A conjugated Graetz problem of the double-pass flat-plate heat exchanger with internal recycle at uniform wall temperature was solved analytically using the orthogonal expansion technique for the power-law fluid. The mathematical formulation was derived for a fully developed laminar flow through the flat-plate channels by ignoring axial conduction and assuming temperature-independent fluid properties. A constant wall temperature, and both the continuous temperature and the same heat flux at the interface of the two adjacent subchannels made by inserting an impermeable sheet in between, were considered as the thermal boundary conditions. Experiments were carried out in order to validate the proposed mathematical formulation and the results can be very satisfactory. It is found that the recycle ratio and the impermeable-sheet position play significant influences on the efficiencies of this double-pass flat-plate heat exchanger. But, if the power consumption is also evaluated, the performance declines for the double-pass heat exchanger with large reflux ratios. The heat-transfer efficiency enhancement for the power-law fluid with a smaller power-law index is found to be less than that with a larger one, however, if both the heat transfer efficiency and the power consumption increment are considered together, the fluid with a smaller index would have a higher performance.
International Communications in Heat and Mass Transfer 50, pp.44–51