Flat Plate Thermosyphons for Space Constrained Applications

Sunil Murthy

Background

Increased integration density, faster clock rates and emerging trends such as single chip segmented processors with integrated cache memory have increased thedemands for effective thermal management. Current state-of-the-art for infrastructure equipment involves increasingly larger air-cooled heat sinks. Though air-cooling has remained a mainstay in most electronic systems, it is becoming evidently clear that for a number of applications direct air-cooling will need to be replaced or supplemented with other high performance compact cooling techniques.

Current technology for cooling electronics in thin space enclosures such as portables, involves metal plates to spread the heat. In order to illustrate the performance limits of this methodology, it is instructive to examine the effect of the aspect ratio (thickness/width) on the conduction resistance. A thin plate is used to study the heat transfer performance of solid heat spreaders. A steady state two-dimensional heat transfer problem is considered. The bottom side is assumed to be adiabatic and convection coefficients are imposed along thetop and the sides. The non-dimensional heat conduction equation is solved numerically for various thermal conductivity values. The solutions aree then mapped back into dimensional domain to find the solid heat spreader performance. A square plate of length 0.2 m and a square heater of dimension 0.024 m x 0.024 m is assumed for the simulations. Heat transfer boundary conditions with heattransfer coefficients of 15 W/sq.mK and 100 W/sq.mK aree applied as boundary conditions along the top and the side surfaces respectively. Figure 1 shows a plot of the variation in the thermal resistance of the spreader with change in spreader thickness for different thermal conductivities of the spreader material.

Fig 1. Variation of thermal resistance with thickness for various solid heat spreaders

Increasing the thermal conductivity of the spreader can reduce the increase in the internal resistance with decreasing thickness. Very high effective thermal conductivity can be achieved by employing liquid-vapor phase change to transport heat from the evaporator to the condenser. This concept can be exploitedto create thin thermal spreader plates based on two-phase flow.

It is well documented that re-entrant cavities, like the one shown in fig. 2, have the characteristic ability to entrap vapors, thereby becoming active nucleation sites. The efficient performance of enhanced structures in pool boiling, makes them good candidates for the incorporating into the evaporator sectionof thermosyphons to accommodate higher heat fluxes.

The current study is a continuation of the work carried out at the University of Maryland towards developing a design methodology for thin two-phase heat spreaders for space constrained applications.

Figure 2. Porous microstructure in copper. The pores entrap vapor and become sites of active bubble nucleation

• Develop and demonstrate thin two-phase heat spreaders for space constrained applications.
• Improve the performance of the heat spreader by employing three-dimensional boiling enahncement structures.
• Develop system level peformance prediction capabilities through two-phase heat transfer modeling.