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

Abstract

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.

Numerical methodology for the assessment of relative and absolute deterministic flow structures in the analysis of impeller–tongue interactions for centrifugal fans

“…At the bottom, the left plot reproduces the axisymmetric flow (throughflow), revealed as a chordwise distribution in both impellers. The results for the free impeller (blue curve) show a higher mean value of the distribution than the obstructed one, as expected from previous works reporting a 60-40% ratio of the total flow rate for the squirrel-cage fan [20]. Additionally, the flow non-uniformities in both reference frames are also illustrated in the figure.…”

“…Due to the sliding mesh technique employed, it is possible to analyze both time-averaged results (related to fan performance and overall efficiency) and unsteady maps (to understand in a closer look the energy transfer processes). In particular, previous investigations by the authors [20,27] have already presented the main flow characteristics for low (0.6 Q n , 212 m 3 /h), nominal (Q n , 352 m 3 /h) and high (1.5 Q n , 542 m 3 /h) flow rate for baseline operation (both impellers running normally) and fail operation mode (blocking the free inlet of the impeller, a typical industrial problem for these small units). These studies conclude the strong influence of the tongue over the impeller flow, specially distorted and susceptible to flow separation, and the large levels of unsteadiness distributed practically through the whole impeller.…”

“…The two parts are mounted producing a shift of the 23 blades, so that the mechanical resonances are minimized. However, it has been reported both experimental and numerically [3,20] that due to the different aspirating conditions for both parts, a 60%-40% ratio in the flow rate distribution is observed in the impeller outlet, thus providing a non-symmetric flow. The main geometrical variables and working parameters at the nominal conditions are shown in Table 1, for a rotational speed of 3600 rpm.…”

“…In the present article, this numerical methodology is extended to analyze blade-to-blade planes in this type of centrifugal fans for the first time. Using the numerical model presented and validated in [20], a comprehensive decomposition of the unsteady flow in both relative and absolute frames of reference is carried out in order to provide a deep understanding of the tongue-impeller interaction, revealing non-linear components in the phase dependent interaction. Interpolation routines and relocating algorithms, relating circumferential and temporal discretizations, are employed to segregate the different velocity components in both fixed and rotating frames of reference.…”

Purified orbit diagram and numerical study for a failure analysis of a Sirocco fan

“…A purified orbit diagram (POD) technique is used to complement previous efforts of the research group, according to different previous publications. 9,10,11,[20][21][22][23] It can be considered in the frame of previous works by the same working team, see, for instance, Velarde-Sua´rez et al, 9 Ballesteros-Tajadura et al, 11,20 Gonza´lez et al 21 and Delgado et al 22,23 Experimental set-up and measurements For the particular application, a squirrel cage fan unit to be studied is built with a pair of twin impellers' unit, arranged as shown in Figure 1 (left sketch). The two impellers are placed on both sides of the electrical motor and supported by the two sides of the shaft in cantilever, from its central position.…”