Solar drying is a common method for preservation of food materials and improving thermal efficiency of solar collectors is important as the main part of a solar air heating system. One the most used collectors in solar drying system is flat plate collector and in these collectors the main problem is the low heat flux through the absorber plate, which reduces thermal and drying efficiency of the dryer. In this research, the heat transfer process of the solar dryer was investigated at three conditions non-porous system, with porous system and with porous and recycling system with three methods (experimental, computational fluid dynamics (CFD) and thermo-graphical). The flat plate collector was divided into four equal sections and then the amount of heat transfer coefficient, heat flux and total heat transfer from the collector over the time were calculated for each section. The dryer performance was also evaluated using a heat exchanger that placed at the inlet of the collector. Moreover, the air velocity distribution and temperature counters inside the collector were simulated using computational fluid dynamics method. The results showed that the heat transfer for all cases increased from 10:00 and reached to maximum level at around 12:00-13:00. Due to the reduction of heat accumulation at the end of the collector, heat transfer was more homogeneous for porous collector. Moreover, the results showed that the amount of total heat transfer increased about 16.50% in porous system and 21.19% porous and recycling system compared to non-porous system. The average drying efficiency for solar dryer were about 51%, 53% and 54% for non-porous system, porous system and porous and recycling system, respectively. The results of computational fluid dynamics simulation and thermo-graphical test showed a good agreement between the experimental data and the mentioned methods (R-2 > 0.96 and R-2 > 0.98).