There is a gap in the fundamental understanding of developing flow, especially in the field of phase change heat exchangers, where the tube walls of the heat exchanger are held at a uniform temperature. To date, no study has investigated the local development of mixed convective flow, and only a few have been able to analyze fully developed flow. Relationships have been developed that attempt to predict the heat transfer characteristics of the flow, but as will be shown in this paper, there is a level of uncertainty associated with them. This study experimentally investigated the local development of flow through a 5 m long tube with an inner diameter of 4.9 mm. The experimental setup used a water bath system to control the wall temperatures and measure the fluid temperatures along the tube at 1 m intervals, making it possible to measure the local development of the flow. Experiments were conducted between Reynolds numbers of 600 and 2 400 with tube wall temperatures maintained 30°C and 35°C. The results indicate that even with wall temperature non-uniformities, the experimental correlations could still accurately predict the heat transfer results. Furthermore, it was determined that a constant wall temperature could only be achieved at low Reynolds numbers. When a perfect UWT was enforced, the local and average Nusselt number results no longer correlated with literature. This indicated that experimental wall temperature non-uniformities do not necessarily void the assumption of UWT flow but are in fact an integral part of experimental investigations. These findings highlight the uncertainties with previous UWT experimental and the fact that care should be taken when comparing experimental and numerical results.