Heat transfer by free convection inside horizontal elliptic tubes with different axis ratios and different orientation angles

Free convection heat transfer from the inside surface of a horizontal elliptic tube of different axis ratios and different orientation angles with a uniformly heated surface is investigated experimentally. The axis ratio is changed from 1.5 to 3.5 and the orientation angle is changed from 0 degrees to 90 degrees with steps of 15 degrees. The experiments covered a range of Rayleigh numbers (Ra) from 6.5 x 10(5) to 1.13 x 10(8). The local and average heat transfer coefficients and Nusselt number (Nu) are estimated for different axis ratios and different orientation angles at different Rayleigh numbers. The results showed that the surface temperature increases with the increase in axial distance from both ends until a maximum value at the middle of the elliptic tube at a constant heat flux. The surface temperature decreases with the increase in the axis ratio or the orientation angle; consequently the local Nu increase with the increase in the axis ratio or the orientation angle at the same axial distance and Ra. The average Nu increases with the increase in the axis ratio or the orientation angle at the same Ra. The results obtained are correlated by dimensionless groups and with the available data of the horizontal elliptic tube. (C) 2009 American Institute of Physics. {[}DOI: 10.1063/1.3207799]