The difference between the axial fan and radial fans can be divided into two parts, namely geometry and fluid dynamics.
An axial-flow fan has blades that force air to move in a parallel direction to the shaft around which the blades rotate. For a radial fan, the air flows in on a side of the fan housing, then turns 90 degrees and accelerates due to centrifugal force as it exits the fan housing. These differences in air flow direction have design implications. For example, a radial fan can blow air across a PCB more efficiently, and using less space, than mounting an axial fan to blow air down onto a board.
The fluid flow rate through an electronics system, e.g. enclosure, is determined by the intercept between the fan and system curves that plot air pressure drop over volumetric flow rate. A system’s air flow curve can be calculated using 1D fluid mechanics, or it may require the use of high performance CFD or experimental data. In general, for the same power and rotation speed the radial fan can achieve a higher pressure head than an axial fan. However, an axial fan can achieve a higher maximum flow rate that a radial fan.
In theory, this same approach applies when using two fans in series or in parallel. When the fans are in series, the maximum flow rate should stay the same as for the single fan, but the maximum pressure head doubles. When using two fans in parallel, the maximum pressure head should remain the same as for the single fan, but the flow rate doubles. In real situations, though, the fans may interfere with each other, thus providing lower than expected results. Thus actual experimentation is typically needed.
More information on the performance differences between fans and blowers in electronic systems can be found in the Qpedia article at www.qats.com or by calling 781-769-2800.