This paper summarizes an analytical investigation of the effects of free convection on fully developed laminar flow in horizontal circular tubes with uniform heat flux. Solutions for heat transfer and pressure drop, with both heating and cooling, were obtained for water with two limiting tube-wall conditions. The infinite-conductivity tube exhibits higher Nu and f than the glass tube, with Nu being over five times the Poiseuille value at Gr ∼ 106. With the glass-tube boundary condition, significant circumferential wall temperature variations exist. A correlation for the difference between the wall temperatures at the top and bottom of the tube is presented. Design correlations for Nu and f are developed from the analytical solutions. The analytical predictions bracket recent experimental data for water.

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