Defrost Control Mechanism And Experimental Research Based On Gray Scale Graph Theory

Cold stores can provide a low-temperature and high-humidity environment for long-term preservation of agricultural products,seafood,and other products.When evaporator temperature is lower than the air dew point temperature,the moisture in air condenses.Frosting will occur in the evaporator coil when the evaporation temperature is below 0℃.With the increase of frost thickness,the heat transfer resistance of evaporator coil will be increased,and the airflow channel will be blocked,thereby reducing the heat transfer performance of the evaporator and the system efficiency.Therefore,the frosting problem of evaporator seriously affects the operation efficiency of the system.Effective defrosting methods and control strategies are essential to ensure efficient and stable operation of the system.On the basis of summarizing of the development and research of evaporator defrosting technology and control methods at home and abroad,this paper puts forward a new method based on the principle of gray scale graph theory in the detection of frosting amount and proposes a new characteristic parameter S_t,which combines Grayscale contrast normalization parameterξto predict the optimal defrosting start/stop point.Firstly,a frost performance test bench to conduct frost experiments was built.The variation patterns of system parameters were investigated and discussed under different frost conditions and found out the suitable system parameters as the basis for judging the optimum defrosting time of the system.Then,the application of differential pressure sensors was described in evaporator frost measurement and a dimensionless differential pressure factor（PLF）was proposed.It qualitatively described the growth of frost on the evaporator surface based on the constant flow energy equation of unity flow.Five differential pressure measuring points were arranged on the evaporator surface for experiments under different frosting conditions,and the average PLF was calculated based on the data measured at the five measuring points.The average PLF was compared with the PLF calculated from the data measured at the five measuring points to obtain the relative error and standard error,which were used to determine the ideal installation position of the differential pressure sensor in the evaporator.Secondly,based on the theory and foundation of existing computer image processing technology,a new defrost control method based on gray-scale map theory was developed.A dimensionless parameter St of evaporator frost characteristics was proposed to characterise the frost degree on the evaporator surface,and the critical defrost time of the system was determined jointly with the Grayscale contrast normalization parameterξof the frost image,and a defrost control experiment bench for this control strategy was built.Finally,the new defrosting control method was experimentally verified and discussed under the five frosting conditions.First,under frosting condition 1,the optimal defrosting time of the system was determined based on the change of system operating parameters.Secondly,based on the optimal defrosting time,the thresholds of the frost degree coefficients S_tandξwere determined,so as to determine the critical defrosting time,and comparative experiments were carried out to verify the reliability of the new defrosting control method by changing the frosting conditions.The results show that this method could avoid the phenomenon of"mal-defrosting"of the system and achieve precise defrosting.In this paper,a new solution for defrost control methods for evaporators in refrigeration systems（air source heat pumps）.The method could reduce the influence of illumination intensity on frosting image recognition,and the system could accurately judge the defrosting start point,thereby realising accurate defrosting.