Design And Experiment On Artificial Radiation-frost Chamber Based On Temperature Difference Between Leaves And Air Dew Point

For some economic plants sensitive to low temperature, occasional frost freezing could result in its growth obstacle, However It is difficult for field study to control the freezing factors in freezing-related research including low temperature response and anti-frost control. Therefore an artificial freezing chamber is required. The meteorological data and frost formation process during the typical radiation frost night was monitored and analyzed; the model structure and main parameters were confirmed and the leaf-air temperature difference was simulated by CFD method; after finishing the calculation and manufacture, the frost forming performance was tested. Main research and contents are as follows.(1) Observation and analysis of natural frost formation processBased on the radiation frost theory and frost formation condition, the wind speed, solar radiation, tea canopy temperature and humidity were recorded, The frost formation on the leaves was analyzed. the weather of January 26 th was cloudy and calm, February 4th was calm and cloudless, March 1st was cloudless and calm.The result showed that during 16:00-18:00, the variation of solar radiation in observed days were similar, the initial value was about 200 W/m2,and the decreasing ratio was about 90 W/m2 per hour. The decreasing ration of rapid decreasing period was about 3-5℃,the slow period was about 0.4-1 ℃. The variation of the tea canopy humidity is in contrast with the temperature variation, the rapid decreasing ratio was 15%-30% per hour, the slow decreasing ratio was 6%-15% per hour. The biggest temperature difference of leaf and air dew point temperature was-1.2 ℃ at June 26 th,while the fluctuation was big due to the cloud, the desublimation speed of air vapor was less than sublimation speed of frost, so the frost could not form on the leaf. The biggest temperature difference was-2.3 ℃ at February 4th, and the fluctuation was small, the desublimation speed of air vapor was larger than sublimation speed of frost, so the frost could easily form on the leaf. The biggest temperature difference was-1.0 ℃ during the calm period at March 1th, the desublimation speed of air vapor was nearly equal to the sublimation speed of frost, so the frost formation was not obvious.(2) Radiation frost chamber model and main parametersBased on the observation results, the main components including basic cooling unit, radiation sink and separation unit were designed, the main technical parameters including radiation sink temperature and heating power were theoretically estimated to be-30 ℃and 40 W respectively. The CFD simulation showed that leaf-air temperature difference ofestimated value was 1.5 ℃,at which frost could not occur according to the field experiments, thus the heating power and the radiation sink temperature is corrected and resimulated, the simulation result showed that 40 W heating power and-40℃ radiation sink is more reasonable.(3) Manufacture of frost chamberBased on the designed parameters, the basic cooling chamber was selected and improved, the radiation cooling system including heat load calculation, thermodynamics calculation and cooling unit were finished; the circuit was designed. the calculated heat load was 189 W. Selected compressor type was PH60G0C( Rated power is 345 W, cooling ability is 985 W and COP is 2.86), fin and tube type was selected(heat transfer area is 2 m2,maxmium wind speed is 8 m/s).(4) Frost formation research of frost chamberThe performance including radiation sink temperature, temperature and humidity in separation part, temperature and humidity in the basic cooling temperature were tested. The results showed that leaf temperature slowed down after a rapid decrease, which is similar to the natural condition, and the frost formation on the leaf is obvious, but in the first 0.5 hours, the decreasing speed is much larger.The temperature decreasing speed in the chamber is improved to simulate the natural condition. a 2 factor and 3 level experiment is adapted, the leaf temperature variation affected by both basic cooling and radiation is analyzed, the leaf temperature affected by only radiation cooling is analyzed. The results showed that the radiation ability has little relation with basic cooling temperature, and the temperature decreasing speed is similar to natural condition in the first 1 hour. A leaf adjusting schedule was proposed and the results showed that the leaf decreasing speed is similar to the natural condition.