The problem of laminar flow heat transfer in channels of annular cross section formed by concentric equilateral triangles is studied. Numerical solutions for friction factor, and magnitude of minimum limiting Nusselt numbers for both constant heat input per unit length and constant wall temperature are obtained. The results obtained by the numerical method are compared with the available solutions for the limiting geometries of equilateral triangle and parallel plates. Experimental data for isothermal pressure drop and constant wall temperature boundary condition at the outer wall are presented. Test sections with length to equivalent diameter ratios of 13.85, 15.12 and 20.8 are employed in the present investigation. The Prandtl numbers are varied from 4 to 65 applying Glycerol-water mixture as coolant. Empirical correlations for isothermal friction factor and Nusselt number for constant wall temperature boundary conditions are presented.