Study On The Heat And Mass Transfer Models Of Coupled Hot Air And Microwave Drying

Heat and mass transfer inside the dried material is the basic problem of the drying theory.Coupled hot air and microwave drying is one of the novel drying methods. It is difficult to testthe moisture content and temperature change online during its drying periods as other dryingmethods. Therefore, it is significant to build mathematical models for parameter optimizationand process control. Based on the new type laboratory equipment and the energy balanceequations. Heat and mass transfer models for cylindrical and spherical samples wereestablished and solved by the implicit finite difference method to predict the temperaturedistribution and moisture variation. Carrots and peas were selected and made into thedesigned shape as the test samples, Then the temperature and moisture content in the samplesduring the actual drying process were measured and compared with the predicted value toverify the feasibility of the models.A coupled hot air and microwave drying rig was designed and microwave powerabsorption was solved by the Maxwell’s equations and Lambert theorem. The size of therectangular waveguide and dryer cavity were determined and the electric field strength wasestimated to satisfy the uniformity requirements.Heat and mass transfer models were established for cylindrical sample for coupled hotair and microwave drying using carrot as experimental material. Different drying conditionswere set to compare the predicted values with the experimental ones so as to validate thefeasibility of the models. Three different microwave power density (0.9;2.0;2.6W/cm3) anddrying air temperature (40;50;60°C) were set to study their effects on the temperaturevariation and moisture distribution when the air velocity was maintained at4m/s. Comparedwith the measured value, the predicted temperature was a bit higher and the moisture contentwas a bit lower Compared the drying rate of two different size sample under the same dryingconditions, the value was higher in the small size, indicating that the inside mass transferresistance was smaller when the size was small.Heat and mass transfer models were established for spherical sample for coupled hot airand microwave drying using the peas as experimental material. Three different microwavepower density (0.3;0.8;1.5W/cm3), air temperature (30;50;70°C) and air velocity (4;5;6m/s) were set to study their effects on the temperature variation and moisture distribution. Theresults showed that microwave power density and the hot air temperature were more eminentthan the hot air velocity in the effect of the drying rate. The same as we have seen in thecylindrical, the predicted value agreed well with the measured value. With respect to4m/s, thedrying rate was lower in the final stage when the air velocity was6m/s, this may be due to thehot air as a cooling medium in the final stage.