Thrust measurements to assess the performance and the characteristic curves of ducted axial fans: A first experiment

In the field of industrial ventilation, some operators consider the standard parameter fan static pressure p fs as the term representing the operation and performance of a ducted axial fan, neglecting (or ignoring) the meaning of the fan pressure pf. This misunderstanding persists since more than 60 years and is a common cause of fans mismatching the required duty, when installed in an air system. Trying to clarify this incomprehension, understanding the operation of a ducted axial machine would possibly be easier for any operator if the characteristic curves were obtained with measurements of fan thrust. With this didactic aim, an original experiment was arranged to compare the characteristic curves of a ducted axial fan as obtained according to the ISO 5801 standard (in terms of fan pressure and fan static pressure) with the one resulting from measurements of the fan thrust. An inlet chamber test rig was built and arranged to perform simultaneously the conventional standard measurements of air pressure and those of the thrust experienced by a small ducted axial fan under test, thus permitting an immediate comparison of the characteristic curves. Tests according to the ISO 13350 standard with the fan in jet-configuration were performed as well, including and omitting the bell-mouth to assess the performance drop caused by the absence of a suitably shaped inlet. The first results show a promising agreement between the characteristic resulting from the thrust measurements and the fan pressure pf one in the stable part of the curves, providing the operators a different method to understand the actual performance of a ducted axial fan. In addition, the tests on the machine in jet configuration show that the omission of the bell-mouth causes a 14% and 16% loss on the jet-fan performance and efficiency, respectively.