3D numerical simulation of laminar forced-convection flow subjected to asymmetric thermal conditions. An application to solar thermal collectors

Abstract

This paper looks at the problem of fluid flow within pipes subjected to thermal asymmetrical boundary conditions, a problem faced in many real industrial situations, such as those related with solar thermal devices. But despite the asymmetry, much simpler bidimensional models based on radial symmetry are currently used for pipe design purposes, ignoring the important consequences of the asymmetry that exists.For this purpose, a 3D steady-state analysis is made of the laminar forced-convection heat transfer process for a fluid flowing through a round pipe when radially asymmetrical external conditions are applied to the tube external surface (a known uniform temperature to the upper surface and adiabatic condition to the lower), taking into consideration axial heat conduction in the fluid. The differential equations are solved numerically. Also, experimental measurements of temperatures are made in thermal solar devices.The results are depicted by reference to the temperature profile, which depends on the radial-spatial coordinate. The isotherms form parallel-like shapes in the top half of the tube, but become distorted in the lower half, where loops appear due to the conduction effects of the pipe wall, illustrating the non-uniformity of the temperatures within the fluid. When the characteristic length of the problem was considered, the 2D approach was found to be no longer valid when asymmetrical boundary conditions existed in the pipe. Our theoretical/mathematical results closely agreed with the experimental measurements made in solar devices. The knowledge gained in this study of the real non-uniform flux inside pipes has many potential implications for the design of solar thermal devices, both flat plate and CPC plants, as well as a wide range of industrial equipment and facilities.