Study On Heat Transfer And Propulsion Characteristics Of Lithium-cooled Nuclear Power System For Aviation

The aviation nuclear propulsion system has the advantages of carrying heavy equipment and achieving long-endurance cruise under the condition of high power output.The development of high efficiency heat transfer and large thrust-weight ratio nuclear propulsion system is of great significance to the development of propulsion system.At present,the conventional heat transfer mode of the nuclear propulsion system is mainly open cycle.Compressed air is directly heated through the reactor core,and then work is done to produce thrust.Although this kind of heat transfer mode is simple in structure,it causes the risk of nuclear fuel leakage and radioactive pollution.Aviation nuclear power system is a high-altitude flight system,which is obviously limited by weight and volume.The coupling heat transfer between reactor and engine is one of the important bottlenecks.In the process of reactor heat output,efficient heat transfer method should be adopted to reduce reactor volume and realize miniaturization of nuclear power system.The phase-change heat transfer of the power-driven lithium heat pipe is an effective means to realize efficient coupling heat transfer in the core.In addition,the existing nuclear heat exchanger has a large volume,so it is necessary to discuss the new type structure of heat exchanger to realize the integrated coupling between the nuclear heat exchanger and the engine.In this thesis,systematically studies the key scientific issues of efficient coupling heat transfer between nuclear propulsion system reactor and engine.For a certain type of small aviation turbojet engine with Ma<l,a new aviation nuclear power system based on power-driven heat pipe and closed cycle was designed.Using the lithium channel of the power-driven heat pipe,the heat is carried out of the core and transferred to the nuclear heat exchanger by means of phase change heat transfer.The designed nuclear heat exchanger has multi-annular structures,which is placed in the annular space of the original combustion chamber to realize heat transfer between air and heat source.The feasibility of the new nuclear power system is explored and the scheme design and systematic evaluation are carried out.The main research contents and results are as follows:(1)Based on the nuclear propulsion system of lithium-cooled reactor,the heat transfer and propulsion scheme design and scheme adaptability research were carried out.Based on the design of the dynamic phase change circuit,the selections for the new heat transfer system are analyzed in detail.For example,the heat transfer mode selection in the reactor,the multi-annular structure characteristics of the nuclear heat exchanger,the coupling heat transfer mode selection between the phase change channel and the core,the coupling heat transfer mode selection between the phase change channel and the nuclear heat exchanger,etc.The analysis shows that the power-driven heat transfer system realizes the flexible arrangement of the reactor and nuclear heat exchanger.The engine structure can be fully utilized,which is of great significance to the optimization of the thermal transmission mode for the nuclear propulsion system.(2)The mechanical analysis of phase change heat transfer in the channel with lithium as working medium was carried out.The basic transient characteristics of two-phase flow phase change heat transfer,the influence of gravity on two-phase flow passage in conventional flight state,the coupling heat transfer characteristics of two-phase flow passage and core,and the coupling heat transfer characteristics of two-phase flow and nuclear heat exchanger are analyzed in detail.The results show that for the boiling phase transition in the core,the vapor is mainly concentrated in the adiabatic zone,and the evaporating zone is the main phase of the liquid.It will not keep the air-liquid muddy.The core temperature distribution is uniform,and the hottest temperature is near the side wall.In the nuclear heat exchanger,the temperature of the phase-change channel decreases continuously and becomes liquid at the bottom.Finally,the heat in core can be effectively carried out,and the heat transfer system can achieve stable operation.(3)The structure optimization and propulsive characteristic analysis of multi-annular nuclear heat exchanger were carried out.The flow characteristics in the nuclear heat exchanger are analyzed by using various soft wares.The optimum structure with maximum heat transfer coefficient and thrust is obtained.The sensitivity factors affecting the propulsion performance of nuclear power engines are comprehensively evaluated.The results show that there is an optimal structure near the normalized intersection point of thrust F and heat transfer coefficient h.It ensures the same propulsion performance of the original engine with the minimum loss of core flow resistance and it also meets the performance requirements from cruising to maximum thrust flight states.The thrust is more sensitive to the change of control variables under the condition of keeping the structure unchanged.The increase of air intake is more beneficial to the improvement of system performance.The main innovations of this paper are as following:(1)A new type of heat transfer system which is suitable for the aviation nuclear thermal propulsion system has been carried out.Vapor phase change heat transfer mechanism analysis is completed in the heat transfer channels for the super-high temperature liquid metal lithium.(2)An efficient multi-annular nuclear heat exchanger is developed to optimize the performance of the propulsion,so that the nuclear heat exchanger can achieve the same propulsion performance as conventional fuel engine by using the original combustion chamber space.(3)The sensitivity analysis of propulsion performance improvement is carried out for the new aviation nuclear propulsion system.The relationship and sensitivity between the heat transfers,propulsion characteristics and the change of external between the heat transfers,propulsion characteristics and the change of external control variables are proposed during the process of performance improvement.It will provide necessary reference for the design and optimization of aviation nuclear thermal propulsion system.This thesis is the basic research on the heat transfer and propulsion performance optimization for the nuclear propulsion system.Based on the current research results,it can provide necessary reference and support for the performance optimization of the nuclear propulsion system.