Numerical simulation and experimental verification on humidity field for pipeline humidifying device

Maintaining the humidity in the environment is very important for guaranteeing the qualities and prolonging the shelf-life of fresh products. In order to obtain the distribution characteristics of humidity field when the pipeline humidifying device was working, a transport container for fresh-keeping transportation with a pipeline humidifying device based on differential pressure principle was investigated. In this study, navel orange was used as the test material. A three-dimensional numerical turbulent model of 1/4 ratio of the container was established. The structure of this model was built in PRO/E, and the mesh was generated in ICEMCFD by unstructured mesh method; the maximum size of the cell was 0.05 m and the minimum size was 0.005 m. The humidity distribution in the container with pipeline humidifying device was simulated by adopting Porous Medium Model, Species Transport Model, SIMPLE algorithm and Enhanced Wall Treatment Function method to solve the model in FLUENT. After that, the humidifying rate and the cloud charts of humidity distributions on the longitudinal and cross sections in the container and on the surface of products were obtained. With pipeline humidifying device, the relative humidity in the container could be increased from 75% to 90% in 246 s. The distribution of humidity field in container was uniform and the difference of relative humidity was less than 2%, and the difference of relative humidity on goods surface was no more than 3%. A test in a real container with 75 kg navel oranges was done to verify the accuracy of the model. Experimental validation showed that the test results were in good agreement with the simulation results. The difference between the simulated value and experimental value was less than 1.2%, which proved that such model could be used to study the effects of pipe diameter, number of holes, air velocity of air duct on humidity distribution on products surface. Some comparison and analysis were done for the results, and some conclusions could be drawn. It showed that humidifying rate increased as the velocity of air duct and the diameter of pipe increased; and when the relative humidity at the monitoring point was 90%, the humidifying time was 258, 246, 242 and 236 s, respectively as the velocities of air duct were 2, 4, 6 and 8 m/s , while 255, 252, 246 and 242 s respectively as the diameters of pipe were 12.5, 19.0, 25.4 and 38.1 mm. The effect of the number of holes on the humidifying rate was not significant. The maximum difference of humidity on products surface decreased as the velocity of air duct increased, and the differences were 3.09%, 2.45%, 2.05% and 2.01%, respectively. But it decreased first then increased as the number of holes increased (2, 4, 6 and 8), and the differences were 2.53%, 2.18%, 2.31% and 2.45%, respectively. And it increased first then decreased as the diameter of pipe increased, with the differences being 2.38%, 2.57%, 2.45% and 2.02%, respectively. The results from this study have a certain reference value for the optimization of humidifying device for fresh-keeping transportation containers.