It has been speculated that a forced pipe flow is always assisted by free convection owing to the dependency of fluid properties on its temperature. The purpose of the current study is to experimentally examine the effect of different-sized smooth horizontal pipes on mixed convection of water in internal flows under uniform heat flux (UHF) wall conditions. Infrared thermal imaging is used to measure outer surface temperature in axial and circumferential directions. Reynolds number range is taken between 1000 and 18,000 on three test sections of the diameter of 8 mm, 13.8 mm, and 17.8 mm. The outcome of varying tube diameter, mass flux, and heat flux on mixed flow characteristics is studied. The strength of free convection is illustrated by the ratio of top to bottom local heat transfer coefficient. It is found to be maximum at the tube outlet by 50% and 80% for 8 mm and 13.8 mm tube diameter than the inlet. This enhanced the laminar Nusselt number by 3 to 6 times the analytical value of Nu = 4.36 under UHF condition. The Nusselt number increases with the increase in the tube diameter. The Nusselt number increased by 36% when the surface area increased from a tube diameter of 8 mm to 17.8 mm. Also, the temperature distribution in the turbulent regime remains constant from the highest point to the bottom point. However, it significantly differs in laminar flow. A suitable correlation is suggested for the variation of the Nusselt number under the laminar regime showing the emphasis of free convection on forced convection.