Potential of static pressure recovery of low pressure axial fans

Authors

Hauke Witte  - Technische Universität Braunschweig, Germany
Christoph Bode - Technische Universität Braunschweig, Germany
Friedrichs Jens - Technische Universität Braunschweig, Germany

Abstract

Utilized in a wide range of applications, low-pressure axial fans account for a significant share of global energy consumption and thus, especially in view of climate change, the need for efficiency improvements is steadily increasing. Typically, low pressure axial fans are installed in a rotor-only free-exhaust-configuration, where the rotor is discharging directly into a free atmosphere. In such a configuration, the static pressure recovery obtained in guide vanes by decelerating the swirl component of the flow is missing. Consequently, the flow discharging into a free atmosphere has a strong swirl component which contains a considerable part of the kinetic energy imposed on the flow by the rotor. Usually the entire swirl component in the discharge is considered as loss. However, simple considerations show, that a significant part of this kinetic energy contained in the swirl component is recovered as static pressure in the far field. Considering the radial equilibrium immediately downstream of the rotor, the swirl component of the flow results in a negative pressure towards the core of the flow, while in the shear layer at the outer edge of the discharge, atmospheric pressure is imprinted. Thus, the average pressure in a plane immediately downstream of the rotor is below atmospheric pressure. At a plane far downstream of the rotor, the swirl component of the flow is dissipated and thus, the negative pressure at the core of the flow has declined. Consequently, the average pressure must have increased from a plane immediately downstream of the rotor to a plane far downstream of the rotor. Although this additional pressure increase must be directly considered as work done by the fan, is has been sparsely researched so far.In literature, this static pressure recovery is quantified with up to 8 % of the total pressure increase of the fan. In this paper, numerical RANS-simulations of the free atmosphere downstream of the rotor are carried out to estimate the axial extent as well as the amount of the pressure recovery, validating the potential quantified in literature. To evaluate the quality of the static pressure recovery obtained in the free field, a comparison between the rotor-only configuration and a configuration with guide vanes is considered. This comparison between two configurations allows for the formulation of a total pressure loss coefficient, which the guide vanes must have, such that the configurations achieve an equal pressure recovery. This total pressure loss coefficient serves as an evaluation criterion of the pressure recovery for the free-exhaust-configuration. Considering that the distribution of the swirl component downstream of the fan is the central driver for the negative pressure in the exhaust, a variety of generic outlet velocity profiles with a constant operating point are investigated towards their influence on the quality of the static pressure recovery in the free atmosphere. The results of this variation show a promising potential in terms of pressure increase and consequently an increase in efficiency for new designs of low pressure axial fans.