Influence of temperature control limits on passively cooled computing system performance
Passively-cooled electronic systems such as tablet computers and cell phones commonly are thermally limited by the allowable touch temperature of their surfaces. These systems typically cannot cool maximum processor power in steady state and simultaneously meet surface temperature restrictions. The primary surface temperature control method for thin mobile devices is to limit power, typically resulting in lower computing performance. Improved thermal designs allow higher power dissipation and therefore increased computing performance. Thermal and mechanical design engineers typically operate in terms of temperature for a given power, and this is not usually translated to computing performance. It can be difficult to justify a better thermal design if it is not quantified in its utility to the user. This paper quantifies the tradeoffs between thermal solution performance/temperature control limits and computing performance for two tablet computers and one cell phone that all use the same type of SoC processor.
A study of the maximum theoretical power dissipation of tablets under natural convection conditions
This study determines the limits of natural convection cooling for ideal handheld devices of various sizes based on simulations. Four popular tables were tested to determine the surface temperature rise under heavy computational and graphics loads. The thermal performance of each tablet based on hot spot temperature is measured against the performance of an ideal tablet of the same size and a figure of merit for the efficiency of heat dissipation is calculated to see how well they perform against the ideal tablet. The factors affecting the maximum possible power dissipation are the available surface area and surface finishes, selection of the outer shell materials, thermal interface materials, heat spreaders and air gaps. In most cases, the limiting factor in the thermal design of these devices is not the temperatures of the internal components but the temperature of the external surfaces since these are in direct contact with the skin of the user. This study presents a method for analyzing the efficiency of the thermal design of these devices.
Intensification of Bubble Disintegration and Dispersion by Mechanical Stirring in Gas Injection Refining
Water model experiments were performed for establishing highly efficient gas injection refining processes. Mechanical stirring was applied to disintegrate the injected bubbles and to disperse them widely in the bath. The bubble disintegration and dispersion were investigated by changing rotation mode (direction of rotation), rotation speed, blade size of the impeller and gas flow rate. Forward rotation of the impeller induced a stable tangential flow and could not disperse bubbles in the bath due to formation of a vortex around the impeller shaft. The tangential flow could be suppressed by forward–interrupt rotation, which could reduce the vortex formation to some degree. However, forward–interrupt rotation could not disperse the bubbles widely in the bath. Forward–reverse rotation could prevent the vortex formation completely and create a strong shear stress field, which intensified the bubble disintegration and dispersion in the bath. Higher impeller rotation speed and larger blade length in forward–reverse rotation could enhance the bubble disintegration and make the dispersed bubbles smaller. The bubble dispersion zone became wider with larger blade length. The bubble size tended to be larger at higher gas flow rates. However, its dependence on the gas flow rate became smaller at higher impeller rotation speed.