Optimization Of Refrigerated System For Commercial Display Cabinets And Numerical Simulation Of Temperature Fluctuation Of Food Stored

With the application of food cold chain technology becoming more and more popular,display cabinets as the cold chain terminal equipment draws more and more attention.The study of energy-saving optimization of display cabinets refrigerated system and temperature stability inside cabinet has become a significant subject.In this paper,based on the research of the traditional vertical open display cabinet refrigerated system,few new display cabinet refrigerated systems were introduced,and the mathematical method of graph theory was used to optimize the design of the refrigerated systems,and improve the performance and energy efficiency of the systems.Besides,the half-wave functions and waveform characteristic values analysis were applied to the numerical simulation of food temperature in the display cabinet to analyze the influence on the food temperature fluctuations in the cabinet under several different working conditions,so as to offer reference data for the optimization of the display cabinet system and the improvement the quality of food stored.After simulation and test,the research results are obtained as following:1.Basing on graph theory,the thermodynamic cycle of a novel combined refrigerated system including low temperature application(LT),medium temperature application(MT)and air conditioning was simplified and transformed to an adjacency matrix,which help to establish a mathematical model.A numerical simulation of refrigerated system with three different kinds of configuration and three different refrigerants was done,basing on the analysis of the calculated results,the following items were concluded:under the condition of+50 C~o condensing temperature,the comprehensive efficiency of refrigerated units using R22refrigerant is the highest,using R407C is in the middle,and using R404A is the lowest.Currently,the energy efficiency of popular environment-friendly refrigerants cannot compete with old R22 refrigerant.Among the three configurations,the cooling capacity of plan A is basically meets the requirements and also with a high energy efficiency;the cooling capacity of the MT and LT display cabinet in plan B is a little inadequate,which cannot meet the design requirements;the cooling capacity for display cabinets of plan C is relatively oversize,and the energy efficiency of the condensing unit is the lowest.Therefore,according to the requirements,plan A is the best one.In addition,the parallel two MT compressors of plan A are with the same displacement,so that the oil balance of the compressor crankcase is better and the return gas distribution is more equally,it avoids the disadvantage that the lack of oil in small displacement compressor crankcase when the different displacement compressors are assembled in parallel.By the numerical simulation and analysis basing on graph theory,the optimal best system configuration plan is effectively selected,which greatly shortens the design time and reduces the design cost.2.The mathematical model of the temperature field response in the display cabinet and the corresponding mathematical response model of the food temperature fluctuation were established.Under the initial conditions of the pulse function of supply air response and periodic defrost response,the temperature field in the display cabinet and the fluctuation characteristics of stored liquid food were numerically simulated.The simulation results show that the fluctuation range of food temperature in the defrosting cycle is much larger than that in the refrigeration cycle,and has a more negative impact on the quality of food.With the increase of defrosting heating time,the fluctuation time of the central position of liquid food increases correspondingly,and the peak height of temperature rise increases gradually too.The temperature fluctuation of liquid food from the surface to the first internal 20mm position is changed most sharply,and the temperature increase or drop is also fastest.From the internal20mm point to the central(30mm)point,the change trend of each characteristic value is gentle,the values are with a small change or even basically no change.The shorter the defrosting heating time,the smaller the on-off ratio R value,the greater the water content of the food,the temperature fluctuation time will be shorter and the fluctuation peak will be smaller.It is valuable to optimize the operation condition of display cabinet and improve the quality of food by selecting appropriate defrost mode,on/off ratio basing on the moisture content of different foods.3.The centralized air supply display cabinet system(CASDC)was put forward,which can run with three modes of"centralized air supply by refrigerated system(CAR)","centralized air supply of natural cold resource(CRNC)"and dual mode,which can reduce cabinet temperature fluctuation and system energy consumption,and improve system energy efficiency ratio.A test rig of the centralized air supply display cabinet system was built,it can be switched between the three working modes of CAR,CRNC and dual.The air supply velocity to two cabinets can be adjusted through air valves control and fan frequency conversion.Meanwhile,a comparison between the CASDC system and the traditional display cabinet system(TDC)was done.In this paper,the comparative analysis of defrost cycles of CASDC system with CAR mode and the traditional cabinets was done,the impact of air velocity on CASDC cabinet temperature and air curtain velocity was investigated,the air curtain velocity and temperature distribution inside cabinet of CASDC system with two working mode and traditional cabinet was compared and studied.The power consumption of CASDC system with CRNC mode was tested and compared with the TDC.Based on the air curtain model with the improved two-fluid method,the whole CASDC system was simulated,and the velocity field and temperature field distribution of the air supply duct and cabinet were calculated respectively.The numerical simulation results are consistent with the measured values.Due to a bigger fin pitch evaporator and high-power electric defrosting heater is used in the CASDC system,the frost amount and defrosting time are significantly reduced,the defrosting interval time can be extended from 6 hours to 9 hours comparing with the traditional display cabinet,and the maximum temperature rise in the cabinet is reduced from 4.0?to 2.4?in a 24-hour working cycle.The velocity distribution and temperature distribution of cabinet in CASDC system with two modes are similar to TDC and CASDC can work well.In cold season,the CASDC system with CRNC mode can save about 43%energy compared with the TDC,and the energy saving effect is significant.Through numerical simulation,it can be found that the velocity field and temperature field of the CASDC under the two working modes are similar to that of the TDC,which is manifested as the entrainment of hot air outside the cabinet and the stratification of the temperature outside the cabinet driven by heat pressure.By comparing the numerical simulation temperature value and the measured value,the maximum average deviation and the maximum deviation of the simulation results under the two operating modes are-0.8?and 0.7?respectively.The calculated deviation is within the acceptable range,and the simulation results are consistent with the measured values.4.Combined with the simulation methods,the overall optimization strategy is proposed for the whole display cabinet refrigerated system.On the one hand,the strategy of saving the total energy consumption of the whole system and improving the energy efficiency ratio is studied and discussed;on the other hand,the strategy of reducing the temperature fluctuation of stored food in the cabinet is studied and discussed.Specific strategies are as following:for complicated multiple combined refrigerated system,the thermodynamic cycle can be transformed as a graph matrix description and help build a mathematical model,and the refrigerated system with different configuration plans will be simulated,power consumption,energy efficiency ratio and cooling capacity etc.data will be outputted.Through the analysis of calculation results,the best construction plan matching with system can be selected.For CASDC system and water-cooled single frequency converter system,the shortest path algorithm of graph theory can be used to optimize pipeline path,reduce the resistance inside pipes and heat leakage to ambient,so as to save system energy consumption,improve energy efficiency ratio,save materials and reduce installation cost.Through the analysis of the advantages and disadvantages of the CAR and CRNC two air supply mode,the necessary of dual work mode is confirmed.And the air supply strategy for CASDC system is summarized,solve the problems of regional restrictions with CRNC mode,expanded the application area of CASDC system all over the country.Through the analysis of simulation results,it can be seen that defrost condition and on-off ratio of the display cabinet refrigerated system are very important factors to reduce the fluctuation of food temperature in the display cabinet,and also the best objects of optimization strategy.Optimizing defrost conditions,such as using CASDC system or hot air(hydrothermal)defrost method;reducing the on-off ratio,such as using PID thermostat,electronic expansion valve and variable frequency compressor,will significantly reduce the temperature fluctuations in the cabinet of food,extend the storage time of food,improve the quality of food stored.At last,the author puts forward the deficiency of this paper,and looks forward to the direction and the content of further research.