Performance Analysis of Multistage Cooled Cooling Air System with Series Air-Air and Air-Fuel Heat Exchangers in Aero Engine

The rise of turbine inlet temperature of advanced aero-engine greatly increases the cooling requirement of turbine blades. In addition, the pressure ratio of high-pressure compressor (HPC) becomes higher, which could weaken the cooling capability of bleed air from HPC. In this background, it must bleed more cooling air from HPC while the excess air bleed will offset the benefits brought by the increase in turbine inlet temperature and pressure ratio. This contradiction will bring severe challenges to the cooling of hot-end components. Relying on the development of material temperature resistance and cooling technology hardly solves such a demanding thermal protection problem, and new cooling solutions need to be developed. Cooled Cooling Air (CCA) technology uses low-temperature heat sink to cool the cooling bleed air from HPC, thereby improving the quality of the cooling air and the cooling efficiency without changing flow rate of the cooling air. Heat sinks used in CCA technology include bypass air, fuel, and ram air. In CCA technology, the use of air-air or air-fuel heat exchanger has been well studied. By using heat exchanger simulation model, this study analyzes the advantages and disadvantages of CCA thermal management system using only air-air or air-fuel heat exchangers under a certain working condition. Results indicate that a single air-air heat exchanger could exceed the limitation of flow resistance under the given heat transfer capacity. Meanwhile, the scheme only applying fuel as heat sink could not satisfy the cooling requirement of hot air under the limitation of fuel temperature (150 ℃). Under the same working condition, the scheme of air-air and air-fuel heat exchanger in series is first proposed, and design parameters of the double heat exchangers are adjusted to obtain suitable results that meet the requirements of heat transfer, pressure drop, and temperature limitation of fuel. The bypass air is used as the heat sink of air-air heat exchanger, and the fuel required by combustion chamber is used as the heat sink of air-fuel heat exchanger. The heat-exchange system is designed to control the cold side flow resistance of the air-air heat exchanger within 2% and the hot side flow resistance of the heat exchange system within 4%, and the temperature drop of the cooling air can also meet the cooling requirement.