collector efficiency of upward-type double-pass solar air heaters with fins attached

the collector efficiency of upward-type double-pass flat plate solar air heaters with fins attached and external recycle is investigated theoretically. the double-pass device was constructed by inserting the absorbing plate into the air conduit to divide it into two channels (the upper and lower channels). the double-pass device introduced here was designed for creating a solar collector with heat transfer area double as well as the extended area of fins between the absorbing plate and heated air. moreover, the advantage of external recycle application to solar air heaters is the enhancement of forced heat convection strength, resulting in considerable device heat transfer performance improvement. this advantage may compensate for the remixing at the inlet which decreases the heat transfer transfer-driving force decrement (temperature difference).