Experimental Investigation On The Heat Transfer Characteristics Of A Downward Curved Heated Surface Under Natural Circulation Condition

Once the serious accident occurs in a nuclear power plant,the temperature of the core material in the pressure vessel rises rapidly,and even melts and collapses into the bottom of the lower head of the pressure vessel where the molten pool will be formed.The molten pool continuously transfers heat outward through the outer wall surface of the pressure vessel due to the decay heat.Different from the second-generation nuclear power plant,the third-generation nuclear power plant adopts In-Vessel Retention(IVR)External Reactor Vessel Cooling(ERVC)to ensure the integrity of the head,that is,flood the pressure vessel by injecting water into the heap chamber,and rely on the natural circulation flow to take away the decay heat inside the pressure vessel.In this case,if the decay heat is too high,film boiling outside the pressure vessel may happen,which will lead to deterioration of heat transfer conditions and damage of the pressure vessel wall.Therefore,determining the Critical Heat Flux(GHF)of the lower head surface of a pressure vessel under natural circulation conditions has important engineering significance for nuclear power plant safety.A number of studies have been conducted internationally on this measure.Due to the large size of the prototype pressure vessel,it is difficult to carry out the full-scale experiment.Some scholars have carried out the two-dimensional slicing experiment with the same radius of the prototype pressure vessel to obtain the critical heat flux value at different angles of the lower head of the pressure vessel under natural circulation conditions,which provides data references.Some researchers have used small-scale two-dimensional slices to conduct experiments,and obtained critical heat flux values under forced circulation conditions.In addition,there are few studies on the effect of natural circulation height on CHF values.In summary,there are few related studies on two-dimensional slicing,natural circulation,different scales and different natural circulation heights.Therefore,it is necessary to carry out the CHF value of the pressure vessel under the above comprehensive conditions.This paper uses small-scale two-dimensional experimental slicing,using natural circulation to study fluid temperature,two-phase flow pattern and CHF value under different natural circulation heights and inlet temperature.In order to obtain different heat flux values at different angles,the inclined downward curved heating plate consists of 9 segments of different thicknesses.The results show that under the natural circulation conditions,the CHF value obtained by this experiment is about 30%lower than that of the full-scale two-dimensional slice experiment.Compared with the similar scale two-dimensional slice,the CHF value obtained by the natural circulation condition is about 32%lower than the forced circulation.In addition,under the low inlet water temperature condition,the CHF value of the lowest natural circulation height condition is the highest.When the inlet water temperature rises,increasing the natural circulation height has an effect on the improvement of the CHF value.This experiment evaluates the influence of the size,circulation mode and natural circulation height of the experimental section on the flow characteristics,and can provide a certain uncertainty reference for the application of different types of gantry in engineering.