Heat transfer by free convection from the inside surface of the vertical and inclined elliptic tube

Abstract Free convection from the inside surface of vertical and inclined elliptic tubes of axis ratio ( a : b ) 2:1 with a uniformly heated surface (constant heat flux) is investigated experimentally. The effects of orientation angle ( α ) and inclination angle ( ϕ ) on the heat transfer coefficient were studied. The orientation angle ( α ) is varied from 0° (when the major axis is horizontal) to 90° (when the major axis is vertical) with steps of 15°. The inclination angle ( ϕ ) is measured from the horizontal and varied from 15° to 75° with steps of 15°. The vertical position is considered as a special case of the inclined case when ϕ  = 90. The experiments covered a range of Rayleigh number, Ra from 2.6 × 10 6 to 3.6 × 10 7 . The local and average Nusselt numbers are estimated for different orientation angles and inclination angles at different Rayleigh numbers. The results obtained showed that the local Nu increased with the increase of axial distance from the lower end of the elliptic tube until a maximum value near the upper end, and then, it gradually decreased. The average Nu increases with the increase of α or ϕ at the same Ra . The results obtained are correlated by dimensionless groups and with the available data of the inclined and vertical elliptic tubes.