Experimental Facility Design And Experimental Study On Flow And Heat Transfer Characteristics In A Narrow Rectangular Channel Of Plate-fuel Elements

Since the 1960s,experimental research on flow heat transfer in narrow rectangular channels has been conducted.In terms of nuclear energy utilization,it has gradually transformed from large-scale nuclear power plants to miniaturized equipment.In some cases,it is not feasible to build large nuclear power plants,such as submarines,large mobile equipment power supply,island power supply,etc.It is considered to change the shape of the core fuel assembly and the size of the coolant channels.The core consists of a plate fuel assembly and narrow rectangular coolant channels,such as NP engineering reactor,Japan Research Reactor No.3,etc.The size effect enhances heat transfer in the narrow rectangular channel.The coolant always has a complex flow regime and heat transfer phenomenon in a narrow rectangular channel due to size effect.In this paper,based on NP engineering reactor and the requirements of experiments,a set of thermal hydraulic experimental loops was established.The experimental system can achieve the system pressure of 1.0 MPa,the temperature can reach 200 °C,and the loop volume flow rate is up to 1.7 m3/h.The experimental loops had many functions of pressure control,flow regulation,heating and condensation.According to real reactor operational parameters and experimental requirements,the flow direction was designed to be downward and test section was heated on both sides.The test section with gap size of 2.3 mm,channel width of 67 mm(the heated width of 62 mm)and channel length of 1000mm(the heated length of 750 mm)was adopted for the experiment.The isothermal forced circulation experiment was carried out on the vertical downward flow resistance characteristics.The transition of the Reynolds number(Re)was obtained,Re=3000.Compared with the pipe,the critical Reynolds number was larger,the transition region disappeared,and then the data was processed to obtain the empirical equations for laminar region and turbulent flow region.Compared with the existing empirical relationship,it is found that in the region of Re≤1600,the experimental data is in good agreement with the calculation formula of the laminar flow analytical method.In the region of 2300(Re<3000,the experimental data and the Kays formula,Majian’s formula is quite consistent.In the Re>3000 region,due to the influence of the aspect ratio on the friction coefficient,the test value is smaller than the calculated value of the existing empirical relationship.For the single-phase forced convection heat transfer characteristics in a vertically narrow rectangular channel,a series of single variable experiments were carried out with different heating power,different inlet temperature,and different volume flow rates.The trend of forced convective heat transfer coefficient was as follows:Under vertical downward flow and double-sided heating conditions,the improved heating power,the increase of the inlet temperature and the increase of volume flow rate would make the heat transfer coefficient bigger.Then the existing convective heat transfer Nu calculation relation is evaluated.It is found that the calculation results of Dittus-Boelter relation and Gnielinski relation can predict the heat transfer coefficient well,and the average relative error is±11%.Finally,the two-phase instability characteristics were studied.The experimental results show that the early behavior(generation,combination,slip,etc.)of the bubble would make the pressure drop produce a short-term fluctuation and then recover relatively stable.As the heat flux density increases,the pressure drop oscillation amplitude increases.For the influence of the inlet temperature,the increase of the inlet temperature causes the pressure drop oscillation to occur in advance,the oscillation amplitude to increase,and the flow rate to fluctuate periodically.For the volume flow,the larger the volumetric flow rate,the higher the heat flux density when the pressure drop oscillation occurs.Through the visualization,increasing the volume flow can reduce the bubble oscillation displacement in the channel and enhance the ability of the mainstream to take away the bubble during the downward flow.In this paper,the single-phase and two-phase flow heat transfer characteristics under vertical downward flow and double-sided heating in a narrow rectangular channel are studied,and the characteristics of pressure drop heat transfer are obtained.The experimental results could enrich the research data of coolant in narrow rectangular channels,and provide the foundation for the optimization of thermal hydraulics in the NP reactor.