Study On Heating Performance Of An Air Source Heat Pump With Multiple Drying Modes

Drying is a key technique in modern industry,which can promote the value of agricultural products and prolong the storage time.In recent years,air-source heat-pump has been widespread use for drying by reason of strong adaptability,high efficiency,energy-saving and environmentally friendly.However,there are still some problems with the existing studies: single drying equipment can not meet the drying needs of multiple types of agricultural products;The prediction of thermal performance of heat exchanger under complex working conditions;The performance of air-source heat-pump is strongly restricted by the environment.Therefore,the research contents of this paper include the performance experiment and simulation of the air-source evaporator under multiple drying conditions,the numerical simulation of the condensation characteristics of the liquid film,and the numerical simulation of the adaptation characteristics of the evaporator under different air conditions.The following results have been achieved:1)An air source heat pump drying device with multiple drying modes was presented,which can realize three drying modes: heating mode,dehumidifying mode and heating and dehumidifying mode.The performance of heat pump evaporator under different working conditions was tested experimentally.The results showed that increasing the inlet air velocity of the evaporator can effectively increase the refrigerant heat gain of the evaporator,but the dehumidification capacity of evaporators to unit mass air decreased with the increased of air velocity.Besides,the heating and dehumidification mode,not only can provide high heat for the refrigerant,but also provide the low-humidity airflow to the drying cabinet.2)A CFD numerical heat and mass transfer model for two-stage evaporator was established.The coupling methods of Eulerian Wall Film model and Species Transport model were used.The maximum error of air temperature was ±2 ℃,and the maximum error of air relative humidity was ±7%.The numerical simulation fitted well with the experimental data.3)The condensation characteristics on the evaporator’s fin were simulated and analyzed.The distribution of the liquid film and the effect of environment on the mass transfer coefficient were studied.With a 10% increase in the relative air humidity,liquid film thickness and temperature increased by 65% and 1.5%,respectively.When the air velocity increased by 1.0 m/s,the liquid film thickness and temperature decreased by 47% and 6.7%,respectively.The mass transfer coefficient increased with increasing air velocity.4)The environmental adaptation characteristics of the evaporator were simulated and analyzed,and the operation characteristics of the evaporator under critical condition were obtained.The results showed that the heat gain quantity of the evaporator for 1℃heat-transfer temperature difference was decreased sharply with increasing evaporation temperature in the sudden-change zone,but the heat gain quantity showed a reverse trend when the evaporation temperature was not within the sudden-change change zone.The heat gain quantity,at 1℃ temperature difference for the frost condition,was higher than the non-frost condition.