Research On The Mechanism And Optimization Of Naturally Stratified Water Storage System With Large Flow Rate And Large Temperature Difference

Naturally stratified technology is the most economical and effective method in the chilled water storage and its core is the design of water diffuser.The better the performance of diffuser is,the higher the storage efficiency of chilled water is,and however,the storage efficiency is significantly affected by the operation flow rate and temperature difference.With the development of science and technology,many scientific experimental facilities come into being.Most of them need to be equipped with the large-scale cooling system because of their high power dissipation,and most of cooling system will choose the mode of water storage in order to save the operation cost and reduce the scale of chillers.While compared with the common air conditioning water storage system,these large-scale cooling systems are often characterized as the extreme operating conditions and strict requirements of cooling temperature.The maximum flow rate of the water storage system in Steady High Magnetic Field Facilities(SHMFF)is more than 1000m3/h and its temperature difference is close to 20℃.The chilled water storage temperature is 6℃ and it is invalid when the temperature is above 7.5℃.Based on this system,this paper will make an intensive study on the naturally stratified water storage method with large flow rate and large temperature difference,and to solve the problem that the storage efficiency of large-scale chilled water storage system with extreme operating conditions is not ideal.SHMFF puts forward to the urgency to improve the chilled water storage capacity,but it is difficult to realize through its original water storage mode.Based on the existing double water storage tanks,it is point out that the problem of insufficient chilled water storage capacity can be fundamentally solved if the naturally stratified method is adopted.The key lies in the design of efficient diffuser which is suitable for the operation conditions of large flow rate and large temperature difference.Based on the conservation of energy,the one-dimensional unsteady differential equation for energy conservation was proposed for a naturally stratified water storage tank.By establishing a discrete thermal conduction equation and combining the actual operation conditions of the system,it is pointed out that the heat conduction is not the most important factor to cause the thermocline while the convection doping related to the design of the diffuser is the primary factor affecting the thickness of thermocline.An optimization research was made on the scheme of multi ring octagonal diffuser with nozzles which was early used to verify the feasibility of naturally stratified method for the chilled water storage tank of SHMFF.Through the numerical analysis,the better diffusion angle and outlet mode of the nozzle were given.Considering the optimization measures cannot be verified in the actual water storage device,a performance test platform of diffuser was designed and installed.Based on the platform,the differences between octagonal diffuser and radial diffuser are analyzed.The results show that the matching of disc diameter and slit height has a significant effect on the stratification effect of radial diffuser at the same outlet velocity,and the radial water distributor with large Reynolds number can also achieve better stratification effect.In addition,at the same outlet velocity and under a certain tolerance,the storage efficiency of octagonal diffuser is better,but the difference between these two kinds of diffusers is not significant.Furthermore,through the experimental analysis,it was pointed out that the octagonal diffuser can obtain the ideal storage efficiency by equipping the uniform flow orifice plate.However,the use of nozzles should consider the matching of water pump and the operability of overhaul because of a large number of nozzles are inconvenient to maintain and will also cause flow resistance.Based on the operation characteristics of large scientific facilities,the design scheme of multi ring octagonal diffuser with uniform flow orifice plate was determined for the buffer tank.The performance comparison between new and old diffuser schemes and the experimental research on the influence of flow rate and temperature difference on the storage efficiency were carried out.It was verified that the storage efficiency of the device can reach to 88%under extreme conditions and its performance is obviously better than the original diffuser scheme.It was suggested that the diffuser design for the large-scale water storage device should not only consider high storage efficiency,but also be simple in structure and easy to maintain.Based on the scheme of octagonal diffuser and uniform flow orifice plate,the simulation and optimization of the outlet mode of water distribution pipe and the opening parameters of orifice plate were carried out.According to the numerical simulations on the different volume water storage tanks,the comprehensive analysis shows that,the diffusion performance of circular opening is better than slit opening,the thickness of thermocline is thinner when the opening ratio of the orifice plate is 20%than the case of 30%and 45%,and the hole diameter should not be too small under the same orifice ratio.Combined with the above research results,the diffuser scheme for the future tank was proposed,and the feasibility of the scheme was verified by numerical simulations.