Thermal Characteristics And Temperature Field Simulation During High Pressure Processing

High pressure processing is a rapid developing and promising food processing technology.During high pressure processing,the pressure in the high pressure chamber is uniform,but the thermal effect due to adiabatic compression heating is not uniform,which has a complicated effect on high pressure processing.Due to the lack of test equipment resisting high pressure,the basic data related to high pressure processing of food is limited.This has become a bottleneck for the development and application of the new technology.This thesis focused on the thermal effects of high pressure processing,and try to solve the difficulties of test and correction of key parameters such as pressure and temperature under high pressure,and to develop thermal characteristics in-situ measuring equipments under high pressure conditions.Based on the thermodynamics of high pressure processing,a high pressure heat transfer model was constructed.And the thermal characteristics under high pressure obtained through experiments were used to study temperature field distribution and variation during high pressure processing by computer simulation technology.The thesis innovates in engineering and fundamental research to improve the theory and practical application of high pressure processing.The research contents,methods and results are as follows:1.A real-time pressure and temperature acquisition system under high pressure was established.It provided technical support for the study of thermal effects under high pressure.A high pressure test system was designed based on the laboratory high pressure equipment(HPP/600MPa/5L).Pressure correction and pressure-resistant temperature detecting devices were installed.The real-time collection of pressure and temperature in the high pressure chamber was realized by a data acquisition instrument.A polyformaldehyde insulation container was designed and fabricated to control the sample temperature under high pressure.The experimental results of phase transition pressure-temperature of water-ice Ⅰ and adiabatic compression heating of water determination under high pressure indicated that the test system was accurate and reliable.2.The adiabatic compression heating of selceted materials under different high pressure processing conditions(pressure from 0.1 to 400 MPa,initial temperature: 5℃ and 25℃ for organic solvents,and 15℃ ~ 50℃ for food materials)were determined.The objectivity of adiabatic compression heating was mastered.A prediction model for temperature of materials based on pressure and initial temperature under high pressure-adiabatic condition was established.Adiabatic compression heating is a direct source of thermal effect in high pressure processing.The designed test system under high pressure was used to determine the adiabatic compression heating characteristics of six organic solvents such as methanol and three food materials including tofu,wheat flour dough and pork.The results indicated that the adiabatic compression heating of organic solvents under high pressure processing was larger(up to 44℃/400MPa).The temperature increase rate decreased with increasing pressure,and initial temperature had little effect on the adiabatic compression heating values.For water and food materials,the adiabatic temperature increase rate did not decrease significantly with increasing pressure.And the higher the initial temperature,the larger the adiabatic compression heating.The established temperature prediction model under high pressure-adiabatic conditions based on pressure and initial temperature fitted well for experimental data.3.A fast measuring thermal probe for thermal conductivity measurement of foods under high pressure was developed.The thermal conductivity of selected foods under different high pressure conditions(pressure from 0.1 to 400 MPa,and temperature from 25℃ to 55℃)were obtained.And a prediction model for thermal conductivity of foods was established.Based on the test system under high pressure and mastering the adiabatic compression heating characteristics of materials during high pressure treatment,a thermal conductivity probe for food material was developed according to the transient heat transfer theory of line heat source,combined with the specialty of high pressure condition.1.5% agar gel was used for calibration test of the thermal probe.And then the thermal conductivity coefficients of tofu,wheat flour dough and pork under high pressure were measured.The results indicated that there is no correlation between the calibration factor of the thermal probe and the pressure,temperature and its interaction.The calibration factor of the thermal probe was 1.006.When the pressure and temperature are constant,the thermal conductivity of food is mainly determined by its composition.When the temperature is constant,the thermal conductivity of food increases with increasing pressure,but the increase rate decreases gradually.When the pressure is constant,the thermal conductivity of food increases with increasing temperature.The proposed quadratic polynomial model based on pressure and temperature could predict the thermal conductivity coefficient of the tested foods well under certain conditions(pressure from 0.1 to 400 MPa,and temperature from 25℃ to 55℃).4.The high pressure chamber was digitally modeled,and the thermodynamic process involved in the high pressure processing was analyzed.The temperature field distribution and variation in the high pressure chamber was then simulated by computational fluid dynamics software Fluent.The simulation result was verified by experimental test.It established foundation of the method for dynamic temperature field simulation of high pressure processing of foods.The results indicated that computer numerical simulation technology could analyze the temperature field distribution and variation in high pressure chamber during high pressure processing conveniently and efficiently.The simulation results were in good agreement with the experimental results.Both pressure and initial temperature could significantly affect the temperature field distribution and variation in high pressure chamber during high pressure processing.With the increase of pressure,the temperature filed distribution became more uneven and changed faster.With the increase of initial temperature,the temperature field was toward uniform in space and changed faster.5.The temperature field distribution and variation in high pressure chamber during high pressure processing of pork(selected as the representative of food material)was simulated by Fluent.The effects of pressure,initial temperature and sample size on the temperature field were analyzed.The results indicated that there is a significant temperature gradient among the food materials,the pressure transmitting medium and the high pressure vessel during high pressure processing of foods.The temperature gradient varies with time and space constantly.The numerical simulation technology could dynamically analyze and show the distribution and variation of the temperature field.The higher the pressure,the more uneven the temperature field distribution of the processed food samples,and it changed faster.Increasing the initial temperature had little effect on the temperature field during high pressure processing of pork.Under the same conditions,increase the size of the sample would make the temperature field more uneven during pressure holding time.