Abstract :
Passive solar air heaters, such as solar chimneys and Trombe Walls, rely on solar-induced buoyancy-driven (natural) convection to
produce the flow of air. Although buoyancy-driven convection is well understood for a single vertical plate, buoyancy-driven convection
in an asymmetrically-heated channel is more problematic, and in particular, the effects of the channel height on the flow rate and heat
transfer. This paper reports on experiments on test rigs resembling lightweight passive solar air-heating collectors. The test rigs were of
heights 0.5, 1.0 and 2.0 m, with adjustable channel depths (20–150 mm) and heat inputs (up to 1000 W/m2). Measurements were made of
the air, plate and cover temperatures, and air velocities. Results are presented as dimensionless correlations of mass flow (as Reynolds
number) and efficiency against heat input (as Rayleigh number), channel depth and height. Thermal efficiency is shown to be a function
of the heat input and the system height, but not of the channel depth; mass flow is shown to be a dependent on all three parameters.
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