Progress with development of foil and slot thermal transfer technology

Abstract

Foil And Slot Thermal Transfer (FASTT) devices have a unique set of characteristics and the technology may have an important role in future thermal engineering. The absence of freezing and boiling issues, together with the potential for very wide operating temperature range from below 100 K to above 1000 K, as well as their use of low cost, environmentally friendly materials are among those features which have already been mentioned in literature. The geometry of FASTT devices is currently limited to planar or extrudable sections, however, such formats are found in a number of important applications, such as high power, high flux cooling of semiconductors.The core of any FASTT device is the slotted region within a thermally conductive material through which solid foils move and, in a similar fashion to fluids, carries heat away to a larger, lower flux zone where the heat may be dissipated by convection. This allows very high input cooling flux levels to be achieved. There are a number of differences between FASTT and pumped fluid heat transfer. “Equivalent” heat transfer coefficients comparable to those in pumped liquid systems can be obtained but at much lower media velocities (0.05 cf 3.00 ms−1). Narrow channel flow for liquids can be limited by viscosity and allowable operating pressure level. The thin solid foils used in FASTT can run with very low friction at foil to slot gaps below 10 µm, and with channel widths below 100 µm. This ensures low circuit parasitic losses and low pressure operation. The foils can provide both primary heat transfer as well as acting as the extended output surfaces in unsealed variants of these devices. This can remove the need for fluid channels/pipework and thus, potentially, reduce system mass.Limiting flux levels for simple air blast cooled finned arrays are considered, as are theoretical flux limits for FASTT devices, using a selection of foil material and gap media. Results from an unsealed prototype device are given which show that flux values of over 250 Wcm−2 at 70C temperature differences are possible with this type of device operating at room temperature, even in prototype form. Future proposals include sealed devices operating at cryogenic temperatures where the technology is predicted to show significant advantages over existing cooling systems.