Convective Heat Transfer Process Research Articles

Experimental investigation on the stability and heat transfer enhancement of phase change materials composited with nanoparticles and metal foams

Conventional phase change materials (PCMs) possess the challenge of low thermal conductivity, which hinders the energy exchange rate and storage efficiency. PCMs composited nanoparticles and metal foams have been a proven solution to enhance the thermal characteristics of PCMs. In this work, three enhanced PCM composites including nanoparticle/paraffin, metal-foam/paraffin and nanoparticle/metal-foam/paraffin were prepared using fusion blending and vacuum impregnation method. The effects of different nanoparticles with different mass fractions on the stability and heat storage and release characteristics of nanoparticle/paraffin were investigated. Compared with paraffin, the melting time and solidification time of nano-metal/paraffin are reduced by a maximum of 30.18 % and 43.15 %, respectively. In addition, the effect of pore density of metal foam on natural convective heat transfer process was evaluated. The results showed that the stability in descending order is as follows: nano‑aluminum, nano‑copper and nano‑silver. The optimal mass fraction of nanoparticles with good stability was 1.5 wt% (copper), 1.0 wt% (silver), 1.5 wt% (aluminum), respectively. The metal foam makes the internal temperature distribution of PCMs more uniform in the heat transfer process, while inhibiting the convective heat transfer process. With the increase of pore density, the effect on natural convection heat transfer process rises. When the PPI (pores per linear inch) of foamed metal is greater than 40, the natural convection process can be inhibited, obviously. The foam metal has a more significant effect on the heat transfer performance of nanoparticle/metal-foam/paraffin than that of the nanoparticles. The economic benefits of nanoparticle/paraffin, metal-foam/paraffin and nanoparticle and metal foam composite phase change materials are progressively decrease, with MFP being suitable for applications with high economic budgets.

Thermal Performance of Hybrid PV/T-TEC Air Collector with Perforated-Finned Absorber Plate

PV/T-TEC collector is a device that can produce thermal energy and electrical energy at the same time. The electrical performance may be increased by absorbing residual heat from the PV surface. Due to the temperature difference available between the hot and cold sides of this collector, additional electrical energy can be also generated by using the TEC to increase the total efficiency. The purpose of this study is to evaluate the thermal effect of using both fins and TECs acting as a heatsink. Thus, the perforated zone with a dimension of 100 x 50 x 2 mm was manufactured by lancing and bending processes. Furthermore, 44 pieces of TECs were placed inside the perforated-finned zone and were attached underneath the PV surface to allow a convection heat transfer process. The model collector was then tested by using a solar simulator under 1100 W/m2 with different fluid mass flow rates ranging from 0 to 0.339 kg/s. To meet the test requirements, environmental fluid flow was also implemented and simulated by a fan with an air velocity of 1 m/s. The results showed that increasing the air mass flow rate as a working fluid by five times can reach the operating temperature of the model collector by about 64 °C (24% decrease) and 74 °C (26% decrease) for both with and without environmental fluid flow, respectively. Meanwhile, under the same test conditions, the maximum value for the temperature difference of TEC sides is found to be 5.6 °C for both with and without ambient fluid flow.

Study on the convective heat exchange coefficient of concrete surface considering the surface roughness

A heat exchange experiment of concrete surface under different roughness conditions was conducted, and the convective heat exchange coefficient model of concrete surface influenced by many factors was deduced and developed. The model comprehensively considers the effects of concrete surface roughness, wind speed, air humidity and thermal conductivity on the surface heat exchange performance. We obtained the model parameters from experimental data, which captured the convection heat transfer process under controlled laboratory conditions. The comparison between the calculation result and the measurement data is good enough, and prove the selection value of the model parameters are very good. The new convective heat exchange model can better reflect the heat exchange of the concrete structure surface under complex boundary conditions, provide support for more accurate simulation analysis of concrete temperature field, and provide the possibility for better calculation of concrete temperature stress and control of temperature cracks.

Numerical Investigation of Double-Diffusive Convection in an Irregular Porous Cavity Subjected to Inclined Magnetic Field Using Finite Element Method

Purpose—This study aims to perform an in-depth analysis of double-diffusive natural convection (DDNC) in an irregularly shaped porous cavity. We investigate the convective heat transfer process induced by the lower wall treated as a heat source while the side walls of the enclosure are maintained at a lower temperature and concentration, and the remaining wall is adiabatic. Various factors, such as the Rayleigh number, Darcy effects, Hartmann number, Lewis number and effects of magnetic inclination are evaluated for their influence on flow dynamics and heat distribution. Design/methodology/approach—After validating the results, the FEM (finite element method) is used to simulate the flow pattern, temperature variations, and concentration by solving the nonlinear partial differential equations with the modified Rayleigh number (104 ≤ Ra ≤ 107), Darcy number (10−4 ≤ Da ≤ 10−1), Lewis number (0.1≤Le≤10), and Hartmann number 0≤Ha≤40 as the dimensionless operating parameters. Findings—The finding shows that the patterns of convection and the shape of the isotherms within porous enclosures are notably affected by the angle of the applied magnetic field. This study enhances our understanding of how double-diffusive natural convection (DDNC) operates in these enclosures, which helps improve heating and cooling technologies in various engineering fields. Research limitations/implications—Numerical and experimental extensions of the present study make it possible to investigate differences in thermal performance as a result of various curvatures, orientations, boundary conditions, and the use of three-dimensional analysis and other working fluids. Practical implications—The geometry configurations used in this study have wide-ranging applications in engineering fields, such as in heat exchangers, crystallization, microelectronics, energy storage, mixing, food processing, and biomedical systems. Originality/value—This study shows how an inclined magnetic field affects double-diffusive natural convection (DDNC) within a porous system featuring an irregularly shaped cavity, considering various multiphysical conditions.

Numerical investigation of three-dimensional natural convection heat transfer on corrugated plates of variable height

PurposeThe purpose of this study is to numerically investigate the geometric influence of different corrugation profiles (rectangular, trapezoidal and triangular) of varying heights on the flow and the natural convection heat transfer process over isothermal plates.Design/methodology/approachThis work is an extension and finalization of previous studies of the leading author. The numerical methodology was proposed and experimentally validated in previous studies. Using OpenFOAM® and other free and open-source numerical-computational tools, three-dimensional numerical models were built to simulate the flow and the natural convection heat transfer process over isothermal corrugation plates with variable and constant heights.FindingsThe influence of different geometric arrangements of corrugated plates on the flow and natural convection heat transfer over isothermal plates is investigated. The influence of the height ratio parameter, as well as the resulting concave and convex profiles, on the parameters average Nusselt number, corrected average Nusselt number and convective thermal efficiency gain, is analyzed. It is shown that the total convective heat transfer and the convective thermal efficiency gain increase with the increase of the height ratio. The numerical results confirm previous findings about the predominant effects on the predominant impact of increasing the heat transfer area on the thermal efficiency gain in corrugated surfaces, in contrast to the adverse effects caused on the flow. In corrugations with heights resulting in concave profiles, the geometry with triangular corrugations presented the highest total convection heat transfer, followed by trapezoidal and rectangular. For arrangements with the same area, it was demonstrated that corrugations of constant and variable height are approximately equivalent in terms of natural convection heat transfer.Practical implicationsThe results allowed a better understanding of the flow characteristics and the natural convection heat transfer process over isothermal plates with corrugations of variable height. The advantages of the surfaces studied in terms of increasing convective thermal efficiency were demonstrated, with the potential to be used in cooling systems exclusively by natural convection (or with reduced dependence on forced convection cooling systems), including in technological applications of microelectronics, robotics, internet of things (IoT), artificial intelligence, information technology, industry 4.0, etc.Originality/valueTo the best of the authors’ knowledge, the results presented are new in the scientific literature. Unlike previous studies conducted by the leading author, this analysis specifically analyzed the natural convection phenomenon over plates with variable-height corrugations. The obtained results will contribute to projects to improve and optimize natural convection cooling systems.

Climatology of Synoptic Non-Gaussian Meteorological Anomalies in the Northern Hemisphere during 1979–2018

The analysis of spatial and temporal variability in the number of non-Gaussian extreme anomalies of climatic parameters was carried out for both the initial time series and synoptic variability in the troposphere of the Northern Hemisphere over the period 1979–2018, based on ERA-Interim reanalysis data. There are predominantly three types of empirical distribution densities at 850 hPa, each characterizing the processes of advective and convective heat transfer. At the beginning of the 21st century, compared to the end of the 20th century, there was an increase in the number of anomalies in vertical wind speed and specific humidity for the Northern Hemisphere. Additionally, there is an increase in the number of zonal wind speed anomalies in the low and middle latitudes. Regions with the maximum number of anomalies are primarily located over the continents, while for vertical wind speed anomalies, they are predominantly over the oceans. The application of R/S analysis and multifractal analysis has established that the identified tendencies (which are persistent processes) will continue in the identified regions. The time series of non-Gaussian anomalies (both initial and synoptic scales) exhibit a long-term memory of approximately four years, and synoptic extreme anomalies were found to be more predictable.

Optimization analysis of nuclear thermal coupling for a small nuclear thermal propulsion (SNTP) reactor

As a main structure, tie-tube elements are used both in the traditional and modern LEU nuclear thermal propulsion reactors. This complex structure and its corresponding complex convective heat transfer process have been studied, including CFD calculations. But these studies did not effectively evaluate the applicability of the models and typical problems. In this study, a general small nuclear thermal propulsion (SNTP) reactor model is established by using three-dimensional Monte Carlo code and commercial CFD program Fluent. The applicabilities of material properties, turbulence models and grids are clarified by analyzing various influencing factors. The calculation results show that due to the small enthalpy difference and heat transfer, the convection effect of inner flow based on hydrogen can be ignored. There is no factor that can significantly reduce the maximum fuel temperature, but the porosity of the insulator can significantly improve the heat transfer ratio. In terms of thermal safety and overall performance, 2:1 support ratio is still the best choice. Based on the simulation results, the phenomenon of steep temperature rise of outer tie-tubes is found, and the SST k-omega model with Y + <3 is recommended for simulation of similar problems.

Numerical Simulation of Heat Transfer of Porous Rock Layers in Cold Sandy Regions

The heat transfer characteristics of porous rock layers (PRLs) have significant seasonal differences. This feature has been used to protect the permafrost subgrade under highways and railways from degeneration. However, in cold sandy environments, the transformation law of heat transfer characteristics of PRLs on account of climate warming and aeolian sand filling needs to be solved. This work developed a coupled heat transfer model for the soil–PRL system aimed at analyzing the convective heat transfer process and mechanism of a closed PRL. Furthermore, the impact of climate warming and sand filling on the cooling performance of the PRL under different mean annual air temperatures (MAATs) of −3.5, −4.5, and −5.5 °C was quantified. The numerical results indicated that the natural convection of the closed PRL occurred only in winter, and the effective convective height of the rock layer decreased with the sand-filling thickness. As the thickness of sand filling increased, the critical temperature difference for the occurrence of natural convection increased, accompanied by decreases in the Rayleigh number, the duration, and intensity of natural convection. When the sand-filling thickness exceeded 80 cm, natural convection would not occur in the PRL. Under a warming scenario of 0.052 °C·a−1, the cooling performance of the PRL could offset the adverse impact of climate warming and raise the permafrost table in the first 20 years. Moreover, the closed PRL can be more effective in permafrost regions with colder MAATs. For cold sandy permafrost zones, sand-control measures should be taken to maintain the long-term cooling performance of the PRL. This study is of great significance in guiding porous rock embankment design and road maintenance along the Qinghai–Tibetan Railway.

Computational fluid dynamics analysis of PCM-based ocean thermal engine with external rib turbulators

The concept of extracting energy from ocean thermal gradients for the long-term deployment of underwater profiling vehicles is promising. One potential method to capture ocean thermal energy is leveraging the volume change of phase-change materials (PCMs). However, the low thermal conductivity of PCMs and the low heat transfer coefficient between heat exchangers and seawater limit the performance of PCM-based ocean thermal engines (OTEngs). In this paper, a new type of heat exchanger structure with external rib turbulators is proposed. Detailed computational fluid dynamics analysis is conducted for the external forced convection heat transfer process of the heat exchanger with Reynolds number in the range of 240,000 to 600,000. In addition, a solver for calculating the phase-change process of the PCM inside the heat exchanger was developed based on OpenFOAM. The influence of the ribs on the heat transfer and energy conversion processes of the OTEng was analyzed. Results show that (i) the heat transfer coefficient of the ribbed heat-exchanger wall significantly improves, especially at low Reynolds numbers; (ii) 2.5–5 mm high ribs can effectively enhance heat transfer through the wall. When rib weight exceeds 5 mm, the heat transfer coefficient does not increase with the height; (iii) increasing the rib spacing reduces the water resistance and heat transfer coefficient of the wall; and (iv) the ribbed OTEng has a higher net power output compared with the conventional engine.

Studying Alumina–Water Nanofluid Two-Phase Heat Transfer in a Novel E-Shaped Porous Cavity via Introducing New Thermal Conductivity Correlation

Investigating natural convection heat transfer of nanofluids in various geometries has garnered significant attention due to its potential applications across several disciplines. This study presents a numerical simulation of the natural convection heat transfer and entropy generation process in an E-shaped porous cavity filled with nanofluids, implementing Buongiorno’s simulation model. Analyzing the behavior of individual nanoparticles, or even the entire nanofluid system at the molecular level, can be extremely computationally intensive. Symmetry is a fundamental concept in science that can help reduce this computational burden considerably. In this study, nanofluids are frequently conceived of as a combination of water and Al2O3 nanoparticles at a concentration of up to 4% by volume. A unique correlation was proposed to model the effective thermal conductivity of nanofluids. The average Nusselt number, entropy production, and Rayleigh number have been illustrated to exhibit a decreasing trend when the volume concentration of nanoparticles inside the porous cavity rises; the 4% vol. water–alumina NFs yield 17.35% less average Nu number compared to the base water.

Study on unsteady natural convection heat transfer of crude oil storage tank based on fractional-order Maxwell model

When considering the natural flow processes in crude oil in static storage tanks, researchers usually treat the nonlinear heat and mass transfer and flow using integer-order models. However, this ignores the nonlinear convection and heat-transfer phenomena that occur; these are dependent on time and space due to the viscoelasticity of crude oil, which is a non-Newtonian fluid. In this work, we examined the time and space dependence of the nonlinear natural convective heat-transfer processes in crude-oil storage tanks. Fractional-order Maxwell and Cattaneo–Vernotte models were applied to the traditional momentum and temperature equations considering the Boussinesq approximation. Based on the L1 discrete format, the fractional-order pressure-correction equation was derived, and an associated solver, CaputoTemporalFractionalFoam (CTfracFoam), was developed to simulate the convection processes in static crude-oil storage tanks. The results show that when α equals 0.1, the maximum velocity in monitoring area 2 is 0.1404 m/s, and the velocity gradually increases as α increases with a maximum value of 1.1468 m/s at 0.9. When the β is 0.1 and 0.2, the temperature changes in the transition region are −5.204 and −0.165 °C, respectively, while when β is greater than 0.2, the temperature changes are all positive values with a maximum value of 1.924 °C at 0.9. Compared with integer-order Fourier heat-transfer and Newtonian fluid-flow models, the developed fractional-order model can describe the natural convection phenomena of non-Newtonian fluids such as crude oil more flexible and accurately.

INFLUENCE OF THE DIAMETER OF CYLINDRICAL SAMPLES MADE OF DIFFERENT GRADES OF ALUMINUM ON THE COOLING KINETICS

The influence of the diameter of cylindrical samples from various grades of technical aluminum (A0, A5, A6), aviation AB98 and high-purity aluminum A5N on their characteristic cooling times and rates, as well as on heat transfer processes in a wide temperature range, was studied. It was revealed that cooling of the samples occurs due to convective heat transfer and radiation, which have characteristic times. An assessment of the magnitude of characteristic cooling times due to convective heat transfer and radiation has been carried out. It is shown that the characteristic cooling time due to radiation is less than the characteristic cooling time due to convection. Using experimental data on the cooling rate of the studied samples and theoretically calculated values of their heat capacity according to the Neumann-Kopp rule, heat transfer coefficients for the processes of convective heat transfer and radiation depending on temperature were determined. It has been established that with increasing temperature, the contribution of radiation to the total heat transfer coefficient increases, and the coefficient of convective heat transfer with increasing temperature first increases, then after passing a maximum it decreases. The contribution to the overall heat transfer coefficient from radiation is noticeable at high temperatures. It was revealed that the characteristic cooling times due to radiation and convection increase nonlinearly with increasing sample diameter (V/S). This dependence is explained by a decrease in the heat transfer coefficient with increasing sample diameter. Compared with the patterns of influence of sample size on the thermophysical properties of spherical samples. It was revealed that spherical samples cool faster than cylindrical samples of the same mass, and the heat transfer coefficients are higher.

Quadratic Thermal Convection in Magneto-Casson Fluid Flow Induced by Stretchy Material with Tiny Particles and Viscous Dissipation Effects

This investigation is based on the phenomenon of quadratic thermal convection in Magneto-Casson fluid flow induced by stretchy material with tiny particles and viscous dissipation effects. The study aims to understand and optimize the complex behaviour of fluid flow and heat transfer in this system, which has significant implications for engineering and industrial applications. The Magneto-Casson fluid, characterized by its non-Newtonian behaviour and yield stress, interacts with stretchy material and tiny particles, introducing unique flow characteristics and thermal properties. Viscous dissipation, resulting from internal friction, further influences the convective heat transfer process. Mathematical models are developed and solved by employing a numerical technique through the Runge-Kutta Fehlberg scheme coupled with the shooting method to investigate these phenomena comprehensively. The results are deliberated using several graphs on the dimensionless profiles. The results showed that there is a decline in the momentum boundary film as the magnetic field improves due to the effect of the Lorentz force. Also, an increase in the Casson fluid term raises the viscosity and thereby resists the fluid motion.

Effects of the Broken Kernel on Heat and Moisture Transfer in Fixed-Bed Corn Drying Using Particle-Resolved CFD Model

To investigate the pore structure distribution and the coupled heat and moisture transfer during the drying process of the grains, this study focuses on fixed-bed corn drying with varying levels of broken kernel rate. A model of internal flow and conjugate heat and mass transfer was established for the drying process. Random packing models of whole and half corn kernels with different proportions were generated using rigid body dynamics (RBD), and the porosity, airflow distribution, and coupling of temperature and moisture transfer in fixed beds with different levels of broken kernel rate were analyzed. A fixed-bed corn drying device was developed, and the effects of broken particle contents of 0%, 10%, 20%, and 30% on drying characteristics were studied. The research findings reveal that the radial porosity in the fixed bed exhibits an oscillating distribution, with the localized porosity decreasing as the broken kernel rate increases. Increasing the broken kernel rate intensifies the curvature of the airflow paths within the fixed bed, increasing the pressure drop in the bed. The broken kernels fill the gaps between the whole kernels, improving the uniformity of the velocity distribution within the fixed bed. Under various packing models, the average discrepancy between pressure drop obtained from Particle-resolved Computational Fluid Dynamics (PRCFD) simulations with experimental remains below 15%. The increase in broken kernel rate within the fixed bed enlarges the heat transfer area, resulting in an elevation of the transient heat transfer characteristic parameters during drying. Simultaneously, the broken kernel rate increases the surface area of mass transfer, thereby enhancing the moisture transfer rate within the fixed bed. Compared to the fixed bed without broken kernels (0%), which requires 560 min to dry the corn pile to a safe moisture of 14% (d.b.), the drying time is reduced by 60 min, 100 min, and 130 min for the respective broken kernel contents of 10%, 20%, and 30%, respectively. The PRCFD method successfully simulates the processes of convective heat and mass transfer in the fluid phase and thermal and mass diffusion in the solid phase, exhibiting a strong correlation with experimental data.

Correlations for forced convective heat transfer coefficients at the windward building façade with vertical louvers

Vertical louvers are a widely used external shading system to control natural lighting and solar heat gain for buildings. However, when installed on a building façade, they can disrupt the flow field around the façade and affect the external convective heat transfer process of building's exterior surfaces. To this end, this paper conducted CFD simulations to clarify the forced convective heat transfer patterns in a vertical louver-facade system based on the k-ω SST model with a detailed verification. The extended Newton's cooling equation is used to describe the heat exchange process between louvers, building façade, and air, which generates four heat transfer coefficients: hwa, hwb, hba, and hbw. Then the impacts of dimensionless temperature difference (θ), louver angle (φ), louver slat width (B), and installation distance (W) on the heat transfer coefficients are investigated. The results show that θ does not affect the average heat transfer coefficients. hwa reaches a minimum value and hba reaches a maximum value when φ = 75o. In addition, hwa increases by 55.3% and hba decreases by 21.9% as B increases from 0.3 m to 1 m. When W increases from 0.25 m to 1 m, hwa decreases by 5.6% and hba increases by 39.6%. The variation in the heat transfer coefficients (hwb and hbw) between the louvers and building façade is also explained. Finally, new correlations of paired heat transfer coefficients are established as a function of φ, B, W, and wind velocity (U10). The accuracy of these correlations is evaluated by comparing calculated values with simulated results.

AUGMENTATION HEAT TRANSFER IN A CIRCULAR TUBE USING TWISTED - TAPE INSERTS: A REVIEW

Heat transfer enhancement is the process of increasing the heat-transfer coefficient, which enhances the system's performance. Enhancing heat transfer is a major problem for saving energy and is also beneficial economically. Many passive devices are used inside tubes to improve heat transfer such as twisted tape inserts, rough parts, extended surfaces, additives for liquids wire plugs, etc. This research reviewed one of the most effective passive devices which are twisted tape inserts. Since it has many advantages such as simple fabrication, simple operation, and ease of maintenance. The twisted tape inserts generated swirl flow and vortex inside the tube. Therefore, the internal convective heat transfer process is significantly improved. The current research article provides an overview of different twisting tape inserts that can improve heat transfer rates. By reducing boundary layer thickness near tube walls. Which lead to reduce the size and cost of many industrial applications, including heat exchangers, refrigeration systems, air conditioners, reactors, thermal power plants, spacecraft, and automobiles. A summary of previous experimental and numerical studies is presented as well. The primary results indicated that the twisted tape inserts are demonstrated to be efficient in enhancing heat transfer inside the tube for laminar and turbulent flow. But during a turbulent flow, twisted tapes increased pressure loss more than laminar flow because of flow obstruction.

The Effect of Vibrating Inner Cylinder on Natural Convective Heat Transfer in Concentric and Eccentric Horizontal Cylindrical Annuli

In this study, natural heat convection caused by centric and vertically eccentric long horizontal cylinders under the influence of vibration is experimentally investigated. The internal wall of the annulus is heated and kept at a constant heat flux, while the outside wall is cooled and maintained at a constant temperature. The vibration frequency impacts the annular convection heat transfer process and the effects of the Rayleigh number, heat flow, and eccentricity. This work employed a moderate, laminar-ranging Rayleigh number from (5×104 to 6.48×106), while the eccentricity range is varied from (-0.667, 0, and+0.667). The investigation is carried out at five different frequencies (0, 2, 5, 10, 15, and 20 Hz); therefore, it was decided to compare the case under the same circumstances in both the absence and presence of vibration. The present results' verification worked exceptionally well in concordance with previous studies. When heat fluxes are considered, the study demonstrates that the temperature difference along the gap (radial difference) between the two cylinders significantly decreases for negative as opposed to positive eccentricities for each Rayleigh number. For different eccentricities, it was discovered that the temperature difference decreased as the Rayleigh number increased. Along with these reductions, the temperature difference was promoted as the vibration frequency increased, which is significant at (20 Hz) within the range considered for controlling parameters. It was also observed that the decrease in temperature difference is higher for the negative eccentric position than for the centric and positive positions. At vertical positive eccentricity at a low Rayleigh number, the gain in vibrational average Nusselt number caused by applying vibration had a minimum value of 22.50601475%, While at the higher value of the Rayleigh number, the maximum increase in negative vertical eccentricity was 86.66933125%. However, the gain in the average Nusselt number depends on the position of the heated inner cylinder, the Rayleigh number, and the vibrational frequency.

The effect of two main vortexes generator on heat transfer enhancement in mini rectangular ducts of airborne thermoelectric conversion system

To enhance the heat transfer process of supercritical pressure fluid in rectangular channels of the airborne thermoelectric conversion system, the combined effect of the Reynolds stress, properties variation and longitudinal fins on convective heat transfer process in ducts under supercritical pressure is investigated. Then, a method to induce the two main vortexes structure in rectangular ducts is proposed. Results show that, in smooth ducts, the two main vortexes structure is formed due to the effect of density variation. By comparing the flow and heat transfer process in the smooth duct and ridged duct with trapezoidal fins attached on the lower wall, it is found that, due to the effect of new vortexes induced by trapezoidal fins, the two main vortexes structure is inhibited, which makes local wall temperature increases about 50 K. Accordingly, the thermal performance enhancement factor of rectangular duct with trapezoidal fins is lower than 1. To enhance the two main vortexes structure, a method that using triangular fins to induce two main vortexes structure is proposed, and the duct with one triangular longitudinal fin attached on the upper wall is designed, which has better thermal performance than ducts with trapezoidal fins. Due to the effect of triangular fins, the dominant distance of two main vortexes structure increases by 416%. Accordingly, in ducts with triangular fins, local heat transfer coefficient is enhanced about 23.5%, and the maximum of thermal performance enhancement factor is about 1.29.

Thermodynamic performance of Al2O3-Cu-CuO/water (W) ternary nanofluids in the full-flow regime of convective heat transfer

To explore the convection mechanisms of ternary nanofluids in the full-flow regime (laminar-transition-turbulent regime), the thermodynamic performance of Al2O3-Cu-CuO/W ternary nanofluids (with a volume fraction of 0.02 vol%) and their corresponding mono nanofluids was investigated in Reynolds number (Re) range of 800–8000 in a cylindrical tube. The results showed that the presence of suspended particles in the Al2O3-Cu-CuO/W nanofluids caused the transition regime to shift to a lower Re value of ∼ 1900. As Re was increased, the heat transfer performance exhibited a fluctuating upward trend, with a maximum at Re = 5250. Further, the ternary nanofluid with a mixing ratio of 30:30:40 showed a Nu 1.73 times that of the base fluid, with the corresponding values for the CuO/W and Al2O3/W nanofluids being only 1.36 and 1.39 times, respectively. Moreover, the thermal conductivity of the ternary nanofluids had a substantial impact on heat transfer performance in the laminar regime, with a correlation coefficient, r, of 0.6429, but less of an influence in the turbulent regime, with an r value of only 0.5. Finally, the optimum mixing ratio for the convective heat transfer process was determined to be 30:50:20 based on the values of Nusselt number enhancement rate and enhanced heat transfer factor, as well as on thermodynamic analysis of the system entropy generation and exergy.

Experimental study on convective heat transfer of an open-loop borehole heat exchanger

Open-loop borehole heat exchanger (OBHE) is a single well geothermal heat exchanger with an open-loop structure that can realize the geothermal energy extraction without mining the geothermal water. In this paper, a sandbox experiment is designed to simulate the convective heat transfer process in the reservoir area of OBHE. The mechanism of convective heat transfer in the reservoir area is studied, and the key factors that affect the convection heat transfer intensity are analyzed. The results show that the convection heat transfer of OBHE in the reservoir area is affected by both the driving effect of fluid flow inside the screen tube and the buoyancy effect. In the forward flow mode, the two effects have the opposite direction. While in the backward mode, the two effects have the same direction. The backward flow mode is more conducive to convective heat transfer. In addition, many factors influencing significantly the convective heat transfer of OBHE include inlet temperature, inlet flow rate, reservoir temperature, fluid flow direction and inner tube diameter.