Study On Cold Storage Characteristics Of Multi Walled Carbon Nanotubes/Water Nanofluids

Cold storage technology is used in most parts of China widely,storing cold energy in the trough of electricity consumption,releasing the cooling capacity at the peak of electricity consumption to reduce the power consumption during the peak period of electricity consumption,realizing the peak shift of power use and filling valleys,regulating the imbalance of energy supply and demand,the main improvement direction of phase change cold storage technology is to optimize the cold storage characteristics of cold storage materials.Water is one of the main materials of the cold storage system,because water has a high cold storage density,stable performance,phase change process approximate constant temperature,water in the icing process with the ice thicker and thicker,will be more and more dependent on heat conduction,and its own often has a large degree of subcooling,in practical applications will be limited greatly.In the current research,there are few studies on optimizing the cold storage characteristics of water with carbon nanotubes as thermal conductive enhancers and nucleating agents and ultrasonic assistance,and it is necessary to strengthen the study of the cold storage characteristics of carbon nanotubes/water nanofluids with different concentrations and specifications and ultrasonic nanofluids after introducing different sound intensities.In this paper,three different diameters of 5-12 nm,10-20 nm,and20-30 nm were added to water to prepare 7 nanofluidic samples.The cold storage test bench was set up to carry out cold storage experiments on different samples,after that different frequencies of ultrasonic sound were placed to explore the change law and characteristics of cold storage characteristics,the COMSOL numerical simulation software was used to explore the temperature distribution uniformity,temperature and phase changes of ionized water at different initial temperatures and cooling temperatures,and the main conclusions were summarized as follows:(1)At the beginning of the cold storage process,in the height direction,the temperature in the middle of the fluid decreases the fastest,while the temperature at the top decreases the slowest.Point B,located at the middle height of the nanofluid,cools down the fastest and has the strongest heat transfer ability.After the nanofluid freezes,the nanofluid at the bottom cools down the fastest and has the best heat transfer force.The subcooling of the nanofluid gradually increases from the bottom to the top;Multi walled carbon nanotubes can increase the heat transfer capacity of deionized water;The addition of multi walled carbon nanotubes can improve the thermal conductivity of ice.As the concentration increases,the improvement effect becomes better.However,In the late stage of icing,the improvement effect is not significant;A small amount of multi walled carbon nanotubes can effectively improve the uneven distribution of undercooling in various parts of deionized water;As the concentration increases,the undercooling of the nanofluid gradually decreases,the undercooling rate increases,the completion time of the B-point phase transition shortens,and the nucleation time advances.The nanofluid with a concentration of 0.05% reduced the average undercooling of deionized water by 32.5%,increased the average undercooling rate by 12.5%,shortened the phase transition completion time by 13%,and shortened the nucleation time by21.2%.Moreover,when the concentration increased from 0.05% to 0.2%,the average undercooling decreased by 23%,the undercooling rate increased by 33.3%,the phase transition completion time shortened by 5%,and the nucleation time shortened by 3.8%.As the diameter of multi walled carbon nanotubes decreases,the heat transfer capacity of the nanofluid increases,the undercooling decreases,and the distribution of undercooling in height becomes more uniform.A small pipe diameter can affect the heat transfer ability of nanofluids in both liquid and solid states.(2)The addition of ultrasonic sound intensity has an enhanced heat transfer effect on multi walled carbon nanotubes/water nanofluids without ultrasonic sound intensity.As the level of ultrasonic sound intensity increases,the enhanced heat transfer ability of ultrasound on nanofluids during the fluid cooling stage increases by 0.5 times;The undercooling decreases with the increase of ultrasound intensity level.Compared to the absence of ultrasound field,the undercooling of multi walled carbon nanotubes/water nanofluids with added first level ultrasound intensity decreases by 1.2 ℃,while at fourth level ultrasound intensity,the undercooling of nanofluids decreases by 3.2 ℃ compared to the absence of ultrasound;As the level of ultrasound intensity increases,the supercooling rate of the nanofluid increases,and the amount of supercooling rate increase gradually decreases.Compared to the nanofluid without ultrasound field,adding a level 1 ultrasound intensity increases the supercooling rate by 0.0064 K/s,an increase of 27.3%.When the ultrasound intensity increases from level 2 to level 4,the supercooling rate only increases by 2%.;When the ultrasonic intensity level is between 0 and 2 levels,the nucleation time of multi walled carbon nanotubes/water nanofluids is significantly shortened,with a reduction of 2.8 minutes,which is 32.2% lower than when the ultrasonic intensity level is 0 level.However,when the ultrasonic intensity level is raised from 2 to 4 levels,the nucleation time of the nanofluids is shortened by 0.1 minutes,and the nucleation time is reduced by 1.7%.The change in nucleation time is not significant.The effect of different ultrasonic intensities on the completion time of phase transition is not significant,and the relative average completion time of phase transition is only 4.5%(3)Simulation and experimental comparison show that the phase transition time error is22%,which is within the allowable range of error;The uniformity of the temperature distribution of the deionized water will become worse with the decrease of the initial temperature and cooling temperature.The temperature gradient is the largest near the junction of the solid phase and liquid phase of the deionized water,the temperature distribution gap is the largest,and the heat transfer is the fastest;The decrease in initial temperature and cooling temperature will accelerate the heat transfer process,shorten the start and end time of phase change.Lower initial temperature has a promoting effect on the conversion between liquid and solid phases,and lower cooling temperature has a better effect on the conversion between liquid and solid phases.