Mixed Convection Heat Transfer Research Articles

Experimental analysis of electrical field combining nanofluids to enhance the mixed convective heat transfer in the rectangular channel

Owing to the disadvantages of low thermal conductivity and large viscosity, the mixed convective heat transfer efficiency of transformer oil can not meet the cooling demand of transformer. Furthermore, the electrical field force could manipulate the motion of nanoparticles, so as to further strengthen the heat transfer of nanofluids. Therefore, the electrical field and nanofluid are combined in the present study to enhance the thermal performance for cooling transformer. Meanwhile, the effects of electrical field strength, the nanofluid concentration and heat flux on heat transfer as well as mechanism are discussed. Then, a semi empirical correlation is proposed by considering the effects of electrical field and nanoparticles. The result shows that the heat transfer is greatly strengthened up to 65% with the increment of the electrical fields, and there exists an optimal concentration of nanofluid for the best enhanced effect. According to the mechanism analysis, the electrical field force pulls nanoparticles to high potential area and the fluid molecules move with nanoparticles, which impacts the boundary layer. Then, disturbances of nanofluids increase and the boundary layer thickness is decreased. Thus, the boundary layer thickness is reduced and fluid disturbance is strengthened, which significantly enhances heat transfer. Moreover, the predicted values calculated by the semi empirical equations fit the experimental data well.

Comment on the paper “Influence of nanoparticles on the electromagnetic hydrodynamic mixed convection flow and heat transfer of a polymeric FENE-P fluid past a Riga plate in the presence of Arrhenius chemical reaction, Razi Khan, Adeel Ahmad, Journal of Magnetism and Magnetic Materials, 567, (2023), 170352”

Some errors exist in the above paper.

Stability Analysis of Unsteady Mixed Convection Flow Near the Stagnation Point with Buoyancy Effect

This numerical study investigates the behaviour of the unsteady mixed convection flow and heat transfer near the stagnation region past a vertical plate in Al2O3-Cu/H2O hybrid nanofluids. By choosing an appropriate similarity transformation, an ordinary differential equations system is obtained, hence scrutinized via the bvp4c package embedded in the MATLAB program. The results show that as the nanoparticle volume fraction and unsteadiness parameter increase, the fluid's skin friction coefficient also increases. Also, an increase in the nanoparticle volume fraction and unsteadiness parameter has broadened the range of solutions hence, delayed the progress of boundary layer separation. In addition, the presence of the unsteadiness parameter provides a significant result in the thermal system. The first solution is declared stable by the stability analysis.

Mixed Convection in a Horizontal Channel–Cavity Arrangement with Different Heat Source Locations

Several researchers are very interested in mixed convection heat transfer because of how widely it is used, particularly for solar thermal collectors, cooling electronic equipment, and chemical process instruments. Using COMSOL-Multiphysics, this article establishes laminar coupled mixed convection heat transfer characteristics across a horizontal channel–cavity architecture. Investigations are conducted into the effect of heat source location on isotherms, velocity distribution, pressure, temperature, average and local Nusselt numbers, and air density. The intake airflow Reynolds number is assumed constant on 2.8814. The enclosure with an isothermally heated right wall in the shape of a “<” as a heat source in three configurations (heat source in the base (1st case), in the upper step (2nd case), and the below step (3rd case). The obtained numerical results present that the higher heat transfer is performed in case two because the heat source is near the contact surface between the channel and the cavity. With the hot sources’ locations being altered, the velocity distribution seems to be unchanged. The increase in the positive y axis has no impact on the pressure distribution throughout the channel. Changing the position of the heated source does not seem to have any impact on the pressure distribution. Air density profiles start to diverge across cases around y = 0.035 m; the third example has a larger value than the second case, and the latter case has a larger value in the density distribution than the former. The contact between the enclosure and the channel (y = 0), where the greatest Nusselt number also occurs, exhibits the highest heat transfer. The maximal Nusselt number falls as y’s absolute value rises.

Stacked Heterogeneous Ensemble Learning Model in Mixed Convection Heat Transfer from a Vertically Oscillating Flat Plate

In this study, the effects of mixed convection heat transfer from a moving vertical flat plate with an experimental and stacked heterogeneous ensemble learning approach are analyzed. In the experimental work, the effects on both natural and forced convection of dimensionless oscillation amplitude (Ao), dimensionless oscillation frequency (Wo) and Rayleigh number (Ra) are investigated. In the experiments, the vertical movement of the plate is provided by a flywheel-motor assembly. The average Nusselt numbers (Nu) on the fixed plate and the moving plate surface were obtained. Additionally, this study is focused on the prediction of heat transfer of a moving flat plate using single-based algorithms (Gradient Boosting, AdaBoost, Multilayer Per-ceptron) and a stacked heterogeneous ensemble learning model. The statistical per-formance of the single-based algorithms and the stacked ensemble model is meas-ured in the prediction of mixed convection heat transfer. The results show that the stacked-based ensemble learning model yielded the MSE = 2.01, RMSE = 1.42, MAE = 1.1 and R2 = 0.99 values. Overall, this study reveals that the proposed stacked en-semble machine learning model can be used successfully for modeling convection heat transfer of a moving plate.

Effect of cavity aspect ratio on mixed convective heat transfer phenomenon inside a lid-driven cavity due to elastic turbulence

This study performs extensive numerical simulations to investigate how the aspect ratio (AR) of a lid-driven cavity influences the onset of elastic instability and elastic turbulence and the subsequent mixed convective heat transfer rate inside it. To this end, we utilize the finite volume method based open source code OpenFOAM along with Rheotool to solve the mass, momentum, energy, and viscoelastic constitutive equations. We find that the dependency of the cavity AR on the heat transfer rate is highly complicated depending upon the values of the Richardson (Ri) and Prandtl numbers (Pr). At low values of Ri, the heat transfer rate continuously decreases with AR irrespective of the value of the Prandtl number and the fluid type, i.e., Newtonian or viscoelastic. The same trend is also observed at high values of Ri and low values of Pr. At these combinations of Ri and Pr, the heat transfer rate is always higher in viscoelastic fluids than in Newtonian fluids due to the presence of elastic turbulence in the former fluids. However, a different trend is observed at high values of both Ri and Pr. At this combination of Ri and Pr, the heat transfer rate increases with AR in Newtonian fluids, whereas it decreases in viscoelastic fluids. Therefore, at high values of AR, Ri, and Pr, the heat transfer rate is higher in Newtonian fluids than that in viscoelastic fluids despite the presence of elastic turbulence in the latter fluids. This is in contrast to the assumption that the elastic turbulence phenomenon always increases the rate of transport processes. A possible explanation for this behavior is provided in this study. Along with the heat transfer aspects, we also provide a detailed discussion on how the cavity aspect ratio influences the corresponding flow dynamics inside the cavity. In particular, we find that the onset of the elastic instability (and the subsequent elastic turbulence) phenomenon is delayed to higher values of the Weissenberg number as the cavity aspect ratio increases. This is in line with prior experimental studies reported in the literature.

Mixed Convection Heat Transfer in a Channel-Open Cavity with Two Heat Sources

Mixed Convection Heat Transfer in a Channel-Open Cavity with Two Heat Sources

Mixed convection heat transfer and entropy generation of MHD hybrid nanofluid in a cubic porous cavity with wavy wall and rotating cylinders

The current simulations are for the three-dimensional (3D) Fe3O4/MWCNT-water hybrid nanofluid flow within a 3D cubic enclosure filled with a porous medium. It was assumed that the flow area contains two rotating cylinders and has an upper wavy wall. Also the magnetic field is also applied to the problem due to magnetic nanoparticles in the base water. The Galerkin finite element method (GFEM) with a triangular element shape was applied to solve the governing equations. The findings were shown for a range of flow parameters such as angular speed of the cylinder (Ω=-500–1000), Hartmann number (Ha = 0–10), Darcy number (Da = 10−5–10−2), and direction of cylinders rotations and their positions in the cavity. The influence of the various parameters on flow, thermal transport, and entropy production is illustrated by the stream function, isotherms, and isentropic contours. Higher values of Ω Da and lower values of Ha enhanced the heat transfer and Nusselt number. Entropy production is mostly due to heat transfer; however, fluid-friction and magneto effects also contribute.

Experimental and numerical study on the heat transfer enhancement over scalene and curved‐side triangular ribs

AbstractThe present study examines the turbulent flow of mixed convection heat transfer enhancement within a rectangular channel considering three different novel shapes of ribs (smooth, scalene, and curved‐side triangular). The investigations were conducted experimentally by developing a new test facility, while the numerical computations were carried out using the finite volume method. The experimental work involves constructing of the channel, ribs, and all equipment and measurement instruments. The numerical work is based on ANSYS FLUENT considering the k–ε turbulent model. The results are presented and compared in terms of Nusselt number, friction factor, and performance factors for Reynolds numbers ranging between 3000 and 12,000. By comparing the average values of the numerically obtained Nusselt number with experimental measurements, the data showed a close agreement with a maximum difference of 5%. It also found that scalene triangular ribs (STRs) provide better performance in terms of heat transfer, although introducing a slight increase in friction losses. STRs showed (20%) increase in Nusselt number compared with smooth channel, and 3%–6% increase in Nusselt number compared with curved‐side triangular ribs (CTRs). In contrast, CTRs have a lower friction factor value of 5% compared with STRs at a low value of a Reynolds number of 3000. Furthermore, the Nusselt number changes significantly (250% increase) by increasing the value of the Reynolds number from 3000 to 12,000. A thermal performance factor of up to 1.28 was achieved for the STRs at the lowest range of Reynolds' number of 3000. The findings from the present study are of practical importance for industries requiring heat transfer enhancement techniques to improve heat transfer equipment performance.

Mixed convective heat transfer characteristics and mechanisms in structured packed beds

The structured packed bed is considered a promising reactor owing to its low pressure drop and good heat transfer performance. In the heat transfer process of thermal storage in packed beds, natural convection plays an important role. To obtain the mixed convective heat transfer characteristics and mechanisms in packed beds, numerical simulations and coupling analyses were carried out in this study on the unsteady process of fluid flow and heat transfer. A three-dimensional model of the flow channel in the packed bed was established, and the Navier–Stokes equations and Laminar model were adopted for the computations. The effects of the driving force on fluid flow around a particle were studied in detail. The differences in velocity and density distributions under different flow directions due to effect of the aiding flow or opposing flow were intuitively demonstrated and quantitatively analyzed. It was found that the driving force strengthens the fluid flow near the particle surface when aiding flow occurs and inhibits the fluid flow when opposing flow occurs. The boundary layer structure was changed by the natural convection, which in turn influences the field synergy angle. For the aiding flow, the coordination between the velocity and density fields is higher than that for the opposing flow. By analysis the effects of physical parameters on mixed convective heat transfer, it is indicated that with an increase in the fluid-solid temperature difference or the particle diameter, or a decrease in the fluid temperature, the strengthening or inhibiting effect of natural convection on the heat transfer became more significant.

MHD Mixed Convective Non-Newtonian Stagnation Point Flow Over an Inclined Stretching Sheet: Numerical Simulation

This paper includes numerical simulation upon MHD mixed convective heat transfer properties of stagnation point flow across an angled stretched sheet. Boundary value problem is solved using similarity transformation approach with shooting technique. The impact of different corporeal constraints like mixed convection parameter, thermal radiation parameter, chemical reaction parameter, Brownian motion and thermophoresis, Casson parameter upon velocity and temp profile as well as skin-friction coefficient , Nusselt number, Sherwood number on velocity, temp concentration profile are shown graphically. Casson parameter increases velocity and diminishes temperature profile. Chemical reaction term decreases Sherwood number and increases concentration profile.

Significance of thermo-diffusion and chemical reaction on MHD Casson fluid flows conveying CNTs over a porous stretching sheet

The current work aims to study the magnetic field on mixed convection heat transfer with Casson fluid flow over a porous medium in the presence of the thermo-diffusion mechanisms with CNTs. CNTs have a wide range of industrial, biomedical, energy production, and space cooling applications because they can improve the thermal properties of the base material. A non-linear governing equation is mapped to a set of ordinary differential equations via a similarity transformation. The highlight of the work is the Casson fluid using CNTs over a Darcy porous media through momentum and temperature profile with the support of an analytical process. The various plots are also used to discuss the effects of physical parameters such as Casson fluid; mass transpiration, Darcy number, and Dufour-Soret number on the profiles, which are depicted through graphical representations. Modulating temperature distribution in different industrial applications, including solar collector design, electronic cooling, building ventilation, etc. The study reveals that the thermal radiation, Casson fluid; mass transpiration, Darcy number, Dufour-Soret number, Lewis number, and Prandtl number increase the values of Richardson number, also buoyancy ratio, magnetic parameter, and volume fraction decrease the values of buoyancy ratio, also controls the transfer of heat. Studies are done on boundary effect studies that have an impact on fluid flow. The behavior of physical factors, such as the Lewis, Prandtl, Dufour, and Soret numbers influencing temperature and concentration field, is determined and presented using graphs. Highlights The analytical method is used to executive equations are converted in the highly non-linear ODEs are obtained from PDEs. The importance of SWCNTs and MWCNTs in modifying MHD, Richardson number, Buoyancy ratio Casson nanofluid parameter and Heat source/sink, carbon nanofluid flow is examined. Stretching/shrinking sheet under the impact of Magnetic parameter, Prandtl number, and mass transpiration non-Newtonian heating is considered. Water is a base fluid mixed with SWCNTs and MWCNTs. The impact of magnetic parameter, Casson fluid, Darcy number, Lewis number, Dufour number, Soret number, mass transpiration, and nanoparticle volume fraction parameter studied.

Non-Similar Solutions of Dissipative Buoyancy Flow and Heat Transfer Induced by Water-Based Graphene Oxide Nanofluid through a Yawed Cylinder

The fluid flow through blunt bodies that are yawed and un-yawed frequently happens in many engineering applications. The practical significance of deep-water applications such as propagation control, splitting the boundary layer over submerged blocks, and preventing recirculation bubbles is explained by the fluid flow across a yawed cylinder. The current work examined the mixed convective flow and convective heat transfer by incorporating water-based graphene oxide nanofluid around a yawed cylinder with viscous dissipation and irregular heat source/sink. To investigate the heat diffusion across the system of buoyancy effects, the mathematical formulation of the problem was modeled in terms of coupled, nonlinear partial differential equations. The boundary value problem of the fourth-order (bvp4c) solver was operated to find the non-similarity solution. The outcomes indicated that the velocity in both directions enlarged owing to the higher impacts of yaw angle for the phenomenon of assisting flow but decreased for the instance of opposing flow, while the temperature of nanofluid increased because of heightened estimations of yaw angle for both assisting and opposing flows. In addition, with larger impacts of nanoparticle volume fraction, the shear stresses were enhanced by about 0.76% and 0.93% for the case of assisting flow, while for the case of opposing flow, they improved by almost 0.65% and 1.38%, respectively.

Darcy-Forchheimer Flow of a Nanofluid Over a Porous Plate with Thermal Radiation and Brownian Motion

This research looked at the effects of thermal radiation and Brownian motion on the mixed convection heat and mass transfer of nanofluid over a porous plate in a Darcy-Forchheimer flow. The controlling partial differential equations are converted to ordinary differential equations and numerically solved by using Runge-Kutta fourth-order alongside shooting approach. Different physical parameters’ effects on temperature, velocity, and concentration distribution are studied. In addition, the results are also graphically represented. The reduced local Nusselt number and Sherwood number are shown and compared to previous investigations; the comparisons are extremely close. According to the results, the rate of heat transfer is lowered by 63.83% when the Brownian parameter is increased from 0.2 to 1 (in case of suction) and by 83.31% when the Brownian parameter is increased from 0.2 to 1 (in the case of injection). Also, in the case of suction, thermophoresis parameters lowered it by 50.48%, and in the case of injection, it was reduced by 65.08%.

Mixed convection heat transfer and transient flow of high-pressure gas in a large-scale cavity with rotating blade

Mixed convection heat transfer phenomenon, incorporating natural and forced convection process, occurs widely in natural and industrial fields. Nevertheless, few works have investigated numerically the turbulent mixed convection heat transfer within the large-scale structure under high-pressure condition. In this study, a three-dimensional numerical model of a large-scale closed cavity with two rotating blades and reactor has been built on account of the finite volume method, aiming to explore the mixed convection heat transfer and turbulent flow of internal high-pressure gas. Compressible gas with varying thermophysical properties was chosen as the heat transfer medium and the numerical model was solved by Unsteady Reynolds-Averaged Navier-Stokes (URANS). Turbulent flow and heat transfer, inside the enclosure and reactor, were compared through the variation of the Reynolds number (Re), gas pressure (P) and heating surface temperature (Th). The results signify that for given P and Th, varying Re has a consistent impact on the transient flow and heat transfer process inside the enclosure and reactor. The turbulent flow and thermal transport are enhanced as increasing the Re. When the Re = 900,000, an enhanced forced convection effect is found inside the reactor. Dissimilar impacts on the flow field and thermal transport inside the enclosure and reactor are found with changing gas pressure owing to the interaction of natural and forced convection. The flow field attenuates gradually as the increment in P, and thereby larger high-temperature zone is achieved at a smaller pressure inside the enclosure, whilst inside the reactor, faster heat conduction is attained at a bigger pressure. Changing heating surface temperature has a more significant influence on the flow field and heat transfer inside the reactor compared to the enclosure. Additionally, increasing the Re and Th has a smaller effect on the average Nusselt number (Nuavg) while a positive correlation between the Nuavg and P is obtained.

Mixed convective and radiative heat transfer of LiF-BeF2-ThF4-UF4 in a vertical circular tube

Molten salts are promising fluids for applications involving heat transfer and thermal energy storage. The buoyancy effect and radiative heat transfer effect are frequently ignored in previous studies that concentrate on forced convective heat transfer of molten salts. However, these effects might not be negligible, particularly for laminar flows of optically semi-transparent fluids, such as molten salts. Therefore, this study tries to understand the heat transfer characteristics of molten salts, including mixed (forced and natural) convective heat transfer and radiative heat transfer. A two-dimensional axisymmetric model is created in STAR-CCM+ for assisting (buoyancy-assisted) laminar flows of LiF-BeF2-ThF4-UF4 (67.5–20.0–12.0–0.5 mol%) in a 5-m long, 15-mm-inner-diameter vertical circular tube. The effects of several non-dimensional parameters on the velocity, temperature, and Nusselt number are examined, including the Reynolds number (Re¯=2.0×102to1.8×103), Grashof number (Gr¯=3.1×103to5.4×104), optical thickness (τ=0.1τ0to10τ0, τ0 = 0.27, 2.11, and 446.47 for the three bands λI = 0.10 to 3.36 µm, λII = 3.36 to 5.42 µm, and λIII = 5.42 to 100.00 µm, respectively), and emissivity (ε=0.0to1.0). A flow regime map is also suggested to assess the radiative heat transfer effect in molten salts. Our analysis demonstrates that: (1) the predicted results in this study deviate from published experimental data by 14.5 to 45.1 % without taking the buoyancy effect and radiative heat transfer effect into account, however, these differences are dramatically decreased to 10.1 to 17.4 % while considering the buoyancy effect or 7.5 to 14.7 % while considering the buoyancy effect and radiative heat transfer effect; (2) the radiative heat transfer effect tends to mitigate the buoyancy effect, but their net effect enhances heat transfer; (3) the buoyancy effect factor fbuoy and thermal radiation effect factor frad are, respectively, up to 38.5 % and 19.5 % under the conditions investigated, illustrating the need to evaluate the two effects for molten salts; (4)fbuoy becomes larger for smaller values of Re¯, τ, and ε, and larger values of Gr¯ and non-dimensional axial location x/Di. frad becomes larger for smaller values of Re¯ and larger values of Gr¯, τ, ε, and x/Di; and (5) the flow regime map provided in this study aids in determining whether the radiative heat transfer effect in molten salts is negligible.

Numerical investigation of turbulent mixed convection flow and heat transfer characteristics of 2 × 2 subchannels with spacer grids

AbstractIn turbulent mixed convection conditions, flow and heat transfer can deteriorate under specific conditions, thus affecting the performance of the safety of the system, which could imperil the safety of the nuclear reactor. In this paper, by analyzing the velocity, temperature distributions, and other thermal‐hydraulic parameters, the steady‐state computational fluid dynamics approach with the SST k–ω turbulence model used in this study was verified. Then, flow and heat transfer characteristics in turbulent mixed convection inside 2 × 2 subchannels with and without spacer grids were investigated. The results showed that the velocity profiles were distorted and the heat transfer capacity was impaired due to the influence of buoyancy. The Nusselt number ratio (Nu/Nuf) between turbulent mixed convection and forced convection increased with the increase of Reynolds number (Re) until unity but decreased with the increases of Grashof number (Gr) and Buoyancy parameter (Bo*).

Entropy Generation of CuO-Water Nanofluid in a Cavity with an Intruded Rectangular Fin

Entropy generation and heat transfer in cavities have received significant interest due to the ever-increasing demand for enhancing thermal performances in many scientific and engineering fields. In particular, nanofluids are being used increasingly in engineering applications and real-life problems, as they exhibit significantly better thermal properties than basic heat transfer fluids, for example, water, oil, or ethylene glycol. This study investigates the entropy generation and heat transfer of a nanofluid in a confined cavity with a moving top wall and a rectangular fin at the bottom. Here, a macro-homogeneous model based on a previously developed model is employed for investigating the mixed convective flow and heat transfer of CuO-water nanofluid. Various fin geometries, Rayleigh numbers, Reynolds numbers, and nanofluid concentrations have been employed. Present results indicate that the heat transfer rate can be improved, while entropy generation can be minimized using nanofluids instead of conventional heat transfer fluids.

Mixed Convection Heat Transfer From Swirling Open Spherical Cavity

Abstract This work reports a numerical study on mixed convection flows around a swirling spherical shaped open vessel in air within the laminar regime. This investigation is quite important and relevant in various industrial operations like centrifugal casting, formation of shield surfaces, thermal processing of different food stuffs, etc. This study aims to characterize the fluid flow and heat transfer behavior from both inner and outer surfaces of the open cavity. Governing differential equations, such as continuity, momentum, and energy are solved by using finite volume technique to describe the effect of relevant pertinent parameters over wide range: Rayleigh number (103≤Ra≤107), height to diameter ratio (0.15≤h/D≤0.95), and Reynolds number (0≤ReD≤300). It is observed that the plume is deformed greatly by swirling effect at higher ReD and lower Ra for a fixed h/D. The percentage of increase of heat transfer rate from ReD=0 to ReD≠ 0 is significantly higher at lower Ra for all cases of h/D. Lastly, a suitable correlation for average Nusselt number is proposed as a function of Ra, h/D, and ReD, which shows a satisfactory agreement with numerical data. This correlation is expected to be helpful for academic and industrial purposes.

Impact of size and location of outflow opening vent on mixed convective heat transfer induced by two aligned heated cylinders immersed in a partially open channel

AbstractThe current work deals with the numerical investigation of mixed convection occurring from two hot cylinders placed inside vented ducts under different sizes of inflow and locations of outflow opening vents. The mathematical formulation is solved based on the finite volume approach. The location of the outflow section is changed to left, center, and right of enclosure. Impacts of inflow section sizes (1 = 0.125 and 1 = 0.25), locations of outflow section ( = ), space between cylinders (), Reynolds number (), and Richardson number () are implemented. Findings display that the average Nusselt number (Nu) is exceeded by 20% for low due to the effect of nonsymmetrical inflow and outflow sections as compared with the case of equal opening vents. For large Ri, a higher Nu is obtained for nonequal opening vents in comparison with the case of equal opening vents by 10%. The difference in the average Nu between left and right cylinders is about 11%. Findings display that the enhancement in Nu due to the large spacing size between cylinders as compared with those of low spacing size is about 35%. The variations of Nu display the opening vents ratio, Re, , and have an extreme action on the characteristics of flow and temperature fields.