Effect of external recycle on the performances of flat-plate solar air heaters with internal fins attached

The influence of external recycle on the collector efficiency in solar air heaters with internal fins attached, has been investigated theoretically. The application of external recycle operation to solar air heaters actually has two conflict effects. One is the increase in fluid velocity to decrease the heat-transfer resistance, which is good for performance, while the other is lowering the driving force (temperature difference) of heat-transfer, due to the remixing at the inlet, which is bad for performance. It is found that considerable improvement in collector efficiency is obtainable if the operation is carried out with an external recycle, where the desirable effect overcomes the undesirable effect. The enhancement increases with increasing reflux ratio, especially for operating at lower air flow rate with higher inlet air temperature.