2D transient natural convection in diode cavities containing an electronic equipment with discrete active bands under constant heat flux A. Baïri a,⇑ , J.M. García de María b , I. Baïri c , N. Laraqi a , E. Zarco-Pernia a , N. Alilat a a Université Paris Ouest, LTIE-GTE EA 4415, 50, rue de Sèvres, F-92410 Ville d’Avray, France b Universidad Politécnica de Madrid, Ronda de Valencia 3, E-28012 Madrid, Spain c Ecole Nationale des Ponts et Chaussées, 6–8 Avenue Blaise Pascal, F-77455 Marne-La-Vallée, France article info Article history: Received 23 October 2011 Received in revised form 7 April 2012 Accepted 16 April 2012 Available online 1 June 2012 Keywords: Transient natural convection Imposed heat flux Electronic equipment Parallelogrammic cavity Discrete heat sources Finite volume method Experimental data abstract The thermal behavior of airborne electronic equipment submitted to natural convection in closed parallelogrammic air-filled cavities is examined in this study. The cold active wall of the enclosure is maintained isothermal. The hot wall, representing the electronic device, is composed of three parallel dis- crete bands generating a constant heat flux, separated by two adiabatic bands of equal dimensions. Both walls remain always vertical. The channel is considered adiabatic and the aspect ratio of the cavity is equal to unity. Many configurations are examined while varying the inclination angle of the top and bot- tom walls of the channel. When the angle is positive the convective heat transfer is favored in comparison with the case of the right cavity, but, on the contrary, it is reduced for negative angles. The resultant enclosures are so called diode cavities in the convective heat transfer sense of the word. The experimental part of the study is achieved with a setup based on electrical data and temperature measurements on the walls. The numerical approach using the finite volume method allows to complete the experimental results with the thermal and dynamical characteristics of the 2D flow. The temperature fields show the thermal behavior of the device during the transient phase after switching it on. The convective results concerning the imposed heat flux treated in this study differ from those corresponding to impose the temperature on the hot bands. The distribution and evolution of the Nusselt number allow to characterize the natural convection occurring in the cavity. The results of this work are consistent with previous stud- ies and allow to predict the thermal behavior of the electronic equipment during the transient phase. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Temperature control is essential for equipments in electronic engineering, especially in aeronautics given the high reliability re- quired in this domain. Heat transfer must be controlled during the transient state following the power on of the device until steady state. This study examines heat exchanges by natural convection, whose advantages are well known by engineers, especially when the equipment is contained in closed cavities. The work is focused on the transient phenomena that affect on-board electronic equip- ments with very low heat capacity, contained in air-filled closed cavities of parallelogrammic section whose both hot and cold ac- tive walls remain always vertical. The passive top and bottom walls of the channel are inclined with respect to the horizontal with an angle of inclination that can be positive or negative. Thus the lat- eral walls of the cavity have a parallelogram-shape. The phenom- ena occurring in this type of cavities are very different from those taking place in the more common rectangular cavities used in electronic engineering. Many configurations are examined while varying the inclina- tion angle respect to the horizontal direction. When this angle is negative, the hot wall is higher than the cold one, so the convective heat transfer is smaller than in the case of the right cavity (zero an- gle). The cavity is then called ‘‘insulating’’ in the convective sense of the word. For positive values of the angle, the hot wall is lower than the cold one. For these configurations, the natural convection is favored, with a maximum heat transfer for an angle of about 30°. The cavity is then qualified as ‘‘conductive’’. Such combinations of cavities behave like a diode and are called diode cavities in the con- vective heat transfer sense of the word. The thermal control of the device is essential to ensure its correct operation during transient and steady-state phases at the given thermal and geometrical con- ditions. The convective flow depends on many physical parame- ters. The most important ones are the hot and cold temperature values, the difference between these temperatures, the inclination of the top and bottom walls with respect to the horizontal, the dimensions of the cavity, the value of the imposed flux and the thermophysical properties of the air. Some previous studies have 0017-9310/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2012.04.032 ⇑ Corresponding author. E-mail address: abairi@u-paris10.fr (A. Baïri). International Journal of Heat and Mass Transfer 55 (2012) 4970–4980 Contents lists available at SciVerse ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt