The heat-transfer Graetz problem of a double-pass parallel-plate heat exchanger for the power-law flow subject to the hybrid boundary condition, was investigated theoretically. An impermeable barrier without thermal resistance was placed between two parallel plates, whose thermal boundaries were preset at constant temperature at one side, and constant heat flux at the other; it is called the hybrid boundary condition. The mathematical model was solved analytically using the separation of variables, together with the superposition principle and an orthogonal expansion in the power series. A dimensionless number ( ) is defined as the ratio of the temperature difference between the inlet-flow and the wall at one side over the heat flux at the other side. It can be found that the value might render significant influences on the heat transfer efficiency. The experiments were conducted to justify the theoretical simulations. The theoretical results show that the fluid with lower power-law indices might possess greater heat transfer efficiency for the double-pass heat exchanger in this study.
