Experimental investigation on the effects of body force environment on flat heat pipes thermal performance

Abstract

This paper reports on the effects of body forces environment : gravitation, vibration and acceleration forces with step changes in input power on the thermal performance of a flat copper/water heat pipe. The experimental results on the orientation effects show that the heat pipe is hardly affected by the gravitation forces and exhibits nearly the same thermal performance whatever the tilt angle for input heat powers lower than 20 W. For input heat powers higher than 20 W, there is a slight heat pipe thermal performance dependency on gravitation. It is also found that the heat pipe thermal performance is hardly affected by vibration whatever the mounting direction on the shaking table. An investigation into the effects of transient acceleration forces with constant input heat loads on the heat pipe thermal performance has demonstrated a decrease in the heat pipe thermal performance with increasing acceleration. However, the heat pipe successfully reprimed under continuous input heat load, with a suppression of acceleration. In all cases, the increase of the heat pipe thermal resistance does not exceed 60 %. The maximum heat pipe thermal resistance obtained under lOg acceleration level remains an acceptable value for the electronic package safety. INTRODUCTION Packaging and thermal management of electronic equipments have become an important problem because of increased power levels and simultaneous miniaturization of the devices. This may cause difficulties to use the latest packaging technologies available particularly in avionics or space applications. Indeed, more functions are being levied on aircraft systems. The raise of functional density increases the electronic component density. To meet the added functional requirements, electronic devices must shrink to allow more devices on a module and to allow the increase of processing/interconnection speeds. This, however, produces : (i) an increase of card and module total heat dissipation so that the heat Copyright © 2000 The American Institute of Aeronautics and Astronautics Inc. All rights reserved. dissipated is greater than 100 W for a module, (ii) and an increase of local heat densities for highly integrated components so that heat fluxes underneath the chips could be higher than 25 W/cm. The typical cooling techniques for avionics are based on cooling with conduction and with forced or natural convection. These techniques may provide no convenient source of cooling to prevent the electronics from operating at temperatures surpassing those required for maximum component life. New cooling techniques will have to be developed to maintain components within the temperature limits. Heat pipe technology may be able to provide sufficient cooling in these types of situations. Heat pipes have traditionally been operated in environments free of adverse body forces such as vibrations and high acceleration forces. Recently, heat pipes have been proposed to be used aboard fighter aircraft to act as heat sinks for electronic packages. During combat, transient acceleration fields up to 10 g could be present on the aircraft. Acceleration force fields may be transient and coupled with transient heat loads. Therefore, characterizing the steady state and transient performance of heat pipes under elevated acceleration fields is of importance to designers of the electronics packages in need of cooling and will require experimental approaches. Acceleration values should be obtained from the aircraft structural loads analyses. Figure 1 shows the six acceleration directions to which the aircraft can be submitted during maneuvering and combat. Suggested acceleration levels for different avionics vehicles are presented in Table 1 (Standard Military specifications: MIL-STD-810D Procedure II : operational test). Moreover, vibrations cannot be avoided in the aircraft equipments. Therefore, some study of the effects of vibrations on the performance of heat pipes must be carried out to determine if vibrations will either aid or hinder the operation of the heat pipe. American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.