| In high altitude,or high latitude or low temperature areas,water or water vapor in the environment easily adheres to material surface and freezes into ice.The phenomenon of icing is very common.And ice adhesion is very common in engineering field,but it brings many hazards,such as increasing the load on the part and changing the topography and surface characteristics of the component.It affects the safety,reliability and efficiency of the operational equipment,which brings serious harm to life and production.The de-icing method in engineering can be divided into three categories: mechanical,heating and chemical.And these methods have many shortcomings during the actrual use,such as high cost and energy consumption,environmental pollution.Anti-icing coating is one of focuses in developing the anti-icing technology.However,many scholars have found that anti-icing coatings have many disadvantages,such as poor durability,environmental pollution and poor adhesion between materials.It demonstates that the anti-icing coating cannot be applied into the engineering.So,a new type of anti-icing method is urgently needed in engineering.In order to develop a new anti-icing method and understand the freezing process and the formation law of water,a multi-dimensional microscopic observation device was built to observe the freezing process of quantitative water on cold surface.The freezing parameters were collected during the freezing process,such as the inner temperature of water droplet,height,volume and cross-sectional diameters at different heights.In experiment,the inner temperature would increase suddenly during the freezing process,accompanied by the increase of volumes.The freezing process of water could be divided into three stages: wetting and spreading stage,icing stage and stabilizing stage.Combined with the “frost wedging” phenomenon in cold area,the test apparatus was established to measure the axial swelling force during the freezing process of water.The swelling force generated by freezing water in the container with a diameter of 30 mm and a depth of 3.5 mm was measured at-25 ?.The swelling force reached 46.38 N.The experiment showed that the elastic boundary at the top of the container would be subjected to swelling force load during a short period,and the expansion load could be divided into non-load stage,expansion load increase and decrease,and stability stage.In fact,ice adhesion on the surface of material is a special interface science during low temperature environment.Compared with the conventional anti-icing method which used additional power to remove the accumulated ice on the material surface,this paper adopted the swelling force during the freezing process to disturb the stability of the interface between ice and material,and achieved the purpose of reducing the ice adhesion strength.An active anti-icing model based on phase change time dfference was proposed.It meant that the pit for filling the aqueous solution with low freezing point was processed on the material surface,and the model took an elastic film as the ice-solid interface.At low temperature,the water on the material surface would freeze into ice firstly,and forms a stable adhesion interface with the material,followed by freezing of the aqueous solution in the pit.So,the swelling force generated by the freezing solution in the pit was used to impact the ice-solid adhesion interface.In order to eliminate the influence of thermal conductivity on this anti-icing model,the test employed 6061 aluminum alloy and polymethyl methacrylate(PMMA)with thermal conductivities of 237 W/m·K and 0.2 W/m·K respectively,to represent materials with good and poor thermal conductivities.The experimental model were prepared and frozen for 1 h at-25 ?,and ice adhesion strength were respectively measured on the surface of model substrate and conventional sample by using self-built device.The experimental results shown the ice adhesion strength on samples with the solution-filled were substantially lower than that in smooth specimen;in particular,ice adhesion strength of sample PMMA filled with water was 0 N,with 100% ice adhesion strength reduction rate.And ice adhesion strength on aluminum alloy surface filled with 10% ethanol solution was reduced by 81.42%.Under the same experimental conditions,ternary quadratic regression orthogonal experiment method was adopted to design the experiment conditions,as the mass concentration of etanol solution and the size of the pits,and the mathematic model was set up to analyze the relationship between the experimental factors and the ice adhesion strength.The regression analysis method was used to solve the regression equation,and the model parameters with the greatest impact on ice adhesion strength were determined.The optimal structural samples were prepared.Under the same conditions,the optimal verification test was carried out.The active anti-icing model based on phase-change time difference proposed in this paper needed to process the large-sized pit on the substrate surface,which did not meet with engineering application requirements.Based on the study of the active anti-icing model depended on the phase change time difference,this paper proposed another new anti-icing model: the phase change temperature gradient was built on the interface between ice and material.That meant that many pits with small size were manufactured on substrate surface,and solution with different freezing points were filled in the different pits,so a phase change temperature gradient on the interface was set up.The samples were frozen for 1 h at-10 °C,-15 °C,-20 °C,and-25 °C,respectively,and the self-built device was used to measure the ice adhesion strength on the anti-icing model surface and the conventional material surface.The experimental results showed that ice adhesion strength on the substrate with phase change temperature gradient was 0 k Pa compared with the ice adhesion strength on the smooth sample surface,and the reduction rate of ice adhesion strength was 100%.The self-peeling of the accumulated ice on sample surface was realized.In order to investigate the influence of the phase change expansion force generated by water freezing on the stability of the contact surface between ice and material,the phase change swelling force acting on the top elastic boundary during freezing process was collected by using the self-built device.The sweeling force curve was fitted by using MATLAB software,and the expansion force fitting equation was solved.The influence of swelling force on the stress and strain of interface between ice and material was analyzed by adopting the finite element method.Combined the freezing law of water on the cold surface,the swelling force generated by the freezing process of water,the results and the phenomena during the model test,with the finite element simulation,the effect of the swelling force on the stability of the interface and the mechanism of reducing the ice adhesion strength were analyzed and summarized.An active anti-icing theory based on the swelling force produced by the freezing process to disturb the stability of the interface was proposed.The study would provide a multi-functional active anti-icing solution for engineering field with low cost,low energy consumption and non-pollution. |
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