Mechanism Analysis Of Heat Transfer And Equipment Design For Soil Steam-injecting Disinfection

In recent years,the problem of soil continuous cropping obstacles has become increasingly prominent in China and soil-borne diseases are an important cause of soil continuous cropping obstacles.Soil steam-injecting disinfection technology is one of the effective means to alleviate soil-borne diseases.A large amount of clay in the agricultural planting in the Yangtze River Delta region,due to the small pores and weak permeability of the clay,steam diffusion is difficult when the soil is disinfected and heated by the steam-injecting steam disinfection machine.Thus,the soil temperature distribution between the disinfection pipes is uneven,which leads to the problem of excessive energy consumption in the disinfection system.Therefore,this paper takes clay as the object,uses CFD numerical simulation technology and soil steam disinfection test to study the heat transfer mechanism of soil steam-injecting disinfection.Through fine rotary tillage,the steam diffusion and heating range can be improved and the effective disinfection area can be enlarged.The uniformity of temperature distribution can be Improved by changing the distance between the steam pipe spacing.Based on the study of the heat transfer mechanism,a soil rotary tillage steam disinfection machine suitable for clay soil was designed.The main research contents and conclusions were:(1)The diffusion form of the single-pipe injection steam heat flow affects the distribution of the soil temperature field and the effective range of the disinfection area.Through the use of CFD numerical simulation technology,the single pipe injection steam heat flow diffusion law under different soil aggregate sizes was simulated and analyzed,and the soil particle size<2mm was set as the control group.Studies have shown that under the same steam flow and disinfection time,with the increase of soil aggregate size and pore size,the steam diffusion form gradually changes from small pore matrix flow to non-uniform large pore steam flow with elliptical shape(horizontal short radius/vertical long radius=0.65)and irregular pattern.The soil high temperature area(above 80?)was transported from the lower middle layer(-10?-20cm)to the soil surface layer(0?-5cm),and the deep soil heating rate is gradually lower than the steam heat transfer rate.When the vertical distance was 0?-20cm,the horizontal distance of the disinfection effective area was changed to 13cm,and the heat transfer efficiency of steam in this area was the highest.(2)The test investigated the effect of different soil aggregates sizes(2?7mm,7?17mm,17?27mm and>27mm)on the steam heat flux diffusion range,soil temperature and water content changes under single pipe steam disinfection condition,and the soil particle size of<2mm was set as control group.The test results showed that under the same steam flow and disinfection time,as the soil aggregate size increased,the soil pores also increased gradually.A large amount of steam will directly diffused along the macropores and accumulated on the surface of the soil to form a "superheated" region.The diffusion and heat transfer rate of steam in the vertical direction was greater than that of the horizontal direction.As the soil aggregate size decreased,the soil pore flow channel became fine,and the steam mainly diffused uniformly around the soil in the form of matrix flow.The horizontal diffusion distance was enhanced,the temperature distribution become gradually uniformized and the scope of effective disinfection area steadily increased.The soil high-temperature area of the>27mm and 17?27mm treatment group has an irregular shape,which is mainly concentrated in the horizontal direction of 0?10cm,and the distance of vertical direction is 0?-20cm.The soil high-temperature area of the<2mm treatment group is 0?-20cm in the vertical direction and the horizontal direction is up to 12cm.The high-temperature area is basically 1/4 elliptical(horizontal short radius/vertical long radius=0.6),which is basically consistent with the soil temperature field simulation results.(3)In order to improve the uniformity of soil temperature distribution between pipes,the effects of different pipe spacings(12,18,24 and 30 cm)on heat flow diffusion range,soil temperature and water content were studied.The test results showed that under the same steam flow,as the steam pipe spacing decreased gradually,the variation range of soil temperature and water content gradually decreased in the horizontal direction and gradually increased in the vertical direction,indicated the uniformity of temperature distribution gradually increased in the horizontal direction and gradually decreased in the vertical direction.The high temperature and high water content areas of the soil surface became smaller and the area with the temperature and water content of the deep soil became higher.The soil surface treated with a pipe spacing of 24 and 30 cm and the deep soil treated with a pipe spacing of 12 cm may exhibit "overheating" and the soil temperature distribution was uneven.The relative unit energy consumption and disinfection time ratio indicates that the relative unit energy consumption value of the treatment at pipe spacings of 30,24 and 12 cm were higher than 2.18 kJ/(kg·?),and these of the disinfection time ratio were lower than 0.8.Compared with other pipe spacing treatment groups,the soil temperature distribution with the pipe spacing of 18 cm was the most uniform(Cv<0.4),the system energy consumption was the smallest(Wt=2 kJ/(kg·?)),and the steam utilization efficiency was the highest(?=0.85).(4)Based on the research of the steam-injecting disinfection mechanism,a soil rotary tillage steam disinfection machine was designed.The machine mainly includes a steam generating device,a power device,a control system,a rotary tilling ridge device and a disinfecting film covering device,which can complete soil rotary tillage,ridge shaping,soil disinfection and film covering operations at the working time.To effectively verify the effectiveness of soil heating and sterilization temperature after disinfection,soil rotary tillage,pipe spacing and film processing field experiments were carried out respectively,and the soil heating curve,average temperature and fungi killing rate are analyzed.The field verification test results showed that after the soil was finely ground and the pipe spacings were optimized(pipe spacing=18cm),the high-temperature zone is mainly concentrated in the middle and lower layers of the soil.The soil in the disinfection zone can fully utilize the heat of the steam to rapidly heat up.Disinfection for 6 minutes,the average temperature of the soil could reach 84.2? After 30 minutes of disinfection,the average temperature of the soil treated by the film sheet method of the optimum treatment group could reach 60?,and the soil heat storage capacity was enhanced to meet the requirements of soil steam disinfection temperature.