Sandra Velarde-Suárez

Numerical Calculation of Pressure Fluctuations in the Volute of a Centrifugal Fan

In this work, a numerical model has been applied in order to obtain the wall pressure fluctuations at the volute of an industrial centrifugal fan. The numerical results have been compared to experimental results obtained in the same machine. A three-dimensional numerical simulation of the complete unsteady flow on the whole impeller-volute configuration has been carried out using the computational fluid dynamics code FLUENT®. This code has been employed to calculate the time-dependent pressure both in the impeller and in the volute. In this way, the pressure fluctuations in some locations over the volute wall have been obtained. The power spectra of these fluctuations have been obtained, showing an important peak at the blade passing frequency. The amplitude of this peak presents the highest values near the volute tongue, but the spatial pattern over the volute extension is different depending on the operating conditions. A good agreement has been found between the numerical and the experimental results.

Noise Prediction of a Centrifugal Fan: Numerical Results and Experimental Validation

Centrifugal fans are widely used in several applications, and in some cases, the noise generated by these machines has become a serious problem. The centrifugal fan noise is frequently dominated by tones at the blade passing frequency as a consequence of the strong interaction between the flow discharged from the impeller and the volute tongue. In this study, a previously published aeroacoustic prediction methodology (Cho, Y., and Moon, Y.J., 2003, “Discrete Noise Prediction of Variable Pitch Cross-Flow Fans by Unsteady Navier-Stokes Computations ,” ASME J. Fluids Eng., 125, pp. 543–550) has been extended to three-dimensional turbulent flow in order to predict the noise generated by a centrifugal fan. A three-dimensional numerical simulation of the complete unsteady flow on the whole impeller-volute configuration has been carried out using the computational fluid dynamics code FLUENT ® . The unsteady forces applied by the fan blades to the fluid are obtained from the data provided by the simulation. The Ffowcs Williams and Hawkings model extension of Lighthill’s analogy has been used to predict the aerodynamic noise generated by the centrifugal fan from these unsteady forces. Also, the noise generated by the fan has been measured experimentally, and the experimental results have been compared to the numerical results in order to validate the aerodynamic noise prediction methodology. Reasonable agreement has been found between the numerical and the experimental results.

Unsteady Flow Pattern Characteristics Downstream of a Forward-Curved Blades Centrifugal Fan

The results of an experimental investigation of the flow at two exit radial locations of a forward-curved blades centrifugal fan are presented. Hot wire techniques were used to obtain steady velocity components and velocity unsteadiness levels (rms value of the components of velocity fluctuation) for different operating conditions, Globally speaking, the data reveal a strong flow asymmetry, with considerable changes in both magnitude and direction along the different circumferential positions. Particularly, big differences appear between the circumferential positions closer to the volute tongue and the other ones. The periodic character of the velocity signals due to the passing of the blades, clearly observed around the impeller, is missed in the vicinity of the volute tongue, where the main contribution to the velocity fluctuations appears to be random. Based on the measured velocity signals, velocity unsteadiness of the flow is determined analyzing the main contributions as a function of the flow rate and the measurement position. High levels of velocity unsteadiness were observed near the volute tongue, mainly at low flow rates.

Experimental Study on the Aeroacoustic Behavior of a Forward-Curved Blades Centrifugal Fan

In this paper, an aeroacoustic study on a forward-curved blades centrifugal fan has been carried out. As a first step, the fan performance curves, i.e., total pressure, power, efficiency and sound power level versus flow rate were obtained, showing its unstable behavior over a wide operating range. Second, the fan sound power level spectra for several working conditions were determined. For this purpose a normalized installation for testing in laboratory was designed and constructed. Afterwards, the velocity and pressure fields, both at the inlet and outlet planes of the impeller were measured using hot wire probes and pressure transducers, for different operating conditions. Finally, the aeroacoustic behavior of the fan was determined measuring the vorticity field at the impeller outlet, which is known to be related to tonal noise generation. This relation is worked out using the theory of vortex sound, developed by several authors during the second half of this century. The paper shows that the generation of tonal noise is produced at the blade passing frequency and it increases with the flow rate. Although the main contribution to fan noise generation is due to mechanical sources, the bands in which aerodynamic noise is generated by these fans correspond to frequencies especially unpleasant to the human earl Therefore, the research presented in this paper may be of considerable interest, establishing a starting point for the design of quieter and more efficient fans.

Total unsteadiness downstream of an axial flow fan with variable pitch blades

Variable pitch axial flow fans are widely used in industrial applications to satisfy variable operating conditions. The change of the blade pitch leads to a different rotor geometry and has a major influence on the unsteady operation of the machine. In this work, an experimental research on an axial flow fan with variable pitch blades has been carried out. First of all, the fan performance curves has been obtained. Then the flow field has been measured at ten radial locations both at the inlet and exit rotor plane using hot wire anemometry. Velocity components and total unsteadiness were determined and analyzed in order to characterize the influence of pitch blade and operating conditions on the flow structure

Numerical Model for the Unsteady Flow Features of a Squirrel Cage Fan

This paper shows a numerical research on the unsteady flow field inside a squirrel cage fan. The studied features are both the instantaneous flow fields and the average fluid flow associated to the blade passage frequency. The squirrel cage fan studied is a small centrifugal fan with a twin impeller configuration, each with 23 forward curved blades. The blades chord is 0.013 m and each impeller has a diameter of 0.08 m and a width of 0.094 m. The impellers operate inside an external spiral casing with a rectangular exit, followed by the outlet duct. A first series of experimental tests were performed in order to characterize the unit. The performance curves (head, power and efficiency versus flow rate) were measured. These tests show a nominal flow rate at around 0.098 m3 /s and a specific speed ωs = 1.9. From there on, three different flow rates were considered to study different flow behaviours in the impeller. In parallel with the mentioned experimental study, the unsteady 3D flow field inside the fan equipped with the same impeller was modelled for the referred flow rates, by means of the commercial CFD code FLUENT. To facilitate the modification of any geometrical feature, the mesh of the modelled fan was divided in several regions: inlet duct, impeller, volute and diffuser with outlet duct. The main goal of the paper is to show the numerical results obtained on the absolute and relative frames. Three main flow features will be analysed: the inlet flow distribution, the blade to blade field and the impeller exit flow. At the fan inlet, special care will be taken to detect possible recirculation or separation zones. On the other hand, and for each studied flow rate, the distribution of outlet flow field is also analysed. Conclusions on flow uniformity are drawn.© 2009 ASME

Numerical Simulation of the Unsteady Flow Patterns in a Small Squirrel-Cage Fan

In this work, a numerical simulation on the main flow features in a squirrel-cage fan, used in automotive air conditioning units, has been carried out. A 3D unsteady model has been developed for the entire machine. The flow in this geometrical model has been solved using the commercial code FLUENT®. Some of the analyzed features are the performance curves, the flow distribution over the different aspiration sections, the pressure and velocity distributions in selected surfaces, and the forces on the blades and on the whole impeller. The numerical results have been compared with the available experimental data, showing a reasonable good agreement.© 2006 ASME

Numerical Prediction of the Aerodynamic Tonal Noise in a Centrifugal Fan

In this work, a numerical study about the aerodynamic tonal noise generation in an industrial centrifugal fan with backward curved blades has been carried out. A three-dimensional numerical simulation of the complete unsteady flow on the whole impeller-volute configuration has been performed. Special attention has been focused on the impeller-volute interaction phenomena, analysing the influence of the distance between the impeller and the volute tongue. The numerical results have been contrasted using previous experimental investigations carried out in the same machine.Copyright

Experimental Determination of the Tonal Noise Sources in a Centrifugal Fan

In this work, an experimental study about the aerodynamic tonal noise sources in an industrial centrifugal fan with backward-curved blades has been carried out. Acoustic pressure measurements at the fan exit duct and pressure fluctuation measurements on the volute surface (specially in the vicinity of the volute tongue) have been made for different flow rates. The correlation of both pressure signals can lead to determine the zone of effective acoustic radiation. As was expected, the tonal noise generation is concentrated on the meridional plane of the impeller discharge, near the volute tongue and presents a strong dependence with the flow rate.Copyright

Flow Analysis and Deterministic Decoupling in a Squirrel Cage Fan

Squirrel cage fans are often used as blowers for automobile applications or for small industrial equipment. The flow in this kind of fans happens to be quite complex and with unsteady features, that makes it quite difficult to be studied. In particular, unsteady flow separation at the machine inlet or at the impeller blades and a variety of flow induced vibrations is found for most of the operation conditions. The deterministic stress analysis becomes an interesting tool in analyzing the main flow features from an existing numerical model. In this paper, the analysis of the internal flow and the periodic phenomena resulting from a deterministic study is intended. To accomplish this goal, a series of numerical routines were performed for different flow rates, including also failing operating modes of the fans (blocked inlets as a typical problem for this type of arrangements). The squirrel cage fan studied is a small centrifugal fan with a twin impeller configuration, each with 23 forward curved blades. The blades chord is 0.013 m and each impeller has a diameter of 0.08 m and a width of 0.09 m. The performance curves (head, power and efficiency versus flow rate) was obtained numerically in previous works and confirmed experimentally in normalized test campaigns. These tests have shown a nominal flow rate at around 352 m3 /h and a specific speed ns = 1.9. The main goal of the paper consists on the evaluation of the non-uniformities induced by the volute tongue over the blade to blade distributions within the impeller. As a consequence, fluctuation levels in the blade loadings, derived from deterministic non-uniformities can be provided in the relative frame of reference. The practical applications of the conclusions do imply a progress in the knowledge of the working parameters for machines that affect in a direct way to the passengers comfort.© 2011 ASME