Heat exchangers and pumps can be made smaller, lighter, and less expensive.

Experiments have demonstrated the feasibility of heat exchangers with flow channels containing open-cell metallic foams. Nonmetallic foams could also be used. One proposed design calls for an open-cell foam in the annular region between an inner and an outer tube, with a heat-transfer fluid flowing along the annular region while either the outer or the inner tube is heated by flowing hot gas. Heat would be transferred from the heated tube wall to the fluid with an efficiency greater than that of a simple tube heat exchanger. The increase in heattransfer efficiency would result from the large surface area of the foam in contact with the cooling fluid and from turbulence induced by the ligaments of the foam in the flow path. Thermal conduction between the wall and the foam would also make a small contribution.

Greater heat-transfer efficiency should make it possible to design smaller, lighter heat exchangers for some applications. Another advantage of foam heat exchangers is that all flow paths through foam are interconnected, so that if a flow becomes obstructed by debris lodged in foam at one place in a channel, the flow can continue elsewhere in the channel; in other words, complete blockage of a channel is unlikely. This is in contrast to the situation in a heat exchanger containing narrow brazed tubes and/or narrow, precisely machined channels, which can become blocked more readily. Moreover, in comparison with machined-channel and brazed-tube heat exchangers, foam heat exchangers can be fabricated more easily and at lower cost.

Remarkably, a properly designed foam heat exchanger exhibits less back pressure than does a machined-channel or brazed-tube heat exchanger of equal heat-transfer capacity. Thus, a smaller pump can be used to circulate the heattransfer fluid.

Lewis Research Center Cleveland, Ohio

This work was done by Arthur J. Fortini and Robert H. Tuff as of Ultramet for Lewis Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Physical Sciences category.