The study of compact fan installations with variable circulation distribution along blade length

DOI: 10.34759/vst-2020-1-30-42

Аuthors

Glushkov T. D.

Central Aerohydrodynamic Institute named after N.E. Zhukovsky (TsAGI), 1, Zhukovsky str., Zhukovsky, Moscow Region, 140180, Russia

e-mail: dianno@bk.ru

Abstract

At present, there is an undeniable demand for developing new prospective layouts of various cooling systems and lifting complexes for air-cushion units, in which a flat barrier of substantial size (a screen, air duct, radiator) is being placed behind the axial fan. This problem can be solved by the effect of kinetic energy conversion of the swirling flow behind the impeller into the static pressure, observed in axial- radial diffusors, formed by the fan outlet manifold and a flat barrier, upon which the flow is ingoing. Implementation of such structures of fan installations allows not only preserving high energy-efficiency of the fan installations but as a whole, but significantly reduce their axial size as well.

The main parameter affecting the efficiency of the swirled flow dynamic pressure into static pressure conversion is the flow swirl intensity, characterized by the Rossby number, since with its increase, the total pressure loss in the axial radial diffuser decreases. The article demonstrates that namely fans with the said circulation distribution along the blades length implementation, whereby the flow is swirled by the law of the solid body, is expedient in such kind of fan installations. These fans swirling intensity can reach much higher intensity compared to those, for which classical methodology for the constant circulation is used while aerodynamic design.

Based on the available experimental data on the swirling flow total pressure loss in axial radial diffusers, calculation was performed for aerodynamic parameters of compact fan installations with variable circulation in the wide range of calculated parameters such as flow rate and hub-to-shroud rate, which finally determine the blade shape geometry. According to the obtained results, the installations under consideration can develop rather high for axial fans static pressure rate at a minimum axial size.

An additional analysis of fans with variable circulation revealed two limitations that significantly narrowed the range of design parameters.

The first limitation is stipulated by the criterion of the aerodynamic load limit of blades system, characterized by the value of equivalent diffusion cascade D_eq. Exceeding the D_eq maximum value for peripheral cascades may lead to the high intensive separated flow path of the rotor. Unlike the classical fans with constant circulation, the diffusion cascade criterion for the fans under consideration does not depend on the design parameters, and, eventually, determines the minimum value of the axial velocity, at which this limitation is fulfilled.

The second restriction is determined by the energy balance condition: the total kinetic energy of the flow should not exceed the energy transferred to the flow by the rotor blades. This problem manifests especially pointedly in the near-hub sections, since unlike the fans with the constant circulation, the quantity of energy transferred to the flow by the blades in the fans, which swirl the flow by the solid body law, reduces from shroud to the hub. Overall, this limitation determines the maximum value of the axial velocity coefficient and the range of optimal design parameters of considered fans.

With account for the analysis being done, the aerodynamic designing of the experimental fan was performed and studied experimentally. The obtained results reflected the main concepts used in aerodynamic design. Significantly higher values of pressure ratio and flow rate were obtained on the experimental fan installation compared to the similar compact fan units, designed employing the classical technique for constant circulation.

Keywords:

Axial fan, fan installation, rotor, axial-radial diffuser, variable circulation

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