An experimental study on flow patterns and heat transfer enhancement in low-Reynolds-Rayleigh-number channel flow

An experimental study was conducted on buoyancy-induced flow patterns and heat transfer characteristics of airflow through a horizontal rectangular channel. The channel had an aspect ratio of six, and its bottom and sidewalls were heated, whereas the top of the channel was cooled. The experiments were conducted at Reynolds numbers of 40 and 100 and Rayleigh numbers ranging from 100 to 4200. The Nusselt number and the temperature distributions on the top surface of the channel were measured simultaneously at different thermal/flow conditions, and the heat transfer characteristics of the channel were evaluated, together with the flow patterns in the channel. The results showed that due to the heated sidewalls, which was an 'imperfect' factor comparing with the classic Rayleigh-Benard channel, longitudinal vortex rolls can occur at a Rayleigh number Ra=100, starting with number of rolls N=2 and then N=4 as Ra increases, rather than the N=6 mode for the same channel with 'perfect' sidewalls. It was demonstrated that the high modes can be excited actively in the channel to produce significant heat transfer enhancement in low Reynolds and Rayleigh number flow.