Improve Heat Transfer Efficiency Research Articles

Analysis of thermohydraulic flow and enhancement heat performance in 3D dimple tube based on varying geometrical configurations

AbstractThis research work investigates how different dimple designs affect the flow field and thermal performance of three‐dimensional pipes. The study focuses on the effect of the number of improved dimples NOD (3, 4, and 5), different groups numbers DGNs (1, 2, and 3 groups), arranged around the pipe, and different distances between dimples (DBDs). Dimple geometry affects flow: Changing dimple parameters alters the velocity and pressure distribution within the pipe. Performance evaluation factor (PEF) varies with dimple configuration: The PEF, which balances heat transfer enhancement and pressure drop penalty, ranges from 1.187 to 1.23 for NOD and from 1.292 to 1.31 for DGN, and also from 1.26 to 1.302 for DBD. Reynolds number range, Re = 4000–15,000; turbulence model, standard k–ε model; numerical scheme, second‐order upwind scheme; test tube conditions, inlet temperature (Tin) = 25°C; pipe diameter D = 23 mm; thickness = 2 mm; heat flux q = 25,500 W/m²; and material (Cu). This research focuses on improving heat transfer efficiency in pipes using dimples. Dimple size and arrangement significantly impact flow dynamics and heat transfer. PEF is used to evaluate the overall performance considering both heat transfer improvement and pressure drop penalty. The study found a specific range for PEF under various conditions for different dimple configurations. The average enhancement in Nusselt number for model 2 was 15.16% compared with a smooth pipe and the heat transfer performance by 10.028%–28.963% at the effect of NOD, the DGN has slightly higher Nu values than smooth pipes, indicating improved heat transfer due to the dimples (around 7%–58% at Re 4000–15,000 and 9%–13% at Re 12,000), and at DBD (13.5%) at a Reynolds number of 12,000 and 4.6%–59% at Re 4000–15,000.

Boosting thermal energy transport across the interface between phase change materials and metals via self-assembled monolayers

Thermal energy storage using phase change materials (PCMs) has great potential to reduce the weather dependency of sustainable energy sources. However, the low thermal conductivity of most PCMs is a long-standing bottleneck for large-scale practical applications. In modifications to increase the thermal conductivity of PCMs, the interfacial thermal resistance (ITR) between PCMs and discrete additives or porous networks reduces the effective thermal energy transport. In this work, we investigated the ITR between a metal (gold) and a polyol solid–liquid PCM (erythritol) at various temperatures including temperatures below the melting point (300 and 350 K), near the melting point (390, 400, 410 K, etc) and above the melting point (450 and 500 K) adopting non-equilibrium molecular dynamics. Since the gold-erythritol interfacial thermal conductance (ITC) is low regardless of whether erythritol is melted or not (<40 MW m−2 K−1), self-assembled monolayers (SAMs) were used to boost the interfacial thermal energy transport. The SAM with carboxyl groups was found to increase the ITC most (by a factor of 7–9). As the temperature increases, the ITC significantly increases (by ∼50 MW m−2 K−1) below the melting point but decreases little above the melting point. Further analysis revealed that the most obvious influencing factor is the interfacial binding energy. This work could build on existing composite PCM solutions to further improve heat transfer efficiency of energy storage applications in both liquid and solid states.

Flow reversal and multiple states in turbulent Rayleigh–Bénard convection with partially isothermal plates

This paper examines turbulent Rayleigh–Bénard convection in a two-dimensional square cavity with partially isothermal conducting plates on the horizontal walls. The study reveals that controlling the relative locations of the partially isothermal plates can accelerate or completely suppress the reversals of large-scale circulation. The heat transfer efficiency, which is characterised by the time-averaged Nusselt number, is generally higher than that of the traditional Rayleigh–Bénard convection, and can be further enhanced when the reversal is fully suppressed. The reversal in our cases is mainly caused by the competition between the two alternately growing ‘corner’ vortices, fed by the detaching plumes from the hot/cold plates. This differs from those reported in traditional Rayleigh–Bénard convection. Fourier mode decomposition of the kinetic energy, reflecting the diverse contributors, in the reversing cases further emphasises the distinction between the current system and traditional Rayleigh–Bénard convection. In addition, multiple states were observed where the conducting plates were positioned at specific relative locations and had different initial conditions. It has been observed that the difference in Nusselt numbers between the anticlockwise and clockwise states increases linearly with the distance between the upper cold and lower hot plates. Moreover, the analysis of the buoyancy moment and the stability of the primary roll structure suggests that the higher heat transfer efficiency between the two states is strongly linked to a more stable primary roll structure. This study presents a new approach for controlling flow reversal and improving heat transfer efficiency by modifying the non-global conducting boundary and initial conditions.

Heat transfer mechanism of different vortex generators arrangements embedded in heat exchange tubes

Heat exchange is widely used in many fields to improve heat transfer efficiency and save energy, while longitudinal vortex generator (VG) is widely used in many heat transfer heat exchangers. In this work, the effect of VGs with different numbers and different pitch ratios (PR) on the heat transfer characteristics in the tube was studied numerically. The results show that when the fluid flows through the VG, the reflux region will be generated, and the fluid velocity in the reflux region will be inhibited, resulting in uneven temperature distribution and increased Nu. When multiple VGs are arranged in a row, the number of vortices is increased, and the area affected is increased. This improves the overall heat transfer performance. With the decrease of PR, the number of VG rows increases, and the heat transfer performance increases first and then decreases with the decrease of PR.

Analysis of enhanced thermal transmission mechanism with microstructures by field synergy theory

This study numerically examines the impact of oblique ribs and staggered herringbone microstructures on the heat transfer performance of microchannels. The results indicate that the Nux values of the oblique rib (Type A) and staggered herringbone (Type B) microchannels exhibit significantly higher performance compared to smooth rectangular (Type C) microchannels. The oblique ribs and staggered herringbone microstructures improve heat transfer efficiency along the flowing direction, leading to an evident increase followed by horizontal fluctuations in Nux . In contrast, the Nux of Type C without microstructures decreases significantly at the inlet and subsequently remains relatively constant. The bottom temperature Tw of Type A and B are lower than the temperature of Type C. The results of field cooperation analysis indicate that the synergistic fields of the oblique ribs microchannels primarily exist along both sidewalls, while they are present at the fluid center for the staggered herringbone microchannels. Type A exhibits a slightly stronger heat transfer effect despite having a smaller synergistic area compared to Type B.

Optimizing heat transfer efficiency with injection in ventilated cavity in the presence of internal thermal effects

Venturing into unexplored territory, the study explores the dynamics of mixed convection flow within a square cavity, influenced by a ventilated channel and an injection port. By conducting numerical simulations, the impact of internal heat generation/absorption and viscous dissipation on heat transfer efficiency is thoroughly examined. Employing a finite difference method alongside an alternate direction implicit scheme and iterative successive under-relaxation (SUR) technique, the study systematically solves the governing equations. The effect of physical parameters, namely the Reynolds number ( 50 ≤ Re ≤ 500 ), Richardson number ( 0.01 ≤ Ri ≤ 10 ), volumetric internal heat generation ( △ * > 0 )/absorption ( △ * < 0 ), Eckert number ( 0 ≤ Ec ≤ 0.3 ), and especially variation of injection strength, on the streamlines, isotherms, and average Nusselt number are discussed graphically. The findings demonstrate a significant improvement in heat transfer efficiency, ranging from 17% to 73% with injection, underscoring its substantial implications for advancing thermal management across diverse engineering domains.

Analytical analysis of water and engine oil base nanofluid flow with the influence of viscous dissipation and variable viscosity on stretching surface

AbstractIn this study, we will look at the analytical characterization of water and motor oil‐based nanofluid flow and the effects of viscous dissipation and variable viscosity on stretching surfaces. The primary goal of this research is to improve heat transfer efficiency in a range of systems, including cooling applications, refrigeration systems, and heat exchangers. The addition of nanoparticles to the base fluid increases its thermal conductivity, which boosts heat transfer rates. The flow system considers the impact of viscous dissipation. In addition, this methodology accounts for temperature and velocity slips during stretching. We utilized the proper transformations to convert a set of PDEs to NLODEs. To solve this system of equations, we use hybrid nanofluid. The impact of different parameters obtained from temperature and velocity equations involving the porosity parameter, power law number, ratio velocity, dynamic viscosity, Forchheimer parameter, and Eckert number input factors are shown in the form of graphs.

Experimental comparison of finned tube heat exchangers for heat rejection under natural convection conditions

This article presents experimental analysis of heat removal capabilities of a wavy finned tube heat exchanger operating under natural convection conditions Two types of heat exchangers are considered in this work: standard construction exchangers, most often found in waste heat rejection devices and exchangers based on a novel geometry, which supports intensification of heat transfer processes under natural convection. The aim of the research was to experimentally verify the novel conception and to determine the thermal efficiency of heat exchangers under the influence of selected design parameters. The factors analyzed in the research include: temperature difference between the heat transfer surface and the surrounding air, different angles of the units transferring heat and the height of the exchanger casing. Four variants of the casing height were considered in this work. The conducted research also allowed to determine the influence of the fin spacing on the performance of the heat exchangers under natural convection conditions. The test results obtained show that the novel geometry allows for significance improvement of heat transfer efficiency. The proposed solution allowed to increase the density of the heat flux from 12% in (fin spacing of 2.8 mm) up to 24% (fin spacing of 5.6 mm). It was also observed that the height of the casing has significant impact on the heat transfer efficiency. For each of the analyzed cases, the heat exchanger without a casing was characterized by the lowest thermal efficiency, while the highest heat flux was removed from the exchanger with the highest casing (up to four times higher heat transfer rate). It was determined that the largest amount of heat was removed from exchangers located horizontally, while the exchanger with an angle of 60°, was characterized by the lowest rate of heat removal. Another factor which was determined to have significant impact on the performance of the heat exchangers is the spacing between the fins. The research results obtained shows that increasing the spacing of the finned surface from 2.8 mm to 5.6 mm resulted in an average twofold increase in thermal efficiency in the case of four-row exchangers and an almost threefold increase in the case of two-row exchangers. The obtained results can be used as basic guide for the development of finned tube heat exchangers operating under natural convection conditions.

Effects of varying heat transfer rates for borehole heat exchangers in layered subsurface with groundwater flow

The existence of groundwater flow adds a convective heat component to shallow geothermal heat pump systems, significantly improving heat transfer efficiency. Numerous analytical and numerical methods have been proposed to assess the impact of groundwater flow on ground heat exchangers. However, numerical simulations often require significant resources and time, while conventional analytical solutions overlook the time and depth-varying heat transfer rates, leading to inaccurate temperature predictions. This study proposes a computationally efficient semi-analytical solution by extending the line source solution and coupling it with a thermal resistance model. This approach addresses variations in heat transfer rate over time and depth, overcoming the limitations of conventional analytical solutions. Compared with a three-dimensional numerical model constructed in COMSOL Multiphysics, the average absolute percentage error (MAPE) of borehole wall temperatures for a U-shaped borehole heat exchanger (BHE) is approximately 1 %, across a wide range of water flow velocities and thermal conductivity ratios between two contacting geological layers. A significant advantage of the proposed solution is its computational efficiency. The proposed solution can complete long-term simulations with over 9,000 time steps in about 10 min, as opposed to numerical models that typically require several hours. The excellent computational efficiency and substantial accuracy make the proposed solution a simple and effective tool for the design and optimization of BHEs in engineering applications.

Numerical investigation of 3-D rotating hybrid nanofluid Forchheimer flow with radiation absorption over a stretching sheet

Examining the movement of MHD hybrid nanofluid flows along linearly extending regions provides valuable insights for improving heat transfer efficiency. Many researchers have investigated various factors, such as the Forchheimer number, porosity, radiation, aligned magnetic field and mixed convection, in the context of incorporating hybrid nanofluids into these flows. The existing literature lacks discussion on the effect of radiation absorption along with porosity, magnetic parameter and an aligned magnetic field on the Forchheimer flow of a 3-D rotating Ag−CuO/H2O hybrid nanofluid through a linearly expanded sheet. The current research endeavours to address this gap and offer valuable insights into these particular factors.BVP-5C shooting scheme through MATLAB is utilized to solve the system that governs nonlinear PDEs by converting them to nonlinear ODEs. The influences of dimensionless parameters, including radiation absorption (1 ≤ Q ≤ 2), Forchheimer number (0 ≤ Fr ≤ 9), porosity parameter (0 ≤ K ≤ 4), magnetic parameter (0 ≤ M ≤ 4), aligned magnetic field (0 ≤ α ≤ π/2), on the velocity along x and y-axes, temperature profile and concentration distributions are discussed. Also, skin friction, heat and mass transfer rate impacts are demonstrated traverse tables. Velocity along both x and y-axes is decreasing as Forchheimer number increases. As the values of radiation absorption and porosity parameter rise, there is a corresponding decrease in the temperature field. Greater Forchheimer number and aligned magnetic field leads to increase in the concentration profile of Ag−CuO/H2O hybrid nanofluid. The Nusselt number demonstrates an upward trend when Forchheimer number, porosity parameter, radiation absorption, magnetic parameter and aligned magnetic field experience an increase, whereas the Sherwood number exhibits a contrary behaviour with these identical parameters. The current study demonstrates a compatibility rate of 99.9% with the preceding research across diverse values of the stretching ratio parameter.

Study on heat transfer performance of cold plate with grid channel

The utilization of cold plate radiators as a prevalent method for indirect liquid cooling has been extensively investigated and implemented in server cooling systems. However, there is a lack of comprehensive study on the application of this technology at the chip size, indicating a need for more development and exploration in this area. A proposal was made for a grid-channel chip cold plate heat sink to facilitate the dissipation of heat from a chip. tests were conducted to investigate the impact of the flow rate of the cold plate and the layout of the inlet and outlet on various thermal parameters, including the average temperature, maximum temperature, thermal resistance, and uniformity coefficient of the cold plate. The tests were specifically conducted under a chip power of 150W, and the accuracy of the simulation was confirmed through the use of FLUENT. The findings indicate that the cold plate effectively regulates the temperature of the chip, ensuring it remains below 85 °C throughout all experimental groups. In contrast to the single in single out configuration, the single-in multiple-out layout exhibits a higher degree of temperature uniformity within the cold plate. Nevertheless, it is important to note that augmenting the quantity of exits does not guarantee an improvement in heat transfer efficiency. This outcome is contingent upon the presence of a longitudinal flow channel shared by the outlet and intake, as well as the dispersion characteristics of the outlet. Enhancing the dispersion of the exit can significantly enhance the thermal transfer efficiency of the cold plate. Furthermore, a strategy for adjusting the aperture of the orifice is proposed as a solution to address the challenges related to flow uniformity and the issue of high pressure drop in the cold plate.

Dual solutions of Darcy-Forchheimer radiative-convective magneto-nanofluid flow over an exponentially stretching/shrinking surface: Stability and entropy generation analysis

A numerical investigation was conducted to examine the existence of dual solutions over exponentially stretching or shrinking permeable surfaces. The study aimed to analyze the impact of thermal and second-order velocity slip on magnetohydrodynamic (MHD) mixed convective radiative nanofluid flow within a Darcy-Forchheimer porous medium while considering heat generation. Two distinct types of nanofluids, namely Cu-water and TiO 2/water, were employed in the investigation. To address the nonlinear partial differential equations (PDEs), similarity transformations were utilized to transform them into a system of ordinary differential equations (ODEs). The resulting coupled higher-order nonlinear ODEs were then solved using a MATLAB-based boundary value algorithm known as bvp4c. Two distinct solutions are observed within a restricted range of the stretching/shrinking parameter ξ ( ξ > ξ c ) and suction parameter S ( S > S c ), terminating respectively at ξ = ξ c or S = S c . The critical values are identified as ξ c = − 1.1952 ( Cu − water ) , − 0.9304 ( Ti O 2 − water ) and S c = 1.6437 ( Cu − water ) , 1.8900 ( Ti O 2 − water ) , maintaining the fixed values of the remaining physical parameters. Notably, the critical value for Cu-water is lower than that of TiO 2-water. Moreover, in both solution branches, the range of skin-friction coefficient and local Nusselt number for Cu-water surpasses that of TiO 2-water. An evaluation of the temporal stability of these solutions reveals that only the first solution remains stable. This is supported by the positivity of the smallest eigenvalues ( γ 1 > 0 ), indicating stability. Conversely, for the second solution, the smallest eigenvalues ( γ 1 < 0 ) are negative, signifying its instability. Also, Cu-water nanofluid exhibits higher fluid velocity and lower fluid temperature than TiO 2-water nanofluid for stable solutions and the opposite behavior for unstable solution branches. Significantly, within the initial solution branch, there is an escalation in velocity profiles attributed to the magnetic parameter, Grashof number, porous parameter, and first- and second-order velocity slip parameter. Conversely, a decrease is observed for the Forchheimer number, while the opposite holds true for the second solution branch. For both solution branches, the temperature profiles experience a reduction due to thermal slip and stretching/shrinking parameters, whereas enhancements are noted for thermal radiation and heat generation parameters. Furthermore, an increase in the suction parameter leads to elevated skin friction coefficient and local Nusselt number in the first stable solution, thereby promptly improving heat transfer efficiency. Moreover, improvements in entropy generation arise from factors such as thermal radiation, magnetic field, and nanoparticle solid volume fraction, presenting opportunities to optimize heat transmission in cooling and heating systems.

Analytical Investigation of Arrhenius Kinetics with Heat Source/Sink Impacts along a Heated Superhydrophobic Microchannel

Exothermic Arrhenius-controlled chemical reactions are widely employed for heating applications, such as combustion in heating systems and fuel-burning motors. These effects are critical for manufacturing high thermal systems required for high thermal performances. Therefore, this paper presents the analytical investigation into the Arrhenius-controlled heat-generating/absorbing fluid of a hydromagnetic flow along a heated upstanding plate in a microchannel. One wall had a superhydrophobic surface and temperature jump conditions, but the other was unaltered. The regular perturbation approach investigated the nonlinear, coupled governing equations. With the help of graphical plots, the impacts of crucial and relevant parameters embedded in the flow are described. This investigation concludes that chemically reacting parameters' impact significantly elevates the micro-channel's thermal and hydromagnetic flow. However, applying the heat generation parameter increases fluid velocity, whereas the heat absorption parameter produces the contrary effect. The outcomes of this study can be relevant to the field of biomedical sciences and devices for improving heat transfer efficiency, chemical synthesis, enhancing the performances of micro-electro-mechanical systems (MEMS) and mini-devices, heating and energy generation, designing efficient energy conversion processes, and so on.

Studies on heat transfer performance of miniature cross-flow heat exchangers with perforated fins for direct methanol fuel cells

Despite the widespread industrial application of finned micro heat exchangers, there is still a need to enhance the heat transfer efficiency. This study focuses on fin perforation research to improve heat transfer efficiency. In this study, a miniature cross-flow heat exchanger (heat source is a heat stream at 351.35 K) was simulated. By employing a combination of experimental methods and numerical simulations, the investigation delves into the impact of mixed shapes and innovative spatial distributions of perforations on fin convective heat transfer characteristics. The findings demonstrate that square and square-and-diamond perforated fins exhibit a temperature drop increase of 10.1 % and 8.9 %, respectively, in comparison to unperforated fins during forced convection. Fin temperature drop is augmented by 12.3 % with differentially staggered perforations on adjacent fins (spatially staggered perforations) in contrast to parallel perforated fins, and by 23.6 % compared to non-perforated fins. Furthermore, the optimal performance of the fin heat transfer is observed at 5.74 % perforated area occupancy. The study reveals a positive correlation between the Nusselt and Reynolds numbers. Moreover, the spatial distribution of perforations significantly enhances the performance evaluation criteria compared to the perforation shape. Ultimately, the implementation of fin perforation technique is considered to offer more consistent power output and an extended range for direct methanol fuel cells.

Pore size optimization and fluid channel analysis of composite wick and anti-gravity study

Flat plate micro heat pipes (FPMHPs) utilize the vapor–liquid two-phase circulation of the working medium to transfer heat and play an essential role in heat dissipation systems. But the effects of capillary force and porosity on the flow channel’s heat transfer characteristics and the working medium’s anti-gravity properties are not clarified. It is found in the heat transfer theory that balancing the capillary force and porosity of the composite wick can improve heat transfer efficiency, increase the maximum thermal power and anti-gravity of the FPMHP. FPMHPs with composite wicks of different pore sizes were developed and tested by the established test platform. The results indicated that the capillary force and porosity of the composite wick constrained each other, and the FPMHP made of the composite wick sintered with 80 mesh copper powder (CW80-FPMHP) had the best heat transfer characteristics and anti-gravity properties. The maximum thermal power of CW80-FPMHP is 38.6 W at 0°, which is 5.4%, 3.6%, 3.5% and 17% higher than that of CW60-FPMHP, CW70-FPMHP, CW90-FPMHP and CW100-FPMHP, respectively. At a tilt of 45°, the maximum thermal power of CW80-FPMHP is 26.27 W, which is 17.6%, 24.3%, 53.9%, 53.9%, and 51.8% higher than CW60-FPMHP, CW70-FPMHP, CW90-FPMHP, and CW100-FPMHP, respectively. In addition, the thermal resistance of the vapor channel in CW80-FPMHP is lower than that of the liquid flow channel. Gravity affects the thermal conductivity of the vapor and liquid flow channels, which in turn affects the performance of the FPMHP. Balancing the capillary force and porosity can improve the performance of the FPMHP under various operating angles.

Nano-Iron Oxide Coating for Enhanced Heat Transfer in Gas–Solid Fluidized Bed Systems

This study explores using iron oxide coatings on glass beads to improve heat transfer efficiency in fluidized bed reactors. Techniques such as BET surface area analysis, SEM imaging, and X-ray diffraction were used to characterize the coated beads. Results showed the successful creation of a crystalline iron layer on the beads’ surface and increased thermal conductivity, especially at elevated temperatures. The study also quantified the impact of air surface velocity and heating power on the heat transfer coefficient, revealing substantial improvements, especially at higher velocities. It was found that the heat transfer coefficient for 600 µm glass beads increases significantly from 336.4 W/m2·K to 390.3 W/m2·K when the velocity is 0.27 m/s and the heating flux is 125 W. This demonstrates the effectiveness of the iron oxide coating in improving heat transfer. The results of this study emphasize the efficacy of iron oxide coatings in augmenting heat transmission characteristics, particularly in fluidized bed reactor.

Application of performance dynamic equationin numerical simulation and optimization of waste heat utilization and storage system

In order to improve the current situation of double energy loss in waste heat recovery and utilization, according to the factors such as phase change temperature, thermal conductivity, phase change latent heat, economy, and safety of thermal storage materials, 70# phase change thermal storage balls produced by XX company were selected as phase change thermal storage materials, and differential scanning calorimetry was used to test and analyze them, and the melting point, freezing point and phase change latent heat value of PCM were obtained, provide data reference for simulation in the following text. The shell and tube heat exchanger is selected as the core component of the mobile heat storage system, and its heat storage and release processes are simulated using FLUENT software. The results show that when the heat transfer equipment is charged for 7 hours, almost all of the inner-heat exchangers have completed the heat supply, and only the phase change products at both sides and bottom of the heat supply have ?dead zone?, which is the major effect of heat storage. In order to improve the heat storage and release rate and break the ?bottleneck? of heat storage, the heat transfer was strengthened by changing the diameter size, arrangement, and adding fins of the heat exchange tube. The effects of straight fins, T-shaped fins, as well as the number, height, thickness, and width of fins on the heat storage performance were explored. The results indicate that, adding fins cannot only improve heat transfer efficiency, but also inhibit natural-convection. The heat storage and release time decreases to varying degrees with the increase of fin width, thickness, height, and the number of circumferential fins in a single heat exchange tube.

Automotive Radiators: An Experimental Analysis of Hybrid Nanocoolant

A hybrid nanocoolant is a novel type of heat transfer enhancement medium that has the potential to enhance the performance of automotive radiators by improving heat transfer efficiency and heat dissipation. The focus of the present work was to investigate the effect of different hybrid nanocoolant mixing ratios on Reynolds number, Nusselt number, Friction factor, heat transfer coefficient and convective heat transfer on heat transfer performance. Single and its hybrid nanocoolant were tested through a commercial-sized automotive radiator and a scaled-down automotive radiator to determine its laminar convective heat transfer. The nanocoolants are prepared with a fixed volume fraction of 0.01 vol% and for hybrid nanocoolants, different ratios of CNC and CuO nanoparticles are formulated. The studies utilised flow rates of 0.75, 1.00, and 1.25 LPM with a radiator inlet liquid temperature of 80°C. The experimental results show that the Reynolds number, Nusselt Number, heat transfer coefficient and convective heat transfer are proportionally related to the volumetric flow rate, while the friction factor decreases when there is an increase in the flow rate. A scale-down radiator with a low-volume concentration of hybrid nanofluids able to improve the heat transfer efficiency by 92.43% compared to conventional fluids in a commercial-sized car radiator.

Structure and migration mechanism of thin liquid film in vicinity of advancing contact line

Thin liquid film near the gas-liquid-solid three phase contact line is the core area of oil and gas production, phase-change heat transfer, and material synthesis systems. Although there are many experimental studies on fluid dynamics in the contact line region, the prediction of contact angle is still difficult, and the bottleneck lies in the special structure of thin liquid film in the contact line region. Because the microstructure of thin liquid film is not well understood, the prediction of dynamic contact angle is always controversial. At present, the main controversial points focus on whether the microscopic contact angle changes with speed, and whether the microscopic contact angle is the same as the macroscopic contact angle. Therefore, it is crucial to monitor the dynamic process of the microscopic contact angle in the thin liquid film region of the contact line. In this work, the wetting system of 50 nm liquid droplets on different solid surfaces is constructed by molecular dynamics simulation, and the structure and migration mechanism of thin liquid film are studied. The structure of the precursor liquid film in the completely wetting droplet advancing contact line region and the nanoscale convex structure in the partially wetting droplet advancing contact line region are obtained. The precursor liquid film is 2–3 molecular layers in thickness, leading the droplet to move forward. However, there is no precursor liquid film in a partially wetting system, and the convex nano-bending larger than 10 nm is formed in the wetting process, resulting in the microscopic contact angle. By comparing the difference between the absolute smooth surface dynamic wetting process and the actual solid surface dynamic wetting process, the dynamic evolution law of the micro contact angle and the macro contact angle with time are obtained for the first time in the simulation. The liquid molecules in the contact line region are tracked and statistically analyzed by means of particle tracer. It is revealed that the liquid molecules in thin liquid film change from sliding mode to rolling mode with speed increasing under the action of solid surface friction, and then the air entrainment at the bottom of the contact line leads to slip and sputtering. The research results are expected to provide theoretical guidance for the following three directions: 1) improving heat transfer efficiency of micro and nano device based on wettability control; 2) improving the imbibition displacement efficiency of shale oil micro-nano matrix based on wettability regulation; 3) constructing universal contact angle prediction model.

EHD flow and heat transfer of hybrid nanofluid in a free surface cavity fitted with an internal hot obstacle

The objective of this study is to numerically investigate the effects of electrohydrodynamics (EHD) on coupled free convective thermocapillary flow. The study focuses on a free-surface cavity filled with a hybrid nanofluid (MWCTN- Fe3O4-thermal oil) and containing a hot rigid obstacle. An electric field is applied between the obstacle and the lower cavity wall to induce additional instabilities in this region. This technique aims to eliminate the dead zone typically present in traditional natural convection and is expected to greatly improve heat transfer efficiency. To achieve this objective, the equations governing the EHD phenomenon, as well as the equations defining the physicochemical properties of the hybrid nanofluid, are solved using the finite volume method (FVM) coupled with a blocked-off region procedure. A parametric study is conducted, involving the Marangoni number (surface tension), the thermal Rayleigh number (buoyancy force), the electrical Rayleigh number (electrical force), the nanoparticle concentration, the size and the position of the obstacle. It has been demonstrated that applying of an electric field to the convective-thermocapillary flow of the nanofluid (combining two enhancement techniques) significantly enhances heat transfer, resulting in a more than 230 % increase in the Nusselt number.