Effects of relative humidity on heat and mass transfer characteristics of carrot during hot air drying

Humidity, as an important drying medium parameter, has significant influence on heat and mass transfer during drying process. It has the most significant influence on the heat and mass transfer during the drying process. Relative Humidity (RH) is often used to describe the humidity content of the drying medium under constant drying temperature and atmospheric pressure. The available research reported that the pressure difference of moisture vapor can be enlarged as RH decreased so that the drying force was enhanced for better drying efficiency. Additionally, step-down RH can accelerate the drying rate to prevent surface casehardening in the porous agriculture products whose surfaces were easily crusted during drying. Step-down RH drying means that a high RH is selected to pretreat the material until the temperature increases to a high level, and afterwards a decreased RH with a low value is obtained to increase surface moisture evaporation. Now, step-down RH has been successfully applied into the drying processing of yam slices, and American ginseng root. This study aims to reveal the mechanism for improved drying efficiency with step-down RH drying. Carrot slabs were selected to explore the convective heat transfer (ht), convective mass transfer (hm), and surface micro-pore structure under the drying condition of constant RH and step-down RH with constant drying temperature 60 ℃ and constant air velocity 3.0 m/s. The results showed that the increase of RH significantly enhanced ht, so that the material temperature increased quickly to a high value. With 20%, 30%, 40%, and 50% RH, ht was 42.9, 64.7, 135.1, and 178.9 W/(m•℃), respectively. The ht value of 50% RH was 3.17 times that of 20% RH. During 0-15 min with 50% RH, the drying rate was small and little moisture was evaporated. The carrot obtained the least amount of heat of 1 159.3 J, of which 33.0% was used for water evaporation. During 0-15 min with 20% RH, the drying rate was high and more moisture was evaporated. The carrot obtained the most amount of heat of 1 387.9 J, of which 64.5% was used for water evaporation. Both absorbed energy and percentage of moisture evaporating decreased as RH increased. The percentage of energy consumption at material temperature increased as RH increased. When the RH was 20%, hm was 1.01×10-6-2.54×10-6 m/s, whereas, when the RH was 50%, hm was 0.26×10-6-1.12×10-6 m/s, indicating that the decreasing RH significantly increased the hm coefficient. When 50% RH was kept 30 min and then decreased to 20%, the hm value was the maximum, compared with the other three holding time with high RH. With 50% RH drying condition, it was beneficial for keeping the material surface porous structure. However, when the RH was 20%, the moisture diffusion duct was easily shrunken and blocked, due to a high drying rate. Therefore, the mechanism of improved drying efficiency with step-down RH drying can be expressed as follows. Firstly, the ht value was improved with the high RH in an increasing stage of drying rate. Secondly, the surface of the porous structure was also well kept with the high RH in the increasing stage of drying rate. Thirdly, the hm increased with the low RH in constant and falling drying rate. Such investigation can be expected to serve as a theoretical foundation to calculate the ht and hm during the drying process. Meanwhile, the specific mechanism of improved drying efficiency can provide technical support for the wide use of step-down RH drying into agriculture products.