Impact of the Blade Profiling on the Performance of Low-Pressure Axial Fans

Abstract

Abstract Low-pressure axial fans have a very wide range of applications in the field of heating, ventilation and air conditioning as well as refrigeration (HVAC&R). Due to stricter legal requirements, increasingly higher minimum fan efficiencies are required. Due to the simpler manufacturing efforts, unprofiled, simply bent sheet metal blades are often used. However, these simple sheet metal blades have the disadvantage of unfavorable flow separation and stall comparable to that of a thin plate. For this reason, a profiled blade contour is used in axial fans in order to obtain an optimal flow with low flow separations and delayed detachment. The starting point of the present investigation is the axial impeller, which was designed and optimized based on the Euler equation for turbomachines and a subsequent iterative optimization approach, see [1]. In this pre-design, the impeller blades were designed as thin sheet metal blades with a constant thickness. This axial impeller was modeled with the ANSYS BLADE MODELER and simulated with the commercial CFD solver ANSYS CFX 2021R1. After that, different profiles with different symmetrically thickness distributions were applied to the designed blade contour. First, profiles from the NACA four-digit-series were used. The maximum profile thickness was varied between 6%, 12% and 18%, i.e. NACA0006, NACA0012 and NACA0018. However, these NACA four-digit-series profiles have a fixed position of the maximum profile thickness at 30% of the profile length. They were developed for Reynolds numbers of the order of 106. Therefore, the impact of the location of the maximum profile thickness for axial fan blades at lower Reynolds numbers was investigated in this work. Typical Reynolds numbers for the axial fans considered in the application are around 105 and thus one order of magnitude lower. Therefore, in addition to the well-known NACA profile, a self-defined University of Coburg profile, shorten called “CoPro”, was developed in this work that consists of an ellipse from the leading edge to the point of maximum thickness and of a parabola from the point of maximum thickness up to the trailing edge. With the CoPro profiles the maximum profile thickness as well as its position can be parameterized and changed with no restrictions. In addition to the NACA profile study, a detailed parameter variation was also performed with the CoPro profiles. Analogous to this, the maximum profile thicknesses of 6%, 12% and 18% were set. In addition, however, the position of the maximum profile thickness was varied between 15%, 30%, 45%, 60% and 75% of the profile length. These profiled axial fans were also computed with ANSYS CFX. In order to determine the influence of the profiling, the integral characteristics such as the pressure characteristic and the efficiency characteristic were compared. In addition, the flow field around the blade profile was examined in post-processing and the areas of flow detachment and stalling were evaluated. Finally, the study showed that for the present application of the axial fan, a profile with a maximum profile thickness position further to the trailing edge shows a significantly higher efficiency. The method and the results are discussed in detail.