Large eddy simulations of heat transfer and thermal oxidative coking of aviation kerosene in vertical U-tube at a supercritical pressure

The onboard fuel in an advanced aero engine can be used to cool the highly compressed hot air in a heat exchanger to restore the air's cooling capacity. In the process, the high fuel temperature leads to thermal oxidative reactions and surface coking. Large eddy simulations have been conducted in this paper to study the combined effects of buoyant and centrifugal forces on heat transfer and thermal oxidative coking of the aviation kerosene, RP-3, in vertical U-bend tubes at a supercritical pressure of 4 MPa. Numerical results indicate that buoyancy exerts its impact on fluid flows and heat transfer mainly in vertical straight sections of the tube, promoting turbulence generation in the downward flow and suppressing turbulence in the upward flow. This causes differences of the heat transfer and surface coking rate in the up-down and down-up cases. Centrifugal force plays an important role in fluid flows and heat transfer in the U-bend section. Strong secondary flows driven by the centrifugal effect significantly enhance heat transfer in the outer region but weaken heat transfer in the inner region of the U-bend section, resulting in up to 300 K circumferential wall temperature difference in the present cases. The large wall temperature gradient and high wall temperature lead to significant thermal oxidative coking inside the tube near the entrance and in the inner region near the exit of the U-bend section. Results herein provide fundamental understanding and have practical implication for the design and safe operation of an air-fuel heat exchanger in aerospace applications.