Calculation and validation of moisture uniformity in drying process based on Shannon-Wiener index

Abstract: Belt dryers are the most popular equipment for betel nut drying. One of the drawbacks of this dryer is the non-uniformity in the desired moisture content of end product. In order to improve the structure of flow field and the moisture uniformity for the belt dryer, on the theoretical basis of computational fluid dynamics, the effects of the accumulative thickness, air velocity, air temperature and moisture content of hot air on the moisture uniformity of betel nut dried were simulated and discussed by using FLUENT software package. The orthogonal table about 4 factors and 3 levels was designed. In this paper, the betel nut was considered to be a wet porous medium, and the air flow through the areca layer was seen as the flow inside the porous medium, as well as the flow in the layer. By adding momentum equation source term to the momentum equation, the porous medium model was formed. The momentum equation was divided into 2 parts which included viscous item loss and inertia item loss. Experimental verification was carried out. Experimental test was finished in a multilayer belt dryer designed by Hunan Chain Co. Ltd.. The dryer was a five-layer mesh belt with cross flow drying system. Observation hole was set in the 1st path (top layer) and the 5th path (bottom layer), and the hole position was X=0.8, 2.4, 4.0, 5.6, 7.2 m, respectively. After washing, soaking, air-curing, separating, removing the film, rinsing and draining, betel nut was dried in dryer. The QDF-2B hot ball type electric wind speed meter was set to test the wind speed of the dryer, and the measured data had a good agreement with the calculation results. Based on the Shannon-Wiener index, the water content uniformity index was put forward, which was compared to the conventional index and the calculation result. Two deflectors with different shape (traditional deflector and wing-shape deflector) were put forward and analyzed. Simulation results showed that in the length direction of top path, the wind speed at the positions of X=4 m and X=6 m was relatively larger, which was 0.8 and 0.7 m/s respectively. The wind speed when X was equal to 4-6 m was gradually reduced, while that when X was equal to 4.5-5.5 m was the lowest (0.4 m/s). The wind speed curve had the "camel peak" shape. In the length direction of bottom path, the hot air was divided into 2 parts, most of which flowed along the length direction, and the remaining moved upward through the material layer. The speed was gradually reduced in bottom channel, and the speed through the material layer was maintained above 1.05 m/s. Results showed that the moisture content of the betel nut layer with a thickness of 80 mm was more uniform than 40 or 60 mm. Therefore, the thickness of betel nut had most effect on the uniformity of moisture content than the other 2 factors. The general trends of water content uniformity index, conventional index and "camel peak" curves were similar, but that of the water content uniformity index was more close to the calculation result. It indicated that layer thickness of 80 mm, inlet temperature of 70 ℃, flow rate of 1.5 m/s, moisture content of 0.24 were more conducive to moisture uniformity. The deflector could obviously improve the uniformity of moisture. Installing simple deflector mildly expanded the contact area of air inlet and slightly reduced the moisture of betel nut, yet there still had partial areas without drying. These areas were mainly distributed in the middle and sides of the dryer. After equipping with wing deflector at the entrance of dryer, hot air was evenly distributed to the entire first path, while moisture content of the betel nut layer decreased from 0.285 to 0.215, and moisture uniformity was improved. In the case of shortening the drying time by 5 min, the water content uniformity index was 0.926, and the drying efficiency was promoted.