Numerical thermohydraulic investigation of developing mixed convective laminar flow through horizontal tubes with a uniform wall temperature

Abstract

A numerical investigation of simultaneously developing laminar mixed convective flow in a horizontal tube under a uniform wall temperature (UWT) boundary condition was conducted. Two numerical models were created to examine the differences between experimental and numerical approaches. The first replicated an experimental UWT setup, while the second implemented an ideal UWT boundary condition. Simulations were performed in ANSYS Fluent with temperature-dependent water properties. The Reynolds number and Grashof number ranges were 500-2000 and 0.18×103-9.43×103, respectively. Length-to-diameter ratios varied between 1020 and 1632. The circulation strength of buoyancy-driven vortices was calculated to quantify free convection effects. Higher Grashof numbers intensified the circulation strength and shifted the peak circulation strength upstream. Increased Reynolds numbers delayed the occurrence of peak circulation strength without altering its magnitude. A boundary layer analysis indicated longer thermal and hydrodynamic entrance lengths with higher Grashof numbers or Reynolds numbers. Furthermore, the local Nusselt number decreased from a maximum at the tube inlet to a trough and then increased to a peak before declining to a value of 3.66. Elevated Grashof numbers amplified these trends and shifted the troughs and peaks upstream, whereas increasing Reynolds numbers delayed these extrema points without significantly affecting their magnitudes. Based on circulation strength and boundary layer behaviour, seven thermohydraulic regions were defined: (1) hydrodynamic-merge, (2) free convection increasing, (3) free convection dominating, (4) free convection settling, (5) sustained free convection, (6) hydrodynamically fully developed forced convective flow, and (7) thermally fully developed forced convective flow. HIGHLIGHTS • Seven distinct thermohydraulic regions defined for laminar UWT mixed convective flow. • Thermal and hydrodynamic boundary layer analysis for developing UWT flow. • Quantification of mixed convective flow using circulation strength of buoyancy-induced vortices. • Mathematical justification of increasing local Nusselt numbers for mixed convective flow. • Influence of Grashof and Reynolds number on local Nusselt numbers of mixed convective UWT flow.