Efficiency enhancement mechanisms of a new type of full reversible axial-flow fan

Abstract

A row of airfoil-shaped support struts has been proposed to be installed on both sides of the rotor, so as to remedy the configuration asymmetry of a conventional fully reversible fan in its forward and reverse flow modes. In the forward mode, the upstream support struts act as inlet guide vanes (GVs), and the downstream ones as outlet GVs; whereas in the reverse mode, the roles played by the two support-strut rows switch over. Among the five representative airfoils, symmetric airfoil NACA0012-64 is found to be the most suitable airfoil used as the section of the two rows of the airfoil-shaped support struts. Detailed numerical investigations on four such new fans, whose design points cover a wide range, reveal that the installation of the two rows of blades improves the performance of the new type fan, when compared with the one-rotor fully reversible axial-flow fan without such support-struts. The upstream blade row has little influence on the direction of incoming flow and causes little aerodynamic losses. By contrast, the downstream blade row plays an important role in efficiency improvements: (1) the downstream blade row, which acts as outlet GVs, significantly reduces the rotor-exit tangential velocity and recovers part of the swirl energy in the flow downstream of the rotor, although sometimes the aerodynamic losses caused by such support-strut-like outlet GVs may be high at a large incidence; (2) due to the obviously reduced tangential components of the flow velocities downstream of the outlet GV, the air travels a much shorter distance toward the exit of the annular diffuser, compared with that in the fan without outlet GVs. This leads to a markedly reduced friction loss. In addition, the larger the equivalent divergence angle of the annular diffuser is, the more effectively does the installation of the two rows of the support-strut-like blade improve the performance of the new type fans.