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Cooling Technologies Research Center
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CTRC Breakthroughs

Thin-Film Transport, Wicks and Heat Pipes

Analysis of Thin Vapor Chamber for Thermal Management under Transient Heat Loading

Transient Mobile breakthroughs figure Hot spot mitigation in electronic devices under dynamic scenarios is a critical thermal challenge. While vapor chambers have long been an effective method to deal with thermal hotspots, the predominant design approaches have been founded on a core assumption of steady state operation; there is urgent need to overhaul these approaches, and the understanding of transient vapor chamber operation, so as to allow design for practical scenarios. In previous work in the CTRC, a novel analytical model was developed that enables rapid assessment of transient vapor chamber behavior in a variety of dynamic loading scenarios. This model was used to reveal the mechanisms governing the transient performance of a vapor chamber. Based on this new understanding, a refreshed set of design guidelines have been proposed based on the thermal performance of vapor chamber under transient condition. The design procedure yields the working fluid and vapor chamber dimensions that minimize the hotspot temperature in response to a transient step heat input. Furthermore, to facilitate immediate adoption of the understanding derived from the model, closed-form analytical expressions for the effective anisotropic thermal conductance have been derived. These expressions can be plugged into any transient conduction solver as material properties to accurately simulate the thermal response of a vapor chamber.

Transient Analysis of Thin Vapor Chambers for Mobile Thermal Management

Highly effective heat spreading is needed in portable electronic devices to mitigate transient hotspots. While thin vapor chambers provide extreme heat spreading performance potential, their transient thermal behavior in response to dynamic heating conditions was previously not well understood. A first-of-its-kind analytical model was developed that simulates the transient thermal behavior of thin vapor chambers at extremely low computational cost, without compromise in accuracy or functionality. The model revealed the primary mechanisms that govern the transient behavior of vapor chambers; these newly identified mechanisms were verified experimentally. The validated model, which was used to compare the relative performance of a vapor chamber versus a solid heat spreader over a range of operating conditions, will allow engineers to make an informed selection of heat spreader type for transient applications

Transient Analysis of Thin Vapor Chambers for Mobile Thermal Management

Transient Mobile breakthroughs figure A low cost, transient vapor chamber transport model is a necessity to simulate the thermal response of vapor chambers to transient heat loads in mobile electronic devices and other applications. The developed model has three to four orders of magnitude lower computational cost while maintaining good accuracy. The model can accurately compute the three-dimensional and transient temperature, pressure and velocity fields in a vapor chamber heat spreader, when subjected to multiple time varying heat inputs. The model will allow members of the industry to simulate vapor chamber prototypes as heat spreaders for their applications and compare their performance with each other.

Thin Heat Pipes for Low Power Applications

Thin Heat Pipes breakthroughs figureUsing analytical and numerical models, the performance of thin heat pipes have been evaluated and compared to that of a baseline copper heat spreader. The results of this study will generate comprehensive guidelines for the range of geometries and boundary conditions under which ultra-thin heat pipes may provide a comparative performance benefit against the current solid heat spreaders used in industry. Prior to tackling practical manufacturing challenges (namely, wick structure, fluid charging, mechanical robustness) the study assess the theoretical limits and design objectives.

Characterization of Composite Heat Spreaders

Composite HS breakthroughs figureThe characterization of composite heat spreaders using the 3-omega method has been used widely to provide accurate thermal conductivity measurement of materials in the past several decades. The total thermal resistance of a cooling package can be reduced by replacing traditional Al/Cu heat spreaders with suitable low cost new materials. Furthermore, carbon nanotubes (CNTs) are synthesized on heat spreaders to improve heat transfer at interface. The results indicate that, using an Al/Ti/Fe tri-layer catalyst, a high-density CNT array can be directly and firmly bonded to heat spreader surface. The figure shows CNT array synthesis on silicon substrate.

Thin Film Evaporation

Thin Film breakthroughs figureEvaporation, taking place at the very thin solid-liquid-vapor junction, is claimed to be the dominant mode of heat transfer in a liquid-vapor phase change process. Micro-PIV experiments are being performed to study the flow pattern near the evaporating meniscus. The differential evaporation sets up a temperature gradient between the corners and the center of meniscus which generates the thermocapillary convection seen in the form of two counter-rotating vortices. This strong convection is expected to increase the total heat transfer by providing better mixing of the fluid.

Experimental Investigation of the Transport Properties of Wicks

Heat Pipes breakthroughs figureHeat pipes are commonly used in electronics cooling applications due to their ability to move high amounts of heat over reasonable distances with only a small drop in temperature. Porous wick structures imbedded inside heat pipes provide the interfacial tension necessary to drive the working fluid. Improved measurements and observation of transport processes are also aiding in the design of miniaturized heat pipes. Two dedicated heat pipe wick-testing facilities have been established. One measures mass transport in a variety of flat wick structures, including sintered copper and aluminum grooved wicks. The second facility is an instrumented thermosyphon test bed to measure conductivity values for wick structures under varying degrees of wick saturation.