Heat Flux Transfer Research Articles

Microfluidic controllable synthesis and size-dependent flow boiling heat transfer of silica nanofluids

Microchannel flow boiling heat transfer with nanofluids is an effective approach to enhance the heat transfer performance of high-power electronics. However, conventional nanofluids are plagued by laborious fabrication procedures, poor stability, and uncontrollable nanoparticle sizes, which compromise the overall heat transfer efficiency. To address these challenges, a facile microfluidic synthesis strategy is developed to prepare highly stable and size-controllable silica nanofluids in a continuous, efficient, and high-throughput methodology. The introduction of silica nanofluid aims to facilitate surface bubble formation and delay coalescence, ultimately enhancing the critical heat flux (CHF) and heat transfer coefficient (HTC) for efficiently cooling silica chips. Through the specifically designed spiral microchannel reactor, nanofluids with controllable nanoparticle sizes ranging from several tens of nanometers to a few hundreds of nanometers can be synthesized with long-term (>30 days) dispersion stability at both room temperature (25 °C) and high temperature (75 °C). Importantly, with negligible changes in pressure drop, the CHF and HTC of the synthesized nanofluids increase by maximum of 55 % and 63 % respectively compared to the basic fluid. Furthermore, our results suggest that smaller-sized nanoparticles outperform larger ones at higher flow rates. These findings not only provide important guidelines for the controllable preparation of nanofluids, but also shed new light on the rational design of efficient nanosystems for two-phase cooling.

Drift current-induced tunable near-field energy transfer between twist magnetic Weyl semimetals and graphene

Abstract Both the nonreciprocal surface modes in Weyl semimetal (WSM) with a large anomalous Hall effect and the nonreciprocal photon occupation number on a graphene surface induced by the drift current provide a promising way to manipulate the nonreciprocal near-field energy transfer. Interestingly, the interactions between nonreciprocities are highly important for research in (thermal) photonics but remain challenging. In this study, we theoretically investigated the near-field radiative heat flux transfer between a graphene heterostructure supported by a magnetic WSM and a twist-Weyl semimetal (T-WSM). The nonreciprocal surface mode could be changed by the separation space between two Weyl nodes and the twist angle. Notably, we found that in the absence of a temperature difference between two parallel plates, nonequilibrium fluctuations caused by drift currents led to the transfer of near-field radiative heat flux. Furthermore, these nonreciprocal surface modes interacted with the nonreciprocal photon occupation number in graphene to achieve flexible manipulation of the near-field heat flux size and direction. Additionally, graphene adjustable flux in the case of a temperature difference between the two plates was also discussed. Our scheme can provide a reference for near-field heat flux regulation in nonequilibrium systems.

Influence of smooth heater size on critical heat flux and heat transfer coefficient of saturated pool boiling heat transfer

In order to study the influence of smooth heater size on the performance of pool boiling heat transfer, saturated boiling experiments of FC-72 were conducted on eight sized smooth silicon chips (the test surfaces are denoted as S5 ∼ S30, and their dimensionless dimensions are in the range of Lh/Lc = 7 ∼ 42). The bubble dynamics during boiling were observed with a high-speed camera. The results indicated that in natural convection regime, the heat transfer coefficient decreases with increasing heater size at a given heat flux. Besides, the larger surfaces enter into the nucleate boiling regime earlier. In a fully developed nucleate boiling regime, S5 surface displays the optimal heat transfer performance. The reality is that no vapor columns are produced even in the high heat flux range due to its small bubble departure diameter and high bubble departure frequency. For S8 ∼ S15 surfaces, the wall temperatures increase continuously with the increase of heat flux, while S20 ∼ S30 surfaces present a trend of first keeping constant and then increasing. The active nucleation site densities on S15 ∼ S30 surfaces follow the law of Na = 0.45q2 in the whole nucleate boiling regime. By contrast, for S5 ∼ S12 surfaces, the nucleation site densities abide by this law only in a fully developed nucleate boiling regime. In addition, the departure diameter and departure frequency of bubbles increase with the increase of heat flux for all surfaces. It is observed that with the increase of smooth heater size, CHF presents a changing trend of first increasing (in range of Lh ≤ λD, λD is the most dangerous Taylor wavelength), then decreasing (in range of λD < Lh ≤ 3λD) and finally leveling off (in range of Lh > 3λD). Accordingly, the CHF value maximizes at the heater size Lh = λD. In the segment of decreased CHF, the decline rate of CHF in range of λD < Lh ≤ 2λD is larger than that in range of 2λD < Lh ≤ 3λD. Meanwhile, the influencing mechanisms of heater size on CHF were explained by consideration into bubble patterns and behavior.

Coupling dynamic thermal analysis and surface modification to enhance heat dissipation of R410A spray cooling for high-power electronics

With high critical heat flux (CHF) and heat transfer coefficient (HTC), spray cooling is considered as one of the most promising thermal management technologies for high-power electronic devices. To increase its cooling performance, a closed-loop experimental rig was constructed to study the effects of spray and system parameters on heat transfer enhancement by R410A. The best cooling performance can be achieved under optimal subcooling degree of 17 °C and nozzle diameter of 0.56 mm. When the compressor frequency reaches the upper limit of 90 Hz, maximum CHF and HTC on flat surface are 301.6 W/cm2 and 91.7 kW/(m2·K). To further improve CHF, mechanism of heat transfer enhancement by square pin finned surface was revealed in terms of droplet splashing. With fin width of 0.5 mm and height of 3 mm, CHF as high as 522.1 W/cm2 and peak HTC of 407.0 kW/(m2·K) are reached, while maintaining the cooling surface temperature lower than 55.6 °C. Compared to flat surface, CHF and HTC are enhanced by around 73.4% and 3.5 times, respectively. Based on the experimental data, CHF correlation applicable to pin finned surface was obtained with precision of ±12.4% by introducing fin height and width.

Experimental investigation into flow boiling heat transfer and pressure drop in porous coated microchannels

The flow boiling heat transfer and pressure drop in porous coated micro-channels were experimentally investigated at four porous coating particle sizes (i.e. 75 μm, 48 μm, 25 μm, and 13 μm), three mass flow rates (150 kg/m2s, 275 kg/m2s, and 400 kg/m2s), and a range of wall heat fluxes (0–1285 kW/m2). The comprehensive heat transfer (CHT) performance in the porous micro-channels was obtained and compared to the smooth micro-channel. The flow patterns in the micro-channels were visualized to analyze the flow boiling heat transfer characteristics near wall region. The results show that the heat transfer coefficient and critical heat flux were significantly enhanced and the pressure drop was increased in porous coating micro-channels as compared to the smooth micro-channel. As the size of sintered particle increases, the heat transfer coefficient on porous coating surface is increased, while the pressure drop in the micro-channel is increased. Due to its good liquid storage and re-wetting ability, the porous coating surface has a higher hydrophilicity, resulting in a higher critical heat flux in the porous micro-channel than the smooth channel. However, the CHT performance of the porous coating surface is not always better than the smooth surface. Among the studied porous-coating surfaces, the micro-channel with the averaged sintered particle size 13 μm has the best CHT, while the surface with 75 μm sintered particles has the highest critical heat flux and heat transfer coefficient. Compared to the smooth surface, the critical heat flux and heat transfer coefficient on the 75 μm porous coating surface were enhanced by 53.2 % and 114.5 %, respectively.

Gravity modulation, thermal radiation and viscous dissipation impact on heat transfer and magnetic flux across gravity-driven magnetized circular cylinder

Heat transfer in burners, combustion engines and energy consumption through nuclear explosions is significantly influenced by radiations. In missile nozzles, nuclear power plants for aerospace uses, and gaseous-core nuclear rocket systems, the radiations are considered for evaluating heating significance. The significance of present study is to compute heat and magnetic flux behavior across various angles π/6, π/4, π/3, π/2, π and 3π/2 of gravity-driven magnetized cylinder under gravity modulation, thermal radiation and viscous dissipation. The aim of this research is to examine wave oscillations in heat and current density along magnetic-driven circular cylinder. The computational and mathematical model is evaluated in terms of coupled periodic partial differential equations (PDE). The appropriate dimensionless variables are used to convert governing periodic model into non-dimensional form. The dimensionless formulations are transformed into steady and oscillatory form. To explore physical and numerical findings, the finite difference method with primitive formulations is applied for smooth algorithm in FORTRAN language. The significant results are explored for various pertinent parameters. The oscillatory magnetic flux and heat transmission are plotted by using steady state temperature and magnetic boundary layers. It is found that the prominent amplitude in velocity increases as reduced gravity increases at each angle. The maximum oscillating amplitude in heat transfer enhances at each angle as viscous dissipations and solar radiation increases.

Effects of Brownian Motions and Fractal Structure of Nanoparticles on Natural Convection

The study simulated heat transfer in alumina-water nanofluid in a natural convection flow and Rayleigh-Benard configuration considering the Brownian motions and fractal structure of the nanofluids. The simulations were based on a two-dimensional, Eulerian-Eulerian method. Many simulations have been performed to examine the effect of aspect ratio, heat flux, and para-meters related to the structure of the nanoclusters including size, fractal dimension, and volume fraction on the natural convective heat transfer coefficient. The comparison between the simulation results and the experimental data of heat transfer coefficient indicates a good agreement. The simulation results indicated that the enhancement of aspect ratio, heat flux, and fractal dimension increases the heat transfer coefficient. On the other hand, the reduction of nanoclusters and nanoparticle size decreased this coefficient. Moreover, the simulation results showed that in high heat transfer fluxes, the heat transfer coefficient first increases by increasing the nanoparticles solid volume fraction and then decreases. However, heat transfer coefficient decreased steadily with the increase in the nanoparticles solid volume fraction in low heat transfer fluxes. The results suggested that using the nanoparticles Brownian motion mechanism along with their fractal structure can be well-applied in natural-convection heat transfer modelling of nanofluids.

Heat transfer and turbulent heat flux budgets in cooling films

Statistics from large-eddy-simulation (LES) of cooling films with different cooling holes are used to evaluate budgets in the transport equation of turbulent heat flux. The capability of LES has been assessed by comparing simulated results with experimental data, while the correctness of the procedure generating turbulent heat flux budgets has been examined on a turbulent boundary layer. The mechanism of heat transfer has been preliminarily studied throughout the three-dimensional flow field at different blowing ratios using a general outer scaling, to recognize specific regions with corresponding heat transfer patterns. A compressible version of budget terms in the transport equation of turbulent heat flux is then explored to show the thermal behavior of flow downstream from the cooling film holes. Characteristics of each budget term are presented in a defect scaling consistent with the scaling suitable for Reynolds stresses and Reynolds stress budgets. Furthermore, these budget terms are compared among an array of different cooling hole shapes combined with different blowing ratios to explore self-similarity. Results show that the dynamic balance of all budget terms is significantly influenced by the cooling hole shape, while each turbulent heat flux budget term may scale with the velocity defect, temperature deficit, and the normalized streamwise distance to the power of an exponent dependent on the hole shape.

Oscillatory and transient role of heat transfer and magnetic flux around magnetic-driven stretching cylinder under convective boundary conditions

The magnetic outcomes are frequently implemented for assessing fluid specifications around stretching magnetized surfaces and energy producing technology of thermodynamics. Magnetohydrodynamic reactors and magneto-hydrodynamic generators are two primary methods for producing magnetic flux through ionized fluids along a warmed and magnetized stretched surface. The desired outcomes of the current work are to investigate magnetic flux and heat transmission in the presence of convective boundary conditions using an electrically-conducting fluid over a stretchable cylinder. Most of the parameters are considered in the discussion with intensity and variance due to time dependent methodology. The suggested fluctuating dynamic computational framework is formulated for associated forms of coupled model under boundary variables. The appropriate non-dimensional variables are used to transform the complicated mathematical expression into a non-dimensional representation. To make the non-dimensional representation easier to work for efficient statistical calculations, it is reduced further. Later, for a variety of variables, significant computations are performed via the implicit finite difference methodology. The main finding of current communication is to explore the prominent slip effect in temperatures for every Biot number around all positions. From a physical standpoint, the phenomenon was predictable since surface heat flux is employed as sporting source to increase heat transmission in electric-conducting fluids. For every possible angle, the field of magnetism increases with minimal intensity of surface heat flux. Since, magnetizing functions referred to as an insulating layer that diminishes high temperatures across surfaces and fluids.

Critical heat flux and heat transfer behaviors on aged RPV outer wall surface in flow boiling under IVR conditions

Critical Heat Flux (CHF) plays an important role in the nuclear power plant safety strategy both during normal operation and under accident conditions. During normal operation, the outside wall is exposed to high-temperature environment. The surface is oxidized by air, which is so-called “aged surface”. To specify the characteristics of aged surface and the effect on CHF, the specimen aging experiments and flow boiling experiments under External Reactor Vessel Cooling (ERVC) were conducted. The results of specimen aging experiments show that surface characteristics get steady after 500 h aging and the oxide layer is the Fe-Fe2O3-Fe3O4 mixture. After aging, the surface gets rougher and has better hydrophily. In the flow boiling experiments, characteristics of heat transfer and CHF are studied. Heat transfer is enhanced by extended surface area and increased nucleation sites at lower angle. The influence of capillary suction is not obvious and the enhancement of CHF is limited. As the inclination angle increases, the departure of bubbles is more frequent and capillary suction is dominant to assist liquid supply into the liquid sublayer and delay the boiling process. At higher angle, CHF is more obviously enhanced but heat transfer gets worse. Overall, CHF of RPV outer wall is increased after surface aging, and it is beneficial to improving the safety margin of IVR mitigation measures.

Oscillatory and Periodical Behavior of Heat Transfer and Magnetic Flux along Magnetic-Driven Cylinder with Viscous Dissipation and Joule Heating Effects

Several primary mechanisms are less utilized in engineering and recent technologies due to unsustainable heating. The impact of viscous dissipation and Joule heating is very important to examine current density and heat rate across a magnetized cylinder. The key objective of this examination was to insulate excessive heat around the cylinder. The present effort investigated the impact of viscous dissipations, Joule heating, and magnetohydrodynamics (MHD) on the transitory motion of convective-heat transport and magnetic flux features of dissipative flows throughout a magnetized and warmed cylinder at suitable places. The suggested turbulent dynamical structure of mathematics is offered for an associated method of partial differentiation equations impacted by boundary values. The complex equations are translated via non-dimensional shapes by using relevant non-dimensional numbers. The non-dimensional representation has been improved to make it easier to conduct uniform computational calculations. The computational answers for these linked dimensionalized formulations have been achieved using the Prandtl coefficient Pr, Joule heating parameter ζ, Eckert number Ec, the magneto-force number ξ, the buoyancy parameter λ, and multiple additional predefined factors. The important contribution of this work is based on non-fluctuating solutions that are utilized to examine the oscillating behavior of shearing stress, rate of fluctuating heat transport, and rate of fluctuating magnetic flux in the presence of viscous dissipation and Joule heating at prominent angles. It is shown that the velocity of a fluid increases as the buoyancy parameter increases. The maximum frequency of heat transmission is illustrated for each Eckert variable.

Heat transfer distribution and pressure drop fluctuations in subcooled flow boiling at subatmospheric system pressure

The transfer of higher heat flux at subatmospheric system pressure is relevant to different applications where the higher surface temperature is a constraint. There is a scarcity of literature on subcooled flow boiling at subatmospheric system pressure. The experiments are performed at 50, 75 and 101 kPa absolute system pressure in an SS-304 tube of 5.85 mm radius, heated length of 1500 mm and 0.5 mm thickness. The mass flux of 90–300 kg/m2s is streamed at the varying heat flux of 48–218 kW/m2. The heat transfer coefficient in single-phase flow is not impacted significantly by the subatmospheric system pressure. The subcooled flow boiling heat transfer coefficient is increased at the lower subatmospheric system pressure. The more nucleation sites due to higher wall superheat are activated at the higher heat flux at subatmospheric system pressure. The surface temperature is radially symmetric at subatmospheric system pressure for all values of the Froude number. The gravitational force is suppressed at subatmospheric system pressure. The onset of nucleation is achieved earlier in the axial direction at the higher heat flux. The mass flux has no significant effect on the subcooled flow boiling heat transfer coefficient. The pressure drop and fluctuations are enhanced at the higher mass flux, higher heat flux and lower subatmospheric system pressure.

Dropwise condensation heat transfer on vertical superhydrophobic surfaces with fractal microgrooves in steam

Despite dropwise condensation witnesses the superior heat transfer performance with in-time removal of condensate, pinning and flooding phenomena significantly limit its application at high subcooling. In this study, we design and fabricate a self-similar fractal groove superhydrophobic surface inspired by the self-similar fractal spines of cactus thorns. A visualization and condensation heat transfer measurement experimental platform is established to investigate droplet dynamics and heat transfer performance on the vertical self-similar fractal groove superhydrophobic surface. Dropwise condensation is analyzed and compared with that on plane and groove superhydrophobic surfaces. The fractal grooves promote spontaneous coalescence-induced jumping of mismatched condensate droplets at low subcooling, resulting in a higher probability and frequency of droplet jumping. Furthermore, second-level grooves constrain the shape of large droplets more effectively and facilitate condensate removal after the failure of first-level grooves at high subcooling. In agreement with droplet dynamics, the heat transfer measurements demonstrate that the fractal groove superhydrophobic surface achieves the highest condensation heat flux and heat transfer coefficient among the three investigated superhydrophobic surfaces as subcooling increases.

An open superhydrophilic microchannel heat sink for thin film boiling with a high coefficient of performance

Currently, the existing two-phase boiling heat transfer technologies have limitations in extremely narrow spaces and cannot satisfy the high heat flux dissipation requirements of highly integrated and ultrathin devices. In this study, a high-efficiency and low-cost open superhydrophilic microchannel heat sink with a micro/nanocomposite structure, negligible pumping energy, and a high coefficient of performance (COP) was achieved. The wettability and microscopic morphology of the different heat sinks as well as the variation in the critical heat flux (CHF) and heat transfer coefficient (HTC) at different mass fluxes were discussed. When the mass flux was 0.05 kg/m2⋅s, the CHF of the microgrooved composite nanostructured copper (MCNC) heat sink was 173.90 W/cm2, which was 182.80% higher than that of the smooth copper (SC) heat sink and 20% higher than that of the microgrooved copper (MC) heat sink. In addition, the highest CHF of 325.90 W/cm2 was obtained for the MCNC at a 0.15 kg/m2⋅s mass flux. The high CHF of thin film boiling was achieved at a low inlet water pressure. This resulted in a COP of 695,600 for this boiling method, which is a larger value than that in most reports thus far. This study provides a preliminary investigation of the thin film boiling performance for an open microchannel. This may provide a new solution to the challenge of high heat flux dissipation in extreme spaces.

A hybrid substrate for practical applications in dropwise condensation enhancement

We introduce a durable hybrid substrate consisting of superhydrophilic micropillars surrounded by superhydrophobic depressions for practical industrial applications. The proposed surface can be mass-produced via a facile and affordable method. Moreover, the stability tests show that the wettability properties of fabricated surfaces do not vary after the imposition of hot steam flow for 110 h. Two hybrid samples with different patterns of micropillars are compared with superhydrophobic and bare aluminum samples to explore the physics behind the condensation improvement ability of hybrid surfaces. The results reveal that the heat transfer coefficient and heat flux can be significantly increased with the incorporation of micropillars with optimized dimensions. Among the tested surfaces, the hybrid one, whose pillar's diameters are 500 μm, increases the heat transfer coefficient by 33.50% and 19.60% with respect to the superhydrophobic and bare surfaces, respectively, at a subcooling temperature of 18.50 °C.

Cool skin effect and warm skin phenomenon observed by shipboard radiometer in the Northwest Pacific

Sea surface temperature (SST) is an important variable in the study of ocean boundary layers and heat exchange. The accurate simulation and measurement of skin effects are vital to air–sea model processing and satellite SST retrieval. Shipboard measurements from eleven cruises in the Northwest Pacific between August 2015 and October 2018 were used to estimate the cool skin effect and compare model results. The temperature difference ΔT between the sea surface skin temperature (SSTskin), as measured by an infrared radiometer, and the sea surface depth temperature (SSTdepth) at around 4 meters showed a mean difference and a standard deviation of the same 0.2 K, with a total of 5-min 39909 measurements. Both daytime and nighttime ΔT values were compared to physical model simulations and were found to have relatively larger mean values. A set of new coefficients for an exponential parameterization of the cool skin effect was derived in the research area, which performed well in comparison to previous empirical models. In nighttime observations from two summer cruises, the reverse process of heat flux transfer from the air to the sea in the form of a warm skin was distinguished. There were 667 positive ΔT values out of the 1917 nighttime observations, with magnitudes ranging from around 0 to 0.3 K. A high proportion of the cases of the warm skin phenomenon occurred when the air was very humid and much warmer than the sea surface.

Molecular Dynamics Simulation Study on Thermal Transport in Graphyne/Polyaniline Composite System

AbstractGraphyne (GY) is a new carbon material with excellent electrical conductivity and low thermal conductivity (TC). The doping of GY into polymers to improve the thermoelectric properties of the material has become a hot research trend. In this study, molecular dynamics (MD) and nonequilibrium MD are used to study the effect of the number of oxidation units of polyaniline (PANI) on TC and heat transfer of PANI and GY/PANI systems. The geometric structure of polymer, interaction energy, and heat transport of all systems are studied and analyzed. It is found that (1) as the number of oxidation units of PANI increases, the interchain and intrachain heat transfers of PANI are decreased thereby decreasing the heat transfer of the PANI chains; (2) the weak interaction energy at the interface hinders the heat flux transfer, and the phonon vibration of GY and PANI mismatch at the interfaces; eventually the above reasons lead to low interface TC; (3) the doping of GY can effectively reduce the TC of the system. This study provides some research ideas and theoretical exploration for the application of polymer doped with GY composites in the field of thermoelectricity.

Pool Boiling Heat Transfer Enhancement through Biphilic Stepped Microchannel

In present study, pool boiling heat transfer (PBHT) from biphilic stepped microchannel comprising: hydrophobic fin top and hydrophilic channel region is investigated. The biphilic stepped microchannel is prepared by mechanical polishing and thermo-catalytic etching. The improved liquid supply pattern, increased nucleation site density, retarded bubble coalescence between the adjacent channel and decreased wall forces acting on the bubble meniscus resulted in the PBHT enhancement. Contact angle of the water droplet on hydrophobic and hydrophilic surface is 74.02° and 22.5°, respectively. Enhancement in critical heat flux and heat transfer coefficient by the biphilic stepped microchannel is 195.52% and 367.00%, respectively.

Oscillatory Behavior of Heat Transfer and Magnetic Flux of Electrically Conductive Fluid Flow along Magnetized Cylinder with Variable Surface Temperature

The present study deals with electrically conductive fluid flow across a heated circular cylinder to examine the oscillatory magnetic flux and heat transfer in the presence of variable surface temperature. The proposed mathematical formulation is time-dependent, which is the source of the amplitude and fluctuation in this analysis. The designed fluctuating nonlinear computational model is associated with the differential equations under specific boundary conditions. The governing equations are converted into dimensionless form by using adequate dimensionless variables. To simplify the resolution of the set of governing equations, it is further reduced. The effects of surface temperature parameter β, magnetic force number ξ, buoyancy parameter λ, Prandtl number Pr, and magnetic Prandtl parameter γ are investigated. The main finding of the current study is related to the determination of the temperature distribution for each inclination angle. It is seen that a higher amplitude of the heat transfer rate occurs as the surface temperature increases. It is also noticed that the oscillatory magnetic flux becomes more important as the magnetic Prandtl number increases at each position. The present magneto-thermal analysis is significantly important in practical applications such as power plants, thermally insulated engines, and nuclear reactor cooling.

Gradient microstructures for flow-boiling enhancement

Herein, we apply grayscale lithography to produce novel gradient microstructures, demonstrating the enhancement of flow boiling through the manipulation of drag forces upon the forming bubbles. The use of engineered 3D anisotropy allowed directional control of boiling enhancement, with respective critical heat flux and peak heat transfer coefficient enhancements of up to 20% and 93% observed over the unstructured substrate. It is our suspicion that the sheltering of bubbles inside of cavity microstructures deteriorates their departure characteristics, and that the combination of induced turbulence and available surfaces to capture passing liquid is beneficial for protruding microstructures. We demonstrate the promising novel implementation of gradient microstructures in the boiling discipline, which may serve as a useful foundation for future work. The enclosed approach to boiling enhancement shows a potential future direction for engineered boiling microstructures, wherein bubble dynamics are directly manipulated on bespoke, 3-dimensional substrates, allowing the transfer of large heat fluxes at low wall temperatures.