Convective Heat Transfer Enhancement Research Articles

Steady and unsteady numerical investigation of mixed convective heat transfer enhancement in a channel with baffles attached to the heated wall

Steady and unsteady numerical investigation of mixed convective heat transfer enhancement in a channel with baffles attached to the heated wall

Statistical and Numerical Investigations of Convective Heat Transfer Enhancement of Nanofluids Inside a Rectotrapezoidal Enclosure in the Presence of Horizontal Periodic Magnetic Field

Statistical and Numerical Investigations of Convective Heat Transfer Enhancement of Nanofluids Inside a Rectotrapezoidal Enclosure in the Presence of Horizontal Periodic Magnetic Field

Mathematical modeling of convective heat transfer enhancement using circular cylinders in an inverted T‐shaped porous enclosure

Mathematical modeling of convective heat transfer enhancement using circular cylinders in an inverted T‐shaped porous enclosure

Preferential enhancement of convective heat transfer over drag via near-wall turbulence manipulation using spanwise wall oscillations

This study investigates the manipulation of convective heat transfer through spanwise wall oscillations in a turbulent channel flow. Direct numerical simulations are performed at Reτ=180 and Pr=1.The primary focus of this work is to explore the heat transfer response to oscillation parameters that promote drag increase, a regime that has received limited attention. By adopting an extended oscillation period (T+=500) and amplitude (W+=30), which have been reported to enhance drag, a remarkable dissimilarity between momentum and heat transport emerges. Under these conditions, the convective heat transfer undergoes a substantial 15% intensification, while the drag increases by a comparatively moderate 7.7%, effectively breaking the Reynolds analogy. To elucidate the physical mechanisms responsible for this dissimilar behaviour, a comprehensive statistical analysis is conducted. The control effect on the near-wall streaks and the associated mixing of momentum and heat is investigated by examining the energy distribution across scales and wall-normal locations. This analysis provides valuable insights into the control’s impact on the turbulent structures. Furthermore, the correlation between wall-normal velocity fluctuations and both streamwise velocity and temperature fluctuations is scrutinized to understand the modification of sweep and ejection events, which drive the transport of momentum and heat. The Fukagata–Iwamoto–Kasagi (FIK) identity is employed to identify the contributing factors to the changes in drag and heat transfer. The analysis highlights the importance of the pressure term in the streamwise velocity equation and the linearity of the temperature equation. Further investigation is necessary to fully unravel the complex mechanisms governing the decoupling of heat and momentum transport. The results of this study underscore the potential of using unconventional spanwise wall oscillations parameters to preferentially enhance convective heat transfer while minimizing the associated drag penalty.

Effect of Forced Convection Heat Transfer on Vapor Quality in Subcooled Flow Boiling

Abstract A new theoretical model based on energy balance and well-known heat transfer correlations is proposed for predicting vapor quality in subcooled flow boiling. This model takes into account the enhancement of forced convection heat transfer due to the presence of vapor. It is shown that the vapor quality predicted by our model is much less than that by a previous model for low pressure. This result demonstrates that the convective heat transfer coefficient cannot be constant, and the effect of gas phase on forced convection heat transfer cannot be neglected even for subcooled flow boiling, particularly at low pressures. However, the difference between the present and previous models decreases as the pressure increases, because (i) the increase of the convective heat transfer coefficient is weakened, and (ii) boiling heat transfer becomes dominant. Furthermore, the present model has the capability of locating the point at which bulk boiling commences.

Dynamic characteristics of a climate-adaptive radiant cooling and fresh air supply integrated system with zeotrope R290/R600a

Enhancing climate-adaptability stands as a pivotal focus in the advancement of air-conditioning technology. The nuanced control over thermodynamic properties of zeotropic mixture emerging as a critical factor in effectively accommodating variable loads. This paper proposes a radiant cooling and fresh air supply integrated system, featuring dynamic adapting to various operating conditions by composition adjustment with zeotropic mixture of R290/R600a. By synchronizing the cooling capacities of dual cooling sources with indoor thermal-humidity load, the climate-adaptive operation is accomplished by tuning the secondary fluid of the liquid separation component. A physics-based dynamic model is constructed on the liquid separation condenser in the sequence of control volume-pipeline-path based on finite volume approach. The numerical analysis indicates the spatial distribution of mass fraction and thermophysical properties of the zeotropic mixture over time. To facilitate intuitive analysis of the climate-adaptive operation, the enthalpy humidity ratio (ε) that the system can handle is developed for evaluating the applicable scenarios and operating conditions of the dual cooling source system. The results demonstrate that the liquid separation component exhibits a rapid response owing to its low thermal inertia, lagging less than 30 s to adapt to the airflow step from 0.45 to 0.4–0.5 kg/s over a total duration of 120 s. The enhancement of convective heat transfer and reduction in heat capacity both lead to a decrease in thermal inertia, thereby shortening the response time. Increasing the airflow rate shortens the response time to 60 % of its initial duration. R290/R600a at mass fraction ratio of 0.6/0.4 exhibits the highest sensitivity to airflow adjustment. The capacities of the dual cooling sources respectively fluctuate between −55 % and 62.5 %, and −84.6 % and −0.02 %, as a result of the mass flow rates of the refrigerant varying between −60 % and 120 %, and −85.2 % and −48.1 %, with the mass fraction of R290 ranging from −20 % to −10 %, and −11.8 % to −5.9 %. The system can provide fresh air of 19 ± 16 %℃ and 12 ± 23 %g/kg and chilled water supply for the radiant terminal of 15 ± 20 %℃. The system is capable of handling air-conditioned spaces with ε ranging from 1450 to 20000 kJ/kg by adjusting the airflow rate within a range of ± 12 %, and offering an adjustable dehumidification capacity ranging from 0.2 to 2.5 kg/h. The system constitutes a promising prospect of thermal-humidity decoupling technology that adapts to various climatic conditions and personalized need.

Forced air flow through a rectangular channel with 3D turbulence enhancers: visualization of flow structures by laser sheet scattering

In recent times, the design of heat transfer devices is strongly evolving due to both the diffusion of additive manufacturing techniques, that use a wide variety of materials, and Computational Fluid Dynamics (CFD) -based shape optimization techniques. Considering high-aspect ratio rectangular channels, that represent the geometry of heat transfer devices used, among other applications, for batteries cooling systems and compact automotive heat exchangers, the first step of a project towards the design of optimized turbulence enhancers is the validation of the CFD methodology. In particular, since it is important to have a fast-running 3D simulation with physical meaning, several techniques of increasing complexity and computational requirements and times are currently under scrutiny, and their validation is mainly carried out by experiments that measure both local and global heat transfer enhancement. In this context, flow visualization is a powerful tool to get insights of the physical mechanisms that generate convective heat transfer enhancement. In this paper, the setup of a laser sheet scattering method to visualize turbulent structures in a Plexiglas, 1:10 aspect-ratio, rectangular channel ribbed on one of the main surfaces is presented, and results on two different cutting planes are presented for 90° and V-down squared ribs for Re = 500.

Natural convective heat transfer of Al2O3-Cu/water hybrid nanofluid in a rectotrapezoidal enclosure under the influence of periodic magnetic field

This study seeks to examine quantitatively the two-dimensional unsteady free convective heat transfer enhancement of Al2O3-Cu/H2O hybrid nanofluid within a rectotrapezoidal-shaped structure. The lower wall is both uniformly and non-uniformly heated with different thermal boundary conditions, the top horizontal wall is adiabatic, and the vertical and inclined walls are assumed chilly. The enclosure is penetrated by a periodic magnetic field. A set of variable transformations is used to render the governing equations dimensionless, which makes it possible to identify the model parameters that control it. The finite element method is then used to numerically solve these equations. To verify the code's numerical precision, comparisons with formerly available works are performed and, a high level of agreement is obtained among the results. The generated outcomes are shown for the principal parameters that control the flow in the context of streamlines, heat lines, isotherms, average Nusselt number, friction factor, and thermal efficiency index. The findings demonstrated that the efficiency index, friction factor, and heat transmission are all considerably influenced by the Rayleigh number (Ra). The lowest Ra yields the maximum thermal efficiency. The results also revealed that the hybrid nanoparticle volume fraction and period of the magnetic field amplified the heat transfer rate and efficiency index due to the increased pumping force. It is observed that when the percentage of the mixing ratios for Cu nanoparticles rises from 0% to 100%, the heat transport in Al2O3-Cu/H2O hybrid nanofluid increases for a fixed volume fraction of hybrid nanoparticles. As compared to parabolic and sinusoidal boundary conditions, the uniform temperature boundary setting for Ra = 105 exhibits the highest average rate of heat transport of 10.04. The average Nusselt number increases by 25.12% at Ra = 103 with a 3% volume fraction of hybrid nanofluid as compared to pure water and the mixing ratios of Al2O3:Cu = 0:300 gives the highest increased rate of 16.30%. The results further illustrate that Al2O3-Cu/H2O hybrid nanofluid has the highest heat transfer rate compared to the pure water and Al2O3– H2O nanofluid.

Effect of Magnetic Flow and Convective Heat Transfer Enhancement Using Hybrid Nanofluid: A Structured Review

Convective heat transfer is vital in a variety of engineering applications, including thermal management systems, electronic refrigeration, and energy conversion devices. Improving the rate of heat transfer in these systems is of the utmost significance for increasing their efficiency. This review focuses on the single and combined effects of these parameters on improving heat transmission in such systems. The heat and mass transfer of nanofluid is greatly influenced by various factors, including the intrinsic features of the nanofluid, the process used for synthesising the nanofluid, the impact of magnetic force, the concentration and size of nanoparticles, and the Reynolds number (Re). Furthermore, it is important to note that the material characteristics, thermal properties, and performance of magnetic nanofluids are significantly influenced by slight variations in the magnetic force and magnetic field gradient. Multiple research projects have reached the agreement that the inclusion of a magnetic field within magnetic nanoparticles enhances the convective heat transfer capabilities of a nanofluid, resulting in an improvement ranging from around 13% to 75%. Moreover, several applications of hybrid nanofluids in thermal systems have been introduced.

Controlling TPMS lattice deformation for enhanced convective heat transfer: A comparative study of Diamond and Gyroid structures

This study investigates the enhancement of convective heat transfer in Triply Periodic Minimal Surfaces (TPMS), focusing on Gyroid and Diamond lattice structures. An innovative lattice structure control method, employing a Deform-Control parameter (β), is proposed to modulate lattice deformation within their mathematical frameworks. Numerical simulations validated by experimental measurements were conducted to analyze the impact of varying β values from 0 to 0.75 on the thermo-hydraulic performance of both lattice types at flow rates ranging from 0.4 m3/h to 2 m3/h. The results indicate significant improvements in heat transfer efficiency with increasing β. For Gyroid structures, increasing β from 0.1 to 0.75 led to a 3.8% to 32.7% rise in convective heat transfer coefficients and a 3.2% to 23.7% increase in Nusselt numbers. Diamond structures also demonstrated enhancements, with coefficients and Nusselt numbers climbing by 2.9% to 10.4% and 3% to 7%, respectively. Notably, both Gyroid and Diamond structures showed increase in flow resistance at higher β values, highlighting a critical balance between heat transfer efficiency and fluid flow considerations. This research presents a promising control method for optimizing heat transfer in TPMS structures, offering opportunities for more efficient and effective heat dissipation solutions.

Multiobjective optimization of Vortex Generators for heat transfer enhancement in turbulent flows

Convective heat transfer can be enhanced by streamwise vortices and coherent flow structures produced downstream Vortex Generators (VGs). The thermal performance depends on the VG shape which controls the intensity and topology of the vortices. Thus, VG shape optimization is a major challenge in designing sustainable and efficient heat exchangers. In the present study, a multi-objective optimization is performed through large space exploration design to find optimal VG shapes that adapt to different flow regimes. Simulations are carried out in parallel plate turbulent flow for three different Reynolds numbers; Re1=4600,Re2=9920, and Re3=30050. The present numerical results are first validated against numerical and experimental data from the open literature. Then, and for the first time, seven design parameters are investigated, based on VG shape dimensions. The new optimized VG show a thermal enhancement factor between 11 and 35%, relative to a smooth channel case, and between 3 and 14%, relative to different VG shapes from the recent open literature. The present results are analyzed in details by focusing on the physics of the flow structure and its correlation to the convective heat transfer enhancement using local criteria for flow visualization and vortex intensity quantification.

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.

Buoyancy and Velocity Field Synergy Principle in Convective Heat Transfer and Its Role in Thermo-Hydraulic Performance Improvement

Abstract This study proposes the buoyancy and velocity field synergy principle and aims to enhance thermo-hydraulic performance in convective heat transfer. A mechanical energy conservation equation concerning synergy between buoyancy and velocity was derived, which describes the mechanical energy transport and dissipation in convective heat transfer. Two new field synergy numbers, FsU,g and FsU,∇p, were proposed to characterize the degree of synergy between velocity and buoyancy, and the degree of synergy between velocity and pressure gradient over the fluid domain, respectively. The pressure drop of a channel subjected to convective heat transfer is related to not only Gr/Re2 but also FsU,g. Under a same Gr/Re2, a larger |FsU,g| leads to a smaller |FsU,∇p|, and thus the pressure drop is decreased. Furthermore, the multifield synergetic relationships among buoyancy, velocity, temperature gradient and pressure gradient were analyzed for convective heat transfer in channels. The correlation between FsU,∇p and (Gr/Re2)FsU,g, and the correlation between FsU,g and a traditional field synergy number characterizing convective heat transfer capability, Fc, were derived, which reveals the coupled mechanisms of mechanical energy dissipation and thermal energy transport in convective heat transfer. The proposed principle was applied in typical channel flows subjected to convective heat transfer, and its benefits were demonstrated. It is noted that both pressure drop reduction and convective heat transfer enhancement can be achieved in convective heat transfer using the proposed principle. This paper provides a new insight for improving thermo-hydraulic performance of heat exchangers.

MHD radiative natural convective heat transfer enhancement for Casson nanofluid flow on a horizontal circular cylinder

The current numerical analysis investigates the sensitivity of heat transfer enhancement in the magnetohydrodynamic radiative natural convection flow of a Casson nanofluid over a cylinder, which has significant ramifications for different fields of science, e.g., biomedical devices, aerospace engineering, nanofluid-based cooling systems, energy harvesting devices, and drag reduction in marine engineering. The effects of numerous physical parameters on heat transfer characteristics are investigated using numerical and statistical simulations. Here methanol is taken as the base Casson fluid, while enhancement is achieved by introducing the nanoparticles such as graphite oxide, along with single-wall carbon nanotubes and multi-walled carbon nanotubes. The transformed governing differential equations are numerically solved using a second-order accurate implicit finite difference scheme known as the Keller-Box technique. To conduct the sensitivity analysis RSM is used to explore the impact of various physical quantities on heat transfer. The influence of Nusselt number (Gr−1/5Nu) and skin friction (Gr1/5Cf) against various physical quantities associated with the behaviour of Casson nanofluid flow are shown in terms of graphs and tables. One of the key findings is that an increase in the Casson parameter leads to a reduction in Gr1/5Cf and an increase in the Gr−1/5Nu. Moreover, among the nanoparticles studied, multi-walled carbon nanotubes prove to be the most effective in enhancing heat transfer.

Turbulent Heat Transfer of Hybrid Nanofluid with Nano-Encapsulated Phase Change Material in a Square Channel

Investigation of turbulent mixed convection of different working fluids, namely, pure water, CuO/water nanofluid, n-octadecane/water nanofluid (or slurry), and a hybrid water-based suspension of CuO and n-octadecane nanoparticles in a uniformly heated vertical square channel is carried out. The shear stress transport/kappa-omega model is applied to simulate the turbulent flow. Eulerian–Lagrangian two phase model and homogeneous model are employed to simulate the nanofluid flow. It is shown that Eulerian–Lagrangian two-phase model predicts higher Nusselt number and friction factor than single-phase model and the results of two-phase model are closer to experimental data. It is found that a new hybrid nanofluid with dispersed nano-encapsulated phase change material particles provides better thermal and hydrodynamic performances compared to other nanofluids. Slurry containing 5% n-octadecane provides an improvement of nearly 37.2%, while hybrid nanofluid (1% CuO + 5% n-octadecane) offers added convective heat transfer enhancement of almost 60%. With increasing volume fraction, friction factor becomes slightly larger than that of pure water. For 5% n-octadecane/water and 1% CuO + 5% n-octadecane/water nanofluids, friction factor increases by 2.5% and 1.7%, respectively. Due to dominance of thermal entropy generation, total entropy generation reduces by increasing Reynolds number and volume fraction.

Performance improvement in a heat exchanger tube using discrete X–V baffle (DXVB) turbulators

Thermal performance improvement in a heating tube (HT) using discrete X–V baffle (DXVB) as turbulators is presented. The DXVB is designed with three main goals: 1. To decrease the pressure drop in the tested tube and 2. To increase the mixing quality in the heat exchanger tube and 3. To produce powerful vortex flows. Lower pressure drop, generated vortex flows and higher mixing quality will augment a heat transfer rate and thermo-hydraulic efficiency. This study examines effects of DXVB structures, DXVB distances and placements on laminar air flow and thermal structure with Reynolds number from 100 to 2000. The current problem is numerically investigated using the finite volume technique of a commercial program/code (ANSYS-FLUENT V.2022). The created model of the HT fitted with the DXVB is numerically validated. From the numerical results, the DXVB installed affects the air flow structure in the tested tube. The vortex streams are observed. The change in fluid flow influences thermal characteristics. The vortex streams destroy some parts of the thermal boundary layer and help to increase mixing quality. These perturbed thermal boundary layer (TB-layer) and better mixing quality are two important factors that contribute to convective heat transfer enhancement. In addition, the maximum heat transfer rate and pressure loss are 10.17 and 45.75 times upper than that in the circular plain tube, respectively, while the best thermal enhancement factor (TEF) of 3.09 is observed at 15b5g, +x at Re = 2000.

Thermal performance analysis of triangular solar air heater duct having broken V-shaped ribs

In the present investigation, a numerical study has been done to analyse the effect of attachment of broken V-shaped ribs on the heating surface of the triangular-shaped solar air heater duct. The roughness and flow parameters include relative roughness pitch (between 8 and 14), angle of attack (between 30 and 75 degrees) and Reynolds number (between 4000 and 20,000). The effect on heat transfer and flow characteristics has been observed due to variation in parameters. Considerable enhancement in convective heat transfer is observed along with frictional effect and indicated by Nusselt number and friction factor values. All the variations resulted in an increase in the performance of a solar air heater, which was indicated by the value of the thermohydraulic performance parameter being more than 1. Abbreviations: CFD, Computational fluid dynamics; SAH, Solar air heater; THPP, Thermohydraulic performance parameter

EFFECT OF GEOMETRICAL PARAMETERS ON THE THERMAL-HYDRAULIC PERFORMANCE OF INTERNAL HELICALLY RIBBED TUBES

In this paper, single-phase heat transfer characteristics of water inside internal enhanced tubes were investigated. Refrigerants are boiling or condensing outside the tube. The experimental tubes have internal helical rib heights of 0.25-0.36 mm, helix angles of 40-60°, rib base thicknesses of 0.40-0.79 mm, rib tip thicknesses of 0.078-0.283 mm, and N<sub>s</sub> (starts number per circle) of 40-50. It shows that the heat transfer enhanced ratios usually range from 2.3 to 3.64. The friction factors relative to the smooth tube are about 1.8 to 3.3 times higher. Analyzing the effect of rib geometry on flow and heat transfer, it was found that the higher the height of the internal rib, the better the enhancement of convective heat transfer in the tube. Also, the greater the thickness of the rib tip and base, the more detrimental to the friction factor in the tube. There was no noticeable influence on the heat transfer performance as the helix angle increased from 45° to 50°. For the increase of N<sub>s</sub>, it appears that 45 ribs per circle is the best value in the present study when considering the increase in pressure loss. The thermal-hydraulic performance of 11 tubes was also evaluated. It shows that Tube-1 had the best performance in the condensing tubes and Tube-7 had the best performance in the boiling tubes.

Local heat-transfer coefficient estimation in cross-helix corrugated tubes under turbulent regime

In a wide variety of engineering applications, convection heat transfer enhancement plays a major role in reducing the heat exchanger’s size and costs and increasing their efficiency. The passive methods for increasing convective heat transfer are the most interesting since they don’t require any external power to achieve the desired enhancement. For this reason, this approach is preferred in the industrial sector of heat exchangers. For the enhancement of the performances of these apparatuses, in particular, the tubular ones used in the pharmaceutical, food, and chemical, the most adopted passive solution is the corrugation of their tube walls. Two pipes characterized by cross-helix corrugation have been experimentally analyzed both investigating the global heat transfer performance and studying the local heat transfer coefficient distribution to profoundly examine the thermal performance enhancement mechanisms of this passive technique. It has been observed that at Re = 16,000, the heat transfer rate in the smaller pitch tube is 18.18% higher than that in the larger pitch tube. The inverse heat conduction problem in the tube wall is addressed to achieve the local coefficient distribution method by using temperature readings collected from an infrared camera on the outer surface of the tube as the initial data.

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%.