Enhanced Heat Transfer Efficiency Research Articles

“Enhancing Automotive Radiator Performance through Optimal Heat Transfer with Nanofluids”

Abstract: The automotive industry continually seeks innovations to improve vehicle efficiency and sustainability. One critical aspect is optimizing the performance of vehicle cooling systems, especially the radiator, to enhance heat transfer efficiency and overall thermal management. This study explores the integration of nanofluids, colloidal suspensions of nanoparticles in traditional engine coolants, into automobile radiators to achieve this goal. Nanofluids, engineered at the nanoscale, have garnered significant attention due to their exceptional thermal properties. By judiciously selecting and dispersing nanoparticles within the coolant, substantial improvements in thermal conductivity and heat capacity can be achieved. This leads to superior heat transfer capabilities compared to conventional coolants. In this research, a comprehensive experimental investigation is conducted to evaluate the thermal performance of nanofluid-infused radiator systems. The study encompasses the selection of suitable nanoparticles, their dispersion within the coolant, and subsequent testing within a controlled automobile radiator environment. The experimental results showcase substantial enhancements in heat transfer efficiency, demonstrating the potential of nanofluids to optimize cooling system performance. The findings suggest that nanofluid-based cooling systems have the capacity to significantly elevate heat transfer rates within the radiator. This enhancement holds promise for improving the overall efficiency of automotive cooling systems, consequently positively impacting vehicle performance and sustainability. The use of nanofluids in radiator applications stands as a viable solution to meet the ever-growing demands of efficient thermal management in modern automobiles. This research not only contributes to advancing the understanding of nanofluid applications in automotive engineering but also underscores the potential for practical implementation in vehicle cooling systems. The results advocate for further exploration and integration of nanotechnology within the automotive sector to drive advancements towards sustainable and energy-efficient transportation solutions

Heat transfer efficiency enhancement of gyroid heat exchanger based on multidimensional gradient structure design

Nowadays, a bio-inspired heat exchanger incorporating a triply periodic minimal surface (TPMS) structure has demonstrated great potential in the fields of new energy research and aerospace, which necessitates achieving a balance between low volume and high heat transfer efficiency while maintaining low pressure drop. In this paper, the adjustable heat efficiency of the heat exchanger with TPMS in gradient thicknesses is specially designed. A steady-state conjugate heat transfer (CHT) model is coupled with computational fluid dynamics (CFD) analysis. In addition, the temperature profile and velocity streamline are also checked to analyze the fluid flow behavior of the radiator. The results show that the convective heat transfer coefficient of the Gyroid with gradient level set values is 26.02–60.10% higher than that of the uniform Gyroid model, and the pressure drop is decreased by 9.66–18.05%. Both high heat transfer efficiency and low pressure drop can be achieved when the thickness is 0.2–0.3 mm and Re is 100–125. The heat exchangers with a TPMS thickness gradient in the ratio of 2:4:6 demonstrate a remarkable enhancement in overall heat transfer efficiency, achieving a 30.22% improvement compared to those with a TPMS thickness gradient in 6:4:2.

Effects of particle size on the pyrolysis of waste tires during molten salt thermal treatment

Recently, waste tires gain a growing recognition of the potential as value-added fuel via pyrolysis. In the present study, molten salt was proposed to be used for the waste tires pyrolysis, aims to enhance heat transfer efficiency, and then reduce energy input for the mechanical shredding of waste tires into small particles before pyrolysis. In detail, the conversion and products formation behavior of two waste tires (of different particle sizes) were compared by a constructed molten salt thermal treatment system. Results show that H2 and CH4 contents of the pyrolysis gas increased substantially, and smaller tire particles produced more H2 and CH4 (up to 500%), while CO2 content dropped to zero. Also, the contents of benzene increased remarkably with molten salt added (growth rate is between 1100% and 1800%) for electro-mobile tires, while caprolactam was disappeared. Specifically, benzene yield increases and then decreases with increasing particle size. In addition, molten salts were also effective in removing sulfur and ash from pyrolytic char, with more than 60% of the sulfur fraction being removed. These findings could provide theoretical support for the development of low-cost thermal treatment technology for waste tires.

The influence of surface topography on thermal contact resistance of 7075-T6/beryllium bronze

The calculation of thermal contact resistance is crucial for studying heat transfer between solids. Surface topography has a significant impact on thermal contact resistance, however, the influence of surface topography parameters on thermal resistance is still limited to the parameters such as roughness. The effect of other topography parameters still lacks. Therefore, in this paper, randomly non-Gaussian rough surfaces with specified root mean square roughness value, kurtosis, skewness, and wave lengths are generated by two-dimensional digital filtering method combined with Johnson transformation system. The contact mechanical deformation, heat transfer performance and thermal contact resistance between 7075-T6 and beryllium bronze QBe2 are studied using ABAQUS. The results show that thermal contact resistance is correlated with real contact area. The thermal contact resistance decreases with the increase of pressure, and gradually gets a rise with the increase of root mean square roughness value. Normal distribution of asperities, the isotropic surface, and zero or a low negative value of skewness all can enhance heat transfer efficiency. Our results provide guidance for designing surface topography when heat transfer need be considered in application.

Heat Transfer Investigation in Plus-Shaped Enclosure Using Power Law Fluid: A Finite Element Approach

The main purpose of this study is to investigate the thermal behavior of power law fluid within a plus-shaped cavity under the influence of natural convection, also taking into account the Darcy number and magnetohydrodynamics (MHD). The problem is formulated as a system of partial differential equations considering the power law fluid’s rheological behavior. The left-side walls are maintained at a specific low temperature while the lower and the right-side walls have uniform maximum temperatures. The boundary condition is designed to enhance heat transfer efficiency within the cavity, utilizing advanced thermal insulation methodologies. Finite element method (FEM) simulations are conducted, and a grid independence test is performed to validate the results. The impact of relevant parameters on the variation in momentum and thermal distributions is investigated using streamline and isothermal contour plots. The results indicate that as the Rayleigh number increases, the kinetic energy also increases, whereas the viscosity and circulation zones expand with an increase in the power law index. The Nusselt number exhibits a higher value in the shear-thinning case (n = 0.7) compared to the Newtonian (n = 1) and shear-thickening (n = 1.2) cases. This empirical observation underscores the vital role that fluid rheology plays in molding the overall heat transfer performance within the cavity. The study concludes that there is a distinct correlation between the heat transfer rate and the Rayleigh number (Ra). As Ra increases, there is a significant improvement in the heat transfer rate within the flow domain. Furthermore, the fluid behavior and heat transfer performance within the cavity are significantly influenced by the presence of magnetohydrodynamics (MHD) and the Darcy effect.

Multi-objective optimization of thermo-hydraulic behavior of heat exchanger with v-cut twisted tape in axial and radial direction using NSGA-II

ABSTRACT In pursuit of optimizing the thermo-hydraulic behavior of a heat exchanger, this study employs twisted tape inserts, a well-known method for enhancing heat transfer rates in heat exchange devices compared to plain tubes. Experiments involving v-cut twisted tapes with v-angles of 15°, 30°, and 45° in both axial and radial directions were conducted within a double pipe heat exchanger, encompassing a Reynolds number range from 5500 to 10,000, with water as the flowing medium. As part of the modeling approach, response surface methodology (RSM) was harnessed to construct mathematical models for the heat transfer rate, quantified as the Nusselt number, and the friction factor. This modeling accounted for independent variables including Reynolds number, Prandtl number, axial angle, and radial angle. The RSM models demonstrated high statistical significance and reliability, yielding an R-squared value exceeding 97%, with the linear terms of Reynolds number and cut angles identified as the most influential factors. To attain optimization, a multi-objective approach was adopted, employing the NSGA-II algorithm. This algorithm efficiently explored the design parameter space, seeking the optimal combination that maximizes heat transfer rate and minimizes the friction factor simultaneously. Quantitative results from the experimentation revealed that, at a Reynolds number of 10,000, the Nusselt number achieved its peak value of 96.51. Conversely, the highest friction factor (0.0582) and performance evaluation criteria (PEC) of 1.65 emerged at a Reynolds number of 5500, specifically when employing a 45-degree radial v-cut angle. Notably, this configuration led to substantial enhancements: the Nusselt number increased by 98.7%, the friction factor by 38.9%, and the PEC by 39.4% compared to a plain tube. These findings underscore both the effectiveness of the NSGA-II algorithm in optimizing heat exchanger designs and the valuable insights gained into the influence of twisted tape insert parameters on heat transfer performance and hydraulic behavior. The optimized design parameters obtained through this study hold significant potential for enhancing heat transfer efficiency across diverse industrial applications. Overall, this research advances heat exchanger optimization techniques and provides a framework for further investigations in this field.

Optimizing Solar Parabolic Trough Receivers with External Fins: An Experimental Study on Enhancing Heat Transfer and Thermal Efficiency

Several researchers have shown that the heat transfer performance of solar parabolic trough (SPT) receivers may be improved by increasing their surface area or by adding internal fins to the tubes. Unfortunately, the manufacture of internally finned tubes involves complex processes, resulting in significant cost increases. On the other hand, the addition of external fins to tubes is more technically and economically feasible in a low-resource setting. This study investigates the potential benefits of integrating external fins on the receiver tubes of a low-cost SPT collector system. Experiments were conducted using an SPT system with a focal length of 300 mm and a collector length of 5.1 m, and they were positioned by an automated Sun tracking system. Tests were undertaken using both smooth and externally finned receiver tubes operating at five different water flow rates. The solar receiver with a finned tube was able to provide a maximum water temperature of 59.34 °C compared with that of 56.52 °C for a smooth tube at a flow rate of 0.5 L per minute. The externally finned absorber tube was also found to have a maximum efficiency of 18.20% at an average daily solar intensity of 834.61 W/m2, which is approximately 48% more efficient than the smooth tube. The calculations indicate that the experimental SPT system using finned tubes potentially avoids 0.2726 metric tons of CO2e per year, with finned tubes outperforming smooth tubes by up to 44%. The results show that using externally finned receiver tubes can significantly enhance the thermal performance of SPT collector systems.

Electrothermal transport of water conveying copper, silver and alumina nanoparticles through a vertical wavy microchannel

This study emphasizes the significance of optimizing heat transmission, energy conversion, and thermal management in electronic devices, renewable energy systems, and emerging technologies like thermoelectric devices and energy storage systems. The aim is to enhance heat transfer efficiency for improved performance and lifespan of electronic equipment. The research utilizes a mathematical flow analysis to study a water-based ternary nanofluid’s flow and thermal characteristics in a vertical microfluidic channel driven by peristalsis and electroosmosis. The ternary-hybrid nanofluid (THNF), comprising copper, silver, and alumina nanoparticles dissolved in water, is examined considering induced magnetic fields. The study delves into fluid flow, heat absorption, and mixed convection, using Debye–Hückel, lubrication, and long wavelength approximations. Results show that THNF exhibits superior heat transmission compared to pure water. Increasing solid volume fraction of nanoparticles decreases THNF’s temperature. Induced magnetic fields impact the system. This research could influence thermal pipe heat sinks and bioengineered medical devices design.

Entropy generation optimization of cilia regulated MHD ternary hybrid Jeffery nanofluid with Arrhenius activation energy and induced magnetic field

This study deals with the entropy generation analysis of synthetic cilia using a ternary hybrid nanofluid (Al–Cu–Fe2O3/Blood) flow through an inclined channel. The objective of the current study is to investigate the effects of entropy generation optimization, heat, and mass transfer on ternary hybrid nanofluid passing through an inclined channel in the proximity of the induced magnetic field. The novelty of the current study is present in studying the combined effect of viscous dissipation, thermophoresis, Brownian motion, exponential heat sink/source, porous medium, endothermic–exothermic chemical reactions, and activation energy in the proximity of induced magnetic field is examined. The governing partial differential equations (PDEs) are transformed into the ordinary differential equations (ODEs) using appropriate transformations. Applying the low Reynolds number and the long-wavelength approximation, resultant ODEs are numerically solved using shooting technique via BVP5C in MATLAB. The velocity, temperature, concentration, and induced magnetism profiles are visually discussed and graphically analyzed for various fluid flow parameters. Graphical analysis of physical interest quantities like mass transfer rate, heat transfer rate, entropy generation optimization, and skin friction coefficient are also graphically discussed. The entropy generation improves for enhancing values of Reynolds number, solutal Grashof number, heat sink/source parameter, Brinkman number, magnetic Prandtl number, and endothermic-exothermic reaction parameter while the reverse effect is noticed for chemical reaction and induced magnetic field parameter. The findings of this study can be applied to enhance heat transfer efficiency in biomedical devices, optimizing cooling systems, designing efficient energy conversion processes, and spanning from renewable energy technologies to aerospace propulsion systems.

Rheological characteristics and thermal studies of EG based Cu:ZnO hybrid nanofluids for enhanced heat transfer efficiency

The present work describes an experimental investigation of Cu-doped ZnO–EG hybrid nanofluids for better viscous nature and thermal efficiency in cooling systems. The initial phase involves synthesizing copper (Cu) doped zinc oxide (ZnO) nanoparticles using chemical precipitation method. The structural and morphological properties of these nanoparticles are determined using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Thermo-physical measurements are conducted to analyze the viscosity and thermal conductivity of Cu-doped ZnO nanofluids based on ethylene glycol (EG) at varying particle concentrations (ranging from 0.01 wt% to 0.08 wt%) with various temperature ranges of 298 K to 328 K. The observed rheological properties showed that the viscosity of tested Cu-doped ZnO-EG nanofluids exhibited non-Newtonian shear-thinning behavior due to the alignment of nanoparticle agglomerates with increasing shear rate. Similarly, the thermal properties of the prepared Cu:ZnO-EG nanofluids are examined, demonstrating a smooth increase in thermal conductivity with changes in particle concentration and temperature. The thermal conductivity ratio of the nanofluids suggests that the newly developed hybrid nanofluids exhibit superior heat transfer characteristics.

Performance Analysis of Prepaid Nano Fluid for Cooling-Vehicle Motor

With the growing demand for electric vehicles (EVs), the importance of efficient thermal management systems has significantly increased. These systems play a crucial role in maintaining optimal performance and prolonging the lifespan of critical components, particularly electric motors. However, traditional cooling methods often struggle to meet the high heat dissipation requirements of EV motors, resulting in reduced overall efficiency and reliability. To address this issue, the integration of nano fluids as a cooling medium has emerged as a promising solution. Nano fluids, which are engineered colloidal suspensions, consist of a base fluid (such as water or oil) and nanoparticles with sizes ranging from 1 to 100 nanometers. Through the dispersion of nanoparticles within the base fluid, nano fluids possess unique thermal properties that significantly enhance heat transfer efficiency compared to conventional fluids. This abstract provides an overview of the application of nano fluids in E-vehicle motor cooling, emphasizing their potential benefits, challenges, and future prospects.

Influence of notched baffles on aerothermal performance behaviors in a channel

The proper designs of modified heat transfer surfaces or turbulence enhancement inserts for heat transfer augmentation are extremely important for improving overall aerothermal performances relating to the energy-saving capabilities of thermal systems. A major challenge is to control friction loss as little as possible while maintaining reasonable heat transfer enhancement. Transverse baffles with rectangular notches or notched baffles (NBs) were applied for improving aerothermal performance in a channel with a constant aspect ratio of 3.75 while notch height-to-baffle height ratio (a/e) ranged from 0.125 to 0.5. Reynolds number ranged from 6000 to 24,000, in experiments. Heat transfer enhancement, pressure loss, and aerothermal performance in a rectangular channel with notched baffles were examined. Compared to the solid transverse baffle (SB, a/e = 0), the NBs with a/e = 0.125 increased the heat transfer rate while lessening the pressure loss, as shown by the experimental findings. Obviously, Nusselt number, friction factor and aerothermal performance increased as the a/e ratio decreased. The NBs with the smallest notch height-to-baffle height ratio (a/e = 0.125) exhibited the highest aerothermal performance of 1.17, which can be attributed to the efficient heat transfer enhancement by the strong multi-jet impingements and the moderate friction loss penalty resulting from the presence of notches (spaces) on the baffles.

A hybrid prediction approach for enhancing heat transfer efficiency of coal-fired power plant boiler

Predicting future fouling status is a crucial but tough topic in the applications of energy conservation and pollution reduction at coal-fired power plants because of the significant influence that ash slag has on the heat transfer efficiency of boilers in coal-fired power plants. For the prediction of gray areas in heated areas, a hybrid system based on complementary ensemble empirical modal decomposition, gray models, and long short-term memory networks is presented. This is because the time series of ash pollution degrees is not linear and not smooth. Initially, using a complementary ensemble empirical modal decomposition, the original sequence after wavelet threshold denoising is divided into a number of subseries components. The projected values for the cleanliness factor were then generated by superimposing the predictions from the IMF and residual components. The experimental findings support the model’s precision and dependability and demonstrate that the CEEMD-GM-LSTM model does, in fact, perform very well in forecasting the ash situation in the heated zone.

Numerical investigation of droplet condensation and self-propelled jumping on superhydrophobic microcolumned surfaces

This paper investigates the processes of droplet condensation and self-propelled jumping on microcolumn-structured superhydrophobic surfaces with various size parameters. Using a three-dimensional (3D) multiphase lattice Boltzmann method, a novel phenomenon of secondary coalescence jumping is identified, and the underlying mechanisms are analyzed in detail. The simulation results show that wettability has a significant influence on droplet jumping. As the hydrophobicity of the surface increases, the droplets tend to jump from the substrate. However, structure parameters, such as the microcolumn spacing and height, have non-monotonic effects on droplet jumping. The structure parameters determine whether droplet coalescence occurs under the bottom–bottom droplet coalescence mode or the bottom–top droplet coalescence mode. Bottom–bottom droplet coalescence is shown to promote droplet jumping. Based on the simulation results and kinetic analysis, the optimal spacing-to-width and height-to-width ratios of the microcolumns for droplet jumping are found to be approximately 0.6 and 1.0, respectively. We believe the results of this work will provide valuable guidance in the design of self-cleaning surfaces and enhancing heat transfer efficiency.

Wall-sheared thermal convection: heat transfer enhancement and turbulence relaminarization

We studied the flow organization and heat transfer properties in two-dimensional and three-dimensional Rayleigh–Bénard cells that are imposed with different types of wall shear. The external wall shear is added with the motivation of manipulating flow mode to control heat transfer efficiency. We imposed three types of wall shear that may facilitate the single-roll, the horizontally stacked double-roll, and the vertically stacked double-roll flow modes, respectively. Direct numerical simulations are performed for fixed Rayleigh number $Ra = 10^{8}$ and fixed Prandtl number $Pr = 5.3$ , while the wall-shear Reynolds number ( $Re_{w}$ ) is in the range $60 \leqslant Re_{w} \leqslant 6000$ . Generally, we found enhanced heat transfer efficiency and global flow strength with the increase of $Re_{w}$ . However, even with the same magnitude of global flow strength, the heat transfer efficiency varies significantly when the cells are under different types of wall shear. An interesting finding is that by increasing the wall-shear strength, the thermal turbulence is relaminarized, and more surprisingly, the heat transfer efficiency in the laminar state is higher than that in the turbulent state. We found that the enhanced heat transfer efficiency at the laminar regime is due to the formation of more stable and stronger convection channels. We propose that the origin of thermal turbulence laminarization is the reduced amount of thermal plumes. Because plumes are mainly responsible for turbulent kinetic energy production, when the detached plumes are swept away by the wall shear, the reduced number of plumes leads to weaker turbulent kinetic energy production. We also quantify the efficiency of facilitating heat transport via external shearing, and find that for larger $Re_{w}$ , the enhanced heat transfer efficiency comes at a price of a larger expenditure of mechanical energy.

Optimizing Device Performance of Multi-Pass Flat-Plate Solar Air Heaters on Various Recycling Configurations

Various external-recycle configurations of multi-pass flat-plate solar air collectors were studied theoretically to examine the optimal thermal performance under the same working dimension and operation conditions. An absorber plate and insulation sheet were implemented horizontally and vertically, respectively, into an open rectangular conduit to conduct a recycling four-pass operation, which the device lengthens the air flow channel and increases the air mass flow rate within the collector, and thus, a more heat transfer efficiency is obtained. Four recycling types with different external-recycle patterns were introduced and expected to augment the heat transfer rate due to the turbulent convective intensity through four subchannels in the present study. Coupling energy balances into one-dimensional modeling equations were derived by making the energy-flow diagram within a finite element, which the longitudinal temperature distributions for each subchannel were obtained. The theoretical predictions show that the improved four-pass device is accomplished due to the multiple heating pathways over and under the absorber plate, from which the turbulence intensity augmentation results in the heat transfer rate as compared to that in the device without inserting the absorber plate and insulation sheet (say a downward-type single-pass solar air collector). The theoretical results also show that the external-recycle configuration (say Type C in the present study) acts as an optimal collector thermal efficiency and leading to a beneficial design in multi-pass solar air collectors for improving heat-transfer rate and increasing resident time under the same operation conditions. Theoretical predictions show a higher heat-transfer efficiency for the present recycling configurations up to a maximum 115% device enhancement in comparison to that of a single-pass device. Examination of implementing the absorber plate and insulation sheet on the heat-transfer efficiency enhancement as well as the hydraulic dissipated power increment were also delineated, and deliberated the suitable external-recycle configuration with respect to an economic consideration.

Numerical Simulation of Heat and Moisture Transfer in Corrugated Walls Dryer

The primary goal of the current work is to model heat and mass transfer during mango drying in a wavy airflow dryer. By modifying the dryer walls, we were able to produce the undulating airflow (with V-shaped obstacles). With convective boundary conditions applied to all product surfaces, the explicit finite difference approach was used to study heat and mass exchanges in two dimensions during the drying of mango slices. During drying, the transfer coefficients are thought to fluctuate. Using EasyCFD software, the external flow, temperature, velocity, and pressure fields were then analyzed. This provided the profile of the heat transfer coefficient. These profiles were then utilized to calculate the mass transfer coefficient using the Shilton-Colburn analogy. Moreover, the code created to determine the heat and mass transfer coefficients in the product was used to derive the evolution of temperature and moisture content over time. The results allowed for the discovery of a new air flow in dryers called an undular flow and demonstrated how modifying the drying air stream enhanced heat transfer efficiency. By changing the air flow in the dryer, it was possible to achieve heat transfer coefficients ranging from 47.55 W/m²K to 357.38 W/m²K and mass transfer coefficients of 3.21 x 10-5 to 3.21 x 10-4 m²/s. When the outcomes of this investigation were compared to experimental results from the literature (under identical drying circumstances), a reasonable level of adequacy was discovered.

Predicting the thermal conductivity of unsaturated soils considering wetting behavior: A meso‑scale study

Unsaturated soils are three-phase porous materials that consist of the solid phase, the liquid phase and the gas phase. While accurate predictions to the thermal conductivity of unsaturated soils are crucial for the analysis and design of geothermal systems, the distribution characteristics of the different phases can also significantly affect the thermal performance. In addition, the fact that soils are heterogeneous materials also adds to the challenges in the evaluation of the thermal conductivity of unsaturated soils. In this paper, a methodology is proposed to calculate the thermal conductivity of unsaturated soils. This methodology starts from identifying representative soil parameters based on microscopy images of soil specimens. The meso‑structures that consist of the different phases are then reconstructed in a finite-element model based on the identified characteristics. The Monte-Carlo-based finite-element analyses are then carried out to calculate the thermal conductivity of unsaturated soils while considering the wetting behavior between the solid and liquid phases, which is characterized using the hydrophilia coefficient. After validating the methodology using both the experimental and numerical results reported in the literature, a parametric analysis is conducted. The results indicate that a high quartz content improves the heat transfer, but the quartz content does not affect the heat transfer skeleton. Furthermore, an increase in the solid content and degree of saturation results in an increase in the density of the heat transfer skeleton, which leads to enhanced heat transfer efficiency. A reduction in the hydrophilia coefficient and the formation of liquid-bridges have a similar effect. Last, the effects of wetting are more evident in soils with high porosity and a low degree of saturation.

Rayleigh–Bénard instability in nanofluids: effect of gravity settling

Understanding the mechanism of thermal instability in nanofluids is of fundamental importance to explore the reasons behind the enhancement of heat transfer efficiency. Since Buongiorno (ASME J. Heat Transfer, vol. 128, 2006, pp. 240–250) proposed his theoretical model of nanofluids, most studies focusing on the thermal instability analysis exclusively considered Brownian motion and thermophoresis as the main diffusion mechanisms of nanoparticles. All the analyses concluded that a nanofluid layer is much more unstable than its pure counterpart as it is heated from below. However, a recent experimental observation on Rayleigh–Bénard convection appears to contradict the theoretical prediction, implying that some mechanisms neglected in the previous model may have a significant impact on the onset of thermal convection. In the present study, we revise the convective transport model of nanofluids proposed by Buongiorno and find that the gravitational settling of nanoparticles is a crucial factor influencing the thermal instability behaviour of nanofluids. By performing a linear stability analysis based on the novel model, the effect of gravity settling exhibits a stabilizing mechanism to resist the destabilizing effect of thermophoresis. Furthermore, the onset of instability can be delayed once the nanoparticle diameter exceeds a certain threshold, which explains the phenomenon observed in experiments. Particularly, the oscillatory mode is found to emerge and dominate the flow instability when the gravity settling effect is competitive with the effect of thermophoresis.

Development of predictive models for density of hybrid nanofluids using different machine learning techniques

Nanofluids are unique thermal fluids with enhanced heat transfer efficiency relative to conventional thermal fluids. The density of nanofluids is an important thermophysical property that determines the heat transfer coefficient of fluids. Hybrid nanofluids are known to show superior thermal properties compared to normal nanofluids, yet investigation on the predictive modeling of the density of hybrid nanofluids is scarce in the literature. In this report, the application of machine learning (ML) for predicting the density of hybrid nanofluids is examined. The considered hybrid nanofluids consist of Al2O3/SiO2, TiO2-SiO2, Fe3O4-MWCNT, Al2O3-CNT, Al2O3-MWCNT, TiO2-MWCNT, CeO2-MWCNT, ZnO-MWCNT, MgO-MWCNT, CuO-MWCNT, Co3O4/rGO, TiO2-MgO, Ag-GNP, and ND-Fe3O4 nanoparticles suspended in H2O, GB, DW, and W-EG (60:40%). The ML algorithms examined in this report include: support vector regression optimized with genetic algorithm (SVR-GA), Gradient Boost Regression Algorithm (GBR) with Grid Search Optimization (GBR-GSO), Decision Tree (DT), and Voting Ensemble (VE). These models were developed using the following input parameters: temperature, volume concentration, the density of individual nanoparticles, and density of the base fluid. Excellent correlations of 99.81%, 99.76%, 96.36%, and 95.05% were obtained for the SVR-GA, GBR-GSO, VE, and DT, respectively. This result shows that the SVR-GA had the highest correlations with experimental results. The development of a highly accurate predictive model for the density of hybrid nanofluids is essential because it can facilitate the rapid design of heat transfer devices. Developing a diffusivity model for hybrid nanofluids using a machine learning approach is a possible extension of the present study.