AN ENGINEERING APPROACH TO BLADE DESIGNS FOR LOW TO MEDIUM PRESSURE RISE ROTOR-ONLY AXIAL FANS

Low to medium pressure rise axial fan equipment of the arbitrary vorte x flow rotor-only type is widely used in industrial and commercial app lications, with many of the installations and rotor designs being far from optimum. Complex computational methods exist for analyzing flows in, for example, high-speed axial flow compressors with multistage bla de rows; however, the designers and manufacturers of low-speed, genera l-purpose axial flow fan equipment have been reluctant to embrace this technology. A simpler yet reliable design technique is presented that allows this category of ducted axial fan rotors, in the presence of s wirl-free inlet flow, to be designed to achieve a specified duty with sufficient accuracy for engineering purposes. Practical blade design r ecommendations and limits, similar to those that exist for free vortex flow axial rotors, have been established for the arbitrary vortex flo w rotor-only case. The technique employs a straightforward engineering approach to arbitrary vortex flow axial fan rotor design, and the equ ation set can be solved by using relatively simple numerical methods. Estimates of pressure rise and shaft power characteristics for a propo sed fan/rotor design can be computed and the design loop iterated unti l an acceptable set of blade parameters is identified. It is also poss ible to analyze the performance of an existing axial fan installation as a prelude to the design of a more efficient and effective replaceme nt rotor. Experimental data used in validating the design and analysis techniques are also presented. These data include comprehensive Cobra pressure probe surveys of local flow parameters downstream of three d ifferent low boss ratio, low solidity, arbitrary vortex flow rotors (a ll with circular arc camber line type blades) as well as fan performan ce characteristics for one of the experimental rotors configured as a direct-exhaust fan unit. Installation-dependent factors such as direct -exhaust losses and tip clearance effects are also examined. The analy tical technique is shown to provide acceptable estimates of fan/rotor pressure rise performance and shaft power characteristics over a moder ately wide range of blade angles and operating conditions.