Investigation On Solidification Characteristics Of Supercooled Sodium Acetate Trihydrate Induced By Mechanical Vibration

The use of inorganic hydrated salt supercooled energy storage is an effective means to solve the instability and intermittentness of renewable energy such as solar energy,and is of great significance to reducing the use of fossil energy and improving energy utilization.In this paper,the mechanical vibration trigger method is used to conduct experiments and simulations on the characteristics of the hydrated salt phase change material Sodium acetate trihydrate(SAT).Experiments are performed on the effect of mechanical vibration with two kinds of vibration devices on inducing the solidification and release of heat of the stable supercooled SAT solution in the rounded rectangular heat storage unit.First,a steel ball was used to knock different positions of the unit to summarize the rule of induction time and form the induction time map.Second,electromagnetic vibration device was used to investigate the change law of induction time under different vibration frequencies and input voltages.and the action intensity of the two vibration devices was tested and analyzed.The results showed that the 17 knock positions can be divided into three groups according to the induction time map.i.e.the vicinity of the sealing cover,four edges and the center of the unit,with the corresponding average induction time being 10.2s,30s and 50s,respectively.In particular,the induction time for positions between the sealing cover and the edges are shortest,which are both within 10s.With the strength test,the vibration acceleration and displacement of the present electromagnetic vibration device at the same position on the unit surface at the frequency of 1Hz are about 1/4 that of the steel ball.As the electromagnetic vibration frequency increases within 0?20Hz,the induction the decreases monotonically and the rate of change shows a decreasing trend as a whole.Also.20Hz is the better trigger frequency with the induction time of only 33.3s.The vibration power is increased for higher input voltage and then the input energy per unit time is larger with shorter induction period.The induction time for an input voltage of 36V is about 1/3 that of 12V.Further,based on the AUTODYN finite element analysis software,the pressure,density and velocity changes of the supercooled SAT solution in the heat storage unit after the steel ball knocking are simulated.The results showed that positive and negative pressure fluctuations appear in the solution after being hit by a steel ball.After a period of time,the amplitude of the positive pressure peaks,and then the decrease tends to be stable.The density change is basically proportional to the pressure change,while the vertical velocity change presents an oscillating increase trend.The maximum velocity at the peak pressure is 11.667 m/s.According to the experiment,the critical height for the successful triggering of the steel ball is between 0.4 and 0.5 m.When the height is 0.45 m,the simulated peak pressure of the solution at the center of unit is 5.322 kbar,which is basically consistent with the theoretical calculation result of 5 kbar,which can be used as critical pressure condition to trigger nucleation of solution.The simulated peak pressure of the solution at the edges and corners of the unit is much smaller than the center position and the smallest at the corners,which is 7.675×104 kPa.It can be inferred that the heterogeneous nucleation at the unit corners due to the interface structure is dominant,which is the main reason for the corner effect of the heat storage unit.