Heat recovery from aircraft engines

The paper deals with the problem of heat recovery from aircraft engines, with specific reference to jet engines and turbo propeller engines. In actual propulsion systems, the trend for cycle maximum temperatures is for very high values, with a consequent increasing of discharge temperature, due account also given to operative modalities of the discharge nozzle, often working in off design conditions. Thus, the heat discharged strongly influences the performances of the system, affecting them even if already optimized with reference to other cycle parameters. In this way, heat recovery appears an obliged way for further increments of performances. In the space sector, however, the recovery of the discharged heat appears strongly problematic because of engine configuration and specific limitations, as weight, overall dimensions, maximum reliability, etc. Nevertheless, this possibility shows great interest. The present paper evaluates first the possible modalities to utilize the recovered heat, emphasizing how the regenerative processes should be considered among the most attractive ones. Then, the different types of jet engines are analyzed to evaluate the possible obtainable recoveries. The analysis shows how the recovery not only influences positively efficiency and therefore specific fuel consumption, but allows also to amplify the importance of other operative parameters. Finally, the practical possibility of heat recovery is evaluated, trying to define the configuration of suitable heat exchangers and to identify the most appropriate location within the propulsive system.