Effects of viscous shear stress on thermoregulation of electronics. Transient free convection in diode enclosures induced by discrete heat bands under constant heat flux

Abstract

Thermal and dynamic phenomena that occur in the immediate vicinity of electronic components during operation generate viscous shear stresses due to velocity gradients. When thermocouples used for thermal regulation of these assemblies are installed in this environment, temperature measurements may be erroneous. It is therefore essential to take into account viscous effects in the boundary layer when dealing with thermal control of electronics subjected to natural convection. These phenomena are particularly pronounced and complex when generation of heat at the active wall is not uniform. That is the case for the real device treated in this work. The natural convective flow is generated by a vertical wall composed by alternated adiabatic and heated bands under constant heat flux, representing a working electronic equipment. The 2D transient boundary layer near the vertical active hot wall of parallelogram-shaped enclosures is treated in order to determine the viscous shear stress. Results are obtained by numerical approach using the finite volume method. Many geometrical configurations are treated while varying the inclination angle of the top and bottom passive adiabatic walls. The very different local distributions of viscous shear stresses and vertical thermal gradients confirm the necessity to take them into account to properly install the sensors used for thermoregulation.

Free convection in parallelogram-shaped enclosures with isothermal active walls: viscous shear stress in active systems

“…The normalized ratio τ * α τ * 0 is about 0.92 for α = +15 • and reduces to 0.36 for α = +75 • . The stress values obtained in this work are consistent with those obtained in the case of a hot wall constituted by discrete bands under constant heat flux (Baïri 2011a). A comparison has been made for the same geometrical and thermal conditions at the steady state.…”

“…These studies confirm the importance of taking into account the local phenomena and provide valuable information on a correct disposition of the thermocouples used for thermal regulation of active systems, as is the case of the solar building sector. The effects of viscous stress were discussed recently for the same type of cavity treated in the present study (Baïri 2011a). However, in that work, the hot wall consisted of three heat sources in the form of horizontal stripes separated by adiabatic bands of the same height.…”

“…This detailed description based on numerical simulation outcomes has been validated by measurements on the inner walls of the channel. Details of the experimental setup can be found in previous studies (Baïri et al 2010;Baïri 2011a). In brief, the passive walls of the experimental cavity are made of polyurethane foam panels (thickness 100 ± 2 mm) whose average thermal conductivity measured in the laboratory at the considered temperature range is 0.041 ± 0.007 W m −1 K −1 .…”

Numerical heat flow visualization analysis on enhanced thermal processing for various shapes of containers during thermal convection

“…subjected to various thermal boundary conditions of the walls. A number of works (Moukalled et al , 1993; Bairi et al , 2010; Moukalled and Darwish, 2010; Bairi, 2011; Bairi et al , 2012; Moukalled and Darwish, 2013; Bairi et al , 2017) are based on natural convection in parallelogrammic/rhombic containers. A few works are detailed as follows.…”