A boundary layer solution is presented for fully developed laminar flow in a horizontal circular tube, assuming large Prandtl number and temperature-dependent viscosity and density. The solution is given by Nu = C1 Ra1/4, where C1 is a function of a nondimensional viscosity parameter and the heat flux boundary condition. The heat transfer predictions for large values of the viscosity parameter are 50 percent above the constant viscosity predictions. The present analysis is in good agreement with experimental data for water and ethylene glycol flowing in electrically heated tubes which approximate the boundary conditions assumed in the analysis.

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