Mixed Convection Heat Transfer Research Articles

Enhancement of carbon nanotubes/kerosene nanofluids on mixed convective heat transfer in rectangular enclosures

With the objective of understanding how the mixed convective and the geometrical parameters together influence the nanofluid behavior (enhancement or deterioration) and finding the best configuration of the moving walls for engineering purposes, this paper investigates the single lid-driven mixed convective flow of carbon nanotubes/kerosene nanofluids confined in vertical and horizontal cavities with non-uniform boundary conditions applied to the vertical walls. The findings of this paper can provide crucial conclusions into the behavior and optimization of nanofluids in mixed convective flows in enclosed systems for engineering applications. The system of the governing equations is tackled by a validated in-house code, where we compared the code accuracy with previous experimental and numerical works. The governing parameters are -50 ≤ Pe ≤ 50, 0.5 ≤ A ≤ 4, and 0 ≤ φ ≤ 3%. The effective nanofluid properties are calculated using special experimental models. The outcomes revealed that the carbon nanotube effect on heat transfer depends on the aspect ratio. The greater the aspect ratio, the more pronounced the negative impact of the nanotube concentration where Nusselt number is reduced by 4.49% and 16.17% for φ = 0.01 and 0.03, respectively, in the case of Pe = 0 and A = 4. The Peclet number also has a significant impact on the heat transfer, especially for higher aspect ratios, and diminishes the critical aspect ratio where the nanofluid's behavior changes. An augmentation in the aspect ratio and Peclet number brings about an augmentation in the Nusselt number. Additionally, the Peclet number can reverse the conjugated effect of carbon nanotube volume fraction and aspect ratio on nanofluid enhancement. When Pe = 50 in C1, heat transfer rate is 1.75% and 1.11% enhanced for φ = 0.01 and 0.03, respectively, and A = 4. Moving horizontal walls exhibits higher heat transfer enhancement compared to vertical ones.

Mixed convective heat and mass transfer of Carreau fluid flow past over a permeable stretching disk: thermal and concentration analysis

Mixed convective heat and mass transfer of Carreau fluid flow past over a permeable stretching disk: thermal and concentration analysis

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

Numerical Approach of the First Instability Appearance in Inclined Taylor–Couette System

The present study numerically investigates the three-dimensional forced and mixed convection heat transfer in an inclined Taylor–Couette system (η=0.5 and Γ=20) submitted to a radial temperature gradient. The main objective of this study is to determine the effect of the angle inclination duct on the thermal and dynamic fields. Several cases have been dealt with depending on the inclination angle to detect the critical Reynolds number in each case. The model of the conservation equations with their boundary conditions is numerically solved by the finite volume method with a second-order spatiotemporal discretization. The results show that, in forced convection, the effect of the inclination angle is inexistent on the velocity field. However, with the presence of buoyancy effects, which impact flow stability and the transition to turbulence, the inclination influences both velocity and temperature fields. It also shows that selecting the vertical position of the annulus is preferable to obtain hydrodynamic stability in mixed convection. At the same time, from the thermal point of view, it is preferable to select the horizontal position to get dynamic and thermal stability.

Research on heat transfer characteristics and prediction methods of supercritical liquified natural gas flowing horizontally under the influence of buoyancy

An exhaustive elucidation of the heat transfer performances and mechanisms of supercritical LNG under water-bath heating conditions is paramount for optimizing heat transfer and designing related heat exchangers. This study employs experiments and numerical simulations to explore the heat transfer mechanisms and prediction methods of supercritical LNG flowing horizontally under buoyancy. The experimental investigation was executed within a small SCV using the N2-Ar mixture as a substitute medium for safety considerations. Concurrently, the accuracy of the numerical model employed is rigorously authenticated through a comparative analysis with the empirical findings obtained from the experimental endeavor. The investigation delves into the impacts of operating conditions, structural parameters, and medium composition on heat transfer through numerical simulations. The influence mechanism of buoyancy on heat transfer is meticulously explicated by examining the distribution of flow fields and thermophysical properties within the boundary layer. The introduction of a novel criterion, Bufull, derived from the analysis of wall shear stress variation caused by buoyancy, facilitates a quantitative assessment of the impact of buoyancy on heat transfer. Additionally, a simplified buoyancy criterion, FBu, is introduced, dependent solely on fundamental operating parameters and tailored for engineering applications, with a determined critical value of 1/ FBu = 60.3. Based on the dramatically variable thermophysical and transport properties, that occurs near the critical point, as well as buoyancy correction, a correlation is formulated for predicting mixed convection heat transfer, exhibiting an MAE of merely 3.04% and accurately predicting 81.28% of the data within a ± 5% margin of error, surpassing existing correlations. Finally, the proposed heat transfer calculation method is validated using five groups of operational data from two in-service SCVs. The maximum absolute discrepancy between the computed outlet temperature and the monitored value is 1.95K. The findings of this paper can support practical applications.

Machine learning algorithms on predicting the turbulent mixed convection flow in a driven-cavity with two horizontal cylinders

Turbulent mixed convection flow and heat transfer properties in a driven cavity with two circular cylinders arranged one above another are analyzed numerically with the incorporation of a finite element scheme, based on the Galerkin method of weighted residuals. A comparison of streamlines, isotherms, and local, average Nusselt number is provided to illustrate how the Richardson number Ri, Reynolds number Re and the eddy viscosity ε affect the transport phenomena inside the cavity. The local and average Nusselt number have been evaluated and it is found that strong convection at the top wall causes the average Nusselt number to progressively rise with respect to Ri and ε, while steady or slightly varying profiles are seen with regard to Re. In order to predict the thermal distribution inside the cavity due to these hot cylinders, Artificial Neural Network (ANN) and Gaussian Process Regression (GPR) have been developed with these model parameters as input and average Nusselt Number as target values. Several plots have been depicted to come to a conclusion that both models have been trained very effectively with the minimal observed Mean Square Error (MSE) values of 0.0026018 and 0.0026428 for ANN and GPR, respectively. In support to these findings, the coefficient of determination R2 suggests that ANN (R2 = 0.89) outperforms GPR (R2 = 0.87) in testing accuracy, hence it is recommended for prediction task.

Numerical investigation on flow characteristics and heat transfer of mixed convection in a packed bed

Flow characteristic and heat transfer of mixed convection in the packed bed were investigated numerically comparing with the previous experimental studies. For the fixed packed bed and sphere diameters of 0.06 m and 0.01 m, the ratios of bed height to sphere diameter and Reynolds numbers were varied from 5 to 20, and from 1 to 300, respectively for both buoyance aided and opposed flows. The temperatures difference between fluid and sphere was 78.85 K. The packed bed structure was modeled by generating location coordinates through the random number with an in-house code and computational analyses were performed using ANSYS FLUENT software. Laminar mixed convection behaviors were observed: heat transfer enhancement for buoyancy aided flow and impairment for buoyancy opposed flow. For larger bed heights, even under a laminar flow condition, turbulent behaviors were observed: heat transfer impairment for buoyance aided flow and enhancement for buoyance aided flow. The same trend was observed in the analysis of turbulence intensity. Local flow analysis within the packed bed confirmed that this is due to the vortices and the wakes resulting from the interaction between mixed convection flows and the randomly stacked spheres in the packed bed. This study unveiled mixed convection phenomena accompanied by the geometric effects of the packed bed on vortices and wakes, which exhibit turbulence-like behavior.

Mixed convective nanofluid flow and heat transfer induced by a stretchable rotating disk in porous medium

AbstractEnhancement of “heat transfer” using “nanofluid” has diverse potential applications in heat exchangers, thermal management of electric devices, cooling of tractors, solar thermal systems, manufacturing of paper, and many others. Hence, the aim of the current investigation is to explore the impacts of “mixed convection” on “nanofluid flow” over a permeable rotating disk, which is stretched radially in a porous medium. Variable wall “temperature” and “convective boundary conditions” are also considered here. This makes the present investigation different from others. The suitable “similarity transformations” are imposed to alter the governing partial differential equations into a set of coupled ordinary differential equations (ODEs). Then, these ODEs are solved numerically by the “4th order Runge‐Kutta method” using the “shooting technique” with the help of the bvp4c package in MATLAB software. The effects of fluid controlling “parameters” on “flow and thermal fields” as well as “skin friction coefficient” and “Nusselt number” are presented graphically and explained physically. Due to enhanced rotation of the disk, the radial and azimuthal velocity of the fluid increase and the temperature of the fluid decreases. Most importantly, it is observed that when the disk rotates faster than the stretching rate, the temperature of the nanofluid decreases rapidly, which has wider applications for cooling purposes. It is also noted that when the suction parameter increases its value from −1 to 1, for Ag–water nanofluid, the “skin friction coefficient” decreases by 73.56%, and the Nusselt number also decreases by 24.11%, and for Fe3O4–water nanofluids, the “skin friction coefficient” decreases by 71.25% and the Nusselt number decreases by 24.47%.

Correction: Heat and mass transfer in double-diffusive mixed convection of Casson fluid: biomedical applications

Correction: Heat and mass transfer in double-diffusive mixed convection of Casson fluid: biomedical applications

Lid-Driven Cavity Flow Containing a Nanofluid

In this paper, we consider the flow of a nanofluid in an enclosed lid-driven cavity using a single-phase model. Two cases are considered: one in which the top and bottom walls are kept at adiabatic conditions, and a second case in which the left- and right-side walls are kept in adiabatic conditions. The impact of different viscosity models on the mixed convection heat transfer is examined, and numerical methods are used to obtain solutions for the Navier–Stokes equations for various parameter ranges. Using our robust methods, we are able to obtain novel solutions for large Reynolds numbers and very small Richardson numbers. Using water as the base fluid and aluminium oxide nanoparticles, our results suggest that heat transfer enhancement occurs with increasing particle concentration and decreasing Richardson numbers. There are also significant differences depending on the viscosity model used in terms of the impact of reducing corner recirculation regions in the cavity.

Thermophoresis & Soret-DuFour on MHD Mixed Convection of a Nano Fluid with a Porous Medium over a Stretching Sheet with a Non-Uniform Heat Source/Sink

This work aimed to examine the effects of thermal diffusion (Soret) and diffusion-thermo (DuFour) on MHD mixed convective flow of a viscous nanofluid across a stretching sheet under a magnetic field embedded in a porous medium in the presence of non-uniform heat source/sink and chemical reaction. The governing equations are transformed into a set of ordinary differential equations using the similarity transformation approach. They are subsequently resolved computationally by use of the efficient Keller box method. The effects of different physical factors on concentration, temperature, and velocity profiles are graphically shown. Increasing Du values decreases temperature, although concentration profiles indicate the reverse. As temperature rises, the chemical reaction parameter Kr values increase, while the concentration profile decreases. The temperature was found to rise when the space-dependent (A1) and temperature-dependent (B1) parameters for heat source/sink increased. Additionally, a tabular presentation of the skin friction coefficient, Nusselt number, and Sherwood number behaviour is provided. Mixed convection heat and mass transfer flows are very important in manufacturing for designing reliable equipment, nuclear power plants, gas turbines, and various propulsion devices for aircraft, rockets, satellites, and spacecraft. The effects of non-uniform heat sources/sinks, thermophoresis, and chemical reactions on mixed convection flow play an important role in space technology and high-temperature processes.

Influence of solid cylinders on fluid flow and thermal analysis in a curved channel with constant magnetic field

The current study explores the numerical simulation of constant mixed convection heat transfer and fluid flow within a channel, driven by the inlet velocity. The investigation focuses on a curved irregular channel where the presence of three differently sized circular obstacles introduces an unpredictable element to the heat transfer process. For the two-dimensional physical scenario, mathematical equations are formulated to analyze velocity and temperature. Employing appropriate dimensional variables, the coupled partial differential equations are transformed into dimensionless form. Subsequently, the Finite Element Method (FEM) is employed to simulate the results for non-dimensional physical parameters such as Reynolds number, Richardson number, Hartmann number, nanoparticles' volume fraction, and various states of the circular obstacles on heated lines, flow structure, velocities, and temperatures at mean positions generating isotherms, streamlines, velocity, and temperature profiles for various parameter increments. The analysis reveals significant influences of Reynolds number (Re) and heated circular obstacles on the temperature profile, manifesting as heated lines within the system. The analysis brings to light the substantial impacts of nanoparticles on the temperature profile at both the vertical and horizontal mean positions. An elevated Hartmann number signifies a more robust magnetic field than viscous forces. A heightened magnetic field can hinder fluid movement and modify the patterns of heat transfer. Based on the graphical results, we infer that temperature differences tend to arise between strata as the Richardson number increases, leading to a heightened and more evident stratification.

Mixed convective heat transfer in a square cavity filled with power‐law fluids under active flow modulation

AbstractThe current study presents a computational investigation of mixed convective heat transfer in a square enclosure containing power‐law fluid. An active flow modulator is employed in the form of a flat plate with negligible thickness, and the mixed convection is achieved through clockwise rotation of the plate. The rotation of the plate is modeled by incorporating a moving mesh technique. The solution is then obtained by applying the Finite Element Technique under the arbitrary Lagrangian–Eulerian framework. Numerical validation is performed with contemporary research studies consisting of rotating plates to justify the accuracy of the present study. The study is conducted at constant Prandtl number Pr = 1.0 and Reynolds number Re = 500 while varying the power‐law index (0.6 ≤ n ≤ 1.4) and the Richardson number (0.1 ≤ Ri ≤ 10.0). The results have been presented in terms of the flow and thermal fields, spatially averaged Nusselt number, spatially averaged power consumption by the plate, and the velocity and temperature profile in the enclosure. The numerical findings indicate that a higher Richardson number encourages heat transfer. For the shear‐thinning fluid, a 37% thermal augmentation is observed in comparison to the Newtonian fluid at Ri = 10. However, in the case of shear‐thickening fluid, thermal performance was reduced by 21.13%. Small thermal oscillations are observed in naturally dominated mixed convection for shear‐thinning fluids, but none are observed for shear‐thickening or Newtonian fluids. In addition, the findings demonstrate that the flow modulator has a positive impact on heat transfer for the shear‐thickening fluids (n > 1) and an adverse effect for the shear‐thinning fluids (n < 1). Furthermore, the power consumption decreases as Ri increases, and it becomes negative beyond Ri = 1.0 due to the increase in natural convection strength.

Artificial intelligence approach in mixed convection heat transfer under transverse mechanical vibrations in a rectangular cavity

In the current research, the mixed convection heat transfer in a rectangular chamber with walls with sinusoidal oscillations and mechanical vibrations has been investigated. Mechanical vibrations on the chamber, sinusoidal oscillations of the hot wall and flow due to buoyancy are considered. The finite volume method is utilized for simulation. The efficacy of changes in governing parameters, such as frequency, oscillation amplitude, Ra, chamber length to width ratio, change of fluid type on Nu has been investigated. The results indicate that there is an optimal ratio of chamber dimensions that has the maximum Nu in the fixed Ra, and this ratio depends on the type of fluid. In the presence of sinusoidal oscillations of the hot wall and transverse mechanical vibrations of the cylinder, it increases the heat transfer by about 96 % and 75 %, respectively, compared to the state without vibration. The increase in the frequency and amplitude of oscillations in the case where the hot wall oscillates sinusoidally is negligible on the Nusselt number. Increasing the frequency and amplitude of oscillations of transverse vibrations of the chamber has a significant efficacy on Nu, and the amplitude of oscillations has a greater efficacy than the frequency of oscillations on heat transfer. Based on the available data and using artificial intelligence, GMDH, Nu has been estimated. The results indicate that this modeling has been able to estimate the Nusselt number with good accuracy with R2=0.948.

Expression of Concern: Entropy analysis and MHD mixed convection heat transfer of the magnetic nanofluid in an asymmetric T-shaped cavity using the lattice Boltzmann method

Expression of Concern: Entropy analysis and MHD mixed convection heat transfer of the magnetic nanofluid in an asymmetric T-shaped cavity using the lattice Boltzmann method

Computational Analysis of MHD Nanofluid Flow Across a Heated Square Cylinder with Heat Transfer and Entropy Generation

Abstract The mixed convection heat transfer of nanofluid flow in a heated square cylinder under the influence of a magnetic field is considered in this paper. ANSYS FLUENT computational fluid dynamics (CFD) software with a finite volume approach is used to solve unsteady two-dimensional Navier-Stokes and energy equations. The numerical solutions for velocity, thermal conductivity, temperature, Nusselt number and the effect of the parameters have been obtained; the intensity of the magnetic field, Richardson number, nanoparticle volume fraction, magnetic field parameter and nanoparticle diameter have also been investigated. The results indicate that as the dimensions of nanoparticles decrease, there is an observed augmentation in heat transfer rates from the square cylinder for a fixed volume concentration. This increment in heat transfer rate becomes approximately 2.5%–5% when nanoparticle size decreases from 100 nm to 30 nm for various particle volume fractions. Moreover, the magnitude of the Nusselt number enhances with the increase in magnetic field intensity and has the opposite impact on the Richardson number. The findings of the present study bear substantial implications for diverse applications, particularly in the realm of thermal management systems, where optimising heat transfer is crucial for enhancing the efficiency of electronic devices, cooling systems and other technological advancements.

Heat and mass transfer in double-diffusive mixed convection of Casson fluid: biomedical applications

Heat and mass transfer in double-diffusive mixed convection of Casson fluid: biomedical applications

Mixed convective heat transfer in an open cavity with fins

AbstractThis work numerically explores the mixed convective heat transfer in an open square enclosure containing conducting fins fixed to the heated vertical wall. This kind of work with fins has enormous potential due to its applications in research, engineering, and current industries. Therefore, the current work is highly significant to understand the impact of mixed convection. The external flow enters from the hole in the bottom wall and leaves from the hole in the upper wall. The left vertical wall of the enclosure is heated isothermally, and the fins are attached to the heated walls at a uniform height. Both the upper and lower walls are adiabatic, whereas the right sidewall is at a lower temperature. The non‐dimensional transport equations are resolved by using the finite element method. The study is accomplished for the wide control variables range, such as Reynolds number (50 ≤ Re ≤ 200), Richardson's number (0.1 ≤ Ri ≤ 10), the length of the fins (Lf = 0.2, 0.4, and 0.6), the size of the outlet opening (Wout = 0.1, 0.2, and 0.3), and the gaps in between the outlet hole and left heated wall (S = 0, 0.45, and 0.9). The results show that the thermal performance of the open enclosure is meaningfully affected by the control parameters. The maximum and minimum heat transfer happens when the position of the outlet opening is at the left (S = 0) and right (S = 0.9), respectively. The heat transfer improves by raising the Ri and Re, whereas increasing the fin's length and distance between the outlet opening and left wall reduces heat transfer significantly. The rises 13% with a decrease in the fin's length from 0.6 to 0.2 at Re = 200, S = 0 due to the improvement of the convection on the heated wall. Also, increases by 15% when Ri increases from 1 to 9.

Investigation of Laminar Forced and Mixed Convection in Horizontal Mini Tubes with Uniform Wall Heat Flux Boundary Condition

In this study, a numerical model is established to study the forced and mixed convection heat transfer in the laminar region for horizontal mini tube with different diameters (1.2 mm to 3.0 mm) under the uniform wall heat flux boundary condition. The Reynolds number is set from 100 to 2,000 and the uniform wall heat flux boundary condition is set at 10 and 20 kW/m2. The model is verified with well-established correlation to guarantee good accuracy. The existence of mixed convection is examined by (1) the ratio between the heat transfer coefficients at the top and the bottom of the tube, and (2) the temperature contour plot. Based on the simulation results, the existence of buoyancy superimposed on the main flow is strongly related to the tube diameter, the dimensionless location, the Reynolds number, and the heat flux applied. Under the same heating condition, for small diameter tube with insufficient length, buoyancy effect can never establish. The existing flow regime maps for macro tube were used to predict the forced and mixed convection regimes from the simulated mini tube results and discrepancy was found. A new flow regime map is hence developed for mini tubes, and it classified the simulation results with good accuracy.

Numerical investigation of multiple affecting parameters on mixed convective heat transfer for arrays of buildings

This study aims to explore relationships between wind speed, wind direction, exterior surface roughness, temperature difference, plan area density and convective heat transfer coefficients (CHTCs) for external surfaces of arrays of buildings. The present investigation provides correlations for CHTCs at the windward, leeward, top, left, and right lateral surfaces. Three different plan area densities of 0.25, 0.111 and 0.027 are considered and 249 numerical simulations are conducted to investigate the influential parameters including density (λp), wind angle (θ), wind speed (U10), temperature difference (ΔT), and surface roughness (ε). Findings reveal that higher plan area density results in lower CHTC due to heightened airflow obstruction. Increase of the wind angle amplifies CHTC on leeward and right lateral facades while reduces it on the left lateral side. With increased wind speed, CHTC experiences significant increase. The results also shows that the temperature difference's impact is pronounced at lower air velocities, leading to heightened CHTC. Besides, increased surface roughness enhances heat transfer coefficients. Finally, the present study employs extensive simulations to present correlations for different facades of the buildings in the studied arrays offering insights into CHTC variations in terms of plan area density, wind angle, speed, temperature difference, and roughness height.