of the effective thermal conductivity which most of previous researchers presented. Especially, we experimentally first observe the thin porous coating layer formed by nanoparticles suspended in nanofluids at wick structures. Based on the observation, it is first shown that the primary mechanism on the enhancement of the thermal performance for the heat pipe is the coating layer formed by nanoparticles at the evaporator section because the layer can not only extend the evaporation surface with high heat transfer performance but also improve the surface wettability and capillary wicking performance.     

Surface effects of ribbon heaters on critical heat flux in nanofluid pool boiling

This paper deals with a study of enhanced critical heat flux (CHF) and burnout heat flux (BHF) in pool boiling of water with suspended silica nanoparticles using Nichrome wires and ribbons. Previously the current authors and other researchers have reported three-digit percentage increase in critical heat flux in silica nanofluids. This study investigates the effect of various heater surface dimensions, cross-sectional shapes as well as surface modifications on pool boiling heat transfer characteristics of water and water-based nanofluids. Our data suggest that the CHF and BHF decrease as heater surface area increases. For concentrations from 0.1 vol% to 2 vol%, the deposition of the particles on the wire allows high heat transfer through inter-agglomerate pores, resulting in a nearly 3-fold increase in burnout heat flux at very low concentrations. The nanoparticle deposition plays a major role through variation in porosity. The CHF enhancement is non-monotonic with respect to concentration. As the concentration is increased, the CHF and BHF decrease prior to increasing again at higher concentrations. Results show a maximum of 270% CHF enhancement for ribbon-type heaters. The surface morphology of the heater was investigated using SEM and EDS analyses, and it was inferred that the 2 vol% concentration deposition coating had higher porosity and rate of deposition compared with 0.2 vol% case.     

Pool boiling heat transfer of functionalized nanofluid under sub-atmospheric pressures

A water-based functionalized nanofluid was made by surface functionalizing the ordinary silica nanoparticles. The functionalized nanoparticles were water-soluble and could still keep dispersing well even at the mass concentration of 10% and no sedimentation was observed. An experimental study was carried out to investigate the pool boiling heat transfer characteristics of functionalized nanofluid at atmospheric and sub-atmospheric pressures. The same work was also performed for DI water and traditional nanofluid consisted of water and ordinary silica nanoparticles for the comparison. Experimental results show that there exist great differences between pool boiling heat transfer characteristics of functionalized and traditional nanofluid. The differences mainly result from the changes of surface characteristics of the heated surface during the boiling. A porous deposition layer exists on the heated surface during the boiling of traditional nanofluid; however, no layer exists for functionalized nanofluid. Functionalized nanofluid can slightly increase the heat transfer coefficient comparing with the water case, but has nearly no effects on the critical heat flux. It is mainly due to the changes of the thermoproperties of nanofluids. Traditional nanofluid can significantly enhance the critical heat flux, but conversely deteriorates the heat transfer coefficient. It is mainly due to effect of surface characteristics of the heated surface during the boiling. Therefore, the pool boiling heat transfer of nanofluids is governed by both the thermoproperties of nanofluids and the surface characteristics of the heated surface.     

梯度孔密度金属泡沫的池沸腾传热性能研究

实验研究了梯度孔密度通孔金属泡沫的池沸腾传热性能。工质为去离子水,梯度孔密度金属泡沫材质为铜和镍, 孔隙率为0.98,泡沫厚度为4-14 mm。实验结果表明：相比于单层泡沫,梯度孔密度金属泡沫显著的增强了沸腾传热能力,但增强程度受孔密度变化梯度、泡沫厚度和材料的影响;梯度孔密度泡沫的池沸腾传热性能随着表面活性剂SDS浓度的增大而减小,而且SDS降低了梯度孔密度金属泡沫的临界热流密度; 添加Al2O3纳米颗粒严重的削弱了梯度孔密度铜泡沫的池沸腾传热能力。     

Surface wettability change during pool boiling of nanofluids and its effect on critical heat flux

The pool boiling characteristics of dilute dispersions of alumina, zirconia and silica nanoparticles in water were studied. Consistently with other nanofluid studies, it was found that a significant enhancement in critical heat flux (CHF) can be achieved at modest nanoparticle concentrations (<0.1% by volume). Buildup of a porous layer of nanoparticles on the heater surface occurred during nucleate boiling. This layer significantly improves the surface wettability, as shown by a reduction of the static contact angle on the nanofluid-boiled surfaces compared with the pure-water-boiled surfaces. A review of the prevalent CHF theories has established the nexus between CHF enhancement and surface wettability changes caused by nanoparticle deposition. This represents a first important step towards identification of a plausible mechanism for boiling CHF enhancement in nanofluids.     

Boiling time effect on CHF enhancement in pool boiling of nanofluids

Using TiO₂–water nanofluids as the test liquid, pool boiling experiments were carried out to investigate the dependence of the nucleate boiling heat transfer, surface wettability and critical heat flux (CHF) on the boiling time in nanofluids. In the experiments performed at sufficiently high nanoparticle concentrations, the boiling heat transfer first degraded, then improved, and finally reached an equilibrium state. It was hence supposed that the present nanofluids had competing effects to deteriorate and enhance the nucleate boiling heat transfer. As for the surface wettability and CHF, the static contact angle asymptotically decreased whilst the CHF asymptotically increased with an increase in the boiling time. The maximum CHF enhancement measured in the present experiments was 91%, and strong correlation was found between the contact angle and the CHF. Although the boiling time needed to achieve the maximum CHF enhancement was less than a minute at high particle concentrations, a longer time of the order of 1 h was necessary at the lowest particle concentration tested in this work. This experimental result indicated that sufficient attention should be paid to the boiling time effect particularly in industrial applications of nanofluids to emergency cooling.     

Sorption and agglutination phenomenon of nanofluids on a plain heating surface during pool boiling

The pool nucleate boiling heat transfer experiments of water (H₂O) based and alcohol (C₂H₅OH) based nanofluids and nanoparticles-suspensions on the plain heated copper surface were carried out. The study was focused on the sorption and agglutination phenomenon of nanofluids on a heated surface. The nanofluids consisted of the base liquid, the nanoparticles and the surfactant. The nanoparticles-suspensions consisted of the base liquid and nanoparticles. The both liquids of water and alcohol and both nanoparticles of CuO and SiO₂ were used. The surfactant was sodium dodecyl benzene sulphate (SDBS). The experimental results show that for nanofluids, the agglutination phenomenon occurred on the heated surface when the wall temperature was over 112 °C and steady nucleated boiling experiment could not be carried out. The reason was that an unsteady porous agglutination layer was formed on the heated surface. However, for nanoparticles-suspensions, no agglutination phenomenon occurred on the heating surface and the steady boiling could be carried out in the whole nucleate boiling region. For the both of alcohol based nanofluids and nano-suspensions, no agglutination phenomenon occurred on the heating surface and steady nucleate boiling experiment could be carried out in the whole nucleate boiling region whose wall temperature did not exceed 112 °C. The boiling heat transfer characteristics of the nanofluids and nanoparticles-suspensions are somewhat poor compared with that of the base fluids, since the decrease of the active nucleate cavities on the heating surface with a very thin nanoparticles sorption layer. The very thin nanoparticles sorption layer also caused a decrease in the solid–liquid contact angle on the heating surface which leaded to an increase of the critical heat flux (CHF).     

Pool boiling heat transfer of non-Newtonian nanofluids

This paper reports on the investigation of pool boiling heat transfer of γ-Al₂O₃/CMC non-Newtonian nanofluids. To prepare nanofluids, γ-Al₂O₃ nanoparticles were dispersed in CMC solution (carboxy methyl cellulose in water) using ultrasonic mixing and mechanical mixer. Different concentrations of CMC non-Newtonian fluids and γ-Al₂O₃/CMC non-Newtonian nanofluids were tested under nucleate pool boiling heat transfer conditions. Experiments were carried out at atmospheric pressure. Results show that the pool boiling heat transfer coefficient of CMC solutions is lower than water. The decrease in boiling heat transfer is more pronounced at higher CMC concentrations and, as a result, higher solution viscosity. Adding nanoparticles to CMC non-Newtonian solutions results in an improved boiling heat transfer performance. The enhancement in the boiling heat transfer coefficient increases with the nanoparticle concentration; at a concentration of 1.4 wt.%, the boiling heat transfer coefficient increases by about 25% when compared to the base fluid.     

Development of Empirical Correlation for Heat Transfer to Nanofluids

Nanofluids are emerging as alternative fluids for heat transfer applications due to enhanced thermal properties. Several correlations are available in open literature for heat transfer coefficient (HTC) and thermophysical properties of nanofluids. Reliability of correlations that use effective properties for estimation of HTC needs to be checked. Comparison of experimental HTC and that estimated from existing correlations is the main objective of the present study. An empirical correlation is developed with experimental data of the HTC for zinc–water and zinc oxide–water nanofluids. Experimental HTC is compared with that estimated from developed correlation and existing correlations. The range of Re considered for the study is 4000 to 18,000. Comparison indicated large deviation in experimental values and the values estimated from existing correlations. Based on comparison results, it can be concluded that the single‐phase models of forced convective heat transfer cannot be extended to nanofluids.     

Effect of Vibration on Forced Convection Heat Transfer for SiO2–Water Nanofluids

In the process of heat transfer, the fluid type and external parameters have a significant impact on heat transfer performance. For this reason, the physical properties, pressure differences, and heat transfer rates of SiO₂–water nanofluids have been experimentally investigated in a straight circular pipe. Experimental results revealed a great difference in physical properties between SiO₂–water nanofluids and purified water. The friction factor of low-volume-concentration nanofluids was slightly increased for laminar flow and tended to be almost independent of the Reynolds number for turbulent flow. The heat transfer coefficient can be enhanced either by adding nanoparticles to purified water or by imposing a transverse vibration on the heat transfer surface. Using these two methods at the same time (compound heat transfer enhancement), heat transfer performance is much better than that with either method alone. The largest increase of about 182% was observed under conditions of compound heat transfer enhancement.     

Experimental investigation on pool boiling heat transfer of ZnO,and CuO water-based nanofluids and effect of surfactant on heat transfer coefficient

Comparison of boiling performance of nanofluids and mixtures of nanofluids with surfactant is an objective of this research. Experimental investigation has been performed with different heat flux and concentrations of nanoparticles and surfactant. CuO and ZnO water-based nanofluids are used and sodium dodecyl sulfate (SDS) is used as surfactant. The size of nanoparticles is measured from Field Emission Scanning Electron Microscopy pictures. Roughness of rod heater is calculated by using Atomic Force Microscopy picture. The pure water is tested after each run with nanofluid and mixture of nanofluid with SDS and the results are presented. Result from experiments demonstrates that the addition of SDS to nanofluids solution resulted in improving boiling performance. Experimental results also show an inefficient process by excluding surfactant. An optimum value for heat transfer coefficient is found by increasing of surfactant concentration within CuO nanofluid (0.01wt%CuO). Pictures of coated and clean surface in boiling pure water are employed for understanding the dynamics of bubbles. Gorenflo constant (h₀) is utilized to show the effect of addition of nanoparticles and SDS in boiling performance of base fluid.     

Experimental study on the pool boiling CHF enhancement using magnetite-water nanofluids

This paper described the effects of a magnetite-water nanofluid (MWNF) on the critical heat flux (CHF) enhancement using an Ni–Cr wire in pool boiling. All experiments were performed at a saturated condition under atmospheric pressure. The CHF values between the MWNF and the other nanofluids with several volume concentrations were compared to evaluate the effect of the MWNF on the CHF enhancement. The CHF values of the MWNF were enhanced from approximately 170% to 240% of pure water as the nanoparticle concentration increased. In addition, the CHF for the MWNF showed the highest value among the evaluated nanofluids. In this paper, three methods were introduced to elucidate the mechanism underlying CHF enhancement. First, scanning electron microscope (SEM) images were obtained to explain the CHF enhancement mechanism due to the deposited nanoparticles, which is related to the surface wettability of the heating surface during the pool boiling. Second, bubble formation in pool boiling was analyzed using image processing to demonstrate the relationship between bubble dynamics and CHF enhancement. Finally, the magnetic field was analytically calculated using the Biot–Savart law to evaluate the effects of the magnetic field on the CHF.