Forward sweep to improve the efficiency of rotor-only tube-axial fans with controlled vortex design blades

Abstract

Rotor-only axial fans feature rotors designed according to different vortex criteria. Nowadays the literature does not exhaustively clarify when a specific swirl distribution has to be used and which are the advantages/drawbacks in terms of fan performance and efficiency. A review of the experimental performance of rotor-only axial fans designed with different vortex criteria is summarized here in Φ − Ψ and σ − δ (specific speed-specific diameter) graphs to identify the best operating conditions of each design. Four rotor-only axial fans (two free-vortex, a constant-swirl and a rigid-body swirl one) are tested on an ISO-5801-A rig. For two of them, flow velocities at rotor exit are measured with a 5-hole probe. The result is an experimentally based map around the Cordier curve for rotor-only axial fans. Indications on the best Φ − Ψ range for fans designed using different vortex criteria are provided and explained. The effects of increasing the tip clearance on the rotor performance at design duty are investigated as well. KEYWORDS Rotor-only axial fans, Vortex criteria, Axial-fan design, Arbitrary vortex, Non-Free Vortex, Cordier curve NOMENCLATURE const generic constant c chord length [mm] c tip tip chord length [mm] r radius [m] R tip radius [m] tc tip clearance [mm] D fan diameter [m] b c airfoil camber [%] T aerodynamic rotor torque [Nm] q v flow-rate [m 3 /s] c u local swirl velocity [m/s] c a2 local axial velocity [m/s] c a = qv π D 2 4 ·(1−ν 2) mean axial velocity [m/s] v m = qv π D 2 4 velocity at fan exit [m/s] n rotational speed [rpm] Re tip = ρ·(πnD 60)·c tip µ Reynolds number DF Lieblein Diffusion Factor FVP = 1 2 ρv 2 m fan velocity pressure [Pa] ∆p t−s fan total-to-static pressure rise [Pa] OPEN ACCESS Downloaded from www.euroturbo.eu 1 Copyright c ⃝ by the Authors FTP=∆p t−s +FVP fan total pressure [Pa] ν hub-to-tip ratio ω angular velocity [rad/s] Φ = qv (π D 2 4)(πnD 60) flow-rate coefficient Ψ = (F T P or ∆p t−s) 1 2 ρ(πnD 60) 2 pressure coefficient η = (F T P or ∆p t−s)·qv T ·ω efficiency σ = n·q 0.5 v F T P 0.75 specific-speed δ = D·F T P 0.25 q 0.5 v specific-diameter ρ air mass density [kg/m 3 ] ξ stagger angle (with respect to fan axis) [ • ] µ air dynamic viscosity [Pa s] Γ circulation [m 2 /s] Σ a = c a2 ca dimensionless axial velocity ϵ s = cu ca dimensionless tangential velocity INTRODUCTION In this paper the performance of 30 rotor-only axial fan designed according to different vor-tex criteria is analyzed with the aim of providing fan designers with indications on the suitable choice of swirl distribution for a given duty. The rotor-only configuration is largely the most common for low-to-medium pressure-rise axial-fan applications. In this layout fixed vanes and diffuser are absent and the only aerodynamic components are the impeller and the external casing. The resulting simplicity corresponds to cheapness of purchase and maintenance but it is paid with the loss of the dynamic pressures associated with the axial and tangential velocities at rotor exit (except the small amounts con...

Free and Non-Free Vortex Design for Axial Fans with Circumferential Sweep through CFD Techniques

“…12, are consistent with the typical behavior of fans designed with the Non-Free Vortex condition. In the bibliography (Vad 2012;Masi et al 2016;Corsini and Rispoli 2004) it is reported that rotors designed with this condition have the disadvantage of the presence of the strongest radial component, especially in the hub region. The presence of the radial component of the velocity is associated with the total pressure losses and, consequently, with the fan performance.…”

“…Literature review shows that the incorporation of the forward sweep of the blade is beneficial for rotors with a non-free vortex design (Vad, 2011(Vad, , 2012Vad et al 2015 andMasi et al 2016). The sweep effect in rotor designs with a non-free vortex design contributes to the attenuation of the radial component of the flow which reflects in the reduction of losses caused by the flow separation, an efficiency improvement, and noise reduction.…”

Aerodynamic Design of Low-Speed Axial-Flow Fans: A Historical Overview

“…In 2014, Lindemann et al extended the design method to account also for swept geometries for aeroacustic reasons [67]. Conversely, Masi et al [68,69] proposed a simple design method for swept blades of constant-swirl design (i.e., c u = const) with the aim of improving performance and efficiency: the method is based on the computation of the radial motion related to the non-free-vortex distribution and involves tilting the blades according to the inclination of the inter-blade meridional flow; the method provided promising results from the experiments [70]. Meanwhile, design exercises are more and more leaving the scene in favor of computer-based optimization techniques.…”

Editorial for Special Issue on Air Movement Fan Technology

“…Masi et al. 9 studied the use of controlled vortex blading when attempting to increase the efficiency of a rotor-only axial fan. Air movement fans have classically been designed around a free vortex aerodynamic design.…”