Experimental And Numerical Study On Inner Flow In Dewar Bellows For Superconductor Cable

With the development of industries and expansion of cities,the demand for electricity grows day by day,which calls for a corresponding grow of the transmission capacity of cables.Yet the application of superconducting cable will probably be a useful way to fix this problem.High temperature superconducting materials and cryogenic technologies have been developing very rapidly in recent years,and the application of superconducting cable is on the way.Due to the critical transition temperature of the high superconductor,it needs to be immersed in cryogenic fluid to dissipate heat and therefore to allow itself working properly.The temperature of LN2 just meet the working condition requirement of superconducting cable,and fortunately LN2 is commercially available in low cost and is manipulated safely,which makes it competent for superconducting cable application.Bellows has been widely chosen as the shell tube for superconducting cable to transport LN2 due to its good quality of flexibility,contractility and pressure proof.The pressure loss of LN2 is essential for the design of superconducting cable since it decides the distance between the two cryogenic stations for cryocooler and pump,the power needed for the pump and bearing capacity of the bellows.A system is constructed including LN2 supply,subcooler,test section and measurement module.Experiments are performed on a straight pipe and a bellow,and the inner diameter of which are 8.5mm.Also,a 5-mmdiametered insert is inserted in and tested.And the result shows that when 8kg/h<flowrate<195kg/h and 2300<Re<50000,the pressure loss grows with the flowrate,the rank of pressure loss under the same flowrate is straight pipe<empty bellow<bellow with an insert.The Darcy friction factor goes down as the Reynold number grows up,and the rank of Darcy friction factor under the same Reynold number is straight pipe<bellow with an insert<empty bellow.A correlation was fitted as follow:f=0.04235+0.33517exp(-Re/5750).It can be used in similar situations for the calculation of friction factor.In order to study further and wider on the bellows which match the size of superconducting cables,numerical studies are performed using ANSYS FLUENT to simulate the flow in bellows with different diameter(d),pitch(p)and peak(e).The results shows that when the rate of e/d and p/e stays the same,the changing of diameter hardly has any affection on the Darcy friction factor,when e/d keeps still,the growing of pitch will make the Darcy friction factor grow in low Reynold number section and decrease in high Reynold section.When 10000<Re<80000,the transition flow occurs.As the Reynold number grows,the Darcy friction factor increases at first and decreases if the Reynold number grows further.And inserts with the diameter of 20mm,40mm and 60mm are considered in an 80mm-inner diameter bellow,and found that the pressure drop increases and the Darcy friction factor decrease as the diameter of the insert grows,and the thinner the insert is,the more similar is the inserted bellow comparing to an empty one.To verify the numerical results,an interpolation is performed with the simulation data to get an interpolated curve of the bellow used in the experiment,and the curve fit the experimental data well when Reynold number is high,but not so well when the Reynold number is low.The size of an insert is interpolated in and also fit the experimental data well.