Mohamed Chaker Zaghdoudi

Thermal control of electronic equipment by heat pipes

Abstract In the frame of the BRITE-EURAM european programme (KHIEPCOOL project), a literature survey on the main heat pipe and micro heat pipe technologies developed for thermal control of electronic equipment has been carried out. The conventional heat pipes are cylindrical, flat or bellow tubes, using wicks or axial grooves as capillary structures. In the field of micro heat pipes, three and four corner tubes have been developed. The pipes are mounted on single chips, in-line chip arrays or integrated into the component interconnection substrate. The best performances were achieved with Pleschs axially grooved flat miniature heat pipe [1], which is able to transfer a heat flux of about 60 W·cm −2 . Theoretical models have shown that the performance of micro heat pipe arrays increase with increasing tube diameter, decreasing tube length and increasing heat pipe density. The heat pipe technologies are classified and compared according to their geometry and location in the system. A list of about 150 references, classified according to their subjects, is presented.

Electric Field Effects on Pool Boiling

This paper deals with an experimental study of the influence of a DC uniform electric field on the nucleate boiling heat transfer. EHD effects are quantitatively investigated by performing experiments on various liquids with different properties. In these experiments, n -pentane, R-113, and R-123 are used as working fluids, and the boiling phenomenon takes place on a horizontally oriented copper surface. The experimental results show (1) an enhancement of the nucleate pool boiling heat transfer, (2) an increase of the critical heat flux (CHF), and (3) disappearance of the hysteresis phenomenon. The basic enhancement mechanisms are analyzed and quantified. For nucleate pool boiling and CHF regimes, heat transfer laws based on dimensionless numbers are proposed and determined from the experimental data. The results obtained with the proposed EHD models are in good agreement with experimental results.

Theoretical and experimental study on the thermal performance of flat miniature heat pipes including rectangular axial grooves

Experimental and theoretical studies are realized in order to verify the Mini Heat Pipe (MHP) concept for cooling high power dissipation electronic components, and determine the potential advantages of constructing mini channels as an integrated part of a flat heat pipe. In the experimental part of this study, a Flat Mini Heat Pipe (FMHP) prototype including a capillary structure composed of parallel rectangular microchannels is manufactured and a filling apparatus is developed in order to charge such FMHPs. The heat transfer improvement obtained by comparing the heat pipe thermal resistance to the heat conduction thermal resistance of a copper plate having the same dimensions as the tested FMHP is demonstrated for different heat input flux rates, heat sink temperatures, and orientations. In the theoretical part of this work, a detailed mathematical model of a FMHP with axial microchannels is developed in which the fluid flow is considered along with the heat and mass transfer processes during evaporation and condensation. The model is based on the equations for the mass, momentum and energy conservation, which are written for the evaporator, adiabatic, and condenser zones. The model, which permits to simulate several shapes of microchannels, can predict the maximum heat transfer capacity of FMHP, the optimal fluid mass, and the flow parameters along the microchannel. The comparison between experimental and model results shows the good ability of the numerical model to predict the axial temperature distribution along the FMHP.

Thermal simulation approach to the cooling of a power IGBT by heat pipes

In this work, we have developed a model in order to simulate the cooling of a power IGBT module by heat pipe systems. The IGBT is modeled by a RC thermal circuit approach on the basis on its thermal characteristics delivered by the manufacturer. The heat pipe is also modeled by a RC thermal circuit. The thermal resistances and capacitances of the heat pipe model are determined by both experiments and theoretical calculations. The model aims to determine the junction temperature of the IGBT as well as the heat pipe temperatures in response to a periodic heat input power as a function of different parameters such as the cyclic ratio and the switching frequency. The simulations results indicate that for, a given switching frequency, the cyclic ratio affects the junction temperature which oscillates between a minimum value and a maximum one. Indeed, the maximum as well as the minimum junction temperatures increase with increasing cyclic ratio. For a given cyclic ratio, the junction temperature is also affected by the switching frequency. The maximum junction temperature increases with the switching frequency, however, the minimum junction temperature decreases as the switching frequency increases. In all cases, the junction temperature value remains less than the maximum value allowed for the safety operation of the IGBT.

Thermal performance modeling of loop heat pipes with flat evaporator for electronics cooling

Abstract This work deals with modeling of the thermal performance of a copper-water loop heat pipe (LHP) with a flat evaporator operating in steady state operation. The model is based on steady-state energy and momentum balance equations for each LHP component. Modeling the heat transfer in the evaporator was particularly considered, and the evaporation heat transfer coefficient is determined from a dimensionless correlation which is developed on the basis of experimental data from literature. The validation of this model consists in comparing the experimental results and those obtained by the model for different cooling temperatures. Finally, a parametric study is presented to show the effects of different key parameters such as the radii and the lengths of the liquid and vapor lines, the length of the condenser, the heat sink temperature and heat transfer coefficient as well as the ambient temperature and the heat losses to the ambient.

Use of flat miniature heat pipes for the thermal management of electronic packages

An experimental study is realized in order to verify the mini heat pipe concept for cooling high power dissipation electronic cards. Two kinds of card substrates are considered: alumina and FR4 epoxy, and the chip on board technology is used. Different prototypes of configurations on reporting the chip on the card are tested. The thermal measurements show that the use of heat pipes allows for significantly reduced temperature gradients and maximum chip temperature decrease.

THEORETICAL INVESTIGATION ON THE THERMAL PERFORMANCE OF FLAT TWO-PHASE HEAT SPREADERS WITH MICROCHANNELS

A detailed mathematical model of a two-phase heat spreader with axial microchannels is developed in which the fluid flow is considered along with the heat and mass transfer processes during evaporation and condensation. The model is based on the equations for the mass, momentum and energy conservation, which are written for the evaporator, adiabatic, and condenser zones. The model, which permits to simulate several shapes of microchannels, can predict the maximum heat transfer capacity of the two-phase heat spreader, the optimal fluid mass, and the temperatures and pressure gradients along the microchannel. The effect of shear stresses at the free liquid surface in a microchannel due to the frictional liquid-vapor interaction on the liquid flow is taken into consideration. The heat transfer through the liquid films in both evaporator and condenser is accounted for in the model, which is described with respect to the disjoining pressure, interfacial thermal resistance, surface roughness, and curvature. The thermal resistances of the evaporator and condenser are determined by accounting for the longitudinal distribution of the meniscus curvature, which is dependent on heat load and heat spreader inclination.