The new device of double-pass concentric-tube heat exchangers with sinusoidal wall fluxes and external recycle has been developed theoretically in the present study. The approximation solutions to the resultant mathematical formulations for such conjugated Graetz problems were achieved by using the orthogonal principle and Frobenius method. The theoretical results of the temperature distribution and heat transfer efficiency are represented graphically and compared with those in single-pass devices (without an inner tube inserted) of the same working dimensions. The results show that introducing recycle operation can enhance the heat-transfer efficiency in designing double-pass concentric-tube heat exchangers with sinusoidal wall heat fluxes. The influences of the inner tube location and Graetz number on the heat-transfer enhancement and power consumption increment have also been discussed.
