| Solar drying technology has attracted widespread attention in the academia for its clean environmentally friendly,energy-saving,and renewable.But solar energy is highly dependent on weather conditions,as a result,solar energy is discontinuity and instability,thus the development of solar drying is still slow.Applying solar simulator to the research of solar drying can effectively solve the problem that the research process is disturbed by weather factors,and promote the development of solar drying.However,most of the existing solar simulators are developed for application scenarios such as aerospace,high-energy reaction,and optical component testing.The application costs are relatively high,making it difficult to apply to agricultural solar drying technology research.Considering the above situation and characteristics of drying,this paper designed a solar simulator based on thermal radiation equivalent,and tested the performance of the solar simulator.This paper not only tested the applicability of solar simulator with bitter gourd,but also constructed a mathematical model of solar simulator and solar drying system.Then this paper did a numerical simulation of the system and analyzed the energy and mass transfer phenomena of drying process.The main conclusion from the study are as follows:Firstly,according to the characteristics of solar drying process,the design standards for the solar simulator were determined.The COMSOL Multiphysics was used to analyze and optimize the light arrangement of the solar simulator.The paper also designed and processed the solar simulator,and its performance was tested.Test results of solar simulator show that the solar could provide a maximum irradiance of 1216.1W/m~2,it is higher than the theoretical maximum irradiance on the earth’s surface of 1000 W/m~2;the solar simulator irradiance non-uniformity is within 10%,it meets International Electrotechnical Commission(IEC)’s requirements of class C;the solar simulator irradiance temporal-instability is within 10%,it meets IEC’s requirements of class C;the effective irradiated area of the solar simulator is larger than material stacking area of0.2m~2 in general drying experiments.Over all,the solar simulator designed in this paper meets the requirements.Then,using bitter gourd as experimental material,this paper design 9 groups of experiments by three slice thickness(3mm,5mm,7mm).each group of experiments was divided into outdoor experiments and indoor experiments.In the indoor experiments,the solar simulator was used to simulate the change of solar irradiance under the corresponding outdoor conditions.The experiment was also analyzed from three aspects:drying characteristic curve,effective moisture diffusitivity,activation energy.The experimental results show that the drying rate of indoor experiment and outdoor experiment is accompanied by a small degree of alternating change,but the trend of drying rate change is consistent.and the change of moisture ratio is completely consistent,after excluding the data points of closed to absolute dryness,the moisture ratio relative error of each point is within 10%;the relative error of the effective diffusivity of 8th group is the largest,it is 10.71%,and the relative error of the effective diffusivity of remaining8 groups are all within 10%;the relative errors of the activation energy of bitter gourd slice thickness of 3mm,5mm,and 7mm are 11.9%,10.5%,and 11.3%,respectively.The activation energy of indoor experiments and outdoor experiments are within reasonable ranges.The solar simulator designed in this paper can be used to study the drying process.Finally,the energy and mass transfer characteristics of drying experimental system are analyzed.According to Fick’s law,Darcy’s law,energy conservation laws,and Gauss’law,this paper establish the mathematical model of energy and mass transfer for drying experimental system by introducing the porous media model.The model was verified by the 6th group experiments,and the changes of the main physical quantities during the solar drying process were analyzed.The results show that moisture ratio curve of simulation and experiment is basically consistent,and the largest deviation is 0.159;the water vapor concentration in the drying chamber increase first and then decrease,and the water vapor concentration in the center area of the drying chamber is higher than it in the edge area of the drying chamber;the material exists dry zoon,wet zoon,and evaporation zoon during the drying process,and the dry zoon and the wet zoon are separated by the evaporation zoon,and the evaporation zoon gradually migrates from the outside to the inside of the material;the average temperature of the material rose rapidly at the beginning of the experiment,it indicated the material had a preheating time during the drying process;in the first few seconds of the drying process,the hot air at the bottom of the drying chamber has a clear upward trend,and cold air flows in at air inlet,as the hot air continues to accumulate on the top of the drying chamber,the air pressure in the drying chamber gradually increase,after a period of time,the flow field in the drying chamber tends to stabilize,however,it still fluctuates slightly with irradiance. |
