Najib Laraqi

SIMULTANEOUS ESTIMATION OF FRICTIONAL HEAT FLUX AND TWO THERMAL CONTACT PARAMETERS FOR SLIDING CONTACTS

In the dry friction between two solids in imperfect contact, the determination of temperature distribution within solids needs the knowledge of: (1) heat flux generated by friction, (2) thermal contact conductance, and (3) intrinsic heat partition coefficient. In this article, numerical study is conducted in order to estimate the values of these parameters from simulated temperatures. Numerical investigations and a sensitivity analysis are performed in order to show the feasibility and the needed conditions for an accurate estimation. In order to take into account the measurements noise, statistical investigations using a stochastic approach and Monte Carlo method are also performed.

Diagrams for fast transient conduction in sphere and long cylinder subject to sudden and violent thermal effects on its surface

Analytical solutions for fast transient conduction in spherical and cylindrical geometries subject to sudden and violent thermal effects on its surface are presented in this paper. The numerical solutions are presented in the form of simple diagrams that can be easily and readily used in some engineering applications such as aeronautics, electronics, fire dynamics, tribology, metallurgy or food and agricultural technologies. These data are useful for the optimization of numerical codes in fluid mechanics in association with heat transfer and inverse methods for the determination of thermal characteristics of the surface phenomena in various cases. These diagrams are for specific ranges of Fo and Bi numbers corresponding to the fast transient problems characterized by weak Fourier numbers, associated with a large combination of dimensions of the body and values of thermal surface conductance (large range of Bi). These diagrams constitute a special implementation (violent and sudden thermal effects) of the well known Heisler’s charts and are very useful for understanding and teaching transient heat transfer. A numerical solution is proposed. It overcomes the problems due to a too slow convergence. The main difficulty is encountered when solving characteristic equations based on a combination of the parameters involved in the particular equations of temperature and energy. That may take asymptotic values for the specific phenomenon addressed in this study. The results are successfully compared to those based on a different calculation procedure.

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).

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...

Fast transient conduction in infinite plate subject to violent thermal effects

Abstract This work is an implement to a study that was conducted for long cylinders and spheres undergoing fast processes characterized by low Fourier numbers (Fo). It allows to solve some complex 2D and 3D problems involving the specific thermal boundary conditions examined in this survey. Analytical solutions for fast transient conduction in infinite plate subject to sudden and violent thermal effects on its surface are presented in this paper. The numerical solutions are presented in the form of simple diagrams that can be easily and readily used in some engineering applications. These data are useful for the optimization of numerical codes in fluid mechanics in association with heat transfer and inverse methods for the determination of thermal characteristics of the surface phenomena in various cases. These diagrams are for specific ranges of Fo and Bi numbers corresponding to the fast transient problems characterized by low Fourier numbers, associated with a large combination of dimensions of the body and values of thermal surface conductance (large range of Bi). These diagrams constitute a substantial improvement (especially for violent and sudden thermal effects) of the well known Heisler’s charts. A numerical solution is proposed. It overcomes the problems due to a too slow convergence. The main difficulty is encountered when solving characteristic equations based on a combination of the parameters involved in the particular equations of temperature and energy which may take asymptotic values for the specific phenomena addressed in this study. The results are successfully compared to those based on a different calculation procedure. It is for example the case for thermo-mechanics problem due to the braking using the tablet-disk system.

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.

Electronic board modeling by the means of DELPHI compact thermal model of components

Board-level simulation considers the challenging process of analysing the impact of the vicinity of high and medium powered devices on the sensitive ones. This level of design is often considered in the industry sector as an unnecessary luxury. This damaging approach is becoming untenable with the rising use of miniaturized high-powered devices and High density Interconnection electronic board, which intensifies the coupling effect of neighbouring components. So, perform board-level thermal simulation at the earliest stage of the design process makes sense today, more than ever. To minimize the computation times, from days to minutes, the concept of compact thermal model was defined in 1996, by the European consortium DELPHI1 Unfortunately, DELPHI project ended with a methodology restricted to steady-state compact thermal model for mono-chip electronic component. Emerging problematic such as multi-chips module or transient thermal model were not addressed, which remains today for worldwide companies a non-trivial challenge. Since 2009, Thales is implementing these missing methods. The present paper summarizes the comparison of a “state-of-the-art” numerical detailed model and its deducted compact model with the help of infrared experimental results in order to promote a modelling guideline. (1)DEvelopment of Library of Physical model for an Integrated design environment.

Dynamic Compact Thermal Model for stacked-die components

The present work proposes an approach to generate Dynamic Compact Thermal Models or “DCTMs” dedicated to electronic components. This one is based on the European project DELPHI, which defined the first comprehensive methodology concerning the generation of thermal behavioral model, Boundary Condition Independent, called Compact Thermal Models or “CTMs”. Unfortunately, the scope of “CTMs” was limited to the steady state as well as for single chip packages. But, the latest trend toward higher and higher density packaging using several chips requires henceforth a methodology capable to take into account the transient regime for 3D integration technologies like stacked-die solution. Following the CTMs modus operandi the DCTMs were conceived to propose a RC network able to predict a set of sensitive component temperatures with a minimized difference during component duty cycle. This work suggests the use of the genetic algorithms fitting technique that turns out relevant for the realization of DCTM, as well as the conventional DELPHI CTM.

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.