Thermal sensitivity analysis of avionic and environmental control subsystems to variations in flight condition

The operation of fast jet military aircraft spans a large flight and atmospheric envelope. This study is the analysis of an avionic thermal management system for a typical fast jet military aircraft across changing operating conditions. The system which governs avionic module temperature is only partially active; therefore the efficiency and heat rejection capability is almost completely dependent on the system inputs of flight and atmospheric conditions. The thermal sensitivity to variation in system inputs is assessed with the use of experimental testing, one-dimensional thermodynamic modelling and energy flow calculations. The avionic module is the final component of the thermal management flow path and to understand the performance at component level, every subsystem upstream must be considered through a complete systemic approach. The facility used to deliver this analysis considers the total system energy consumption, Environmental Control System (ECS), cabin and three avionic subsystems as a single airflow path. The system is subjected to a typical fast jet flight profile, including a take-off, climb, cruise, combat, landing and ground operation cases. The flight profile is considered across three atmospheric conditions; ISA standard, hot and cold. It is found that the system heat rejection is stable with variations in flight conditions; therefore the system efficiency is inversely proportion to total energy consumption. The highest system efficiency is delivered at high altitude low load cruise conditions, with the lowest efficiency found at high speed low attitude flight. The system is most efficient when thermal safety factor is lowest. This is hot atmospheric and low load conditions, where the ECS bypass flow rate is low and avionic module exhaust temperatures are high. The ground ops condition in a hot atmosphere is the worst case scenario for avionic module exhaust and cabin temperatures. Considerable system gains could be made by introducing an element of active control, such as limiting bleed and ram air consumption when avionic temperatures are low and ECS bypass flow rate is high.