Investigation on the Maximum Total to Static Efficiency of Axial Fans With and Without Diffusers
3.4 Fan Design for Improved Efficiency and Extended Operating Range
The range of kinetic energy at the outlet of a fan depends on the fan design and the operation point. In most configurations the kinetic energy at the outlet is a loss and thus affects the fan efficiency. Diffusers can be installed to reduce these losses. In this study the maximum static efficiency
related to the total efficiency is analyzed for a fan with and without outlet guide vanes. In order to achieve a further efficiency increase, the effect of a diffuser is studied. The kinetic energy can be divided into two components, a meridional and a circumferential component. Both components depend on the hub-to-tip ratio and the swirl distribution. In this study the common free vortex design is considered and the hub-to-tip ratio is chosen according to the Strscheletzky criteria. In the first part, the maximum static efficiency of fans with and without outlet guide vanes is analyzed. The outlet guide vanes lead to the elimination of swirl and thus increase the efficiency. A further possibility to increase the efficiency is the installation of an annular diffuser
downstream the axial fan. The diffuser reduces the kinetic energy at the outlet and thus the Carnot losses. A large reduction of the kinetic energy is achieved by a big area ratio of the diffuser. Since the opening angle of the diffuser is limited this can lead to long components. There is a correlation
between area ratio, length and losses in the diffuser. The static efficiency is analyzed for a diffuser length which corresponds to the outer fan diameter. There is a significant increase compared to the latter configurations.