Heat transfer enhancement for the power-law fluids through a parallel-plate double-pass heat exchangers with external recycle

The conjugated Graetz problem of the double-pass heat transfer through a parallel-plate device with uniform wall temperature and external recycle in the outlet channel was solved analytically with the use of the orthogonal expansion technique for the power-law fluids. The mathematical formulation of the heat transfer problem was developed for fully developed laminar velocity profiles through the parallel-plate channels by ignoring axial conduction and assuming fluid properties of temperature independence. The constant outer wall temperature and continuous temperature and heat flux between the two subchannels with inserting impermeable sheet were considered for thermal boundary conditions. The analytical results show that the recycle ratio and impermeable sheet position play significant influences on the efficiencies of the heat transfer as compared to that in a single pass (without an impermeable sheet inserted and without recycle). The outlet temperature of the heat exchanger seems to be independent of the power-law index of the fluid, while the average Nusselt number could not be enhanced significantly with the lowering power index. The power consumption increment owing to the cross-sectional area reduction from single-pass to double-pass was also taken into account for comparisons.