Experimental Study Of Free Convection Research Articles

Experimental study of free convection from vertical tubes with curved extended surfaces

Herein, the free convection from vertically aligned tubes with curved extended surfaces is experimentally investigated. Heat transfer rates from tubes with curved extended surfaces to the ambient air are quantified under various conditions, including under varying tube wall temperatures, numbers of extended surfaces, and heights of extended surfaces in the radial direction. A correlation for calculating the Nusselt number is suggested, drawing upon the experimental findings. The difference between the experimental outcomes and the estimates generated by this correlation falls within a 10% margin. Using the correlation, a contour diagram is generated to show the thermal resistances across different numbers and thicknesses of extended surfaces. Finally, the thermal resistance of tubes featuring curved extended surfaces is evaluated against tubes with conventional straight extended surfaces. It is found that the tube with curved extended surfaces has a thermal resistance that is 20% less than that with straight extended surfaces.

Numerical and experimental study of free convection through a horizontal open‐ended axisymmetric cavity

AbstractThis paper summarizes a numerical and experimental investigation of free convective heat transfer in an open‐ended cavity between two horizontal parallel circular plates. The upper plate is maintained at an ambient temperature and the lower one is heated. Air is used as the heat transfer medium. The numerical model equations are solved using a control volume‐based finite differences method, and the experimental study was performed using holographic interferometry. Streamlines and isotherm patterns are presented and discussed for different aspect ratios (A) and Rayleigh numbers (Ra). Heat transfer at the surface of the lower plate is thoroughly inspected in the ranges and . Useful correlations of Nusselt numbers in terms of and A are given with their validity ranges. Also, an investigation of both numerical and experimental results is performed. It shows similar temperature field aspect with some differences in the radial boundary layer thickness and a small deviation in the heat transfer.

Numerical and experimental study of free convection on wire-bonded QFN64b electronic package

PurposeThe wire-bonded version of the quad flat non-lead with 64 leads (QFN64b) is increasingly integrated in modern arrangements, given its thermal and electrical characteristics suited for specific applications. Temperature control is thus essential for its proper operation, particularly when the heat exchange with the environment is done by natural convection. This work aims to consider a conventional assembly consisting of a large printed circuit board (PCB) on which is welded a QFN64b generating a power in the range 0.01-0.1 W. The PCB could be inclined at an angle varying between 0° and 90° (horizontal and vertical positions, respectively) according to the intended application.Design/methodology/approachThe 3D numerical approach done by means of the finite volume method is complemented by thermal and electrical measurements for all the configurations numerically processed. The low deviations obtained between the calculations and the measurements validate the adopted model. These results complement recent work that considers the same assembly equipped with a tilted and low-powered QFN64 basic model subjected to free convection.FindingsThe surface temperature in any part of the assembly has been determined. The influence of the power generated by the device and the PCB’s inclination angle relative to the gravity field have been quantified. The work shows that the radiative heat transfer is negligible given the temperatures reached and that the thermal state of the considered assembly is different from the one equipped with the QFN64 basic model. The QFN’s temperature is lowered, while that of the PCB is increased. The temperature distribution is also different from that of assemblies equipped with other QFN models with and without wire-bonding.Originality/valueThe correlations proposed in this survey help optimize the thermal design of the QFN64b electronic package used in many engineering fields.

Experimental study on free convection of sodium in a long cylinder

The operation experience of sodium fast reactor shows that during design-basis validation of pipelines and equipment it is necessary to take into account the sodium free convection in the enclosures. The paper presents the results of experimental study of free convection of liquid sodium in a long thermo-insulated cylinder with the end heat supply and removal. The sodium-filled cylinder diameter is 168 mm, length is 850 mm. Three experiments for horizontal, inclined (at 45° to a vertical line) and vertical position of the cylinder were compared in detail. The Rayleigh number (based on cylinder diameter) is approximately same for three experiments and is equal to 5 × 106. The structure of large-scale and small-scale flows was analyzed. Nusselt number estimations being the intensity measure of heat transfer in case of free convection were obtained. A relationship between the flow structure and the Nusselt number is revealed.

Experimental Study of Free Convection in Open-cell Aluminum Foam

Free convection in air-saturated open-cell aluminum foam was studied experimentally. The influence of pore density, foam height and bonding technologies are examined. For this research, the base plate temperature is varied between 55°C and 95°C. The ratio of the sample length-to-width is kept to a fixed value of 10. The investigated foams have a pore density of 10 and 20 PPI (Pores Per linear Inch) with a porosity of 93%. The foam height is varied between 6 and 40mm. The bonding of the foam is done by either brazing, or by applying a single epoxy.An effect of the bonding methods is observed. The brazed samples showed a higher heat transfer rate in all cases. The influence of the pore density is more pronounced. However, the strongest influence can be attributed to an increase in sample height.

Effect of the nonmonotonic temperature dependence of water density on convection under uniform heating

This paper presents the results of an experimental study of free convection in a closed rectangular tank with fresh water whose initial temperature is lower than the constant temperature of air outside the tank. Convective instability is shown to play a significant role in the process of heat transfer from the boundary layers on the walls to the bulk of water. In the case where the initial water temperature is higher than the temperature at which the density is maximal (about 4°C), convective instability occurs only in the boundary layer at the bottom of the tank. At a water temperature below 4°C convective instability also exists for some time in the boundary layer on the cover. Quantitative information is given on the variations in temperature and water density in time and on the vertical coordinate.

Experimental Study of Free Convection in Coiled Tube Heat Exchanger with Vertical Orientation

An experimental study has been conducted on steady-state natural convection heat transfer from helical coil tubes in vertical orientation. Water was used as a bath liquid without any mixing and cold water was used as a coolant fluid. A straight copper tube of 6 mm ID, 8 mm OD and 3 m length was bend to fabricate the helical coil. Four coils are used in this experiment has different curvature ratios and pitches. The data were correlated using tube diameter as the characteristic length. The results show that the overall heat transfer coefficient and Nusselt number increase when the flow rate of coolant and curvature ratio increase. The effect of coil pitch was investigated and the results show that when of the coil pitch (angle of inclination) increases Nusselt number increase. A correlation was presented to calculate the outside average Nusselt number of coil.

Experimental study of free convection in a window with a heated between-panes blind

An experimental study has been conducted to examine free convection in a window with an enclosed aluminum venetian-type blind. The unique feature of this experiment was that the blind slats were heated electrically to simulate absorbed solar radiation. Convective heat transfer measurements and temperature field visualization were obtained using a Mach–Zehnder laser interferometer. Optical measurements were made for two glazing spacings, two blind slat angles, two blind heat flux levels, and two glazing temperature differences. Both local and average convective heat flux data were obtained in the center region of the tall air-filled enclosure. At the widest glazing spacing, the temperature field was found to be unsteady. For these cases, the temporal fluctuation of the local convective heat transfer was time-averaged using a high speed camera. The experimental results have been compared to a simplified method in the literature for predicting the center-glass heat flux for this configuration.

Experimental study of free convection in an elliptical annular enclosure in blunt and slender orientations

In this paper, free convection heat transfer between two elliptical cylinders having different elliptical ratios in blunt and slender situations was studied experimentally. Three pairs of elliptical cylinders having the same radius ratio of 2.0, the same surface area and different elliptical ratios of 0.662, 0.866 and 0.968 were cut using the computed numerically controlled wire-cut machining. The tests were carried out by keeping a constant heat flux on the inner cylinder while cooling the outer one to be isothermal. The effects of vertical eccentricity, lateral eccentricity, angle of attack of the inner cylinder on natural convection for both blunt and slender situations of each pair were investigated. Empirical correlation was deduced within an acceptable uncertainty for the experimental results. Compatible and satisfactory to the conscience agreement was found in the comparison among the results of present and previous works. In the vision of the comparison, it was found that; the vertical eccentricity can enhance free convection by about 15% than the concentric case, the horizontal eccentricity can enhance natural convection by about 10% than concentric case and slender situation offers about 40% enhancement in free convection than the blunt situation for the same elliptical ratio.

Experimental study of free convection from a pair of vertical arrays of horizontal cylinders at very low Rayleigh numbers

Steady laminar free convection from a pair of vertical arrays of equally-spaced uniformly heated horizontal cylinders set in free air is studied experimentally. A specifically developed experimental facility is used to perform heat flux and temperature measurements and thermal field visualization by means of Schlieren technique. Experiments are performed for pairs of tube-arrays consisting of five circular cylinders, for center-to-center horizontal and vertical spacing ranging from 2 to 145 cylinder-diameters and from 4 to 12 cylinder-diameters respectively, and for values of the Rayleigh number based on the cylinder-diameter in the range between 2.4 and 11.9. It is found that any cylinder may exhibit either enhanced or reduced Nusselt numbers with respect to the case of single array, depending on its location in the array, on the geometry of the array, as well as on the Rayleigh number. The visualization has allowed to relate a number of observed effects to the fact that side-by-side placement of the arrays produces a lateral air suction into the space between them.

Experimental Study of Free Convection from a Vertical Flat Plate in Porous Media

A set of experiments is done to study the phenomenon of free convection heat transfer from an isothermal vertical flat plate embedded in a saturated porous medium in steady state condition. The porous medium consisting of 0.8 cm spheres. The aspect ratio of the isothermal flat plate, H/W, is equal to 2. Where H is the height and W is the width of the vertical plate. The investigations were cared out for Darcy modified Rayleigh number between 100 and 500. The results indicate that heat transfer increases linearly with increasing the Darcy modified Rayleigh number. In addition, the present results are in good agreement with the higher-order boundary layer theory obtained by Cheng and Hsu [1].

An Experimental Study of Free Convection in an Inclined Rectangular Channel Using Radiant Barrier

This article reports an experimental study of free convection in an inclined rectangular channel open ended at various aspect ratios. The upper plate is heated by a uniform constant heat flux while the lower plate is attached to a radiant barrier. The channel width and length are 0.7 and 1.5 m, respectively. Tests were made for different aspect ratio (air gap to channel length, S/L) by varying air gap space and different constant heat fluxes. The tilt angle was fixed at 15°. The objective is to determine the heat transfer coefficient and the induced airflow rate. The correlation equations of Nusselt and Reynolds numbers as function of Rayleigh number and aspect ratios are also developed to evaluate the heat transfer coefficient through the channel. The correlationships are valid in the range 1 × 105 < Ra sin 15 < 1 × 108. Heat transfer enhancement due to the radiant barrier was assessed by comparison to a channel equipped with gypsum board.

Natural convection in a porous layer bounded by impervious domains: from numerical approaches to experimental realization

This work presents the results of a numerical and experimental study of free convection in a porous layer bounded by impervious domains subjected to a vertical temperature gradient. This configuration has been chosen as a reference for some geological structures. The main parameters of the experimental configuration are a slope of 15°, an aspect ratio of 10 for the porous layer and a thermal conductivity ratio (λ p⧹ λ np) of 0.7. For that case, a good coherence is found concerning the flow patterns observed through the different methods. In all results, an anti-clockwise unicellular flow (with a layer rising from left to right) is observed at low filtration Rayleigh numbers Ra∗. At higher Ra∗, the three-dimensional (3D) flow which develops is composed of seven transversal (horizontal ) rolls and five to seven longitudinal rolls. However, depending on the resolution method, the critical filtration Rayleigh numbers for the transition from 2D to 3D flows are significantly different. Moreover, the experimental temperature fields have some special features: a high amplitude of the unicellular flow and, for 3D flows, a low amplitude for the rolls in the central part of the layer and an even number of longitudinal rolls. Numerical resolutions taking into account the thermal heterogeneities of the experiment prove the significant influence of lateral thermal conditions on flow patterns and on the critical filtration Rayleigh number.

Experimental Study of Free Convection at an Indoor Glazing Surface with a Venetian Blind

A Mach-Zehnder interferometer was used to measure the laminar free convective heat transfer rate from an isothermal vertical surface adjacent to a set of aluminum Venetian blinds. Local and average heat transfer data were obtained at three different blind-to-plate spacings, and at four different blade angles. Data are presented up to a Rayleigh number of Ra =3 × 107, based on the distance from the leading edge. Flow visualization results that illustrate the cellular flow patterns between the blades are also presented. Overall, it was found that the flow field and local heat transfer distribution on the surface can be significantly affected by the presence a Venetian blind. When placed close to the surface, the blind causes a strong periodic variation in the local Nusselt number distribution. However, for all cases studied the average Nusselt number was within 13% of an isolated vertical flat plate at the same Rayleigh number.

Experimental study of free convection and radiation in horizontal fin arrays

An experimental investigation of interaction of free convection and radiation in a horizontal fin array is carried out. A differential interferometer is used to obtain free convection heat transfer and radiation is calculated by solving the integro-differential equations numerically. Results are presented to show the effects of various parameters such as emissivity of the fin surfaces, fin spacing, fin height and base temperature. Correlations are suggested in terms of non-dimensional parameters, based on a large number of experiments. The main conclusion to be drawn from the present study is that radiation—convection interaction invalidates additive approaches in which convection and radiation contributions are independently calculated assuming all surfaces to be isothermal and then adding these to obtain the total heat loss from the fin array.

Large-scale convection induced by the Soret effect.

We present an experimental study of free convection in binary mixtures when convection is theoretically predicted to appear in the form of rolls of large extent. Quantitative measurements of the very slow velocities associated with this convective pattern are presented.

An Experimental Study of Free Convection in Compound Parabolic Concentrator (CPC) Cavities

An experimental study of the free convection heat transfer between the cylindrical absorber and the flat top of a compound parabolic concentrator (CPC) is described. Results are obtained for a range of absorber temperatures and four CPC cavity heights. For similar conditions of operating temperature, the heat transfer from a cylinder in free air is about 30 to 50 percent higher than in a CPC cavity. Two correlations for Nusselt and Grashof numbers have been obtained using the equivalent length and the cavity height as the characteristic length of the system.

Theoretical and experimental study of free convection from a vertical frustum of a cone of a finite length

A theoretical and experimental study has been conducted to investigate free convection from a vertical frustum of a cone of a finite length. Fully elliptic calculation scheme has been proposed to solve the full Navier-Stokes equations. Calculation results are compared with local heat transfer data obtained using a Mach-Zehnder interferometer. A flow visualization has been also attempted using lead stearate powder as a tracer. Both the prediction and measurements suggest that the local heat transfer rate over the lateral surface of the frustum is influenced significantly by the presence of the upper and lower end surfaces of the frustum. It is especially interesting to note that the recirculation region appearing above the upper end surface tends to suppress the diffusion rate of the thermal boundary layer, and consequently leads to a significant augmentation of the local heat transfer rate from the lateral surface.

Convection in rectangular cavities with differentially heated end walls

This paper describes an experimental study of free convection in an enclosed rectangular cavity, the end walls of which are maintained at uniform but different temperatures. The experiments are carried out for a variety of Rayleigh numbers, R = αgΔTh4/κνl, and aspect ratios, L = l/h, for fluids with Prandtl number σ ≥ 10. For R ∼ O(103) it is shown that the basic structure of the flow field is a single two-dimensional cell for 0·25 ≤ L ≤ 9. When R &gt; O(104) the boundary layers on the vertical walls control the flow field, but the basic overall structure remains unicellular. At greater values of R secondary vortices appear for all L ≥ 0·5. As R increases the intensity and then the number of these vortices increases. Measurements of the end-wall boundary-layer profiles at different values of R and L confirm Gill's boundary-layer analysis. The effects of variations of viscosity with temperature are discussed in the context of the observed boundary-layer profiles.Core shear profiles and mass flux measurements are also reported. For L = 1 the observed shear profiles are in good agreement with numerical solutions of the Boussinesq equations. However, when L &gt; 1 the observations suggest that the horizontal boundary layers have a significant effect on the core flow field. The stream function is demonstrated to be L-dependent in the boundary-layer regime, where variations due to R are second order. Similarities between the results of the present work and earlier observations by Elder and by Seki, Fukusaka &amp; Inaba for tall slender cavities (L [Lt ] 1) are discussed.

Experimental study of free convection with porous blowing and suction on a vertical surface

The paper presents the results of an experimental study of the heat transfer and the temperature field in a free-convection boundary layer on a vertical surface with uniform porous blowing and suction and Tw=const. The method of measurement is described and the experimental data are compared with the theoretical results obtained by the authors in an earlier work. The experiments were carried out on a Zender-Mach interferometer. The theoretical and experimental results are found to be in good agreement. Critical values of the parameters are found, which determine the beginning of the transient and developed turbulent regions as a function of the rate of blowing or suction.