Heat transfer of conjugated Graetz problems with laminar counterflow in double-pass concentric circular heat exchangers

A device of external recycle at the ends of double-pass concentric circular heat exchangers with uniform wall temperature, resulting in substantially improving the heat transfer, has been designed and studied theoretically. The theoretical analysis on heat transfer efficiency improvement has been developed using orthogonal expansion technique in power series. The analytical results are also represented graphically and compared with that in an open conduit (without an impermeable plate inserted and without recycle). Considerable improvement in heat transfer is obtainable by employing the external recycle at both ends with a suitable adjustment of the impermeable-sheet position and recycle ratio, instead of using an open conduit.