Analysis of the hot fuel return characteristics for a fuel thermal management system with multiple temperature limit points
YANG Shiyu, LIN Yuanfang, YU Haiyu, XU Xianghua, LIANG Xingang
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, School of Aerospace Engineering, Tsinghua University, Beijing 100084, China
Abstract：[Objective] With the rapid increase in aircraft thermal loads and the flight Mach number, combustion fuel and ram air can no longer effectively cool the fuel thermal management system (FTMS). Recently, hot fuel return has become an important way to enhance the cooling capacity of the FTMS, and the regulation of the hot fuel return flow has garnered significant attention. However, the impact of the hot fuel return flow on the limited temperatures still requires systematic research, and the thermal load limits of the FTMS have not been explored. Therefore, in this study, the hot fuel return characteristics of the FTMS with multiple temperature limit points were investigated to improve system performance. Considering the scalability of the models, a steady-state simulation model for the FTMS was developed using Python based on the thermal fluid network method and solved using the damped Newton method to guarantee the convergence of the flow and heat transfer iterations. In addition, the program was verified via experiments to guarantee its accuracy. A complete FTMS flow path was designed, and a standard condition was set for subsequent calculations. First, the temperature variations of the temperature limit points with the hot fuel return flow were calculated under the standard condition. Subsequently, by increasing the airborne thermal load under the standard condition, the maximum airborne thermal load that the system can withstand under the action of the hot fuel return (airborne thermal load limit) was investigated, and the occurrence condition of the limit state was analyzed. Lastly, the maximum total thermal load for normal operations (total thermal load limit) was explored under the standard condition, and the condition for reaching the total thermal load limit was discussed by changing the aeroengine thermal load. The increase of the hot fuel return flow may not decrease all the limited temperatures, even inducing the outlet temperature rises of the fuel tank and the hot fuel return valve. Excessive or insufficient hot fuel return flow may result in the overtemperature of the FTMS, and there exists a change interval for it to meet the multiple temperature limitations. For the outlet temperatures of the airborne thermal load heater and the fuel nozzle, there exists a critical flow of the hot fuel return, which indicates that the two outlet temperatures will not change once the hot fuel return flow reaches the critical flow. Combined with the outlet temperature rises of the fuel tank and the hot fuel return valve, when the hot fuel return flow surpasses its critical value, further increasing the hot fuel return flow will only increase the risk of system overtemperature. Moreover, the FTMS exhibits similar hot fuel return characteristics under different airborne thermal loads, and the critical flow of the hot fuel return rises with increasing airborne thermal load. However, as the critical flow of the hot fuel return rises slower than the lower boundary of the limited interval for the hot fuel return flow with the increased airborne thermal load, the airborne thermal load limit corresponds to the critical state when the size of the limited interval for the hot fuel return flow mutates into zero, and the lower boundary of the limited interval of the hot fuel return flow is just the system critical flow of the hot fuel return in this condition. Furthermore, the calculation results reveal that when the outlet temperatures of the airborne thermal load heater and the fuel nozzle reach their respective limit values in the limit state, the total thermal load limit can be achieved. In addition, to fully utilize the total thermal load limit of the FTMS under the action of an unreasonable aeroengine thermal load, the intermediate loop in this paper can be used to achieve the mutual transfer of the system thermal loads by the heat exchangers and refrigerating devices. As long as the total thermal load does not exceed the total thermal load limit, the FTMS can ensure that the system works normally through the intermediate loop to adjust the new aeroengine thermal load transferred into the fuel. This study explains the temperature variation regularity of multiple temperature limit points and the thermal load limits under the effect of the hot fuel return, providing a reference for the design of the thermal load distribution and the regulation strategy of the hot fuel return flow.
杨世宇, 林远方, 于海育, 徐向华, 梁新刚. 多温度限制点条件下燃油热管理系统热回油特性分析[J]. 清华大学学报（自然科学版）, 2024, 64(5): 841-851.
YANG Shiyu, LIN Yuanfang, YU Haiyu, XU Xianghua, LIANG Xingang. Analysis of the hot fuel return characteristics for a fuel thermal management system with multiple temperature limit points. Journal of Tsinghua University(Science and Technology), 2024, 64(5): 841-851.
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2024, Vol. 64 
