Heat Transfer Performance Study Of Phase Change In Evaporative Tube Of Underwater Thermal Power System

Underwater vehicles play a very important role in ocean development,ocean exploration and ocean strategy.In current society with increasingly fierce maritime disputes,countries pay more and more attention to it.The main performance indexes of underwater vehicle,such as range,speed and depth,depend on well-performing power system.The quality of water vapor acts a pivotal part in the performance of underwater vehicle with closed cycle thermal power system,powered by high temperature and high pressure steam.The flow of supercooled water in the evaporation tube of the power system and the heat transfer performance of subcooled flow boiling are the key fundamental problems in the design of efficient and compact thermal power system.Therefore,under the condition of thermal power system,the flow characteristics and heat transfer performance of subcooled boiling in the evaporative tube of an underwater vehicle are studied numerically.Firstly,based on parameters of the frequency of bubble departure,the nucleated site density and the bubble departure diameter,which close the heat flux distribution model of RPI boiling model embedded in Eulerian multiphase model,a set of numerical calculation models predicting subcooled flow boiling in evaporating tubes of underwater vehicles are proposed.Based on the experimental data from the literatures,the numerical models proposed above are compared and verified.It is found that there is maximum prediction accuracy when Cole model,Lemmert-Chawla model and Unal mode are used for the frequency of bubble departure,the nucleated site density and the bubble departure diameter,respectively.Secondly,based on the above validation model,flow characteristics and heat transfer performances of water in a smooth helical tube during the subcooled flow boiling are studied,and the influences of operation conditions and structural parameters on local heat transfer coefficients,liquid temperature and distribution of vapor volume fraction along the flow direction are analyzed and discussed.It is found that the reduction of inlet mass flux and inlet subcooling,and the increase of heat flux can promote the process of subcooled flow boiling in the tubes.Compared with the straight tube,the bulk flow in the helical tubes reaches the saturation temperature earlier,the outlet vapor volume fraction is smaller,and the maximum wall temperature is higher.The maximum local heat transfer coefficient in the helical tube with75°helical angle increased by 20.75%compared with that of the straight tube.However,the helical angle has little influence on the variation of axial vapor volume fraction and the liquid temperature.Similarly,the effect on secondary flow intensity could be also ignored.Finally,based on the above analysis on the flow and heat transfer mechanism of subcooled flow boiling,a novel helical tube with convex ribs is proposed,and the flow characteristics and heat transfer performances of subcooled flow boiling is discussed under various cases with different center angleφof convex rib,diameter d_rof convex rib,diameter d_a of connecting arc and rib number n.Then,overall thermal-hydraulic performance is evaluated.It is revealed that the overall performance is the best whenφis 90°,d_r is 0.8mm,d_a is 0.12 mm,and n is 4,respectively.Based on the smooth helical tube,the heat transfer coefficient of the novel evaporative tube increased by 50%and the evaluation factor p is up to 1.26.