Combined forced and natural convective heat transfer from spheres at small reynolds number.

Experiments on heat transfer from spheres to liquids have been carried out in the range of 2.0×10-4<Re<900, 7.0<Pr<2.4×104 and 5.7×10-3<Gr<107 for combined convection. A sphere, 46.03 mm in diameter, composed of eleven copper segments, and two brass spheres (19.03 and 9.534 mm in diameter) have been used. Test liquids used were 0.96%, 2.59%, 2.62% CMC aqueous sol. and 0.69%, 0.97%, 1.07% MC (methylcellulose) aqueous sol. The experimental data are summarized by the following relation: Num-2 = [{(126Re+57Re3/2)Pr1/3/(1+52Re1/2+100Re)}3/2 + (0.44Gr1/4Pr1/4)3/2]2/3(μb/μs)1/4 where μb is the viscosity at the bulk liquid temperature tb and μs is the viscosity at the sphere surface temperature ts. This equation has been shown to represent measured values of Num within an average deviation of 13.6 per cent.