Search Results (18)

Spray cooling, of which the essence is droplet impacting, is an efficient thermal management technique for dense electronic components in unmanned aerial vehicles (UAVs). Nanofluids are pointed as promising cooling dispersions. Since the nanofluids are unstable, a dispersant could be added to the fluid. However, the added dispersant may influence the droplet, thereby impacting behaviors. In this work, the effects of dispersant on the nanofluid droplet-impacting dynamics are studied experimentally. The base fluid is deionized water (DI water), and Al₂O₃ is the selected nanoparticle. Sodium dodecyl sulfate (SDS) is used as the dispersant. Five different concentrations of nanofluids are configured using a two-step method. Droplet impacting behaviors are observed by high-speed imaging techniques. The other effects, i.e., the nanofluid particle volume fraction and the Weber number on droplet impact dynamics, are also systematically investigated. The results illustrate that the surface tension of the Al₂O₃ nanofluid increases with increased nanofluid concentrations. The surface tension of Al₂O₃ nanofluid with SDS is lower than that of DI water. And the increase in droplet impact velocity increases the spreading morphology. Nanofluid droplets exhibit spreading and equilibrium process when SDS is added. Furthermore, as the concentration of the nanofluid increases, the spreading process is inhibited. Whereas without SDS, the droplets undergo spreading, receding, and equilibrium processes. Moreover, there is no appreciable change in the impacting process with concentration increase. The empirical models of maximum spreading factor should be established without SDS and with SDS, respectively. This study can provide theoretical basis and specific guidance for experimental characterization of UAVs’ electronic devices based on the mechanism of nanofluid droplet impact on the wall. Full article

Machining high-strength structural steels often requires challenging processes. It is essential to improve the machinability of such materials, which are frequently needed in industrial manufacturing areas. Recently, it has become necessary to enhance the machinability of such materials using different nanopowders. In this study, different cooling/lubricating (C/L) liquids were prepared with cellulose nanocrystal (CNC) nanopowder. The aim was to improve the machinability properties of Dillimax 690T material with the prepared CNC-based cutting fluids. CNC nanopowders were added to 0.5% distilled water by volume, and a new nanofluid was produced. Unlike previous studies, base synthetic oil and CNC-based cutting fluid were sprayed on the cutting area with a double minimum quantity lubrication (MQL) system. Machinability tests were carried out by milling. Two different cutting speeds (Vc = 120–150 m/min), two different feed rates (f = 0.05–0.075 mm/tooth), and four different C/L environments (dry, MQL oil, CNC nanofluid, MQL oil + CNC nanofluid) were used in the experiments. In the study, where a total of 16 experiments were performed, cutting temperature (Tc), surface roughness (Ra), tool wear (Vb), and energy consumption results were analyzed in detail. According to the test results, significant improvements were achieved in the machinability properties of the material in the experiments carried out using CNC nanofluid. In particular, the hybrid C/L environment using MQL oil + CNC nanofluid improved all machinability metrics by over 15% compared to dry machining. In short, using CNC nanopowders offers a good milling process of Dillimax 690T material with effective lubrication and cooling ability. Full article

Nanofluid composite electrostatic spraying (NCES) is a new clean machining technology for minimum quantity lubrication. The base fluid of external fluid and voltage are the two important parameters that affect its performance. This study presented the effect of base fluid of external fluid on milling force and temperature of NCES to determine the suitable base fluid and the best external/internal fluid. Herein, castor oil, castor oil-based nanofluid, sunflower oil, and sunflower oil-based nanofluid were employed as external fluid, and water and water-based nanofluid as internal fluid. Atomization experiments were conducted to determine the common voltage for different external/internal fluids to generate an applicable atomization mode. Under this voltage, morphology of applicable atomization mode, current and standard deviation, droplet speed, and electrowetting contact angle were explored to discuss the effect of base fluid on NCES milling. Next, the best external/internal fluid was used to further investigate the milling force and temperature under various voltages. Sunflower oil was the suitable base fluid for NCES, and sunflower oil-based nanofluid/water-based nanofluid was found to be the best external/internal fluid causing a significant reduction in force and temperature. Compared to castor oil, sunflower oil as the base fluid lowered the milling force and temperature by 5.4–10.8% and 6.3–7.9%, respectively. Within the voltage range of applicable atomization mode, raising the voltage lowered the milling force and temperature by 2.4% and 3.9%, respectively. Full article

The PAO/MgO nanofluids-based dielectric fluid DF(MgO-20) has significantly increased corrosion resistance as a coating. Electrochemical studies show that the DF(MgO-20) coating has protection efficiency of up to 99% for steel, copper, and aluminum. This coating is capable of providing corrosion protection for steel samples for up to 120 h in salt spray tests, and printed circuit boards (PCBs) for more than 20 days in salt spray tests in accordance with the ASTM B117 standard. The DF(MgO-20) coating fully meets the moisture resistance and fungal resistance standards required by the MIL-1-46058 C standard. The coating also demonstrates water displacement, meeting the requirements of the MIL-PRF-81309G standard. The DF(MgO-20) coating is able to protect electronic equipment working in underwater environment for up to 20 days. The aforementioned outstanding protection properties are achieved thanks to the nanofluid effect of the DF(MgO-20) dielectric fluid with the presence of MgO nano-additives that increase its overflow ability. The coating penetrates deeply and adheres tightly to the metal substrates, helping to separate them from moisturizing agents and corrosive agents. The research results aim to apply this coating to protect electronic equipment working in the tropical marine climate of Vietnam. Full article

The current work aims to experimentally evaluate the effect of the size of circular superhydrophobic regions of biphilic surfaces on the bubble dynamics under pool boiling conditions. Biphilic surfaces are structured surfaces with tunable wettability, presenting an array of hydrophobic small spots in a hydrophilic surface or vice versa. The factors that affect the bubble dynamics are of geometric nature such as the diameters of the bubbles, their volume, and the height of the centroid, and of more complex nature such as the departure frequency of the bubbles and the rate of evaporation mass transfer. In this study, the bubble dynamics and boiling performance were evaluated by adjusting the diameter of the single circular superhydrophobic regions. A stainless steel AISI 304 foil was used as the base hydrophilic region, and the superhydrophobic regions were made by spray coating the NeverWet^® superhydrophobic solution over well-defined masks. The main conclusion was that the bubble dynamics are clearly affected by the diameter of the superhydrophobic spots. The smaller spots favored the generation of more uniform and stable bubbles, mainly due to the border surface tension forces’ dominance. With the increase in the diameter of the bubbles, the surface tension acting at the border with the much larger hydrophilic region impacts the process less. Thus, the smaller superhydrophobic regions had higher evaporation mass transfer rates. The region with the best pool boiling performance along with improved bubble dynamics was the superhydrophobic region with an 0.8 mm diameter, corresponding to a superhydrophobic area to total area ratio of 0.11%. Moreover, this experimental work confirmed that the bubble dynamics’ impacting factors such as the diameter at the various stages of development of the bubbles can be modulated according to the final objectives of the design and fabrication of the biphilic surfaces. The research significance and novelty of this work come from the comprehensive study of the geometrical pattern of the heat transfer surface in pool boiling conditions and its impact on the bubble dynamics and heat transfer capability. We also suggest further studies considering nanoscale superhydrophobic spot arrangements and the future usage of different working fluids such as nanofluids. Full article

Spray cooling is a heat transfer technology that has already shown its advantages and limitations. There has been increasing interest from academia and industry in combining this technology with nanofluids as coolants, owing to their potential for heat transfer enhancement. Nevertheless, there is a lack of understanding of the physical mechanism leading to this enhancement with the presence of technical problems that prevent the use of nanofluids in spray cooling applications. In this study, we investigate the effect of water-based TiO${}_2$ nanofluids on both spray characteristics and heat transfer using an industrial full-cone pneumatic nozzle. For this purpose, three mass concentrations (0.05 wt.%, 0.1 wt.%, and 0.2 wt.%) were prepared and tested. We monitored the droplet sizes and velocity profiles with a particle dynamics analysis system. Moreover, the temporal temperature decrease of a heated aluminum block from 190 to 65 °C was measured via an infrared camera to calculate the heat transfer rate and heat transfer coefficient. The presence of nanoparticles is shown not to substantially alter the spray characteristics. Moreover, heat transfer is augmented mainly in the boiling regime due to more nucleation sites formed by the deposited nanoparticles. However, in the non-boiling regime, the contribution of adsorbed nanoparticles to the heat transfer enhancement diminishes. Overall, the aluminum block is cooled down 6%, 12%, and 25% faster than the DI water by the nanofluids at 0.05 wt.%, 0.1 wt.%, and 0.2 wt.%, respectively, including boiling and non-boiling regimes. Full article

The efficiency of PV (photovoltaic) modules is highly dependent on the operating temperature. The objective of this work is to enhance the performance of PV by passive cooling using aluminum fins that have been nanocoated (like those on an automobile radiator). A rise in the cell temperature of the module PV leads to a decrease in its performance. As a result, an effective cooling mechanism is required. In this work, the performance of the PV module has been improved using natural convection, which was achieved by placing three similar PV modules next to each other in order to test them simultaneously. The first panel will be the base panel and will be used for comparison purposes. An automotive radiator (with aluminum fins) was firmly fixed onto the rear of the other two PV modules, and the fins of the third PV panel had titanium oxide (TiO₂) water-based nanofluid applied to them. The power produced by the PV modules, as well as their rear side temperatures, were recorded every 30 min over four months. A temperature reduction of 4.0 °C was attained when TiO₂ water-based nanofluid was sprayed onto the panel’s finned rear side. This was followed by the scenario where the rear side was only finned, with a temperature drop of 1.0 °C. As a result of the temperature reduction, the percentage of power produced by the coated-finned PV and the finned PV increased by 5.8 and 1.5 percent, respectively. This caused an increase in PV efficiency of 1.1 percent for coated-finned panels and 0.4 percent for finned PV. Full article

In recent years, technical advancements in high-heat-flux devices (such as high power density and increased output performance) have led to immense heat dissipation levels that may not be addressed by traditional thermal fluids. High-heat-flux devices generally dissipate heat in a range of 100–1000 W/cm² and are used in various applications, such as data centers, electric vehicles, microelectronics, X-ray machines, super-computers, avionics, rocket nozzles and laser diodes. Despite several benefits offered by efficient spray-cooling systems, such as uniform cooling, no hotspot formation, low thermal contact resistance and high heat transfer rates, they may not fully address heat dissipation challenges in modern high-heat-flux devices due to the limited cooling capacity of existing thermal fluids (such as water and dielectric fluids). Therefore, in this review, a detailed perspective is presented on fundamental hydrothermal properties, along with the heat and mass transfer characteristics of the next-generation thermal fluid, that is, the hybrid nanofluid. At the end of this review, the spray-cooling potential of the hybrid nanofluid for thermal management of high-heat-flux devices is presented. Full article

In this paper the mathematical and physical correlation between fundamental thermophysical properties of materials, with their structure, for nanofluid thermal performance in spray cooling applications is presented. The present work aims at clarifying the nanofluid characteristics, especially the geometry of their nanoparticles, leading to heat transfer enhancement at low particle concentration. The base fluid considered is distilled water with the surfactant cetyltrimethylammonium bromide (CTAB). Alumina and silver are used as nanoparticles. A systematic analysis addresses the effect of nanoparticles concentration and shape in spray hydrodynamics and heat transfer. Spray dynamics is mainly characterized using phase Doppler interferometry. Then, an extensive processing procedure is performed to thermal and spacetime symmetry images obtained with a high-speed thermographic camera to analyze the spray impact on a heated, smooth stainless-steel foil. There is some effect on the nanoparticles’ shape, which is nevertheless minor when compared to the effect of the nanoparticles concentration and to the change in the fluid properties caused by the addition of the surfactant. Hence, increasing the nanoparticles concentration results in lower surface temperatures and high removed heat fluxes. In terms of the effect of the resulting thermophysical properties, increasing the nanofluids concentration resulted in the increase in the thermal conductivity and dynamic viscosity of the nanofluids, which in turn led to a decrease in the heat transfer coefficients. On the other hand, nanofluids specific heat capacity is increased which correlates positively with the spray cooling capacity. The analysis of the parameters that determine the structure, evolution, physics and both spatial and temporal symmetry of the spray is interesting and fundamental to shed light to the fact that only knowledge based in experimental data can guarantee a correct setting of the model numbers. Full article

Cooling by impinging droplets has been the subject of several studies for decades and still is, and, in the last few years, the potential heat transfer enhancement obtained thanks to nanofluids’ use has received increased interest. Indeed, the use of high thermal conductivity fluids, such as nanofluids’, is considered today as a possible way to strongly enhance this heat transfer process. This enhancement is related to several physical mechanisms. It is linked to the nanofluids’ rheology, their degree of stabilization, and how the presence of the nanoparticles impact the droplet/substrate dynamics. Although there are several articles on droplet impact dynamics and nanofluid heat transfer enhancement, there is a lack of review studies that couple these two topics. As such, this review aims to provide an analysis of the available literature dedicated to the dynamics between a single nanofluid droplet and a hot substrate, and the consequent enhancement or reduction of heat transfer. Finally, we also conduct a review of the available publications on nanofluids spray cooling. Although using nanofluids in spray cooling may seem a promising option, the few works present in the literature are not yet conclusive, and the mechanism of enhancement needs to be clarified. Full article

Spray impingement on smooth and heated surfaces is a highly complex thermofluid phenomenon present in several engineering applications. The combination of phase Doppler interferometry, high-speed visualization, and time-resolved infrared thermography allows characterizing the heat transfer and fluid dynamics involved. Particular emphasis is given to the use of nanofluids in sprays due to their potential to enhance the heat transfer mechanisms. The results for low nanoparticle concentrations (up to 1 wt.%) show that the surfactant added to water, required to stabilize the nanofluids and minimize particle clustering, affects the spray’s main characteristics. Namely, the surfactant decreases the liquid surface tension leading to a larger wetted area and wettability, promoting heat transfer between the surface and the liquid film. However, since lower surface tension also tends to enhance splash near the edges of the wetted area, the gold nanospheres act to lessen such disturbances due to an increase of the solutions’ viscosity, thus increasing the heat flux removed from the spray slightly. The experimental results obtained from this work demonstrate that the maximum heat convection coefficients evaluated for the nanofluids can be 9.8% to 21.9% higher than those obtained with the base fluid and 11.5% to 38.8% higher when compared with those obtained with DI water. Full article

The statistical characterization of sprays is an essential way of organizing data on drop size and velocity to provide reliable information on the spray dynamics. A clear presentation of data using statistical tools provides evidence of a clear research question underlying the spray characterization. In this article, a review of the best practices to build histograms is presented, as well as three relevant details on spray characterization: (i) the application of information theory to assess if we have enough information (not data); (ii) the link between mathematical probability distributions and the physical interpretation of spray data; (iii) and introducing, for the first time, the concept of drop size diversity, with the quantification of the polydispersion and heterogeneity degrees. Finally, the view presented is applied to the characterization of nanofluid sprays for thermal management. Full article

Nanofluids are attracting attention as future energy carriers owing to their high performance for improving combustion and heat transfer. In this study, the macroscopic characteristics of nanofluid jets in a subsonic gaseous crossflow were investigated by focusing on the influence of nanoparticle additives on the breakup process. Based on a distribution map of the image grayscale standard deviation, we propose an improved method to process transverse injection shadowgraphs. A simplified model of the transition mechanism from column breakup to surface breakup at a small Weber number was established. The effects of nanoparticles on the jet trajectory and column fracture position were analyzed according to the deviations from the pure liquid. To interpret the effects of the nanoparticles, a new nondimensional parameter was introduced into the empirical correlation of the column fracture position. The results indicated that at low concentrations of nanoparticles, the surface tension of the nanofluids increased slightly, while the viscosity increased significantly (by up to 23%). These changes in the physical properties had little effect on the breakup regimes or jet trajectory. Moreover, the nanoparticles promoted cavitation inside the liquid column, resulting in an additional primary breakup mode for the nanofluids. Consequently, the length of the column fracture was reduced by up to 20% compared with that of the basic fluid. Full article

The gradient of surface temperature is known as Marangoni convection and plays an important role in silicon melt, spray, atomic reactors, and thin fluid films. Marangoni convection has been considered in the liquid film spray of carbon nanotube (CNT) nanofluid over the unsteady extending surface of a cylinder. The two kinds of CNTs, single-wall carbon nanotubes (SWCNTs) and multiple-wall carbon nanotubes (MWCNTs), formulated as water-based nanofluids have been used for thermal spray analysis. The thickness of the nanofluid film was kept variable for a stable spray rate and pressure distribution. The transformed equations of the flow problem have been solved using the optimal homotopy analysis method (OHAM). The obtained results have been validated through the sum of the total residual errors numerically and graphically for both types of nanofluids. The impact of the physical parameters versus velocity, pressure, and temperature pitches under the influence of the Marangoni convection have been obtained and discussed. The obtained results are validated using the comparison of OHAM and the (ND-solve) method. Full article

Carbon nanotubes play a significant role in improving the thermal efficiency of common liquids. The objective of this research is to examine the thin film spray over the surface of a vertical tube through carbon nanotubes (CNTs) nanofluids. Processes for the preparation of the nanofluid and the stable dispersion of the CNTs in water were followed from the available experimental literature. The thickness of the spray pattern was kept variable to control the stability of the spray pattern and to accomplish the suitable heat transmission under the effects of a magnetic field. The pressure supply and rate of the spray were also calculated as a function of the liquid film thickness. The basic governing equations were transformed into nonlinear differential equations by using suitable similarity transformations. The numerical outcomes were obtained by means of the BVPh 2.0 package of the optimal scheme. The influences of the physical quantities like spray rate and variable thickness on the dimensionless velocity, temperature, pressure distribution, Nusselt number were investigated and the results are compared with the existing literature. The comparison was found to be in good agreement. The present results showed that the single-walled carbon nanotubes are more efficient in the enhancement of heat transfer rate compared to the multi-walled carbon nanotubes. Full article