Convective Heat Transfer Characteristics Research Articles

Evaluation of nanoparticle shape factor on a laminar forced convective heat transfer characteristics of various nanofluids flow in a tube using single-phase numerical model

Nanotechnology is advantageous in improving thermophysical properties and enhancing heat transfer rate compared with conventional fluids due to their superior thermophysical properties. These properties vary with many parameters such as concentration and shape. In this study, the effect of nanoparticles shape is numerically explored on heat transfer performance under a laminar flow regime (Re=500 and 2000) through the smooth tube. Heat transfer enhancement capability of the nanoparticle shapes with targeted reference to average Nusselt number, average Darcy friction factor, pumping power, and performance evaluation criterion have been investigated. Three types of nanofluids (Fe3O4/water, Al2O3/water, and GO/water) with various nanoparticle shapes (brick, cylindrical, platelet, and spherical) and different nanoparticle volume fractions (φ=1.0, 2.0, 3.0, and 4.0%) have been used as heat transfer fluid in analyzes. Numerical results show that heat transfer performance was greatly influenced by changing nanoparticle shapes. The highest average Nusselt number was obtained for GO/water nanofluid with platelet nanoparticle shape and φ=4.0%. Compared to water, Fe3O4/water, and Al2O3/water, the average Nusselt number in GO/water increased by 64.34%, 54.02%, and 43.41%, respectively. The highest performance evaluaton criterionwas obtained for the GO-water nanofluid with platelet nanoparticle shape at Re=2000. On the other hand, it is found that the Fe3O4/water nanofluid with platelets nanoparticle shape causes the highest pumping power compared with other analyzed nanofluids.

Laminar flow and convective heat transfer characteristics of plain and wavy wall cross confined circular cylinder

An attempt is made to identify the convective heat transfer characteristics of the plain wall (Pw) and wavy wall (Ww) cross confined circular cylinder in the laminar flow regime (20 ≤ Re ≤ 100). The study has been extended to identify the wake heat transfer characteristics for various confinements (L = 0.5D, 1D, 1.5D and 2D), such that the weaker and stronger heat transfer region in the downstream of cross-confined cylinder has been investigated. CFD solver Ansys Fluent (Version 18.0) is used for assessing the field variables, the results are analyzed using streamline plots, λ 2-criterion vortex identification method, Nusselt number and field synergy method. The wall confinement is highly significant on heat transfer, the maximum Nusselt number is observed at high confinement (L = 0.5D) for all the Reynolds number studied. The changes in Nusselt number with respect to confinements is more significant at high confinements (L = 0.5D and L = 1D), become less significant at low confinements (L = 1.5D, 2D). It is found around 73% and 50% rise in Nusselt number for the confinement height, L = 0.5D as compared to L = 1D at Reynolds number 20 and100, respectively. Besides, in order to identify the heat stagnation region in the wake, contour of zero temperature gradient along the flow direction is observed. From the contour, it is identified that more heat stagnation region in the wake of Pw confined cylinder as compared to Ww confinement which is due to the three-dimensional behavior of flow in the wake.

Investigation on heat transfer enhancement of supercritical hydrocarbon fuel in rotating channel interpolated with spiral wire

Hypersonic vehicles carry their hydrocarbon fuels as the only regenerative fluid because the existing air-cooled turbine blades cannot meet the stringent heat transfer requirements, including aerodynamic heating in overload environments. In this study, supercritical-pressure n-decane was used as the working medium, and a spiral wire with a wire diameter and pitch of 0.1 and 10 mm was inserted into a smooth circular tube (inner diameter = 2 mm; length = 200 mm) for studying the convective heat transfer characteristics under different rotating speeds, mass flow rates, inlet temperatures, and heat fluxes and investigating the enhanced heat transfer effect of the spiral wire. Experimental results indicate that the overall heat transfer performance of the spiral-wire structure tube was improved by more than 1.3 times compared to that of the smooth tube. Further, at the trailing edge surface at 300 rpm, the Nusselt number increases by an average of two times; in the meanwhile, the PEC value for the reinforced tube is obtained as 1.25–1.68, indicating a better enhanced heat transfer capability in the rotating channel. The experimental results are compared and verified using a computational fluid dynamics method with SST k-ω turbulence model, revealing the maximum relative deviation to be within 13.56 %. The numerical simulation results explain the mechanism of the temperature differences between the leading- and trailing-edge surfaces within the centrifugal section and the reinforcing effect of the interpolated spiral wire. This study provides a novel orientation towards the application of aviation fuel for turbine blade cooling and the efficient heat transfer performance of cooling channels under accelerated conditions.

Estimation of natural convection heat transfer characteristics of rack server in a cavity: experimental and numerical analyzes

Estimation of natural convection heat transfer characteristics of rack server in a cavity: experimental and numerical analyzes

Natural and Forced Convective Heat Transfer Enhancement for Solid Cylinders with Different Geometrical Shapes

Enhancing heat transfer for both natural and forced convection is a common issue for any heat transfer process. Experimental studies have been carried out for six different geometrical shapes of solid bars for natural convection and forced convection with four different air velocities while keeping the same perimeter and length of the solid bars, which means the lateral surface area of the bars is the same. Results reveal that both the natural and forced convective heat transfer characteristics are greatly influenced by the geometrical shape in terms of Nusselt number (Nu), heat transfer coefficient (h), and heat transfer rate (q). In addition, isosceles and cylindrical shape geometry contribute to the lowest and highest heat transfer, respectively. As well, it is obtained from the results that convective heat transfer characteristics are directly related to the cross-sectional area, even if the perimeters are the same. Moreover, among the different geometrical shapes, the isosceles and hexagonal shapes take the shortest and longest duration to attain the steady-state condition in the conductive heat transfer process. The convective heat transfer characteristics are well-validated, with available results for both natural and forced convection heat transfer.

Characteristics of natural convection heat transfer in composite cavity

In order to study the influence of the nature of the mining airspace on the natural convection heat transfer in the unventilated working face, we establish a numerical model of natural convection in a composite cavity containing porous media by CFD method. The effects of different Rayleigh numbers (Ra), porous media thicknesses, and porosities ε on the natural convection temperature and velocity fields and heat dissipation from the heat source were comparatively analyzed. The results show that the larger Ra is, the stronger the natural convection flow and heat transfer in the composite cavity; when the thickness of porous media increases from 0.2 to 0.75, the average Nussel number (Nuave) of the high-temperature wall surface decreases by 40.38%, the Nuave of the interfaces decreases by 6.15%, the maximum dimensionless velocity of the boundary layer in the porous region decreases from 10,771 to 3775, and the dimensionless horizontal line in the center of cavity decreases by 0.05, indicating that the natural convection temperature and velocity fields and heat dissipation from the heat source are affected by different Ra. Temperature decreased by 0.05, demonstrating that the increase in the thickness of porous media will weaken the strength of the fluid side of the flow and heat transfer; porosity increased from 0.2 to 0.8, and the interfacial Nuave increased by 5.61%, demonstrating that the larger porosity makes the porous media area flow and heat transfer enhancement, but has a certain weakening effect on the flow and heat transfer in the fluid domain. The results of the study can be used for the natural convection flow and heat transfer in the unventilated coal mining face, and provide some reference value for avoiding the thermal damage of mechanical equipment.

Experimental study on the cooling effect of fine water mist on the thermal runaway in a single lithium ion battery

Lithium ion battery (LIB) has been widely utilized, owing to the various advantages. However, extreme thermal hazard, namely the thermal runaway (TR), can be triggered under abusive conditions, leading to the fire or even explosion hazard. Unfortunately, efficient mitigation strategy is still lack, especially for the confined spaces. In this work, the fine water mist (FWM) is introduced into the air flow, and the cooling effect has been comprehensively study. Results show that the cooling capacity can be significantly improved with FWM droplets addition. The maximum improvement reaches 29.5 %, and the local evaporation of droplets will dominate the cooling process when the FWM flow rate exceeds 0.6 mLs−1. With the increase of air speed, the cooling capacity also increases, but the retention time of droplets in the high temperature region has been significantly shortened, leading to the decreased cooling efficiency. Inhomogeneous cooling process has been revealed, and the cooling power is found to decrease with the decreasing temperature. Moreover, the FWM cooling presents basic characteristics of convection heat transfer. The average cooling coefficient shows close relationship with the droplet content. The FWM can be used to efficiently mitigate the TR hazard, which is promising for practical utilization.

Lattice Boltzmann modeling of convective heat and solute transfer in additive manufacturing of multi-component alloys

Additive manufacturing (AM) is a remarkable breakthrough technology, allowing for the direct fabrication of three-dimensional components through the layer-by-layer stacking of materials. A novel free Surface lattice Boltzmann (LB) model is developed to simulate the heat and solute transfer in AM of multi-component alloys. The behavior liquid phase is described by using the free surface LB model, and the phase transitions between solid and liquid are modeled by using the LB-enthalpy method. A LB equation is directly constructed, which accounts for solute transfer in a certain multi-component alloys system. The thermodynamic information used in the calculation of phase transitions are determined by an extensive thermodynamic database. The model is validated via several benchmark examples of fluid flow and heat transfer. The characteristics of convective heat and solute transfer within various AM are investigated. Finally, the non-equilibrium convective heat transfer, phase transitions, and macroscopic segregation are discussed within the AM melt pools. The melt pool expands and convective heat transfer is enhanced by the Marangoni effect. Thus there is a consistent decrease in solute segregation. The solute segregation is more severe near the surface of the deposit layer. Additionally, the thermal convection experiences cyclic intensification attributed to the successive impact of multiple droplets in wire feeding and melting AM. This continuous impact serves to diminish the solute segregation. The results underscore the significant potential and advantages of LB method in accurately simulating the AM, which provides valuable insights for understanding the underlying mechanism of heat and solute transfer in materials and manufacturing.

Performance evaluation and enhancement of turbulent flow and convective heat transfer characteristics for turbine blade internal cooling

For turbine blade internal cooling, the performance of turbulent flow and convective heat transfer in the cooling channel is numerically investigated via the computational fluid dynamics method, where the cooling channel adopts the combination of transverse rib arrangements and round-edged rib configurations or the combination of oblique rib arrangements and round-edged rib configurations. Under these two combination conditions, the simulation result shows that in the wide Reynolds number range of Re = 20 000–80 000, the heat transfer enhancement and the flow loss reduction, such as the larger normalized Nusselt number of Nu/Nu0 and smaller normalized friction factor of f/f0, are simultaneously realized by the oblique rather than transverse rib arrangement. In the oblique rib channel, the relationship between turbulent flow and convective heat transfer is for the first time revealed by the relationship between secondary vortices and turbulent kinetic energies. Based on the relation between secondary vortices and turbulent kinetic energies, the overall performance of turbulent flow and convective heat transfer for the oblique rib channel is first optimized by various normalized rib height, e/Dh, and rib spacing, p/e, and then evaluated by both overall performance factors of OPT1 and OPT2. Both OPT1 and OPT2 are larger at e/Dh = 0.062 and p/e = 15.00 in the wide range of e/Dh = 0.047–0.101 and p/e = 10.00–15.00. As a result of the comprehensive evaluation of OPT1 and OPT2, the combination of e/Dh = 0.062 and p/e = 15.00 is eventually employed by the oblique rib channel.

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.

Study of the flow characteristics and augmentation of heat performance in a horizontal tube with and without spring inserts

In this project, heat transfer and thermal performance factor characteristics in a tube fitted with coil wire and twisted tape inserts, using air as working fluid are investigated experimentally. Enhancement forced convection heat transfer characteristics is studying for air Reynolds number range from 10.328×103 to 11.769×103 and 10.927×103 to 12.029×103 in the tube without and with coil wire inserts. Effects of various hydrostatics parameters on the heat transfer are considered in this work with heat flux are constant conditions of 600 W/m² and 800 W/m² with using the coil wire and twisted tapes with three ratios (y/w = 3, 4 and 5). The test section is a horizontal annular passage with outer diameter 10 cm. The results indicated that the enhancement of heat transfer coefficient factors in tube with coil wire inserts provide local heat transfer coefficient percentages of (5.1% to 7.8%) for heat fluxes (600) W/m² and (800) W/m² respectively. While tube with swirl inserts provide local heat transfer coefficient percentages of (3.6% to 6.1%) for heat fluxes (600) W/m² and (800) W/m² respectively. The obtained results show that mean Nusselt number and heat transfer coefficient in the tube with twisted tape and coil wire increase with decreasing twisted ratio (y/w). It is also observed that the percentage of heat transfer coefficient for the coil wire when compared with that of swirl inserts tube were ranged between (10.6%) to (16.5%) and (11.5%) to (17.3%) for the heat flux (600) W/m².

Studies on the melting dynamics of PCM in a semicircular cavity with straight and wavy heating surfaces

Several kinds of curved confinements for phase changing material storages are found application in natural systems, latent heat storages, metal furnaces, and thermal control devices. Semi-circular enclosures have an inherent advantage of decreasing unmelt volume on top with an increase in melt height when heated from bottom. Hence the melt dynamics differs from conventional rectangular enclosures of similar orientation and realizes faster thermal response during the entire regimes of melting. Experimental and numerical studies on Rayleigh Benard convection in a semi-circular enclosure having straight, corrugated, and wavy bottom surface with constant heat flux conditions are presented. Melt front evolution, thermal convection, and unsteady surface heat transfer characteristics of a phase changing material contained in a semicircular confinement are presented in this study. Enthalpy-porosity method based finite volume solver is used to simulate the unsteady thermal convection of PCM in semi-circular enclosure. Experimental thermal response at various locations inside the confinement and shadowgraph images are used to verify the numerical results quantitatively and qualitatively. Present results and correlations (103 < Ra < 109) are useful in design, analysis, and thermal characterization of PCM enclosures of minimum volume and better thermal response finding application in electronic cooling devices and latent energy storages.

Heat transfer characteristics of R134a flow boiling in a microfin tube under typical ORC pressures based on comparison with a smooth tube

The experimental investigation on the flow boiling heat transfer of an organic working fluid under typical organic Rankine cycle (ORC) operating conditions is insufficient, which directly affects the performance of the ORC system. In this paper, the flow boiling characteristics of R134a in a horizontal microfin tube were experimentally investigated in the reduced pressure range 0.6 ∼ 0.9. The general convection heat transfer characteristics of R134a flow boiling, along with the effects of heat flux, mass flux, pressure, and rewetting conditions, were investigated in a microfin tube and compared with that in a smooth tube. Results show that the main flow regime in the microfin tube under typical ORC operating pressures is the stratified flow with dry-out occurring at the top surface. The microfin effectively enhances the heat transfer and restrains the wall temperature separation through inducing strong helical flow due to buoyancy and centrifugal forces. With the heat flux increasing, the micro-fin strengthens the nucleate boiling by enhancing the vaporization cores, leading to more pronounced heat transfer enhancement compared with that in the smooth tube. The heat transfer on the bottom surface shows almost negligible sensitivity to variations in mass flux. However, in regions of high vapor quality, an increase in mass flux results in higher heat transfer coefficients on the top surface. Due to the interaction between intensified nucleate boiling and weakened gas convection when pressure is increased, it slightly affects the heat transfer coefficient. Unlike the smooth tube, the rewetting in the microfin tube can be maintained within a larger range of vapor quality due to the spiral flow caused by the micro-fin.

Forced Convection Heat Transfer Characteristics of Developed Laminar Flow in the Octagonal Channels of Octo-Square Asymmetric Particulate Filters

Forced Convection Heat Transfer Characteristics of Developed Laminar Flow in the Octagonal Channels of Octo-Square Asymmetric Particulate Filters

Influence of normal stress‐induced three‐dimensional rough fracture aperture heterogeneity on nonlinear seepage‐heat transfer coupling processes

AbstractA clear understanding of the dependence of the heat transfer process on the flow field in three‐dimensional (3D) rough fractures is crucial for many underground projects. In this study, 3D rough rock fractures with heterogeneous apertures and mechanical effects are established. Navier‐Stokes flow and local heat balance theory are used to simulate the seepage‐heat transfer coupling process. Five normal stresses and five flow velocities are set to quantify the influence of aperture variability on the flow field and heat transfer behavior of the fractures. The results show that the change in fracture aperture caused by normal stress is the direct cause of the nonlinear distribution of the flow field and temperature fluctuation. A quantitative parameter of temperature fluctuation degree in the fracture is proposed, which decreases exponentially with increasing fracture aperture and increases with greater flow velocity. An empirical model is established to describe the relationship between the heat transfer coefficient and the ratio of hydraulic aperture to mechanical aperture. The correctness of the model is verified by published seepage‐heat transfer coupling test results. The model is extended to establish the relationship between the heat transfer coefficient and the volume fraction of eddy. It is confirmed that the heat transfer coefficient increases with the augmentation of nonlinear flow, and the reason why the results of evaluating the heat transfer coefficient by roughness alone in related experiments are inconsistent is revealed. The results enhance the understanding of convective heat transfer characteristics in 3D rough rock fractures.

Buoyancy influencing convective heat transfer characteristics of supercritical CO2 in a serpentine solar receiver tube at low Reynolds number

The tubular receiver is one of the most promising forms of supercritical CO2 (S-CO2) receiver. Due to the high-temperature and high-pressure working condition of S-CO2, the pressure resistance of solar receiver tube faces great challenges. The convective heat transfer performance of S-CO2 will be significantly improved by using the serpentine solar receiver tube instead of the traditional straight solar receiver tube in the S-CO2 tubular solar receiver, which will offset the negative effects of increased thermal resistance caused by increasing wall thickness to improve pressure resistance. Centrifugal buoyancy and gravitational buoyancy are important factors affecting the convective heat transfer characteristics of S-CO2 in serpentine tube which are numerically studied in this paper. The results show effect of buoyancy in the serpentine tube can significantly improve the convective heat transfer performance with a maximum value of 23% and reduce the entropy generation with a maximum value of 22% compared with straight tube. The convective heat transfer performance of S-CO2 for the downward flow is stronger than the upward flow due to the effect of gravitational buoyancy on the intensity of turbulence kinetic energy near the wall. The dominant position of centrifugal buoyancy and gravitational buoyancy affects the convective heat transfer characteristics of S-CO2 in serpentine tube. The synergistic/competitive relationship between centrifugal buoyancy and gravitational buoyancy will promote/suppress the change of the flow field which affects the convective heat transfer characteristics of S-CO2 in serpentine tube. The conclusions of this work can provide theoretical guidance for the design and optimization of tubular receiver using S-CO2 as heat transfer fluid.

Preparation and application of rice husk-based SiO2/water nanofluids in heat transfer enhancement under pulsation

Rice husk was used as raw-material to produce nano-SiO2 by the heat treatment method, and the structural characteristics of rice husk SiO2 (R-SiO2) was compared with the commercial SiO2 (C-SiO2) by various characterization methods. SiO2/water nanofluids were prepared by a two-step dispersion method using various sources SiO2 (R-SiO2 and C-SiO2) and applied in the convective heat transfer of corrugated tube channels in a self-built experimental platform. The experimental results showed that the specific surface area of the rice husk-based silica was high, reaching 335 m2/g. For the same nanofluids concentration, the thermal conductivity enhancement rate of R-SiO2/water nanofluid was higher than that of C-SiO2/water and the enhancement rate of 0.5% R-SiO2/water at 25 and 40 °C reached 14.01% and 12.16%, respectively. The convective heat transfer coefficient of R-SiO2/water was slightly better than that of commercial C-SiO2/water, which indicated that R-SiO2 has great potential to be a substitute of C-SiO2 for application in heat exchangers. The convective heat transfer characteristics of R-SiO2/water nanofluids in corrugated tube at different Re and volume concentrations were compared. It was found Nu augmented as Re increased. And a higher nanofluids concentration might result in a drastic increment in convective heat transfer coefficient. When a pulsating wave was imposed, the larger pulsation amplitude resulted in a higher heat transfer enhancement rate. And the enhancement rate at lower Re was better. Under the conditions of Re = 385, f = 0.67 and A = A15, the maximum average convective heat transfer enhancement rate reached 143%.

Primitive and gravity modulation of periodical heat transfer along magnetic-driven porous cone with thermal conductivity and surface heat flux

The significant goal of present problem is to evaluate oscillating frequency and amplitude in heat and magnetic flux with thermal conductivity and heat flux effects across magnetized porous cone under lower gravitational region. To maintain heat transport efficiency, the thermal conductivity of conducting fluid is assumed as temperature dependent. The magnetic-driven cone is placed in lower gravitational region. To improve the heating rate across the magnetic-driven porous cone, the convective heating conditions are applied. The implications of thermal conductivity, surface heat flux, and reduced gravity on the periodic behavior of convective heat transfer characteristics of conducting fluid across porous gravity-driven cone is the novelty of this research. Using the appropriate non-dimensional variables, the model's nonlinear governing partial differential has been converted into a dimensionless form. The proposed model is solved later with the help of finite difference approach. The dimensionless form of the equations is reduced to the convenient form for numerical algorithm. The influence of adjusting parameters, such as thermal conductivity parameter ζ, reduced gravity parameter Rg, Biot number Bi, Prandtl number Pr, mixed convection parameter λ, porosity parameter Ω, magnetic prandtl number γ and some others stationary parameters has been highlighted. By using FORTRAN tool, the numerical outcomes are explored in graphs and tabular form. It is depicted that magnetic field enhances as Biot number decreases because magnetic field insulates the excessive heating across the magnetic-driven porous cone. It is noticed that significant amplitude in fluid velocity is noticed with prominent changes as reduced gravity increases. It is concluded that fluctuations in heat transport increases as parameter γ decreases under thermal conductivity because magnetic material insulates the extreme heat across the porous cone. The recent thermal conductivity and lowered gravity problem is significant in the fields of radioactive waste, cooling electronic components, storing nuclear, catalysts, heat exchangers, the underground storage of radioactive or nonnuclear materials, aircrafts and space vehicles.

Convective heat transfer characteristics within a multi-package during precooling

Convective heat transfer characteristics within a multi-package during precooling

Comparison of the use of different nanofluids for heat transfer from the outer surface of spiral tubes: Energy, exergy, and exergoeconomic (3E) analysis

Heat exchangers play a crucial role in industrial processes by facilitating the efficient transfer of thermal energy. The thermophysical properties of nanofluids, which consist of nanoparticles dispersed in conventional heat transfer fluids, are enhanced. Their remarkable thermal conductivity and convective heat transfer characteristics have the potential to significantly improve heat transfer processes. In the present investigation, a numerical study has been carried out to investigate the effect of different nanofluids on heat transfer from outer surface of a spiral coil based on the first and second laws of thermodynamics. The numerical solutions are checked against previous experimental and numerical research in the field. Nusselt number, Euler number, exergy efficiency, irreversibility, and equipment cost are studied with respect to concentration, mass flow ratio (Rm), and shell and tube inlet temperature for a variety of nanofluids, including Al2O3/H2O, CuO/H2O, Fe2O3/H2O, and TiO2/H2O. Compared to water, the exergy efficiency is lower while using nanofluids, as shown by the results, especially in higher nanofluid concentrations, and among the different nanofluids, Al2O3–H2O nanofluid was found to have the highest thermodynamics second law efficiency. The exergy destruction rate experiences linear growth by increasing the nanofluid concentration. A higher pressure drop based on the Euler number is expected for nanofluids. In addition, the consumption of nanofluids on the shell side results in a considerable reduction of the purchased equipment cost (PEC) value compared to water. As opposed to the tube-side input temperature, variations in the shell-side inlet temperature found to have a greater impact on the heat exchanger's exergetic performance.