A. Baïri

Importance of radiative heat exchange in 2D closed diode cavities applied to solar collectors and building

The aim of this work is to determine the radiative thermal exchange in 2D closed diode cavities in which a natural convective flow is established. The distinction between heat transfer by natural convection and radiation is necessary for these cavities, considering their particular geometry. The hot and cold walls of the cavity have the same size (height H) and are at the origin of the natural 2D convective flow. Being at a distance L, varying according to the treated case (shape factor A = L/H) and remaining always vertical, they are maintained isotherm with an imposed difference of temperature ΔT. The four other walls of the cavity have a very high thermal conductive resistance (i.e., adiabatic). The upper and lower walls are inclined in relation to the horizontal with an angle slope α that can be positive or negative. Thus, the lateral walls of the cavity have a parallelogram shape in most of the cases. The configurations of the cavities corresponding to various combinations of ΔT, α and A = L/H parameters give very different natural flows being able either to favor the convection or, on the contrary, to decrease it (convective diode effect). According to the treated case, it entails sensi-tive differences of natural convection and radiation contributions. In this survey, we put the emphasis on the radiative aspect. We present detailed calculations based on radiosity method coupled with thermal measurements obtained experimentally. The results of this work put in evidence the importance of radiation in the global exchange. These cavities can be used in several engineering domains. However, this work consists in adapting their use in solar energy techniques like building and flate-plate solar collectors.

THREE-DIMENSIONAL CALCULATION OF TEMPERATURE IN A ROTATING DISK SUBJECTED TO AN ECCENTRIC CIRCULAR HEAT SOURCE AND SURFACE COOLING

This article derives, for the first time, an appropriate analytical solution for the calculation of three-dimensional temperature distribution inside a rotating disk subjected to an eccentric circular heat source and surface cooling. This problem is encountered in, among others, the pin-on-disk frictional device, thrust ball bearing, or machining in a lathe. In order to obtain this solution under an explicit and simple form, we assume that the cooling occurs over the whole frictional surface of the disk, including the heated region. A numerical study, using the finite-volume method, is also conducted in order to define the limits of validity of the above assumption. It is shown that the analytical solution is accurate for a wide range of heat convection coefficients ( h ≤ 1,000 W/m2K for high Peclet numbers and h ≤ 5,000 W/m2K for low ones).

Detailed correlations on natural convective heat transfer coefficients for a QFN32 electronic device on inclined PCB

ABSTRACT Correlations allowing the calculations of the convective heat transfer coefficient on all elements of an electronic assembly are proposed in this work. The active element is a Quad Flat Non-Lead-type QFN32 package that may be welded in any position of a printed circuit board (PCB) inclined with respect to the horizontal at an angle ranging from 0° (horizontal position) to 90° (vertical position) by steps of 15°. The power generated by the QFN32 varies between 0.1 W and 0.8 W, corresponding to its normal operating range. Six distinct surfaces are considered in this work and the power exchanged between each of them and the environment is quantified. This survey details a previous one in which it is quantified the global convective heat transfer concerning the whole assembly. The correlations proposed in the present work provide a better modeling of this conventional device widely used in electronic applications. They thus optimize its design while controlling its temperature during operation.

Nu-Ra correlations for natural convection at high Ra numbers in air-filled tilted hemispherical cavities with dome oriented upwards. Disk submitted to constant heat flux

Purpose – The purpose of this paper is to propose correlations between Nusselt and Rayleigh numbers for the case of inclined and closed air-filled hemispherical cavities. The disk of such cavities is subjected to a constant heat flux. The study covers a wide range of Rayleigh numbers from 5×107 to 2.55×1012. Design/methodology/approach – Correlations are obtained from numerical approach validated by experimental measurements on some configurations, valid for several angles of inclination of the cavity between 0° (horizontal disk) and 90° (vertical disk) in steps of 15°. 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 correlations provide solutions for applications in several fields of engineering such as solar energy, aerospace, building, safety and security. Originality/value – The new relations proposed are the first published for high...

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

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 resu...

Nu–Ra–Pr correlations for nanofluidic natural convection in tilted hemispherical enclosures with an active disk

ABSTRACT This work qualifies and quantifies the natural convective phenomena occurring in a hemispherical enclosure filled with ZnO-Water monophasic nanofluid with a volume fraction varying between 0% (pure water) and 10%. The dome of the cavity is kept isothermal while its circular base generates a power varying between 0.5W and 400W. The disc can be inclined with respect to the horizontal plane by an angle varying between 0 and 180° (horizontal disk with a dome facing upwards and downwards respectively). The 3D numerical approach is carried out with the finite volume method based on the SIMPLE algorithm. The temperature and velocity distributions are presented and the convective heat transfer is examined for all processed configurations. The nanofluidic convective heat transfer is quantified by means of correlations of the Nusselt-Rayleigh-Prandtl-tilt angle type. They allow to optimize the thermal design of these cavities used in the field of electronics.

Thermal impedance of multi-finger microelectronic structures: exact analytical model

An exact analytical expression for the complex thermal impedance Z of multi-finger microelectronic components is presented in this paper. The integral transform technique has been used to obtain this expression and solve the three dimensional heat conduction equation directly in the frequency domain. Calculations were first performed for a single-finger on a single-layer structure in order to compare the results with those available in the literature and hence validate the solution. Generally, the comparison shows good agreement between our results and those given in most publications. When the structures are composed of several layers, the thermal impedance changes with the thermal conductivities and the thicknesses of the different layers. It is also affected by the thermal contact resistance between the layers. Some results illustrate the influence of these parameters. The case of a multi-finger component is then treated and the influence of distances between fingers is investigated. For all cases, the Nyquist diagram (i.e. Im(Z) versus Re(Z) for different pulsation values ω) is plotted. Mainly two zones are observed: one for the high frequencies and the other for the lower ones. The substrate dimensions are found to largely influence the scale of the low frequency zone whereas the distance between the fingers influences the higher one. Finally, the solution is applied to a multi-finger device in contact with a heat sink.

On the junction temperature of the QFN64 electronic package subjected to free convection: Effects of the encapsulating molding compound

ABSTRACT The thermal conductivity of the molding compound (resin) used to encapsulate the QFN64 package significantly affects the thermal behavior of this electronic package during operation when it is subjected to natural convection. These effects are quantified in this work by varying the conductivity between −80% and +100% of its average value. The 3D numerical solution carried out by means of the control volume method clearly shows that the maximal temperature reached in the junction of the device is affected by this parameter for a wide range of the generated power and various inclinations relative to the horizontal plane. The correlation proposed in this work allows optimizing the thermal design and increases the reliability, durability, performance, and correct operating of this electronic device widely used in various engineering fields.

Free convective overall heat transfer coefficient on inclined electronic assembly with active QFN16 package

Purpose – The purpose of this paper is to determine the overall free convective heat transfer coefficient for an assembly constituted by a Quad Flat Non-lead QFN16 welded on a Printed Circuit Board (PCB) which may be inclined with respect to the horizontal plane by an angle varying between 0° and 90° corresponding to the horizontal and vertical position, respectively. This electronic device widely used in electronics generates during its effective operation a power ranging from 0.1 to 0.8 W. The assembly is installed in an air-filled cavity. Design/methodology/approach – Calculations are done by means of the finite volume method for many configurations obtained by varying the generated power, the inclination angle and the position of the QFN16 on the PCB. The dynamic and thermal aspects are presented and commented. Findings – The study shows that the thermal state of the electronic device is influenced by the previous three physical parameters. A correlation between the global convective exchange coeffici...

Transient free convection in parallelogrammic cavities applied to the thermoregulation of avionics

This work addresses momentum and heat transfer phenomena that take place in closed cavities filled with air. The cavities are formed by two vertical active walls, one hot consisting of bands that are alternately insulated and heated at temperature Th and other cold, isothermal at Tc. The closing upper and lower passive walls can be tilted an angle αeither above or below the horizontal plane. When α is positive (hot wall below the level of the cold one), the air movement is favoured while the opposite occurs for negative angles. These cavities are thus either transferring or insulating, which confers them the quality of a so-called convective diode. Such geometry is applied in this work to the thermoregulation of the on-board aviation electronics. The flows corresponding to different angles and temperature differences ΔT = Th – Tc are examined both numerically and experimentally. We consider in particular the transient transport phenomena which occur at the very beginning, a critical stage in the operation of the on-board materials.