Turbulent Natural Convection From a Vertical Cylinder to an Array of Cooled Tubes

1993 ◽  
Vol 115 (4) ◽  
pp. 928-937 ◽  
Author(s):  
D. M. McEligot ◽  
C. M. Stoots ◽  
W. A. Christenson ◽  
D. C. Mecham ◽  
W. G. Lussie
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Test Section ◽  
Vertical Cylinder ◽  
Turbulent Natural Convection ◽  
Cooling Tube ◽  
Natural Convection Problem ◽  
Perforated Tube

In order to determine whether available correlations are adequate to treat a complicated, turbulent natural convection problem encountered in industrial practice, experiments were conducted by resistively heating a slender, vertical cylinder centered inside a concentric perforated tube, which was, in turn, surrounded by an array of three larger-diameter cooled tubes. The ratio of the test section temperature to the cooling tube temperature was varied up to 2.6; and the Rayleigh number, based on tube diameter and properties evaluated at the cooling tube temperature, ranged from 2.9×104 to 9.2×105. Results indicate that the convective heat transfer parameters for the perforated tube are about 15 percent higher than for the smooth bare tube centered in the same position relative to the array. The Nusselt number for convective heat transfer across the annulus between the heated test section and the perforated tube corresponded approximately to parallel laminar flow (i.e., Nus≈1).

The Effect of Thermal Wall Properties on Natural Convection in Inclined Rectangular Cells

1982 ◽  
Vol 104 (1) ◽  
pp. 111-117 ◽  
Author(s):  
B. A. Meyer ◽  
J. W. Mitchell ◽  
M. M. El-Wakil
Keyword(s):  
Heat Transfer ◽  
Cell Wall ◽  
Natural Convection ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Conductive Heat ◽  
Numerical Work ◽  
Aspect Ratios ◽  
Convection Problem ◽  
Natural Convection Problem

The effects of cell wall thickness and thermal conductivity on natural convective heat transfer within inclined rectangular cells was studied. The cell walls are thin, and the hot and cold surfaces are isothermal. The two-dimensional natural convection problem was solved using finite difference techniques. The parameters studied were cell aspect ratios (A) of 0.5 and 1, Rayleigh numbers (Ra) up to 105, a Prandtl number (Pr) of 0.72 and a tilt angle (φ) of 60 deg. These parameters are of interest in solar collectors. The numerical results are substantiated by experimental results. It was found that convection coefficients for cells with adiabatic walls are substantially higher than those for cells with conducting walls. Correlations are given for estimating the convective heat transfer across the cell and the conductive heat transfer across the cell wall. These correlations are compared with available experimental and numerical work of other authors.

Investigations on Transient Natural Convection in Boron Nitride-Mineral Oil Nanofluid Systems

2012 ◽  
Author(s):  
Shijo Thomas ◽  
C. B. Sobhan ◽  
Jaime Taha-Tijerina ◽  
T. N. Narayanan ◽  
P. M. Ajayan
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Transfer Coefficient ◽  
Natural Convection ◽  
Boron Nitride ◽  
Transfer Coefficient ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Convective Heat Transfer Coefficient ◽  
Mineral Oil

Nanofluids are suspensions or colloids produced by dispersing nanoparticles in base fluids like water, oil or organic fluids, so as to improve their thermo-physical properties. Investigations reported in recent times have shown that the addition of nanoparticles significantly influence the thermophysical properties, such as the thermal conductivity, viscosity, specific heat and density of base fluids. The convective heat transfer coefficient also has shown anomalous variations, compared to those encountered in the base fluids. By careful selection of the parameters such as the concentration and the particle size, it has been possible to produce nanofluids with various properties engineered depending on the requirement. A mineral oil–boron nitride nanofluid system, where an increased thermal conductivity and a reduced electrical conductivity has been observed, is investigated in the present work to evaluate its heat transfer performance under natural convection. The modified mineral oil is produced by chemically dispersing boron nitride nanoparticles utilizing a one step method to obtain a stable suspension. The mineral oil based nanofluid is investigated under transient free convection heat transfer, by observing the temperature-time response of a lumped parameter system. The experimental study is used to estimate the time-dependent convective heat transfer coefficient. Comparisons are made with the base fluid, so that the enhancement in the heat transfer coefficient under natural convection situation can be estimated.

Transient free convection within air-filled hemispherical enclosures. Nu-Ra-Fo relationships for isothermal and inclined disk with dome oriented upwards

2015 ◽  
Vol 25 (3) ◽  
pp. 629-638 ◽  
Author(s):  
Abderrahmane Baïri
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Convection Heat Transfer ◽  
Numerical Approach ◽  
Content Type ◽  
Turbulent Natural Convection ◽  
Higher Temperature ◽  
Fourier Type ◽  
Volume Method

Purpose – Nusselt-Rayleigh-Fourier type correlations are proposed to quantify the temporal evolution of convective heat transfer occurring within air-filled hemispherical enclosures whose disk, initially at ambient temperature, is suddenly maintained at a higher temperature. The temperature difference imposed between this hot wall and the isothermal cold dome involves Rayleigh number varying between 104 and 2.55×1012. Depending on the application, the disk can be inclined with respect to the horizontal plane by an angle varying between 0° (horizontal disk) and 90° (vertical disk) in steps of 15°. The paper aims to discuss these issues. Design/methodology/approach – The results are obtained by means of a numerical approach based on the finite volume method. The proposed correlations linked to the steady state Nusselt-Rayleigh internships recently published, concerning the same inclination angle and Rayleigh ranges. Findings – The statistical analysis of a large number of calculations leads to reliable results covering laminar, transitional and turbulent natural convection heat transfer zones. Practical implications – The proposed relationships can be applied in several engineering fields such as nuclear technology, solar energy, security and safety electronics, building, domotics or aeronautics. Originality/value – The new relationships proposed in this paper provide important information on the evolution of convective heat transfer during the transient regime.

Convective Heat Transfer Studies in Mini-Channels Using Digital Interferometry

2009 ◽  
Author(s):  
V. Sajith ◽  
Divya Haridas ◽  
C. B. Sobhan ◽  
G. R. C. Reddy
Keyword(s):  
Heat Transfer ◽  
Image Processing ◽  
Temperature Distribution ◽  
Convective Heat Transfer ◽  
Digital Image Processing ◽  
Convective Heat ◽  
Digital Image ◽  
Test Section ◽  
Average Heat ◽  
Mini Channels

Convective heat transfer in micro and mini channels has been recommended as an effective heat removal method for various electronic packages and systems. Experimental and theoretical investigations on the thermal performance of micro and mini channels have gained immense attention and hence, heat transfer studies in mini channels are of great importance. Some of the experimental results found in the literature on heat transfer in small-dimension channels are of contradicting nature even though some generally agreeing results are also found. One of the probable reasons for such deviations is the intrusive nature of the measurement techniques used. The traditional method of temperature measurement in channels uses the thermocouple probe, and for obtaining temperature distribution across the channel either a number of probes or a moving probe technique is required, both of which disturb the flow field and cause measurement errors. Hence a non intrusive measurement technique, such as an optical method is preferable for temperature measurement in small channels. In the present work, convective heat transfer studies have been performed on water flowing through a mini channel of hydraulic diameter 4 mm, using the non-intrusive technique of laser interferometry, coupled with digital image processing. The channel is fabricated using high quality optical glass and aluminum blocks. Mach Zehnder Interferometry is used for obtaining the temperature distribution in the channel. The experimental arrangement consists of two identical channels, one placed in the test section and the other in the reference section of the interferometric set up. As the test section is heated, a density variation is produced in the medium, which causes a refractive index variation, deforming interference fringes. This enables the calculation of the temperature distribution inside the channel. The interferograms are grabbed using a CCD camera and an AVT Fire package software. Digital image processing technique, using MATLAB software is used for locating the fringe-centers, and calculating the temperature distribution. The temperature profiles are obtained at different sections of the channel for various values of the average Reynolds number and various heating levels. The local and average heat flux values are obtained from the constructed temperature distributions. Variations of the local and average heat transfer coefficients and Nusselt number are determined and discussed. Results of parametric studies are compared and contrasted with relevant entry length solutions from the literature.

Entropy Generation in Laminar and Turbulent Natural Convection Heat Transfer From Vertical Cylinder With Annular Fins

2017 ◽  
Vol 139 (4) ◽  
Author(s):  
Jnana Ranjan Senapati ◽  
Sukanta Kumar Dash ◽  
Subhransu Roy
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Entropy Generation ◽  
Vertical Cylinder ◽  
Turbulent Regime ◽  
Maximum Heat ◽  
Turbulent Natural Convection ◽  
Maximum Heat Transfer ◽  
Wide Range ◽  
Annular Fins

Entropy generation due to natural convection has been calculated for a wide range of Rayleigh number (Ra) in both laminar (104 ≤ Ra ≤ 108) and turbulent (1010 ≤ Ra ≤ 1012) flow regimes, for diameter ratio of 2 ≤ D/d ≤ 5, for an isothermal vertical cylinder fitted with annular fins. In the laminar regime, the entropy generation was predominantly caused by heat transfer (conduction and convection) and the viscous contribution was negligible with respect to heat transfer. But in the turbulent regime, entropy generation due to fluid friction is significant enough although heat transfer entropy generation is still dominant. The results demonstrate that the degree of irreversibility is higher in case of finned configuration when compared with unfinned one. With the deployment of a merit function combining the first and second laws of thermodynamics, we have tried to delineate the thermodynamic performance of finned cylinder with natural convection. So, we have defined the ratio (I/Q)finned/(I/Q)unfinned. The ratio (I/Q)finned/(I/Q)unfinned gets its minimum value at optimum fin spacing where maximum heat transfer occurs in turbulent flow, whereas in laminar flow the ratio (I/Q)finned/(I/Q)unfinned decreases continuously with the increase in number of fins.

Radiative and convective heat transfer coefficients of the human body in natural convection

2008 ◽  
Vol 43 (12) ◽  
pp. 2142-2153 ◽  
Author(s):  
Yoshihito Kurazumi ◽  
Tadahiro Tsuchikawa ◽  
Jin Ishii ◽  
Kenta Fukagawa ◽  
Yoshiaki Yamato ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Human Body ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Convective Heat Transfer Coefficients

Mixed Convection Heat Transfer From Thermal Sources Mounted on Horizontal and Vertical Surfaces

1990 ◽  
Vol 112 (4) ◽  
pp. 975-987 ◽  
Author(s):  
S. S. Tewari ◽  
Y. Jaluria
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Heat Source ◽  
Mixed Convection ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Finite Width ◽  
Heat Sources ◽  
Wide Range ◽  
Thermal Sources

An experimental study is carried out on the fundamental aspects of the conjugate, mixed convective heat transfer from two finite width heat sources, which are of negligible thickness, have a uniform heat flux input at the surface, and are located on a flat plate in the horizontal or the vertical orientation. The heat sources are wide in the transverse direction and, therefore, a two-dimensional flow circumstance is simulated. The mixed convection parameter is varied over a fairly wide range to include the buoyancy-dominated and the mixed convection regimes. The circumstances of pure natural convection are also investigated. The convective mechanisms have been studied in detail by measuring the surface temperatures and determining the heat transfer coefficients for the two heated strips, which represent isolated thermal sources. Experimental results indicate that a stronger upstream heat source causes an increase in the surface temperature of a relatively weaker heat source, located downstream, by reducing its convective heat transfer coefficient. The influence of the upstream source is found to be strongly dependent on the surface orientation, especially in the pure natural convection and the buoyancy dominated regimes. The two heat sources are found to be essentially independent of each other, in terms of thermal effects, at a separation distance of more than about three strip widths for both the orientations. The results obtained are relevant to many engineering applications, such as the cooling of electronic systems, positioning of heating elements in furnaces, and safety considerations in enclosure fires.

scholarly journals Ultrasounds Used as Promoters of Heat-Transfer Enhancement of Natural Convection in Dielectric Fluids for the Thermal Control of Electronic Equipment

Acoustics ◽  
2020 ◽  
Vol 2 (2) ◽  
pp. 279-292
Author(s):  
Carlo Bartoli ◽  
Alessandro Franco ◽  
Massimo Macucci
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Natural Convection ◽  
Transfer Coefficient ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Convective Heat Transfer Coefficient ◽  
Enhancement Effect ◽  
Electronic Board ◽  
Experimental Apparatus

We present an experimental investigation of the effect of ultrasound application to increase the heat-transfer coefficient for natural convection of a dielectric fluid. An experimental analysis is carried out to estimate the increase of the convective heat-transfer coefficient between an electronic board and a refrigerant fluid, the Fluorinert Electronic Fluid FC-72. For this purpose, an experimental apparatus composed of an electronic board, its electronic control circuit, and data acquisition systems have been designed and implemented. The data collected appear to confirm in some situations of practical interest the enhancement effect of the convective heat-transfer coefficient in connection with the use of ultrasound. The most favorable condition was observed with the fluid in quite low subcooled conditions.

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