Study On The Nucleation Mechanism Of Graphene Oxide Nanofluids Under Acoustic Levitation

With the increasingly severe energy situation in the world,the ice storage air conditioning system with ice slurry as the heat transfer medium has developed rapidly.Previous liquid-liquid ice slurry production methods have high power consumption and low ice-making efficiency,so they can not be widely used in engineering fields.Traditional phase change materials also have some shortcomings such as high undercooling and low heat transfer efficiency in ice storage process.In order to solve the above problems,a gas-liquid direct contact system was used to produce ice slurry.The system structure was simplified and the environmental pollution was avoided.It could generate greater thermal disturbance and improve energy efficiency in heat transfer.As a new phase change material,nanofluids can effectively suppress the supercooling of ice storage solution and enhance heat transfer.In this paper,the factors affecting heat transfer in gas-liquid direct contact ice storage,the factors affecting supercooling of Nanofluids and the inhibition of supercooling of ice storage solution by nanofluids relative to ethylene glycol will be explored.The specific work is as follows:(1)Designing the system of gas-liquid direct contact ice storage system,completing the design,calculation and manufacture of evaporator,ice storage bucket and dryer.After debugging,the experimental system can run continuously and steadily,laying the foundation for the next experiment,analysis,measurement and data acquisition.(2)Using deionized water as base liquid,nanofluids were prepared by "two-step method".The nanoparticles were evenly dispersed into the base liquid by ultrasonic oscillation,and graphene oxide nanofluids with different concentrations were prepared.In order to avoid the influence of the wall surface on the liquid supercooling,the acoustic suspension system was used to measure the liquid droplet supercooling.The supercooling degree of nanofluids droplets and deionized water with equal volume was measured at the cooling temperature of-20?.The experimental results show that the average supercooling degree of deionized water and nanofluids is 9.9? and 5.5?,respectively.The decrease of supercooling degree of nanofluids is close to 45%.In addition,the step cooling curves of droplets of nanofluids in acoustic suspension were compared with those of deionized water.The results show that graphene oxide nanofluids have a greater inhibition on supercooling.(3)Graphene oxide nanofluids with concentration of 10mg/100 ml,15mg/100 ml and 20mg/100 ml were prepared,and their supercooling degree was measured at-20? cooling temperature.The experimental results show that the mean undercooling of nanofluids droplets at three concentrations is 6.9?,5.5? and 4.8? respectively.The supercooling degree of nanofluids decreases with the increase of concentration,and the rate of decrease slows down gradually.The supercooling degree of nanofluids with concentration of 20mg/100 ml has been measured several times at different cooling temperatures.The results show that the average supercooling degree is 2.4? when the cooling temperature is-10?,3.6? when the cooling temperature is-15?,and 4.8? when the cooling temperature is-20?.The supercooling degree of nanofluids increases with the decrease of cooling temperature.(4)The air ejector with different diameter of nozzle is used to store ice at high intake.From the heat transfer effect,the small-diameter air ejector is slightly better than the large-diameter air ejector,but there is little difference between them,which indicates that the diameter of nozzle has little influence on the heat transfer effect at high intake.Experiments at different liquid level show that the volume heat transfer coefficient varies with the liquid level.High liquid level height can make heat transfer sufficient,and low liquid level will lead to incomplete heat transfer.(5)Experiments were carried out with 5% and 10% ethylene glycol solution and nano-fluid solution respectively.The results show that compared with water and ethylene glycol solution,nano-fluid solution has lower undercooling and better heat transfer effect,and can produce loose and delicate ice slurry.When the concentration of nanofluids is 10%,the supercooling degree is 0.9?.It takes 25 minutes to reach the freezing point,which is almost half the time of ethylene glycol solution.