Experimental Study On Boiling Heat Transfer Characteristics In Helically-coiled Tubes Under Ocean Conditions

This article takes the heat transfer characteristics of the helically-coiled tube direct current steam generator in the floating platform of offshore nuclear power as the research background,and conducts experimental research on the single-phase convective heat transfer characteristics,subcooled boiling starting point characteristics,dryout point characteristics,and saturated twophase boiling heat transfer characteristics inside the helically-coiled tube.The influence of the superposition of ocean conditions on the heat transfer characteristics of the helically-coiled tube is also studied.The experimental part was conducted under hot conditions,and the main research work completed is as follows:(1)Studied the single-phase convective heat transfer characteristics in helically-coiled tube under ocean and non ocean conditions.Firstly,it is necessary to ensure that the outlet of the helically-coiled tube is in a supercooled state.Under non ocean conditions,the single-phase convective heat transfer characteristics should be studied by changing the heat flux density and mass flow rate.The results indicate that both heat flux density and mass flow rate affect the single-phase convective heat transfer characteristics,and the single-phase convective heat transfer coefficient increases with the increase of heat flux density and mass flow rate.Under ocean conditions,the fluctuation of small acceleration has little effect on the single-phase convective heat transfer coefficient.Under rolling conditions,the single-phase convective heat transfer coefficient will experience periodic fluctuations,and the fluctuation period is called the rolling period.There is no significant difference in the average heat transfer coefficient of single relative flow between non ocean and ocean conditions.The experimental results were compared with two empirical formulas for single-phase convective heat transfer coefficients in helicallycoiled tube,and the deviation results were both less than 20%.These two empirical formulas can be used to predict the experimental results.And compared to straight pipes,the heat transfer coefficients of these two empirical relationships are both greater than those of straight pipes.(2)The variation of the starting point position of subcooled boiling in a helically-coiled tube under ocean and non ocean conditions were studied.Exploring the changes in the location of the onset point of subcooled boiling and the size of heat transfer coefficient by changing heat flux density,mass flow rate,and ocean conditions.The results indicate that under non ocean conditions,the higher the heat flux density,the closer the distance between the boiling start point and the inlet of the helically-coiled tube,and the greater the heat transfer coefficient of the subcooled boiling start point.The larger the mass flow rate,the farther the starting point of subcooled boiling is from the inlet of the helically-coiled tube,and the greater the heat transfer coefficient of the starting point of subcooled boiling.Under ocean conditions,the fluctuation of small acceleration does not affect the position of the initial point of subcooled boiling and the heat transfer coefficient.The swing condition has little effect on the position of the starting point of subcooled boiling,but it can enhance the heat transfer coefficient of the starting point of subcooled boiling.(3)The variation law of critical void fraction and heat transfer coefficient at the dryout point inside a helically-coiled tube under ocean conditions was studied.The results indicate that the influence of ocean conditions on the critical gas content at the dryout point is not significant,and the swing motion increases the critical gas content by an average of about 0.019%.Compare the experimental results of the critical gas content at the dryout point under static and ocean conditions with the relationship equation of the critical gas content at the dryout point inside the helically-coiled tube,and the maximum deviation is about 9.396%.This relationship can be used to predict the results of this experiment.(4)The characteristics of saturated two-phase boiling heat transfer in helically-coiled tube under ocean and non ocean conditions were studied.On the premise of ensuring that the helically-coiled tube outlet is in a saturated two-phase state,the changes in local and average heat transfer coefficients in the saturated two-phase boiling zone are explored by changing the heat flux density,mass flow rate,and ocean conditions.The results indicate that within a certain range of heat flux density,the higher the heat flux density,the greater the local and average heat transfer coefficients.The influence of mass flow rate on local and average heat transfer coefficients is not significant.The fluctuation motion of small fluctuation acceleration has little impact on local and average heat transfer coefficients.The fluctuation motion with relatively large fluctuation acceleration,swing motion,and coupled motion increase the local and average heat transfer coefficients,enhancing the heat transfer capacity of the saturated two-phase boiling zone.Compare the experimental values of local heat transfer coefficients in the twophase region with the calculated values of the relationship equation under static state.It was found that when the void fraction is greater than 0.5,the deviation between most experimental values and calculated values is less than 20%.Therefore,this saturated two-phase boiling heat transfer relationship can effectively predict the experimental results in the high void fraction region.