Experimental investigation of a sustainable natural convection solar air dryer with an internally ringed finned tube

Abstract The purpose of this study is to design, fabricate, and experimentally evaluate a sustainable natural convection solar air heating system suitable for passive drying and heating applications without the use of auxiliary power sources. A natural convection solar air heater was developed using a rectangular wooden enclosure integrated with a circular tube fitted with internally placed triangular cross section rings to enhance heat transfer and air mixing. A thin aluminum plate supporting a black coated V shaped absorber was positioned above the tube to effectively capture solar radiation. A transparent glass cover was used to minimize convective and radiative heat losses. Experiments were conducted under inlet air temperatures of 28–32°C and solar radiation intensities ranging from 610 to 950 W/m 2 . Key parameters such as tube surface temperature, outlet air temperature, buoyancy driven mass flow rate, and instantaneous thermal efficiency were evaluated. The results show that the internally ringed finned tube significantly enhances convective heat transfer compared to a smooth tube due to improved turbulence and secondary flow formation. The natural convection mass flow rate increased by approximately 25–30%, while the maximum temperature rise between inlet and outlet air ranged from 18 to 22°C under peak solar radiation. A maximum instantaneous thermal efficiency of about 46% was achieved at peak solar irradiance of 950 W/m 2 .