High Temperature Heat Exchanger Development

Gas turbine engines have long used fuel on its way to the combustor as a coolant for engine components. Advanced aircraft and engine technologies are demanding more cooling capacity than conventional fuel delivery systems can offer. To meet these new cooling requirements, technologies are being pursued to allow fuel to reach significantly higher temperatures without the traditional limitations associated with coking. A compact fuelair heat exchanger is a critical component in these new high-temperature fuel systems. Both tubular metal foam and hollow-truss cellular metal heat exchangers were examined for performance, size, and weight benefits as compared to conventional shell-tube technology. The tubular metal foam heat exchanger behavior was characterized with scaled laboratory and full-scale rig experiments. The data were used to calibrate an analytical model based on data in the open literature. The results indicate that sintered metal foam can provide as much as twice the heat transfer as a plain bank of staggered tubes. However, the corresponding 4X increase in pressure drop almost neutralizes the heat transfer benefit. Furthermore, it was determined that metal foam manufacturing constraints combined with the small high pressure fuel tubes would result in a design which has no sizeweight or performance benefit. However a 40 reduction in the number of tubes may provide improved reliability. A preliminary investigation of a hollow-tube cellular metal heat exchanger found that when elliptical non-flowing rods are used to augment the tube-bank they can increase the heat transfer by as much as 1.5X over a plain staggered tube-bank without a significant increase in pressure drop. It is projected that 10 weight and 30 size reductions can be achieved with hollow-truss cellular metal as compared to shell-tube heat exchanger. However, realization of hollow-tub cellular metal may require a costly manufacturing process.