The local heat transfer characteristics associated with mixed convective developing flow through a horizontal tube exposed to a uniform wall temperature boundary condition

Extensive research has been conducted on the heat transfer characteristics related to the boundary conditions present in phase-change applications. However, there remains a fundamental gap in understanding the local heat transfer characteristics of mixed convective laminar flow exposed to a uniform wall temperature boundary condition. Furthermore, there is a disparity between numerical and experimental studies investigating this boundary condition. This study addresses these gaps by being the first to experimentally investigate the local heat transfer characteristics of developing laminar flow through a horizontal tube exposed to a uniform wall temperature boundary condition. A novel experimental setup was developed to measure the mean fluid temperatures along a 5 m-long copper tube with an inner diameter of 4.9 mm. While the local results indicated an increase in wall temperature along the test section, the average Nusselt numbers correlated well with literature, indicating that similar temperature trends existed in prior experimental studies. The local heat transfer characteristics for developing laminar uniform wall temperature flow were divided into four regions: (1) Free Convection Developing, (2) Free Convection Governing, (3) Sustained Free Convection, and (4) Diminishing Heat Transfer. Free convection effects were found to increase near the inlet of the tube and the associated secondary flow assisted the flow in becoming fully developed. However, due to the decreasing wall-fluid temperature differences, free convection effects could not be sustained, and heat transfer eventually diminished as the fluid temperatures approached the wall temperatures.