Natural convection heat transfer of alumina-water nanofluid in vertical square enclosures: An experimental study

An experimental study has been undertaken concerning natural convection heat transfer of a nanofluid in vertical square enclosures of different sizes, whose dimensions, width × height × length (mm), are 25 × 25 × 60, 40 × 40 × 90, and 80 × 80 × 180, respectively. The nanofluid formulated in the present experiment is water dispersed with various volumetric fractions of the alumina (Al₂O₃) nanoparticles ranging from 0.1 vol.% to 4 vol.%. The Rayleigh number varies in the range of 6.21 × 10⁵–2.56 × 10⁸. A correlation analysis based on the thermophysical properties of the nanofluid formulated shows that efficacy of applying the nanofluid for natural convection heat transfer enhancement in enclosure is inferred to be generally infeasible. The experimental results for the average heat transfer rate across the three enclosures appear generally consistent with the assessment based on the changes in thermophysical properties of the nanofluid formulated, showing systematic heat transfer degradation for the nanofluids containing nanoparticles of c_v ≥ 2 vol.% over the entire range of the Rayleigh number considered. However, for the nanofluid containing much lower particle fraction of 0.1 vol.%, a heat transfer enhancement of around 18% compared with that of water was found to arise in the largest enclosure at sufficiently high Rayleigh number. Such enhancement cannot be explained simply based on the net influence due to relative changes in thermophysical properties of the nanofluid containing such low particle fraction, thus strongly suggesting other factors may come into play.     

An experimental study on pool boiling of milk

This research paper reports the results for convective heat transfer coefficient and nucleate boiling heat flux for pool boiling of milk during khoa making. Various indoor experiments were conducted for different heat flux inputs varying from 9638.55 to 14457.83 W/m². Experimental data obtained for pool boiling of milk were analyzed by using the Rohsenow correlation with the help of simple linear regression analysis. The convective heat transfer coefficients were estimated in the range of 334.48 to 837.78 W/m² °C for the given heat inputs. The results for heat flux were found to be varying from 3344.8 to 8377.8 W/m² at 10 °C excess temperature of the aluminum pot surface above the saturation temperature of the milk. The experimental errors in terms of percent uncertainty were also calculated. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20336     

Experimental investigation and CFD modeling of the dynamics of bubbles in nanofluid pool boiling

This work investigated the dynamics of bubbles in pool boiling of nanofluid with coated and sodium dodecyl sulfate (SDS) solution with different nanoparticles. Also, computational fluid dynamics (CFD) module was used for mathematical modeling of bubbles in pure water boiling.Different macroscale parameters such as: shapes, numbers and contact angle of bubbles also were investigated experimentally and verified by CFD modeling results. Porous layers of nanoparticles on stainless steel substrate in conjunction with SDS additions were shown to modify formation of bubbles in comparison to reference condition. Improvement in surface hydrophilic conditions and boiling performance was observed by Multi-wall Carbon Nanotube (MWCNT) porous layers, in spite of coated surface by CuO and Al₂O₃ (γ) water based nanofluid. Growth time of bubbles also changes by the presence of porous layers and surfactant solution which resulted from change in surface tension force. Number of bubbles increased by MWCNT and SDS solution and decreased by CuO and Al₂O₃ nanofluid boiling. Results showed the comprehensive change in bubble dynamics and surface wettability by porous layer of nanoparticles and SDS solution.     

Visualization of pool boiling heat transfer of magnetic nanofluid

To increase heat transfer, ferrofluids have been utilized to study the effective parameters of pool boiling. Changes and possible enhancement of pool boiling heat transfer of magnetic fluids is a function of magnetic field and concentration of nanoparticles. To the best knowledge of the authors, no systematic experiments have been conducted to visualize the phenomena during the boiling of ferrofluids with different concentrations. In this study an experimental investigation has been conducted, by designing and fabricating a novel hele‐shaw vessel with glass sides, to explore via visualizations some details in the pool boiling of ferrofluids. Boiling patterns of ferrofluids at various concentrations have been visualized –both in the presence of a constant magnetic field and without any magnetic field. Pure water tests were performed as a baseline, and the experimental program has been conducted at four different concentrations, namely 30, 40, 50, and 500 ppm. The primary focus of the visualization is to study how different concentration of ferrofluid affects the boiling ebullition cycle through a high‐speed camera. The results showed that in the boiling process of ferrofluids with a low concentration (10 to 50 ppm), the rising bubbles lead to enlarge the active nucleation sites and create cavities. The formation of cavities changes the solid layer of the surface to a porous medium and enhances the wettability of the surface and boiling heat transfer coefficient. In the ferrofluid boiling with high concentration (500 ppm), bubbles rising is hindered by nanoparticles.     

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%.     

An experimental investigation of flow and heat transfer characteristics over blocked surfaces in laminar and turbulent flows

Flow and heat transfer measurements were obtained over a blocked surface mounted on a low speed wind tunnel in order to investigate the combined the effects of free stream velocities and the different size of rectangular blocks on the flow and heat transfer characteristics. Mean velocity and turbulence intensities were measured by a constant temperature anemometer and wall temperatures by copper-constant thermocouple and static pressures by a micro-manometer, respectively. It was found that the flow separations and reattachments were occurred before the first blocks, on the first blocks, between blocks and after the last blocks. The blocked surface area and flow separation caused not only heat transfer enhancement but higher turbulence levels as well. The average Stanton numbers, for block heights of 10, 15 and 20 mm, were higher than those of flat surface by 82%, 95%, 113% in laminar and 27%, 38%, 50% in turbulent, respectively. These results showed that heat transfer enhancement on the blocked surface increased with block heights and become more pronounced in laminar than that of turbulent flows.     

Preparation and pool boiling characteristics of copper nanofluids over a flat plate heater

The copper nanoparticles of average size of 10 nm have been prepared by the sputtering method and characterized through atomic force microscopy (AFM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The pool boiling heat transfer characteristics of 0.25%, 0.5% and 1.0% by weight concentrations of copper nanoparticles has been studied. Different copper based nanofluids were prepared in both, distilled water and distilled water with 9.0 wt% of sodium lauryl sulphate anionic surfactant (SDS). The pool boiling heat transfer data were acquired for the boiling of nanofluids over a 30 mm square and 0.44 mm thick stainless steel plate heater. The experimental results show that for the critical heat flux of pure water is 80% higher than that of water–surfactant fluid. Also, it was found that the critical heat flux for 0.25%, 0.5% and 1.0% concentrations of copper nanoparticles in copper–water nanofluids are 25%, 40% and 48% higher than that of pure water. But in the case of copper–water with surfactant nanofluids comparing with pure water, the CHF decreases to 75%, 68%, and 62% for respective concentrations of copper nanoparticles. The heat transfer coefficient decreases with increase of nanoparticles concentration in both water–copper and water–copper with surfactant nanofluids.     

Performance of different structured surfaces in nucleate pool boiling

Several structured surfaces have been developed in-house for the augmentation of boiling heat transfer using distilled water as test fluid under atmospheric pressure. These surfaces have either a number of parallel tunnels or orthogonally intersecting tunnels. Effect of design parameters like tunnel inclination and different cavity structure at the tunnel base on the boiling heat transfer has been investigated. Three different structures namely circular groove, rectangular groove and rounded base have been used at the end of the tunnels. Heat flux is varied in the range of 0–250 kW/m². Experimental results showed tunnels inclined at an angle 60° with the horizontal provide better augmentation compared to straight vertical tunnels. Amongst different base geometry the circular pocket produced most conducive condition for the boiling heat transfer. The use of tunnels also increases the degree of augmentation. The highest augmentation was obtained from the surface having intersecting inclined tunnels with a circular base.     

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.     

Explosive boiling of liquid argon films on flat and nanostructured surfaces

Abstract

Because of the effects of the nanostructure, phase change behaviors on flat and nanostructured surfaces display distinct features. In this work, the molecular dynamics simulation method is employed to investigate the onset temperature of explosive boiling (T_s) with various film thicknesses, pillar heights, and wettability. The simulation results show that T_s decreases with the film thickness on both wettability flat surfaces. However, the decreasing rates have the significant distinctions, where the difference between two surfaces of T_s with the identical film thickness decreases. In addition, the simulation results demonstrate that all the values of T_s on nanostructured surfaces are lower than those on flat surfaces with the same film thickness. With the increase of the film thickness, T_s presents a decreasing trend on both wettability and nanostructured surfaces, especially with the liquid film with the thickness over 6?nm, where a completely opposite conclusion compared to the flat surface is represented.     

An experimental investigation on heat transfer performance of nanofluid pulsating heat pipe

The effect of SiO2 particles on heat transfer performance of a pulsating heat pipe(PHP) was investigated experimentally.DI water was used as the base fluid and contrast medium for the PHP.In order to study and measure the character,there are SiO2 /H2 O nanofluids with different concentration and applying with various heating powers during the experiment investigation.According to the experimental result,the high fraction of SiO2 /H2 O will deteriorate the performance of PHP compared with DI water,i.e.the thermal resistance and the temperature of evaporation section increases.It is in contrary in the case of low fraction of SiO2 /H2 O.Finally,the comparison of the thermal performances between the normal operation system and the static settlement system is given.It is found that both the thermal resistance of nanofluid PHP and the temperature of the evaporation section increase after standing for a period,and it is the same trend for the temperature fluctuation at the identical heating power for PHP.     

Experiments on pool boiling of a dielectric fluid on extended surfaces

An experimental study on pool boiling heat transfer from finned copper surfaces immersed in a saturated dielectric liquid (Galden HT-55) is presented. Two extended surfaces of different dimensions were tested using vertical and horizontal orientations. The effects of nonboiling waiting period, pressure and spine dimensions on boiling behavior were examined.A marked enhancement of heat transfer performance was observed on passing from plane to extended surfaces. Increasing the pressure improved the heat transfer coefficient and critical heat flux. The nonboiling period, orientation and pressure significantly influenced the boiling incipience and the hysteresis phenomenon that accompanies increasing and decreasing heat fluxes. Experimental data are expressed in terms of enhancement ratios of extended surfaces as a function of base surface superheat and pressure.     

Vapor bubble dynamics and heat transfer characteristics over the spherical surface during saturated pool boiling

The present study deals with numerical investigations of the boiling phenomena over a spherical surface at different degrees of superheat (ΔT), varying from 10 to 500 K. Various phenomena like vapor sliding, bubble formation, pinch-off, induced vorticity have been illustrated for a deep understanding of the boiling process over a spherical surface. The effect of the degree of superheat on the bubble pinch-off time and volume is also investigated. Further, reported the spatial observation of vapor sliding and retention over the surface with time scale, overall and average characteristics. The fast Fourier transform of a spaced average void fraction of liquid and Nusselt number showed the dominance of film boiling with respect to the degree of superheat. As the degree of superheat increases, the vapor generation rate also increases, which produces a more vapor–liquid interface. Further, with an increase in the degree of superheat, the vapor generation progression shifted from linear to nonlinear patterns. A sphere with ΔT = 500 K generated 32.59 times more vapor than a sphere with ΔT = 10 K. It is found that the vapor generation is dependent on the degree of superheating and exposed time for heating. Thus, a correlation and artificial neural network model have been developed to predict vapor generation during boiling over the spherical surface as a function of time and degree of superheat.     

加热板上液滴沸腾实验研究

采用高速摄像技术研究了不同加热表面上液滴蒸发和沸腾的相变特性和壁面温度变化特性,讨论了局部相变行为对壁面温度变化的影响.同时定量的研究了三种不同表面特性的加热板对沸腾和传热的影响,以及液滴初始体积对相变的影响.结果表明,表面特性和液滴尺寸对沸腾传热有较大影响.     

Wettability of heated surfaces under pool boiling using surfactant solutions and nano-fluids

The wettability of the heated surface under pool boiling of surfactant solutions and nano-fluids has been investigated. Tri-sodium phosphate (TSP, Na₃PO₄) solutions (0.01, 0.05, 0.1, 0.3, 0.5, 0.8 wt.%) and Aluminum oxide (Al₂O₃) nano-fluids (NF) (0.5, 1, 2, 4 vol.%) were prepared for experiments. Stainless steel (SUS 304) strips (30 × 30 × 3 mm) were heated by an alcohol lamp and quenched in the prepared solutions. Before complete quenching, when the surface temperature was 150 ± 10 °C (nucleate boiling region), the strip was taken out and excessive liquid on the surface was removed. Contact angles of pure water and the solutions on the quenched surface and fresh surface were measured. Contact angles of pure water on the quenched surfaces (5°–25°) were much smaller than those on the fresh surface (65°–70°). The solutions (TSP, NF) on the quenched surface shows the smallest contact angle (5°–15°). Surfaces deposited TSP and nano-particle could affect surface energy of the strips and enhance hydrophilicity of the surfaces. Several implications of the experimental results on the pool boiling CHF model and CHF enhancement using TSP and NF were discussed.     

An experimental investigation of bubble nucleation of a refrigerant in pressurized boiling flows

An experimental investigation is performed to determine the effect of system pressure and heat flux on flow boiling and associated bubble characteristics of a refrigerant in a narrow vertical duct. A high-pressure flow boiling test loop was built and TLC (thermo-chromic liquid crystal) was applied to the back of the heater foil for high resolution and accurate measurement of heater surface temperature. Refrigerant R-134a is used as the test fluid at different pressures ranging from 690 to 827 kPa and different heat fluxes to quantify their influence in bubble characteristics such as bubble nucleation, growth, departure, and coalescence. Two synchronized high resolution and high-speed cameras are used to simultaneously capture TLC images as well as bubbling activities at high frame rates. By varying flow rate and system pressure, TLC and bubble images were captured and analyzed. Results show that the bubble generation frequency and size increase with heat flux. An increase in pressure from 690 to 827 kPa increased the bubble frequency and size by about 32 Hz and 20 μm, respectively. Bubble coalescence was also observed after departure from the nucleation site.     

Nucleate pool boiling heat transfer over a bundle of vertical tubes

An experimental investigation has been carried out to determine the heat transfer coefficient during pool boiling of water over a bundle of vertical stainless steel heated tubes of 19.0 mm diameter and 850 mm height. The p/D of bundle was 1.66 and was placed inside a glass tube of 100 mm diameter and 900 mm length. The data were acquired for the heat flux range of 2–32 kWm^− 2.     

Enhancing the onset of pool boiling by wettability modification on nanometrically smooth surfaces

Pool boiling experiments are performed with degassed water on highly smooth surfaces of two different wettabilities: hydrophilic and hydrophobic cases. Boiling curves and visual observations on the boiling have been performed. The onset of nucleate boiling (ONB) has been measured and the influence of the wettability has been quantified. As the inherent mean roughness of the glass substrates was lower than one nanometer it was possible to show the sole effect of the wettability. No hysteresis in the boiling curve was observed for both cases. The ONB was observed after 3.5 °C superheat on the hydrophobic case and the heat transfer coefficient (HTC) changed suddenly from the one of a convection regime (1.5 kW/m² K) to the one of a nucleate boiling regime (4 kW/m² K). On the contrary for the hydrophilic case, despite superheat above 37 °C and presence of boiling, the HTC was kept as the one of the convection regime.     

Numerical and experimental investigation of pressure drop characteristics during upward boiling two-phase flow of nitrogen

The characteristics of overall pressure drop during upward boiling two-phase flow of nitrogen with constant mass flux and varying heat flux are experimentally investigated using a vertical tube with an inner diameter of 6.0 mm and numerically simulated using the two-fluid model with new closure correlations. Comparison of the numerical results against the experimental data shows that prior to the transition point in the curve of pressure drop evolution, the predicted pressure drop is in a satisfactory agreement with the experimental data. The present study demonstrates that both the wall lubrication force modeling and the bubble diameter modeling have significant effect on the pressure drop prediction. Both the theoretical analysis and the experimental evidence suggest that the transition point in the pressure drop evolution curve reflects the bubbly-to-slug flow regime transition.