Convective drying of shrinking hydrogel with a constant temperature stage: Experimental and numerical investigations.

• Drying characteristics and mechanisms of hydrogel with a constant temperature stage are clarified and revealed. • A maximum duration (3 h) of constant temperature stage with an evaporation heat flux 268.7 W/m2 is observed in the hydrogel drying. • The maximum reduction in duration of stage II is 100 % when initial Biot number (Bi 0) increases from 0.39 to 0.47. • Under the same drying temperature, a critical Bi 0 (Bi 0,c) exists for the same hydrogel below the Bi c the drying rate increases with Bi 0 significantly. In this study, the convective drying characteristics of hydrogel with a constant temperature stage at different drying temperatures and initial Biot numbers (Bi 0) are experimentally presented. Additionally, a numerical model coupling the mass, energy conservation equations and solid momentum balance equation is developed and validated to reveal the drying mechanisms. The results show that there are three drying stages, i.e., the temperature rise (stage I), constant temperature (stage II), and second temperature rise (stage III). Numerical results demonstrate that the duration of stage II is related to the internal moisture transport resistance which increases rapidly and leads to the end of stage II. A maximum duration of 3 h for stage II with evaporation heat flux of 268.7 W/m2 is observed at drying temperature of 35 ℃. The three drying stages persist as the drying temperature increases, but the duration of stage II decreases rapidly. In particularly, under the same drying temperatures, there is a critical Bi 0 (Bi 0,c) below which the drying rate increases with Bi 0 significantly, but above it, the drying rate increases slightly for the same hydrogels. The duration and temperature of stage II decrease with increasing Bi 0. [ABSTRACT FROM AUTHOR]