Pure Natural Convection Research Articles

Research on Melting and Solidification Process of Phase Change Storage Energy Materials

Investigate a melting and solidification of heat storage and release which used in building retaining structure, material of storage energy and phase change. In the process of phase change, liquid phase is used as two ways of heat exchange: pure heat conduction and natural convection, and we exploit CFD software to carry out numerical simulation. To compare and analyze under melt and solidification, the proportion of solid and liquid, the time of phase change, the interface of phase change and motion temperature image of liquid-region. To probe into the effect of natural convection under the heat transfer from phase change and the impact of regulation of by heating and cooling conditions, and it verified by experiments.

Research on Melting and Solidification Process of Phase Change Storage Energy Materials

Investigate a melting and solidification of heat storage and release which used in building retaining structure, material of storage energy and phase change. In the process of phase change, liquid phase is used as two ways of heat exchange: pure heat conduction and natural convection, and we exploit CFD software to carry out numerical simulation. To compare and analyze under melt and solidification, the proportion of solid and liquid, the time of phase change, the interface of phase change and motion temperature image of liquid-region. To probe into the effect of natural convection under the heat transfer from phase change and the impact of regulation of by heating and cooling conditions, and it verified by experiments.

A critical analysis of turbulent natural and forced convection in a plane channel based on direct numerical simulation

The balance equations for mixed convection in plane channel flow, based on the Boussinesq assumption for buoyancy, are carefully analysed by considering the two limits of pure forced and natural convection. Additional information about the flow is gained from the mechanical energy equation and the entropy equation (the second law of thermodynamics). Since the Boussinesq approximation assumes , i.e. a vanishing dimensionless temperature difference between the hot and the cold wall, certain inconsistencies in modelling the flow appear for ε ≠ 0. They are identified and discussed with respect to their importance. Numerical solutions gained by a direct numerical simulation (DNS) approach give detailed information about the friction factors and Nusselt numbers including the influence of those terms in the equations that turn out to be the source of the inconsistency in the modelling for ε ≠ 0.

Velocity measurements inside the concentration boundary layer during copper-magneto-electrolysis using a novel laser Doppler profile sensor

The evolution of the velocity boundary layer during the initial phase of copper electrolysis under the influence of a magnetic field is studied by using particle image velocimetry and a novel laser Doppler velocity profile sensor. With this new sensor, time-resolved velocity measurements within 400 μm of a vertically aligned cathode in an aqueous 0.05 M CuSO 4-solution are presented. In this way, the complex interaction of Lorentz force and opposing buoyancy-driven convection was studied by measuring the resulting velocity profile inside the concentration boundary layer with a spatial resolution of 15 μm. It is shown that the Lorentz force-driven convection only dominates the velocity boundary layer during the early phase of electrolysis and induces a linear velocity profile near the cathode. The linear relationship between the velocity gradient and Lorentz force is determined. With the onset of the opposing buoyancy-driven convection at the cathode, a duplex structure of the boundary layer appears. Its characteristic quantities, given by the horizontal distances, δ max and δ v = 0 , where the velocity reaches the maximum and where it is equal to zero, remain nearly unchanged, while the maximum velocity, v max , in spite of the counteracting Lorentz force, increases faster as compared to pure natural convection, depending on the current density.

Combined Heat Transfer of Radiation and Convection on Wall Sintered with Open Celled Metallic Foams

Experimental investigation of the combined radiation and natural convection heat transfer on wall surface sintered with open celled metal foams at different inclination angles is presented.The effects of wall inclination angle,Rayleigh number Ra* and thickness on heat transfer performance are studied.The heat transfer results for vertically placed smooth cooper plate are in good agreement with the available data in the literature to show the feasibility of the test rig.With increasing of the inclination angle under the same heat flux,the pure natural convection heat transfer coefficient and total heat transfer coefficient increase firstly to a turning point and then decrease.The maximum heat transfer performance occurs at an inclination angle between 60° and 80°.The radiation heat transfer accounts for 33%～44% of the total heat transfer.When placed vertically,the ratio of radiation heat transfer of metallic foam sintered surface in the total heat transfer is less than that of the smooth surface,and is improved with increasing of metal foam thickness.Compared to the smooth surface,the mean Nusselt numbers of combined heat transfer and pure natural convection heat transfer of vertically placed sintered surface increase by 1.52～1.98 and 1.16～1.66 times respectively.The temperature distribution on foam surface photographed by infrared camera are analyzed with image processing technique to validate the thermal radiation heat transfer calculation model based on average foam surface temperature.

Characteristics of mixed convection heat transfer in a lid-driven square cavity with various Richardson and Prandtl numbers

In mixed convection flows, a common knowledge is that the heat transfer in a cavity is increased with increasing Grashof or Reynolds number when its respective Reynolds or Grashof number is kept at constant. On the other words, the heat transfer would increase if the flow proceeds toward pure natural convection or forced convection dominated regimes. An unanswered question is that would the heat transfer be increased continuously with simultaneously increasing both Grashof and Reynolds numbers, while keeping the Richardson and Prandtl numbers constant. And to what extent the mixed convection flows would change from laminar to chaos. These questions motivate the present study to systematically investigate the flow and heat transfer in a 2-D square cavity where the flow is induced by a shear force resulting from the motion of the upper lid combined with buoyancy force due to bottom heating. The numerical simulations cover a wide range of Reynolds (10 ≤ Re ≤ 2200), Grashof (100 ≤ Gr ≤ 4.84 × 10 6), Prandtl (0.01 ≤ Pr ≤ 50), and Richardson (0.01 ≤ Ri ≤ 100) numbers. The average Nusselt numbers are reported to illustrate the influence of flow parameter variations on heat transfer, and they are also compared with the reported Nusselt number correlations to validate the applicability of these correlations in laminar flow regimes. Time traces of the total kinetic energy and average Nusselt number are presented to demonstrate the transition of the flows from laminar to chaos.

Effect of surface radiation on conjugate natural convection in a horizontal annulus driven by inner heat generating solid cylinder

This paper reports a numerical study of the laminar conjugate natural convection heat transfer with and without the interaction of the surface radiation in a horizontal cylindrical annulus formed between an inner heat generating solid circular cylinder and an outer isothermal circular boundary. Numerical solutions are obtained by solving the governing equations with a pressure correction method on a collocated (non-staggered) mesh. Steady-state results are presented for the flow and temperature distributions and Nusselt numbers for the heat generation based Grashof number ranging from 10 7 to 10 10 , solid-to-fluid thermal conductivity ratios of 1, 5, 10, 50 and 100, radius ratios of 0.226 and 0.452 and surface emissivities of 0–0.8 with air as the working medium. It is observed that surface radiation reduces the convective heat transfer in the annulus compared to the pure natural convection case and enhances the overall Nusselt number.

Analysis of a Synthetic Jet-Based Electronic Cooling Module

This article presents a numerical study of an electronic cooling module using a periodic jet flow at an orifice with net zero mass flux, known as a synthetic jet. The two-dimensional time-dependant numerical simulation models the unsteady synthetic jet behavior, the flow within the cavity and the diaphragm movement while accounting for fluid turbulence using the shear-stress-transport (SST) k-ω turbulence model. Computations are performed for a selected range of parameters and the boundary conditions to obtain the heat and fluid flow characteristics of the entire synthetic jet module. The numerical simulation aptly predicts the sequential formation of the synthetic jet and its intrinsic vortex shedding process while accurately illustrating the flow within the cavity. It is indicated that the thermal performance of the synthetic jet is highly dependant on the oscillating diaphragm amplitude and frequency. At the heated surface, this jet impingement mechanism produces a very intense localized periodic cooling effect that reaches a peak with a time lag relative to the top displacement position of the diaphragm. The overall heat transfer rate of the synthetic jet module is about 30% better than an equivalent continuous jet. When compared to pure natural convection the enhancement varies from 20 to 120 times in the range of parameters studied. The study clearly identifies the outstanding thermal potential of the synthetic jet module for intense electronic cooling applications and its ability to operate without additional fluid circuits.

Conduction-combined forced and natural convection in lid-driven enclosures divided by a vertical solid partition

Numerical simulations of the conduction-combined forced and natural convection (mixed convection) heat transfer and fluid flow have been performed for 2-D lid-driven square enclosure divided by a partition with a finite thickness and finite conductivity. Left vertical wall of enclosure has two different orientations in positive or negative vertical coordinate. Buoyancy forces are taken into account in the system. Horizontal walls are adiabatic while two vertical walls are maintained isothermal temperature but the temperature of the left moving wall is higher than that of the right stationary wall. Thus, heat transfer regime between moving lid and partition is mixed convection. Conduction occurs along the partition. And, pure natural convection is formed between the partition and the right vertical wall. This investigation covers a wide range of Richardson number which is changed from 0.1 to 10, thermal conductivity ratio varies from 0.001 to 10. It is observed that higher heat transfer was formed for higher Richardson number for upward moving wall for all values of thermal conductivity ratio. When forced convection becomes effective, the orientation of moving lid becomes insignificant. Heat transfer is a decreasing function of increasing thermal conductivity ratio for all cases and Richardson numbers.

Turbulent mixed convection in a shallow enclosure with a series of heat generating components

Turbulent mixed convection flow and heat transfer in a shallow enclosure with and without partitions and with a series of block-like heat generating components is studied numerically for a range of Reynolds and Grashof numbers with a time-dependent formulation. The flow and temperature distributions are taken to be two-dimensional. Regions with the same velocity and temperature distributions can be identified assuming repeated placement of the blocks and fluid entry and exit openings at regular distances, neglecting the end wall effects. One half of such module is chosen as the computational domain taking into account the symmetry about the vertical centreline. The mixed convection inlet velocity is treated as the sum of forced and natural convection components, with the individual components delineated based on pressure drop across the enclosure. The Reynolds number is based on forced convection velocity. Turbulence computations are performed using the standard k–ϵ model and the Launder–Sharma low-Reynolds number k–ϵ model. The results show that higher Reynolds numbers tend to create a recirculation region of increasing strength in the core region and that the effect of buoyancy becomes insignificant beyond a Reynolds number of typically 5 × 10 5 . The Euler number in turbulent flows is higher by about 30 per cent than that in the laminar regime. The dimensionless inlet velocity in pure natural convection varies as Gr 1 / 3 . Results are also presented for a number of quantities of interest such as the flow and temperature distributions, Nusselt number, pressure drop and the maximum dimensionless temperature in the block, along with correlations.

Effect of thermophoresis particle deposition on mixed convection from vertical surfaces embedded in saturated porous medium

PurposeThe aim of this paper is to formulate and analyze thermophoresis effects on mixed convection heat and mass transfer from vertical surfaces embedded in a saturated porous media with variable wall temperature and concentration.Design/methodology/approachThe governing partial differential equations (continuity, momentum, energy, and mass transfer) are written for the vertical surface with variable temperature and mass concentration. Then they are transformed using a set of non‐similarity parameters into dimensionless form and solved using Keller‐box method.FindingsMany results are obtained and a representative set is displaced graphically to illustrate the influence of the various physical parameters. It is found that the increasing of thermophoresis constant or temperature differences enhances heat transfer rates from vertical surfaces and increases wall thermophoresis velocities; this is due to favorable temperature gradients or buoyancy forces. It is also found that the effect of thermophoresis phenomena is more pronounced near pure natural convection heat transfer limit, because this phenomenon is directly temperature gradient‐ or buoyancy forces‐dependent.Research limitations/implicationsThe predicted results are restricted only to porous media with small pores due to the adoption of Darcy law as a force balance.Originality/valueThe paper explains the different effect of thermophoresis on forced, natural and mixed convection heat, and mass transfer problems. It is one of the first works that formulates and describes this phenomenon in a porous media. The results of this research are important for scientific researches and design engineers.

Interaction of turbulent natural convection and surface thermal radiation in inclined square enclosures

Two-dimensional numerical studies of flow and temperature fields for turbulent natural convection and surface radiation in inclined differentially heated enclosures are performed. Investigations are carried out over a wide range of Rayleigh numbers from 108 to 1012, with the angle of inclination varying between 0° and 90°. Turbulence is modeled with a novel variant of the k–e closure model. The predicted results are validated against experimental and numerical results reported in literature. The effect of the inclination of the enclosure on pure turbulent natural convection and the latter’s interaction with surface radiation are brought out. Profiles of turbulent kinetic energy and effective viscosity are studied to observe the net effect on the intensity of turbulence caused by the interaction of natural convection and surface radiation. The variations of local Nusselt number and average Nusselt number are presented for various inclination angles. Marked change in the convective Nusselt number is found with the orientation of enclosure. Also analyzed is the influence of change in emissivity on the flow and heat transfer. A correlation relevant to practical applications in the form of average Nusselt number, as a function of Rayleigh number, Ra, radiation convection parameter, N RC and inclination angle of the enclosure, φ is proposed.

Laminar conjugate mixed convection in a vertical channel with heat generating components

Conjugate mixed convection arising from protruding heat generating ribs attached to substrates (printed circuit boards) forming channel walls is numerically studied. The substrates with ribs form a series of vertical parallel plate channels. Assuming identical disposition and heat generation of the ribs on each board, a channel with periodic boundary conditions in the transverse direction is considered for analysis. The governing equations are discretised using a control volume approach on a staggered mesh and a pressure correction method is employed for the pressure–velocity coupling. The solid regions are considered as fluid regions with infinite viscosity and the thermal coupling between the solid and fluid regions is taken into account by the harmonic thermal conductivity method. Parametric studies are performed by varying the heat generation based Grashof number in the range 10 4–10 7 and the fan velocity based Reynolds number in the range 0–1500, with air as the working medium. Results are obtained for the velocity and temperature distributions, natural convection induced mass flow rate through the channel, the maximum temperatures in the heat sources and the local Nusselt numbers. The natural convection induced mass flow rate in mixed convection is correlated in terms of the Grashof and Reynolds numbers. In pure natural convection the induced mass flow rate varies as 0.44 power of Grashof number. The maximum dimensionless temperature is correlated in terms of pure natural convection and forced convection inlet velocity asymptotes. For the parameter values considered, the heat transferred to the working fluid via substrate heat conduction is found to account for 41–47% of the heat removal from the ribs.

Use of a vibrating plate to enhance natural convection cooling of a discrete heat source in a vertical channel

A numerical investigation was conducted into an alternative method of natural convection enhancement by the transverse oscillations of a thin short plate, strategically positioned in close proximity to a rectangular heat source. The heat source is attached to a mounting board in a vertical channel. Two-dimensional laminar flow finite element studies were carried out with the oscillation parameters, the oscillating plate-heat source mean clearance spacing, and the oscillating plate position varied. Significant cooling was found for displacement amplitudes of at least one-third of the mean clearance together with frequencies ( Re / Gr ) of over 2π with the displacement being more critical to the cooling level. For the parameters investigated, up to a 52% increase in the local heat transfer coefficient relative to standard natural convection was obtained. The results indicate that this method can serve as a feasible, simpler, more energy and space efficient alternative to common methods of cooling for low power dissipating devices operating at conditions just beyond the reach of pure natural convection.

근적외선 농수산물 건조기의 복합열전달특성에 관한 수치적 연구

Mixed heat transfer in an indirected NIR (Near Infrared Ray) dry chamber was investigated numerical analysis. It is Important that the miked heat transfer effects on double parameters which the Reynolds number and the position of emit lamp. Reynolds number are based on the outer diameter of the cylinder range from 103 to <TEX>$30{\times}105$</TEX>. Four difference heat transfer regimes of behavior are apparent: forced convection and radiation on the outer surface of the cylinder, pure conduction, pure natural convection and radiation between lamp surface and inner surface of the cylinder. The temperature and flow patterns are illustrated by iso-contour lines for the double parameters. Also presented are results on the convective heat transfer flux and the radiative heat transfer flux as increased with Reynolds number.

Combination technique for improving natural convection cooling in electronics

The combination of an appropriately placed cross-flow opening and a strategically positioned transversely vibrating plate is proposed as a means of augmenting pure natural convection in a vertical channel. This method is intended to provide a more efficient, reliable, and consumer conscious alternative to conventional techniques for lower power dissipating devices where standard natural convection cooling proves insufficient. Two-dimensional numerical simulations are employed to investigate this combination method using models consisting of a vertical channel containing two rectangular heat sources which are attached to a vertical mounting board, as well as a transversely oscillating plate and a cross-flow opening in the mounting board area between the two heat sources. Varied parameters and geometric configurations are studied. The results indicate the combined effects of the vibrating plate and the opening flow have the potential to cause significant improvement in the thermal conditions over pure natural convection. As much as a 70% improvement in the local heat transfer coefficient from that for a system with a board opening but without a vibrating plate was attained.

A Theoretical Investigation Into the Optimal Longitudinal Profile of a Horizontal Pin Fin of Least Material Under the Influence of Pure Forced and Pure Natural Convection With a Diameter-Variable Convective Heat Transfer Coefficient

Abstract The increased importance of heat sinks in electronic cooling applications has resulted in a revived interest in extended surfaces, or fins. Also, space and cost constraints provide impetus for optimizing thermal performance for a given, or least, amount of material. The current research focuses on a pin fin design of least material, where the diameter of the pin fin varies axially to maintain the axial heat flux constant; thus all fin material is utilized equally. Although such fins have been studied in the past, the convective heat transfer coefficient was assumed to be constant, which is not entirely true since it is known to be a function of diameter for cylindrical bodies. The current research shows that an optimal fin based on a variable convective heat transfer coefficient yields a true optimal profile, and utilizes material better; that is, it uses a lower volume of material to achieve the same heat dissipation rate. This improvement in material utilization is show to be anywhere from approximately 3% to 14%.

Feasibility Study of Unconventional Cooling of Electronic Components by Vibrating Plates at Close Proximity

ABSTRACT In this study, an unconventional approach to electronic cooling is described, and its feasibility is investigated. This approach makes use of the transverse oscillations of a short plate placed in close proximity to the electronic component to provide for focused and localized cooling. This method can be applied for intermediate cooling rates, above those of natural convection, in order to extend the no-fan regime. To estimate the cooling potential, a numerical simulation of the temperature field in a channel formed by an oscillating surface and a fixed constant heat flux surface is performed. The results show that transverse oscillations have the potential for significant cooling enhancement compared to pure natural convection. The cooling effect is found to increase with the oscillation amplitude and channel length to mean width ratio and, to a lesser extent, with the oscillation frequency.

A numerical study of natural convection in a horizontal enclosure with a conducting body

The physical model considered here is a horizontal layer of fluid heated below and cold above with a conducting body placed at the center of the layer. The body has dimensionless thermal conductivities of 0.1, 1 and 50. Two-dimensional solution for unsteady natural convection is obtained using an accurate and efficient Chebyshev spectral methodology for different Rayleigh numbers. Multi-domain technique is used to handle a square-shaped conducting body. The results for the case of a conducting body are also compared to those of adiabatic and neutral isothermal bodies. When the dimensionless thermal conductivity is 0.1, a pattern of fluid flow and isotherms and the corresponding surface- and time-averaged Nusselt number are almost the same as the case of an adiabatic body. When the dimensionless thermal conductivity is 50, a pattern of flow and isotherm and the corresponding surface- and time-averaged Nusselt number are similar to those of neutral isothermal body. The results for the case of dimensionless thermal conductivity of unity are also compared to those of pure natural convection.

Start-up and steady thermal oscillation of a pulsating heat pipe

As a novel electronic cooling device, pulsating heat pipes (PHPs) have been received attention in recent years. However, literature survey shows that no studies were carried out on the start-up and steady thermal oscillation of the PHPs. In the present paper, the copper capillary tube was being bended to form the snake-shaped PHP. Heating power was applied on the heating section, and transferred to the condensation section and dissipated to the environment by the pure natural convection. The inside diameter of the capillary tube is 2.0 mm and the working fluid is selected as FC-72. A high speed data acquisition system was used to detect the start-up and steady thermal oscillation of the PHP. Two types of the start-up process were observed: a sensible heat receiving start-up process accompanying an apparent temperature overshoot followed by the steady thermal oscillation at low heating power, and a smooth sensible heat receiving start-up process incorporating a smooth oscillation period at high heating power. For the steady thermal oscillation, also two types were found: the random thermal oscillation with a wide frequency range, indicating the random distribution of the vapor plug and liquid slug inside the capillary tube at low heating power, and the quasi periodic thermal oscillation with the same characteristic frequency for both heating section and condensation section, indicating the uniform distribution of the vapor plug and liquid slug inside the capillary tube at high heating power. The power spectral density (PSD) was used to analyze the thermal oscillation waves. The frequency corresponds to the time that a couple of adjacent vapor plug and liquid slug passing through a specific wall surface.