Experimental Study On Flow Resistance Properties Of High Temperature Molten Salt Loop

Molten salt reactor loop system,mainly composed of circulating pump,molten salt heat exchanger and pipeline,is the key facility to transfer the nuclear heat in reactor core.In order to ensure that the molten salt pump works near the highest efficiency point when the loop is running,the calculated value of loop resistance should be consistent with the rated head of pump as far as possible.Since the calculation formula of pressure drop for heat exchanger and pipelines are given on basis of hydraulic experiments via water media,whether they are suitable in molten salt condition or not is indistinct.Therefore,the evaluation of those formula's applicability is carried out in the FLiNaK molten salt test loop under variable flow conditions.In order to meet the needs of loop resistance regulation,a set of molten salt control valve was developed and a high temperature molten salt test bench was built for testing as well.Finally,the variation curves of flow coefficient and flow resistance coefficient of valve with different opening degrees and the flow head curve of pump in molten salt medium were given.It was also found that those curves of valve and pump got in water media can be applied in molten salt condition,and the error range of those curves was also determined.The specific research contents are as follows,Firstly,the upgrade of FLiNaK molten salt test loop is completed and the pressure drop of the loop is recalculated subsequently.By adjusting the frequency of the circulating pump,the resistance characteristics of the system under different flow rates were studied.In the experimental range of 19.3-24.3m~3/h,sum of the calculated value of pressure drop of electric heater and that of the pipe section from heater outlet to pump inlet is deviated from the measured value of pressure gauge with the degree of±5%.Therefore,the Esso method for calculating the shell layer pressure drop of shell and tube heat exchanger,in addition with the formula for calculating the resistance along the pipeline and the one for calculating local resistance are proved to be suitable for molten salt medium.As limited by the experimental conditions,the head value of the pump in this loop cannot be directly measured,so the flow head curve of the pump tested in water condition is compared with the calculated pressure drop of the loop,which shows that the deviation between them is within±28%.Secondly,an angle plunger type control valve is developed.CFD method is used to simulate the flow characteristics of the valve under 20%,40%,60%,80%and 100%relative opening degree,and the corresponding flow coefficient and flow resistance coefficient are given.The flow coefficient is about 444m~3/h when the valve is fully opened.Through a water test bench,the relationship curves between the flow coefficient,flow resistance coefficient and relative opening of the control valve in water medium are obtained.When the valve is fully opened,the flow coefficient is about485m~3/h.Compared with the simulated results in molten salt medium,it is found that the variation trend of flow coefficient and flow resistance coefficient with relative opening of valve is highly consistent,which proves the accuracy of CFD method.When the relative opening is not less than 20%,the test results of flow coefficient are greater than the simulation ones,and the maximum deviation between them is about 14%.Since the flow resistance coefficient is proportional to the reciprocal of the square of the flow coefficient,the test results of the flow resistance coefficient are smaller than the simulation ones,and the deviation between them are expanded to within-24%.Finally,the hydraulic characteristics of the control valve and molten salt pump were tested by a molten salt test bench.By adjusting the valve opening and changing the pump frequency,a series of operating points with different flow rates and resistance are obtained.The flow characteristic curves of the control valve with different opening are obtained,which are in good agreement with the CFD simulation results and those ones measured in water condition.When the relative opening of valve is changed from20%to 80%,the test results of flow coefficient and flow resistance coefficient are between the results measured in water condition and the simulatd ones.With regard to the flow coefficient,the deviation between the results tested in molten salt medium and the simulation ones is less than 6%,while the deviation between them and the results measured in water medium is less than-8%.The flow resistance coefficient test results are also between the simulation results and the ones measured in water medium,but the deviation is relatively larger.The deviation from the measured results in water is less than 16%,and the deviation from the simulation results is less than-11%.On the other hand,this experiment also confirmed that the pump flow head curve measured in water medium is still suitable for high temperature molten salt condition,although it will become a bit flatter.In the frequency range of 25-50Hz,the error range between them is about±17%.What's more,when the flow rate is relatively small,it shows negative deviation,otherwise it shows positive deviation when the flow rate is relatively large.