Mixed Convection Heat Transfer Characteristics 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.

Mixed Convective Heat Transfer Characteristics of Graphene Nanofluid Strengthened by Periodically Direction-Switching Electric Field

Abstract Transformer-oil with low thermal conductivity and large viscosity has poor heat dissipation capability, which leads to the thermal drive failure caused by transient overload. To improve its cooling capability, this paper has proposed first the method which combined the periodically direction-switching electric field and graphene nanofluid to enhance the mixed convective heat transfer properties of transformer-oil, and analyzed the effects of switching periods, nanofluid concentration, electric field strength, heat flux, and Reynolds number on mixed convection heat transfer experimentally. The results show that the heat transfer characteristic of transformer-oil is improved up to 52% by the periodically direction-switching electric field and graphene nanofluid. As the switching period decreases, the thermal performance of the suspension is enhanced more significantly. Moreover, by analyzing the heat transfer mechanism, the periodically direction-switching electric field causes the nanoparticles to move reciprocally, repeatedly impacting and breaking the boundary layer of the heat exchange surface to enhance the perturbation, thus enhancing the heat transfer effect. Meanwhile, the predicted correlation has been proposed on the basis of influence factors, which are in good agreement with the experimental data.

Numerical investigation on forced, natural, and mixed convective heat transfer of n-decane in laminar flow at supercritical pressures

The present study numerically investigates the forced, natural, and mixed convective heat transfer characteristics of supercritical n-decane in a tube in laminar flow, focusing on the law of mutual coupling of forced and natural convection in mixed convection. Effects of heat flux (100 W/m2–8000 W/m2) and flow direction on heat transfer are investigated. For forced convection, deterioration occurs when the wall temperature reaches the pseudo-critical temperature and the local Nusselt number decreases by up to 17.4% compared to the theoretical value at the heat flux of 2400 W/m2. For natural convection, heat transfer is continuously enhanced with the increase of heat flux. For mixed convection in upward flow, heat transfer is enhanced by the buoyancy of increasing the fluid velocity gradient near the wall. As for the downward flow, buoyancy illustrates the opposite effects. Based on numerical simulation results, the Nusselt number correlations are proposed for forced and natural convection respectively and the correlation for mixed convection is modeled employing a function of the forced and natural convection.

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.

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.

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*).

Mixed Convection Flow and Heat Transfer of Two Rotating Cylinders in a Trapezoidal Enclosure Filled with Porous Medium

The goal of this research is to investigate numerically mixed convection flow and heat transfer characteristics from two rotating cylinders in a trapezoidal enclosure filled with porous material for a variety of factors including rotation speed and direction, Richardson number, and the location of the cylinders. The results of this presentation were checked against accessible data and determined to be very similar. The heat transmission properties within the trapezoid were found to be substantially dependent on the Richardson number, speed, and direction of the spinning cylinders, as well as the location of the cylinders according to the findings of this study. Furthermore, it was discovered that the CW-CW casing produced more heat transfer as compared to the CCW-CW scenario for various Richardson numbers when the cylinders placed vertically. Furthermore, this research revealed that, for higher Richardson values, the rotation speed has no effect on the heat transfer increase inside the trapezoidal enclosure. The effect of the cylinders’ location on the average Nusselt number was also studied in this investigation. It was found the placing the cylinders in the vertical direction produced higher heat transfer compared with horizontal location for a trapezoid heated from below.

Numerical analysis of unsteady mixed convection heat transfer characteristics of nanofluids confined within a porous lid-driven L-shaped cavity

The numerical analysis of unsteady combined convective heat transfer properties of kerosene-alumina nanofluid restricted in an L-shaped porous lid-driven cavity has been examined in this research. Fluid flow governing equations and conjugate heat transport along with the corresponding boundary conditions are used to express the physical problem mathematically. Firstly, boundary conditions and governing equations are transformed into non-dimensional forms using a series of appropriate transformations. Later on, the transformed non-dimensional equations are solved by utilizing the Finite Element Method based on the Galerkin residuals. A comparison with earlier available work is shown and turned out to be a great agreement. Numerical solutions are acquired in a wide variety of governing variables. The governing variables in this study are the Darcy number ( Da ), Reynolds number ( Re ), Richardson number ( Ri ), solid volume fraction (δ), time parameter (τ), etc. The numerical findings of flow and thermal fields are represented graphically in respect of isotherms, streamlines, and average Nusselt number at the hot surface of the cavity. The Re and Da are found to have an extensive impact on the isotherms and streamlines in the cavity, and also in determining the average heat transmission rate.

Analysis of triple solutions in mixed convection flow and heat transfer characteristics of Ag-water based nanofluid over porous shrinking/stretching sheet

The two-dimensional laminar boundary layer flow and heat characteristics of silver (Ag) water based nanofluid with buoyancy effect over porous shrinking/stretching sheet has been considered. The shooting technique is used for numerical solutions to address the considered physical flow parameters. The numerical investigation shows that the triple solutions exists at the distinct ranges of parameters. Therefore, the analysis of stability is carried out to explore the reliability of solutions. The important results related to the stable solutions indicate that the skin friction declines for λ>0 and enhances for λ<0 when suction, and nanoparticles volume fractions are enhanced. An increase in rate of suction decreases velocity and the temperature profiles. Besides, an increase in silver nanoparticles volume fraction increases velocity and the temperature profiles, respectively. An augmentation in thermal slip factor reduced the temperature. The decrement in heat sink/source constraint resulted the lower temperature.

Mixed Convective Heat Transfer Characteristics and Mechanisms in Structured Packed Beds

Mixed Convective Heat Transfer Characteristics and Mechanisms in Structured Packed Beds

Unsteady conjugate heat transfer with combined effects of MHD and moving conductive elliptic object in CNT-water nanofluid with ventilation ports

PurposeThe purpose of this paper is to analyze the unsteady conjugate mixed convective heat transfer characteristics in a vented porous cavity under the combined effects of moving conductive elliptic object and magnetic field.Design/methodology/approachThe finite element method and arbitrary Lagrangian-Eulerian (ALE), impacts of Reynolds number, Hartmann number, aspect ratio of the conductive ellipse and moving speed of the object on the hydro-thermal performance are analyzed.FindingsIt was observed that the dynamic characteristics of the local and average Nu number of each hot wall are different. Magnetic field strength increment resulted in the enhancement of average Nu number for bot steady and transient case while the optimum case for best hydro-thermal performance is achieved for highest Ha number and non-dimensional time of 10. Higher value of average Nu and lower pressure coefficient are achieved for aspect ratio of 4 and non-dimensional time of 10. When the moving velocity of the conductive ellipse is considered, 42% enhancement in the average Nu is obtained at non-dimensional time of 20 and object velocity equals to 0.012 times entering fluid velocity in the negative y direction while the pressure coefficient is higher. The moving object is used as a useful tool to control the dynamic features of heat transfer in a vented cavity.Originality/valueThe present method of convective heat transfer control inside a vented cavity with a moving elliptic object is novel and can be used as an effective tool with magnetic field effects owing to diverse use of convection in cavities with vented ports in many practical thermal engineering systems.

An LBM-based investigation of thermal buoyancy and arrangement angle on flow characteristics and heat transfer over four heated square cylinders

In this work, the effects of thermal buoyancy and rotation angle on the hydrodynamics and mixed convection heat transfer characteristics over four heated identical square cylinders placed in a parallel horizontal channel are numerically studied. For the flow field, the Lattice Boltzmann method (LBM) with multiple relaxation time collision operator (MRT) is implemented and the Bhatnagar-Gross-Ktook collision operator (BGK) is adopted for the thermal field simulation. The distance between two adjacent square cylinders is fixed for a parametric study and a uniform velocity is imposed at the inlet region. Ranges of the influence parameters are defined as: α = 45°, 90°, 120°; 50 ≤ Re ≤ 150; 0 ≤ Ri ≤ 0.5. Several flow and heat transfer parameters including the time-averaged lift/drag coefficients, the global time-averaged Nusselt number and the distributions of local Nusselt number are analyzed, respectively. The vorticity and isotherm patterns are also plotted to illustrate the visual disturbance of the thermal buoyancy and the rotation angle on the flow and thermal fields. Numerical results show that the flow and thermal patterns for all arrangements become disordered and nonsystematic with the introduction of thermal buoyancy, and the thermal pattern for the downstream cylinders has an upward drift in the wake region. The time-averaged lift coefficient for each square cylinder is relatively sensitive to the influence parameters and the arrangement, and it is always less than zero when the Richardson number is not equal to zero. The global time-averaged Nusselt number is directly proportional to the Reynolds number and Richardson number, and the local Nusselt numbers on the front and rear faces for each square cylinder reach a maximum and minimum, respectively. It is also observed that the distribution of local Nusselt number for all rotation angles is intensively affected by the thermal buoyancy.

Enhanced magnetohydrodynamic thermal convection in a partially driven cavity packed with a nanofluid‐saturated porous medium

Convective heat transfer characteristics in the purview of fundamentals of flow physics in a two‐sided partially driven cavity (PDC) are addressed in this work. The effect of partial translational motion on the sidewalls of PDC can significantly alter the convective heat transfer characteristics. To explore this novel idea, a square enclosure packed with Cu–water nanofluid‐saturated porous media under the influence of the external magnetic field, heated around a protruded heater mounted at the bottom wall. The cavity is cooled at the upper half of the two sidewalls—partially translating upward or downward direction. Fundamentals of the evolved flow physics and mixed convection heat transfer characteristics are analyzed numerically using extensively validated FORTRAN code based on the finite volume approach. The impacts of the relevant parameters on fluid flow and heat transfer are investigated systematically. The governing parameters are wall speed (Reynolds number) and its direction, convection regime (Richardson number), permeability (Darcy number and porosity), magnetic field strength (Hartmann number), and nanoparticles volume concentration. The results show that the thermal performance of PDC is greatly influenced by all of the above parameters. Heat transfer is enhanced significantly at higher Reynolds number, Richardson number, Darcy number, and porosity and increases in the nanoparticles volume fraction for the opposing flow case.

Numerical simulation of mixed convection flow and heat transfer in the lid-driven triangular cavity with different obstacle configurations

There are several industrial applications, particularly lid-driven walls, for mixed convection heat transfer characteristics across various cavities. In order to increase the effectiveness of cooling, electrical, electronic and nuclear devices and to monitor the fluid flow and heat exchange of solar thermal installations and thermal storage, such a problem requires further investigation. The main goal of this profound study is to examine the convective heat transfer nature of thermal convection on Newtonian MHD fluid in a lid-driven triangular cavity subjected to heating by a thick triangular wall, including the effects of varying Richardson number, Reynolds number, Hartmann number, and cold circular obstacle. Graphical illustration shows that the upper wall having temperature Th is moving from left to right, whereas inclined sidewalls are adiabatic. Further, a cold circular obstacle containing temperature T∗∗ is placed near the left and right wall of the triangular cavity with T∗∗ < Th. The governing flow equations are tackled by the Galerkin Finite Element Method (GFEM) to prepare the desired results. Velocity contours, isotherms and heat transfer rates demonstrate flow kinematics and heat transport processes. The high estimate of the Reynolds number has been found to provide a decent rate of convective heat transfer to a good liquid progression. For a higher number of Grashof, which achieves the maximum convection intensity, it is checked that natural convection dominates within the convection system. The Richardson number is a rising function of the Nusselt number, while the opposite trend is observed due to the rise in the Hartmann number.

Effect of partial wall motion on MHD mixed convection heat transfer undergoing in a porous cavity filled with Cu–water nanofluid with a centrally mounted heat source

The Magneto-hydrodynamics (MHD) convective heat transfer process in a partially driven cavity (PDC) containing a centrally mounted heat source is focused in the present study. The cavity is filled with Cu–water nanofluid saturated porous medium. Heated cavity fluid is cooled through the upper half of the two sidewalls. These cold sidewalls are allowed to move in upward direction at the same velocity. The horizontal walls and lower part of the sidewalls are insulated. The cavity subjected to an externally imposed uniform magnetic field. The flow structures and mixed convection heat transfer characteristics are analyzed numerically utilizing developed CFD code adopting FVM and SIMPLE algorithm. The thermo-fluid phenomena are analyzed under a range of governing parameters like Richardson number (Ri), Hartmann number (Ha), concentrations of nanoparticles (ϕ). The results reflect that the heat transfer characteristics of the PDC are greatly influenced by all of the above parameters. Heat transfer is enhanced significantly at the higher Ri and higher volume fraction.

Mixed convective heat transfer characteristics of microcapsule slurry around a horizontal heated cylinder

Mixed convective heat transfer characteristics of microcapsule slurry around a horizontal heated cylinder

UNSTEADY MHD FLOW OF RADIATING CASSON FLUID THROUGH A PERMEABLE CHANNEL WITH SLIP, BUOYANCY FORCE AND HEAT SOURCE

Analytical investigation is performed into an unsteady Magnetohrodynamics mixed convective Casson fluid flow and heat transfer characteristics with thermal radiation, wall slip, heat source and buoyancy force in a permeable vertical channel. The fluid is injected into the left wall of the channel and sucked out at right wall. The governing momentum and energy balance equations are achieved and tackled analytically. The effects of numerous thermophysical parameters on the temperature profiles, velocity, Nusselt number as well as skin friction are presented graphically and discussed qualitatively. The results show that a temporal decline in the pressure gradient causes both the temperature and fluid velocity to decrease. Moreover, the enhancement in heat transfer due to wall injection/suction also causes the skin friction to decrease.

MHD boundary layer heat and mass transfer flow of nanofluid through porous media over inclined plate with chemical reaction

PurposeSteady-state mixed convection boundary layer flow, heat and mass transfer characteristics of Buongiorno's model nanofluid over an inclined porous vertical plate with thermal radiation and chemical reaction are presented in this analysis.Design/methodology/approachThe governing nonlinear partial differential equations represent the flow model that can be converted into system of nonlinear ordinary differential equations using the similarity variables and are solved numerically using finite element method.FindingsThe rates of nondimensional temperature and concentration are both decelerate with the higher values of thermophoresis parameter (Nt).Originality/valueThe work carried out in this paper is original.

Periodic mixed convection flow along the surface of a thermally and electrically conducting cone

The main aim of the present work is to highlight the significances of periodic mixed convection flow and heat transfer characteristics along the surface of magnetized cone by exerting magnetic field exact at the surface of the cone. The numerical simulations of coupled non-dimensional equations are computed in terms of velocity field, temperature and magnetic field concentration and then used to examine the periodic components of skin friction, ?w, heat transfer, qw, and current density, jw, for various governing parameters. A nice periodic behavior of heat transfer qw is concluded for each value of mixed convection parameter, ?, but maximum periodicity is sketched at ? = 50. It is also computed that the lower value of magnetic Prandtl number ? = 0.1 gets poor amplitude in current density but highest amplitude is sketched for higher ? = 0.5. The behavior of heat and fluid-flow in the pres?ence of aligned magnetic field is associated with the phase angle and amplitude of oscillation. It is also noted that due to the increase in magnetic force parameter, ?, there are wave like disturbances generate within the fluid layers. These disturbances are basically hydromagnetic waves which becomes more prominent as the strength of magnetic force parameter is increased.

Periodic mixed convection flow along the surface of a thermally and electrically conducting cone

The main aim of the present work is to highlight the significances of periodic mixed convection flow and heat transfer characteristics along the surface of magnetized cone by exerting magnetic field exact at the surface of the cone. The numerical simulations of coupled non-dimensional equations are computed in terms of velocity field, temperature and magnetic field concentration and then used to examine the periodic components of skin friction, ?w, heat transfer, qw, and current density, jw, for various governing parameters. A nice periodic behavior of heat transfer qw is concluded for each value of mixed convection parameter, ?, but maximum periodicity is sketched at ? = 50. It is also computed that the lower value of magnetic Prandtl number ? = 0.1 gets poor amplitude in current density but highest amplitude is sketched for higher ? = 0.5. The behavior of heat and fluid-flow in the pres?ence of aligned magnetic field is associated with the phase angle and amplitude of oscillation. It is also noted that due to the increase in magnetic force parameter, ?, there are wave like disturbances generate within the fluid layers. These disturbances are basically hydromagnetic waves which becomes more prominent as the strength of magnetic force parameter is increased.