Modeling mixed convection in laminar pipe flow

Laminar pipe flow is becoming an important experimental test case for new high efficiency heat carriers like nano- and ferrofluids. Here, a new scaling approach for mixed convection in laminar pipe flow with constant heat flux is proposed. The model relates the radial temperature gradient at the wall, represented as the local Nusselt number, with local Reynolds, Prandtl, and Grashof numbers. The proposed scaling approach is successfully employed to collapse data from different test rigs with horizontally oriented pipes and operated with water. Influences of differing strengths following from free convection are gathered with the new scaling. Moreover, the new scaling approach is successfully utilised to value experimentally obtained heat transfer data of nanofluid flow. In this regard, the impact of nanoparticles, namely the suppression of heat transfer in mixed convection, is experimentally shown and theoretically analysed. Finally, the influence of pipe orientation (vertical / horizontal) is discussed.