Experimental investigation and theoretical analysis of inclined tubes with and without internal rings for natural convection in an energy efficient stove

This study investigates the heat transfer performance of an energy efficient smokeless biomass stove enhanced by natural convection through inclined tubes. The stove incorporates a cylindrical combustion chamber (406.4 mm in diameter and 762 mm in height), enclosed within an insulated outer housing to minimize thermal losses. A mild steel inclined tube, 20–24 inches in length and positioned at a 75° inclination, facilitates passive air supply to the combustion zone via buoyancy-driven flow. Two configurations of the inclined tube were examined: a plain tube and a tube fitted with internal circular rings to induce turbulence and enhance convective heat transfer. Five K-type thermocouples were installed along the length of each tube at 120 mm intervals to monitor temperature gradients. The internally ringed tube consistently recorded wall temperatures 8–18 °C higher than those of the plain tube, attributed to intensified flow disruption and improved heat transfer from flue gases to the tube surface. This temperature rise increases the buoyancy effect, reducing the density of incoming air and thereby enhancing the natural draft into the combustion chamber. Improved air inflow promotes more efficient combustion and reduces smoke generation, without relying on mechanical assistance. The experimental results are supported by a mathematical model based on buoyant flow theory. The findings confirm that simple internal modifications to inclined tubes can substantially improve natural convection and heat transfer, making this approach a viable and low cost solution for sustainable cooking, especially in rural settings where fuel efficiency and indoor air quality are essential.