During different airplane flight modes, various effects may appear that need to be analyzed for both the oil and the fuel system at steady-state and transient operating modes. The effects, which relate to the cold temperature, associated with fuel freeze or wax point, cause a malfunction in the fuel pumps, nozzles, and other areas of the fuel system. On the other hand, high fuel temperature also leads to negative effects — the most common failure of high-flow fuel systems is cavitation, or “vapor-lock.” The combination of too much heat or too much inlet restriction can create this operating condition, where the liquid fuel literally boils inside the fuel pump. These effects are eliminated by the fuel/oil heat exchange system. In case of low fuel temperature, the fuel is used as a refrigerant to cool down hot oil coming from bearings. And in case of high fuel temperature, the oil serves as a coolant.
This paper considers the method of evaluating normal and critical aircraft engine operation modes of the oil supply system with a fuel-oil heat exchanger utilizing an unsteady-state thermal-fluid network approach. The analyses are done based on the aircraft engine example to evaluate fuel and oil systems parameters variation in time under different flight conditions — the amount of fuel in the tank, inertial thermal effects, and the response time of the system to the regulation of the heat exchanger. The article is focused on sudden switching from a high to low gas engine operating mode. Fuel consumption to the engine is reduced abruptly, but the heat transfer from the bearings to the oil is still high due to thermal inertia. In this situation, a large amount of heated fuel must be returned to the fuel tank. At a certain point in time, the temperature of the fuel can reach a critical value. At the same time bearing cooling becomes ineffective, which leads to overheating. The calculation of thermal management system was performed at nominal conditions to obtain the initial data for low power settings analysis. As results of analysis at the low power settings mode the oil temperature before fuel cooled oil cooler is reached above 138 °C, which is high value. The failure of flow return valve is considered. The variations of oil temperature after the tank and increasing of fuel temperature at the tank in case of emergency situation are obtained. The influence of cooled fuel amount on the system thermal management is analyzed.