Heat exchangers are used for the transfer of heat between two or more media. The media may be separated by a solid wall so that they never mix, or they may be in direct contact. Heat exchangers may be used in HVAC systems, power plants, and mechanical devices and systems that require heat transfer. Examples of systems where heat dissipation would be desirable are various electromechanical systems, and electronics applications such as conventional computing systems. In a typical computing system that includes a CPU, one or more memory devices, and other circuitry, cooling of the CPU in particular may be an important design consideration.
In a conventional CPU cooler, the heat transfer bottleneck is the boundary layer of “dead air” that clings to the cooling fins. Within this boundary layer region, diffusive transport is the dominant mechanism for heat transfer. The resulting thermal bottleneck largely determines the thermal resistance of the heat exchanger. Another longstanding problem is inevitable fouling of the heat exchanger surface over time by particulate matter and other airborne contaminants. Heat sink fouling is especially important in applications where little or no preventative maintenance is typically practiced.
Another drawback to conventional heat exchangers is inadequate airflow resulting from restrictions on fan noise. Small- and medium-sized fans have relatively poor mechanical efficiency; unproductive expenditure of mechanical work on the surrounding air results in high noise levels.
The Sandia Cooler architecture developed in this work simultaneously eliminates all three of the drawbacks of conventional air-cooled heat exchanger technology: it provides a several-fold reduction in boundary layer thickness, intrinsic immunity to heat sink fouling, and drastic reductions in noise. It is also expected to be very practical from the standpoint of cost, complexity, ruggedness, etc.
In this new device architecture, heat is efficiently transferred from a stationary base plate to a rotating (counterclockwise) structure that combines the functionality of cooling fins with a centrifugal impeller (see figure). Dead air enveloping the cooling fins is subjected to a powerful centrifugal pumping effect, providing a 10× reduction in boundary layer thickness at a speed of a few thousand rpm. Additionally, highspeed rotation completely eliminates the problem of heat exchanger fouling.
The device can be mounted in any direction. In the radial direction, the heat sink impeller is directly supported by a long-life roller bearing assembly; in the axial direction, a spring-loaded mechanism is used for retention of the heat sink impeller and preloading of the hydrodynamic air bearing.
Under typical operating conditions, the current device has a free-delivery volumetric flow rate of 1700 L/min (60 CFM), and a maximum pressure rise of 100 Pa (0.4 in-H2O). The impeller design and rotational speed can be altered to achieve higher or lower flow rates and pressure rises if desired.
The “direct drive advantage” in which relative motion between the cooling fins and ambient air is created by rotating the heat exchanger provides a drastic improvement in aerodynamic efficiency. This translates to extremely quiet operation. The benefits have been quantified on a proof-of-concept prototype.