Free convection heat transfer inside vertical and inclined elliptic tubes with different axis ratio and different inclination and orientation angles

Abstract

Experimental investigation of free convection heat transfer from the inside surface of vertical and inclined elliptic tubes of different axis ratios, different orientation angles and different inclination angles with a uniformly heated surface are presented. The axis ratio is changed from 1.5 to 3.5, the inclination angle is changed from 15° to 75° and the orientation angle is changed from 0° to 90°. The experiments covered a range of Rayleigh number, Ra from 6.85 × 10 5 to 1.3 × 10 8. The local and average heat transfer coefficients and Nusselt numbers are estimated for different axis ratios and different orientation angles at different Rayleigh numbers. The experimental results showed that the axis ratio has a significant effect on the local and average heat transfer coefficients. The local Nusselt number ( Nu) increases with the increase of axial distance from the lower end of the elliptic tube until a maximum value is reached near the top of the elliptic tube, and then, the Nu gradually decreases. Also, the local Nu increases with the increase of α, ϕ and AR at the same axial distance. The average Nu increases with the increase of orientation angle, inclination angle and axis ratio (AR) at a constant Ra. Correlations of the average Nusselt number for free convection inside vertical and inclined elliptic tubes with the orientation angles ( α), inclination angles ( ϕ) and axis ratios (AR) are presented.