Theoretical Simulation And Experimental Studies Of Column Vegetables During Vacuum Cooling

The mathematical model for the heat and mass transfer process of column vegetablesduring vacuum cooling is established in this thesis, with radiation and water evaporation asits important boundary condition, according to the actuality that the research on the principleof vacuum cooling of vegetables is incomplete both domestic and abroad. Taking celery assample and using finite volume approach, the variation histories of vegetable temperature andmass, as well as the total pressure of the vacuum system are achieved by numericalcalculation performed on MATLAB. The experimental equipments are founded. Roots andwater ring pump unit is used to replace rotary vacuum pump. Agilent 34970A data collectionequipment is used to record the temperature of vegetable, while the pressure of the vacuumchamber is read from the vacuum gauge and the weight of vegetable is measured byelectronic balance. Several column vegetables such as spinach, celery, Chinese cabbage andcarrot, are selected to perform the vacuum cooling experiments and the results are comparedwith that of spherical vegetable cabbage. It is shown from the experimental results that thecooling effect of column vegetables is almost as same as that of spherical vegetables, whichis mainly determined by the ratio between area and volume and the thickness of the skin ofvegetables. Therefore, the cooling effect of leafy vegetables is much better than that ofrhizomatic vegetables. The cooling effect of different species of leafy vegetables isdetermined by the leaf compactness. Comparing the modeling results with the experimentalresults, it is found that the tendencies of the simulated pressure and experimental pressure areconsistent, while the difference between them is little when the pressure decreases. Thesimulated final pressure is 613 Pa, while the experimental one is 660 Pa. The tested central,surface and average temperatures of celery are 3.23℃, 1.76℃and 2.5℃, while the simulatedvalues are 1.14℃, 0.78℃and 0.99℃. The calculated water loss of vegetable during thewhole cooling process is 5.94%, and the tested value is 5.2%. It is proved by all the abovethat the mathematical model is reasonable. Also the research results provide some basic rulesfor flow and heat transfer about the characteristics of evaporation heat transfer of productduring vacuum cooling.