Heat transfer characteristics in double tube helical heat exchangers using nanofluids

In the present work a three-dimensional analysis is used to study the heat transfer characteristics of a double-tube helical heat exchangers using nanofluids under laminar flow conditions. CuO and TiO₂ nanoparticles with diameters of 24 nm dispersed in water with volume concentrations of 0.5–3 vol.% are used as the working fluid. The mass flow rate of the nanofluid from the inner tube was kept and the mass flow rate of the water from the annulus was set at either half, full, or double the value. The variations of the nanofluids and water temperatures, heat transfer rates and heat transfer coefficients along inner and outer tubes are shown in the paper. Effects of nanoparticles concentration level and of the Dean number on the heat transfer rates and heat transfer coefficients are presented. The results show that for 2% CuO nanoparticles in water and same mass flow rate in inner tube and annulus, the heat transfer rate of the nanofluid was approximately 14% greater than of pure water and the heat transfer rate of water from annulus than through the inner tube flowing nanofluids was approximately 19% greater than for the case which through the inner and outer tubes flow water. The results also show that the convective heat transfer coefficients of the nanofluids and water increased with increasing of the mass flow rate and with the Dean number. The results have been validated by comparison of simulations with the data computed by empirical equations.     

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.     

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.     

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     

Finned tube performance evaluation with nanofluids and conventional heat transfer fluids

The performance of hydronic finned-tube heating units with nanofluids is compared to their performance with a conventional heat transfer fluid comprised of 60% ethylene glycol and 40% water, by mass (60% EG) using a mathematical model. The nanofluids modeled are comprised of either CuO or Al₂O₃ nanoparticles dispersed in the 60% EG solution. The finned tube configuration modeled is similar to that commonly found in building heating systems. The model employs correlations for nanoparticle thermophysical properties and heat transfer that have been previously documented in the literature. The analyses indicate that finned tube heating performance is enhanced by employing nanofluids as a heat transfer medium. The model predicts an 11.6% increase in finned-tube heating output under certain conditions with the 4% Al₂O₃/60% EG nanofluid and an 8.7% increase with the 4% CuO/60% EG nanofluid compared to heating output with the base fluid. The model predicts that pumping power required for a given heating output with a given finned tube geometry is reduced with both the Al₂O₃/60% EG and the CuO/60% EG nanofluids compared to the base fluid. The finned tube with 4% Al₂O₃/60% EG has the lowest liquid pumping power at a given heating output of all the fluids modeled.     

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%.     

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.     

Thermal performance of U-shaped serpentine microchannel heat sink using various metal oxide nanofluids

This study aims to evaluate the thermal performance and friction factor characteristics of the U-shaped serpentine microchannel heat sink using three different nanofluids. Two distinct nanoparticles, namely Al₂O₃ (alumina) and CuO (copper oxide), were used for the preparation of nanofluids using water and ethylene glycol (EG) as base fluids. Three nanofluids, namely nanofluid I (Al₂O₃ + water), nanofluid II (CuO + water), and nanofluid III (CuO + EG), have been prepared. The results showed that the thermal conductivity of nanofluids was increased for all concentrations (from 0.01 to 0.3%), compared with base fluids. The theoretical values derived from the relationship between the Darcy friction factor showed a clear understanding of the fully developed laminar flow. Thermal resistance for nanofluid III was lower than other nanofluids, resulting in a higher cooling efficiency. The nanofluid mechanism and the geometry of the U-shaped serpentine heat sink have led to the improvement in the thermal performance of electronic cooling systems.     

Heat transfer enhancement in microchannel heat sinks using nanofluids

Heat transfer enhancement in a 3-D microchannel heat sink (MCHS) using nanofluids is investigated by a numerical study. The addition of nanoparticles to the coolant fluid changes its thermophysical properties in ways that are closely related to the type of nanoparticle, base fluid, particle volume fraction, particle size, and pumping power. The calculations in this work suggest that the best heat transfer enhancement can be obtained by using a system with an Al₂O₃–water nanofluid-cooled MCHS. Moreover, using base fluids with lower dynamic viscosity (such as water) and substrate materials with high thermal conductivity enhance the thermal performance of the MCHS. The results also show that as the particle volume fraction of the nanofluid increases, the thermal resistance first decreases and then increases. The lowest thermal resistance can be obtained by properly adjusting the volume fraction and pumping power under given geometric conditions. For a moderate range of particle sizes, the MCHS yields better performance when nanofluids with smaller nanoparticles are used. Furthermore, the overall thermal resistance of the MCHS is reduced significantly by increasing the pumping power. The heat transfer performance of Al₂O₃–water and diamond–water nanofluids was 21.6% better than that of pure water. The results reported here may facilitate improvements in the thermal performance of MCHSs.     

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。     

A comparison of experimental heat transfer characteristics for Al2O3/water and CuO/water nanofluids in square cross-section duct

The present paper is a comparison between heat transfer characteristics of Al₂O₃/water and CuO/water nanofluids through a square cross-section cupric duct in laminar flow under uniform heat flux. Sometimes because of pressure drop limitations the need for noncircular ducts arises in many heat transfer applications, and a testing facility has been constructed for this purpose and experimental studies were performed on both nanofluids under different nanoparticles concentrations in distilled water as a base fluid. The results indicate that a considerable heat transfer enhancement has been achieved by both nanofluids compared with base fluid. However, CuO/water nanofluid shows better heat transfer augmentation compared with Al₂O₃/water nanofluid through square cross-section duct.     

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.     

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.     

Pool boiling heat transfer on the microheater surface with and without nanoparticles by pulse heating

We study the pool boiling heat transfer on the microheater surface with and without nanoparticles by pulse heating. Nanofluids are the mixture of de-ionized water and Al₂O₃ particles with 0.1%, 0.2%, 0.5% and 1.0% weight concentrations. The microheater is a platinum surface by 50 × 20 μm. Three types of bubble dynamics were identified. The first type of bubble dynamics is for the boiling in pure water, referring to a sharp microheater temperature increase once a new pulse cycle begins, followed by a continuous temperature increase during the pulse duration stage. Large bubble is observed on the microheater surface and it does not disappear during the pulse off stage. The second type of bubble dynamics is for the nanofluids with 0.1% and 0.2% weight concentrations. The microheater surface temperature has a sharp increase at the start of a new pulse cycle, followed by a slight decrease during the pulse duration stage. Miniature bubble has oscillation movement along the microheater length direction, and it disappears during the pulse off stage. The third type of bubble dynamics occurs at the nanofluid weight concentration of 0.5% and 1.0%. The bubble behavior is similar to that in pure water, but the microheater temperatures are much lower than that in pure water. A structural disjoining pressure causes the smaller contact area between the dry vapor and the heater surface, decreasing the surface tension effect and resulting in the easy departure of miniature bubbles for the 0.1% and 0.2% nanofluid weight concentrations. For the 0.5% weight concentration of nanofluids, coalescence of nanoparticles to form larger particles is responsible for the large bubble formation on the heater surface. The microlayer evaporation heat transfer and the heat transfer mechanisms during the bubble departure process account for the higher heat transfer coefficients for the 0.1% and 0.2% nanofluid weight concentrations. The shortened dry area between the bubble and the heater surface, and the additional thin nanofluid liquid film evaporation heat transfer, account for the higher heat transfer coefficient for the 0.5% nanofluid weight concentration, compared with the pure water runs.     

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.     

Experimental thermal–hydraulic evaluation of CuO nanofluids in microchannels at various concentrations with and without suspension enhancers

This experimental study focuses on the effect of two important factors on the heat transfer and flow properties of copper oxide (CuO)/water nanofluids in a parallel microchannel flow configuration. The first factor considered is the solid media (CuO) concentration. In this investigation, concentration values of 0.005%, 0.01%, and 0.1% by volume were tested. The second factor is the use of a surfactant, cetyltrimethylammonium bromide (CTAB), as a suspension enhancer. All together, these two factors led to a total of six types of nanofluids, which were tested in addition to pure water, the reference fluid. The experimental setup allowed for the determination of the hydrodynamic and thermal performance of each nanofluid. In addition, a selection of the nanofluids were characterized by the use of scanning transmission electron microscopy (STEM) and Dynamic Light Scattering (DLS) techniques. The DLS transient settling measurements showed that for a nanofluid with a concentration of 0.1% by volume, the nanoparticle dispersion and suspension is negatively affected unless a surfactant is used. Hydrodynamic losses, which were evaluated by comparing the effect of the imposed pressure drop on the mass flow rate, were not meaningfully affected by the composition of the nanofluids tested. The measurements also showed that nanofluids containing a surfactant generally provided a modest increase in heat transfer rate when compared with tests performed using pure water. The largest increase was about 17% for a fluid with a concentration of 0.01% by volume. Consequently, the gains in heat transfer do not appear to be accompanied by a significant pumping power penalty. The results of this study suggest that the use of a surfactant is essential in maintaining a proper suspension of nanoparticles in the fluid, especially at higher concentrations.     

Experimental and analytical investigation on an automobile radiator with CuO/EG‐water based nanofluid as coolant

In the present study, experimental and analytical thermal performance of automobile radiator using nanofluids is investigated and compared with performance obtained with conventional coolants. Effect of operating parameters and nanoparticle concentration on heat transfer rate are studied for water as well as CuO/EG‐water based nanofluid analytically. The results are presented in the form of graphs showing variations of net heat transfer rate for various coolant flow rate, air velocity, and source temperature for various CuO/EG‐water based nanofluids. Experimental results indicate that with the increase in coolant flow rate and air velocity, heat transfer rate increases, reaches maximum and then decreases. Experimental investigation of a radiator is carried out using CuO/EG‐water based nanofluids. Results obtained by experimental work and analytical MATLAB code are almost the same. Maximum absolute error in water and air side is within 12% for all flow condition and coolant fluids. Nusselt number of nanofluid is calculated using equation number 33[9]. The results obtained from experimental work using 0.2% volume CuO/EG‐water based nanofluids are compared with the results obtained from MATLAB code. The results show that the maximum error in the outlet temperature of the coolant and air is 12% in each case. Thus MATLAB code can be used for different concentration of nanofluids to study the effect of operating parameters on heat transfer rate. Thus MATLAB code developed is valid for given heat exchanger applications. From the results obtained by already validated MATLAB code, it is concluded that increase in coolant flow rate, air velocity, and source temperature increases the heat transfer rate. Addition of nanoparticles in the base fluid increases the heat transfer rate for all kind of base fluids. Among all the nanofluid analyzed in this study, water‐based nanofluid gives highest value of heat transfer rate and is recommended for the heat exchanger applications under normal operating conditions. Maximum enhancement is observed for ethylene glycol‐water (4:6) mixture for 1% volume concentration of CuO is almost equal to 20%. As heat transfer rate increases with the use of nanofluids, the heat transfer area of the radiator can be minimized.     

Nanofluids as the circuit fluids of the geothermal borehole heat exchangers

Application of the CuO water and Al₂O₃ water nanofluids as the working fluids of a geothermal borehole heat exchanger is investigated using numerical simulation. For this purpose, the Reynolds Averaged Navier-Stokes (RANS) equations with SST k-ω turbulence model are numerically solved to model the flow. Physical properties of the nanofluids are obtained using the available correlations. To show the validity of the simulations, the results for pure water are compared with available data in the literature. Results show that there is a specific diameter ratio at which the total water flow pressure loss in the heat exchanger is minimum. The results also show that the CuO-water nanofluid gives higher extracted heat than the alumina-water nanofluid but at the penalty of higher pressure losses and pumping powers.     

Boiling heat transfer on a high temperature silver sphere in nanofluid

To investigate boiling heat transfer characteristics of nanofluids, transient quenching experiments of a high temperature silver sphere in water-based nanofluids with Ag and TiO₂ nanoparticles were performed. A silver sphere with a diameter of 10 mm and an initial temperature of 700 °C was quenched in these nanofluids at a temperature of 90 °C. The results showed a considerable reduction in the quenching ability of nanofluids compared to that of pure water. The presence of nanoparticles in water caused the film boiling mode to vanish at lower temperatures depending on the mixture concentration. Calculated heat transfer rates in nanofluids were lower than those in pure water. In the quenching experiments with an unwashed heated sphere, the film boiling mode did not appear and the hot sphere quenched more rapidly through nucleate boiling. In this case the sphere surface was covered by a thin layer of nanoparticles. It was evident that nanoparticle deposition on the sphere surface prevented vapor film from forming around it and resulted in quick quenching of the hot sphere.     

Thermal performance enhancement studies using graphite nanofluid for heat transfer applications

Enhanced boiling heat transfer using nanofluids is highly relevant due to its potential applications in thermal management of systems producing large heat fluxes. However, the sedimentation of nanoparticles limits their application in heat transfer systems. So, the preparation of a stable nanofluid remains a big research challenge. The stability issues arise due to the large difference in the density of nanoparticle and the base fluid. Graphite nanoparticle is selected in this study, as it has 4.5 times lower density than copper and comparable thermal conductivity. An experimental study is conducted to evaluate the suitability of graphite nanofluid in mesh wick heat pipes, which are devices that utilize boiling and condensation principles to transfer high heat fluxes. Thermal transport properties and boiling heat transfer characteristics showed enhancement and the effect of nanofluid on the device level thermal performance is thoroughly assessed. Experimental results are compared with the published literature. A reduction in thermal resistance by 32.5% and an improvement in the heat transfer coefficient by 48.02% in comparison with base fluid clearly indicate the superiority of the graphite nanofluid for heat transfer applications.