Preparation Of Low-temperature Refrigerant And Study Of Heat Transfer Characteristic During Food Immersion Freezing

With the rapid development of social economy and the improvement of living standard,people are increasingly fond of and inseparable from frozen food.And the quality requirements of frozen food are getting higher and higher.Therefore,the freezing food industry has developed rapidly in recent years.The energy consumption of refrigeration equipment is also increasing year by year.The traditional freezing method has frozen food for a long time and consumed a large amount of energy.Meanwhile,the quality of frozen food is not good.Compared with other freezing methods,the ice crystals formed inside the immersion frozen product are small.And immersion freezing method has better product quality,faster freezing rate,lower energy consumption,and less equipment cost.It is an efficient freezing processing method to develop.Therefore,this paper proposes the immersion freezing technology which is applied in food freezing,develops low-temperature refrigerant which is applied in immersion freezing,and studies on the effects of three different freezing methods of air convection freezing,static liquid immersion freezing and circulating liquid immersion freezing on the freezing time and freezing rate of single vacuum-packaged beef.And on these foundations,the heat transfer characteristic in the freezing process of vacuum-packaged beef in immersion freezing was further studied.The main research work and results are as follows:(1)Based on the analysis and summary of the existing dissertation at home and abroad,the materials that can be used as low-temperature refrigerant were prepared and screened.Several kinds of ternary low-temperature refrigerant and quaternary low-temperature refrigerant suitable for immersion freezing system were developed by step-cooling curve method.Then the mass concentration of multi-component low-temperature refrigerant was changed to study the change rule of freezing point.Eight kinds of low-temperature refrigerants with a temperature lower than-35? were found,and their thermal properties(viscosity,density,thermal conductivity,specific heat capacity)were tested.Finally,DWR-66,a kind of quaternary low-temperature refrigerant,was developed for the immersion freezing research,and the freezing point of DWR-66 was-48.52??(2)The freezing process of single vacuum-packaged beef was numerically simulated and experimented.The physical model of the freezing process of single vacuum-packaged beef was established.The effects of three different freezing methods,air convection freezing,static liquid immersion freezing and circulating liquid immersion freezing,on freezing time and freezing rate of single vacuum-packaged beef were studied.And temperature distribution maps at different times and freezing curve of single vacuum-packaged beef were obtained.At the same freezing temperature,the results of the study showed that the freezing rate of frozen single vacuum-packaged beef using the circulating liquid immersion freezing method was 2.70 times higher than that of static liquid immersion freezing method,and 6.93 times higher than that of the air convection freezing method.Related experiments were carried out on it and showed the validity of the theory.(3)The self-developed new quaternary low-temperature refrigerant DWR-66 was applied to the immersion freezing equipment.The physical model and mathematical model of the freezing tank in immersion freezing equipment was established.And the heat transfer characteristic in the freezing process of vacuum-packaged beef in freezing tank was numerically simulated by ANSYS-FLUENT.The influence rules of the low-temperature refrigerant inlet flow rate,inlet temperature and food spacing in the freezing tank on the velocity distribution,temperature distribution,food freezing completion time and freezing rate in the freezing tank were obtained.The results showed that the low-temperature refrigerant with faster flow rate or lower temperature can make not only the velocity field and temperature field in the freezing tank evenly distributed,but also the shorter freezing time and the faster freezing rate of the vacuum-packaged beef.All of these provide references for practical application and development of the immersion freezing equipment in food freezing technology.