| According to poor ventilation effect in summer,poor heat preservation and temperature increasing performance in winter,and small space,which was not conducive to mechanized operation,three kinds of 20 m long-span plastic tunnels were designed and constructed in this study,namely,"10-10" North-South symmetrical plastic tunnel(NS10-10),"13-7" East-West asymmetric plastic tunnel(WE13-7)and "15-5" East-West asymmetric plastic tunnel(WE15-5).In order to explore the thermal environment of three plastic tunnels under high temperature weather in summer and low temperature in winter,continuous monitoring of air temperature and soil temperature and comparative measurement of air temperature and soil temperature under typical weather conditions were carried out.The temperature field and air flow field in three plastic tunnels were simulated by fluent 16.0 software,and the structural stability of three plastic tunnels was analyzed by ANSYS software.The simulation results showed that:(1)The daily average air temperature of the three plastic tunnels under typical summer weather was WE15-5>WE13-7>NS10-10,and the soil temperature also exhibits the same law at the same depth.Under sunny conditions,the average maximum air temperature of NS10-10 was 33.5?,which was 1.3? and 1.5? lower than that of WE13-7 and WE15-5.The air temperature difference between north and south was 0.4?,and the average maximum temperature of soil was 22.4?,which was higher than WE13-7 and WE15-5 was 0.4? and 0.8? lower,the average maximum air temperature under cloudy conditions was 30.8?,which was 1.4? and 2.4? lower than WE13-7 and WE15-5,the difference of north and south air temperature was 0.5?,and the average maximum soil temperature was 21.1?.WE13-7 and WE15-5 were 0.7? and 1.1? lower.It could be seen that NS10-10 had the best ventilation and cooling effect under high temperature in summer,and the air temperature change was relatively stable.In both sunny and cloudy conditions,WE13-7 had the largest daylighting performance,followed by WE15-5,and NS10-10 had the worst.(2)The daily average air temperature of the three plastic tunnels under typical winter weather was WE13-7>WE15-5>NS10-10,and the soil temperature also exhibits the same law at the same depth.The average maximum air temperature of WE13-7 under sunny conditions was 30.3?,which is 19.4? higher than outside air temperature,1.9? and 5.8? higher than WE15-5 and NS10-10,and the average maximum temperature of soil was 13.4?,which was higher than WE15-5 and NS10-10 by 0.9? and 1.2?,WE13-7 average maximum air temperature was 26.4? under cloudy conditions,18.6? higher than outside,average maximum soil temperature is 12.1?,0.8? and 1.4? higher than that of WE15-5 and NS10-10,and the lowest internal night air temperature was above 7? under sunny and cloudy conditions.The air temperature uniformity was best in NS10-10,and the air temperature difference between north and south was 0.1? in typical weather.During the winter observation period,the minimum air temperature of WE13-7 was above 5? for 52 days,WE15-5 was 40 days,and NS10-10 was 41d;daylighting performance under both sunny and cloudy conditions,WE13-7 had the largest daylighting capacity,followed by WE15-5,and NS10-10 had the worst.(3)The accuracy verification results of CFD models for three plastic tunnels showed that the simulated temperature values were in good agreement with the measured values.The root mean square error(RMSE)between the simulated and measured temperature values of WE13-7 was 0.8467?,that of WE15-5 was 0.7253?,and that of NS10-10 was 0.7265?.The errors were within reasonable range.The CFD models of three plastic tunnels were established.(4)According to the results of finite element simulation of three plastic tunnels,the structural optimization strategy was put forward: For WE15-5,it was necessary to enhance the compressive strength of the web of the trusses in the surrounding area of the short column top and strengthen the connection between the trusses and the ground in the ceiling area on both sides of the south and north;for WE13-7,it was necessary to enhance the compressive strength of the web of the trusses in the area around the top of the short column and strengthen the connection between the truss and the ground in the area at the bottom of the north side of the plastic tunnel,and move the short column from 7m to the vicinity of 7.4m;for NS10-10,it was necessary to enhance the compressive strength of the web of the trusses at the bottom of the arch,and to set the column support in the middle of the arch truss on the east and west sides.(5)The growth,yield and quality of tomatoes in three plastic tunnels in early-spring and autumn-winter were measured.The results showed that there was no significant difference in plant height and stem thickness of tomatoes in early-spring,and the number of leaves and leaf area showed that NS10-10 was significantly larger than WE13-7 and WE15-5,the quality of tomatoes in WE13-7 and WE15-5 was the best,and there is no significant difference in tomato yield in three plastic tunnels;the growth index of tomato in WE13-7 and WE15-5 in autumnwinter was better than NS10-10,and the quality of tomatoes in WE13-7 was the best,and the tomato yield was the largest in WE13-7. |
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