Thermo-fluidic Characteristics And High-efficiency Generation Of Clathrate Hydrate Slurry

In recent years,the electric power demand is increasing rapidly.Therefore,the shortage of electric power supply frequently appears during peak-load time,meanwhile the surplus of electric power supply appears during off-peak time in most of the power gird of China.Recently,the cold storage is proposed as a solution to the aforementioned issue.Such technology stores the cold energy generated in off-peak time and releases this cold energy in peak load time.Moreover,the secondary refrigerant can be implemented in the cold storage air-conditioning system.Consequently,the charge of enviromently harmful primary refrigerant can be reduced significantly,which is in line with the national sustainble development strategies.Recently,a new kind of pahse change material slurry,clathrate hydrate slurry,is proposed as cold storage material,due to its suitable phase change temperature,5-12?,high cols storeg capacity and great fluidity.However,the related research is still insufficient.The present study mainly investigates the thermo-fluidic characteristics of tetra-n-butyl ammonium bromide(TBAB)clathrate hydrate slurry(CHS)and the high-efficiency generation method of TBAB CHS in cold storega air-conditioning sysytem.The main research can be described as follows:(1)Two kinds of TBAB hydrate crystals can be generated from supercooled TBAB aqueous solution.Therefore,the crystallization characteristics of TBAB hydrate crystals and the related heat transfer charcteristics are investigated here.By measuring the temperature variation of TBAB aqueous solution,the supercooling release is determined and the solid mass fraction of TBAB CHS is estimated.Moreover,the microscope is utilized to observe the morphology of TBAB hydrate crystals.In addition,the pre-produced TBAB CHS is dropped into supercooled TBAB aqueous solution,in order to investigate the influence of nucleus on the supercooling release.(2)The sedimentation of TBAB CHS in the cold storage tank is studied experimentally and numerically.In the experiment,the solid mass fraction at different locations and time is measured by sampling method.In the numerical study,the theoretical model is utilized to describe the sedimentation of different kinds of TBAB hydrate crystals in TBAB CHS,and the numerical resulsts are compread with the experimental results to validate the theoretical model.The experimental and numerical results show that the sedimentation velocity of type A TBAB hydrate crystals is much higher than that of type B hydrate crystals,because of the smaller consistency coefficient and higher solid particle density.Moreover,the sedimentaion velocity is slower at higher solid mass fraction of TBAB CHS.In addition,a heterogeneous high-capacity cold storage strategy is proposed based on the sedimentation characteristics of type A TBAB CHS.(3)The flow melting characteristics of TBAB CHS in heated tubes,i.e.straight circular pipe and 90~o elbow pipe,are numerically investigated.The Euler-Euler multiphase model is employed to describe the flow of TBAB CHS,meanwhile the heat and mass transfer between the solid particle and liquid phase caused by the melting of solid particles are considered.Moreover,the diffusion of TBAB salt in the liquid phase is also numerically calculated.The theoretical model is validated by comparing with the experimental results.The numerical results show that the solid mass fraction near the pipe wall is very small,resulting from the particle-wall collison and the melting of solid particles near the pipe wall.In addition,the temperature distribution of TBAB CHS is influenced by the heterogeneous distribution of the solid partilces.In addition,the centrifugal force induces the secondary flow of TBAB hydrate crystals in the 90~o elbow pipe,and vrortex appears at 45~o of the elbow section because of the boundary layer seperation.?(4)The latent heat of type B TBAB clathrate hydrate crystal is higher than that of type A TBAB clathrate hydrate crystal,and the flow and heat transfer characteristics of type B clathrate hydrate crystal is better.Therefore,the high-efficiency generation method of typa B TBAB CHS from 15.0 wt%TBAB aqueous solution is investigated,and many generation approaches are studied comparatively.In the first method,TBAB aqueous solution is chilled untill TBAB hydrate crystals appear,and then TBAB CHS is chilled continuously.In the second method,the refrigerator is turned off for a while when TBAB hydrate crystals appear.In the third method,the refrigerator is turned off before TBAB hydrate crystals appear,and the supercooling is released by the vibration and shock induced by the agitator,corrugated heat exchanger tube,pumping and the impinging of the aqueous solution.In the fourth method,the refrigerator is turned off before TBAB hydrate crystals appear and drop pre-produced TBAB CHS into TBAB aqueous solution in order to induce the supercooling release.The stability,generation time and system energy coefficiency of the four methods are compared,and it is found that system COP of the second and fourth method are 11.7%~14.5%,16.5%~23%,respectively,higher than that of the traditional method..(5)The solid mass fraction of TBAB CHS generated from 15.0 wt%TBAB aqueous solution is limited,so the generation method of type A TBAB CHS from 40.0 wt%TBAB aqueous solution is investigated.Based on the sedimentation feature of type A TBAB CHS,three different storage stategies,i.e.homogeneous,heterogeneous without filter and heterogeneous with filter are utilized.The system energy efficiency and stability of the generation process with different storage strategies are compared,and the distribution of TBAB CHS inside the storage tank is observed.The experimental result shows that the solid mass fraction of TBAB CHS at outlet of the storage tank can be reduced significantly while using heterogeneous storage strategy,meanwhile the pressure drop of TBAB CHS through the heat exchanger also decreases significantly.Therefore,the heat transfer between TBAB CHS and the primary refrigerant is enhanced.Moreover,the solid particles can be significantly prevented by the filter which is placed inside the heterogeneous storage tank.Thus,the fluid at the outlet of the storage tank is almost liquid phase,resulting in better heat transfer.In addition,the Euler-Euler multiphase model is employed to simulate the flow and heat transfer characteristics of TBAB CHS inside the storage tank,and the numerical result fits well with the experimental result.