Effect of external recycle on upward-type solar air heaters

The external recycle effect on the collector efficiency in upward-type flat-plate solar air heaters is theoretically investigated. External-recycling application to solar air heaters has two conflicting effects due to the forced heat convection strength resulting in considerable device heat transfer performance improvement. The first effect is the advantage of decreasing the heat transfer resistance due to increasing fluid velocity. The second disadvantage effect is the remixing at the inlet which decreases the heat transfer transfer-driving force decrement (temperature difference). The theoretical predictions show that considerable improvement in collector efficiency is obtainable if the operation is carried out with external recycling in which the advantage effect overcomes the disadvantage effect. Device performance enhancement increases with increasing recycling ratio, especially when operating at a lower air flow rate with higher inlet air temperature.