The performance of a centrifugal fan with enlarged impeller

The performance of a centrifugal fan with enlarged impeller
Posted by rtyu1yu on 2021/08/24
The performance of a centrifugal fan with enlarged impeller

    The performance of a centrifugal fan with enlarged impeller

    The influence of enlarged impeller in unchanged volute on G4-73 type

centrifugal fan
performance is investigated in this paper. Comparisons are conducted between the fan with original

impeller and two larger impellers with the increments in impeller outlet diameter of 5% and 10% respectively in the numerical

and experimental investigations. The internal characteristics are obtained by the numerical simulation, which indicate there

is more volute loss in the fan with larger impeller. Experiment results show that the flow rate, total pressure rise, shaft

power and sound pressure level have increased, while the efficiency have decreased when the fan operates with larger

impeller. Variation equations on the performance of the operation points for the fan with enlarged impellers are suggested.

Comparisons between experiment results and the trimming laws show that the trimming laws for usual situation can predict the

performance of the enlarged fan impeller with less error for higher flow rate, although the situation of application is not

in agreement. The noise frequency analysis shows that higher noise level with the larger impeller fan is caused by the

reduced impeller–volute gap.


    An implicit, time-accurate 3D Reynolds-averaged Navier-Stokes (RANS) solver is used to simulate the rotating stall

phenomenon in a plastic centrifugal fan. The

goal of the present work is to shed light on the flow field and particularly the aerodynamic noise at different stall

conditions. Aerodynamic characteristics, frequency domain characteristics, and the contours of sound power level under two

different stall conditions are discussed in this paper. The results show that, with the decrease of valve opening, the

amplitude of full pressure and flow fluctuations tends to be larger and the stall frequency remains the same. The flow field

analysis indicates that the area occupied by stall cells expands with the decrease of flow rate. The noise calculation based

on the simulation underlines the role of vortex noise after the occurrence of rotating stall, showing that the high noise

area rotates along with the stall cell in the circumferential direction.


    As the power source of the air and gas system in the thermal power plant, the operation status of the centrifugal fan is

directly related to the safe and economic operation of the power plant. Rotating stall in the centrifugal fan is a local

instabilities phenomenon in which one or more cells propagate along the blade row in the circumferential direction. The

nonuniform flow, the so-called stall cell, rotates as a fraction of the shaft speed, typically between 20% and 70%. This

running mode is responsible for strong vibrations which could damage the blades [1]. Meanwhile, it will increase the

aerodynamic noise.

    In order to reveal the generation mechanism of rotating stall, lots of models and theories have been proposed since the

1960s. Especially, experimental methods were widely used to illustrate the characteristics of internal flow field during

stall. Lennemann and Howard discussed the causes of stall cells in low flow rate condition through the hydrogen bubble flow

visualization method [2]. Lucius and Brenner experimentally studied the speed variation of a centrifugal pump in rotating

stall stage [3]. For the centrifugal turbomachine, multiple factors can affect the characteristics of stall. Vaneless

diffuser, for example, has significant influence on stall. Hasmatuchi et al. experimentally investigated the effect of

blowing technology on the flow field of a centrifugal pump under rotating stall [4]. Rodgers conducted an experimental

research on rotating stall in a centrifugal compressor with a vaneless diffuser and found that the stall margin can be

improved through adjusting the expansion pressure factor [5]. Abidogun carried out an experiment to investigate the influence

of vaneless diffuser on the stall characteristics. The results showed that increasing the length of diffuser can improve the

rotating speed of stall, and the change of width showed no effect on stall [6].

    Further efforts were made to study the stall inception in order to avoid the occurrence or minimize the effect of stall.

As well accepted, two types of stall inception proposed by Camp and Day modal wave inception and spike inception were

investigated experimentally [7]. Leinhos et al. studied development process of stall inception under instantaneous inflow

distortion in an axial compressor [8].


    With the rapid development of computer technology, numerical simulation has become an important method for flow field

research of turbomachine under rotating stall conditions. Gourdain et al. investigated the ability of an unsteady flow solver

to simulate the rotating stall phenomenon in an axial compressor and found that it was necessary to take the whole geometry

into consideration to correctly predict the stall frequency [1]. Choi et al. investigated the effects of fan speed on

rotating stall inception; the results showed that, at 60% speed (subsonic), tip leakage flow spillage occurred successively

in the trailing blades of the mis-staggered blades [9]. Zhang et al. numerically studied the stall inception in a centrifugal

fan, and the results showed that the stall inception experienced probably 50 rotor cycles developing into a stall group. The

inception showed significant modal waveform. The importance of volute for generation of stall inception was illustrated

through flow field analysis [10].

    Aerodynamic noise is mainly caused by vortex and flow separation. So the unsteady behavior of rotating stall may have an

influence on the noise of centrifugal fan. In capturing the physical mechanism of the fan noise associated with rotating

stall, the primary work is to characterize the noise. During the 1960s, the interaction between noise and turbulence was

discussed by Powell, and the vortex sound theory was proposed to explain the generation of acoustic sound. Then, Lighthill

made a breakthrough in aerodynamic noise theory research by proposing the acoustic analogy [11]. Based on these works, Díaz

et al. put forward a prediction of the tonal noise generation in an axial flow fan, and the noise level in the

plastic centrifugal blower

far-field region was estimated by means of acoustic analogy [12]. Scheit et al. analyzed the far-field noise in a

metal centrifugal fan with an acoustic analogy

method and presented design guidelines to optimize the radiated noise of the impeller [13]. The global control of subsonic

axial fan at the blade passing frequency was also discussed by Gérard et al. [14]. He aimed at cancelling the tonal noise by

using a single loudspeaker in front of the fan with a single-input-single-output adaptive feedforward controller. According

to Ouyang et al.’s work, the far-field noise generated by cross-flow fan with different impellers was measured and it showed

the great influence of blade angles on the inflow pattern [15]. Based on the previous research, a new method to predict the

fan noise and performance is developed by Lee et al., and through an acoustic analogy, the acoustic pressures from the

unsteady force fluctuations of the blades are obtained [16].

    In summary, a wide range of flow characteristics on rotating stall in compressor have been investigated and the

researches concentrated on stall inception. The present work focuses on two aspects: simulation of the rotating stall

phenomenon with a 3D flow solver and seeking the deep physical mechanism of this instability in a centrifugal fan. The

numerical method is presented with the model and the particular boundary conditions are used. Results from the whole geometry

simulation are then analyzed. In the first part, aerodynamic characteristics and frequency domain characteristics of the

centrifugal fan under different stall conditions are analyzed. In the second part, the velocity vector field distributions in

the centrifugal fan are discussed. Finally, noise characteristics of the centrifugal fan under different stall conditions are

studied. And the noise characteristics during the circumferential propagation of stall cells are also discussed.


    2. Centrifugal Fan Description


    The configuration of range hood centrifugal fan studied in this work is shown in Figure 1. It is composed of current collector,

impeller with 12 airfoil blades, and the volute. The inlet and outlet diameter of the impeller are 568?mm and 800?mm,

respectively. The inlet and outlet width of impeller are 271?mm and 200?mm, respectively. The nominal rotation speed is

1450?rpm. The volute tongue gap is 1% of the impeller outlet diameter. The width of the rectangular volute is 520?mm, and a

simple antivortex ring is set inside the volute to reduce the generation of vortex. At the design operating point, the volume

flow is 6.32?m3/s and the full pressure is 1870?Pa.


    As shown in Figure 8(b), under the combining influence of both stall cell and volute tongue, the high noise area is

gradually elongated. Due to the propagation of stall cell, it gradually gets away from the area of volute tongue, resulting

in weakening the superimposing effect. As time goes by, the high noise area in Figure 8(c) gets further elongated with a

trend of separation and the sound power level of high noise areas decreases. In Figure 8(d), the high noise areas

corresponding to the vortex noise and volute tongue noise basically separate. And the sound power level corresponding to

volute tongue greatly declines.

    It can be drawn from Figure 8 that while the impeller passes three passages along clockwise direction, the high noise

area passes two impeller passages along the clockwise direction. It indicates that, in the absolute coordinate reference

system, the high noise area occupying about three impeller passages rotates in the same direction with impeller under

rotating stall. It also has the same speed with stall cells, while in the relative coordinate reference system, high noise

area spreads in the opposite direction of the rotation of the impeller.


    Through the analysis above, there are two major sources of noise in a centrifugal fan under rotating stall, namely, the

vortex noise caused by stall and the volute tongue noise caused by the rotation of impeller. When the stall cell spreads to

the volute tongue, due to the superimposing effect of vortex noise and volute tongue noise, the sound power level is the

highest and the high noise area is the largest. While the stall cell is away from the volute tongue, the corresponding high

noise areas separate gradually. Along with that, the sound power level decreases and the high noise area becomes smaller.

Therefore, the aerodynamic noise of the centrifugal fan under rotating stall changes periodically over time, and the

fluctuation period is the same with the rotating period of the stall cell.


    The authors declare that they have no financial or personal relationships with other people or organizations that can

inappropriately influence their work; there is no professional or other personal interests of any nature or kind in any

product, service, and/or company that could be construed as influencing the position presented in, or the review of, this

paper.


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