A New Blade Profile for Bidirectional Flow Properly Applicable to a Two-stage Jet Fan

Abstract

Abstract A reversible axial flow fan called jet fan has been widely used for longitudinal ventilation in road tunnels to secure a safe and comfortable environment cost-effectively. As shifting the flow direction is usually made by only switching the rotational direction of an electric motor due to heavy duty, rotor blades having identical aerodynamic performance for bidirectional flow should be necessary. However, such aerodynamically desirable blades haven’t been developed sufficiently, since most of the related studies have been done from the viewpoint of unidirectional flow. In the present paper, we demonstrate a method to profile the blade section suitable for bidirectional flow, which is validated by studying the aerodynamic performances of rotor blades of a two-stage jet fan experimentally and numerically. Keywords : Blade profile, Wall curvature, Bidirectional flow, Jet fan, Aerodynamic performance, Turbulent flow analysis 1. Introduction A reversible axial flow fan called jet fan has been widely used for longitudinal ventilation in road tunnels to secure a safe and comfortable environment cost-effectively. Various researches and developments of jet fan have been made, and it is said that the fan has the satisfactory performance for the present needs. However, if the future need is concerned, it is still necessary to develop a jet fan having higher aerodynamic performance, for efficient usage of energy in every engineering system is vital in the low carbon society. From the aerodynamic view, a cambered airfoil is regarded as favorable for the rotor (or impeller) blade like a unidirectional axial fan and the variable pitch mechanism is used to set the blade oppositely if the flow direction should be reversed in case of emergency. But, owing to very heavy duty as a safety measure, a reversible fan without this kind of mechanism has been adopted in actual cases. That is, such a method to drive the rotor in the reverse direction is used for switching the flow direction. As the fan should provide almost the same performance in both flow directions, those symmetrical blades without camber are used in the case of single stage fan rotor. The other structure is a 2-stage jet fan where two rotors are assembled to both sides of motor shaft. If higher speed jet from the fan is expected, the latter will be preferable. Though there are vast amounts of studies treating airfoil sections (e.g.: Abbot and von Doenhoff [1]), most of them have been done from the viewpoint of unidirectional flow. Thus, aerodynamically desirable airfoil sections for bidirectional flow are still regarded as unresolved. As one of the typical sections is a symmetrical circular-arc airfoil, this section has been actually used for the rotor blade of a jet fan (Nishioka, et al.[2]). Generally, airflow easily separates from the leading edge at a certain angle of attack for a blade having the sharp edge. However, it is known that aerodynamic performance of the blade doesn’t deteriorate, once the separating flow reattaches the blade surface downstream. This knowledge indicates that the key to secure the performance is attributed to the stable reattachment of the separating flow on the surface. From this view, the wall surface consisting of concave and convex curves shown in Fig. 1 is taken up for consideration. Though it will be nice if this profile is usable for the blade in bidirectional flow, stable reattachment of separating flow shown in Fig. 1 is really accomplished in actual flow cases. Thus, we have investigated the capability of the proposed blade profile for bidirectional flow by using a two-stage jet fan experimentally and