Modeling Phase Change And Heat Transfer Of Water During High Pressure Food Treatment

Base on the phase diagram of water,high pressure food freezing and thawing treatment is implemented by control both pressure and temperature during the phase change.Pressure effect can significant preserve the microstructure of biological substances and reduce phase change time by increase the thermal gradients and reduce the ice crystal size.A survey of the previous work is followed in this thesis by a detailed report on the new study work of the thermophysical properties under high pressure calculation;pressure induced thawing time reduction;thermal evolution influenced by convection currents in the pressure medium;influence on the formation of the ice front by convective fluid motion.A method for the calculation of some thermophysical properties of both liquid water and ice I under high pressure is implemented.These properties,closely interrelated in their physical derivation and experimental measurement,are thermal expansion coefficient,isothermal compressibility coefficient,specific volume and specific isobaric heat capacity.The limits for the set of equations where set in -30 to 120℃and 0 to 500 MPa for liquid water and -30 to 0℃and 0 to 210 MPa for ice I.Their good agreement with many existing experimental data is discussed.In addition,the routines implemented for the calculation of these properties is made by commercial software MATLAB.The routines are used in the later calculations.A computer simulation was conducted to evaluate thawing time reduction of ice during high pressure assisted thawing treatment and the results were compared with those under atmospheric pressure.The release of latent heat was treated by apparent specific heat method. The effects of the different factors on the high pressure assisted thawing treatment were analyzed.The time reductions of different sample dimensions were estimated and the temperature contour of thawing at different operating conditions was obtained.The results show that the temperature gap of phase transition is main factor effecting the thawing time reduction.The second order relationship can be considered between pressure and phase transition time.With the unfrozen temperature boundary condition,two samples with different filling ratio is studied by coupling the N-S equation to convection heat transfer equation.The results show that heat transfer is significant enhanced in the smaller sample(low filling ratio).Strong flow appears in the side of the pressure medium due to the natural convection.The results indicate that heat transfer capability is increases with operation pressure and rate of pressurization and don't change significantly with initial temperature.With the frozen temperature boundary condition,a combination of apparent specific heat method and Gaussian curve is used to deal with the release of latent heat.A sink term is added to the equation of momentum conservation to govern the momentum transfer on the interface between the liquid and the solid phase.Unsteady natural convection during freezing and thawing of water under atmospheric pressure is simulated.The results show that the shape of phase change front is changed by natural convection compare to heat conduction.Because of abnormal behavior of water in the investigated range of temperature(density of water is maximum at 4℃),there are two main vortices separated at the isotherm line of 4℃in the liquid zone.The freezing process is stronger influenced by natural convection compare to thawing process.