Natural circulation served as an indispensable part of nuclear, attracted much more attentions in recent years. It does not need a pump to provide power. The operating principle of natural circulation caused its complexity in analysis process. It was still a difficult issue to reveal the law of natural circulation accurately. Many experiments and calculations had to be conducted to study the basic physical regulation. This paper concentrated upon the heat transfer characteristics in the test section with two different types of heat flux distribution. The two types of heating flux distribution in the test section were linear and chopped cosine along axial direction. Based on a natural circulation experimental facility, physical models and mathematic models were established. RELAP5 code was used to calculate the thermal hydraulic state of natural circulation loop. The variation of heat transfer coefficient along flow direction was different. It was tightly related to heat flux. Some relevant experiments were conducted in many different conditions and steady sate experimental data were achieved to verified theoretical calculations. Experimental data, such as water temperature, wall temperature and flow rate were recorded when the system is stable. The heat transfer coefficients were calculated according to the experimental data. The factors that affected the heat transfer characteristics of natural circulation were analyzed by comparing the heat transfer coefficient under different conditions. The heat transfer coefficient was calculated according to the empirical correlations as well.

After a series of analysis, the results indicated heat transfer coefficient had an obvious difference, which influenced ability of natural circulation. Comparing with experimental data, the evaluation of different empirical correlations was conducted in two test sections. Some empirical correlations turned out to be suitable for the estimation of heat transfer in experiment facility. The increase of heat flux could enhance heat transfer process in the two test section under low pressure. Average heat transfer coefficient increased with the decrease of inlet subcooling degree. The system pressure effected the heat transfer characteristics of natural circulation as well. The increase of mass flux would promote heat transfer while the level was different. RELAP5 had a great agreement with experimental data in single phase flow. Natural circulation ability was influenced by the position of average heat source center, which was slightly different in the research objects. The research would lend strong empirical support to the guideline of experiment and subsequence study in natural circulation.

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