Free Convection Heat Transfer Research Articles

Free convection heat transfer analysis of a suspension of nano–encapsulated phase change materials (NEPCMs) in an inclined porous cavity

In the current study, free convection heat transfer of a suspension of Nano–Encapsulated Phase Change Materials (NEPCMs) is simulated and discussed in an inclined porous cavity. The phase change materials are capsulated in nano-shells layers, while the core stores/releases large amounts of energy during melting/solidification in the vicinity of the hot/cold walls. The governing equations are introduced and transformed into non–dimensional form before being solved by using the finite element method. Simulation results are validated thoroughly. Thereafter, the consequences of the fusion temperature and the Stefan number on the distributions of streamlines, isotherms, and the heat capacity ratio, as well as the heat transfer characteristics, are analyzed for different inclination angles of the cavity. Inspection of the results demonstrates that the best heat transfer performance occurs for the non–dimensional fusion temperature of 0.5 and the inclination angle of 42°. It is found that a decrease in the Stefan number improves heat transfer. The results also show that the presence of the NEPCM particles generally leads to heat transfer improvement.

Analytical solution of free convection heat transfer of hybrid nanofluids over a vertical flat plate embedded in a porous medium

The natural convection heat transfer of hybrid nanofluids over a vertical plate embedded in a saturated Darcy porous medium is analytically investigated. The base partial differential equations were reduced to a set of high‐order nonlinear differential equations by using appropriate similarity variables. A new analytical solution based on the power series–transformation method is presented, and the effect of the presence of hybrid nanoparticles in the form of nondimensional parameters is investigated.

Research of horizontal single-row bundles of ribbon finned tubes at free convection

The intensity of free convection research leads to a wider understanding of the mechanisms and possibilities of using free-convection currents for the heat discharge or heatremoval in many industries, in energy processes, in a large number of devices or systems. Free convection significantly affects the maximum values of heat fluxes, and studying them becomes quite important for problems in which other methods of heat transfer are not applicable or uneconomical. The phenomena of free-convective heat transfer are quite complex, processes of conjugate heat transfer, for example, radiation and convection can be observed. Theoretical methods of description and calculation are used for bodies of simple shape: ball, plate, cylinder. The methods of theoretical analysis for heat transfer of bodies of complex shape are too voluminous and cumbersome. The intensification of free-convective heat transfer is possible through the use of various types of fins. To calculate the natural convective heat transfer of finned surfaces, the criterial dependencies of an empirical nature are mainly used. The article presents the results of a study of free-convective heat transfer of single-row horizontal bundlesof industrial bimetallic pipes with tape finning, the finning factor of 16.8. The position of the pipes in the bundle is characterized by the by transverse steps standardized for heat exchangers for air coolingS1 = 58; 61; 64; 70; 76; 86; 100 mm, to which the relative steps corresponded σ1 = S1/d = 1.208; 1.271; 1.333; 1.458; 1.583; 1.792; 2.083. An equation for calculating the free-convection heat transfer, taking into account the influence of the relative step in the range of Rayleigh number from 30,000 to 350,000 with an average deviation of ± 6.6%, has been obtained.

Improving Building’s Thermal Performance by Means of Porous Media – An Experimental Free Convection Work

The thermal efficiency of the modified Trombe-wall system proposed in this work is improved compared to its conventional version thanks to the adjunction of a porous material to the absorber wall. Thermal measurements were made in a wide Rayleigh number range, with high values reaching 4.52 × 109. The ratio between the matrix’s thermal conductivity of the porous material and that of the air varies between 1 (without porous medium, conventional Trombe-wall system) and about 1888. Moreover, the aspect ratio of the active cavity varies in the 0.1–0.3 range. Influence of these physical parameters on the free convective heat transfer occurring in the active cavity are quantified for this new assembly and compared to the conventional one. The results show that the average convective heat transfer systematically increases with the ratio of thermal conductivities for any Rayleigh number and cavity’s aspect ratio. Furthermore, the simple modification makes it possible to increase the overall thermal efficiency between 4% and 23% on average. This work complements and confirms the numerical results of recent studies realized by means of the control volume method.

Numerical analysis of natural convection of alumina–water nanofluid in H-shaped enclosure with a V-shaped baffle

Numerical analysis of the free convection heat transfer in an H-shaped cavity loaded with alumina–water nanofluid is performed in this work. The entire bottom wall of the H-shaped enclosure and the top rib are at the hot temperature, the two sidewalls are at the lower temperature, and the others are insulated. This study aims to elucidate the influences of different variables on the heat transfer of nanofluid, namely the Rayleigh number (Ra = 104–106), nanoparticle volume fraction percentage (ϕ = 0–2%), aspect ratio (AR = 0.2–0.4), installation of baffle, location of baffle in the cavity, and the types of baffle boundary condition. To model the buoyancy force, the Boussinesq approximation is employed. The characteristics of flow and thermal transmission are presented in streamlines and isotherms. The results show that the heat transmission is improved by Rayleigh number enhancement and nanoparticle volume fraction due to the enhanced temperature gradient and the nanofluid thermal characteristics. In addition, the baffle installation at the bottom rib under a non-adiabatic boundary condition improve the heat transfer because the bottom wall has a larger mean temperature than the top wall.

Using nanofluid saturated porous media to enhance free convective heat transfer around a spherical electronic device

The thermophysical properties of the nanofluid saturated porous media are used in this work to optimize the thermal design of a spherical electronic device. Quantification of free convective heat transfer has been numerically determined by means of the finite volume method using the SIMPLE algorithm. The Rayleigh number based on the component diameter and water characteristics varies between 6.5x106 and 1.32x109, given the power generated during operation of this active component. The latter is disposed in the center of another sphere maintained isothermal. Its cooling is achieved by means of a porous medium saturated with a water based - Copper nanofluid whose volume fraction varies between 0 (pure water) and 10%. The thermal conductivity of the porous material's matrix ranges from 0 to 40 times that of the base fluid (water). Results of this work show that convective heat transfer systematically increases with this ratio according to a function depending on the Rayleigh number in the whole range of the considered volume fraction. The average Nusselt number also increases with the Rayleigh number according to a conventional power type law while influence of the fraction volume is moderate in the 2-10% range. The results are in agreement with those of previous works for particular thermal conditions. In order to optimize the thermal design of this electronic device, a correlation is proposed, allowing determination of the Nusselt number for any combination of the three influencing parameters for applications in various engineering fields, includind electronics.

Free convection heat transfer analysis of slitted fin heat sink of vertical orientation using CFD

ABSTRACT The free convection heat transfer using different kinds of fins is influenced by changes in geometry and interruptions in fluid flow. This paper investigates the enhancement of natural convection heat transfer using a new slitted fin configuration over the vertical plate-fin heat sink. Two different kinds of fins configurations (rectangular plain fin and slitted fin) with a constant fin spacing was selected for investigation. Aluminium with good thermal conductivity was selected for fins and five different heat inputs are considered for the analysis. The numerical results were validated with experimental results and available theoretical correlations. The various flow patterns have been examined for vertical mounted plain and slitted rectangular fin configuration. The results demonstrated that an interruption provided in plain fins reduces the weight and improves the free convection heat transfer rate. The maximum enhancement due to the addition of the slits into the fins was found to be 69%.

Free convective heat transfer in Jeffrey fluid with suspended nanoparticles and Cattaneo–Christov heat flux

Free convection flow of Jeffrey nanofluid past a vertical plate with sinusoidal variations of surface temperature and species concentration is presented. The study of heat transfer and nanofluid transport has been done by employing Cattaneo–Christov heat flux model and Buongiorno model, respectively. Equations governing the flow are non-dimensionalized using appropriate transformations. Furthermore, the method of local similarity and local non-similarity is used to reduce the equations into non-linear coupled system of equations which are then solved by homotopy analysis method. The obtained results are validated by comparing with the existing results available in the literature. The numerical results are found to be in good agreement. The effects of varying the physical parameters such as Deborah Number, Prandtl number, Schmidt number, thermophoresis parameter, Brownian motion parameter and buoyancy ratio parameter are obtained and presented graphically. The effect of sinusoidal variation of surface temperature and species concentration on the skin friction coefficient, Nusselt number and Sherwood number is also shown. Velocity for Jeffrey nanofluid is more than the Newtonian nanofluid while temperature and nanoparticle concentration for Jeffrey nanofluid is less than the Newtonian nanofluid. Raising value of thermal relaxation times leads to an increase in the heat transfer coefficient. It is observed that temperature of Cattaneo–Christov heat flux model is less than that in classical Fourier’s model away from the vertical wall. These types of boundary layer flow problems are found in vertical film solar energy collector, grain storage, transportation and power generation, thermal insulation, gas production, petroleum resources, geothermal reservoirs.

MHD Free Convective Heat Transfer in a Triangular Enclosure Filled with Copper-Water Nanofluid

Two-dimensional time-independent free convective flow and temperature flow into a right-angled triangle shape cavity charged by Cu-H2O nanofluid has been performed. The horizontal side of the enclosure is warmed uniformly T=Th whilst the standing wall is cooled at low-temperature T=Tc and hypotenuse of the triangular is insulated. The dimensionless non-linear governing PDEs have been solved numerically employing the robust PDE solver the Galerkin weighted residual finite element technique. An excellent agreement is founded between the previous, and present studies. The outcomes are displayed through streamline contours, isotherm contours, and local and average Nusselt number for buoyancy-driven parameter Rayleigh number, Hartmann number, and nanoparticles volume fraction. The outcomes show that the temperature flow value significantly changes for the increases of Rayleigh number, Hartmann number, and nanoparticles volume fraction. Average Nusselt number is increased for the composition of nanoparticles whereas diminishes with the increase of Hartmann number.

Effect of Al2O3/water nanofluid on Conjugate Free Convection in a Baffle Attached Square Enclosure

A numerical study of conjugate free convection heat transfer of Al2O3/water nanofluid inside a differentially heated square enclosure with a baffle attached to its hot wall has been carried out. A detailed parametric study has been carried out to analyze the effect of Rayleigh number (104 < Ra < 106), length, thickness and position of baffle, conductivity ratio and volume fraction of the nanoparticle (0<<0.2) on heat transfer. The thermal conductivity ratio of the baffle plays a major role on the conjugate heat transfer inside the enclosure. Higher the baffle length better is the effectiveness of the baffle. The average Nusselt number is found to be an increasing function of both thermal conductivity ratio and volume fraction of the nanofluid. The minimum enhancement of conjugate heat transfer is 30% when Al2O3/water nanofluid of 0.1 volume fraction is used for the entire range of Rayleigh number considered.

Thermodynamic analysis of natural convection supercritical water flow past a stretching sheet using an equation of state approach

The present numerical study examines the free convection heat transfer characteristics of supercritical water flow past a stretching sheet. A suitable equation for the thermal expansion coefficient in a supercritical fluid region is derived based on the equation of state (EOS) approach in terms of compressibility factor, pressure, and temperature. In the present study, the Redlich–Kwong equation of state (RK-EOS) is used to calculate the thermal expansion coefficient in the supercritical region. The values of the thermal expansion coefficient calculated through RK-EOS lies close to the NIST data values when compared to the other equations of state like the Van der Waals equation of state and ideal gas EOS. Also, the behaviour of Nusselt number is studied to characterize the heat transfer characteristics of supercritical water. However, the equations governing the supercritical fluid flow past a stretching sheet are coupled and nonlinear in nature. Hence, the Runge–Kutta fourth-order integration scheme with shooting technique is used to solve these equations. Numerical computations are performed for supercritical water under the influence of various control parameters. Similarity solutions are obtained in terms of flow profiles in the supercritical fluid region. The present study reports that the normal velocity profile decreases and the temperature field increases for increasing values of reduced pressure and reduced temperature. Also, the axial velocity profile shows the dual behaviour for increasing values of unsteady parameter, reduced temperature, and reduced pressure in the supercritical boundary layer region. Furthermore, the component of the normal velocity profile decays for increasing values of the unsteady parameter in the supercritical fluid region. The calculated values of the thermal expansion coefficient using RK-EOS lies in the proximity of NIST data values when compared to Van der Waals and ideal gas equations of state. Also, the local skin-friction coefficient decreases for increasing values of reduced pressure and reduced temperature. Furthermore, the magnitude of local heat transfer rate increases with increasing values of unsteady parameter. The RK-EOS is the suitable EOS approach for predicting the free convection properties of water in a supercritical region.

Two‐phase natural convection analysis and hybrid nanoparticle migration around micromixer blades

AbstractIn this paper, numerical investigations are presented for hybrid nanoparticle migration and free convection heat transfer of two kinds of nanofluids in a micromixer at the fixed propeller condition. The inner blades and outer crust of the micromixer are kept at constant hot and cold temperatures, respectively. Two kinds of hybrid nanofluids, TiO2‐CuO water and ethylene glycol‐(MoS2‐SiO2), are considered. The governing equations including velocity, pressure, temperature formulation, and nanoparticle concentrations are solved by a partial differential equation solver based on the Galerkin finite element method. The results are discussed based on the governing parameters, such as nanoparticle volume fraction, thermal and solutal Rayleigh numbers. The average Nusselt number was found to increase with the increasing nanoparticle volume fractions. Also, increasing the thermal Rayleigh number enhanced heat transfer while the solutal Rayleigh number has an insignificant effect on it. More importantly, increasing the thermal Rayleigh number assisted avoiding the agglomeration of nanoparticles around the blades and ensured more uniform nanoparticle distribution.

Heat Transfer Characteristics of Inclined Helical Coil Tube by Forced and Free Convection : A Review

Aim of this review work is to examine the relative advantage of using an inclined helical coil tube heat exchanger for high-pressure syngas under various industries application. It observed that the heat transfer in the helical coil tube is higher as compared to the straight tube due to their shape. Inclined helical coil tube heat exchanger offers advantageous over straight tubes due to their compactness and increased heat transfer coefficient. The increased heat transfer coefficients are a consequence of the curvature of the coil, which induces centrifugal forces to act on the moving syngas. Due to the curvature effect, the fluid streams in the outer side of the pipe moves faster than the syngas streams on the inner side of the tube. Heat transfer augmentation techniques refer to various methods used to increase the heat transfer rate without changing much the overall performance of the system. The arrangements are useful in a variety of applications where more heat transfer rates desired. Some of the claims involved in heat exchanger used in Thermal Power plants, process industries, air-conditioning equipment, heating and cooling in evaporators, radiators for space vehicles, automobiles, refrigerators. These techniques broadly of three types viz. passive, active, compound heat transfer augmentation techniques. The present paper reviews the experimental investigation of free and forced convection heat transfer from different helical coiled tubes.

Free Convection Heat Transfer Around a Cylinder Embedded in an Enclosure Filled with Porous Media

An experimental and theoretical study of free convection heat transfer for a cylinder placed in an iron test section of dimensions (0.2x0.2x0.2 m3), the test section filled with saturated porous material glass balls (5 mm), and the air is the working fluid with Raleigh number (7692.6 ≤ Ra ≤ 17654). The circular cylinder heater (D = 0.015 m, L = 0.2 m) is heated electrically, made of Copper and located in different positions (in X & Y direction). The theoretical part includes solving the free convection heat transfer using the ANSYS program (fluent). The experimental and theoretical results showed that the surface temperature values around the cylinder perimeter when changing its position within the test section are changing as moving up and down where the effect of buoyancy force appears. The maximum difference between the upper and lower position at the experimental result is 7.22%, and the average Nusselt number increases with Raleigh number and heat flux. Also, the experimental results showed that the use of porous material significantly improves the heat transfer by 48.6%. The maximum percentage change between the experimental and theoretical results is 5.46%. Moreover, experimental correlations were achieved, and a comparison was performed between the present results with the previous studies and it gives a good agreement.

Free convection/radiation and entropy generation analyses for nanofluid of inclined square enclosure with uniform magnetic field

This paper studies the generation of entropy and free convective heat transfer of Al2O3/water nanofluid in an inclined enclosure affected by a magnetic field considering radiation effects. There is a circular quadrant at temperature $$T_{\text{h}}$$ in the bottom section of the left wall of the enclosure. The right wall of enclosure is kept at a fixed temperature $$T_{\text{c}}$$. The other walls are insulated. The governing equations for fluid flow are resolved using the algorithm of SIMPLE. The effect of variations of Rayleigh number (Ra), Hartmann number (Ha), the enclosure angle and concentration of nanoparticles ($$\varphi$$) on the flow field, isothermal field, entropy field, Nusselt number (Nu), Bejan number (Be) and total generation of entropy is investigated. The results indicate that the Nu enhances by 160% and 40% by augmenting the Ra and decreasing the Ha, respectively. Be diminishes by enhancing the Ra and reducing the Ha. The maximum generation of entropy intensifies by 288% and 39% by augmenting the Ra and reducing the Ha, respectively. The highest average Nusselt umber ($${\text{Nu}}_{\text{M}}$$) and an overall generation of entropy occur at the inclination angle of 30°.

Intensification of heat transfer in the cooling systems of radio electronic equipment under free and forced convection

This paper considers the issues of heat transfer intensification in the air cooling system of heat-loaded elements of radio-electronic equipment based on split fins. The results of the study of heat transfer in free and forced convection cooling systems for heat-loaded elements of electronic equipment are presented. The thermal efficiency of systems based on smooth and partially split ribs is compared; the efficiency of split ribs is 16-20%. The dependences of thermal resistance on thermal load are determined. The results of a comparison of mathematical and physical modeling are provided. The influence of the method of supplying heat to split ribs is estimated. Optimization of the work area is presented.

Computational Analysis of Natural Convection Heat Transfer on a Vertical Plate With Discrete Heat Sources

In the present study, buoyancy driven free convection flow along the vertical plate with discrete heat sources is analyzed. To illustrate free convection heat transfer along a vertical plate with finite discrete heat sources a 2-D steady-state model is considered. The thermos-physical properties of fluid are assumed constant except for the buoyancy terms, which are computed using the Boussinesq approximation of Navier-stokes equation. The two-dimensional Navier-stokes equations are solved using SIMPLER algorithm. The dimensionless equations are descretised and solved by using central difference finite difference approach. The development of the air flow caused by buoyancy induced free convective heat transfer has been studied through the progression of velocity and temperature fields. The results are obtained for various Grashof numbers Gr=103 , 104 and 5x 104 , and the influence of Grashof number on flow field has been studied. Average Nusselt number at the plate is also obtained. The effect of variation of Prandtl number at a given Grashof number is also studied.

Free convection and entropy generation of a nanofluid in a tilted triangular cavity exposed to a magnetic field with sinusoidal wall temperature distribution considering radiation effects

Abstract In this research, free convective heat transfer and entropy production of alumina/water nanofluid in a tilted triangular enclosure are studied considering radiation impacts and inclined constant magnetic field (MF). SIMPLE algorithm is used to solve the governing equations. The impact of effective parameters consist of Ra, Ha, MF angle, enclosure angle, radiation parameter and concentration of alumina nano-materials on the convective heat transfer, the entropy production and the Bejan number (Be) are examined. The findings reveal that the heat transfer rate (HTR) increased 2.32 times and the entropy production augmented 14.17 times with growing the Ra. The HTR and total entropy production (TEG) diminish by augmenting the Ha. The Be enhances by decreasing the Ra and growing the Ha. The maximum HTR takes place at MF angle of 90° and also at the enclosure angle of 60°. The HTR enhances 7.3% and the TEG rises by 9.3% when 6% of the nano-materials was added to the water.

Couple Stresses and Thermophoresis Influences on Free Convective Heat and Mass Transfer of Viscoelastic Fluid with Hall and Ion-Slip Effects.

An analysis is presented to investigate the Hall and Ion-slip effects on non-Newtonian couple stress fluid flow between vertical channel in the presence of thermophoresis phenomena. The considered fluid obeys to the viscoelastic second-grade model. The problem is modulated mathematically to describe continuity, momentum, temperature, and concentration equations. The influences of physical parameters of dimensionless equations of velocity components () and (), temperature () and concentration () have been shown in some of figuthe res, as well as skin friction (), Nusselt number() and Sherwood number() have been computed. It is found that the velocity decreases with increasing in Hall parameter and Ion slip parameters.

Numerical Study of Heated Tube Arrays in the Laminar Free Convection Heat Transfer

Laminar free convection heat transfer from a heated cylinder and tube arrays is studied numerically to obtain the local and average Nusselt numbers. To verify the numerical simulations, the Nusselt numbers for a single cylinder were compared to other authors for the Rayleigh numbers of 103 and 104. Furthermore, the vertically arranged heated tube arrays 4 × 1 and 4 × 2 with the tube ratio spacing SV/D = 2 were considered, and obtained average Nusselt numbers were compared to the existing correlating equations. A good agreement of the average Nusselt numbers for the single cylinder and the bottom tube of the 4 × 1 tube array is proved. On the other hand, the bottom tubes of the 4 × 2 tube array affect each other, and the Nusselt numbers have a different course compared to the single cylinder. The temperature fields for the tube array 4 × 4 in basic, concave, and convex configurations are studied, and new correlating equations were determined. The simulations were done for the Rayleigh numbers in the range of 1.3 × 104 to 3.7 × 104 with a tube ratio spacing S/D of 2, 2.5, and 3. On the basis of the results, the average Nusselt numbers increase with the Rayleigh numbers and tube spacing increasing. The average Nusselt number and total heat flux density for the convex configuration increase compared to the base one; on the other hand, the average Nusselt number decreases for the concave one. The results are applicable to the tube heaters constructional design in order to heat the ambient air effectively.