Study On The Thermodynamics Of Ultrasound-Heat Pump Drying System And Drying Characteristics Of Green Soybean Seeds

To solve the problem of low drying quality and high energy consumption during long-time drying of seeds,studies on the investigation of ultrasound-heat pump drying method could provide a new approach to optimize the drying process.The brief structure and main innovation points are as follows:(1)Closed-loop heat pump drying system had its particular characteristics and a simultaneous control strategy was proposed to improve the precision of superheat and drying temperature.The stability of drying temperature guaranted the quality of drying material.And on the premise,superheat should be accurate and stable to improve energy usage.Two fuzzy controllers employing different membership functions and control rules were used to achieve more stable drying temperature.The superheat controller was mainly composed of a PID controller and a conversion emergency controller which accelerated the response of electronic expansion valve.The experimental results showed that the temperatures at the outlet of indoor and outdoor condensers gave smaller fluctuations.One of the parameters Rf representing the value of undershoot ratio was 0.4,0.48 and 0.64 when drying temperature fixed at 30,35 and 38°C,respectively,with a parallel conversion controller.In order to study the application prospects of the parallel conversion control system,a comparison experiment with different compressor frequency range were also conducted.The variations of superheat were investigated when the temperature of drying chamber changed from ambient temperature to 25/30?,from 25 to 30? and from 30 to 25?,respectively.All the experimental data confirmed that the new controller was applicable to decrease the nonlinearities of superheat and drying temperature.(2)For a further understanding of the exergy efficiencies of the heat pump drying system,we employed exergy analysis method to study and optimize its main components.During heating or cooling process,drying chamber and compressor gave the two biggest exergy destructions.Then the exergy destruction was splitted into endogenous/exogenous and unavoidable/avoidable parts to improve our understanding of the thermodynamic inefficiencies and facilitates the improvement of system.(3)Green soybean seed comprises seed coat,cotyledon and embryonic axis(including epicotyl,hypocotyl and radicle)according to functional characteristics.Moisture sorption isotherms of different tissues were all determined using the static gravimetric method at three temperatures(20,30 and 40?).The moisture sorption curves of seed coat and cotyledon conformed to BET ??I type classification while those of embryonic axis exhibited type II behavior.Embryonic axis gave the lowest equilibrium moisture content at constant water activity and temperature.Hysteresis was observed at 20 ? and cotyledon exhibited the largest hysteresis loop.Among the six models tested,GAB model was found superior in predicting the experimental data of all parts.The monolayer moisture content values(d.b.)of seed coat,cotyledon and embryonic axis varied from 0.059 to 0.0654,from 0.0641 to 0.0687 and from 0.0619 to 0.0681,respectively,in the desorption process when temperature decreased from 40 to 20?.The isosteric heat of sorption was calculated from the predicted sorption data by applying Clausius-Clapeyron equation.(4)Intermittent drying was accomplished by periodically changing the input heat and air speed.Based on the well-controlled heat pump drying system,the drying kinetics of green soybean employing seven intermittent drying methods were analyzed.The increase of intermittency ratio caused decrease of effective drying time and increase of specific moisture extraction rate(SMER).SMERs of intermittent drying were 1.21-4.94 and 0.93-5.40 times more than the continuous drying for different initial moisture content(23% and 28%),respectively.Changes of drying rate(DR)and SMER became relatively moderate when intermittent drying with changed intermittency ratio was used.A comprehensive evaluation parameter(CEP),based on DR and SMER,was proposed and calculated.Experimental results indicated that intermittent drying(?=5/6)and intermittency drying with changed ratio gave the best two comprehensive performance.(5)Green soybean seeds were dried at different temperature(33 and 38?),with different ultrasound frequencies(20,28 and 33kHz)and powers(60,80 and 100W)with the new ultrasound-heat pump drying system.The results showed that with the increase of temperature,frequency or power,the drying rate increased but the damage to seeds became greater.