Thermal dynamic and failure research on an air-fuel heat exchanger for aero-engine cooling

This paper presents a novel air-fuel heat exchanger used for the cooled cooling air technology in aero-engines. The helical tube heat exchanger weighing 1.1 kg with an area density of 214 m2/m3 can cool the hot air down 260 K at the air flow of 0.3 kg/s with relative airside pressure drop less than 0.6%. Empirical correlations by multiplying a constant of 1.06 and 0.837 can well predict the pressure drop and convective heat transfer coefficient respectively for hot gas cross-flowing helical tube bundles. Furthermore, in the tube failure tests to simulate the fuel control system fault and flight mode conversion, the straight tube can work continuously for more than 360 s under the extremely high tube wall temperature of over 1200 °C in the heat flux sudden increase test, while the bent tube could continue to glow brightly for 30 min. Coke morphology and chemical composition analysis revealed that increased thermal stress caused by gradually thicker coke layer takes a major cause in the heat flux sudden increase test. The secondary flow in bent tubes can effectively improve the convective heat transfer performance and reduce the formation of coke deposition, thereby improving the tube working life.