Experimental study on drying kinetics and thermal modeling of drying Kohlrabi under different solar drying methods

In all reported work, the constant specific heat capacity of the drying product was presumed for the thermal modeling of the drying system, although this is not the case. This paper focuses on developing a thermal model based on Kohlrabi's temperature-dependent instantaneous specific heat capacity to predict the drying product's temperature under transient and steady-state conditions. The heat transfer (convective and evaporative) coefficients were also determined based on experimental data, including psychometric conditions. Three drying modes were employed: open sun drying, single slope direct solar dryer (SSDSD) with natural air circulation, and SSDSD with forced air circulation. The experimental results showed that the moisture content of the Kohlrabi sample decreased from 91% to 13.67% (on a wet basis) in an open, natural, and forced mode dryer with an average drying rate of 0.0398, 0.0509, and 0.0749 kg/h, respectively. The average convective heat transfer coefficient was determined to be 0.85, 0.63, and 1.61 W/m2.K, and evaporative heat transfer coefficients were 3.34, 9.63, and 17.77 W/m2.K for open, natural, and forced mode, respectively. The results indicated that the higher wet bulb depression (13.56 degrees C), lower relative humidity (40.55%), and higher convective and evaporative heat transfer coefficient signified the highest forced air circulation drying rate compared to other drying modes. The developed thermal model predicted the drying product's temperature agrees with experimental data under steady-state conditions for forced air circulation mode, whereas transient conditions for open sun drying and natural air circulation with 3.19%, 4.23%, and 3.53% root mean square errors, respectively.