Study On Formation Mechanism Of Liquid Hammer In Refrigeration System And Designing Criteria For Preventing Liquid Hammer Impact Failure

For the prevention of the frequently explosion casualties caused by the liquid hammer occurred in the hot ammonia defrosting process,there are some key scientific and technical problems need to solve,the multi-phase stratified flow liquid hammer formation mechanism theoretical model and the corresponding simulation method were built.The formation mechanism of multi-phase liquid hammer with phase change in the hot ammonia defrosting process was revealed by numerical simulation and experimental study,the key control parameters of the liquid hammer was clarified,and the key techniques and methods to suppress the formation of liquid hammer impact were put forward.Based on the safety assessment of the impact of liquid hammer in gas return manifold,the mechanical mechanism of the frequently explosion casualties caused by the liquid hammer occurred in the hot ammonia defrosting process was expounded.The results of the study laid a scientific theoretical basis and technical support for the establishment of the design criteria for the prevention of frost failure in the process of defrosting.In this paper,the theoretical model of gas-liquid two-phase stratified flow formation mechanism considering the influence of rapid phase change in the hot ammonia defrosting process was established firstly,in which a virtual simulation platform for the formation of liquid hammer in the hot ammonia defrosting process was established.The results showed that the two direct driving forces of liquid hammer in gas return manifold in the hot ammonia defrosting process,the first was the sudden change of the hot ammonia vapor inlet velocity by hot ammonia total valve suddenly open,and the second is rapid phase change induced by the contact of high temperature hot ammonia vapor and cryogenic liquid ammonia.In this paper,the correlation relationship between the pressure of liquid hammer in the hot ammonia defrosting process and rapid phase change,the sudden change of the hot ammonia vapor inlet velocity,the residual filling liquid ammonia volume fraction,the key control parameters of the liquid hammer was clarified.The formation mechanism of multi-phase liquid hammer with phase change occurred in the hot ammonia defrosting process was revealed,and the key techniques and methods to suppress the formation of liquid hammer impact was put forward.An experimental study platform for the formation mechanism of liquid hammer in gas return manifold was presented.The prosecution systems and detection data synchronization acquisition display system of hot ammonia vapor inlet velocity,sudden increase speed,the pressure of liquid hammer and liquid level were proposed.The experimental results of gas return manifold liquid hammer in the hot ammonia defrosting process showed that the sudden change of hot ammonia vapor inlet velocity would lead to a significant liquid hammer phenomenon in gas return manifold,and the sudden change of the opening trend of the electric flow control valve caused by the hot ammonia vapor inlet velocity automatic control system overshoot correction would also lead to liquid hammer.It was difficult to eliminate liquid hammer in gas return manifold by the automatic control of the hot ammonia vapor inlet velocity,and the liquid hammer pressure of the experimental test is consistent with the liquid hammer pressure predicted by numerical simulation according to experimental conditions.The slug flow of the multi-phase flow was caused by unstable fluctuations in the layered interface.The direct driving force of the stratified interface is the difference between the velocity difference of the two-phase flow velocity and the rapid phase change in the layered interface.The comparative analysis of the multi-phase stratified flow liquid hammer with or without phase change show that the maximum liquid hammer pressure induced by multi-phase flow with phase change was four to five times more than that without phase change,and it was 65 times more than normal working pressure.So liquid hammer in the hot ammonia defrosting process easily lead to the brittle fracture explosion accidents of burst pipe and head.The theoretical premise of accurate predicting the multiphase flow liquid hammer pressure is that the theoretical model describing the formation mechanism of the liquid hammer is established in which rapid phase transformation driving factor is comprehensively considered in the hot ammonia defrosting process.