Computational study of heat exchange and thermal oxidative coking of supercritical-pressure kerosene with compressed air in counter flows

The Cooled Cooling Air (CCA) technology is an effective approach in an advanced gas turbine engine to reduce the temperature of the highly compressed air and recover its cooling capacity, using the onboard fuel as coolant. Numerical studies have been conducted in this paper to analyze heat exchange between the supercritical-pressure aviation kerosene and the compressed high-temperature air in a double-pipe counter flow configuration, intended for the CCA applications. The thermal oxidative reactions and surface coking of kerosene are taken into consideration using a multi-step reaction mechanism. Results indicate that the air tube diameter should be determined to obtain not only the improved overall thermal performance on the air side, in terms of both heat transfer and pressure loss, but also the properly limited maximum temperature on the fuel side to avoid the strong pyrolytic chemical reactions of kerosene and the resulting fast surface coking process. Although the ribbed and dimpled surface structures are both able to improve the overall thermal performance in the fuel tube and increase the bulk air temperature reduction, they also lead to the increased surface coking rate from the thermal oxidative reactions of kerosene. The thermal oxidative coking process would gradually increase heat transfer barrier and cause an adverse effect on the long-term operation of a heat exchanger. The numerical results obtained in this paper should have fundamental and practical importance in the CCA applications.