Energy Efficiency, Vibrations and Noise of Fan

Energy Efficiency, Vibrations and Noise of Fan: History

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Fans as industrial devices are among the most significant single recipients of driving energy. Therefore, they represent an important area of energy savings to reduce CO₂ emissions. The ubiquity of fans and their operation under conditions different from the optimum provides an opportunity for more significant global reductions in the energy used to drive them. The second important aspect, besides energy efficiency, related to the operation of fans is the noise they generate.

centrifugal fans
fan characteristics
energy efficiency
noise

1. Energy Efficiency—Fan Operation

Fans are among the most significant single energy consumers and therefore represent an essential energy-saving area to reduce CO₂ emissions. In the USA, fans operating in the industrial sector consume about 11% of the electricity of all industrial motor drives ^[1]^[2]. In Sweden, fans consume about 8% and in Great Britain, more than 9% of the energy is produced ^[3]. Figure 1 shows an overview of where energy losses occur in the fan. Aerodynamic and mechanical losses in the flow system (and outside it), power transmission, and control can be observed ^[4].

Figure 1. Places where energy losses occur in the fan.

In order to overcome the losses, the fan generates an appropriate increase in the pressure of the total medium thanks to the mechanical energy supplied to the fan from the drive motor. Electric motors are used almost exclusively to accomplish this task.

This part of the energy transferred from the engine to the gas to realize its flow through the installation is called useful power, as opposed to the power lost in the drive and flow system of the fan.

The exact value of the useful power is calculated from the product of efficiency and total accumulation, and the correction factor ƒ was taken into account for accumulations exceeding 3.6 kPa.

N_{u} = {\dot{V}}_{s} * Δ p_{c} * ƒ

The drive unit supplies Power N_i to the fan’s flow system.

In the flow system, there are power losses caused by the friction of the medium against the walls of the flow channels, as well as the mutual friction of gas molecules against each other in the areas of vortices for both the impeller and the housing. This loss is called flow power loss and means N_p.

In the flow system, a third power loss is distinguished, caused by the friction of the rotating outer walls of the rotor against the gas in the housing. The power loss due to the friction of the rotor discs is called the wading loss and is denoted by N_b. The carrier disc has a significant impact on the amount of wading losses.

On the other hand, N_v as power loss consists of the pressure drop in the impeller inlet chamber multiplied by the leakage flux.

All these three types of losses and the useful power added together are equal to the fan’s internal power ^[5]

N_{u} + N_{p} + N_{v} + N_{b} = N_{i}

There are energy losses in the rotor and housing due to acceleration, deceleration, change of direction, friction, detachment of streams from the walls, and mixing of streams.

In the rotor, the following types of losses are distinguished:

-: Inlet on the funnel;
-: Change of flow direction from axial to radial;
-: Non-tangential flow to the rim;
-: Friction in the interscapular canal.

There are losses in the fan case:

-: Diffusivity of the case;
-: Sudden expansion of the flow;
-: Stream mixing;
-: Friction.

In the Sankey diagram, power flows from the left side to the right. Each component rejects a portion of the input energy due to inefficiencies in the following steps. The energy efficiency of each component is the ratio of the output power to the input power.

The mechanical energy on the fan shaft is used to generate power to the air stream at the outlet. Some input energy is lost due to aerodynamic losses, mechanical losses, and acoustic losses. The ratio of air power to fan shaft power defines the total efficiency of the fan.

Increasing the operational efficiency of rotating machines, including fans, should be a priority goal of international organizations influencing the creation of new market standards. According to the study presented by the International Energy Agency, it can be concluded that the total energy efficiency of rotating machines is a key item determining the possibilities of reducing energy consumption on a global scale ^[6]. An important parameter linked to the fan’s energy efficiency is the rotational speed, usually measured in revolutions per minute (rpm). The rotational speed has a significant impact on the performance characteristics of the fan. Efficiency changes in proportion to the change in rotational speed, pressure is the square of the change, and power demand is the third power. The rotational speed should be considered together with other problems, such as ambient noise, air temperature, fan load variation, and the mechanical strength of the fan. For example, centrifugal fans with forward curved blades require a relatively accurate estimate of the airflow in the installation and pressure requirements, as the wrong speed can result in a reduced capacity or excessive air flow and pressure variations.

The air stream’s temperature has a significant influence. For high temperatures, all materials show lower strength. Since the forces acting on blades, shafts, and bearings are proportional to the square of the rotational speed, high temperatures in the fans result in a low rotational speed. To a large extent, it is the temperature range that determines the type of fan and the selection of the appropriate material. Many materials lose their mechanical strength in high-temperature installations. The stresses increase with increasing fan speed; therefore, the lowest rotational speed is required for systems with high temperatures. Centrifugal fans are built for lower temperatures of the medium.

Table 1 shows the various analyses described in the literature, actions taken, and design measures increasing the efficiency of fans.

Table 1. List of measures taken to improve fan energy efficiency.

Publication	Research Area	Research Results
^[7]	This study looked at the efficiency improvement of axial fans in many areas, including outdoor air conditioning units. The authors analyzed the flow structure occurring at the vane’s tip due to the winglet’s position. The fan used in the experiment is an axial flow fan with a diameter of 370 mm. The fan was connected to the chamber in the same way as it was installed in the outdoor unit of the actual air conditioner.	The authors confirmed the existence of an optimal apical clearance. As a result, this translates into the maximum efficiency of the axial flow fan. The assumption is that this is for a fan with a casing height equal to 30% of the axial chord length. When the fan was partially shielded, winglets on the axial flow fan increased the tip leakage flow rate by 66%. However, it allowed increasing the efficiency by only 0.7%. This is because (the authors suppose) the effect of the tip leakage vortex on the mainstream on the suction side of the blade has been reduced by the winglets. The last case considered is when the cover completely covers the fan. This arrangement increases the tip leak flow rate proportionately to the tip clearance. The highest efficiency can be obtained when the tip clearance is 0.
^[8]	The article considers the influence of the axial gap between the inlet nozzle and the rotor on the performance and the flow pattern. The article presents numerical simulations with various models of turbulence. A unique geometry and mesh pattern were made to determine the effect of this axial gap on the flow pattern as well as very important performance and efficiency.	The authors obtained results showing a high agreement between numerical simulations and experimental data. The numerical simulations showed that the fan with the inlet diameter of 19 cm, which practically coincided with the internal diameter of the rotor, has the highest efficiency of almost 44% on all axial gaps (compared to the other cases discussed in the study).
^[9]	This article examines a slow speed centrifugal fan. The fan was installed inside a rectangular duct. The research was done through measurements and CFD. In the experiment, a rectangular, cuboid-shaped body (serving as a “pressure recovery unit” (PRU)) was mounted behind the (slow-running) centrifugal fan. With it, a significant increase in efficiency can be achieved. This is because it covers most of the space next to the wheel fan, leaving only a small air duct near the duct walls.	Through the introduction of PRU, energy savings were confirmed in experimental studies and numerical calculations.
^[10]	The article presents different strategies and ways to increase energy efficiency, parts, and intelligent improvement techniques. The article also presents new innovative work on the ventilation system and Variable Speed Drives (VSD). This solution allows for a real reduction in energy consumption by the ventilation system.	It has been shown that improvements in the efficiency of the ventilation system can be achieved with the conventional control technique of Variable Speed Drives. During the correct use of components and the motor in the ventilation system, you can save 20% to 30% of energy. VSD also offers an energy-saving advantage compared to other conventional control techniques in a parallel fan system
^[11]	The article deals with a fundamental problem related to ventilation systems in mines and underground facilities. In the case of mines, ventilation systems during operation can account for a significant part of energy costs (from 25 to 40%) and also for 40–50% of energy consumption. The author of this article has found that 40–80% of the energy consumed by fans is used to overcome the resistance of fan components.	The article discusses how to approach the design of machinery and equipment to increase fan installations’ performance and efficiency.
^[12]	In this paper, the characteristics of the recirculating bubble and its full impact on the efficiency of the compressor were investigated. The analysis was carried out using a well-proven three-dimensional numerical simulation.	The conducted tests show that the bubble initiation was carried out within the rotor channel. Decreasing flow velocity is gradually accompanied by gradual stretches in stream and spanwise directions. At the same time, the adverse effects of preheating and pre-swirl remained relatively low. The research shows that in the early stages, the recirculating bubble played a positive role in suppressing the incidence loss, the blade loading loss, and the tip clearance loss, leading to higher compressor efficiency.
^[13]	The article shows the Borda-Carnot sudden expansion study. It is often used in ventilation systems instead of diffusers when it is necessary to increase the cross section. In the beginning, the Borda-Carnot loss for homogeneous and fully developed inlet velocity profiles was experimentally investigated.	It has been shown that the classical loss formula derived for a uniform inlet underestimates the losses. Thus, each sudden expansion can cause a 2–3% error. This is the case in the calculation of losses in ventilation systems. The authors presented a methodology for calculating Borda-Carnot losses in ventilation systems. This method takes into account the loss formula for a fully developed inlet.
^[14]	This experimental study investigated turbulence intensity, pressure recovery ratio, velocity distribution, and flow separation. The study was carried out on a 1:13 model of a diffuser with an octagonal inlet and a square outlet. This type of diffuser can be used in a vertical wind tunnel designed for skydiving. The experiments were carried out both in blowing flow modes and suction.	The results obtained from the conducted tests do not indicate a significant pressure drop and separation in the tested diffuser. The authors noted that using semi-empirical correlations predict the pressure loss in a model diffuser correctly. In order to perform the test correctly, it is necessary to use the large-angle diffuser plane of 10.66◦ instead of the equivalent cone angle in these correlations.
^[15]	The article presents the use of parametric analysis to improve the performance of a centrifugal fan. Using computational fluid dynamics (CFD), the authors conducted a simulation on a specially prepared solid model (the aim was to analyze fluid flow). A simulation was carried out for various data (the number of rotor blades was changed) while maintaining the parameters corresponding to the design conditions. Simulations were performed for Z = 8, 10, 12, 14.	The results of the tests showed that an efficiency of 50.82% is achieved at a total pressure of 1323.07 Pa (at the fan outlet). Simulations for Z = 14 (number of blades) showed an increase in total pressure by 21.77%. For the same conditions, the efficiency increased by 5.74% compared to the design state.
^[16]	This study considered how modal analysis might inform DFAN drive patterns.	The result of the work was the creation of a FEM model of a simplified cylindrical DFAN cavity.

Today’s fans are subject to legal requirements regarding energy efficiency. It is important that they consume as little energy as possible, and this goal is subject to global, EU, and national requirements. In the above studies, the authors use various methods and focus on selected fan elements to optimize the flow, correcting the turbulence that arises. This, in turn, allows for changes to the seals, improvement of clearances, and modifications in the construction of the fans. Selected works also dealt with issues related to using artificial intelligence to improve work efficiency (not only fans but also entire ventilation systems).

2. Vibrations and Noise

In industrial ventilation processes, vibrations and generated noise can be a serious problem. The noise sources are vibrations of the fan impeller, body, piping, foundation, drive motor, and many other elements. High acoustic level causes fatigue in employees. In installations where fans operate, noise is produced by mechanical, electromagnetic, and aerodynamic phenomena ^[17].

The noise generated by the fan depends on the type of fan, air flow speed, and pressure. The increased noise level of a given type of fan is often the result of inefficient fan operation. The sources of vibrations and noise in the fan itself are, among others:

Design, thermal imbalance (heating unevenness, flow unevenness), operational imbalance (deposition of particles of the working medium, rotor erosion);
Bending, e.g., due to high permanent forces and material deflection;
Lost elements, e.g., a spatula;
Rubbing against body elements as a result of high vibrations, permanent bending of the rotor or axial displacement of the rotor as a whole;
Blade resonance leads to resonant vibrations and, consequently, fatigue of the material.

The motors produce considerable noise during operation. However, fan impellers are louder, and the difference between fan and motor sound levels often exceeds 6 dB, so they are inaudible. There are tables ^[5] presenting the noise level of motors of different sizes for different rotational speeds.

Non-stationary gas flow from the nozzle is a typical monopole sound source. The sound power of such a source is proportional to the fourth power of the gas velocity c⁴ and its density ρ, the square of the geometric dimension of the stream l², and is inversely proportional to the speed of sound a_d ^[5].

N ~ \frac{ρ * c^{4}}{a_{d}} * l^{2}

Based on the dimensional analysis, the sound intensity can be represented by a formula ^[5]:

I = ρ * c^{3} = \frac{ρ^{2} * c^{4}}{ρ * c} = \frac{\bar{p^{2}}}{ρ * c}

where

\bar{p}

is the effective sound pressure value RMS (Roor Meane Square).

Real noise, modeled by an acoustic dipole, is produced by stationary and non-stationary forces existing in the source. In the case of fans, the forces are generated when the impeller blades and some casing elements flow around the gas stream with any time-dependent velocity distribution or with a time-independent (stationary) profile.

In the balance of energy losses of the fan, the acoustic power may be omitted. However, the noise associated with the spread of this type of energy is very annoying.

The air particles in contact with the fan surfaces are introduced into vibrations, which, due to the elasticity of the medium, are transmitted to the neighboring particles. The energy transferred to create the vibrating motion of the surrounding particles is the radiated energy.

The sound power radiated to the space around the fan depends in a first approximation on the power of the motor driving the fan, which in turn depends on the efficiency and pressure rise. The sound power level can be calculated with a high approximation from the formula ^[5]:

L_{N} [dB] = 70 + 10 l g N_{s} (k W) + 10 l g Δ p_{c}

where:

N_{s} - electric motor rated power, Δ p_{c} - {fan}^{'} s damming [Pa]

The vibration of fans can also generate noise. The mechanical vibrations of the fan or its components arise as a result of:

-: Unbalance of rotating masses;
-: Non-axial engagement of clutch parts;
-: Work near resonance;
-: Aerodynamic flow disturbance;
-: Damage to bearings and incorrect clearances;
-: Electromagnetic interference affecting the engine;
-: Interference from external vibrations.

Since the fan can operate at any point of its characteristic depending on the installation resistance, the actual operating point in the installation is defined as the operating point. The specific sound power level varies with the flow rate. The lowest noise generation of the fan roughly corresponds to the optimal performance. Particular deviations of the volumetric flow—according to Figure 2—show the effect of the shift of the operating point on the level of generated noise ^[18]. For example, an increase in volume flow of about 40% from the nominal value results in an increase in the specific sound power level of more than 4 dB.

Figure 2. Characteristics of an example fan, where: efficiency—blue line, specific sound power level difference at nominal operating point—green line, and change in specific sound power level difference as a function of change in volume flow—red line.

If a high fan noise level cannot be avoided, measures to reduce the acoustic energy must be used. To reduce noise, engineers use various methods, e.g., duct insulation, rubber or a suitable spring insulator or mounting the fan on a soft base. Another way is to install a sound-absorbing damping material or baffles to absorb vibrations. Active noise control methods are also a very important area of noise reduction.

Table 2 presents the collected literature on the methods of reducing noise generated by fans.

Table 2. Publications and activities related to the reduction of noise generated by fans.

Publication	Research Area	Research Results
^[19]	The article attempts to reduce the noise caused by fluid flow through the axial fan. The fan was installed in the outdoor unit of the air conditioner. The authors analyzed numerically the influence of the vortex occurring in the vicinity of the axial flow fan hub on its blades. It was stated that the size and shape vortex affect the fan’s efficiency.	The conducted study showed a reduced fan efficiency due to the vortex generated by the flow separated from the edge of the hub entrance—due to the impact on the fan blades. The authors of the study indicate a new shape of the hub to reduce the resulting vortices. The performed test proves that changing the shape of the hub allows for a reduction the fan noise by 1.3 dB and the power required for the fan motor by 5% (assuming the same flow rate conditions).
^[20]	This paper addresses a noise comparison of centrifugal fans with backward-inclined airfoil blades and backward-curved blades. Computational aeroacoustics were used to conduct the research. The article explored: sound levels of backward-curved (BC) blade and airfoil (AF) blade centrifugal fans and flow structures used for cooling an electrical machine. The authors used computational aeroacoustics and computational fluid dynamics.	The results show that for the backward-curved blade, there was a flow separation at the leading edge, and also vortices expanded to the suction side of the blade. After replacing the airfoil blade based on a 4-digit NACA, vortices were suppressed, and flow separation was delayed. As a result, the sound pressure level of the airfoil blade decreased sharply in the passing frequency range of the blade. In the next step, the overall sound pressure level of the airfoil blade was reduced by 2.1 dB compared to the aforementioned backward-curved blade.
^[21]	The authors tried to optimize by changing a few key design parameters: air flow rate and radiated noise of an existing multi-blade centrifugal fan and the flow field.	The results of the subsequent simulation showed that, apart from reducing the space occupancy, it can achieve both excellent reductions of low-frequency noise and broadband noise (above 500 Hz) The authors investigated the influence of the mounting position of their composite sound-absorbing structure in a spiral wall. They also suggest that in engineering practice, the optimal installation location should be determined based on the directionality of acoustic radiation (which is important in the far field) as well as a comprehensive consideration of both the benefits of noise reduction and the costs incurred.
^[22]	In this experimental work, four different axial fans with free inflow conditions were examined in terms of their acoustic radiation. They have been modified with different leading edges. The reference fan was a fan with a straight leading edge without serrations. The remaining fans had different spanwise locations of the leading-edge serrations. The greatest attention was paid to the fans with serrations on the outer 1/3 and outer 2/3 of the fan blade. Moreover, a fully serrated fan was part of this scientific research. A standardized test stand with an axial fan with an integrated anechoic chamber was used for the tests. The acoustic pressure on the suction side and the curves of aerodynamic characteristics were measured.	Thanks to this experimental study, it was possible to conclude that using the leading-edge serrations leads to a reduction in the sound pressure level of low-pressure axial fans. However, this has a negative effect on the aerodynamic behavior of the fan. Changing the conditions in the inflow study to higher and lower turbulence intensities would allow for more general statements about the acoustics and aerodynamics of axial fans with serrated leading edges. The authors suggest that the inclusion of axial fans (because they have leading edge serrations only in the inner part of the fan blades and near the fan hub) would give better results and allow for more precise results on the examined acoustic phenomena.
^[23]	In the search for a method to reduce fan speed and noise, examined use of multi-element airfoils has been explored. The authors designed the multi-element airfoil using the viscous/inviscid (MSES/MSIS solver) at a chord Reynolds number of 2 × 105. In addition, the authors presented a new approach to the design of control vortices, increasing the blades’ aerodynamic properties even more.	The designed airfoil allowed to increase in the lift coefficient by 132%. A similar result was observed for resistance (increase by a similar amount). The geometry of the blades obtained in the presented work is ideal for implementing multi-element airfoils. For the combination of the control vortex design and the multi-component blade, relative to the basic vortex design without one component, a total noise reduction of 5.6 dB was demonstrated.
^[24]	This article states that the effects of impeller shape and tip clearance on performance and noise were studied in the design of an electric duct fan.	As a result, the forward swept angle of the blade shape, forward dihedral angle, and narrow tip clearance could contribute to performance improvement and noise reduction.
^[25]	The influence of the type of blade pitch in an axial fan (backward-skewed fan—B, forward-skewed fan—F, and unskewed fan—U) on the acoustic fields at design points was analyzed.	The article observed that a characteristic flow occurs depending on the pitch of the fan blades. It has been shown that there is a large relationship between these flow phenomena and the sound radiation of the tested axial fans. All tests were carried out under the same inlet conditions, with no obstructions in front of the fan. In terms of aerodynamics and aeroacoustics, the fan tilted forward showed the best performance. It has been found that the flow in the tip slot has a large influence on both the flow field and the sound field.
^[26]	The work shows the possibility of reducing the noise of a single-stage axial fan using corrugated front blades. The acoustic and aerodynamic performance of the fan was investigated using the sophisticated unstable Reynolds averaged Navier Stokes (URANS) method as an acoustic analogy (Goldstein equations).	After comparing the baseline fan without significant loss of aerodynamic performance, it was found that the fan with wave-shaped leading-edge blades produces less interaction noise. The paper concluded that the corrugated vanes on the leading edge could improve acoustic and aerodynamic performance. Two different profiles of the leading corrugated edge were shown to effectively reduce the sound power level of the fan tone by 1.2 dB and 4.3 dB, respectively. Fan efficiency was increased by 1% when tested with the wavy leading-edge profile.
^[27]	The possibility of optimizing and redesigning (to improve efficiency in step-flow conditions) of a low aspect ratio transonic fan is presented. An advanced 3D aerodynamic optimization system was used. The system was based on the concept of flow matching in the row of blades and control of the curvature of the pitch line. To discuss the flow for the transonic fan stage, the diagnostic flow method was used.	An innovative rotor blade with continuous curvature was obtained by optimizing the pitch line’s curvature and adjusting the flow angle at the inlet. As a result, the shockwave was skillfully controlled. Thus, flow separation was delayed, thereby increasing the pressure-rising capacity of the fan stage. Everything was done while reducing flow losses. The force of the shockwave in the hub region was reduced, but the separation of the boundary layer was also suppressed.
^[28]	The article describes ventilation systems in the textile industry. The article presents the nature of noise generated in the machine and the forecast of noise generated by centrifugal fans.	The authors’ analyses showed how mechanical loom noise could be reduced by using a damper.
^[29]	Finding faults in the industry can be very problematic. The article presents an analysis of vibrations that can contribute to their detection. A method for detecting vibrations of a forced draft (ID) fan has been proposed.	The proposed vibration measurement detection approach turned out to be a valid tool.
^[30]	The effect of the gap on the fluid mechanics and tonal noise generation of non-volute centrifugal fans are very important issues in the operation of fans. This paper explores these aspects based on different gap geometries. This study was based on existing research for this type of fan, and the study reported that there is tonal noise at the blade passing frequency (BPF) from the gap turbulence. The work presents a simulation in which a hybrid method was used, combining the improved simulation of delayed detached eddies (IDDES) with Farassat Formula 1A. This simulation shows regions with high vorticity values in the channel between two blades.	In this work, authors showed that fan noise is increased by turbulence sucked from the gap area, while aerodynamics is reduced by turbulence inside the blade channel. This is due to the study of pressure fluctuations and vortices at various locations of the fans. It was also observed that the increased turbulence caused by the gap is associated with decreased turbulence near the blade tip.
^[31]	The authors indicate that the main acoustic emission sources are dipole sources on the surface of the spiral and the rotating rotor. It was shown that the radiation pattern of the fan is related to the spiral effect and has a significant influence on it. The authors developed an improved noise study prediction model with a hybrid acoustic calculation method (which refers to a centrifugal fan).	The authors indicate that the main source of dipoles is pressure fluctuations on spiral surfaces. For higher harmonics, the IBEM application allows you to ensure the quality of blade frequency assurance.
^[32]	Work investigated the issue of uneven spacing of the blades. One method was the random modulation method. The goal was to attenuate undesirable tonal noise and reshape the noise spectrum. The investigated effect of blade inhomogeneity on fan noise spectral characteristics was theoretically predicted by calculating the interference function for each discrete frequency. Calculations related to fluid dynamics and aero-acoustic analysis were also carried out. The aim was to test the acoustic performance of each fan blade variant.	In order to achieve lower tonal noise levels and good acoustic frequency distribution, non-uniformly spaced blades were used in this study (in a centrifugal fan with forward-curved blades). The methods of sinusoidal modulation and random modulation were examined in detail. A CFD simulation was also investigated to investigate the effect of non-uniformly spaced blades on the fan noise spectrum.
^[33]	This study was designed to predict the amount of noise emitted by a fan. This can be achieved by identifying quieter blade spacing. Computational Fluid Dynamics (CFD) studies were conducted to obtain transient blade force characteristics for fans with non-uniformly spaced blades. Based on Lowson’s model, a model was built that was used to predict sound harmonics propagating in the far field.	Results showed that predicting tonal noise from blade force characteristics eliminates the need for CFD simulations of fans with unevenly spaced blades.
^[34]	The study concerns a new design of a marine axial fan. The article specifies the optimal parameters for this type of fan. The research was based on numerical simulations and experimental research. They are concerned with analyzing the impact of blade perforation on its internal flow field and aerodynamic noise characteristics.	The analysis showed the possibility of reducing the noise emission by 3 dB by modifying the blade geometry. The perforation in the blade tip area improves vortex propagation at the trailing edge and thus significantly reduces the aerodynamic noise generated by the fan. However, the effect of perforation does not give such unambiguous results. When combined with a change in the pitch of the blades and a change in diameter, these effects are the most beneficial. In addition, the perforation reduces the total pressure but significantly reduces the noise (by 3 dB).
^[35]	The authors of the article presented several ways to reduce noise.	The presented proposals show the key places and areas for further research. Among other things, it was proposed: Proper installation of fans; Using silencers for pneumatic exhausts; Using quieter timing belts and chains with different tooth profiles; Using vibration isolation pads under fan installations.
^[36]	The paper presents research (using PIV—Particle Image Velocimetry) on the impact of the shutter and rotational speed, and thus also the pressure increase in the axial fan, on the emitted noise.	Analyses made in the paper show a very large similarity of flow with increasing rotational speed for a flat and rigidly mounted fan. On the other hand, leakage is determined by the increase in pressure, not the rotational speed.
^[37]	The paper deals with the analysis of improving the efficiency of an axial fan by modifying the blade. The authors studied a bionic shoulder blade modeled on an owl’s serrated wing. The results with the bionic fan blade were compared to the classic shape.	Analyses showed that the belt structure and serrations in the bionic blade of the axial fan mainly improve efficiency. The authors have shown that using biological inspiration for technical applications can give excellent results and present new prospects for the development of fans.
^[38]	The paper includes analyses of noise reduction for a centrifugal fan with a belt drive. For this purpose, the analogy, and the method of Ffowcs Williams and Hawkings were used to simulate vortices based on an algebraic stress model in the sub-grid scale. In the analyses, the air was considered an ideal gas at a constant room temperature of 300 K. Turbulent flows were studied by analyzing the fan blade’s pressure coefficient, friction, and pressure fluctuations.	The proposed method (EASSM) determines turbulent flows very well. It has been shown that flow noise can be reduced using time-averaged sound power. The paper shows that for a centrifugal fan, noise generated can be reduced by design changes when it leads to a reduction in efficiency losses.
^[39]	The paper concerns optimization analyses of aerodynamic and acoustic parameters for insulated centrifugal fan rotors. Microphone measurements and laser Doppler anemometry were used to analyze the noise emitted by the fan.	Thanks to the conducted research, it is possible to gain insight into the processes that generate sound in radial rotors. The article shows that not always improving aerodynamic efficiency (especially insulated rotors) can lead to a reduction in sound emission.
^[40]	The article proposes a model that allows one to characterize the interaction noise of contra rotating (CAR) fans. The paper discusses effective noise reduction methods for CR fans.	This work provides a deeper understanding of noise generation mechanisms and may help develop noise control methods for CR fans. Compared to conventional ones, a much more complicated directionality modal noise interaction was demonstrated for CR fans.
^[41]	This paper presented a case study of how electric motors can be a very relevant noise source on compressor or any other devices. The work deals with issues related to noise generation by PM compressor drive motors used in air-conditioning units. In addition to numerical analyses, experimental research was conducted to reduce noise around the 8 kH band.	Shown possible noise sources due to PM motors were made with a focus on radial magnetic forces. By understanding the excitations on the stator, it was possible to propose an efficient and simple solution to mitigate noise levels on high frequencies, reducing the radiated noise and improving the equipment’s sound quality.
^[42]	This paper shows the application of the cascade clocking technique to a reduction of tone noise generated by multi-stage fans.	The stator clocking alone is insufficient, and the combination of the rotor clocking and the stator clocking is necessary for optimum reduction. The effect of cascade clocking on tone noise reduction can be clearly observed in the case of three stages, and the level of reduction is almost the same as in the case of two stages.
^[43]	Using an experimental measurement using two probes (equipped with a hot wire rotating together with the fan blades), the noise caused by the loose ends of the axial fans was analyzed. The authors proposed a helical velocity fluctuation pattern in a vertical flow to describe the peak frequency generation mechanism. The increase in noise emissions due to tip clearance flow was analyzed. Interference between the leak vortex and the adjacent blade is particularly noticeable at low flow rates.	The results show that the noise due to tip clearance consists of a broadband noise due to velocity fluctuation in the blade passage and a discrete frequency noise due to periodic velocity fluctuation. The rotational speed affects the peak frequency of the air velocity fluctuation in direct proportion. Peak frequency noise was found to be highly dependent on fan speed and is a significant emission source. In order to explain the generation of the discrete component of velocity fluctuation, the authors proposed a spiral pattern of velocity fluctuation.
^[44]	This article described the noise prediction procedure and the result of the developed numerical analysis tool, which couples time-domain acoustic analogy with flow field analysis via the free-wake panel method.	The authors’ work results in obtaining a method of predicting the noise level for an axial fan working with a cover. The received noise forecasts were consistent with the measurement data.
^[45]	In this study, flow-vibroacoustic coupled numerical methods were developed to predict noise radiation from the compressor piping system due to refrigerant flow.	By comparing the measured noise spectrum with the predicted one, the correctness of the proposed method was confirmed. The result reveals that the current numerical methodology can be used as an effective design tool to develop the low-noise compressor piping system.
^[46]	This study has given examples of the specific sound power levels and dependencies of sound power on the speed of different fan configurations. A simplified method has been introduced to use the specific sound power concept for scaling fan noise.	For one operating point of each fan, the results are sound power levels and spectra in the 1/3 octave band.
^[47]	The flow through the data center cooling fan was numerically analyzed in the work by introducing various geometry simplifications. Far-field noise predictions have been studied through the acoustic analogy of Ffowcs-Williams and Hawking. The transient flow field was obtained using the high-fidelity Computational Fluid Dynamic (CFD) method, Large Eddy Simulation (LES).	The radial obstacles (rods) nearest the leading edge that generates turbulence predominantly in tangential directions showed the minimum noise output. The circular obstacles (ring) nearest the leading edge that generates turbulence predominantly in radial directions showed a high tonal noise output.
^[48]	The authors proposed a meta-liner that allows for high-efficiency and broadband sound attenuation. The designed meta-liner consists of a micro-perforated panel (MPP) which is reinforced with metal foam and a series of parallel Helmholtz resonators.	Authors numerically demonstrated that the designed meta-liner can realize a surface wave conversion in a broadband frequency range [1500; 3000] Hz, leading to an enhanced sound absorption performance.
^[49]	This work reviews the familiar noise sources in fans and the procedures for noise attenuation.	Elaboration is presented to illustrate the use of a dissipative silencer.
^[50]	The article is a short analysis of the use of propeller fans for air cooling. The authors cite international research on super-quiet fans. They suggest developing fans with forward-curved blades that can work in industry and have diameters up to 10 m.	The presented new shape of the fan blades would make it possible to eliminate the phenomenon of noise-generating flows. This would reduce the noise to 15 dB(A) for the same fan operating points.
^[51]	In the work, the flow and performance field analyses were carried out using a commercial CFD code. Aeroacoustic characteristics of cross-flow fans were investigated. They are used in the indoor units of a split-type air conditioner.	The authors found that part of the trailing edge generated higher sound pressure than part of the leading edge. The results of the simulations, i.e., predictions of the acoustic spectrum, corresponding to the measured data. This applies not only to tonal noise but also to the broadband level. Work also explored the relationship between the stabilizer and the rotating rotor that generates the tonal sound.
^[52]	In this paper, the effect of a chamber to fan noise was studied numerically. The chamber was modeled as a rectangular box. The vortex-lattice method was used for calculating the unsteady flow of a fan.	The structure changes a fan’s performance, noise source and noise propagation. The noise of a fan propagates differently according to the system where the fan is installed.
^[53]	This paper showed an initial methodology for low-cost modeling of aft-emitted tonal fan noise. Everything took place in isolated and installed configurations.	The present paper discussed only the second harmonic (thanks to tachometer recordings, it is possible to filter signals into a tone and broadband components) due to time and resource limitations. The model captures modification by inserting the plate and the overall directivity of the experimental spectrum. The results indicate the noise source with emission similar to Mach wave radiation from supersonic jets (an extended, wave packet-like nature).

The main source of sound in a ventilation system is the fan. The problem of noise, or rather the noise produced by the fan, is very important, as confirmed by the number of scientific papers indicated above. The noise generated by the operation of the fan can be looked at from an energy point of view—it is closely linked to the efficiency and losses occurring in the system, and from an “aesthetic” point of view—it is not pleasant and affects people who are not far away. The noise generated by the fan depends on many factors. The main one is the design of the fan itself, in which attention should be paid to the number of blades and their shape. The next factors are efficiency and pressure, airspeed, the size and shape of the housing and its rigidity. Noise caused by aerodynamic and mechanical factors can be separated. Aerodynamic noise is directly related to the flow of air through the blade system and the accompanying changes in pressure distribution over the blade surface. Sources of mechanical noise are influenced by rotor imbalance, the incorrect operation of bearings, operation of the electric motor, and possible mechanical vibration of the components due to inadequate, that is, insufficient rigidity of the structure.

This entry is adapted from the peer-reviewed paper 10.3390/en16031042

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