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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. [ABSTRACT FROM AUTHOR]

China is a country that produces and consumes large amounts of fruits and vegetables. During fresh-keeping transportation, energy consumption rises with the increase of traffic volume of fruits and vegetables. The fresh-keeping transportation container with controlled atmosphere by liquid nitrogen injection is an advanced and efficient equipment for transporting fruits and vegetables. However, there is little research on the energy consumption regulations of this kind of fresh-keeping transportation container so this article puts forward a research method. Fresh-keeping transportation with controlled atmosphere by liquid nitrogen injection for fruits and vegetables keeps the temperature, relative humidity, and oxygen volume fraction of the transportation container in a state of relative balance, which could meet the demand for fruit and vegetable fresh-keeping. However, due to the influence of heat transferring towards the container, cold consuming of electrical appliances, cold consuming of fruits and vegetables, heat transferring by door opening, aperture heat leaking, container pre-cooling, and solar radiation in transportation, the equilibrium states of fresh-keeping environment in the container is broken. In addition, heat and mass transferring between the inside and outside of the container has begun. At this time, the fresh-keeping equipments started to keep the equilibrium states of fresh-keeping environment, and then energy consumption was generated. The fresh-keeping equipments of controlled atmosphere fresh-keeping transportation container by liquid nitrogen injection consist of refrigeration, humidification, and controlled atmosphere system, and the energy consumption generated from these three equipments. Finally, the energy consumption model was established based on the analysis of the heat and mass transfer and energy consumption in refrigerating, humidifying, and injecting. After the energy consumption model was set up, the verification experiment was carried out using litchi as testing material and was based on fresh-keeping transportation platform with controlled atmosphere by liquid nitrogen injection for fruits and vegetables. The fresh-keeping transportation platform can adjust the temperature, relative humidity, and oxygen volume fraction in the container automatically and intelligently through refrigeration, humidification, and controlled atmosphere system, creating a suitable fresh-keeping environment for the litchi. Results indicated that the energy consumption was mainly composed of the controlled atmosphere energy, refrigerating energy consumption, and humidifying energy. The theoretical energy consumption obtained through the energy consumption model was basically consistent to the experimental energy consumption with the average relative error of 11.86%±4.29%. The theoretical liquid nitrogen consumption value was basically consistent to the experimental liquid nitrogen consumption mass with the average relative error of 11.60%±3.51%. The total refrigerating capacity from the controlled atmosphere process was associated with the controlled atmosphere volume of the container. What's more, by consuming less energy, the controlled atmosphere could produce a large total refrigerating capacity that accounted for about 22% of the total theoretical refrigerating capacity in the experimental verification. This research provides a reference for optimizing the equipments of controlled atmosphere transportation by liquid nitrogen injection and saving energy of fruits and vegetables during fresh-keeping transportation. [ABSTRACT FROM AUTHOR]

Litchi is special fruit in China. It is important and difficult to keep fresh for litchi during transportation after harvest because it matures in hot seasons and will deteriorate rapidly while stored and transported in normal atmospheric temperature. Foam box with ice, refrigeration, and controlled atmosphere are three main fresh-keeping transportation modes for litchi at present. In order to obtain the effects of different fresh-keeping transportation modes on the quality of litchi fruit, the experiments of these three fresh-keeping transportation modes for "Feizixiao" litchi were carried out based on fresh-keeping transportation platform for fruits and vegetables. The developed fresh-keeping transportation platform can adjust temperature, relative humidity and oxygen volume fraction automatically and intelligently according to initial parameters setting and real time feedback from container to create a suitable fresh-keeping environment for litchi. The main processing of the three experiments is conducted as follows. For the experiment of ice-added transportation in foam box, litchi was packaged with polyethylene bags and then put into foam boxes with appropriate crushed ice (mass proportion of ice to fruit was about 0.30) to maintain low temperature. While for the refrigerated transportation experiment, litchi was packaged with perforated polyethylene bags (aperture ratio was 5%) and then put into plastic baskets, keeping the container environment temperature of 3-5°C and relative humidity of 90%-95%. The package type of the controlled atmosphere transportation experiment was the same as that of the refrigerated transportation experiment and the two experiments were at the same temperature and relative humidity in container, but the oxygen volume fraction should be set to 3%-5%. On this basis, the foam boxes or plastic baskets with packaged litchi were fixed on the vibration device workbench located in the container of fresh-keeping transportation platform, and set the vibration acceleration of 0.25 g (gravitational acceleration) corresponding to the vibration level of 70-80 km/h running on highway. The main fresh-keeping quality parameters of litchi were tested per 24 hours, mainly including mass loss rate, good fruit rate, browning index, total soluble solide content, titrable acidity, firmness of pulp, relative conductivity of pericarp and shelf life quality. Results indicated that the effects of three fresh-keeping transportation modes on the quality of litchi within 1 day of transportation time had no significant difference (p>0.05). However, the refrigerated transportation and controlled atmosphere transportation could effectively delay the quality decline of litchi along with the extension of the transportation time. The controlled atmosphere transportation could significantly delay the decline of pulp firmness, total soluble solids content, pericarp relative conductivity and shelf life quality (p<0.05) after 4 days of transportation compared with the other two transportation modes. It could be concluded that the three transportation modes were all suitable for the litchi quality keeping within 1 day of transportation period; and the refrigerated or controlled atmosphere transportation would be better for the litchi transportation lasting for 2-4 days; however, the controlled atmosphere transportation mode would be the most appropriate selection for the litchi transportation for more than 4 days. The research can provide a reference for the selection of fresh-keeping transportation mode for litchi fruit. [ABSTRACT FROM AUTHOR]