| Unglazed transpired-plate solar air heaters have proven effective in heating air on a once-through basis. However, the basic theory for these collectors has not evolved to the point where the flow distribution can be predicted. The purpose of this thesis is to characterize the flow distribution through the collector so that the underlying principles of the flow field can be established.; The maximum thermal energy gain for vertically oriented collectors occurs when the collector flow rate is established through buoyancy alone. At buoyancy driven flow rates, up to 45% of the heat transfer from the absorber to the air takes place in the collector's plenum. The effect of buoyancy on the inlet flow distribution is that near stagnation flow occurs at the top of the absorber, and relatively high inlet flow (suction) velocities at the bottom of the absorber. The absorber temperature will be high at near stagnation conditions. Reverse flow through the absorber will occur if the collector flow rate is fixed at a level below the collector's buoyancy driven flow rate, resulting in a thermal energy loss. Nonuniformity of the inlet flow distribution does not significantly reduce the absorber's heat transfer rate: as the suction velocity decreases, the air temperature rise increases. This relationship balances out the local heat transfer rate for the air passing through the absorber. For collectors experiencing nonuniform wind induced pressure distributions, the overall result is the same, but the collector flow rate must be increased to prevent reverse flow. A small plenum aspect ratio will reduce viscous and acceleration effects in the plenum, resulting in more uniform flow distributions. This strategy will reduce pumping head requirements, and the thermal efficiency will not be reduced by heat transfer from the plenum air stream to the absorber plate. (Abstract shortened by UMI.)... |
