| The straw-based nutrient seedling-growing bowl tray（referred to as seedling tray）is the core of straw-based nutrient seedling-growing bowl tray seedling and fully mechanized planting technology,and its quality directly affects the promotion and application of this technology.In the production of seedling trays,drying and shaping are important steps in the production process and key factors determining the quality of seedling trays.Selecting appropriate drying equipment is essential to ensure the drying and shaping quality of seedling trays.However,currently there are issues with low drying efficiency,poor uniformity,and low drying quality in seedling tray drying equipment.Therefore,there is an urgent need to develop suitable drying equipment to meet the requirements of seedling tray drying.To this end,based on the advantages of microwave drying technology and hot air drying technology,a temperature-controlled hot air-assisted microwave drying device was designed,which mainly consists of a hot air drying system,a microwave drying system,and a control system.FLUENT software was used to complete the structural design of the air distribution chamber in the hot air drying system,optimizing the uniformity of the outlet velocity of the air distribution chamber.HFSS software was used to simulate the installation method and height of the microwave resonant cavity feed port in the microwave drying system,and the optimal feed port installation method and the height of the feed port with the lowest reflection power were determined.The control system is based on a programmable logic controller（PLC）,which controls the drying rate and drying quality of straw-based nutrient seedling-growing bowl tray by controlling the parameters during the drying process.Considering the characteristics of delayed response,nonlinearity,and time-varying nature of temperature control in the operation of hot air-assisted microwave drying equipment for Straw-based nutrient seedling-growing bowl tray,fuzzy PID control was chosen as the control method for temperature during the drying process based on a comparison of simulation results between conventional PID control and fuzzy PID control.Response surface optimization was used to optimize the drying process of seedling trays.The main research contents are as follows:Firstly,the main components of the drying machine,including the microwave drying system and the hot air drying system,were studied.In the microwave drying system,the microwave resonant cavity is the main working component,and the arrangement and height of the feeders can affect the uniformity of the electric field in the resonant cavity and the reflection power between the feeders.Simulation using HFSS software was conducted to compare different arrangements of the three feeders in the microwave resonant cavity,and the V-L-L arrangement was found to result in the most uniform electric field.Based on the determined arrangement,the optimal height of the feeders was found to be 160mm,resulting in a 78.13%reduction in reflection power compared to a height of 70mm.In the hot air drying system,the air distribution chamber is the main working component.Using the uniformity coefficient at the outlet of the air distribution chamber as the indicator,the height of the deflector chamber,the height of the air distribution chamber,and the bottom diameter of the deflector were optimized using FLUENT software.The optimized air distribution chamber showed a 21.26%improvement in uniformity coefficient compared to the original design.Next,based on the actual structure and control requirements of the hot air-assisted microwave drying machine,the design of the main circuit and control circuit was completed,and the hardware selection for the control system was conducted.The DVP20EX2 series PLC was chosen as the main controller based on the control needs,and the DVP-107CV touch screen was used for human-machine interaction.Suitable fans and frequency converters were selected based on the required air volume during the hot air drying process.Heating tubes and magnetron tubes were chosen based on the required heating power,and appropriate temperature sensors and wind speed sensors were selected based on the actual drying temperature.Furthermore,based on the design of the control circuit for the hot air-assisted microwave drying machine,the software part of the control system was designed,including the hardware configuration and software programming of the upper computer and lower computer,to achieve control over microwave power,wind speed,drying time,and temperature during the drying process.To achieve better temperature control during drying,simulation results of traditional PID control and fuzzy PID control were compared using MATLAB software,and the fuzzy PID control method was ultimately chosen for temperature control during the drying process of the equipment.Finally,after debugging,the results show that the control system meets the expected control requirements and the drying device operates well.Performance tests of the drying device are conducted,comparing the drying performance of the seedling trays under hot air drying,microwave drying,and hot air-assisted microwave drying methods.The test results demonstrate that the drying device exhibits good drying uniformity.Compared to hot air drying and microwave drying,the hot air-assisted microwave drying method achieves higher productivity,smaller tray deformation,and faster drying rate.The deformation values for hot air drying,microwave drying,and hot air-assisted microwave drying are 3,2.092,and 1.875,respectively,while the drying rates are 96.821,203.172,and 378.87,respectively.Compared to hot air drying and microwave drying,the hot air-assisted microwave drying method reduces the tray deformation by 37.5%and 10.37%,respectively,and increases the drying rate by291.31%and 86.48%,respectively.To further investigate the drying process of the straw-based nutrient seedling-growing bowl tray under hot air-assisted microwave drying conditions,experiments are conducted using the seedling trays as the test material.A single-factor experimental design is employed to study the influence of different experimental factors on the drying rate and deformation of the seedling trays under hot air-assisted microwave drying.A three-factor,five-level experimental design is conducted using Design-Expert 8.0 software,and response surface analysis is performed.The results show that under the conditions of microwave power of 1600W,heating temperature of 60°C,and hot air velocity of 15m/s,the seedling trays achieve the fastest drying rate of 4.015%·min-1 and the smallest deformation of1.522cm.This research provides a basis for the design of future hot air-assisted microwave drying devices and the development of rice straw nutrient tray drying technology. |
