Abstract:
Performances of pedestal fans were analysed through experimental measurements of the induced flow field (velocity distribution) and the power consumption by two different types of fans. A test rig was constructed to facilitate the measurement of axial flow velocities in the three dimensional space. In order to predict the jet pattern produced by the fan, axial velocity distributions on sufficient number of planes were measured. These data were used to calculate the jet diameter, flow rate, kinetic energy and the linear momentum of the flow at different planes from the rotor. The power consumption and the angular speed of the rotor for each regulator setting were also measured.
The velocity distributions measured on different planes from the rotor clearly illustrate the general characteristics of the flow generated by free flow fans, including the reverse flow near the rotor plane, jet entrainment and diffusion. The results show that the overall energy efficiency of a fan depends critically on its speed (i.e. regulator setting); efficiency increases with the speed. The results also indicate that the service factor defined by the flow rate at a specified section divided by the power consumption is not a suitable performance index for free-flow fans. Even with a similar axial velocity profile generated by two different fans could result in two different jet developments leading to completely different flow characteristics and performances. The results of the present study indicate the complexity of the jet development and the necessity for detailed experimental measurements of the flow velocity distribution including the swirl components in predicting performance of free-flow fans.
