Mixed Convection Heat Transfer Research Articles

Mixed Convection Heat Transfer Characteristics of Al2O3 – MWCNT Hybrid Nanofluid under Thermally Developing Flow; Effects of Particles Percentage Weight Composition

In this research, an experimental investigation of hybrid nanofluid mixed convection heat transfer characteristics is conducted. The research specifically investigates the effects of percentage weight composition (PWC) of nanoparticles in the hybrid nanofluids on mixed convection heat transfer characteristics along the lamina, transition and turbulent regions. Transition boundaries, thermal entrance effects, and the influence of tube axial position were also critically investigated and analysed experimentally. Three hybrid nanofluids of Al2O3 – MWCNT (i.e., Al2O3 (60%) – MWCNT (40%), Al2O3 (50%) – MWCNT (50%) and Al2O3 (40%) – MWCNT (60%)) were prepared using two-step method and then subjected to a constant heat flux through a horizontal circular copper tube with an internal diameter of 8mm. Results show a significant change in heat transfer characteristics with different PWCs. Al2O3 (60%) – MWCNT (40%) have shown a better heat transfer enhancement among the three fluids investigated. Its Nusselt number has an enhancement of more than 5 % better than the other two fluids. Along the transition regime, critical Reynold numbers (Recr) of the three nanofluids were found to have differed slightly, with Recr = 2020, 2000 and 2100 for Al2O3 (60%) – MWCNT (40%), Al2O3 (50%) – MWCNT (50%) and Al2O3 (40%) – MWCNT (60%) respectively. Mixed convection effects were found to be more significant with Al2O3 (60%) – MWCNT (40%) than with the other two fluids. At the axial position of 63.75, its mixed convection strength (Ὠ) was about 41%, which is the highest among the three fluids. Its strength of mixed convection was also found to deteriorate to about 14.75% with an increase in axial distance from the tube inlet. A similar observation was also noticed with other fluids. Thermal entrance effects were only found to be significant at x/d = 15 and 31.25, as their influence diminishes with an increased x/d distance from the tube inlet. It was concluded that both mixed convection and thermal entrance effects resulted in heat transfer enhancement, especially in the lamina region. Their influences decrease with an increase in axial distance from the tube inlet. Mixed convection influences were only present in the lamina and transition region, and their strength was reduced with an increase in Reynold number and axial position.

Theoretical investigation of the convective heat transfer mechanism along a cantilever shape

The present study aims to investigate the steady state mixed convection flow for a viscous incompressible fluid along a cantilever shape. The study has analyzed the impact of mixed convection boundary layer flow behavior and heat transfer characteristic of the cantilever shape. The leading equations for the said problem are the Navier-Stokes and energy equations. These leading equations are converted into ordinary differential equations using the stream function formulation, and the numerical solution is obtained using the shooting technique with BVP4C and built in MATLAB program. The impact of dimensionless parameters such as n, Pr, and λ on the velocity and temperature profile within the boundary layer of the cantilever cylinder is presented graphically. The results for velocity slope and temperature gradient under the effects of various emerging parameters are presented in tables. The study has concluded that mixed convection parameter has a significant impact on the velocity and temperature distribution, as well as on the velocity slope and temperature gradient rates along a cantilever shape geometry. The outcomes of the current study can be utilized for the design and optimization of various engineering devices involving cantilever shapes. Further, research regarding the impact of other parameters on the cantilever shape including heat transfer characteristics can be conducted using the approach presented in this study. The novelty of the current work can be claimed by saying that for reduced gravity, other forces involved in the flow model induced the fluid motion and temperature gradient.

Investigation of Thermo-Hydraulics in a Lid-Driven Square Cavity with a Heated Hemispherical Obstacle at the Bottom.

Lid-driven cavity (LDC) flow is a significant area of study in fluid mechanics due to its common occurrence in engineering challenges. However, using numerical simulations (ANSYS Fluent) to accurately predict fluid flow and mixed convective heat transfer features, incorporating both a moving top wall and a heated hemispherical obstruction at the bottom, has not yet been attempted. This study aims to numerically demonstrate forced convection in a lid-driven square cavity (LDSC) with a moving top wall and a heated hemispherical obstacle at the bottom. The cavity is filled with a Newtonian fluid and subjected to a specific set of velocities (5, 10, 15, and 20 m/s) at the moving wall. The finite volume method is used to solve the governing equations using the Boussinesq approximation and the parallel flow assumption. The impact of various cavity geometries, as well as the influence of the moving top wall on fluid flow and heat transfer within the cavity, are evaluated. The results of this study indicate that the movement of the wall significantly disrupts the flow field inside the cavity, promoting excellent mixing between the flow field below the moving wall and within the cavity. The static pressure exhibits fluctuations, with the highest value observed at the top of the cavity of 1 m width (adjacent to the moving wall) and the lowest at 0.6 m. Furthermore, dynamic pressure experiences a linear increase until reaching its peak at 0.7 m, followed by a steady decrease toward the moving wall. The velocity of the internal surface fluctuates unpredictably along its length while other parameters remain relatively stable.

Numerical analysis of turbulent mixed convection heat transfer of molten salt in horizontal tubes with uniformly cooled heat flux

Molten salt is a widely used heat transfer medium in the field of energy systems. In the engineering design, mixed convection heat transfer often occurs in the high heat flux tube convection heat transfer process, which can significantly change the convective heat transfer capability. However, optimizing the heat exchange conditions can improve the heat transfer capacity of molten salt. In this study, turbulent mixed convection heat transfer characteristics of Hitec molten salt inside horizontal cooling tubes are investigated numerically using the Reynolds stress turbulence model. The results show that the secondary flow caused by buoyancy can lead to significant nonuniform in heat transfer and wall temperature on the wall surface of the horizontal tube cross section. In addition, the buoyancy parameter Bo is selected to characterize the buoyancy effect on Hitec mixed convection heat transfer inside a horizontally cooled circular tube, and within the Bo range from 2.82 × 10−8 to 1.61 × 10−5, as the Bo increases, the relative average Nusselt number for the upper half tube wall gradually increases, the relative average Nusselt number for the lower half tube wall decreases, and the relative average Nusselt number for the entire tube wall shows the combined results of the upper half wall and the lower half wall relative average Nusselt numbers, which slightly increases and then decreases. Furthermore, based on the numerical data, the critical buoyancy parameters Bocr,1 = 10−7 and Bocr,2 = 6.96 × 10−6 at which the buoyancy force starts to affect heat transfer and the average Nusselt number for the entire tube wall starts to decrease are proposed, and variations of the mixed convection heat transfer against the buoyancy parameter Bo are presented respectively for different regions of the horizontal cooling tubes. Based on this research, a reasonable and widely applicable buoyancy parameter selection criterion can be constructed to guide the future engineering designs.

3D numerical simulations of mixed convective heat transfer and correlation development for a thermal manikin head

The head represents 10% of the body's total surface area. Unprotected, it accounts for a significant portion of overall heat loss when exposed to cold conditions. This study was motivated by a need to clarify how the human head interacts with its environment in terms of heat exchange. Accurate estimations of heat transfer coefficients on the human head are essential for conducting thermal comfort and safety analyses in buildings. In this study, a thermal head resembling a real male human head is utilized to investigate heat transfer between the body and the surrounding environment. A three-dimensional computational fluid dynamics (CFD) model is proposed to simulate steady-state dry heat loss from the human head within a chamber. This model provides predictions for heat flux, temperature, and velocity distribution surrounding the head. A straightforward correlation, derived from numerical and experimental findings, is introduced to forecast the average Nusselt number for the head under combined natural and forced convection. This correlation, relying on dimensionless parameters (Grashof, Reynolds, and Prandtl numbers), offers enhanced accuracy, simplicity, and fewer terms. The predicted average Nusselt numbers from the proposed correlation for mixed convection closely match CFD and experimental results, with relative percentage differences within ±2%, signifying excellent accuracy across a broader range of flow conditions, including temperature differences and air velocities. Additionally, the study explores the impact of head diameter on overall heat transfer.

Nanoparticle’s Shape Effect on Mixed Convection Heat Transfer and Flow of Carbon Nanotubes: Numerical Approach

Nanoparticle’s Shape Effect on Mixed Convection Heat Transfer and Flow of Carbon Nanotubes: Numerical Approach

Direct Numerical Simulation of Mixed Convection Flow in Lid-Driven Cavities

The mixed convection of heat transfer and fluid flow in a - lid drivencubical cavity filled with air is investigated numerically in this study. The computational procedure is based on the finite volume method and a full multigrid acceleration solver. The top wall of the cavity is maintained at a constant high temperature - Th, and it can move with a constant velocity U0. The bottom wall is immobile and maintained at a cold temperature Tc. While, the remaining boundary parts of the cavity are motionless and kept thermally insulated. Several numerical simulations were conducted to investigate mixed convection heat transfer in a sliding cubical cavity for a range of Reynolds numbers from 1000 to 5000 and Richardson numbers from 0.001 to 10. The influence of mixed convection parameters, Reynolds number, Richardson number, and heat transfer rate on the flow behavior was analyzed through parametric studies. The results include flow and heat transfer characteristics, iso-surfaces, and streamlines for the entire range of Richardson numbers and Reynolds numbers investigated. The study shows that as Reynolds number is increased beyond a critical value, the flow becomes unstable and bifurcates.

Mixed convection heat transfer in a partial porous layered split lid-driven wavy wall cavity with Cu–H2O nanofluid

This numerical study investigates mixed convection heat transfer in a two-dimensional irregularly-shaped enclosure with a nanofluid-saturated porous medium. The enclosure has wavy vertical walls, a centered triangular block, and a lid split into two regions moving in opposite directions. Transport equations are solved using the finite element method under varied Darcy (10−5 to 10−1), Reynolds (50 to 200), Richardson (0.1 to 10), amplitude (0 to 0.2), and nanoparticle volume fraction (0 to 0.1) numbers. Results demonstrate the combined effects of forced convection from lid motion and natural convection from temperature differences significantly enhance heat transfer. The wavy walls induce secondary flows and periodic disruption of thermal boundary layers. Increasing Darcy number enables deeper penetration into the porous block. Copper nanoparticles incrementally improve conductive heat transfer. Richardson number effects are more prominent at higher Reynolds numbers as buoyancy forces influence vertical convection. This study provides novel insights into synergistically utilizing wavy geometries, nanofluids, porous media, and active pumping for optimizing heat transfer in irregular enclosures for electronics cooling and thermal management.

Carboxymethyl cellulose-water-based hybrid nanofluid mixed convection flow between porous vertical plates: Entropy generation analysis

This paper analyzes the thermodynamic second law analysis of mixed convection heat transfer of Carboxymethyl Cellulose (CMC)-water based viscoelastic hybrid nanofluid flow in a vertical parallel plate channel filled with porous medium. In mixed convection flow, where both buoyancy and viscous forces play significant roles in the flow behavior, the inclusion of viscous dissipation in the analysis is crucial. The governing equations of the problem are converted into a system of ordinary differential equations using appropriate similarity transformations, which are then solved by the Homotopy Analysis Method (HAM). The behavior of non-dimensional velocity, temperature, skin friction coefficient, Nusselt number, entropy generation and Bejan number profiles for a range of pertinent flow parameter values is displayed graphically and deliberated. Study reveals that a decrease in the dominance of viscous forces relative to inertial forces within the porous medium lowers entropy generation in the system.

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

Expression of Concern: Thermal and hydrodynamic study of mixed convection heat transfer of a nanofluid in an enclosure with unequal-sized baffles using the Lattice Boltzmann method

Expression of Concern: Thermal and hydrodynamic study of mixed convection heat transfer of a nanofluid in an enclosure with unequal-sized baffles using the Lattice Boltzmann method

Deep learning with multilayer perceptron for optimizing the heat transfer of mixed convection equipped with MWCNT-water nanofluid

In the modern era, Artificial Intelligence (AI) has emerged as a powerful tool that can rapidly generate highly accurate data, offering tremendous potential for optimizing system performance. This study focuses on harnessing the capabilities of an artificial neural network (ANN) to determine optimal parameter values for heat transfer in a mixed convection mechanism. The first step of this research involved conducting CFD simulations to investigate the impact of varying Grashof numbers (102,103,and104), Reynolds numbers (1,10,and100), and MWCNT volume fractions (0, 0.01, 0.02, and 0.03) on the Nusselt number within an elliptical enclosure containing a centrally located rotational cylinder. A total of 48 simulations were performed, generating a comprehensive dataset for training the ANN-Multilayer Perceptron (MLP). In the second step, the trained ANN was utilized to generate an additional 700 data points with remarkable accuracy. This enabled efficient exploration of the parameter space, providing valuable insights into the system behavior and facilitating optimization efforts. The findings of this study revealed a 0.03 vol fraction of MWCNT into the water in Grashof numbers of 102,103,and104, the average Nusselt number increased by approximately 46%, 31%, and 12%, respectively. The ANN-based approach successfully identified optimal values for the variables that maximize the Nusselt number.

Dynamic behavior of mixed convection heat transfer among horizontal co-axial fixed pipes at varying temperatures: Efficiency of cylindrical heat sink

The aim of the current research is to highlight the time-dependent properties of laminar convective thermal transmission inside a semi-infinite pipe, with an emphasis on flow frequencies that impact inter-fluid momentum and energy. The implicit finite difference approach is used to solve two-dimensional mathematical model composed of nonlinear partial differential equations. The research includes forecasting thermal performance of time-dependent flow in a pipe over several parameter ranges relevant to the flow model. The obtained forecasts were emphasized graphically. The numerical solutions were separated into steady and unsteady components, with the steady component yielding findings that were then used to determine the time dependent surface shearness and energy shearness in the time dependent phase. The stable component of the investigation revealed that raising the buoyancy force parameter and the impact of viscous dissipation significantly improved the flow velocity pattern and heat distributions throughout the flow domain. While the amplitude of time dependent surface shearness varied somewhat across various buoyancy force parameter values, the amplitude of time dependent heat transmission rates varied significantly. This study sheds light on the interaction of several characteristics, such as buoyancy force effects and internal energy loss, on time-dependent thermal energy flow inside a semi-infinite pipe subjected to laminar convective flow. It is worth noting that the amplitude of the time dependent rate of heat transmission shows more noticeable variations with different values of the buoyancy force parameter, in contrast to the minute change in the amplitude of the time dependent surface sheerness.

Analysis of radiative mixed convective heat transfer for non-Newtonian fluid with heat generation/absorption phenomena

The current work consists of investigating the heat generation/absorption and non-linear thermal radiation effects for two-dimensional mixed convective bi-viscosity fluid flow inside the C-shaped enclosure with uniformly heated left and linearly heated top and bottom walls. The physical flow problem is governed by applying the constitutive laws of bi-viscosity model, conservation of mass, momentum and energy balance equations. The governing non-linear coupled partial differential equations is transformed into the algebraic nonlinear system by using Galerkin's finite-element technique after eradicating the pressure terms via the penalty method, and then the Newton–Raphson technique is used to solve it. It is noted that at a low value of [Formula: see text] (which gives the high plasticity of the bi-viscosity liquid), only the primary circulations with intensity [Formula: see text] appeared, but at a high value of [Formula: see text] decreased the plasticity which caused enhanced buoyancy-driven that generate also the secondary bowls with intensity [Formula: see text] inside the enclosure. The heat transfer rate increased [Formula: see text] by an increase in the Reynolds number from [Formula: see text] to [Formula: see text]. The findings of this study will play a role in improving the thermal performance of microelectronic devices, cooling systems, food processing, solar collectors, and high performance boilers by tracing the energy transport trajectories through heatlines.

Experimental and numerical study of mixed convection heat transfer in a vented cavity partially filled with a porous medium: Effects of reynolds and rayleigh numbers on Nusselt number and flow regimes

This article presents a comprehensive investigation of mixed convection flow and heat transfer within a vented, partially side-heated cubical cavity, incorporating a porous medium with low-conductivity square-shaped inclusions. The study encompasses an extensive range of Rayleigh numbers (106 < Ra < 6 × 106) and Reynolds numbers (200 < Re < 4000) while maintaining a fixed Prandtl number of Pr = 0.71. This investigation spans over three decades in Richardson numbers (Ri = Ra/(Re2 Pr)), aiming to discern the interplay between the Nusselt number, Nu, and the Richardson number. Our understanding of Nusselt number dependencies is enhanced by combining PIV measurements of heat transfer and velocity fields with temperature field data. The study uses both experimental and numerical PIV data, incorporating porous media representation to reveal heat transfer scaling with both Reynolds and Rayleigh numbers. Computational Fluid Dynamics (CFD) methods are used to explore the complex physical behavior under different flow conditions. Three distinct flow and heat transfer regimes have been identified, predicated upon the Richardson number. For Ri < 25, the flow structure and Nusselt number exhibit similarities with pure forced convection, where the Nusselt number scales as Nu ∼ Re0.6, independent of the Rayleigh number. Conversely, for Ri > 70, natural convection dominates the vicinity of the heating wall, rendering the Nusselt number less sensitive to Reynolds number variations and predominantly dictated by the Rayleigh number. Notably, the intermediate regime, ranging from 25 < Ri < 70, witnesses a competition between upward-directed natural convection flow at the heating wall and downward-directed forced flow, culminating in a minimum effective Nusselt number.

Impact of wall velocity ratios on mixed convection heat transfer in porous cavities with nanoliquids

Impact of wall velocity ratios on mixed convection heat transfer in porous cavities with nanoliquids

New closed-form exact solution of action induced magnetic field and fluid wall interaction on radiative mixed convective heat transfer MHD fluid flow inside a vertical walls with suction/injection and pressure gradient

The novelty of this article is not only the adaptation of a new closed-form exact method originally used to solve initial value problems but also to solve boundary value problems whose boundary conditions contain variable values. The boundary conditions of governing equations of velocity, induced magnetic, and temperature for the current study contain different parameters such as (Wall ambient temperature, Knudsen number, and Fluid wall interaction parameter) and, contain gradient of velocity, induced magnetic, and temperature. Using the Laplace transform method, the exact solution of the governing equations was obtained, and the effects of different parameters on its behavior in three different cases of wall-ambient temperature difference ratio was studied. I would also like to clarify that the reason for paying attention to such studies is that they contribute effectively and usefully in many fields and applications, such as molten metal MHD pumps, the design of small pumps, molten salt, and heat exchangers.

Study of Bejan number and entropy generation for mixed convection of nanofluid flow inside a chamber under an inclined magnetic field

Examination of the efficiency and losses in heat exchange devices is pivotal. Therefore, the entropy generation (EYG) and Bejan number (Be) of mixed convective heat transfer of nanofluid flow in a rectangular cavity (RCY) are evaluated in this study. Five baffles are placed at the lower hot wall of the RCY. The cold upper wall moves and causes forced convection flow of nanofluid. The magnetic field (MFD) at different angles affects the nanofluid when the Hartmann number (Ha) changes from 10 to 40. The length of stepped baffles is changed, and the values of frictional EYG, thermal EYG, and total EYG are estimated. The simulations are done using the LBM and an in-house code. The results demonstrate that the improvement in Ha decreases the amount of frictional EYG, thermal EYG, total EYG, and Be. The total EYG and Be also increase as the MFD angle is increased. The increment in the Richardson number (Ri) reduces the frictional EYG and total EYG (61.8 %) and increases the value of thermal EYG. Enhancing the length of the baffles intensifies the amount of Be and total EYG (3.4 %) and thermal EYG, but decreases the frictional EYG.

Influences of blowing and internal heating processes on steady MHD mixed convective boundary layer flows of radiating titanium dioxide‐ethylene glycol nanofluids

AbstractThe present analysis intends predominantly to explore the multiple behaviors of radiating ethylene glycol‐based titanium dioxide nanofluids during their steady boundary layer flows near a vertical permeable surface under the prominent impacts of uniform blowing and nonuniform internal heating processes. By incorporating the dynamical contribution of Lorentz and buoyancy forces in the momentum equation and adopting the single‐phase nanofluid approach along with the constitutive equations of the second‐grade rheological model and Corcione's correlations, the governing partial differential equations and their appropriate boundary conditions are derived mathematically based on the boundary layer theory and other admissible physical approximations. After several simplifications, the aforesaid differential formulation is converted into a set of ordinary differential equations and realistic boundary conditions, which are solved thereafter numerically using GDQ's‐NT method. Moreover, it is demonstrated that the strengthening in the blowing and internal heating processes has a boosting effect on the velocity and temperature profiles. However, the other influencing parameters show dissimilar dynamical and thermal trends. Furthermore, it is witnessed that the mixed convective heat transfer, the viscoelasticity trend of the nanofluidic medium, and the nanoparticles’ loading exhibit an enhancing impression on the surface heat transfer rate.

Opposing Mixed Convection Heat Transfer for Turbulent Single-Phase Flows

Convection, wherein forced and natural convections are prominent, is known as mixed convection. Specifically, when a forced convection flow is downward, this flow is called opposing flow. The objectives of this study are to gain a comprehensive understanding of opposing flow mixed convection heat transfer and to establish the prediction methodology by evaluating existing correlations and models. Several heat transfer correlations have been reported related to single-phase opposing flow; however, these correlations are based on experiments conducted in various channel geometries, working fluids, and thermal flow parameter ranges. Because the definition of nondimensional parameters and their validated range confirmed by experiments differ for each correlation reported in previous studies, establishing a guideline for deciding which correlation should be selected based on its range of applicability and extrapolation performance is important. This study reviewed the existing heat transfer correlations for turbulent opposing flow mixed convection and the single-phase heat transfer correlations implemented in the thermal–hydraulic system codes. Furthermore, the authors evaluated the predictive performance of each correlation by comparing them with the experimental data obtained under various experimental conditions. The Jackson and Fewster, Churchill, and Swanson and Catton correlations can accurately predict all the experimental data. The effect of the difference in the thermal boundary conditions, i.e., uniform heat flux and uniform wall temperature, on the turbulent mixed convection heat transfer coefficient is not substantial. The authors confirmed that heat transfer correlations using the hydraulic-equivalent diameter as a characteristic length can be used for predictions regardless of channel-geometry differences. Furthermore, correlations described based on nondimensional dominant parameters can be used for predictions regardless of the differences in working fluids. The authors investigated the extrapolation performance of the mixed convection heat transfer correlations for a wide range of nondimensional parameters and observed that the Jackson and Fewster, Churchill, and Aicher and Martin correlations exhibit excellent extrapolation performance with respect to natural and forced convection flows, indicating that they can be applied beyond the parameter range validated experimentally.