To provide a forced convection model in an actual use condition capable of accurately predicting heat-transfer / flow loss performance of a heat exchanger.

This outdoor unit for an air conditioner comprises a propeller fan 10 and a flat plate fin tube set 20. In designing the outdoor unit for the air conditioner, it is assumed that a design specification of front wind speed of the plate fin tube set is U, diameter of the propeller fan is D, flow rate measurement positions on a suction side on the rotation axis are x, and radii of flow rate measurement ranges about the rotation axis at the flow rate measurement positions x are r. In at least two different flow rate measurement positions x, an average flow speed Vave and r/D at each flow rate measurement position x are determined based on each flow rate Q measured over a predetermined radius r. Then, a relation between x/D and Vave/U and a relation between x/D and r/D are shown on a dimensionless three-axis Cartesian coordinate where transverse axis is x/D, one longitudinal axis is Vave/U, and the other longitudinal axis is r/D. The front wind speed U as a design target and the diameter D of the propeller fan are fitted to the three-axis Cartesian coordinate graph, and an installation position and front area of the plate fin tube set are determined.