Simulation And Experimental Study On Phase Change Interface During The Freezing Process Of Potato

In this paper, fresh potatoes were chosen for the study, using a combination method ofnumerical simulation and experiment, aimed at the temperature distribution and movementlaw of the phase change interface inside the potatoes during the freezing process, analyzedthe influences on the phase change interface movement,which was caused by the coolingair velocity and temperature according to the thermal conductivity and fitted specific heatcapacity polynomial based on the experiments conducted within the range of30~-40℃.The optimization and control to the plant material freezing process were realized, provideda basis for the freezing technology application in the plant materials freezing andtechnology optimization.The main research points in this paper:1．The study about potato thermal properties measurement, consisted of potatofreezing point, densities before and after freezing, moisture content, temperature-dependentspecific heat capacity within the temperature range of30~-40℃, thermal conductivity,etc., and related polynomials were fitted out.2．Potato freezing numerical model: combined with the results of thermal propertiesexperiments and the fitted polynomials, using the numerical simulation software COMSOLMultiphysics to work out the temperature distribution and the location of phase changeinterface of anytime under variable properties during the freezing process.3．Potato freezing experiments: work out the specified point’s temperature curve ofpotatoes with different sizes under different freezing ways and different freezing rates, alsothe location of phase change interface and temperature distribution inside the potato duringthe freezing process.4．Contrast the simulation results with the experimental results, find out the causes ofthe errors, analyze the phase change interface movement law.The numerical simulation results were as follows:1．The speed of the phase change interface moving towards the center increasesgradually.2．In forced convection, the thermal center of the potato moved downwards thegeometric center gradually. When the temperature of the cooling air was-40℃, the movingspeed of the air increased from1m/s to2m/s，4m/s and8m/s successively, the time of thephase change interface moved from the outermost to the center decreased by700s，300s and400s respectively.3．In natural convection, the thermal center of the potato moved upwards the geometric center gradually. When the temperature of the cooling air decreased from-25℃to-30℃，-35℃and-45℃successively, the time of the phase change interface moved from theoutermost to the center decreased by2000s，1300s and800s.The experiment results were as follows:1．According to the experimental results, divided the thermal conductivity curves intothree segments:30~-2℃,-2~-6℃and-6~-40, fitted the curves with polynomialrespectively. Similarly, divided the specific heat capacity curves into three segments,30~0℃,0~-10℃and-10~-40℃, fitted them all respectively. Results got with fittedpolynomials almost equaled to those got by experiments.2．When the frequency of the cooling fan power was0Hz, the average moving speedof the lower phase change interface was the greatest. In the beginning, the moving speed ofthe upper phase change interface was a bit greater than any of the left or right side. As thefreezing went on, due to the influence of the lower interface, the moving speeds of the leftand right sides overtook that of the upper gradually.3．When the frequency of the cooling fan power was50Hz, the moving speed of theupper phase change interface was the greatest, and the moving speed of the lower phasechange interface was the smallest.4．In the situation that the potatoes exchanged heat with-48℃cold air by naturalconvection, the results obtained by substituting the thermal conductivity and specific heatcapacity, which varied with temperature, into the simulation approached closely to theresults obtained by substituting constant values into it, so the two methods may consideredequal. When the frequency of the cooling fan power was50Hz, the simulation results withthe thermal physical parameter which varied with temperature were closer to theexperimental results.5．When the edge length of the potato was4cm and the frequency of the cooling fanpower was50Hz, the gradient of the temperature inside the potato got the minimum valueduring the whole freezing process; when the edge length was5cm and the frequency was0Hz, the gradient got the maximum value.The conclusions of this paper are as follows:1．The greater the heat exchanging rate between the potato and the surroundings is, thegreater the difference between the experimental results and the simulation results with theconstant thermal properties will be, the more accurate the simulation results with thethermal properties which varies with temperature will be.2．The speed of the phase change interface moving towards the center increasesgradually.3．The faster the cooling air moves, the greater the moving speed of the phase change interface will be. However, as the speed of the wind(ie. the cooling air) get greater, themagnitude of the phase change interface moving speed increasing rate decreases gradually.4．When the potatoes exchange heat with cold air by natural convection, the lower theair temperature is, the faster the phase change interface moves. When the air temperature isat a high level, lower the temperature of the air can enhance the moving speed of the phasechange interface significantly. However, as the temperature of the air decreases, themagnitude of the phase change interface moving speed increasing rate decreases gradually.5．In the situation that the potatoes with the initial temperature of20℃are cooled by-40℃cold air by natural convection, when the size of the potato dice is under3.1cm, thetemperature distribution inside the dice can be considered uniform at anytime, and thetemperature inside the potato can be calculated by lumped parameter method.6．In the situation that the potatoes with the initial temperature of20℃are cooled by-40℃cold air by forced convection, when the air moving speed u and the length of potato ameet the condition, ua≤7.167×10-3m2/s, the temperature distribution inside the potato maybe considered uniform at anytime. According to the calculating results, when a equals to3cm,2cm,1cm, as long as u is under0.239m/s，0.358m/s，0.717m/s, respectively, lumpedparameter method can be used to calculate the temperature inside the potato.