Visualization of upward mixed-convection flows in vertical pipes using a thin semitransparent gold-film heater and dye injection

Abstract

An experimental technique has been developed to visualize water flows in pipes subjected to a uniform wall heat flux (UHF). The technique is based on the use of a thin semitransparent gold-film heater, glued inside a Plexiglas pipe, and dye injection. The capabilities of this technique were used to examine upward mixed-convection flows in vertical pipes. Most of the earlier experimental studies of the UHF boundary condition employed an opaque heating tube. In contrast, the present technique allows direct visualization of fluid-flow phenomena, such as recirculation cells and laminar-turbulent transition, over the entire length of the heated pipe. Flow visualization and numerical results pertaining to one recirculation cell, for values of Re = 90, Gr q = 0.33 × 10 6 (Gr q /Re 2 = 40.7), and Pr = 5.6, are presented in this paper, and the agreement between these results is shown to be qualitatively quite good. Two cases of laminar-turbulent transition are reported in this paper, one for values of Re = 72, Gr q = 0.7 × 10 5, and Pr = 6.0; and the other for Re = 48, Gr q = 0.95 × 10 5, and Pr = 6.0. It appears as if this is the first presentation of such results for a UHF boundary condition.