| Frost formation is a very common phenomenon in refrigeration and cryogenic engineering,wind power generation,aerospace and many other fields,which often leads to the reduction of equipment's operation efficiency and safety.Therefore,it is of great significance to develop efficient defrosting and frost suppression methods both in scientific research and engineering application.The methods of defrosting have been developed to a certain extent up to now,but the problems of high energy consumption,temperature fluctuation and defrosting droplet retention caused by traditional thermal defrosting need to be solved.Because of cavitation effect and surface capillary wave,ultrasonic can cause liquid atomization.In addition,when the ultrasonic wave passes through the medium,huge acoustic pressure and interface stress could be produced,which means it is possible to remove the adherent on the interface.Therefore,ultrasonic has great potential in defrosting droplets and frost layer removing.Existing researches on ultrasonic defrosting often ignore the removal of defrosting droplets as well as lack of visualization and quantitative research of defrosting removing.In this paper,the law of surface defrosting droplets and frost layer removing under ultrasonic excitation was studied theoretically and experimentally.Firstly,the equivalent stress distribution on the surface of plate under ultrasonic excitation is analyzed theoretically and numerically based on ANSYS Workbench,and the visual observation and quantitative analysis of defrosting droplets and frost layer removing under ultrasonic excitation were carried out in combination with the characteristics of equivalent stress distribution.Distribution of surface equivalent stress is greatly effected by the frequency of ultrasonic excitation and the structure of plate.Equivalent stress on the surface of circular plate is distributed in annulus;on rectangular and square plates,the distribution are more complex but centrosymmetric or axisymmetric in geometry;the distribution of surface equivalent stress is independent of amplitude,but the value of equivalent stress at each point is proportional to amplitude;equivalent stress distribution changes with frequency,and the frequency response characteristics of plates with different shapes are different.Atomization characteristics of droplets are mainly affected by the magnitude and trend of the equivalent stress on plate surface,and droplets always spread towards the direction with larger stress under ultrasonic excitation;in the area with larger surface equivalent stress,the droplets are finally atomized completely;if the micro-droplets ejected from the plate fall back to the area with smaller stress,they will gather and finally retain on the surface;The distribution of retained liquid droplets on the surface of the plate in the later stage of atomization has a greater relationship with the distribution of surface equivalent stress.The plate surface can be divided into atomizing regions and non-atomizing regions according to the retention of droplets after ultrasonic excitation,and these two kinds of regions correspond to the peak area and valley area of surface equivalent stress respectively.Three kinds of droplets behavior were observed:in atomizing regions,droplets uniformly spread around and atomize rapidly.Adjacent to these regions,droplets will spread and move toward the nearby atomizing region.When a droplet partially spread into atomizing region,atomization will occur and the rest of the droplet will continuously replenish this region under the action of surface tension.Droplets in non-atomizing regions do not exhibit atomization or migration but only slight deformation if their size are small,part of droplets may coalesce due to disturbance.In the experiment of defrosting droplets removing,if the frosting time is more than 30 minutes,the average diameter of droplets decreased and the distribution range of droplet size narrowed after ultrasonic excitation.However,for the case of 15 min,the maximum and average diameter of droplets increased after ultrasonic excitation due to the coalescence of droplets in non-atomizing regions.No obvious regularity relationship between droplet removal ratio and ultrasonic power or frosting time was found due to the randomness of the location of large droplets formed during frost layer melting.However,when the frosting time is more than 30 min,the removal ratio in each group reaches more than 75%,which means a efficient removal performance even at low power.Thermal effect caused by short-time and low-power ultrasonic excitation is very limited,and the impact of temperature rise on droplet atomization behavior and surrounding environment temperature can be ignored.In the process of ultrasonic defrosting,the surface of frost layer showed a fluctuation consistent with the surface equivalent stress distribution,which means that defrosting behavior are also affected by equivalent stress.Frost layer in regions with larger stress swollen and detached first while which in regions with smaller stress may be retained on the plate;Frost crystal broke at the root and left the plate yet the base of the frost layer could not be removed by ultrasonic because of large adhesion.Prolonging the duration of ultrasonic excitation can improve the efficiency of defrosting,and longer the frosting time is,more obvious this trend is.The increase of ultrasonic power can also effectively improve the removal ratio of frost layer,but under the same defrosting effect,the economy of which was much inferior to the extension of ultrasonic excitation time.With the increase of frosting time,?_c(ratio of frost coverage reduction)decreases while m_d(defrosting quality)and m_r(retained frost quality)increase under the same excitation time and power.When the frost crystal above the base can not be completely removed by ultrasonic excitation for 3 s,?_m (quality removal ratio of frost)maintained at about 45%,which didn't change significantly with the frosting time;when the excitation time reached 5 s and was enough to completely remove the frost crystal above the base,?_m increased with the increase of frosting time. |
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