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.     

Pool boiling characteristics of nanofluid on flat plate based on heater surface analysis

Nucleate pool boiling of Al₂O₃ based aqueous nanofluid on flat plate heater has been studied experimentally. For boiling of nanofluid (< 0.1 vol.%) on heating surface with ratio of average surface roughness to average diameter of particles much less than unity when boiling continue to CHF, the heat transfer coefficient of nanofluid boiling reduces while critical heat flux (CHF) increases. CHF enhancement increased with volume fraction of nanoparticles. Atomic force microscope (AFM) images from boiling surface showed that after boiling of nanofluid the surface roughness increases or decreases depending on initial condition of heater surface. Changes in boiling surface topology during different regions of boiling, wettability and thermal resistance of heater surface owing to nanoparticles deposition cause to variations in nanofluids boiling performance.     

Experimental investigation of the effect of particle deposition on pool boiling of nanofluids

Research on pool boiling of nanofluids has shown contradicting trends in the heat transfer coefficient (HTC). Such trends have been attributed, in part, to nanoparticle deposition on the heater surface. An experimental investigation of the transient nature of nanoparticle deposition and its effect on the HTC of pool boiling of nanofluids at various concentrations has been carried out. Pool boiling experiments have been conducted on a horizontal flat copper surface for alumina (40–50 nm) water based nanofluids at concentrations of 0.01, 0.1 and 0.5 vol.%. Nanofluids boiling experiments have been followed by pure water boiling experiments on the same nanoparticle-deposited (NPD) surfaces. This technique has been employed in order to separate the effect of nanoparticle deposition from the effect of nanofluids properties on the HTC. Contrary to what was expected, boiling of pure water on the NPD surface produced using the highest concentration nanofluid resulted in the highest HTC. A closer look at the nature of the NPD surfaces explained such trend. A new approach using a transient surface factor in Rohsenow correlation has been proposed to account for the transient nature of nanoparticle deposition. The applicability of such approach at different concentrations has been investigated.     

Flow boiling heat transfer in the evaporator of a loop thermosyphon operating with CuO based aqueous nanofluid

An experimental study was carried out to understand the flow boiling heat transfer of water based CuO nanofluids in the evaporator of a thermosyphon loop under steady sub-atmospheric pressures. Experimental results show that both the heat transfer coefficient (HTC) and the critical heat flux (CHF) of flow boiling in the evaporator of the thermosyphon loop could be enhanced by substituting nanofluids for water. The operating pressure has apparent impact on the HTC enhancement of nanofluids. However, the operating pressure has negligible effect on the CHF enhancement. There exists an optimal mass concentration of nanoparticles corresponding to the best enhancement effect. Experimental results show that the CHF enhancement results mainly from the existing of the coating layer on the heated surface formed by the sediment of nanoparticles. However, the HTC enhancement results from the effects of both the existing of the coating layer and the change of thermophysical properties of the working fluid.     

Heat transfer characteristics of Si and SiC nanofluids during a rapid quenching and nanoparticles deposition effects

Experiments were conducted to investigate the effect of nanofluid on a boiling heat transfer during a rapid quenching of a thin platinum (Pt) wire. The typical overall boiling curves have been successfully obtained from the cooling curves of the Pt wire for the water, the silicon (Si) and the silicon carbide (SiC) nanofluids. Meaningful differences in the behavior of the boiling curves between the water and the nanofluids cannot be identified during a quenching. However, the Si nanofluids reveal a slightly higher CHF (critical heat flux) than that for the water. On the other hand, a slight deterioration of CHF is observed in the case of the SiC nanofluids. When the Si and SiC nanoparticle-coated Pt wires are quenched with water, a very high cooling rate is observed and a very different boiling curve from that of the water and the nanofluids appears. Consequently, a considerably large heat transfer coefficient is obtained in a wide range of the wall superheat in the boiling curve.     

Pool boiling heat transfer of water and nanofluid outside the surface with higher roughness and different wettability

In order to investigate the effect of surface wettability on the pool boiling heat transfer, nucleate pool boiling experiments were conducted with deionized water and silica based nanofluid. A higher surface roughness value in the range of 3.9 ~ 6.0μm was tested. The contact angle was from 4.7° to 153°, and heat flux was from 30kW/m² to 300kW/m². Experimental results showed that hydrophilicity diminish the boiling heat transfer of silica nanofluid on the surfaces with higher roughness. As the increment of nanofluid mass concentration from 0.025% to 0.1%, a further reduction of heat transfer coefficient was observed. For the super hydrophobic surface with higher roughness (contact angle 153.0°), boiling heat transfer was enhanced at heat flux less than 93 kW/m², and then the heat transfer degraded at higher heat flux.     

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.     

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     

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.     

Investigations on heat transfer enhancement in pool boiling with water-CuO nano-fluids

The main focus of the present work is to investigate Critical Heat Flux (CHF) enhancement using CuO nanofluid relative to CHF of pure water. To estimate the effect of nanoparticles on the CHF, pool boiling CHF values were measured for various volume concentrations of CuO nanofluid and compared with pure water. CHF enhancement of 130% was recorded at 0.2 % by volume of CuO nano-fluids. Surface roughness of the heater surface exposed to three measured heating cycles indicated surface modifications at different volume concentrations of nanofluid. SEM image of the heater surface revealed porous layer build up, which is thought to be the reason for CHF enhancement.     

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.     

Experimental Investigations of Pool Boiling Heat Transfer Characteristics on a Vertical Surface Using CuO Nanoparticles in Distilled Water

Experiments were conducted on pool boiling heat transfer using dilute dispersions of CuO nanoparticles in distilled water at and above atmospheric pressure. Pool boiling characteristics of CuO nanofluid were studied at different pressures and concentrations. Characterization of the heating surface was done both qualitatively and quantitatively by taking the scanning electron microscopy (SEM) images and by subsequent measurement of surface roughness of the heater. SEM images of the heater surface showed nanoparticle deposition on the heater surface, suggesting surface modification. Thorough visualization showed microcavities on the heater surface, which provide an excellent location for nucleation sites enhancing heat transfer. However, these microcavities, once filled up with the suspended nanoparticles, reduced active nucleation sites, deteriorating the boiling heat transfer coefficient. Based on the experimental investigations it was concluded that there is an optimum thickness of nanoparticles coating at which heat flux is maximum and beyond this coating boiling heat transfer coefficient decreases. At higher pressures, boiling heat transfer coefficient and specific excess temperature remained nearly the same. This showed that pressure has negligible or no role to play in boiling heat transfer using nanofluids.     

Nanocoating characterization in pool boiling heat transfer of pure water

The pool boiling behavior of nanoparticle coated surfaces is experimentally studied in pure water. Nanoparticle coatings were created during nanofluid pool boiling experiments (Al₂O₃–water/ethanol). The nanocoatings developed can significantly enhance the critical heat flux. Ethanol nanofluids created more uniform nanocoatings which outperformed nanocoatings created in water nanofluids. The wetting and wicking characteristics of the nanocoatings are investigated through contact angle measurements and by conducting a dip test. A linear relationship between the CHF enhancement and the quasi-static contact angles of the nanocoatings was revealed. Additionally, a mechanism potentially responsible for nanocoating CHF enhancement is identified.     

Nucleate pool boiling heat transfer characteristics of dilute Al2O3–ethyleneglycol nanofluids

Pool boiling heat transfer coefficients of dilute stabilized Al₂O₃–ethyleneglycol nanofluids as possible coolant fluid are experimentally quantified. The influence of different parameters such as heat flux, heating surface nano-roughness, concentration of nanofluids and fouling resistance on the pool boiling heat transfer coefficient of alumina nanofluids has experimentally been investigated and briefly discussed. Results demonstrated that there are two heat transfer regions with different mechanisms namely free convection and nucleate boiling heat transfer. Studies on the influence of parameter demonstrated that with increasing the heat flux, the pool boiling heat transfer coefficient of nanofluids significantly increases. In contrast, with increasing the concentration of nanofluid, due to the deposition of nanoparticles on the surface, the average roughness of the surface and the heat transfer coefficient dramatically deteriorate, while a significant increase in fouling resistance is reported. Also, studies reveal asymptotic and rectilinear behaviors of fouling resistance parameter in nucleate boiling and free convective domains.     

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.     

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.     

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.     

Pool Boiling Enhancement through Graphene and Graphene Oxide Coatings

ABSTRACT

Boiling has served as an effective means to dissipate large quantities of heat over small areas. Graphene, a two-dimensional material, has garnered significant attention of researchers due to its excellent thermal properties. In this study, copper test chips are dip coated with a solution consisting of graphene oxide and graphene and its pool boiling performance with distilled water at atmospheric pressure was investigated. The surfaces were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy which confirmed the presence of graphene and graphene oxide. The contact angles measured on the coated surfaces indicated hydrophobic wetting behavior. Four heat transfer surfaces were prepared with dip coating durations of 120 s, 300 s, 600 s, and 1200 s, respectively. A Critical Heat Flux (CHF) of 182 W/cm² and a heat transfer coefficient (HTC) of 96 kW/m²°C was obtained with the shortest coating duration which translated to an enhancement of 42% in CHF and 47% in HTC when compared to a plain uncoated surface under similar conditions. Contact angle changes were not seen to be responsible, although roughness was seen as an influencing factor contributing to the enhancement. Further studies are needed to explain the enhancement mechanism.     

低体积分数水基SiO2纳米流体沸腾换热关键物性参数研究

在不添加任何分散剂和改变pH值的情况下,通过两步法将比表面积为150 m~2/g的气相SiO_2纳米颗粒制备成均匀稳定、透明度高、分散性能好的纳米流体。并对该功能性纳米流体进行了导热系数、黏度、表面张力和壁面接触角的测量。低体积分数下,功能性纳米流体较基液的导热系数几乎没有变化,但黏度却有较大改变。传统固液两相混合物黏度模型不再适用功能性纳米流体的计算,其主要原因是传统公式低估了分子间作用力对纳米流体黏度的影响。因此,建立了功能性纳米流体的黏度经验公式。由于纳米颗粒的存在提高了沸腾表面的粗糙度,从而使纳米流体的壁面湿润性能大大提高。实验结果表明,纳米流体的黏性和壁面接触角是沸腾换热发生骤变的关键。     

Experimental investigation of pool boiling characteristics of low-concentrated CuO/ethylene glycol–water nanofluids

Boiling heat transfer performance of nanofluid has been studied during the past few years. Some controversial results are reported in literature about the potential impact of nanofluids on heat transfer intensification. Whereas the mixtures of ethylene glycol and water are considered the most common water-based antifreeze solutions used in automotive cooling systems, the present study is an experimental investigation of boiling heat transfer of CuO/ethylene glycol–water (60/40) nanofluids. The results indicate that a considerable boiling heat transfer enhancement has been achieved by nanofluid and the enhancement increases with nanoparticles concentration and reaches 55% at a nanoparticles loading of 0.5%.