Convection heat loss in vertical chimney

An experimental investigation had been performed to study the buoyancy-opposed mixed convection from an upward flow of hat air to the inner wall of a vertical circular pipe coaled by ambient air. The vertical pipe was used to simulate a tall vertical circular chimney constructed with single-wall metal, which was exposed to a cold ambient environment. Reynolds number (Red) of the flow in the present study ranged from 966 to 14,780, while the Buoyancy parameter (Ω= Grd/Red2) where Grd, is the Grashof number) varied from 0.008 to 2.77. Steady-flow of hot air at moderate pressure and a Prandtl number (Pr) around 0.707 was flowing upward through a vertical steel pipe. Test section of the vertical pipe had a length of 1,625 mm, an internal diameter of 156 mm and an external diameter of 166mm respectively. The vertical pipe was externally surrounded by ambient air at 20°C. Air temperature at inlet of the test section could be controlled at a value between 40°C and 70°C. Radial temperature and velocity profiles of the airflow were measured at both inlet and exit of the test section. Temperatures along the vertical pipe wall were also measured. Non-dimensional expression was developed from the experimental results for prediction of the average heat transfer coefficient of the mixed convection from an upward flow of hot gas to the inner wall of a vertical circular chimney. Convection heat transfer was found to impair when the flow is laminar and was enhanced for turbulent flow condition.